Instrumentation for the UCNB experiment

Instrumentation for the UCNB experiment
Sky Sjue
December 13, 2011
Requirements – protons
Maximum energy O(800 eV) so we need:
0.0016
a=0
a=-0.1
0.0014
bias voltage
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minimal noise
– electronics
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minimal noise
– cryogenic
cooling
0.0012
Probability per eV
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0.001
0.0008
0.0006
0.0004
0.0002
0
0
100
200
a=
300
400
Ep [eV]
1 − λ2
1 + 3λ2
500
600
700
800
Requirements – electrons
Maximum energy O(800 keV) so we need:
0.0025
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thick detectors
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position
resolution
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timing
resolution
Probability per keV
0.002
0.0015
0.001
0.0005
0
0
100
200
300
400
500
Ee [keV]
600
700
800
Requirements – coincidence
Coincidence is necessary to
suppress backgrounds and
possible systematic effects like
electron backscattering.
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segmentation, multipixel
instrumentation
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timing resolution
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data acquisition
dσ
dΩ
∝
R
Ze 2
2µv 2 sin2 (θ/2)
2
Large, thick segmented Si detectors
Particle view:
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D=15 cm
p-type implant contact to minimize dead layer
metallized Al mesh (visible) for faster response
Segmented Si detector
Hexagonal array with 128 pixels, area of a pixel is ∼ 0.8 cm2
Single pixel characterization – α
148 Gd
α source, low energy peak comes from energy loss of an α in
the Al mesh.
Leah Broussard
Single pixel characterization – β
Data
2500
∆N
100
0
-100
2000
Data
Counts
1500
Simulation
1000
500
0
0.5
109 Cd
1
1.5
2
2.5
3
Voltage (V)
3.5
4
4.5
5
source, x rays at ∼ 23 keV, Auger electrons at 63 and 84
keV, PENELOPE simulation versus data
Robert Pattie
Single pixel characterization – low energy protons
1
Ep=26 keV
Ep=30 keV
Ep=34 keV
Ep=38 keV
Scaled counts
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90 100
Channel
Proton spectra taken at TUNL with accelerator built by Seth Hoedl
Proton data – simulations and fits
10000
10000
1000
1000
Counts
Counts
Simultaneous fit of of simulations to four proton spectra
100
10
100
10
1
1
0
20
40
60
Channel
80
100
0
20
40
60
Channel
80
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Fit of simulations to Ep = 26 keV and Ep = 34 keV protons
for fixed calibration with a dead layer of 80 nm
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Fit FWHM=2.4 keV
100
Single pixel characterization – dead layer from protons
Channels per keV
2.65
197
185
156
2.6
2.55
2.5
2.45
2.4
74
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76
78
80
82
Dead layer [nm]
84
86
Global best fit with a dead layer of 80 nm yields χ2 = 155.6
for ν = 158 (P=46%).
Result is 75 ≤dead layer≤ 84 nm at 99% confidence level
Specification was∼100 nm, this exceeds requirements
FET on detector for low noise and no pickup
50 Ω
0.1µf
CF 1pf
BF862
RF 1000M Ω
C1
5pf
P$4
P$3
B
A
P$2
P$1
D
C
IN
AGND
AGND
2.5V
C2
50 Ω
TP3
IC1
−5V
R2 499
4
6
3+
C16
0.02 µf
7
+5V
AD8011
2 −
IC1
R1 249
B
A
P$2
P$1
P$4
P$3
D
C
+5VF
AGND
C3
10µf 0.1µf
AGND AGND
Amplifiers designed by Pat McGaughey and Jacqueline Mirabal
(LANL P-25) perform first stage of amplification with FET at
detector to avoid pickup before further amplification and data
acquisition
8-FET card for 8 pixels toward instrumentation of 128
pixels
Card with 8 FETs for 8 pixels to mount directly on detector (in
testing box). Tail pulse from a function generator put through an
attenuator on the left, feedback on the bottom, output on the
right.
We have four of these in hand right now.
Leakage current frozen by cryogenic cooling
100
Leakage current [nA]
10
1
0.1
0.01
0.001
Measurement
3/2
I∝T exp(-0.543eV/kT)
Amplifier requirement
0.0001
140 160 180 200 220 240 260 280 300
T[K]
Leakage current monitored as a function of detector temperature,
effectively zero below T = 180 K.
Spectrum with FET and Si detector at 137 K
100
background
best fit
90
80
Counts per 335 eV
70
60
50
40
30
20
10
0
20
30
40
50
60
70
Eγ [keV]
I 139 Ce
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spectrum, Ex (Kα) ≈ 33 keV,Ex (Kβ) ≈ 38 keV
Fit gives FWHM = 3.1 keV, but
in UCN experimental area with 50 ft BNC cables to ADC, no
shielding, beam ON.
∆t ≈ 24 hr
Detector mount – initial design
Bias apparatus – initial design
Detector is shielded by a shroud, which is also at voltage.
Cooling apparatus – initial design
This design, slightly modified, will work for an initial run (as we
have demonstrated).
High voltage tests
Initial bias tests performed to 5 kV, discharges monitored on a
scope with a 500 pf capacitor.
High voltage tests with flowing LN2
No indication of any problems associated with flowing LN2 at 5
kV. Discharges appeared smaller if anything.
Summary
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Demonstrated ability to measure protons with energy
corresponding to reasonable bias voltage
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Electronics in hand for multipixel instrumentation with noise
and timing capability sufficient for experimental requirements
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Bias tests, R&D in progress
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Next we will work on operating detectors at bias, at bias in a
magnetic field, at high bias in a magnetic field with all the
pixels, measuring electron-proton coincidences . . .