Performance test of fine-mesh and standard photomultipliers for CLAS central TOF system

by Hoseog Dho (MS Candidate)

Friday Nov 11, 2011, 2:15 PM → 2:30 PM Asia/Seoul

216-1호 (제1과학관)

The updated CEBAF Large Acceptance Spectrometer (CLAS12) is being developed for the JLab upgrade program up to 12 GeV. One important component of CLAS12 is the Central Time-of-Flight system (CTOF). The CTOF will provide us with detection and identification of charged particles emitted at the central angles from 40° to 130°. Its design goal is to achieve time of flight resolution sigma_TOF = 50 ps at the high count rate of 1MHz and magnetic field environment of 1.5 T. This study evaluated timing properties of the CTOF prototype counter. The performance of fine-mesh Hamamatsu photomultiplier tubes R5924-70 and R7761-70 coupled to new upgraded prototype of 3 × 3.2 × 66 cm3 scintillation bar of BC-408 has been measured by three different methods: with 1) cosmic rays, 2) proton beams at MC-50 3) nanosecond LED pulser light source with and without 75 and 58 cm long acrylic light guides. The gain and the timing resolution of FM-PMs were compared with the performance of conventional Hamamatsu R2083 PMs. In the cosmic ray test, the TOF resolutions were measured to be sigma_R2083 ≈ 73.7 ± 0.75stat ± 4stat ps and sigma_FM-PMs  ≈ 79.2 ± 1.3stat ± 4stat ps. The ratios of the TOF resolutions for FM-PMs and ordinary PMs are R = sigma_FM-PMs/sigma_R2083 = 1.08 ± 0.07. In the proton beam irradiation at MC-50 Cyclotron of Korea Institute of Radiological & Medical Sciences (KIRAMS), these values were sigma_R2083 = 35 ± 0.54 ps and sigma_FM-PMs = 39.1 ± 0.55 ps at count rate of 0.15 MHz. The performance remained unchanged at higher count rates up to 1.5 MHz. The ratio was R = sigma_FM-PMs/sigma_R2083 = 1.12 ± 0.02. In LED pulser illumination test, these values were sigma_R2083 = 40.09 ± 0.10 ps and sigma_R7761-70/R5924-70 = 40.95 ± 0.10 ps. The ratio was R = sigma_FM-PMs/sigma_R2083 = 1.02 ± 0.10. These results prove the suitability of fine-mesh PMs, equipped with developed negative high voltage dividers at KNU, for meeting the requirement of fast scintillation systems and operating in the high magnetic field environment.

Thesis Advisor: Prof. Wooyoung Kim