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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
TAUP Munich 8.9.2011 C. Regenfus 2
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
TAUP Munich 8.9.2011 C. Regenfus 3
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.
TAUP Munich 8.9.2011 C. Regenfus 4
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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus
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
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)
TAUP Munich 8.9.2011 C. Regenfus
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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Raw spectrum from LSC
PE collimator
TAUP Munich 8.9.2011 C. Regenfus
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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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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus
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
TAUP Munich 8.9.2011 C. Regenfus 16
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?)
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