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

2. 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

3. 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

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

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

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

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

8. 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

9. Background and Threshold
Still far from expected…

10. 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%

11. 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

12. Neutron background study by Monte Carlo Simulation

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

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

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

16. 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

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

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

19. 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)

20. Ge Detector characteristic peaks
Previous result: 1.3keV, 10.3keV
Current result: 0.923keV, 10.08keV
Ge characteristic X-ray: K series ~ keV, 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

21. Neutron Experiment Relics

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

23. 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

24. Thanks!

25. 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.

29. The electronics fot 20g detectror