CAST axion Mass Taking

Transcription

1 Status and plans of the CAST Experiment 103 rd SPSC meeting Esther FERRER RIBAS (IRFU/CEA Saclay) for the CERN-SPSC / SPSC-SR /10/2011 CAST COLLABORATION CEA Saclay CERN Dogus University Lawrence Livermore National Laboratory Max-Planck-Institut for Solar System Research/Katlenberg-Lindau Max-Planck-Institut für extraterrestrische Physik Max-Planck-Institut für Physik National Center for Scientific Research Demokritos NTUA Athens Institut Ruđer Bošković Institute for Nuclear Research (Moscow) TU Darmstadt University of British Columbia University of Chicago Universität Frankfurt Universität Freiburg University of Patras University of Thessaloniki Universita di Trieste Universidad de Zaragoza 1

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3 Table of Contents 1. Introduction Running in 2010 and a. Status of the experiment... 6 Remainder of the 2010 run... 6 Winter shutdown Shutdown... 9 b. Data taking coverage Data taking in Data taking in c. Grid measurements d. Sun Filming in e. Cryoplant Status The 3 He system a. Future operation with 4 He b. Computational fluid dynamics simulations Detectors a. Micromegas detectors Summary of performance Status of the background simulations and tests in Canfranc Underground Laboratory Characterization of spare detectors Understanding Ultra low backgrounds levels b. The x-ray Telescope and CCD detector Data taking Long term stability and performance of the Detector c. The BARBE detector Data analysis Request and detailed plans for running in Detector interventions Planning

4 Support requested from CERN for 2012 CAST 4 He run CAST magnet running costs Manpower support Plans for the future Improving the Micromegas background and the energy threshold Windows for sub-kev x-rays The framestore pn-ccd detector New Optics Conclusions References Appendix CAST mass coverage completion of the CAST helium program

5 1. Introduction The CAST experiment has been taking data since 2003 providing the most restrictive experimental limits on the axion-photon coupling for a broad range of axion masses. In 2003 and 2004 the experiment operated with vacuum inside the magnet (CAST phase I) and set the best experimental limit on the axion-photon coupling constant in the range of axion masses up to 0.02 ev. Beyond this mass the sensitivity is degraded due to coherence loss. In order to restore coherence, the magnet can be filled with a buffer gas providing an effective mass to the photon. By changing the pressure of the buffer gas in steps, one can scan an entire range of axion mass values. The CAST experiment started this gas program entering its phase II at the end of From 2005 to 2007, the magnet bore was filled with 4 He gas extending the sensitivity to masses up to 0.4 ev. From March 2008 onwards the magnet bore has been filled with 3 He. With the end of the 2011 data taking in July, the CAST experiment has covered axion masses up to 1.18 ev surpassing the initial goal of the phase II which was to reach 1.16 ev. The results of the first part of the 3 He, with a sensitivity up to 0.64 ev, have been finalized and accepted for publication in PRL[1]. Figure 1 shows the exclusion plot with this recent result compared to other measurements, theoretical and astrophysical bounds. Figure 1: Exclusion limit (at 95% CL) from the CAST phase I and part of the phase II data ( 4 He and 3 He up to masses of 0.64 ev), compared to other experimental, theoretical and observational constraints. This report is organized as follows: in section 2 we describe the activities in the experiment as well their present status; section 3 is devoted to the 3 He system; section 4 reviews detector performances; in section 5 we present the status of the data analysis; section 6 is devoted to the request for the 2012 run; plans for the future are given in section 7 and finally in section 8 we gather the conclusions of the proposal. 5

6 Q 2. Running in 2010 and 2011 a. Status of the experiment Remainder of the 2010 run After the last SPSC report in September 2010, CAST experiment took data until the 30 th November 2010 and then made two test fillings of the cold bore to higher densities to compare with CFD simulations. The 3He had to be removed by the 6 th December due to planned power cuts on the 9 th. The running was punctuated with 4 quenches Q (2 natural and two from infrastructure failures). In addition there was a 10 day loss due to a second problem on the cryo roots pump shaft seals and finally 3 days were lost due to a Quench Protection rack threshold tuning which was delayed due to higher priority LHC demands on the expert. A summary of the evolution of the pressure in the cold bore as a function of time is shown in figure 2. Q Q Q j cryo roots j problem j Q QPr n tune delay Q Q Q CFD tests Figure 2: Summary of the end of the He run. Cold bore pressure versus time. Winter shutdown The winter shutdown was planned to be short with no large interventions in order to restart data taking as early as possible in The main activities were: Maintenance and repair of various vacuum pumps 6

7 Annual torque testing of metal chain clamps Upgrading of the 3 He quench overpressure protection system for high pressure running in Integration of CAST power converter control electronics and software into the standard LHC control system However, despite a long-standing request for a prompt intervention for maintenance of the cryo system, end of February 2011, due to higher priority LHC work, the cryo maintenance was only started at the beginning of March This resulted in a number of consequences causing further delays to the start-up of CAST physics. During the CAST vacuum maintenance a number of metal seal-chain clamps suffered stress fractures and failed during the annual torque testing of the chain clamps (on both the 3He and the vacuum system). Several other clamps did not reach the nominal torque and gave cause for concern of an eventual failure during operation. These clamps were two of four clamps on the vacuum pipework directly connected to the x-ray cold windows with no intervening valve and a failure here would cause the breakage of the vacuum (and hence the cold windows) plus a dangerous inflow of air cryo-pumped onto the cold regions inside the cryostat. It was essential to intervene and change the clamps, necessitating warming the magnet towards room temperature from 200K, filling evacuated regions both sides of the cold windows progressively with pure N 2 gas and then changing the 4 clamps and replacing them with a heavier duty model on which higher torques could be applied. The cryo maintenance was completed at the end of week 9 (4 March). On starting the cryo compressors the oil pump failed and its repair and commissioning took 10 days. After the warm up and the intervention on the vacuum system, a further week was needed for pumping before cool down could start. Cool down finally started on 31 March At least 1 month later than had been planned. The late start also constrained the sun filming possibilities to one day s filming. The annual survey GRID was also affected it was not prudent to move the magnet vertically if the magnet is cooling or warming in the range 50K -290K due to additional shear forces on the composite cold feet of the magnet. Cool down was stopped due to vibration problems on the cryo compressor; a restricted GRID was executed during the tests on the compressor with the magnet slowly warming. Only positive vertical angles were measured on 12/13 April. The results were consistent with previous GRID in November The magnet was eventually cooled down and the magnet quench training was initiated on the 20 th April. A quench occurred at 12554A (very low compared with previous years). The following ramp up succeeded in reaching 13050A. Data taking was being prepared when the Roots pump shaft seal problem occurred again. Due to the Easter holiday, the intervention could only be done in the week after Easter and during Easter the magnet was run at lower field and a filling of 0.1 mbar 3He to at least use the time for Physics (exotics). 7

8 Immediately after Easter, the intervention on the shaft seals of the roots pump was made and with the magnet at 4.5K, the cold bore was filled to complete the coverage of small gaps in the previous scans before filling to continue the high pressure scan. In view of the lost time and the requirement to complete the scan by end July 2011, a larger step size was chosen (See Figure 3). Such a step size can be justified at high densities due to the widening of the mass acceptance due to the slight shift in density depending on the vertical angle during tracking. (See report CAST mass coverage completion of the CAST helium program sent to SPSC referees ) Soon afterwards, a problem developed in the wheel bearing of the Jura interior wheel on the chariot. The data taking was stopped to allow an intervention by the PH-DT engineer and mechanical technicians. One of the two wheel bearings had failed and the bearing had begun to disintegrate. Despite this, the wheel was still turning. cryo prob roots wheel prob Wheel shock tests Power cut over 3He recover Bake out Cryo prob 3He recover End 3He Physics Figure 3: Summary of the He run. Cold bore pressure versus time. A rapid intervention was made by PH-DT to remove the wheel and replace the two bearings and at the same time the Airport interior wheel bearings were inspected and found to be OK. A short survey check was then done to ensure that the horizontal encoder reference was not lost. The data taking restarted without incident for a further 10 days when small mechanical shocks occurred on or near the chariot emanating after the end of the evening tracking as the magnet was returning at full speed to the parking position. Data taking was stopped whilst the source of the shocks was thoroughly investigated with the help of the PH-DT team and the Mechanical Measurement lab at CERN. There was concern for the integrity of the cold feet of the magnet and for the personnel safety in case of structural failure of the chariot; particularly the lifting screws suspension points load pins. During the following week, the source was identified and the magnitude of the accelerations induced on the magnet was estimated. The source was a sudden shift of about 1 mm of the wheel axle through the new wheel 8

9 bearings causing a mechanical shock. It was then decided that the data taking could continue and an intervention could be done in the forthcoming shutdown. During this time, measures were taken to reduce these shocks by traversing more slowly back to the parking position. Later the Mechanical Measurement lab instrumented the CAST magnet with accelerometers and made a fast acquisition of the load pin signals. The load pins, which normally show ~ 5.5t loads showed spikes up to at 8.3t, well below the safety limits for these pins (30t safe static overload). The shocks measured near to the composite cold feet of the magnet were < 0.1 g (whereas the vibration specification for the transport of LHC magnets, the vibration level measured during the movement problem must not exceed the limit of 0.3g for longitudinal direction, 0.4g for transversal direction and 0.7g for vertical direction see EDMS ). Data taking between 6th June and 20 th June was smooth, during the vibration measurements mentioned above, on the 23 rd June at 16:18, there was a serious power cut to CERN and the CAST cryo system was tripped off (luckily magnet field was off). The prolonged 400V power cut provoked the 3He PLC to open the 3He safety valves linking the cold bore to the recovery vessel. As a result the 3He was recovered back to the storage vessel. Since CAST was due to make a bake-out of the cold windows (to remove cryo-pumped gases which reduce x-ray transmission), we immediately initiated a bake-out. The bake-out was completed 28 June. After the windows cooled, the cold bores were refilled with 3He followed by a 2-day stabilisation until the final adjustment is made to the density to enable data taking to restart on Saturday evening 2 th July. On the 10th July a further cryo compressor problem caused the magnet temperature to rise unacceptably and the 3He had to be recovered to avoid over-pressurisation of the cold windows. The final refill of the 2011 data run was completed by the 16 th July and data taking continued until the final run on the 22 July. In conclusion, the combination of the late start from the cryo plus several operational problems causing additional delays together with the stoppages due to mechanical movement problems resulted in about two month s loss of available data taking time Shutdown At the end of data taking, before the magnet warm-up, a mini-grid of survey measurements was undertaken (see section on GRID) starting 25 th July. Following this the magnet was force-warmed to 280 K between the 2 nd and 13 th August. Very small out gassing from the region of the x-ray cold windows was recorded indicating low quantities of cryo-pumped contamination. 9

10 Figure 4: Magnet movement Chariot (yellow), wheels inside their forks on inner and outer (guide) rails. Removal of Jura side inner wheel after bearing failure. Close-up of failed bearing. Magnet movement intervention As the magnet neared ambient temperature and vertical movements would soon be permitted, a program of magnet movement tests was initiated. The magnet was first traversed horizontally and various measurements made of the position of the wheel and chariot to the rails. Following this the Jura and Airport interior wheels were removed and the bearings changed and spacers added between the bearings and wheel forks to prevent movements of the wheels along the axle inside the forks (see Figure 4). The wheels were then re-installed and the horizontal angle of the wheels adjusted to point at the centre of rotation. Similarly, the vertical angle was adjusted to towards the nominal 2.2⁰ tilt inwards. These wheels were not well aligned beforehand. The survey of the wheels made after the intervention was that the wheels were aligned within 4 mrad horizontally which given the coarseness of the fixation design is the limit possible for setting the angle. New movements and measurements were initiated and the results are promising the trajectories of the wheels on the rails are reproducible and the hysteresis when changing direction horizontally has been significantly reduced. Furthermore, no shocks were observed during many horizontal traverses and also during the three complete simulations of a solar tracking at different points in the zone. Shocklog data logger accelerometers were installed during these movements and no acceleration was recorded above the threshold set at 0.1g. We consider the inner wheel problem now improved and stabilised. The outer wheels have not been touched. There remain stresses in the system but further interventions are thought to carry too much risk for the potential return. The detailed measurements made will be a reference for future comparisons. The outer wheels will be inspected and lubricated regularly and if the occasion arises the outer wheel bearings will be tested (e.g. during an intervention on the lifting jacks when the magnet is unweighted from the chariot). 10

11 Figure 5: Comparison of the movement of the inner wheels wrt the wheel fork versus the horizontal position of the magnet along the rails both before then after the intervention. The hysteresis is reduced and the trajectories reproducible. 13 ka cable replacement As discussed in previous reports, the outer sheaths of these high pressure water-cooled cables (length 22m see Figure 6) have aged due to repeated movement of the magnet and the torsions introduced by the magnet rotation. Due to the non-ideal original layout the cables, they were also subjected to too-small bending radii in certain regions. Replacing these cables was judged essential by the experts but this implies a substantial undertaking for CAST. The long break between the end of 2011 running and the proposed start of data taking in 2012 gave a near unique opportunity to change the 13kA cable sheaths which has been on hold for some years. A long period (10 weeks) 10 weeks) with the magnet warm is required to make the change plus this period must be outside LHC shutdowns to have availability of the CERN cable experts and specialised contractors. The intervention started on the 19 September 2011 and will be completed early in November There will then be movement tests to check and final adjustments of the cable lengths in the loop between potence cable arm and MFB. CAST wishes to acknowledge the expert support and organisation of the cable replacement by the EN- EL and EN-MEF. 11

12 Figure 6: Cable loop between cable arm-potence and the MFB. Lifting Jacks The magnet movement vertical lifting jacks have passed 45% of the recommended safe internal wear since their installation in late They are projected to reach 55% by the end of the proposed 2012 run. Given the present long shutdown, an opportunity arises to change the jacks and equip CAST for operation for at least another 4 years. Alternatively, the change can be made in the long shutdown foreseen for 1H2013. A drawback of the renewal of the jacks is that after a change in lifting jacks, there is a significant running-in period where the outer diameter of the lifting screws rubs against the new guide protection rings of the jacks causing significant vibrations. A lengthy period of movements is needed to run-in the movement system before a new GRID or data taking can commence. Since the magnet will be un-weighted from the lifting screws and jacks during the change there is also an opportunity to re calibrate the active load pin axles in the suspension point of the lifting screws at the top of the chariot. Every effort will be made to change the jacks in 2011 and recalibrate the load pins however there are conflicts with the availability of mechanical support from PH-DT and of the survey team which may make the intervention on the lifting jacks impractical this year. Full GRID In view of the changes to the inner wheel alignment of the chariot in August and the changes to the 13 ka cables and anticipating the change of lifting jacks, it is considered essential to make a full GRID survey of 90 points (~ 2 full weeks for surveyor team). It is virtually certain that the new GRID will differ enough from the GRID (2002) used in the tracking program to necessitate a change in tracking program reference GRID. Such a change has not been done since the start of CAST and the expert is now in the USA. It will require a significant effort after the GRID results have been analysed and available in order to insert and test the new GRID reference for bugs, check the software, safety end limits of H & V movement and a detailed test program of movements culminating in the next sun filming period March For this reason, the Survey GRID must be completed before the end of 2011 and in particular before the start of the winter LHC shutdown on 8th December 2011 due to non-availability of survey teams from then until early March b. Data taking coverage Data taking in 2010 In 2010, CAST collected data in the period from 5th May to the 30th November. The first part of the run was dedicated to cover all density settings missing due to 3 He leak in The second part of the run with new density settings started on 10th August at the setting that corresponds to 65.1 mbar (axion mass 0.85 ev). In the 12

13 period from 10th August to 30th November, 127 new settings were covered with a density step size of 1.4 dp (dp is the nominal pressure setting of about 0.1 mbar). This leads to the data taking efficiency of 69% (not taking into account breaks due to cryo problems). The run finished at the density setting which corresponds to 82.7 mbar at 1.8 K (axion mass 1.01 ev). Data taking in 2011 The 2011 data taking started covering a mini-gap from the 2010 run (10 days). The data with new density settings were collected in the period 11th May to 22nd July. In order to reach the CAST proposed goal (axion mass 1.16 ev) by the end of July 2011, a new scheme was introduced (the details are explained in the Appendix).Instead of the previously used step size of 1.4 dp (the change was done in the middle of a tracking), the new data were collected with a step size of dp (the change was done between a morning and evening tracking). The run finished at the density setting which corresponds to mbar at 1.8 K (axion mass 1.18 ev), with the data taking efficiency of 72%. The total exposure of CAST during 4 He/ 3 He data taking is shown in Figure 7. Figure 7: Exposure of CAST during the 4 He/ 3 He run covering years from 2008 to c. Grid measurements CAST performs periodically the so-called GRID measurements with the help of the team of surveyors at CERN. These consist in the independent measurement of the position of the magnet in a set of reference coordinates (GRID) previously defined to cover reasonably all range of movements. These measurements are intended to detect any drift in the pointing ability of the system with respect to the initial calibration values measured in 2002, 13

14 the ones which are used by the tracking software to determine the real absolute direction in which the magnet is pointing at any time. With increased confidence in the understanding and monitoring of the movement system over the years, the number of GRID points measured has been reduced to a subset and is referred to as a mini- GRID. The latest measurements were performed in July The system was found substantially unchanged with respect to the September 2007, and generally within the required precision of 1 arcmin in comparison with the reference values of the GRID of The results are shown in Figure 8. However: a. There are some points that have never been included in the mini-grid measurements, for which a bigger deviation has been observed. These points are concentrated in the region where the rail has the maximum deformation, and for vertical angles > 24 o. For these points the deviation is of the order of -4 10m (10% of the solar radius). By chance, they also coincide with the position of the window on the east wall, thus explaining the horizontal deviation that is observed in the solar filming. b. Though the agreement with the 2007 grid is quite good, the system appears to slowly deteriorate with time, showing a bigger dispersion every year. These facts indicate that the mechanics is in need of a revision and a full GRID measurement should then be performed, and the new data should be fed to the tracking program. Figure 8: Comparison of the July 2011 GRID measurements with the ones done in September 2007 (left) and the initial full GRID of 2002 (right). The required precision of 1 arcmin is indicated by the green circle, while the red one represents the 10% of the Sun projected at 10 m. 14

15 d. Sun Filming in 2011 Every year during two time windows it is possible to visually check the magnet pointing to the Sun. This is done by taking pictures during data-taking runs when the Sun can be seen through the experimental hall's window. Usually the window is opened for ten days with a maximum exposure of about 15 min. Up to now the weather conditions were always favorable for Sun filming at least one day during the filming window. The measurement campaign actually starts few days before the window is opened, by aligning the optical axis of the filming set-up with respect to the magnet's one. This is done with the help of CERN surveyors. After having fixed the camera on the magnet, it is connected to the PC dedicated to remotely controlling its parameters. The pictures are taken manually, at rate of two to five pictures per minute. At the end the pictures are analysed by a software written in LabView Vision, and the Sun's centre position is determined. The position obtained is confronted with the intersection of the optical axis (parallel to magnet's one) and the CCD. Last year Sun filming was done during the spring window (March 2011). However also the results from the autumn measurements will be included (September 2010) since these two were taken with the Sun filming system in its actual and possibly final configuration. Figure 9 shows an example of a picture taken during the Sun filming measurements. The obstruction of the field of view by trees is noted. For the moment it is not fundamental to remove them, but in the future they could completely cover the Sun. 15

16 Figure 9: Picture taken on :33. The Sun is behind the trees. Even if the entire disc is not visible the picture could be analysed. However the errors in estimating the Sun's centre are larger than in pictures where the entire solar disk is visible. In the last two measurements campaigns the weather conditions were favorable at least on one day. That allowed us to take a series of good photos well suited for analysis. The analysis of the images of both campaigns shows: very good vertical alignment horizontally the magnet is ahead by ~10% of the solar radius. These results are in agreement with the GRID measurements which show on one hand an excellent vertical precision and on the other hand a deterioration of the horizontal precision that is worse in the region of the Sun filming, corresponding to ~10% of the solar radius. As can be seen from the following graph the magnet pointing is the same, well within error, for both measurement campaigns. This makes us confident that a set-up has been constructed capable of checking the magnet pointing with a sufficient precision and repeatability. Also the alignment has shown good repeatability as observed in the graph below. Since the objective was removed for storage it can be concluded that the supports are stiff enough and machined with sufficient precision thus allowing us to do the Sun filming even without surveyor's support. Figure 10: The magnet is pointing to the right by 100 pixels. The solar disk's radius is 1143 pixels. This gives us a pointing error lower than 10%. 16

17 In order to confirm the repeatability and precision of the set-up more measurements should be done. However no major interventions and improvements are foreseen. The only possible intervention involves cutting the branches and foliage that can be found in the field of view. The set-up is also ready for Star filming. e. Cryoplant Status The roots pump of the Leybold pumping group was revised in September and was found to be in generally good condition despite several years of running close to or at its maximum temperature. The primary pump has not been revised since we made several interventions in 2010 and since then it has operated without a re-occurrence of the problematic shaft seal leak of The principal problem of the 2011 run year was several occurrences of leaks from the main helium compressors oil pump shaft seals. We have therefore taken the decision to replace this type of pump with a magnetically coupled pump, thus removing the shaft sealing problem. This pump and a backup pump have been ordered last week and we will install one as soon as practicable. We would like to install and run it for at least several days before the end of this year. In this way we should be able to debug any problems with its operation before the start of the 2012 run year. We have had several small problems related to the two turbo pumping groups on the magnet this year and we have therefore taken the decision to revise both of the groups during this shut down period. The first group from the MFB end of the magnet has been removed for revision and the second should be removed this week. Both the ports will be left closed by temporary caps. During the present shut down the Cryogenics group s mechanical section are charged with testing the mechanical structure of the main helium compressor in order to identify the source of the abnormal levels of vibration seen since the oil pump failure in April This failure necessitated the replacement of approximately 50 sand bags used for mechanical dampening and oil capture and has led to some noticeable increase in vibration levels. All other tasks associated with normal shutdown revision of the cryoplant will be performed before Christmas 2011, with a view to having the cryoplant ready for restart in the first quarter of

18 3. The 3 He system The 3 He gas system operation in 2011 was successful. The previous upgrades to the system in 2010 allied with the accumulated expertise over the years were fruitful and allowed for a smooth operation in different modes and the rapid resume of data taking after power cuts and cryogenics related problems. In 2011 the gas system covered a total of 205 density settings, reaching a maximum pressure in nominal conditions of mbar (equivalent to 1.18 ev). At the end of the data taking run in July 2011 the 3 He gas was safely recovered from the magnet cold bores. In addition, all the lines of the system were evacuated from gas and the total amount of 3 He is presently stored in the dedicated storage volume. With the end of the 3 He programme for CAST, the 3 He gas will be expedited back to LLNL. In collaboration with the CERN Cryolab, it is now under preparation an extraction system to transfer the gas from the present 3 He gas system into a pressurized cylinder to be transported over to the US. The design of the extraction system is well advanced and most of the necessary components have been procured. a. Future operation with 4 He In 2012 we plan to re-scan the 4 He region with higher sensitivity. The present gas system allows making that transition without major modifications. Due to the existing PLC and control system it will be able to cover all the required different modes of operation in a flexible manner. It will benefit from the experience gained over more than 3 years of operation with 3 He and well established procedures that have been improved over time and smoothly integrated in the shift procedure. b. Computational fluid dynamics simulations A set of computational fluid dynamics describing the present system at CAST have been developed to better understand the behaviour of the gas inside the magnet cold bores in order to qualify and quantity its dynamics. The previous model has been continuously improved to better describe the real model and its complexity. An independent CFD simulation was performed in collaboration with the CFD CERN team taking advantage of their expertise and computing power. Two different base models have been developed to describe the steady state case with the magnet in its horizontal position; these two scenarios correspond to two different run conditions: heated x-ray windows and cold x-ray windows. We have now a good understanding of the convection phenomena that occurs at extremities of the cold bores and how it affects the effective length for axion conversion in a steady condition. Figure 11 shows the gas density in the cold bore. Figure 12 shows the density variation of the effective length at various pressures for hot and cold windows. 18

19 Under way is the development of more elaborate model where the tilting of the magnet is introduced. This kind of transient simulations are much more complex, requiring higher computing power and longer time for full conversion of results, but we hope to have then a great tool for analysis of the gas behavior. Figure 11: Simulation of the gas density in the cold bore. Figure 12: Density variation of the effective length at various pressures for hot and cold windows. 19

20 4. Detectors a. Micromegas detectors The Micromegas detectors were not dismounted during the winter shutdown. The commissioning of the three detectors went rather smoothly as well as the data taking. The three detectors exhibited a very high efficiency both in 2010 and 2011 as can be seen from Table 1. The electronic noise, that had been a problem in the past, was stable and well below the threshold for the three systems. Data taking efficiency (%) SUNRISE SUNSET SUNSET Table 1: Data taking efficiency for the Sunrise and Sunset Micromegas detectors for the 2010 and 2011 data taking. The simulations, discussed later in detail, have concluded that an important fraction of the background comes from the front of the detector via the pipe. Due to this fact, the three systems carried out an upgrade of the shielding within the limited space available. For the Sunrise side the upgrade of the shielding was done before the start of the data taking. It consisted of covering with copper the front of the detector with some copper plates making a thickness of 5 cm. The pipe connecting the detector to the line was also shielding with a 10 cm long cylinder 1 cm thick. Figure 13 shows a picture of the system before and after the shielding upgrade. 20

21 Figure 13: Front of the Micromegas Surise detector before (left) and after (right) the shielding upgrade. The upgrade of the shielding for the Sunset side was done on the 10 th May, after the start of data taking. However, the Sunset Micromegas pipeline is much more compact than the Sunrise side, making it difficult to include extra shielding. The only thing that could be done without disrupting the data taking was to insert a 1.5 cm copper piece in front of the detector. Pictures of the extra layer are seen in Figures 14 and 15. Figure 14: Left: The copper shielding for the Sunset detectors. Two similar pieces are constructed to cover the pipe tightly, and shield the front of the detector. Right: Drawing of the new shielding. 21

22 Figure 15: Photos of the Sunset shielding from bottom and top. Summary of performance In order to complete the previous SPSC report, we present in Table 1 and Table 2 a summary of the 2010 data taking as well as the details for the 2011 data taking. The 2011 data has been analyzed with a quick analysis. The detectors show good stability as can be seen in Figure 16. A preliminary estimation of the background rate is given in Tables 2 and 3. A preliminary background spectrum is shown in Figure SUNRISE Tracking Background Tracking Background Time (h) Counts Mean Rate ± ± ± ± 0.10 (2-7 kev) (c kev -1 cm -2 s -1 ) Table 2: Preliminary performance of the Sunrise detectors in 2010 and In 2011 a total of 52 pressure steps, covering an axion mass range from 1.01 to 1.18 ev. 22

23 SUNSET 1 Tracking Background Tracking Background Time (h) Counts Mean Rate 5.90 ± ± ± ± (2-7 kev) (c kev -1 cm -2 s -1 ) SUNSET 2 Tracking Background Tracking Background Time (h) Counts Mean Rate ± ± ± ± 0.11 (2-7 kev) (c kev -1 cm -2 s -1 ) Table 3: Preliminary performance of the Sunset detectors in 2010 and

24 Figure 16: Stability of the gain for the Sunrise and Sunset Micromegas detectors. 24

25 Figure 17: Preliminary background spectrum for the 2011 data for the Sunrise and Sunset Micromegas detectors. Status of the background simulations and tests in Canfranc Underground Laboratory The achieved background upper bound, < c kev -1 cm -2 s -1, found for the CAST detectors intrinsic radiopurity with the CAST dedicated set-up in the Canfranc Underground Laboratory (LSC) is an encouraging result as it does not limit in practice the expected performance of CAST detectors in the experiment[2]. The available margin to improve our background is still very large. Since a few months ago we are using the especially sensitive set-up in the LSC (thanks to the 20 cm thick of lead shielding) to evaluate the contribution to the background of some internal components of the set-up and external agents. We checked that the stainless steel cathode (the unique piece of the detector made of a non radiopure material) has a second order contribution which is not important at present CAST levels, but it can be convenient to replace it by a copper one if the next upgrade of CAST detectors would have the effect we expect. In contrast we showed that the effect of the radon nearby the detector is negligible with the radon concentration present in the surface. At the same time we have consolidated our primary result as the ultra-low background level was reproduced every time after being disturbed in order to carry out these tests (see Figure 18). 25

26 Figure 18: The ULB achieved in Canfranc, under special shielding conditions, interrupted by the measurements of stainless steel and radon contributions. The fact that the ultra-low background was achieved in the LSC set-up just via shielding upgrade points out the idea that the CAST micromegas background is dominated by the external gamma flux contribution with origin in the natural radioactivity of the environment. At this moment the model built for the background of CAST detectors due to the external gammas from Monte Carlo simulations can be considered practically completed [3]. Only the partial knowledge about the gamma flux in the CAST experimental area does not allow a detailed comparison between simulation and real data. For this reason a new set of flux measurements were disposed during last summer. They have shown the overall radiation level to be roughly half of LSC one and a more precise description is being prepared. Anyway the preliminary background reconstruction with the simulation model shows as main features a final background level around fews 10-6 c kev -1 cm -2 s -1 dominated by steel and copper fluorescences in agreement with the observed one. The most appealing conclusion from the simulations is the huge contribution from gammas which would enter into the detector through the aperture of the shielding that is necessary to connect the detector to the magnet bore: around 70% of the background counts. This weak point of the shielding would be even more dangerous due to the possibility of generation of fluorescence in the inner walls of the stainless steel pipe. This is a clear indication about how to improve the shielding that is going to be applied in the next upgrades of the micromegas set-ups: improving the shielding of the pipe to minimize the solid angle available to external gammas and avoiding fluorescences. One set-up in the LSC and the analysis of the simulated background have tried to estimate the effect of the aperture of the shielding in the background, and therefore showing the possible improvement even maintaining only a 2.5 cm lead thickness as in present set-ups [4]. The M10 detector, which was in the sunrise side during the 2008 data-taking, was installed in the LSC inside a shielding of 2.5 cm thickness but without any aperture. The suppression of the fluorescences peaks is very clear, but the effect of the better closure is expected to be much bigger as we now know that the gamma flux in Canfranc is at least twice it was in the CAST area when the first spectrum was taken (see Figure 19 left). The simulations support the result of this test (see Figure 19 right). 26

27 Figure 19: Left: Comparison between two experimental backgrounds taken with the same detector, first in the sunrise side of CAST magnet (blue line) and later in the LSC (red line) whose shielding has no aperture. In the same graph we represent with a green line the estimation for the background in the CAST area if using a completely closed shielding which is based in the LSC background scaled by the ratio of the external gamma flux intensity in both sites, which has been measured this summer by the CAST team. Right: In blue it is represented the simulated background for the sunrise set-up, and in red the same background after subtraction of the shielding aperture and pipe contribution. Besides this fact, the external origin of the CAST background, suggests that, if first the closure of the shielding is improved, an extension of the shielding thickness should also drive to an important reduction in the background. In the same sense, the last set-up tried in the LSC confirmed that only 10 cm thick lead shielding are enough to get the previous ultra-low background level, and this is a thickness that looks like much more feasible to be installed at surface. Characterization of spare detectors Three new detectors have been built. Two of them have been fully characterized in Saclay. The detectors exhibit nominal energy resolutions of 15 and 16% (FWHM) in 5% Argon-Isobutane. They have no missing strips and show full performances. The third detector will be characterized at 5.9 kev in Saclay at the end of October. These detectors will be tested in the CAST detector laboratory at CERN with the X-ray generator (designed and built in MPE). The tests are expected to take place at the end of 2011 and early This test will allow measuring the efficiencies of the detectors in the range of interest of CAST (2-10 kev) for CAST. The detectors will also be tested with the T2K electronics (3D electronics) and different shielding set-ups in order to optimize the design of the new Micromegas detector. 27

28 Understanding Ultra low backgrounds levels The Micromegas group has been devoting a substantial effort to understand and quantify the very low background levels observed in the detectors installed in CAST, the so called ultra low background (ULB) periods. Since 2008, different ULB have been observed. During these periods, the background level reduced one order of magnitude, reaching a value around 10-7 c kev -1 cm -2 s -1. To clarify the origin of these periods three strategies have been developed in parallel: 1) Shielding tests at ground in CERN and in Saclay; 2) Shielding tests in the Underground Laboratory of Canfranc 3) Simulations of the detector. The origin of this effect is not yet understood. It seems that there is a slightly change of behavior of the detector during the ULB period from an unknown cause. Nevertheless thanks to the different efforts in the last three years we have a much clearer understanding of how we can reach lower levels of background. In particular thanks to the simulations and the shielding tests done at surface and underground, we know that increasing the shielding reduces the background. This points out that the CAST background is dominated by external gammas. Therefore if we improve the shielding coverage, especially in the connection of the detector to the magnet we will reduce our background. We have defined a roadmap of tests to explore ideas and consolidate lower backgrounds. A first step will be to improve the Sunset detectors shielding for the 2012 data taking, as it is discussed in section 6. At the same time we are finalizing a new design for a Micromegas detector for the vacuum phase (see section 7). In parallel, we will study to further improve the intrinsic radioactivity of the detector as well as to complete the background model. 28

29 b. The x-ray Telescope and CCD detector Data taking Since the last report in 2010 the CAST X-ray Telescope demonstrated good data taking reliability and efficiency. However due to some technical problems (DAQ and vacuum system failures) some tracking runs were covered with the other detectors. The overall tracking coverage was of 83%. Most of this off time was caused by DAQ failure. Actions were taken to avoid these failures in the future and the two new systems are now functional. The preliminary statistics of 2010 and 2011 data taking periodes (since last report) for CAST X-ray Telescope are summarized in Erreur! Source du renvoi introuvable.4: Year Dates considered Days of detector operation Covered trackings from possible ones Total Detector live time Total Axion sensitive time > /50 (74%) 4235 hours 131 hours > /55 (91%) days 5.45 days Table 4: Summary of data taking statistics for the X-ray Telescope. In total there are 87 tracking runs with 770 events on the entire chip, in total 28 events were in the expected signal region. These numbers correspond to: counts per tracking on the entire chip corresponding to a flux of events per second per pixel counts per tracking in the spot corresponding to a flux of events per second per pixel The event distribution on the CCD chip is shown in Figure

30 Figure 20: Spatial event distribution of the data acquired with the X-ray telescope in the entire 3 He phase. The intensity is given in counts per pixel and has been integrated over the full exposure time. 30

31 Long term stability and performance of the Detector A sample calibration plot, actually the last one taken before 2011 detector shut-down is displayed in Figure 21. Figure 22 and 23 show the long term stability of certain parameters of the 55 Fe calibration runs taken each morning before the tracking starts. The plots 22 and 23 cover the entire 2010 and 2011 data taking (intervals stated in the first table). In the first box is plotted the Gain [ev/adu] versus time. The Gain is essentially the conversion factor between photon energy and electronic readout signal of the detector. In the second box is plotted Charge Transfer Inefficiency (how efficient is the reading of the signal from the CCD). In the next two boxes are plotted the mean noise and the mean offset in ADU averaged over all pixels. Furthermore in the last 3 boxes, the peak position of the Mn-Kα line, its FWHM and the intensity are displayed. Time is given in standard notation (Universal time, UT) as well as in Modified Julian Day (MJD), which is defined via the Julian Day (JD) by MJD = JD All these plots and the daily quality check of the data and other parameters indicate that the detector has stable performance during long periods of time and provides high quality data, comparable to the level achieved in previous data taking periods. 31

32 Figure 21: Calibration spectra of CCD detector with 55 Fe source. (last day before detector shutdown) 32

33 Figure 22: CCD performance and stability during 2010 (since last report). 33

34 Figure 23: CCD performance and stability during

35 c. The BARBE detector The BaRBE detection system for low energy photons has been taking data in its present form since June We recall here only a brief description of the system since it has already been discussed in greater detail in previous reports. Low energy photons (1-2 ev) propagating in the VT4 beamline in the CAST magnet Sunrise side are collected by a semitransparent mirror and directed toward an optical telescope matching the beamline aperture (41 mm) with the 9 mm aperture of the input collimator of an optical fiber. The mirror is transparent to soft X-ray photons. The optical fiber (40 m long, multimode, 200 μm core) conveys photons to an optical switch alternating, at a selectable frequency (1 Hz presently), the fiber output between two ports. At the moment one of the ports is instrumented with a photomultiplier tube (PMT) while the other is darkened. This arrangement allows for online monitoring of the background and is actually designed for two different detectors taking alternatively live data and background counts. The total acquisition time is set to 5500 s/day during sun tracking and to s/day with the magnet in parking position ( background data). At the end of each daily run the software generates summary histograms, giving the number of 0.5 s periods reporting a given number of counts, and sends them via for preliminary analysis. In 2011 the BARBE system started gathering data in March and continued through July for a total of 3.3x10 5 s of sun-tracking data and of 4.3x10 6 s of background data. The average counting rate in these periods was about 0.4 Hz and there was no statistically significant excess of counts between sun tracking periods and background periods. However, taking advantage of the fact that the detector, thanks to the optical switch setup, looks for 0.5 s at the CAST beamline and for the next 0.5 s at a darkened fiber, one can subtract the instantaneous background from the tracking counts to estimate a differential Dark Count Rate (DCR) of about 5x10-3 counts/s. This DCR can be used to calculate bounds on processes resulting in visible photons propagating in the CAST VT4 beamline. In particular, a significant bound on solar hidden photon (paraphoton) production can be derived using the BaRBE data. A preliminary version of this bound 1 is shown in Figure 24 where the kinetic mixing parameter χ is plotted as a function of the paraphoton mass. BaRBE data exclude all values above the red line. Shaded regions represent existing bounds from astrophysics (medium gray), laboratory experiments (dark gray) and CAST vacuum run X-ray data (light gray). During the CAST shutdown an optics upgrade has been planned to increase the angular field of view of the BaRBE optics from 1.5 mrad to the full 10 mrad needed to cover the angular diameter of the sun. This new feature will significantly improve the bound on hidden photons obtained from the data. 1 The graph has been been generated by S. Troitsky, of the Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia, using flux calculations from the solar model he is developing together with data produced by BARBE-CAST. An updated version of this plot is the object of a forthcoming CAST publication also co-authored by S. Troitsky. 35

36 Figure 24: Preliminary exclusion plot showing the kinetic mixing parameter χ as a function of the paraphoton mass. 36

37 5. Data analysis The first part of the 3 He data corresponding to axion masses between 0.39 ev and 0.64 ev corresponding (first 252 density steps) has been fully analyzed. The data analysis has been performed in a similar manner to our previous results obtained with 4 He gas. The main difference is that we have used the unbinned likelihood method, instead of the binned one used in our previous result. This is motivated by the overall reduction of background rates achieved by CAST detectors with respect to the ones of the 4 He phase, as well as due to the reduced 3 He density setting exposure time (one half that for 4 He). Moreover a best fit value and the deviation of the coupling constant is obtained after the likelihood analysis (for a fixed axion mass), comparing if the fit result is compatible with the absence of positive signal. Indeed, the effective number of background counts in this analysis is about 1 count per density step for the Micromegas detectors, and about 0.2 in the fiducial spot of the CCD/Telescope system. The final result is given in Figure 25 where it can be seen that CAST extends its previous exclusion plot towards higher axion masses, excluding the interval ev down to an average value of the axion-photon coupling of GeV 1. An extended view is shown in Figure 25. It is the first axion helioscope ever that has crossed the axion line for the benchmark KSVZ case. These results have been submitted and accepted to PRL for publication [1]. For the data of exclusion plots for individuals detectors are well advanced. Our goal is to produce a preliminary version before the end of the year, with time to identify a candidate before the 3 He is sent to the United States. 37

38 Figure 25: Top: Exclusion regions in the m a g aγ plane achieved by CAST in the vacuum, 4 He and 3 He phase. We also show constraints from the Tokyo helioscope, horizontal branch (HB) stars, and the hot dark matter (HDM) bound. The yellow band represents typical theoretical models with E/N 1.95 = The green solid line corresponds to E/N = 0 (KSVZ model). Bottom: Expanded view of the limit achieved in the 3 He CAST phase for axion mass range between 0.39 and 0.64 ev. 38

39 6. Request and detailed plans for running in 2012 CAST has enjoyed the sustained development of its detectors towards lower backgrounds during all its lifetime. The latest generation of Micromegas detectors installed at CAST are achieving background levels in the range counts kev -1 cm -2 s -1, a factor 20 better than at the beginning of the experiment in Moreover there are realistic prospects for even lower levels for the near future. This achievement allows us to consider revisiting some of the past data taking configurations with enhanced sensitivity to standard Peccei-Quinn axion models, the main objective of CAST so far. In particular, we consider to take data repeating some of the 4 He settings in 2012 and to take data with vacuum in the magnet bores as in CAST phase I in 2013 and 2014 with upgraded systems[5]. The latter will be discussed in section 7. Due to the low statistics of CAST phase II stepping strategy (each detector has a background per density step of about ~1 count, ~0.2 counts for the case of the CCD) a considered data taking strategy for the new campaign is to focus on a restricted axion mass range, but gathering more statistics per density step. In this way a net improvement in sensitivity to the axion-photon coupling could be achieved for the mass range in consideration. The mass range around the end of the 4 He phase, i.e. 0.4 ev, is probably the most adequate target for this strategy because 1) the improvement in detectors backgrounds with respect to 2006 levels implies an improvement in sensitivity with respect to the current published CAST 4 He exclusion line, and 2) at these mass values, this improvement could be sufficient to cross the benchmark KSVZ axion models, shown as a green line in the exclusion plot shown in Figure 26, which lies close but beyond current CAST limit at these masses. Among the Peccei-Quinn axion models solving the strong CP problem (indicated in the plot by the yellow band with somewhat arbitrary width representing the range of realistic models), the KSVZ ones are classical examples of hadronic axions in which the aγγ vertex arises only through axion-meson mixing. Since data taking in vacuum conditions (CAST phase I) supposes a longer exposure time and therefore the possible achievement of lower backgrounds for the detectors plays an even more important role. In fact, the published CAST phase I limit is largely determined by the x-ray telescope + CCD data, being the system enjoying the best signal to noise ratio at the time, thanks to the gain coming from the focusing of the signal into a few mm 2 area. Revisiting CAST phase I with the current set of detectors would suppose a relatively modest improvement, because despite the improvements in Micromegas backgrounds still the sensitivity of CAST in a long exposure run is in a good part driven by the focusing power of the x-ray telescope. A further improvement in background for the Micromegas detectors (approaching or reaching 10-7 counts kev -1 cm -2 s -1 ) would however bring each of the 3 Micromegas systems to a signal-to-noise ratio comparable to the one of the x-ray telescope / CCD system. In this case, the potential CAST sensitivity would be substantially improved. The interest of a revisited phase I is therefore dependent on the availability of detectors with further improved background in the near future and/or a larger number of detectors that could be coupled to new x-ray telescopes. Therefore, in 2012 we propose a relatively short 4 He run (3-4 months) so that we cover the KSVZ as much as possible and in parallel we concentrate on an ambitious R&D program that is described in Section 7. The R&D program is oriented towards the preparation of the vacuum phase as well as the New Generation of Axion Helioscope (NGAH) [6] where it is expected to gain 1 to 1.5 orders of magnitude in sensitivity. For this purpose a new experiment is being designed based on the innovations already introduced by CAST. At the moment we are preparing a Letter of Intent that will be presented in an independent proposal to the SPSC. The plan for is to revisit the vacuum phase with improved detectors and new x-ray optics to search for solar axions with improved sensitivity. In parallel Micromegas detectors with thin windows and/or the windowless new frame CCD 39

40 with few 100 ev threshold will allow to directly search for solar chameleons or other WISPs. A specific request will be made in a later proposal. In Figure 26 we show the expected region that will be explored by the CAST experiment by revisiting some 4 He density settings. The plots are computed simulating by Monte Carlo the detected background counts and performing a likelihood analysis and computation of the 95% upper limit, using exactly the same experimental parameters (background level and efficiencies) of the current detector setups installed at CAST Two exclusion lines have been simulated: the first one, in pink, including the 4 detectors of CAST and a second one, in blue, with only 3 detectors as a conservative scenario. They must be viewed therefore as realistic estimations. The chosen scanning range and procedure for these curves is such that it starts at ev (corresponding to 15.5 mbar) and proceeds downwards in mass in density steps of 0.1 mbar, remaining 10 tracking per density step during 3.5 months. 40

41 Figure 26: Top: Expected sensitivity lines with current CAST setup by repeating 4 He settings. Bottom: Zoom of the expected sensitivity lines with current CAST setup by repeating 4 He settings. 41

42 Detector interventions No change is planned in the detectors of the Sunrise side. For the Micromegas Sunrise detector we plan to study the variation of the background level due to a radioactive gamma source in the vicinity of the detector. This test will show if the background is affected by external gammas. This result would point out that coating the vacuum pipe in front of the detector with Teflon could reduce the background level. The measurement will be carried out in the next week and only a few days of data taking should be enough. Besides these background tests, we might dismount the detector for calibration in the CAST detector laboratory with x-rays at different energies. This will allow the optimization of the offline analysis and contribute to the understanding of the ULBs. As the Sunset Micromegas detectors have shown a modest energy resolution, we will profit of the winter shutdown to replace them by the available detectors that we have fully characterized in the laboratory and also with the x-ray generator in the CERN detector lab. The performance of the detectors in terms of efficiency and background rejection should be improved. Moreover we plan to upgrade the shielding in the front of the detectors in the sunset sides. Since the compactness of the current sunset pipeline does notallow to improve the shielding, the pipeline will have to be modified. The plexiglass in front of the detector will be replaced by Teflon which will be extended further in the pipe to stop florescence of copper. There will be also an additional copper pipe between the steel vacuum pipe and the Teflon. Around the detector there will be copper and the outmost part will be covered by thick lead shielding (5 cm). The conceptual design can be seen in Figure 28 below. Figure 28: Upgrade of the Micromegas sunset shielding, described in detail in the text. 42

43 Planning Despite the envisaged long shutdown of CAST, there are strong time constraints, particularly before December The overview of the shutdown schedule is shown in Figure 29. Figure 29: Schedule for CAST shutdown The key points: Magnet at RT 13kA cable replacement Movement tests Possible change of lifting jacks Pump down cryostat isolation vacuum Full GRID Extraction of 3 He Purging and Filling with 4 He Repairs to CCD/XTRT vacuum pumps and X-ray finger Dismount the SSMM for upgrade to detectors and shielding Commissioning of the XRT/CCD Vacuum system Add new x-ray finger plus check of alignment of XRT/CCD & finger. Update solar tracking reference GRID and test tracking Sun Filming check of new tracking Install and commission new SSMM Fill 4 He 300 mbar to shape x-ray cold windows Cool down magnet Quench tests when cold Fill cold bore for data taking Allow 6 week reserve at end of data taking in case of unforeseen delays. 43

44 Forced warm-up of the magnet in preparation for the intervention to remove the thin x-ray windows from inside the cryostat in preparation for the future vacuum run. Support requested from CERN for 2012 CAST 4 He run CAST magnet running costs The estimate for the operational costs for operating the CAST magnet in 2012 together with the actual costs in the years of the 3 He run ( ) are shown in Table 5. Item Dept Year Cryogenics M&O AT->TE (incl gases) (hours) Cryogenics power TS->TE (CHF) (hours) Magnet PS Ts->TE (CHF) FSU AB->BE CAST Yearly TOTAL CERN (CHF) Table 5: Magnet support costs for 2008 to 2011 an projections for

45 Manpower support CAST requests from CERN continued support at a similar level to the past years, namely: PH-DT Mechanical engineer consultant. Mechanical technician support for the experimental apparatus including movement system. Electrical technician support for Slow Control and interlocks Applied Fellow responsible for operation of 3 He system TE-VSC TE-EPC Consultant vacuum technician Support for the Power Converter (PC) operation and maintenance TE-CRG Cryolab support for maintenance of the 3He systems and responsibility for any manipulations of 3 He gas Support for the maintenance and operation of the 3 He system PLC Support for the operation of the magnet cryogenics and its control system TE-MPE EN-ICE BE-ABP TE-CV Support for the Quench Protection rack Support for the Power Converter controls system Surveyor support Support for Demineralised Water cooling system for 13kA cables and Power Converter 45

46 7. Plans for the future For the R&D we will focus mainly on three aspects: Finalise the existing new design of the Micromegas detector in order to reach levels approaching 10-7 counts kev -1 cm -2 s -1 with improved energy threshold; Develop thin transparent windows to search in parallel for other WISPS of solar origin in particular hidden sector paraphotons and chameleons[7] ; Develop new optics to be coupled to a Micromegas detector on the existing Sunrise Micromegas line. Improving the Micromegas background and the energy threshold A new Micromegas detector is being designed with improved features. The design will be finalized in December The main improvements are expected from the inclusion of readout electronics providing 3D topological information, increasing the radiopurity of the construction materials and finally building a more compact and full coverage shielding. Two views of the design at its present status are shown in Figure 30. The lead shielding of the actual setup will be replaced by a lead gas-tight vessel, which will contain the gas mixture and will conserve the shielding power. The inner vessel, composed of two pieces of stainless steel and plexiglass, will be replaced by a drift cage of copper and peek. In this way, a uniform drift field will be guaranteed and there will be less and cleaner materials near the readout. Finally, strips will be digitized by the AFTER electronics. This new acquisition system will convert the detector a real TPC which will increase the background reduction. Figure 30: Two views of the new design of the Micromegas detector to be installed in CAST in Left: The drift cage and the flat cable extracting the readout signals as well as the feedthrough connector can be seen. Right: the vessel and the way the electrical connections are done. 46

47 Improving the energy threshold The CAST Micromegas detectors have been running with a gas mixture consisting of Argon and Isobutane. This gas is well adapted for measurements in the 1-10 kev range, providing very good energy resolution and high gains of the order of In order to decrease the energy threshold the signal to noise ratio needs to be increased and higher gain is needed. This can be achieved by replacing Argon with Neon. In this case, the absorption efficiency is still high for photon energies up to 3 kev while the achieved gain is increased of a factor 10. Tests with smaller and simpler Micromegas detectors have been done with this new mixture and a threshold below 300 ev seems feasible [9]. Figure 31 shows the energy spectrum obtained with a Ne based mixture using an 55 Fe source. It can be noted that the escape Ne peak at 910 ev is clearly seen and that the energy threshold is probably less than 300 ev. Before confirming the possibility of using this type of gas mixture for CAST, a full analysis needs to be performed with the CAST Micromegas detector in order to verify that the background rejection is of the same order as the one obtained with our nominal mixtures. Figure 31: Energy spectrum obtained with a Ne based mixture using a 55 Fe source. Windows for sub-kev x-rays Different lines of R&D are on going to develop transparent detector windows for soft x-rays (i.e. below 1-2 kev). One line of R&D is devoted to porous aluminum membranes with the base material being Al 2 O 3. Foils of different sizes (13 to 47 mm in diameter) as well as porous diameter (35 to 200 nm) are available. The porosity of the membranes is from 12% up to 50%. The underlying idea is that a fraction of incident photons can be transmitted either directly or channeled through the pores. This material has already been used for different applications but at rather high energies (8 kev). In order to check the suitability of this type of windows for CAST, the transmission is going to be measured in the Maxim x-ray facility at DESY. 47

48 First transmission measurements with scintillaton detector are scheduled to be made at 4 different photon energies from 6 kev to 14 kev, aiming to detect some collimation effects. The spatially resolved measurements can be done with a narrow beam size of 0.1 mm. A sketch of the set-up is given in Figure 32. Figure 32: The position resolving detector to be used with an absorber to map the footprint of the transmitted beam with respect to sample tilt angle. The camera is equipped with a glas polycapillary system as collimator, allowing only radiation perpendicular to the CCD chip plane to contribute to the image [9]. In parallel, another line of research is dedicated to kapton based thin windows (1 µm). The windows are manufactured using the same techniques as for the Microbulk Micromegas detectors, mainly photolithography and kapton etching in the Rui de Oliveira workshop of CERN. The idea is to produce a thin honey comb of kapton. A first batch of 6 windows has been produced. The thickness and the uniformity of the kapton will be measured as well as their leak tightness. The final step will be to measure the transmission of the windows in an x-ray beam as for the nano windows. The framestore pn-ccd detector A next-generation framestore pn-ccd detector is being tested to be used in the next phase of the CAST experiment. It was developed for future x-ray telescope missions and offers several improvements over the detector currently in use in CAST. In contrast to a normal pn-ccd, the new chip uses an additional framestore area as a buffer during the readout of the image data. By using this buffer, the probability of "out-of-time" events -- that is, the absorption of an x-ray in a pixel during the readout-process -- is reduced to 0.1%. Compared to the pn-ccd currently in use in CAST, the new framestore pn-ccd detector has an improved quantum efficiency for low-energy signals, which is now above 90% in the energy range from 0.3 kev to 11 kev. Provided that a window with good transmission properties for low-energy x-rays is used, this will allow us to increase our sensitivity in the low-energy regime. The energy resolution of the detector has also been improved from 180 ev to 140 ev and is now very close to the theoretical limit of 120 ev given by Fano noise. Another important improvement over the old chip is that the readout-noise has been reduced and that the detector can now be operated efficiently at -60 C, compared to C for the current detector. These reduced cooling requirements simplify the operation of the detector, since 48

49 the cooling load is reduced. The background of the new detector is expected to be reduced by a factor of four over the current detector (design value below counts kev -1 cm -2 s -1 ). However, the performance of the detector still has to be verified experimentally. Figure 33: Vacuum vessel and shielding for the new detector. In order to use the new detector on the CAST x-ray telescope, it has to be mounted in a suitable vacuum vessel and cooled down to its working temperature. During the last two years, considerable work has been done on adapting the new detector for use with the x-ray telescope at CAST. This work has been carried out at TU Darmstadt with support from the University of Freiburg, the Max-Planck Halbleiterlabor, the Max-Planck Institut für Physik, the Max-Planck Institut für extraterrestrische Physik, the University of Zaragoza and CERN (Cryo Lab and TE-MME group). So far, a vacuum vessel, a cooling system, and a graded-z shielding for the detector suitable for CAST have been designed and built and both the vacuum and the cooling system have been tested. In order to use the new detector on CAST, a data acquisition system has to be built. In addition, the detector has to be energy calibrated, mounted on the telescope and aligned. Building the detector system and calibrating the detector will take at least one year; the necessary time will depend on available manpower and access to a facility suitable for energy calibration. Mounting and aligning the detector will require 3-6 months. Some additional time should be used in planning the schedule to take account of unexpected problems or delays. 49

50 New Optics CAST has recently accepted two new institutions, Columbia University and the Danish Technical University- Space (DTU-Space), and together with existing CAST member LLNL, these groups will oversee construction of at least one new x-ray telescope. These groups will be leveraging the significant infrastructure, expertise and close teamwork developed during the conception, fabrication and calibration of x-ray telescopes for NuSTAR, a NASA small-explorer satellite scheduled for launch in February NuSTAR will be the first observatory to utilize focusing optics to image the hard x-ray sky (photon energies E = 6 80 kev). In order to achieve high effective area (the measure of throughput used by x-ray astrophysicists) in this pass-band, NuSTAR telescopes required several innovations over previous generations of x-ray telescopes. First, a new-type of light-weight, high-quality, low-cost substrate was required. The final solution was to take float glass originally developed for flat-panel screens (e.g., the display in laptops or hand-held devices), and thermally heat or slump the glass into the appropriate shape (approximately 1/6 th or 1/12 th of a truncated cone). Columbia University, the PI institution for the NuSTAR x-ray telescopes, pioneered this approach for manufacturing substrates. Next, this mirror substrate had to be coated with a multilayer coating to enhance reflectivity at hard x-ray energies. Multilayers, alternating layers of high- and low-density (and high- and low-atomic number) materials, act like synthetic Bragg crystals. For NuSTAR, multilayer coatings are deposited via magnetron sputtering. DTU-Space developed custom facilities to clean the substrates and deposit extremely uniform, well-controlled and stable coatings. Finally, these coated mirror substrates had to be integrated into a single telescope assembly. In the case of NuSTAR, the final telescope consisted of 133 nested layers with more than 2000 individual mirror segments. The assembly philosophy and the custom integration machines were conceived, designed and utilized by Columbia University staff, with additional support provided by LLNL. Columbia, DTU-Space, LLNL and other institutions first built optics using the approach described above for a balloon mission called HEFT, before NuSTAR was funded by NASA. The teams have been working together for more than a decade, and the optic(s) for CAST will utilize all of the existing facilities and tooling. A new CAST optic The baseline plan is to build a single optic that will be coupled to a Micromegas detector on the existing Sunrise Micromegas line. A detailed optics design will be completed by the end of 2011, and to first order, this new optic will have properties similar to that of the ABRIXAS x-ray telescope already employed by CAST. It is important to note that due to the fundamentally different approach that NuSTAR telescope s are made, the new optic for CAST will be much easier to build than a NuSTAR optic. Specifically, the telescope will consist of nested shells. Additionally, since the CAST magnet bore is small (diameter of 42 mm), a complete telescope does not need to be constructed. Rather, only 1/6 th of a normal x-ray telescope must be built. This means the total number of substrate required will be only two times the number of layers or individual mirror segments. Finally, since the putative solar axion spectrum peaks in the 1-10 kev band, multilayer coatings are not required for high x-ray reflectivity. Rather, simple coatings of nickel and platinum will provide excellent effective area. In Table 6, a summary of the required tasks and the time required to construct a new optic is detailed. 50

51 Task # Description Time required/cumulative 1 glass substrate fabrication and preparation 1 month/1 month 2 Substrate coating: 1 month/2 months 3 Restart of optics assembly facilities(done in parallel with tasks 1-2) 2 months/2 months 4 Optic construction: 1 month/3 months 5 Design and fabrication of mounting fixtures: three months (done in parallel with tasks 1-4) 1 month/3 months 6 Calibration 3 weeks/3.8 months 7 Installation 3 weeks/4.5 months Table 6: Summary of tasks, and time required, to make a new optic. Although the cumulative amount of time needed to fabricate the optic is less than 1 year, we will need time to secure funding from various agencies inside the US and Denmark. Assuming that we start requesting funding in October 2011, we would estimate the latest we would receive funding is in October This would position us to have an optic ready for science observations at CAST in the Spring of

52 8. Conclusions The status of the CAST experiment has been described. The experiment has achieved its original set goals on solar axions by scanning the axion mass region up to 1.16 ev (reaching in practice 1.18 ev) with unprecedented sensitivities while extending its horizon towards particle candidates from the hidden sector (Paraphotons) and the dark energy in the Universe (Chameleons). The experiment has delivered the final results for the first part of the 3 He run with a sensitivity up to 0.64 ev [1]. The Collaboration requests to run in 2012 with 4 He to enhance the sensitivity in the region of 0.4 ev. This gain in sensitivity is expected from our current detectors that exhibit much better performances and at the same time the stepping strategy that will focus on a restricted mass range but with increased statistics per density step. This will allow to improve our current sensitivity obtained with 4 He run and to cross the benchmark KSVZ axion models. In parallel we plan to carry out an ambitious R&D program to be ready for 2013 and 2014 where we would like to take data with vacuum in the magnet bores as in CAST phase I. In order to obtain a significant gain in sensitivity we need detectors reaching background levels of the order of 10-7 counts kev -1 cm -2 s -1 and new x-ray optics. We have a roadmap for Micromegas detectors to reach these background levels and a project of developing one new x-ray telescope. 52

53 9. References [1] CAST Collaboration, CAST search for sub-ev mass solar axions with 3He buffer gas. e-print: arxiv: [hep-ex] accepted for publication in PRL. [2] A. Tomás et al., CAST microbulk micromegas in the Canfranc Underground Laboratory, To appear in the proceedings of the 2nd conference in Technology and Instrumentation for Particle Physics, TIPP [3] A. Tomás et al., The new micromegas x-ray detectors in CAST, (2011), x-ray Spectrometry, 40: doi: /xrs [4] F.J. Iguaz et al., Ultralow background periods in CAST Micromegas detectors and tests in the Canfranc underground laboratory. Published in J.Phys.Conf.Ser. 309 (2011) [5] CAST Collaboration, CAST Physics Proposal to SPSC, CERN-SPSC ; SPSC-SR-085 [6] I.G. Irastorza et al, Towards a new generation axion helioscope, JCAP06(2011)013. [7] P. Brax, A. Lindner, K. Zioutas, astro-ph/ ( ) [8] F.J. Iguaz et al., The discrimination capabilities of Micromegas detectors at low energy, To appear in the proceedings of the 2nd conference in Technology and Instrumentation for Particle Physics, TIPP e-print: arxiv: [physics.ins-det] [9] Joern Donges and Axel Lindner / DESY, Private communication (2011) 53

54 10. Appendix CAST mass coverage completion of the CAST helium program The intrinsic width of the mass acceptance is determined by: Energy spectrum of the axions Density of the buffer gas Magnetic length x-ray absorption in gas Tilt of magnet (hydrostatic pressure gradient) This intrinsic width has been used to compute the step size between settings in order to achieve the original aim of 50% overlap in the mass acceptance (so called 1.0 dp). Note that due to absorption in the gas of the lower energy component as the density increases, the intrinsic width of the mass acceptance increases (see figure 1 below) and hence the dp increases. Figure 1: Value of dp (step size) in mbar versus Setting Number In 2006, the step size used was 1.0 dp for a full tracking. After 1 year, 160 settings had been taken. Continuing in this mode would have required a further 7 years to complete the program. 54

55 With the advent of the versatile 3 He gas system, a step could be introduced mid-tracking and so allow double the number of steps per day. The corresponding reduction of a factor of 2 in exposure time resulted in a 10% worsening in the sensitivity (which was eventually recovered by the continued improvement in the detectors). Hence in 2008, the step change was made after each half tracking and also the step size was increased to 1.2 dp after some months. In 2009, the step size was increased to 1.4 dp in order to compensate for the lost time in the first half of 2009 spent to repair the 3 He leak inside the cryostat, and due to the reduced mass coverage in 2008 caused by the leaks. In summary: Pressure range (mbar@1.8k) Year Gas Tracking per step Step (dp) Approx. Cold Window Temperature (K) He One full He Half He Half He Half He Half He Half see scheme 13 Table 1: Summary of data taking. As the gas density has increased during the 3 He scan, the intrinsic mass acceptance has been strongly modified by the gas dynamics. In view of the different periods taken with different window temperatures, some of the trends have taken some time to discern and then quantify. In addition, the only monitor of the gas dynamics is via the cold bore pressure measured at room temperature outside the cryostat and connected by a small capillary to the cold bore (to prevent thermo acoustic oscillation). This key measurement is complemented by cryogenic thermometry which at the start of the helium running was somewhat basic and has been improved each time the cryostat has been opened. It has taken 3 years of hard work to correlate accurately the pressure measurement outside the magnet to the gas density inside the magnetic length of the cold bore. It has required CAST to: Develop a reliable and detailed model of the 3 He pipework including the cold bore (temperature ranging from 150 -> 1.8K) Source and introduce the thermal characteristics of all the material present in the system over this range of temperatures Source and introduce correctly the equations of state of 4 He and 3 He into the Computational Fluid Dynamics simulation (CFD) Correctly set up the CFD to include hydrostatic and buoyancy Introduce tilting of the magnet 55

56 Make experimental tests at different densities and tilt angle to extract the temperature distributions and cold bore pressure Generate numerous simulations and analyze and interpret the output The CFD group at CERN is still developing a definitive transient simulation to compare with CAST s present static simulations. However we believe that with our static simulations, we have a sufficiently complete understanding of the gas dynamics inside the cold bore. A paper is being prepared to report in detail on the findings so far. From our studies of the gas dynamics, the main trend observed is a shift in central mass value of the cold bore filling depending on the tilt angle of the magnet. At gas densities corresponding to low mass, the effect is small in comparison with the step size. The effect tends to dominate with increasing gas density; the intrinsic mass acceptance of the experiment is widened considerably by this effect over each half tracking. The effect even changes sign between operating with heated windows and unheated windows. As a result in the subsequent physics analyses, the mass value is calculated each minute of the tracking to correct for this effect. A summary of the comparison between the intrinsic width and actual width derived from the CFD analyses is shown in Figure 2. The intrinsic width is equivalent to 1.0 dp. Figure 2: Comparison of intrinsic width and actual width (extrapolated to ev) versus mass. It is clear that in the region above 1 ev mass which we will cover in 2011, stepping with 1.4 dp (~ 0.9 mev) will leave no holes in the mass scan since the actual width is >4 mev (Figure 3). 56

57 In addition, the sensitivity per mev compared with the original aim is largely maintained due to improvements in the detectors. However if one accepts a reduced exposure time per mev, one can increase the step size significantly and still not create gaps in the coverage. This is what we propose. Up to 2011, the daily increase in setting was nominally 2 x 1.4 dp (1.8 mev). We propose to increase this to 4 x 1.4 dp (3.6 mev) per day an effective increase in step size of a factor of two and a corresponding reduction in exposure per mev resulting in a small worsening in the sensitivity of only a factor of ~1.09. Figure 3: Settings 728 & 729. Intrinsic mass overlap two adjacent settings (dotted red/blue) combined to the purple curve. Actual mass overlap two adjacent settings (solid red and solid blue) combined to green curve. For operational, safety and logistical reasons, we propose to do this by the following scheme: 57

58 Table 2: Comparison of the previous and proposed running scheme for The scheme presented in Table 2 was verified using actual data taken to ensure that sufficient overlap between settings exists (see below). Estimates for the 2011 run For reference, using nominal dps for gap and high pressure running (Gap1: 1.2 dp, Gap2: 1.2 dp, HighP: 1.4 dp) and assuming 70% efficiency, the coverage of the two gaps plus the high pressure scan would be completed by 15 th September This would mean 2 months running at highest pressures without a fully qualified 3 He expert. In view of the wider actual mass coverage for each half tracking, an alternative scheme has been studied in order to ensure completion before the end of July 2011 thus maximising the security of the system and the rare gas it contains whilst maintaining a seamless mass coverage. The proposed scheme is as follows: 1. Gap settings The actual width of the mass acceptance around 0.6 ev is about 1.8 times the intrinsic width. Hence any step up to 2x1.2 dp will not cause any gap in the coverage, just a small reduction in sensitivity. a. 26 mbar gap (1.3 mbar) ev 2x1.2 dp at half tracking, morning and evening 6 calendar days with 70% efficiency. 1 day filling + 1 day stabilization b. 35 mbar gap (2.7 mbar) ev 2x1.2 dp at half tracking, morning and evening 58