CsI Veto Detector Performance Study He Dao DMRC. Tsinghua University For KIMS Collaboration

ULE WIMP Experiment WIMP search Ultra-Low Energy wimp experiment –HPGe detector –Expected threshold 100 eV –Correspond WIMP mass limit: ~1GeV

System Structure

CsI detector structure Three parts of crystal –Cylinder top with a slope –Tube side –Cylinder bottom –Optical gel connection Cooler hole Low background PMT Al Shell only for safety

HV Selection Daq: CAMAC; Source Cs-137 ; Output: signal amplitude -1300V selected A reference if HV changed

Incidence position dependency Influence factors: –Big dimension causes serious photon absorption –Conjunction section reflection –Cooler hole –Only one PMT used Daq: MAC Source: Cs-137 Test consideration: height and angular dependency

Source position dependency Height (cm): 1.5, 7, 13, 22, 28 Angle (degree): 180 Height (cm): 5 (near the hole) Angle (degree): 0, 90, 150, 180

Height dependency Peak position –The nearer incident position from PMT, the larger amplitude of output signal –About 70% response while source near the bottom relating to the top position Peak resolution –Round 16%, best situation when source in middle height, top and bottom worse. Main reason: Big size causes photon absorption; pieces section reflection

Angular dependency Peak position –No obvious difference Resolution –The cable hole causes serious resolution difference, 90 degree ’ s response is best. That maybe because the cooler hole is void while I did this test. Fortunately, as a veto detector, the resolution is not very important.

Oscilloscope Calibration (Crystal top) for internal background measurement E(keV) = ( channel - p0 ) / p1 ParamValueError p e e+01 p e e-02 E(keV)channalerror e e e e e e+01 HV: -1200V Source information: Calibration Result:

Internal background Only CsI crystal top 15cmPb, 5cm Cu Daq: Oscilloscope 10 days Peak Pos (channel) Energy (keV) Possible source : Decay chain, internal source, and Radon. More accurate measurement by HPGe detector running in Y2L.

Photon collection efficiency comparison 662 keV gamma HV (minus volt) Oscilloscope scale (mV/div) Peak position (channel) Resolution (%) crystal top All crystal (source near the top) According to HV selection result, if high voltage is set at 1300V the signal amplitude should be about 705/463 to that of 1200V, considering the scale change, the photon collection efficiency of CsI crystal top is 705/463 * 100/50 * 5942/5884 ~= 3 times to that of whole detector!

Summary Working Voltage: –Wide range, -1300V selected Incidence position dependency –Obvious height dependency, weakest situation 70% signal amplitude compared to strongest –Not serious angular dependency Internal background –Decay chain, internal source, and Radon. –More accurate measurement running now by HPGe detector in Y2L Photon collection comparison –Only top 3 times better than whole detector

Plan Pulse Shape Discrimination More accurate internal background content and activity MC simulation Thank you!