On sub-keV Ge detectors and CDEX dark matter searches

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Shin Ted Lin / Sichuan University for the CDEX Collaboration

EDU meeting @ Quy Nhon, VietnamJuly 28, 2017

On sub-keV Ge detectors and CDEX dark matter searches

Shin Ted Lin / Sichuan University , Chinafor the CDEX Collaboration

EDU meeting @ ICISE, Quy Nhon, Vietnam. July 27, 2017

OUTLINE

CDEX Collaboration

Programs : CDEX-1, CDEX-10, CDEX-10X, …

Results from CDEX-1 (Dark matter & Axions )

R&D on key Ge technologies

Outlook & Prospects

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Detection of dark matter with a point-contact germanium(PCGe) array

• Tsinghua University

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China Dark matter EXperiment (CDEX) - EST in 2009

• Sichuan University

• NankaiUniversity

• China Institute of Atomic Energy, CIAE

• Yalong River company (former:EHDC)

• Intensively collaborate with TEXONO group.

CDEX stages

• CDEX-1: Develop a large-mass PCGedetector prototype, data analysis methods, and its background understanding and suppression; since 2011.

• CDEX-10: Performances of a HPGearray detector system and its passive/ active shielding systems; since 2015.

• CDEX-10X: Fabrication of HPGe, Ge crystal growth, VFE and ULB-Cu; since 2016

• ……

• CDEX-1T: Multi-purpose experiment for dark matter and double beta decay.

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CDEX at China JinPing Underground Laboratory

CJPL: The deepest operation Underground Laboratory, located in Sichuan, China.

A new development of CJPL.

See Porf. Henry Wong’s talk July 28 . 5

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The space allocation of CJPL-I

Traffic Tunnel

LBF

CDEX

PandaX

Total space: 4000 m3

Lab Space: 6.5m(W) x6.5m(H) x 42m(L)

CDEX CJPL-LBF PandaX

9 km Ventilation system

CDEX-1 PE RoomLEGEND Collaboration @ LNGS, May. 15-17, 2017

CDEX at CJPL-I

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3

4*5g ULEGe

1kg pPCGe

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Detectors in CDEX -0/1

20 cm PE (B)

1 m PE20 cm Pb

>20 cm OFHC

1 kg-scale pPCGe : low energy threshold & good energy resolution.

NaI, enclosed the cryostat of Ge, served as anti-Compton detector.

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Abnormal surface signal of pPCGe

1kg pPCGe @ CJPL

n+ “inactive layer” is not totally dead; signals finite but slower rise time

Algorithm

Measure energy-dependent signal-retaining (eBS) & background-suppressing (lBS) efficiencies through at least two calibration data & solving the coupled equations.

The new promising analysis is established together with TEXONO group.

CDEX-1A

• C1A 1kg-PPCGe + NaI(Tl)

• Bulk/Surf Discrimination

Q.Yue et al., PRD 90(RC) 091701,2014

B/S disc. to get rid of the Surface events with slow rise-time and partial charge collection;

1/3 background lower with/without AC detector; The best sensitivity by PCGe and Exclude the regions

favored by CoGeNT.

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CDEX-1A 2016: Results of Light WIMPs Searches

Reference: PRD 93 092003 (2016) ; PRD(R) 90 091701(2014) ;

PRD 90 032003 (2014) ; PRD 88 052004 (2013)

Configurations:＊335 kg-days of data @ CJPL (2016)＊Baseline design with NaI(Tl) ＊Fiducial mass : 915 g ＊Analysis above : 475 eV＊Q.F. adopted by TRIM software with 10% systematic uncertainty

~2 times more sensitive than 2014 SI & SD results AM analysis with >1+1 year data going on.

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CDEX-1A : Axion searches

M1 transition from 57Fe from Sun [axion-nucleons]

Compton, bremsstrahlung and axio-RD (recombination & de-excitaion) [axion-electron]

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CDEX-1A : Axion searches

Reference: Majorana demonstrator PRL 2017

S. Liu et al.(CDEX Collaboration), PRD 2017

Formula reference: Pospelov et al. PR D 78, 115012 (2008)

Competitive results below the axion mass of 1 keV.

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CDEX-10 : An array detector

Test of cryogenic system has been done and shipped to CJPL in March 2016.

A germanium array with LN in cryogenic system is commissioning.

The performance of LAr is under study.

PCGe

~9 Kg of PCGe⊕

1.5 Ton of LN/LAr

TargetLN

Inner copper tankcap

Totally 10kg PPCGe under testing and run at CJPL!

Background control on CDEX-10X

• Cosmogenic Backgrounds/ Natural background sources/ Detector responses:

– The rate decreases with energy continuously from sub-keV to 20keV;

– CDEX PCGe: 68Ga ~ 1cpkd; Background Level: ~1cpkkd @ 20keV;

Shorten the Ge crystal and detector fabrication time to less than 0.5 year to lower the 3H continue background: 18 cpkd?? 1 cpkd;

Enrichment (76Ge) and/or UG Ge crystal growth + detector fabrication to lower the Ge-68 0.1cpkkd、 Reduce of 3H concentration further.

LB-VFE & ULB-Copper & A cylinder tank with 14 meter diameter: 1 0.1 cpkkd @20keV、Highly suppress radon gas level around Ge detector.

New technique on surface event at low E.Ground 0.5y +UL 2.5y

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R&D on the Key Ge technologies

• 76Ge enrichment；

• HPGe crystal growth;

• HPGe detector fabrication;

• ULB-VFE including substrate and cables;

• ULB-Copper production in underground

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CDEX-10X

Detector fabrication on two 0.5kg PCGe with <350eV；

Totally new design by CDEX and background control with pure cable, VFE substrate and structure materials；

R&D on two preamp types: JFET and ASIC;

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Detector Fabrication

Successful on 10 g/500 gEnergy resolution : 0.95% @ 59.5 keV(10 g) 0.17% @1173 keV(500 g).

500 g PCGe with home-made fabrication will be studied at CJPL soon.

0.169%@1173keV

0 100 200 300 400

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Co

un

ts

Channel

241

Am

7#点电极

HV=350V，t=4us

Peak=323.99

1.48%@59.5KeV

Low Chan=10

Peak=50

~10KeV

0.95%

@59keV

PCGe Detector-19# (ø50mm)

Bias (V) 800 900 1000 1100 1200

Leakage

Current (pA)1 3 6 7 13

Bias (V) 1300 1400 1500 1600 1700

Leakage

Current (pA)19 39 97 188 922

Mass of 10 g

Front-End electronics design

PPC

HPGeASIC

VD

VO

Ct

G

T

Vacuum Chamber

Silicon

substrate

Wires

HVHV Feedthrough

Signal Feedthrough

Signal cables

HV cable

ROOM Temperature:

HV/

Preamplifier/

Fan out buffersJFET D

S

Cf

Ct

Rh

F

T

H1

H2

CuFlon

substrate

Heating

Resistor

LN Tank

~0.3m

OR

~1.5m

Readout Scheme

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• JFET based readout

Test on JFET bonded on CuFlon substrate

0

5

10

15

20

25

30

35

80 100 120 140 160 180 200 220 240 260 280 300

ENC(e)

温度(K)

ENC @ Cd=0 vs. Temperature

Preamp ASIC bonded on Test PCB

• CMOS ASIC based readout

0 5 10 155

10

15

20

25

30

Cin (pF)

EN

C (

e)

ENC=1.38*Cin+4.46

ENC=1.43*Cin+4.86

Temperature=77K

leakage =0.1pA

shaping time=8us

shaping time=12us

Experiment

R&D on Front-End electronics

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Best sensitive in the range

of 10GeV, ~10-44cm2

76Ge double beta decay

research(join LEGEND

Collaboration)

Projected results

A by-product result to see where we are.

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Outlook & Prospects

CDEX-1@CJPL has been commissioned from 2011.Results of light WIMPs and axion searches from CDEX-0/1 have been achieved. (PRD-2013, PRD*2-2014,PRD-2016, PRD-2017). Results of annual modulation based on background understanding are coming soon.

Three 3 kg arrays detector(with two) in LN@CJPL is intensively studied. Lower background level is going to be pursued.

R&D on the Key technologies for detector fabrication, crystal growth, upgraded electronics as well as electroformed copper.

Welcome to CJPL

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