Status of ULE-HPGe Experiment for WIMP Search in YangYang Li Jin July 2006

Motivation Current weapon: 5g prototype Ge detector 5g Ge 1cpd Current weapon: 5g prototype Ge detector 16.94kg CsI(Tl) veto detector Expected threshold: ~100eV Target

Current System Setup ULE - HPGe detector FADC & VME crate H.V. (CEAN 1527) Amp. (CANBERRA 2026) HPGe: -500V CsI(Tl): -1300V Shaping time: 6us Gain: ×20

System Schematic Diagram Typical signals: HPGe High gain (0~9keV) HPGe Low gain (0~100keV) CsI(Tl) channel (current signal)

HPGe Calibration – high gain channel Peaks: Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray After gain correction and pedestal shift correction

HPGe Calibration – low gain channel Peaks: Np (L X-ray), Ag (K X-ray), Am (alpha decay gamma)

CsI Calibration Gamma energy: Cd-109 (Ag X-ray): 22.577 keV Am-241: 59.5412 keV U-238 (Th-234): 92.6 keV Co-57: 123.66 keV

Time coincidence between HPGe and CsI CsI noise reduction Time coincidence between HPGe and CsI PSD parameter: Threshold ~ 10keV Veto condition Red: background file, Green: neutron file, Blue: Gamma file

Background and Threshold Still far from expected…

Background level Average background level ~ 40 cpd Energy region: threshold(300eV) ~ 100keV Blue: after veto, Red: before veto Average background level ~ 40 cpd Average veto efficiency ~ 60%

Veto Efficiency Main origin of background: neutron Energy region: threshold(300eV) ~ 100keV (1keV/bin) Blue: gamma files, Green: neutron files, Red: background files Main origin of background: neutron

Neutron background study by Monte Carlo Simulation

Energy of Neutron Source Source :The spectrum of neutron in laboratory (same Cf252) Position: top of Pb Shielding

Energy deposited in Ge Detector Electron and positron energy Recoil energy of Ge * 0.25

Energy deposited in Ge Detector 300eV to 100KeV Most events are below 10KeV and Decrease very fast !

VETO efficiency of Cs Dtector The threshold is 20KeV Efficiency is 100% while energy higher 10KeV Efficiency is lower than 30% when energy below 10KeV

The comparison between simulation and data Simulation results The neutron background mainly Below 5KeV ,consists with data Experimental data

PSD – two shaping time Parameter: Background spectrum PSD efficiency Eth ~ 50eV improvement

Noise level & distribution Noise level ~ 105 eV (FWHM) Assumptions: Pedestal events are all generated by electronics, independent of the detector. Detector mass: 5g  1kg (200 times) Predicted threshold: 230eV (70eV improvement)

Ge Detector characteristic peaks Previous result: 1.3keV, 10.3keV Current result: 0.923keV, 10.08keV Ge characteristic X-ray: K series ~ 10.089keV, L series ~ 1.209keV Cu characteristic X-ray: K series ~ 8.905keV, 8.042keV, L series ~ 0.948keV F characteristic X-ray: K series ~ 0.677keV

Neutron Experiment Relics

Summary Detector mass: 5 g (Ge) Background level: 40 cpd Threshold: 300 eV Veto-efficiency: 60% Measured noise level: 107.6 eV (FWHM) The main origin of background may be the neutron from environment based on the simulation study.

Plan in the future… Accumulate more background data physics result Increase the mass of detector(5g 20g) Internal radiation study & neutron recoil experiment Monte-Carlo simulation for neutron and gamma background Neutron shielding

Thanks!

Calibration Energy 1.7KeV The Ta characteristic X-ray comes from X-ray generator itself. The generator uses a kind of Pyroelectric crystal, LiTaO3, as the target and accelerate electrons. When measured spectrum using the X-ray generator there is a 1.7keV energy peak. Among all the possible materials in experiment devices, the M-series X-ray of Ta is the only found and known material that accords to the about 1.7keV peak in measured spectrums. So 1.7eV is assumed to be Ta M-series X-ray and used for calibration.

The electronics fot 20g detectror