1. SUNJI KIM, H. BHANG, M. KIM, K. TSHOO, K. TANIDA, H. FUJIOKA1, Y. SADA1, and H. ASANO1 Seoul National University, 1 Kyoto University Abstract Introduction Conclusions Experiment & Results Seoul National University Department of Physics & Astronomy The World Class University Intermediate Energy Physics Laboratory 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 12 C 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 1x8x15cm 3, we will be able to improve the data acquisition rate of the kaons drastically in the (π +,K + ) trigger events for 12 C(π +,K + ) 12 Λ C 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. ◈ Motivation of WČ Counter inclusion 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. ◈ Principle -Beam : 450MeV positron beam -Beam trigger : t3 ⓧ t4 ⓧ t5 ⓧ t6 -Cerenkov radiation trigger : beam trigger ⓧ WC - Beam trigger size : 1cm x1cm ◈ WČ Counter Structure ◈ WČ counter test @Research Center for electron and photon science a. Setup b. Position dependence measurement with 2”PMT ◈ WČ counter test for new reflector @SNU 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. 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 c. Angle dependence measurement with 2”PMT d. Position dependence measurement with 3”PMT ◈ Single photo-electron measurement (LED calibration) @SNU -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 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 - Incident angle dependence is measured with 2”PMT - The dependence is rather weak ; no problem to cover the large angle scattering kaons e. The number of p.e. at the center of the water box PMTHV [-V] peak pos. pedestal pos. Peak- pedestal 1p.e. ADC Measured p.e. 2”(H6410 No.RD5347) 190060ch12ch48ch8.45.7 3”(H6559 UVB No.ZK4252) 150036ch12ch24ch1.318.5 - 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. Original reflectorModified reflector -At the center, 18.5 p.e. detected - Serious position dependence is observed near PMT ;should redesign the reflector - Efficiency of the measurements=count(total-pedestal) / count(total) > 99% ◈ WČ counter test for different PMT heights @SNU - 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 - 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 12 C(π +,K + ) 12 Λ C reaction at least more than 10 times. 5.7p.e. 60ch 12ch 36ch 18.5p.e.