1. Search for Dark Matter at CJPL with PANDAX
Kirill Pushkin for the PANDAX Collaboration,
10th Rencontres du Vietnam, Very High Energy Phenomena in the Universe,
August 3-9, 2014

2. Contents
Introduction
Experiment
Commissioning. Data taking.
Prospects and summary

3. PANDAX: Particle and Astrophysical Xenon

4. The PANDAX Collaboration
Collaboration started in 2009 (includes ~40 people)
7 institutions
Shanghai Jiao Tong University,
Shandong University,
Shanghai Institute of Applied Physics, CAS,
Beijing University,
Yalong Hydropower Co., Ltd.
University of Michigan (USA)
University of Maryland (USA)

5. China JinPing Underground Laboratory (CJPL)
~2400 m overburden of rock
Deepest underground lab in operation
6720 m.w.e. ~57 muons/(year x m2)
Low radioactivity marble rock
Easy access – located in the middle of the 18 km tunnel
μ – veto system unnecessary
compact shielding structure possible
easy access by road
future plans for CJPL II

6. CJPL
SURF
SURF

7. Current layout of CJPL

8. The PANDAX laboratory Ge counting station
Storage bottles with Gas Xenon
Xe detector

9. The PANDAX Detector Dual phase Xe detector
Probing WIMP-nucleon cross section
Designed for 1 T fiducial mass

10. III stages for PANDAX Stage I: 125 kg xenon target, 400 kg total mass
Stage III
Stage I: 125 kg xenon target, 400 kg total mass
Stage II: 500 kg xenon target, 1.3T total mass
Stage III: 1.5 ton target, 3T total mass
Stage II

11. External shielding Passive shield Rn purge with N2 gas
Reduces background from surrounding
rocks and concrete
Passive shield allows accommodating
a large, 1T, scale detector
Continuous flush with boiled-off LN2
Levels of Rn achieved < 1 Bq/m3 (limited by the
sensitivity of the used Rn detector

12. Cryogenics Cooling power ~180 W (-96 degrees of Celsius) LN cooling
Pulse Tube Refrigerator
Heat exchanger

13. Stage I TPC Detector characteristics: TPC radius: 30 cm
Drift length: 15.4 cm
Total Xenon mass: 400 kg
Sensitive target: Xe mass: 125 kg
Fiducial volume: Xe mass 37 kg
Top PMTs array (1”, R8520): 143
Bottom PMTs array (3”, R11410): 37

14. PANDAX- Stage I

15. Background control: Detector materials counting
Use materials with low radioactivity: 238U, 232Th, 40K, 60Co

16. PANDAX Stage I Progress

17. PMT calibration with LEDs
LED calibrations:
2 Teflon ball diffusers on top PMT array
LED and diffusers are coupled to optical fibers and feedthroughs
Gains balanced at ~2x106

18. Calibration data system
We can insert a gamma or neutron source into the tubing to calibrate the detector
3D position reconstruction important for further background reduction

19. Xenon purity
Electron lifetime ~262 us (drift time us)

20. S2 vs S1 light anticorrelation
80 keV (Xe-131)
40 keV (Xe-129)

21. Gamma background rejection

22. Radioactive background: Kr in Xe
Radioisotope Kr-85, T1/2= years. Beta maximum energy decay 687 keV
We employ a home made distillation column to remove Kr from gaseous Xe.
We use an ultrasensitive Purity Analysis System based on RGA (MKS E-Vision 2).
The amount of Kr-84 in our Xe after the distillation is of the order of ppt which can be subsequently recalculated in to the amount of Kr-85 in Xe.
Kr-Xe distillation system

23. Calibration measurements of Kr-84 in Xe gas
Measured by K. Pushkin and He Min
Purity Analysis System

24. Sensitivity projection (Stage I after 100 days of taking data)
Leff cutoff
below 3 keV
Leff no cut off
below 3 keV
Stay tuned for the Physics Results
(17 days Dark Matter data paper will be released VERY SOON)

25. Upgrade: Stage II Goals:
Upgrade in preparations. Stay tuned for more exciting news
Sensitivity reach 2x10-46 cm2 at 100 GeV
with kg-day.

26. Future plans: CJPL-II

27. Summary
PANDAX has made a good progress in the past year. Currently, the experiment is taking data.
Stay tuned for the coming 17 days Dark Matter data paper (expected to appear VERY SOON on arXiv).
Currently, we are upgrading the detector, Stage I, to Stage II.

28. Thank you for your attention!