Measurement of the BESSY II electron beam energy by Compton - Backscattering of laser photons

Transcription

1 Measurement of the BESSY II electron beam energy by Compton - Backscattering of laser photons R. Klein a,*, P. Kuske b, R. Thornagel a, G. Brandt a, R. Görgen b, G. Ulm a a Physikalisch-Technische Bundesanstalt, Abbestraße, 0587 Berlin, Germany b BESSY GmbH, Albert-Einstein-Straße 5, 489 Berlin, Germany * Corresponding author. Tel , fax , roman.klein@ptb.de Abstract Accurate knowledge of all storage ring parameters is essential for the Physikalisch- Technische Bundesanstalt (PTB) to operate the electron storage ring BESSY II as a primary source standard. One parameter entering the Schwinger equation for the calculation of the spectral photon flux of bending magnet radiation is the electron beam energy. So at BESSY II the electron beam energy is measured by two independent techniques one of which is described in this paper: the photons from a CO - laser are scattered in a head-on collision with the stored electrons. From the spectrum of the backscattered photons that are detected by an energy-calibrated HPGe detector the electron beam energy can be determined. The experimental set-up at the BESSY II electron storage ring as well as the current experimental status are described for operation of the storage ring at the energies of 900 MeV and 700 MeV. PACS: f; 9.0.Dh; 4.75.Ht; 9.30.Kv; Qe;.80. Introduction The Physikalisch-Technische Bundesanstalt (PTB) is the German national metrology institute and entrusted with the realization and dissemination of the legal units. For the realization of the radiometric units in the spectral range from the UV to the X-ray region, PTB uses the 700 MeV electron storage ring BESSY II in Berlin Adlershof as a primary source standard []. At the PTB Radiometry Laboratory [] operated at this storage ring and in the storage ring itself, PTB has installed all equipment necessary for the measurement of all quantities needed for the calculation of the spectral photon flux of bending magnet radiation according to the Schwinger theory [3]. One of them is the electron beam energy. The calculation of the photon flux for photon energy above the characteristic energy (.5 kev for BESSY II operated at.7 GeV) strongly depends on the electron beam energy. Therefore, in order to be able to calculate the photon flux for photon Seite von 3

2 energies up to 50 kev with a relative uncertainty below 0. %, the electron beam energy must be measured with a relative uncertainty of less than The electron beam energy can be measured at BESSY II by two independent techniques: resonant spin depolarization (RSD) [4] and Compton backscattering (CBS) of laser photons. This paper focuses on the latter technique; details of the former technique applied at BESSY II can be found in reference [5]. Although RSD is a very precise and well-established method, it suffers from two drawbacks: the technique requires spin-polarized electrons and therefore can not be applied when the electron storage ring BESSY II is operated at a reduced electron beam energy around 900 MeV in special PTB calibration shifts. At this reduced energy, the build-up of polarization takes too long (around 30 hours). Moreover, it does not work for very low electron beam currents because of the signal-to-noise ratio when the depolarization is detected by the increase in the Touschek scattering rate. This is why the CBS method was already installed and extensively used [6,7] at the electron storage ring 800 MeV BESSY I in Berlin Wilmersdorf, which has meanwhile been closed down and where the PTB operated a radiometry laboratory for almost 0 years [8] for calibrations in the UV, EUV and soft X-ray spectral range.. Compton backscattering The CBS process is described in detail in [6] and in the references therein; a detailed presentation of this theory would exceed the scope of this paper. Here only the basic equations will be given: According to relativistic kinematics, the laser photons of initial energy E and initial angle φ relative to the electron beam, have an energy of β cosφ E = E () β cosθ + E ( cos( θ φ)) / W after scattering, where θ is the angle between the scattered photon and the incident electron beam of energy W = γ m c or velocity β c. e The photons are scattered into a narrow cone around the forward direction of the electron beam (Klein-Nishina differential cross section). The photons scattered exactly in the forward direction ( θ = 0 ) have the highest energy possible and the Compton spectrum shows a distinct cut-off edge at this energy. For a head-on collision ( φ = π ), as in our set-up, this imum energy is given by E = E 4γ. () + 4γ E / m c e So if E is known and E is measured, the electron beam energy can be determined by the above equation. Seite von 3

3 3. Experimental set-up The set-up is similar to that used at the BESSY I electron storage ring [6] and illustrated in Fig. : a CO laser beam is overlapped anti-parallel by the electron beam in one straight section of the BESSY II electron storage ring: The CO laser [9] is set up on a laser table placed next to the radiation shielding wall. The laser beam first passes an attenuator allowing the laser power to be reduced. Thereafter a HeNe pilot laser is superimposed on the invisible CO laser beam for alignment. The laser beams are then deflected by 90 by a plane Si mirror in the horizontal direction. With two 90 deflections by Au coated Cu mirrors, the laser beams are lifted to the height of the electron orbit and directed to the ZnSe viewport mounted on the side of the first mirror chamber of the PTB undulator plane grating monochromator beamline [0]. The first Cu mirror is plane, while the second is curved to focus the CO laser beam into the middle of the storage ring straight section. In the mirror chamber an other Cu mirror can be moved in the vacuum to the beamline axis so as to deflect the laser beams onto the axis of the electron beam in the straight section. Thereafter, the laser beams exit the vacuum through an other ZnSe viewport in the backward direction. The CO laser beam is then monitored in a laser power meter, the HeNe laser beam in a CCD camera. Initial alignment was greatly facilitated by the fact that the PTB undulator U80 [] mounted in the straight section, and normally switched off during energy measurement by CBS, can deliver visible light when the storage ring is operated at a reduced electron beam energy of 900 MeV. This light is also directed in the forward direction of the electron beam and marks the optical path to which the laser beam has to be adjusted. The backscattered photons penetrate the Cu mirror in the vacuum, exit the vacuum at the end of the beamline through a stainless steel blank flange and are detected by an energy-dispersive HPGe detector []. In front of the detector, a collimator is placed to restrict the angular acceptance, since only the photons scattered in the forward direction carry the information of interest. The others would only raise the count rate which should be limited to some khz. The collimator consists of a cylinder 0 cm in length and made of tungsten. On the center axis the cylinder has a hole 4 mm in diameter. This tungsten cylinder is mounted in a rectangular lead block with 0 cm in thickness. Tungsten was chosen for the collimator kernel because it is superior for shielding the high energetic bremsstrahlung that is also scattered in a narrow cone in the forward direction of the electron beam. The collimator is mounted on a computer controlled rotational table. By rotation perpendicular to the center axis of the collimator, the angular acceptance can be so adjusted that the count rate is convenient for the detector system. Collimator and detector are mounted on a computer controlled xy-stage in order to align the center axis of the collimator to the forward direction of the scattered photons. Nevertheless, compared to the BESSY I set-up an additional difficulty had to be overcome: for the normal electron beam energy of BESSY II of 700 MeV and for the described geometry and laser, the cut-off edge at E of the Compton spectrum is at a photon energy above 5 MeV. At this photon energy the energy calibration of Seite 3 von 3

4 the HPGe detector is not so easy because of a lack of suitable radionuclides. (This cut-off energy could have been reduced by a factor of two by a perpendicular excitation geometry ( φ = ±π / ). But this would have required lateral access to the straight section in the storage ring. This on the other hand would have interfered with the PTB undulator installed in this section and was therefore ruled out.) For the energy calibration of the detector a 44 Cm/ 3 C [3] -source is now used. The α-particles from the 44 Cm decay lead through the 3 C(α,n) 6 O reaction to 6 O in an excited nuclear state. The 6.863(4) MeV γ-decay [3] line of this excited state and the corresponding detector single escape line are sufficiently close in energy to the Compton cut-off energy (see fig. 3). This source is placed between collimator and detector during measurements to simultaneously record the Compton spectrum and the calibration lines. Between the calibration source and the detector, a beaker approx. 0 cm in diameter and filled with water is placed to prevent some of the neutrons from the above reaction from reaching the detector. 4. Measurements Fig. and Fig. 3 show typical spectra recorded at BESSY II when operated at an electron beam energy around 900 MeV or 700 MeV, respectively. The Compton cutoff edge as well as the simultaneously recorded calibration lines can be clearly seen in each of the spectra. The spectrum evaluation is illustrated in Fig. 4 and described in detail in [6]. The position of the Compton cut-off edge is given by fitting of the relevant part of the spectrum with a modified error function (left part of fig. 4). The detector calibration is derived from the fitted positions of the simultaneously recorded calibration lines (the two graphs on the right in fig. 4). For the energy calibration of the detector, a 60 Co- or the described 44 Cm/ 3 C - source is normally used for the storage ring operated at 900 MeV or 700 MeV, respectively. These sources have calibration lines that are sufficiently close in energy to the measured Compton cut-off energy. Moreover, additional lines from the natural radiation background, such as the (5) kev line of 40 K, the (9) kev line of 4 Bi or the (3) kev line of 08 Tl can be used for detector energy calibration. This evaluation yields the value for the Compton cuf-off edge E, the electron beam energy can then by determined by equation (). The uncertainty of the electron beam energy is given by the uncertainty of the parameters entering equation () as described in detail in [6]. The σ uncertainties are summarized in table for a typical measurement (measurement no. in table, see later in this paper). The collimator must be placed in such a way that photons scattered in the forward direction ( θ = 0 ) can reach the detector. This can be secured by observing the shape of the spectrum as described in [7]. Therefore an evident misalignment in θ can not occur. The uncertainty is dominated by the uncertainty in the determination of E which is given by the statistical error in the determination of the position of the edge and of the calibration lines in the spectrum. Seite 4 von 3

5 parameter value x i uncertainty u(x i ) dependence u(w) = E kev 0.50 kev 0.5 * W * u( E ) E E ev ev / 0.5 * W * u( E E φ π rad W * u( φ) / 8 ) / * ( ) / c m e c MeV MeV W u mec me contribution to u(w) 0.08 MeV MeV MeV MeV result W 78.7 MeV total uncertainty 0.08 MeV Tab. : Uncertainty of a typical measurement of the electron beam energy Fig. 5 shows the wide dynamic range in the electron beam current to which the method of CBS can be applied. For one fill of the electron storage ring the electron beam current was gradually reduced and the electron beam energy was measured at the values shown in fig. 5. The data acquisition time ranges from approximately 300 s to 000 s for the high and low electron beam currents, respectively. Fig. 6 shows the results for the electron beam energy obtained by the two different techniques: the solid bar shows the result obtained by the RSD technique, the width of the bar giving the ± σ uncertainty. The measurement points with the σ error bars are the results of five successive measurements made by CBS on the same storage ring fill. Between these measurements laser power and angular acceptance were varied to change the detector count rate or detector dead time as shown in table (the detector was shifted approx. 5 cm closer to the collimator after measurement, which explains the rise in dead time). The weighted mean of the five measurements of fig. 6 gives the result for the electron beam energy measured by CBS of whereas the measurement by RSD yielded 78.76(5) MeV, 78.74() MeV. These results are in very good agreement. CBS can be employed not only for the measurement of the electron beam energy but also for the measurement of related parameters such as the electron beam energy spread or the storage ring momentum compaction factor, as described in [6,7]. no. of meas. laser power W electron beam current ma detector dead time % acquisition time s collimator angle degree result MeV (0) (08) (0) () (0) mean of measurements to (5) Tab. : Parameters for the measurements shown in fig. 6. Seite 5 von 3

6 Conclusions The high-accuracy measurement of the electron beam energy is essential for PTB to operate the BESSY II storage ring as a primary source standard. Besides the equipment for electron beam energy measurement by RSD, the set-up for measuring the electron beam energy by CBS of CO laser light was successfully installed. This method can be applied over a wide dynamic range in the electron beam current of approx. four orders of magnitude. For BESSY II operated at the normal energy of 700 MeV, comparison of the result obtained by CBS with the result of the wellestablished and independent RSD technique showed good agreement within the combined relative uncertainty of With CBS the BESSY II electron beam energy can also be measured when the storage ring is operated at a reduced energy of 900 MeV in special PTB calibration shifts. Seite 6 von 3

7 References [] R. Thornagel, R. Klein, G. Ulm, Metrologia 38 (00) in press [] G. Ulm, B. Beckhoff, R. Klein, M. Krumrey, H. Rabus, R. Thornagel, Proc. SPIE 3444 (998) 60 [3] J. Schwinger, Phys. Rev. 75 (949) 9 [4] Ya.S. Derbenev, A.M. Kondratenko, S.I. Serendnyakov, A.N. Skrinsky, G.M. Tumaikin, Yu.M. Shantunov, Part. Accel. 0 (980) 77 [5] P. Kuske, R. Goergen, R. Klein, R. Thornagel, G. Ulm, Proc. of EPAC 000, 77 [6] R. Klein, T. Mayer, P. Kuske, R. Thornagel, G. Ulm, Nucl. Instr. and Meth. A384 (997) 93 [7] R. Klein, T. Mayer, P. Kuske, R. Thornagel, G. Ulm, J. Synchrotron Rad. 5 (998) 39 [8] R. Thornagel, J. Fischer, R. Friedrich, M. Stock, G. Ulm, B. Wende, Metrologia 3 (996) 459 [9] Edinburgh Instruments, Model PL5-S [0] F. Senf, U. Flechsig, F. Eggenstein, W. Gudat, R. Klein, H. Rabus, G. Ulm, J. Synchrotron Rad. 5 (998) 780 [] R. Klein, J. Bahrdt, D. Herzog, G. Ulm, J. Synchrotron Rad. 5 (998) 45 [] EG&G Ortec 00% HPGe detector model GEM-000 [3] Source 3484LM made by Nycomed Amersham plc; Buckinghamshire, England Seite 7 von 3

8 beaker filled with H O xy-stage calibration source mirror chamber ZnSeview port HPGe-detector collimator ZnSeview port flat and curved Cu-mirror storage ring straight section with electron beam HeNe-laser overlap power attenuator power meter CCD-camera Si-mirror CO -laser Fig. : Schematic drawing of the experimental set-up at the PTB Radiometry Laboratory at BESSY II (not to scale) Seite 8 von 3

9 60 Co Count rate / s - Photon energy / MeV Fig. : Measured spectrum of the backscattered photons with the electron storage ring operated at an reduced electron beam energy of about 900 MeV. The cut-off edge of the Compton spectrum can be clearly seen. To the left of the edge, two lines from the 60 Co-source used for the detector energy calibration can be clearly distinguished. The result of the above measurement was 98.53(6) MeV. Seite 9 von 3

10 Count rate / s - 6 O Photon energy / MeV Fig. 3: Measured spectrum of the backscattered photons with the electron storage ring operated at its normal energy around 700 MeV. The cut-off edge of the Compton spectrum as well as the related single escape line left of the edge can be clearly seen. To the right of the edge, the photo peak and the single escape peak of the 6 O-line from the 44 Cm/ 3 C source used for detector energy calibration can be seen. Seite 0 von 3

11 Count rate / a.u. Photon energy / MeV Fig. 4: Illustration of the data evaluation: The electron beam energy can be determined from the fitted position of the cut-off edge (left graph) and the energy calibration of the detector obtained by the fitted position of the simultaneously recorded calibration lines (the two graphs on the right). Seite von 3

12 Electron beam energy / MeV Electron beam current / ma Fig. 5: Measured electron beam energy for different electron beam currents: for one fill of the electron storage ring, the electron beam current was gradually reduced and the electron beam energy was measured at the values shown. This illustrates the wide dynamic range of nearly four orders of magnitude to which the CBS technique can be applied. For convenience, the dotted line shows the mean of the five measurement points. Seite von 3

13 Electron beam energy / MeV No. of measurement Fig. 6: Comparison of the results for the electron beam energy obtained by the two different techniques: the solid bar shows the result obtained by the resonant spin depolarization technique. The width of the bar gives the ± σ uncertainty. The measurement points with the σ error bars are the results of five measurement by CBS. Between these measurements the laser power and angular acceptance were varied to change the detector count rate or the detector dead time. Seite 3 von 3