村松憲仁 東北大学電子光理学研究センター ELPH研究会 「素粒子・原子核実験における全吸収型 カロリメータの実例と応用」 2015年3月10日 BNL-E787/E949 実験で用いた エンドキャップ純 CsI ガンマ線検出器 1 興味のある方は I-H. Chiang et al., IEEE Trans. Nucl. Sci., Vol. 42 (1995) 394. T.K. Komatsubara et al., Nucl. Instr. Meth. A 404 (1998) 315. を参照してください。

BNL-E787/E949 Experiment      : One of the Golden Modes for study of the CKM matrix and CP violation. BR[SM] = (8.0  1.0)  10  11 Range Tail Beam background Signal region Muon Band BR(K + )  |V td | 2 2

BNL-E787/E949 History E E787 Phase-I Phase-II 1992 Japan-US (Phase-II) start endcap photon detector Pb+Sci. w/ long light guide  undoped CsI w/ fine-mesh PMT data taking E E949 approved by DOE beam intensity upgrade new barrel photon veto data taking cancellation by DOE 2005 RSVP cancellation by NSF Most of detector & DAQ system has been transferred to SPring-8. Number of stopped K + 3

▲ : E949 data ○ : E787 data Solid line : E949 signal region Dashed line: E787 signal region K +  + Result (above the K +  +  0 peak) 4

E787/E949 Detectors 710 MeV/c K + Beam (K/  ratio = 3:1) Incoming :  6 MHz, Stopping :  2 MHz Endcap CsI Detector High efficient  -veto in 0  225 MeV range. Lower accidental veto rate near the beam. (old EC: a.v.r.  0.3 MHz stopped K + ) Endcap CsI Det. 1 Tesla B-field 5 Tokiyasu This afternoon

Endcap Undoped CsI Detector Total 143 crystals (upstream : 75, downstream: 68) Dry N 2 gas flow to keep temperature (41.7  C) and humidity (5  15%). Gain monitor by a light pulser. 25 cm (13.5X 0 ) Produced by Crismatec. 6

Fine-Mesh PMT ☺ Work at 1 Tesla magnetic field. (Gain 1 Tesla : 2  10 7  5  10 5 ) 180  300 p.e./MeV (w/ PMT QE of 16%)  old EC : 10 p.e./MeV Short transit time & small time jitter. ☻ Only 40% of single photoelectron hit the 1 st dynode.  Gain variation 7

Undoped CsI Decay Time Constant We observed Fast component :   30 nsec,  300 nm & Slow component :   680 nsec,  450 nm. The ratio of fast component  0.8 UV-transmitting optical filter (U330 by Kenko Co.) from Saint-Gobain Data Sheet 8

Crystal-by-crystal Variation General comments from Prof. Kobayashi The slow component is produced by the radiative recombination of excited electrons, which are captured at lattice defects. The metal impurity at the magnitude of ppm order will affect the crystal growth & quality. 68  88% FOREST pure CsI [very rough analysis]  =44.4 ns (0.732) +  =240.9 ns (0.268) Time [nsec] BNL CsI crystals Pulse height (arbitrary unit) 9

Signal Read-out Trigger, TDC, & ADC lines 250 nsec500 MHz Transient- Digitizers In addition to the Trigger, TDC, & ADC lines, the signal pulse shapes were recorded for 250 nsec by 500 MHz Transient- Digitizers, which were produced based on GaAs CCD at TRIUMF. (More details in D. Bryman et al., IEEE Trans. Nucl. Sci., vol. NS-38 (1991) 295.) 10

Pulse Shape Analysis Detection timing is decided by the leading edge at the constant fraction (0.4). 2 nd pulses on the 1 st pulse tail can be separated above  30 ns.  Overall photon veto inefficiency  10  3 (10  6 for  0 ) 11 1 st pulse: 10 MeV 2 nd pulse: 5 MeV

Time Resolution Timing calibration was done by using K +  +  0 decays. The reference timing is obtained from  + at the Range Stack.  0.7 nsec is achieved at higher energies.  accidental veto rate  20% 12

Photon Energy Measurement Energy calibration was done by using K +  + decays, where the kinetic energy of  + is monochromatic at 152 MeV.  0 energy from K +  +  0 decays : peaked at MeV with  =10.6% (EC-BV combination). Note  =11.8% for BV-BV.  Rough Energy Resolution :  (EC)=12%,  (BV)= 15% at the average energy of 110 MeV. 13 K +  +  0

Summary E787/E949 Endcap undoped CsI  Detector was introduced for the sufficient photon veto ability inside the 1 Tesla B-field. low inefficiency low accidental veto rate Its design is optimized to obtain the timing information with low inefficiency (undoped CsI with the direct connection of fine-mesh PMT, UV-transmitting optical filter, pulse shape analysis) and low accidental veto rate (better time resolution, pulse shape analysis). These techniques may be usable for the photon detectors under the high rate environment. Because of the special purpose, this  detector was not optimized for calorimetry. (1 Tesla B-field, fine-mesh PMT) 14

DE (Direct Emission)IB (Internal Bremsstrahlung) Active Photon Detection for the studies of chiral perturbation theory K +  +  0 , K +  + , K +  +  0  Photon energies are measured by Barrel Photon Detector, & the other activities are vetoed at Endcap. Talk about the Barrel Photon Detector : Tokiyasu this afternoon. 15