SAKASHITA UNDERGROUND COSMIC RAY TELESCOPE

Transcription

1 SAKASHITA UNDERGROUND COSMIC RAY TELESCOPE Cosmic-Ray Research Section, Solar-Terrestrial Environment Laboratory, Nagoya University Nagoya JAPAN Sakashita Tunnel 1

2 I. INTRODUCTION Sakashita Underground Multi-directional Telescope was planned as one of projects of Japan program for International Magnetospheric Study (IMS, ) to study solar modulation of cosmic rays in high energy region. For the Telescope Sakashita Underground Cosmic Ray Observatory, Nagoya University was constructed in 1976 in a disused railway tunnel of Japan National Railways (now Central Japan Railway Co.) at Sakashita-cho, Gifu prefecture, about 100 km north-east of Nagoya. The Underground Telescope was built, extending detectors with three steps in 1977 to 1979 to 4 x 15 m 2 upper and 3 x 14 m 2 lower finally, and full observations of 10 directional cosmic ray intensities started in Dec Fig.1 Map near Sakashita Observatory II. SAKASHITA UNDERGROUND OBSERVATORY The observatory was constructed in the middle of a tunnel at Sakashita (35 35 N and E in geographic coorditnates, 334 m above sea level), which runs 14 westward from the north as shown in Fig. 1. Topography above the observatory is fairly flat as can be seen from the vertical cross sections of the soil in Fig. 2. The vertical depth is estimated at 80 mwe from the topography and also from a comparison between cosmic ray fluxes inside and outside the tunnel (Ueno et al., 1976; Fujii and Jacklyn, 1979). The detector room is 3.6 x 33 m 2 in floor area and has a cross-section of horseshoe shape as shown in Fig. 3. Inside the room, there constructed two layers, upper and lower, with iron-frame and plate to lay out detectors. Thin plastic plate was built along with the coved ceiling of tunnel to protect detector room from water drop out of the brick wall. The humidity is kept 70% by seven 300 W dehumidifiers. The seasonal change of the room temperature is about 1 C from the average 22 C and its daily variation is about 0.2 C. Fig.2 Vertical cross section of the ground 2

3 III. MULTI-DIRECTIONAL UNDERGROUND TELESCOPE a. Detectors and their arrangements The multi-directional telescope consists of two layers of detectors as shown in Fig. 3. Each of detectors is composed of plastic scintillators of 1 x 1 m 2 in area and 10 cm in thickness, which is set at the bottom of a pyramid shaped box of 1.6 mm iron plate and is viewed by a 5" photomultiplier (Hamamatsu Photonics R877). These detectors are set respectively on the upper and lower layers separated by 1.75 m in height without absorber. Configuration of the detector layout was designed in considering the maximum availability of space in the tunnel. Centre of each detector on the lower layer is placed just right under one of the corners of each detector on the upper layer. Each detector is called by the number with initial U or L for the upper or lower layer, as U1, 02,, U60 and L1. L2,, L42. b. Detectors and the adjustments Pulses from each photomultiplier are amplified by about 500 times, and then discriminated from noises at a fixed voltage of 0.5 V by an amplifier/comparator installed in each detector box. These signal pulses are sent to a coincidence circuit board by a coaxial cable (25 m), and reshaped to uniform pulses (5 volts and 200 nsec) to trigger the coincidence circuits. Fig. 4 shows one of plateau curves of detectors, i.e., the dependence of counting rate of the detector on high voltage supplied to the photomultiplier. A plateau is seen at around 850 to 900 volts with a counting rate of about 350 counts per minute, indicating a clear separation of cosmic ray signals from background noises. Based on this character, high voltage of each photomultiplier is adjusted so that the counting rate of every detector is nearly equal to 400 counts per minute. These adjustments are made by inserting a proper resister in series between the photomultiplier and the high voltage supply set at 950 volts. Fig.3 Setting of Telescope in the tunnel. Fig.4 Plateau curve (variation of counting rate with high volate) 3

4 c. Directional telescopes and their counting rates Directional telescopes are composed of selected 2-fold coincidences of signals from upper and lower layer detectors. The selection of the coincidences is illustrated in the top view of the telescope in Fig. 5, where the upper and lower layer detectors are shown by squares of thin solid and thin broken lines respectively. Coincidences for the directional telescopes are indicated by the relative position of the upper layer detectors (thick solid lines) to one representative lower detector or detectors. Central viewing directions and average counting rates of these directional telescopes are summarized in Table I. Two directional telescopes V1 and V2 in the Table are a sum of (NE, SE, SW and WN) and that of (N, E, S and W) respectively. Effective depth and effective median primary rigidity were calculated by Fujimoto et al. (1977) from the yield function of cosmic rays (Murakami et al., 1979) in considering the geometrical configuration of detectors and particle s path length in the rock with a mean density of 2.5 g/cm 3. Fig.5 Configuration of upper and lower layer detectors showing two-fold coincidences for directional telescopes. Table 1. Viewing directions, effective depths, median rigidities and counting rates for multidirectional telescopes at Sakashita 4

5 d. Recording system The output pulses from the coincidence system are fed into 24 channel recorder. Each channel of the recorder consists of 12 bits binary counter with 12-bit gate and 4 digits decimal counter with 4 digits buffer memory. Each binary counter is designed as a sacling factor of 1 to 4096 for decimal counter and is set so that hourly count of the decimal counter becomes about 5000/hour. The accumulated counts in the decimal counter are transferred to the buffer memory once every hour and then the counter is cleared up for the next hour. The contents of the buffer memories as well as the time code are punched every hour on a paper tape. The atmospheric pressure and the room temperature are measured by a digital barograph and thermometers with V-F converters, and are recorded also on paper tape by the recorder. e. Power supplies In order to avoid interruption of observations due to AC power line failure, an AC-DC-AC converter system with a deck of floating batteries (96V, 35AH) is used for the AC power supply. This system can supply 500 W power to the telescope for 3 hours without AC power input. High voltage supply and DC power supplies are highly stabilized against changes of AC voltage and room temperature. The conventional stabilized power supplies are used for the coincidence system and the recording system, as they are not sensitive to the change of the supply voltage,. f. Stability of the telescope All the instrumental gain drift due to the variations of the power supply voltage and room temperature are reflected in the counting rate through slopes of the plateau curve. Fig. 6 shows the plateau curves for five representative coincidence components together with those for total single components of upper and lower layers. Slopes of the plateaus for coincidence components are less than 0.2 % counts per 1 volt at 950 volts. The change of high voltage due to that of AC power line voltage has negligible effect to the counting rate. The main instrumental drift is due to the Fig.6 Plateau curves of various telescopes variation of room temperature. The room temperature causes gain changes of the photomultiplier and amplifier-discriminator system of -0.1 % Gain/ C. Change of 0.1% gain is equivalent to change of -0.1 V of high voltage, and the room temperature effect of the Telescope is estimated to be % counts/ C. As the temperature variation is kept within 0.2 C on daily basis and 2 C on yearly basis, the instrumental drift of the coincidence rate is less than % in a day and % in a year. The other instrumental drift is due to the fatigue of the photomultiplier. The local rainfall has also effect on counting rate by the change of effective ground mass. As no correction for these effects was made, the long term drift of about 1 %/year were observed in the counting rate. g. Automatic check systems To check the malfunction of the Telescope and the long term gain change due to fatigue of photomultiplier, the automatic check system by a PC records daily means of single and coincidence counting rates from each 5

6 detector. This system measure also differential pulse height distribution of each detector by a built-in 256 channel pulse height analyzer, automatically scanning all detectors once every day. Some of the data are checked once every day in Nagoya through a telephone line to the observatory. This automatic check system has been operated since the middle of IV. REDUCTION OF DATA The paper tape is collected once a month and the hourly outputs on paper tape are processed by a computer system in the Institute, to derive the relative cosmic ray intensity variation. Hourly counts N s in the paper tape are converted into the barometer corrected relative intensity (Wp) using the Natural Logarithmic Representation (Wada, 1957) as follows, W P = 10 4 x (In N - In N 0 + WL) - β(p-p 0 ) in unit of 0.01 % where N 0 is average counting rate divided by the scaling factor and WL is artificially added to make the values of Wp around (%). P is the atmospheric pressure, P 0 is 980 hpa and β is the barometric pressure effect coefficient (-0.03 %/hpa for all components). Hourly values of Wp, daily sum and average as well as the 1st, 2nd and 3rd harmonics coefficients of the daily variation of each component are computed and tabulated on daily basis. In the same table, the residual dispersion of the cosmic ray intensities for one day after removing the daily variation (up to 3rd harmonics) is tabulated. Further, these hourly values of Wp are used to produce hourly value plot for every solar rotation period using the X-Y plotter attached to the computer system. Check of the processed data is made, utilizing the hourly data table and inspecting visually the plotting diagrams. The final data are written into a magnetic tape and used as the input to the data processing for final tabulation and plotting for the publication at NAGOYA University Computation Center. REFERENCES Fuji, Z., and R. M. Jacklyn Proc. 16th Coemic Ray Conf. Kyoto (1979) Fujimto, K., K. Murakami, I. Kondo, and K. Nagashitna Proc. 15th Cosmic kay Conf. Plovdiv (1977) Murakami, K., K. Nagasbima, S. Sagisaka, Y. Mishima, and A. Inoue Nuovo Cimento 1-2C 635 (1979) Sagisaka, S., K. Murakami. A. Inoue, Y. Mishima, and K. Nagashima Proc. 16th Int. Cosmic Ray Conf., Kyoto (1979) Ueno, H., K. Fujinroto, Z. Fujii, I. Kondo, and K. Nagashima Proc. Cosmic Ray Symp. of High Energy C.R. Modulation 91, (1976) Wada M. J. Sci. Res. Inst. 51, 201 (1957) 6

7 Appendix 1 EPILOGUE Cosmic-Ray Research Laboratory, Nagoya University found in 1958 as a research institute with cosmic ray telescopes, was reorganized with Research Institute of Atmospherics, Nagoya University to Solar-Terrestrial Environment Laboratory (STEL) in June, Sakashita multi-directional underground telescope continued observations by cosmic ray research section of STEL until closing of observations of about 23 years in March, Appendix 2 FOR USE OF DATA Hourly value data of SAKASHITA Multi-directional underground telescope are available from Web Site of Solar-Terrestrial Environment Laboratory ( via network. One record of the hourly value files (92 bytes) consist of 24 data of 4 digits each, WP in 0.01% and Pressure in 0.1 hpa, shown below SSS none Pressure Z (=N-S) Digit content 1-4 Last digit of year + Day of Year 5 8 Hour and minute (start time of record) 9-12 Upper Single + Lower Single none (for monitoring Telescope) V V NE ES SW WN N E S W NN SS none none NNN 7