study of nuclear recoils in liquid argon - taup conference · study of nuclear recoils in liquid...

TAUP Munich 8.9.2011 C. Regenfus 1

Liquid argon detectors

• Introduction• VUV detection + Calibration• Pulse shapes• Alphaʼs in LAr• n induced NRʼs

Study of nuclear recoils in liquid argon

C. Regenfus Universität Zürich

at zero electric field


Motivation

• Driven by recent developments of LAr technology

• Exploration of the low energy frontier of this technology is ongoing

Energy threshold

Background suppression (PS discrimination)

• LAr has the potential for large and very large projects

• Still space for developments


LAr scintillation - light pulse shape

A

τ1· e−

tτ1 +

B

τ2· e−

tτ2

Fp =

� t0+30ns(t0−20ns) Vp(t)dt� 5µs(t0−20ns) Vp(t)dt

≈ A

A+B

Time [ns]0 500 1000 1500 2000 2500 3000

Sign

al [V

]

-510

-410

-310

-210electrons

!-particles

LAr decay times: 5ns, 1.6µs

Similar to alkali halide crystals (~40ph/keV)

Complicate production process

Argon eximer1Σu,

3Σu Unbound ground state 1Σ+

g + 128±5 nm photon (not absorbed in atomic Ar)

Ar

ArTwo self-trapped exciton states

Ionization density effect

α

µβ/γ

n ≈ 3 pe/keV

Fit example (3rd component neglected)

Prompt Fraction Method:

(arb. scaled)

Second continuum

rate

Hitachi A. et al., Phys. Rev. B, 27 (1983) 5279.


128nm light read out R&D (using GAr)Short wavelength

-> need WLSTPB

Diffusion cell design for ArDM

Semi-reflecting LEM ?

Hamamatsu R5912-02MOD

14 Bialkali 8” PMTs (QE ~ 18%)

TPB evaporated

PMT coatings

Test evaporator (old exsicator)

C.Amsler et al., “Luminescence quenching of the triplet excimer state by air traces in gaseous argon” JINST 3 (2008) P02001

V. Boccone,a, P. Lightfoot, K. Mavrokoridis, C. Regenfus et al., “Development of wavelength shifter coated reflectors for the ArDM argon dark matter detector” , JINST 4 (2009) P06001

Purity quenching

0 2000 4000 6000 8000 100000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Theoretically (CLT; from sqrt(n) x !)

Integration time [ns]

Inte

gra

tio

n e

rro

r [

nV

s]

1pe = 0.2nVs

Trace 1

!6 !4 !2 0 2 4 6 80

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000 3500 4000 50004500

0

0.1

0.2

0.3

0.4

0.5

0.6

IPH /10

leftover charge

3 ! thresh

±2 samples

±10 samples

TAUP Munich 8.9.2011 C. Regenfus

Reconstruction software

5

Clusterfinder

Integration noise (preamp)

Finite integration

Due to the long life time of the triplet state, long integration windows are necessary

Corrections

!2 [ns]

0 200 400 600 800 1000 1200 1400 1600

Inte

gra

l s

ize

[p

e]

0

20

40

60

80

100

120

140

A (singlet comp.)

B (triplet comp.)

LY: 3.75 pe/keV

LY = A+B

τ2· 1600 ns = 3.75 pe/keV

!2 = 1270 ns

Clusterfinder

!int = 4500 ns

Mean 0.1± 170.5

Sigma 0.08± 23.39

IPH [pe]0 100 200 300 400 500 600 700 800

En

trie

s / 1

pe

0

500

1000

1500

2000

2500Mean 0.1± 170.5

Sigma 0.08± 23.39

60keV


Light yield calibration (3” LAr test cell)

6

3 “ LAr cell

83mKr in progress

Impurity quenching in LAr (non radiative destruction of Ar exicmers)

9keV

31 keVLinearity from Compton spectra + 60keV line

Normalized yield

3 “2 “

Res = 1.8 * Sqrt(Npe)

pursued by Alfredo Ferella (DARWIN-UZH postdoc)

0 100 200 300

Ar

electrons

1! width of distribution

pe400

A/A

+B

!

500 600 700 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


NR - ER separation power

7

Multibin likelihood ratio to select nuclear recoils

W. Lippincott et al., Scintillation time dependence and pulse shape discrimination in LAr, Phys. Rev. C 78 035801 (2008)

• Avg. traces for ER and NR are fitted for each energy bin with a sum of twoexponentials convoluted with a Gaussian to parametrize the shapes.

• Here improvement by replacing the Gaussian with the measured meansingle photon pulse shape. Also asymmetric shape in PMTs was de- convoluted by matrix inversion

• As Proof of principle we trained this statistical methods on some older data.Separator improves separation power in this example by ~10 %

Separator ln R (old data)

Method

pursued by Yves Allkofer (UZH postdoc)

NG, alpha, 22Na data Investigate statistical spread

0 0.2

0 1000 50002000 60003000 pe4000

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 MeV

0.68

0.7

0.72

0.74

0.76

0.78

0.8

0.82

0.84

0.86

0.88

Light yield [pe] - energy on ee-scale [MeV]

Co

mp

on

en

t ra

tio

A/A

+B

(in

re

d) DE/dx ASTAR (arb. units)

!

P = polynomial parameterisation

Fit of pol. parameterisationf = a•P(b•x) + c


Alpha particles in LAr (yield, pulse shapes)

8

α

µβ/γ

n ≈ 3 pe/keV

From endpoint of alpha spectrum we find qα~ 0.8

Fit results (errors mainly from systematics):a = 0.13 ± 0.03b = 0.74 ± 0.05c = 0.64 ± 0.1

fn ~ 0.98 nuclear from Lindhardfl ~ 0.75 luminescence

Model seems OK

D.-M. Mei, at al. A model of nuclear recoil scintillation efficiency in noble liquids. Astroparticle Physics, 30(1):12 – 17, 2008

A. Hitachi and T. Doke. Luminescence quenching in liquid argon under charged- particle impact: Relative scintillation yield at different linear energy transfer. Phys. Rev., B46(18), 1992

210Po α-source installed (30 Bq, 5.3 MeV)Influence of dE/dx on triplet - singlet strengths

Prom

pt F

ract

ion

A/A

+B c

ompo

nent

s fr

om fi

ts

ftot = fn x fl


Neutron scattering setup @ CERN

9

Cryo-cooler

Collimator

Neutron shield

Neutron emission

point

xLAr cell

Liquid Scintillators

0.5 - 2m

!

Neutron generator

(D D fusion)

Scattered

neutron

Recoil

nucleusTarget

Neutron

detector

(tagging)

n (2.45 MeV)

Monochromatic neutron generatorDD fusion -> 1D2 + 1D2 -> 2He3 (0.82 MeV) + 0n1 (2.45 MeV) 1D2 + 1D2 -> 1T3 (1.01 MeV) + 1p1 (3.02 MeV)

~ 106 n/s 4πsrUmax = 120 kV, Imax = 10 mA Bremsstrahlung shielded with 4mm Pb

PE shield (1.6t)

Maximal energy transfer in LAr (central collision): 233 keVr

Gas recircu-lation system


Radiation level below 1 µSv/h

Voltage [kV]60 70 80 90 100

n/s

510

610

710IG = 15mA (NSD manual)

X-ray contamination(insuf. Pb shielding)

IG = 10mA (NSD manual)

IG = 10mA

IG = 10mA

IG = 10mA

IG = 14/15mA

IG = 14mA

IG = 14/15 mA

NG Flux in 4!

Rates measured with 5” organic scintillator cell,corrected for solid angles and acceptance (0.8)

with coll., using MPD4

without coll., 1mm Pb, using MPD4

with Coll., 1mm Pb, using MPD4

without coll., 1mm Pb, using MPD4

without coll., MatLab

without coll., 2.5cm Pb, MatLab

without coll., 2mm Pb, MatLab

without Coll., 2.5cm Pb, MatLab

Energy [MeV]

Low energy tail ~ 16%

Entr

ies

per 2

00 k

eV

0 0.5 1 1.5 2 2.5 3-5

0

5

10

15

20

25

30

35

40

Neutron spectrum(preliminary)


Flux and energy spectrum of the neutrons

11

N-spectrum at exit of collimator,unfoldet by the monochr. response

Calibration of LSC with AmBe source

nγ

Long run-in period requiredTypical OP: 80kV / 10mA Max. power : 1.5 kW (cooling)

Now: reliable operation

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(&)!%"*+,-,

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5605

/504

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!5601 !

41

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!+.4++#F=?=9V>?CJ>CAE

#F=?=GCH++?IJAK+W#+8631

"J?CP:>+P=VJ>CG@

O8+?FEA:BAB+9=VG?CG@+F=>AH+Q4;U=G+/8+N)<

%GAE?+@:H+XVXX>APA>>+:;CH+DAE?CP:>

NM.+=J?CP:>+RCGB=R

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,

,

P.O

. Box 1

43

3980 C

C B

unn

ik

The N

etherlands

Tel. 3

1 (0)3

0 6

57 0

312

Fax. 3

1 (0)3

0 6

56 7

563

SC

ION

IX

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Rad

iation D

etectors & C

rystals

SC

ION

IX H

OL

LA

ND

BV

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%!&Oab,*!

O,!&[%,YL&()[%#!%"*

L%O&*(%"*(K[&O,[M(

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Raw spectrum from LSC

PE collimator


DAQ and trigger

12

LeCroy scope DAQ

• Analogue signal splitted (passive)• 4 x 1Gs/s 8bit (Eres)

Trigger logic used:

Min. Bias• Signal in LAr (AND)

Nuclear recoils• Arm on signal in LAr t1 (AND, OR)• Confirm n in LSC (tTrig < t1 +250ns)• LSC trigger derived from MPD4

Trigger threshold / time calibration (22Na)• MPD4 in non-neutron mode

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%GAE?+@:H+XVXX>APA>>+:;CH+DAE?CP:>

NM.+=J?CP:>+RCGB=R

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,

,

P.O

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43

3980 C

C B

unn

ik

The N

etherlands

Tel. 3

1 (0)3

0 6

57 0

312

Fax. 3

1 (0)3

0 6

56 7

563

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(),Y&K

L[,'*K

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+!%!Y&K

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rystals

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ND

BV

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O,!&[%,YL&()[%#!%"*

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Slow control based on LV

Neutron ID

(Poor timing due to unknown position in LSC)

Time[min]

0 10000 20000 30000 40000 50000 60000 70000

Ga

in[n

Vs

]

0.005

0.01

0.015

0.02

0.025

0.03

Time[Day]0 5 10 15 20 25 30 35 40 45

Bottom PMTTop PMT

Time[min]

0 10000 20000 30000 40000 50000 60000

Lig

ht

Yie

ld [

Pe

/ke

Ve

e]

1

2

3

4

5

6

7

8

Time[Day]0 5 10 15 20 25 30 35 40 45

/ ndf 2! 4.725 / 24p0 0.08205± 3.757

/ ndf 2! 4.725 / 24p0 0.08± 3.75


Measurement of NRs - data taking summer 2011

13

• Over 80 runs taken (up to 2000 files each)

• Time, trigger thresh. cal. with 22Na source

• Main LY calibration done with 241Am source

• Additionally with 57Co and 83mKr

• Up to now: 7 angles measured

20 25 30 40 50 60 90 deg

• Data analysis just started

• Basic MC existing (GEANT4)

• Explore basic physical effect with MC

• Use shapes (PDFʼs) derived from real data

pursued by William Creus (UZH PhD student)

change of DAQ

Purity changing between 550 …. 1300 ns

Continuously over 1½ month


Example: raw data @ 40±5 deg (28.5±7 keVr)

14

tTrig

tTrig

Signal~21pe

BG

Inelastics

elastic peak

Inelastics

IPH

CR

CR

BS accidentals

First glimpse on the data

ER (28.5keV) ~ 105 pe

BS

IPH

time

time

Preliminary


Very preliminary

15

• IPH spectra at the 7 angles

• Relative yield calculated relative to 60keV

• No correction done due to the MC line shape

• Value at Er>50keV seems OK

• Error (system.) determined from comparison to different cuts

(First component only, CR cut)

• Horizontal error from MC

0 50 100 150 200 2500

0.1

0.2

0.3

0.4

0.5

Ar recoil energy [keV]

Re

lati

ve

sc

inti

lla

tio

n y

ield

Warp

This work

McKinsey

Preliminary


Summary and Outlook

• We set up and operate a mono-energetic neutron source with a 3” LAr cell (E = 0)

• We cleanly see coincidences from scattered neutrons in LAr and liquid scintillators

• Serious data analysis has just started.

• Data suggest a decrease of Leff towards low energies (as in LXe)

• Further on we measured the LY and pulse shapes for alpha particles

• We plan to upgrade the system with HQE PMTs and a better trigger logic (near future)

• In the LAr sector some home work still has to be done

• However we donʼt expect surprises - things can be inferred from LXe

• Plans exist for adding an E-field and charge extraction in a small new cell (next year?)

study of nuclear recoils in liquid argon - taup conference · study of nuclear recoils in liquid...

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