Ultra-low Energy Calibration of LUX
127
Detector using Xe Electron Capture
Dongqing Huang
Brown University
On behalf of the LUX collaboration
APS April Meeting 2015
1
Time
S1
Drift indicates the depth
S2
LUX Experiment
by Carlos Faham and Samuel Chan
2
S2
S2
LUX Experiment
Time
S2
S1
Particle
by Carlos Faham and Samuel Chan
3
127
Xe decay scheme
Electron capture: p + e- → n + νe
cascade χ-rays
or Auger
The binding energy and expected EC
probability from that shell
- K:
33.2 keV
83.4%
- L:
5.2 keV
13.1%
- M:
1.1 keV
2.9%
- N:
190 eV
0.6%
127
127
Xe
I
γ-ray
νe
- Cosmogenic activation
- Half life: 36days
- Based on the measurement
of Apr222013 data, there are
~0.9million 127Xe atoms in
LUX Xenon volume
52.7% 203keV
17.2% 375keV
4
127
Xe EC Decay Event in LUX Data
S2
TOP PMTs
S2
e-
S1
S1
S2
S2
x-ray(EC site)
eγ-ray
γ-ray
203keV
K-shell χ-ray
33.2keV
BOTTOM PMTs
LUXEventViewerGUI
by James Verbus
5
33.2 keV
5.2 keV
1.1 keV
190eV
Xe EC Events in data
Single
Electron
127
190eV
1.1keV
203keV
33.2keV
µ = 856±2
LUX
Preliminary
375keV
5.2keV
µ = 183±1
33.2 keV
µ = 74.5±0.9
203keV
LUX
Preliminary
375keV
5.2 keV
Note:
(1electron = 10.6phe)
Due to S2 threshold effect, N shell χ-ray events are not
completed found for histogram; Detail analysis for N
shell shown in later slides
6
Ratio Analysis
K 33.2keV
L 5.2keV
LUX
Preliminary
#events used for fitting
M 1.1keV
N 190eV
2515
375
96
16
7517±150
1208±62
245±25
45±11
Expected percentage(%)
83.4
13.1
2.9
0.6
Observed percentage(%)
83.4±2.3
(Result + 0σ)
13.4±0.7
(Result + 0.4σ)
2.7±0.3
(Result - 0.6σ )
0.5±0.1
(Result - 1σ )
Amplitude from fit to decay time
histogram (y intercept)
7
Searching for N-shell χ-rays
Energy Reconstruction of Single-Vertex Events
TOP PMTs
LUX
Preliminary
203keV
BG
SE
2σ
N-shell χ-ray(SE)
2σ
SE
γ-ray
BOTTOM PMTs
8
N shell Fit
●
●
Fiducial r → 22cm
S2 threshold: 250phe
●
All SE-type pulses between S1 and
S2 in drift are histogrammed
○
●
●
●
In this case, more background will be
presented; but no real signal is
missed
LUX
Preliminary
µ = 15.9(1.2+/-1.3)
σ = 7.7(+1.0/-0.8)
N = 106
Total number of observed M-shell χray events with fiducial r 22cm and
S2 threshold 250phe
○
501±22
Total expected number of N-shell
events based on the known ratio
between M and N shell (see slide7):
○
115±11
The best-fit N is within 1 sigma of
what is expected
(1electron = 10.6phe)
9
NESTv98*
Energy
(keVee)
(EC x-ray)
127I
NEST S2c
(phe)
[1][2]
NEST QY
(e/keVee)
& Data
NEST
QY width
(e/keVee)
127Xe Data Fit
S2c
(mean phe)
127Xe Data QY
(e/keVee at 180
V/cm)
127Xe Data QY
width
(e/keVee)
LUX
Preliminary
33.2
(K shell)
8121
23.1
3.42
9093±21
25.8±0.9
4.06±0.15
5.2
(L shell)
1830
33.2
5.18
1943±13
35.2±1.2
7.2±0.3
1.1
(M shell)
633
54.5
12.26
802±10
68.6±2.5
12.5±0.7
0.19
(N shell)
*
131
65.4
32.49
169(+11/-13)
83.7(+6.2/-7.1)
40.6(+5.6/-4.7)
[1] Matthew Szydagis, Adalyn
Fyhrie, Daniel Thorngren, and
Mani Tripathi. Enhancement of
NEST Capabilities for
Simulating Low-Energy
Recoils in Liquid Xenon.
JINST, 8:C10003, 2013. doi:
10.1088/17480221/8/10/C10003.
[2] Brian Lenardo, Kareem
Kazkaz, Aaron Manalaysay,
Matthew Szydagis, Mani
Tripathi. A Global Analysis of
Light and Charge Yields in
Liquid Xenon. arXiv:1412.4417
[astro-ph.IM]
NEST with simulation of detector effects such as the binomial light collection and extraction efficiency
10
Qy Comparison with Tritium[3] and NEST[1]
●
●
●
●
The lowest energy point of
127
Xe is higher than what
can be reached with w value
13.7eV
It indicates that the w value
is not necessary to be a
constant
It implies w value ~12.1eV
with only ion-electron pairs
generated at 190eV
127
Xe points are not used for
run3 re-analysis
LUX
Preliminary
[3] Attila Dobi. Measurement of
ER Fluctuations in Liquid Xenon
with the LUX Detector Using a
Tritium Calibration Source. APS
April Meeting 2015, Volume 60
Number 4.
11
Acknowledgment
Special thanks to My advisor Professor Richard Gaitskell
Special thanks to Alastair Currie(Imperial College London)
Thanks to James Verbus, David Malling, Jeremy Chapman, Simon Fiorucci
12
Appendix slides:
13
127
Xe EC Decay
There are two components in Electron Capture decay:
- γ-ray or I.C. Electron associated with 127I excited state
- χ-ray cascade (in daughter atom) from atomic electron vacancy in the K,L,M or
even N shell.
- Note that Auger Electron processes are subdominant in high-Z atoms.
If a γ-ray is emitted, the mean free path in Xe, will lead to a reasonable probability
that a second vertex separated from the first created.
- 203 keVee → ~1.0cm
- 375 keVee → ~2.2cm
- Note that events where γ-ray escapes the active region completely become low
energy ER background events.
Binding energies of atomic electrons in 127I and percentages:
- K:
33.2 keVee
83.4%
- L:
5.2 keVee
13.1%
- M:
1.1 keVee
2.9%
- N:
0.19 keVee
0.6%
- Note that percentages are calculated by including neutrino-phase space
14
127
Xe EC Event Schematics
●
127
●
In the left two schematics, events
appear as double-vertice
Xe EC events appear in three
scenarios in LUX detector as shown in
figure on right.
electron drift direction
e-
e-
●
In the right schematic, χ-ray pulse and
γ-ray pulse merge into one and events
appear as single-vertex
γ-ray
x-ray(EC site)
e-
eγ-ray
x-ray(EC site)
ex-ray(EC site)
●
Events in the bottom left show as a
big pulse following a small pulse in
data, which are used for the major
part of the analysis
eγ-ray
15
Gamma S2c Histogram
●
●
The first peak is the γ-203keV S2c
histogram
The second peak is the γ-375keV S2c
histogram
LUX
Preliminary
16
Z-Separation
●
●
Figure on the right shows the
scatter plot of z-separation(us) btw
χ-ray and γ-ray of each event.
0.19keV
0.19keV
1.1keV
1.1keV
5.2keV
5.2keV
33.2keV
33.2keV
LUX
Preliminary
1μs = 0.15cm in z direction
17
Z-Separation Histogram
●
Events with γ-203keV S2 size less than the
mean value are used for histogram to
mitigate γ-375keV impact on its mean free
path(see slide16)
●
The histograms show a semi-exponential
trend due to photon attenuation of γ203keV.
●
Data are fitted from the bins which are fully
efficient.
●
The mean free path of the best fit:
○
1.08±0.06cm
The theory predicted mfp:
○
0.928cm
The discrepancy(16%) is due to uncertainty
in electron drift velocity and liquid xenon
density
●
●
LUX
Preliminary
18
N-shell Event Schematics
●
When the size of N-shell χ-ray pulse is too small
(less than S2 threshold), it is then classified as
SE-type pulse; and thus 127Xe EC event that
happens with N-shell χ-ray appears as single
vertex event.
electron drift direction
N-shell x-ray(EC site)
SE
●
Bottom-left schematic shows an N-shell event
with real SEs and χ-ray pulse; and χ-ray pulse
is classified as SE-type pulse due to its small
size
γ-ray
SE
γ-ray
●
Events shown on the right schematic become
background events because the χ-ray pulse
merges into γ-ray pulse; the combined pulse has
size close to the size of γ itself since the size of
γ pulse is significantly greater than that of χ-ray
x-ray(EC site)
SE
N-shell χ-ray(SE)
SE
γ-ray
●
γ-ray signal helps determine what single-vertex
events are needed to be looked into to find Nshell χ-ray signal
19
Energy Histogram of Single-Vertex events
●
●
S2 threshold: 250 phe
Energy reconstruction model:
●
●
●
●
g1 = 0.120±0.002 (phe/ph)
EE = 0.431±0.015
SE = 24.66±0.02 (phe)
g2 = EE*SE
●
●
E = 0.0137 *(S1/g1 + S2/g2)
Esys = ~1.6% at the energy
range shown in the plot
LUX
Preliminary
20
Energy Histogram of Single-Vertex events
●
The second peak is the 127Xe EC events with γ-203keV pulse merged with L-shell χ-ray pulse
○
○
●
The third peak is the 127Xe EC events with γ-203keV pulse merged with K-shell χ-ray pulse
○
○
●
●
data: mean = 215.4keV; sigma = 8.2keV
expected: 203 + 5.2 = 208.2keV predicted by theory
data: mean = 244.2keV; sigma = 8.0keV
expected: 203+33.2 = 236.2keV predicted by theory
The peak means are systematically off from expected values by ~3% which is a factor of 2 greater than the
systematics caused by uncertainty in g1 and g2; the extra systematics is due to the field variation in detector.
Set up a new energy scale relative to the third peak (with most statistics)
○
γ-203keV peak:
■
mean = 212.2 keV; sigma = 8.2keV
●
N-shell χ-rays, which have been classified as SE-type pulse, expects to be found in γ-203keV single-vertex
events
●
The energy cut for signal region (2 sigma):
●
The energy cut for background region:
○
○
[195.8 228.6] keVee
[240 260] keVee
21