The efficiency measurement of the Water Cerenkov Counter for E18

1. The efficiency measurement of the Water Cerenkov Counter for E18 SUNJI KIM, H. BHANG, M. KIM, K. TSHOO, K. TANIDA, H. FUJIOKA1, Y. SADA1, and H. ASANO1 Seoul National University, 1Kyoto University Abstract We have developed a Water Cerenkov(WČ) Counter, which is to be used in the E18 experiment at J-PARC (Japan Proton Accelerator Research Complex), in order to veto the protons produced from the kaon induced reaction on the 12C target. The WČ Counter will be used coupled with the (π+,K+) trigger counters which consist of the time of flight(TOF) counter, the Lucite Cerenkov(LC) counter, and the Aerogel Cerenkov(AC) counter. The contents of the kaons in the (π+,K+) trigger events were only a couple of percent in the previous E508 experiment at KEK (High Energy Accelerator Research Organization) due to the contamination of the protons. With the inclusion of the WČ Counter the inner size of 1x8x15cm3, we will be able to improve the data acquisition rate of the kaons drastically in the (π+,K+) trigger events for 12C(π+,K+)12ΛC reaction at least more than 10 times. The test experiment of the WČ Counter was performed with 450MeV positron beam at ‘Research Center for Electron and Photon Science’ in Tohoku University of Japan. As a result of the experiment, on average, about 18 photo-electrons(p.e.) were detected, and it had more than 99% Cerenkov light detection efficiency. Introduction ◈ Motivation of WČ Counter inclusion ◈ Principle ◈ WČ Counter Structure • Cerenkov radiation arises when a charged particle in a material medium moves faster than the speed of light in that same medium. • The threshold momentum for K+ is 0.55GeV/c, and for proton is 1.06GeV/c with n=1.33 of water. Therefore, only K+ which has typical momentum 720MeV/c can release Cerenkov radiation. • In a momentum range around 720MeV/c, the Aerogel and the Lucite can be used for π/K separation and K/p separation, respectively. • Due to the high energy protons above 850MeV/c and wavelength shifter material doped in Lucite Cerenkov counter, there was a lot of proton misidentification in E508. • Water box – square frame : Becrite • – windows : Al. foil (front) + OHP film (back) • Reflector – Al. foil (inner layer) + thick film (structure) • Supporter – 2mm thickness Al. plate + Al. PMT hole Experiment & Results ◈Single photo-electron measurement (LED calibration) @SNU b. Position dependence measurement with 2”PMT ◈ WČ counter test for new reflector @SNU 60ch Original reflector Modified reflector 5.7p.e. 12ch for 2”PMT(H6410) –absorber : A4 paper • At the center, 5.7 p.e. detected • Serious position dependence is observed near PMT • ;should redesign the reflector c. Angle dependence measurement with 2”PMT • Due to the heavy dependence on the position near PMT, • we have made a new structure reflector. • Beam : cosmic ray • Beam trigger size : 2cm x 2cm • No big difference at the edge of the water box in both cases • Still more, enough p.e. shown near PMT with the original • reflector. • Therefore, previous experiment might have some position error • when we setup the WČ counter at Research Center for electron • and photon science in Japan. • As the absorbers are added, this peak moves towards a pedestal, • and at a certain point, it stops. We define this point is the peak • position of single photo-electron. • The number of channels per one photo-electron for 2”PMT : 8.4ch • The number of channels per one photo-electron for 3”PMT : 1.3ch • Incident angle dependence is measured with 2”PMT • The dependence is rather weak • ; no problem to cover the large angle scattering kaons d. Position dependence measurement with 3”PMT 36ch ◈ WČ counter test @Research Center for electron and photon science a. Setup 18.5p.e. 12ch ◈ WČ counter test for different PMT heights @SNU • At the center, 18.5 p.e. detected • Serious position dependence is observed near PMT • ;should redesign the reflector e. The number of p.e. at the center of the water box • Beam : 450MeV positron beam • Beam trigger : t3ⓧt4ⓧt5ⓧt6 • Cerenkov radiation trigger • : beam triggerⓧWC • Beam trigger size : 1cm x1cm • To cover PMT with a shield for magnetic field protection, • we tested with PMT pulled out 1.5cm or 2.8cm from the • supporter ; no dependence on the heights of PMT • Efficiency of the measurements=count(total-pedestal) / count(total) > 99% Conclusions • The numbers of p.e. we observed are about 4~5.7p.e. with 2”PMT and 14~18.5p.e. with 3”PMT for β=1(450MeV positron beam). • This WČ counter is enough to detect the kaons within any position of the water box and any beam incident angle we measured, because it has no dependence. • With different reflectors or different heights of the PMT, there was no dependence of the p.e. number. • As a result, it had more than 99% Cerenkov light detection efficiency, therefore we will be able to improve the data acquisition rate of the kaons in the (π+,K+) trigger events for 12C(π+,K+)12ΛC reaction at least more than 10 times. Seoul National University Department of Physics & Astronomy The World Class University Intermediate Energy Physics Laboratory