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Scintillation and Ionization yield of
Liquid ArgonRichard Saldanha
On behalf of the SCENE collaboration
24th September 2015
Postdoc position available at UChicago
Work with Prof. Luca Grandi on Xenon1T
For more details please seehttps://inspirehep.net/record/1389800
or talk directly to Luca !
Prior status of Ar Nuclear Recoil calibration
3
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Few measurements of scintillation yield in liquid
argon, especially at low (< 25 keV) energies, with no studies of the effect of
the drift field
One recent measurement of ionization yield
(Joshi, Sangiorgio et. al arXiv: 1402.2037) at 6.7 keVr
7Li(p,n)7Be
4 C.A.Burke et al., Phys. Rev. C 10, 1299 (1974)ATOMIC DATA AND NUCLEAR DATA TABLES 15, 57-84 (1975)
Produces monoenergetic (< 2 MeV) neutronsProton Energy Threshold: 1.881 MeVCross Section Peak: Ep = 2.25 MeV
Experimental Setup
5
6
Proton Beam
• Pulsed Beam(Period = n x 101.5 ns)
• Beam Energy: 10 MeV max.
• Beam Energy Uncertainty(± 1 keV mean, ± 2 keV spread)
• Beam angle at target < 0.006 deg
SCENE parameters1 ns pulses203 ns period (S1 only)406 - 812 ns period (S2)
2.376 - 2.930 MeV
6.3 x 104 p/pulse3 x 10-4 n/pulse (@TPC)
FN Tandem Van de Graaff accelerator at the University of Notre Dame
LiF target
7
0.20 mg/cm2 LiF on 1 mm aluminium backing
0.1 mm tantalum layer between LiF target and aluminium to fully stop protons (Oct run)
~ 20 keV energy loss in target
8
Two-phase liquid argon TPC
2 x 3” R11065 PMTs top and bottom3M foil reflector
ITO-coated fused silica windows
9
Two-phase liquid argon TPC
2 x 3” R11065 PMTs top and bottom3M foil reflector
ITO-coated fused silica windows
10
Energy [keV]0 5 10 15 20 25 30
Counts/0.5 PE
0
200
400
600
800
1000
1200
1400
1600
1800
2000
All scatters
Mult. scatters
Single scatter
median: 10.3 keV
Two-phase liquid argon TPCDesigned to minimize neutron scatters outside active volume
Geant4 simulations of neutron beam predict a clear single scatter peak (after coincidence cuts) above the
multiple scatter background
Simulations of electric field uniformity indicate a ~ 8-10% variation of electric field in the
neutron interaction region
Neutron Detectors
11
EJ-301 Organic Liquid Scintillator5” x 5” coupled to PMT
Provides both timing and γ/n pulse shape discrimination
EJ-301’s mounted on two-axis goniometric stand
Background Discrimination
12
Proton Beam RF Timing
Triggered on coincidence of TPC PMTs (OR/AND)
and any one EJ detector
TPC S1 TimeTPC S1 Pulse Shape
EJ TimeEJ Pulse Shape
Experimental Setup
13
S1 S2[V/cm ] [V/cm ]
0 049.596.4 96.4193 193293 293486 486970
71 cm
73.1 cm (Jun)82.4 cm (Oct)
14Ntof [ns]-20 0 20 40 60 80 100 120 140 160 180
Npsd
0
0.05
0.1
0.15
0.2
0.25
0.3
0
1
2
3
4
5
6
7
8
Neutrons
Photons
(c)
Neutron DetectorTPCtof [ns]
-40 -20 0 20 40 60 80 100 120
f90
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
1
2
3
4
5
6
7
Photons
Neutrons
(b)7 us after S1Require S2 starts later than
LAr-TPC
Selection Cuts
Application of 2 TOF and 2 pulse shape cuts
allows for extremely clean sample of data
Selection Cuts (57.3 keV, 193 V/cm)
15
Before Cuts After Cuts
S1 S2
S1 Trigger Efficiency 16
Signal amplitude [PE]0 2 4 6 8 10 12 14
Trig
ger
Effi
cien
cy
0
0.2
0.4
0.6
0.8
1
Two PMT OR
Two PMT AND
S1 [PE]0 10 20 30 40 50 60
Arbi
trar
y Un
its
0
0.01
0.02
0.03
0.04
0.05 Two PMT OR
Two PMT AND
20.5 keV (1000 V/cm)
Two methods used to measure trigger efficiency
Simultaneously digitize signal and trigger logic, while triggering on
22Na source with external coincidence detector
(Plante et. al. Phys. Rev. C 84, 045805)
Nuclear recoils at 20.5 keV and 970 V/cm were acquired using
both OR and AND trigger
S1 OR Analysis Threshold: 4 PES1 AND Analysis Threshold: 12 PE
Fitting S1 Spectra
• To determine the scintillation yield, the S1 spectra are fit with a Monte Carlo generated recoil spectrum.
• Starting from the neutron production, the entire geometry of the TPC and neutron detectors is simulated
• The same coincidence and timing cuts as used in the analysis are implemented in the simulation to obtain the final sample of nuclear recoils
• The data were then fit to the MC spectra using an independent fixed Leff and resolution for each geometrical setup
17
Fitting S1 Spectra
18
Example of fits to 14.8 keV
nuclear recoils
Kr Reference
19S1 [PE]0 50 100 150 200 250
Counts/2 PE
1
10
210
310All Coincidence Events
After Ntof & Npsd Cuts
After TPCtof & f90 Cuts
(a)
Leff measurements are usually made with respect to electron recoils at null field
83mKr is extremely useful as a reference calibration since it can
easily injected, distributes within the active volume, and decays to a
stable isotope within a few hours
Kr peak
83mKr was continuously injected into the detector to
monitor the light yield stability during the beam runs
Null Field LY:6.3 ± 0.3 PE/keV (June)4.8 ± 0.2 PE/keV (Oct)
Leff Results
20
Recoil Energy [keV]10 20 30 40 50 60
Kr(0V/cm)
83m
S1 Relative to
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
0.32
0 V/cm
290 V/cm
970 V/cm
Drift Field [V/cm]0 200 400 600 800 1000
Kr (0V/cm)
83m
S1 Relative to
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
0.3210.3 keV25.4 keV36.1 keV57.3 keV
Previously believed that the scintillation yield of nuclear recoils is unaffected by external fields
due to the dense nature of the track
SCENE measurements show lower scintillation yield at high fields
Advantageous to run liquid argon TPCs at low fields to improve pulse shape discrimination
Leff Systematics
21
Example of systematics studied at null field
Leff Results
22
0 20 40 60 80 1000
0.1
0.2
0.3
0.4
0.5
Recoil energy [keV]
Leff a
t 0
V/c
m
This workGastlerRegenfus
Gastler et. al. Phys. Rev. C 85, 065811 (2012) Regenfus et. al. J. Phys.: Conf. Ser. 375, 012019 (2012) Creus et. al arXiv:1504.07878v2
Creus
Energy [keV]0 20 40 60 80 100 120 140 160 180 200
S2 [PE]
0
100
200
300
400
500
600
700
800
Quadratic Qy vs Energy
Logarithmic S2 vs Energy
Power Law S2 vs Energy
PRD Submission
Fit Range
500 V/cm486
S2 fitting
Threshold for S2 fitting set by translating S1 threshold using data at each drift field setting
23
S1 [PE]0 20 40 60 80 100 120 140 160 180 200
Mean S2 [PE]
0
100
200
300
400
500
600
S2 OR THRESHOLD: 123 PE
S2 AND THRESHOLD: 196 PE
16.9 keV25.4 keV36.1 keV57.3 keVOR ThresholdAND Threshold
193 V/cm
S2 yield can vary rapidly with energy, significantly even within a
given geometrical setup
Data was fit using several different global functions:
Quadratic, Logarithmic, Power Law
S2 fitting
24
Results for data acquired at 486 V/cm
Qy results
25
Recoil energy [keV]10 20 30 40 50 60
S2 yield [PE/keV]
2
4
6
8
10
12
14
16
96.4 V/cm
193 V/cm
293 V/cm
486 V/cm
/keV]
-S2 yield [e
1
2
3
4
5
Model of energy deposition for electron recoils
26
LET Dependence of Scintillation Yields in Liquid Argon Doke, T. et al. Nucl.Instrum.Meth. A269 (1988) 291-296
Energy (E)
Nex Ni
Nph Ne-
S1 S2
1/W1/Wph -1/W
r (dE/dx,F)1 - r (dE/dx, F)
g1 g2
Excited Ar atoms
Ar ions + electrons
UV Photons
S1 signal
Drift Electrons
S2 Signal
Recombination(depends of track density, drift field)
W : Energy required to produce electron-ion pairWph : Energy required per quanta (Nex + Ni) = Energy required to produce a photon when r = 1r : Recombination fractiong1: Scintillation detection efficiency [PE/UV photon]g2: Electron detection efficiency [PE/electron]
Only recombination (r) depends on the drift field (F)
S1 and S2 are linearly anti-correlated, independent of drift field
27
E
Nex Ni
Nph Ne-
S1 S2
1/W
g1 g2
1/Wph -1/W
r (dE/dx,F)1 - r (dE/dx, F)
S1
E= −g1
g2· S2E
+g1Wph
Is this also true for nuclear recoils ?
S2 vs S1
28
S2 yield [PE/keV]0 5 10 15 20 25 30 35 40 45
S1 yield [PE/keV]
3
3.5
4
4.5
5
5.5
6(a) / ndf 7 / 222!
0.08±(max) [PE/keV] 5.33 phW/1
g
0.003± 0.033 2
g/1
g
Kr during:83m
16.9 keV
25.4/57.3 keV
36.1 keV
S2 yield [PE/keV]0 2 4 6 8 10 12 14 16
S1 yield [PE/keV]
0.8
1
1.2
1.4
1.6
1.8(b)
16.9 keV
25.4 keV
36.1 keV
57.3 keV
(max) [PE/keV]phW/1
g 0.04±16.9 keV: 1.33
0.04±25.4 keV: 1.40
0.04±36.1 keV: 1.49
0.03±57.3 keV: 1.54
Krypton Nuclear Recoils
S1
E= −g1
g2· S2E
+g1Wph
Both electron and nuclear display an S1-S2 anti-correlation, with the same slope (-g1/g2)
g1: 0.104 ± 0.006 PE/photon g2: 3.1 ± 0.3 PE/e-
Assuming Wph(Kr) = 19.5 ± 0.1 eV
S2 vs S1
29
S2 yield [PE/keV]0 5 10 15 20 25 30 35 40 45
S1 yield [PE/keV]
3
3.5
4
4.5
5
5.5
6(a) / ndf 7 / 222!
0.08±(max) [PE/keV] 5.33 phW/1
g
0.003± 0.033 2
g/1
g
Kr during:83m
16.9 keV
25.4/57.3 keV
36.1 keV
S2 yield [PE/keV]0 2 4 6 8 10 12 14 16
S1 yield [PE/keV]
0.8
1
1.2
1.4
1.6
1.8(b)
16.9 keV
25.4 keV
36.1 keV
57.3 keV
(max) [PE/keV]phW/1
g 0.04±16.9 keV: 1.33
0.04±25.4 keV: 1.40
0.04±36.1 keV: 1.49
0.03±57.3 keV: 1.54
Krypton Nuclear Recoils
S1
E= −g1
g2· S2E
+g1Wph
Intercept (g1/Wph) changes with energy for nuclear recoils
Total Quenching
30Recoil energy [keV]0 10 20 30 40 50 60 70
Total quenching factor
0.15
0.2
0.25
0.3
0.35
0.4
This work
Fit to Mei et al.
Lindhard Theory
Nuclear Recoil Energy (Enr)
Nex Ni
Nph Ne-
1/W1/Wph -1/W
r (dE/dx,F)1 - r (dE/dx, F)
Electron Recoil Energy (E)
η (Enr) Some nuclear recoil energy is lost to atomic motion
Lindhard Theory: Quenching attributed to loss of energy to atomic motion
Mei et. al: Quenching attributed to Lindhard + Birk’s quenching
(kB = 5.0 ± 0.2 x10-4 MeV/g/cm2)
Recombination ModelNumber of ions can also be determined using a
recombination model
31
Drift field [V/cm]0 100 200 300 400 500 600
S2 y
ield
[PE
/keV
]
0
5
10
15
20
25
30
35
40
45
16.9 keV25.4 keV36.1 keV57.3 keV
Kr during:m83
16.9 keV25.4/57.3 keV36.1 keV
/keV
]-
S2 y
ield
[e
0
2
4
6
8
10
12
14
Modified Thomas-Imel Box Model (arXiv: 1402.2037)
B: Constant - same for all species, energies
(0.61 ± 0.03)
C, Ni: Constant - different for each energy
Recoil Energy [keV] C [(V/cm)B/e−] Ni/E[keV]16.9 0.58±0.17 8.2±1.925.4 0.50±0.23 7.0±2.536.1 0.45±0.19 5.9±2.057.3 0.42±0.16 4.8±2.64.8± 1.8
Summary• We have performed a measurement of the scintillation and
ionization yield of liquid argon
• Measurement spans energy ranges from 10 - 57 keV recoils with drift fields from 0 - 970 kV/cm
• Simultaneous measurement of the S1 and S2 signals allows for a comparison with quenching and recombination models
• For additional details, including measurements of pulse shape discrimination and directionality, please see publications:
Observation of the dependence on drift field of scintillation from nuclear recoils in liquid argon T. Alexander et. al. (SCENE Collaboration) Phys.Rev. D88 (2013) 9, 092006
Measurement of Scintillation and Ionization Yield and Scintillation Pulse Shape from Nuclear Recoils in Liquid ArgonCao et. al. (SCENE Collaboration) Phys. Rev. D 91, 092007 (2015)
32
Thank you
Energy [keV]0 20 40 60 80 100 120 140 160 180 200
Mea
n S2
[PE
]
0
100
200
300
400
500
600
700
800293 V/cm
16.9 keV25.4 keV
36.1 keV57.3 keVFit to MC Spectrum
293 V/cm
35
Take profile of means in each
S1 slice
Convert S1 axis to energy using Leff
measurement:
S1 = E x LYx Leff
293 V/cm
Assume all events after cuts are single scatter
nuclear recoils in TPC
S1-S2 Consistency Check
36
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