Oshima, A

CERN Document Server

29th International Cosmic Ray Conference Pune (2005) 5, 335338Tests of PMT uniformity for the GRAPES-3 experimentA. Oshima , S.K. Gupta, Y. Hayashi , N. Ito , A. Jain, S. Karthikeyan,S. Kawakami , H. Kojima , T. Matsuyama , P.K Mohanty, T. Nonaka ,K.C. Ravindran, H. Tanaka , S.C. Tonwarand T. Yoshikoshi (a) Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India(b) Graduate School of Science, Osaka City University, Osaka 558-8585, JapanPresenter: T. Nonaka (oshima@alpha.sci.osaka-cu.ac.jp), jap-oshima-A-abs1-og27-posterIt is observed that the response of the PMT varies across the photo-cathode leading to certain non-uniformity.To select PMTs having uniform efficiency across the photo-cathode, an X-Y scanner with appropriate lightsource has been designed to measure PMT photo-cathode response. Here we present the results of uniformitymeasurements on the PMT response.1. IntroductionGRAPES-3 is an extensive air shower experiment operating at Ooty (E76.7  , N11.4  ,2200m asl), in southIndia. It consists of about 300 scintillation detectors each having an area of 1mand 16 large area muondetectors having a total area of 560m. There are two kinds of scintillation detectors. The first type is conicalshaped detector with a PMT directly viewing the scintillator. The second type is flat detectors overlaid withwave-length shifting fibers in parallel grooves called fiber detector. In the case of conical shaped detector, non-uniformity of quantum efficiency is not of any significance because the distance between PMT and scintillatoris large and the photons from scintillator reach PMT surface after several reflections and uniformly illuminatethe PMT photo-cathode. On the other hand, in the flat type detector the photons collected by the WLS fibers aretransferred to the PMT through a optically coupled cookie. Because the WLS fibers are coupled to a local areaof the PMT surface, the response of the PMT varies with the position of the cookie. Thus the response of thePMT becomes non-uniform if the quantum efficiency varies across the face of the PMT. A flat detector has 4scintillator blocks each viewed by a group of 18 WLS fibers and each group is terminated at a separate cookie.Since these 4 cookies are attached to the PMT surface at a different location, non-uniformity of quantumefficiency becomes important for the flat type detector. We are using a fast 2 inch diameter photomultiplier,ETL 9807B with blue-green sensitive bialkali photo-cathode from Electron Tubes [1]. To select PMTs havinguniform quantum efficiency, we have designed a system for measurement of the uniformity using a precisionX-Y scanner.2. SystemIn figure 1 the system used to measure the PMT uniformity is shown. It contains a light tight box where theX-Y scanner and nine sockets facing the same direction, to mount a set of nine PMTs are placed. Thus a totalof nine PMTs can be scanned simultaneously. The main component of this system is the X-Y scanner, which isoperated using two stepping motors to move it along two orthogonal directions (X and Y) with high positionalresolution, in steps of 1  m. There are two plates, placed on the X-Y scanner each having 9 pin-holes of 1mm diameter to serve as a nine beam collimator . Under the lower plate there is another plate on which ninegreen LEDs are fixed as light source. The wave-length of the LED output is 525 nm which is in the range ofsensitivity of the emission spectra of WLS fiber[2]. When triggered, the LEDs produce a light pulse, with awidth of 30 ns. The light pulse emitted from LED is collimated through the two pin holes of the two plates336 A. Oshima et al.above it, finally reaching the PMT surface with small angular spread. We have used a CAMAC based dataacquisition system for generating the trigger pulse for the LED. The operation of X-Y scanner is controlled bya serial interface.X−Y stagecontrollerX−axsisY−axisPMTADCTriggergeneratorCAMACpersonal  computerX−Y stageLEDFigure 1. System for PMT uniformity test. A charge sensitive 32 channel ADC module and a trigger generator module islocated in CAMAC crate. Serial communication is used between PC and X-Y controller to position the X-Y scanner.3. ExperimentThe X-Y scanner is used in a programmed mode to scan a square area larger than the size of the PMT photo-cathode by scanning the PMT on pre-determined step size. At each location data is collected by triggering theLED a fixed number of times as detailed below, which takes about one hour to complete scan.1. Step width in X and Y directions is 1 mm.2. Intensity of LED is adjusted to yield around 100 photo-electron.3. Number of times LED triggered at each position is 200.4. Scanning area is 65  65 mmto cover a total area of PMT surface.Tests of PMT uniformity ... 3374. ResultsIn figure 2 uniformity of the PMT photo-cathode is shown for a typical PMT. Vertical axis represents meanADC counts after subtracting the pedestal. To characterize the variation of the quantum efficiency of the PMTsurface, we determined the mean and rms deviation within the area of a circle of radius 20 mm from the centerof the PMT as shown by an arrow in figure 3. This is necessary because the PMTs display a rapid decrease intheir quantum efficiency near the outer edge. The present PMT shows an rms variation of 6.5%. within 20 mmof the center. In figure 3 the response of the same PMT is shown along two different directions. The plot onthe left shows the PMT response along X-axis and the plot on the right shows the response along Y-axis. In thefigures 2 and 3 one can see a dip in the response and the depth of the dip is about 30 % below the mean value.Almost all PMTs we checked displayed such a dip in their response at the same location. If the area of the dipis excluded then the rms variation of this PMT reduces to 4.4% which may be considered to be fairly uniform.In figure 4 the response of a PMT having large non-uniformity is shown. This PMT has an rms variation of10.7% in the selected fiducial area. 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 0 200 400 600 800 1000 1200 1400PMT0:  AVE=968  :STD=63xyFigure 2. Uniformity of a typical PMT, the rms variation of this PMT is about 6.5%. 0 200 400 600 800 1000 1200 1400-10  0  10  20  30  40  50  60  70ADC countsmmx=30 mm 0 200 400 600 800 1000 1200 1400-10  0  10  20  30  40  50  60  70ADC countsmmy=30 mmFigure 3. Left plot shows the uniformity along the X-axis and the one on the right shows response along Y-axis. Arrowsshow the range of an area used to determine the mean and rms variation.338 A. Oshima et al. 0 200 400 600 800 1000 1200 1400-10  0  10  20  30  40  50  60  70ADC countsmmx=30 mm 0 200 400 600 800 1000 1200 1400-10  0  10  20  30  40  50  60  70ADC countsmmy=30 mmFigure 4. A PMT having large variation in its response. In the right plot response varies from -30% to 40%. from its meanvalue.5. ConclusionThe X-Y scanner developed to measure the response of the PMT has successfully measured the uniformity ofquantum efficiency across the photo-cathode. The X-Y scanner allows identification of the area of uniformresponse on the PMT surface which can be used to locate the WLS fiber cookies. This has permitted successfulutilization of a number of PMTs with rather non-uniform response which otherwise would not have beenpossible.6. AcknowledgementsWe thank K. Manjunath, K. Ramadass, G.P. Francis for help in assembling the X-Y scanner and for conductingvarious tests.References[1] DS 9807B Series Issue, Electron Tubes Inc.[2] Scintillation Materials, Kuraray Co.,Ltd.

Tests of PMT uniformity for the GRAPES-3 experiment

Abstract

Similar works

Full text

Available Versions

CERN Document Server