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Microstructured Semiconductor Neutron Detectors (MSNDs)
Douglas S. McGregor, Steven L. Bellinger, Ryan G. Fronk, Luke Henson, Taylor Ochs, J. Kenneth Shultis
Semiconductor Materials and Radiological Technologies Laboratory (SMART) Laboratory Department of Mechanical and Nuclear Engineering
Kansas State University Manhattan, KS 66506
Tim Sobering, Russell Taylor, David Huddleston
Electronics Design Laboratory
Kansas State University Manhattan, KS 66506
Outline
• Coated Semiconductor Neutron Detectors • Principles of Microstructured Semiconductor
Detectors (MSND) • MSND Results • Next Generation Detectors
Two alternative neutron reactions popular for thermal neutron detection are the 10B(n,α)7Li reaction and the 6Li(n,t)4He reaction.
Thin-Film Detectors
σth = 3840 barns
σth = 940 barns
There is presently a need for neutron detectors dependent upon reactions other than 3He(n,p)3H
Neutron Absorption Cross Sections - 1/v
( )( ) cc
n
cc v
KEEE
EgE ≅Γ+−
ΓΓ
=
4/221
2/1
121
γγ πλσ
YbaXvKEH
EE
aa
a
bba ),(reactionfor;)(
2/1
, ≅
=σ
Radiative capture (Breit-Wigner)
Charged particle capture
Thin-Film-Coated Device Concept
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
( )
( )
00 0
( ) 2 14
10.5 1 1 ;
Fp F Fp F F
FF Fp F
F
D D x
D
F xS D I e dxI L
DF e D LL L
ππ
−Σ −
−Σ
= Σ −
= + − − ≤ Σ
∫( )
( )
( ) ( )
00 0
( ) 2 14
10.5 1 1 1 ;
F Fp F F
p F F
F F Fp F
F
D L L D x
D L L
F e xS D I e dxI L
F e e D LL
ππ
−Σ −−Σ −
−Σ − −Σ
= Σ −
= + − − ≥ Σ
∫
Bragg Ionization Distributionsin Boron-10
Ion Penetration Distance (in microns)0 1 2 3 4 5
Ioni
zatio
n (e
V/ A
ngst
rom
)
0
10
20
30
40
50
60
70
80
840 keV 7Li Ion
1.777 MeV α - Particle
1.470 MeV α - Particle
1.015 MeV 7Li Ion
Ion Penetration Distance (in microns)0 4 8 12 16 20 24 28 32
Ioni
zatio
n (e
V/A
ngst
rom
)
0
5
10
15
20
25
30
35
40
45
2.730 MeV 3H Ion
2.050 MeV α - Particle
Thin-Film-Coated Device Concept: Ranges and Energy Deposition
Ion Penetration Distance, 6LiF Film(in microns)
0 4 8 12 16 20 24 28 32
Tran
smitt
ed E
nerg
y (k
eV)
0
500
1000
1500
2000
2500
3000
2.050 MeVα - Particle
2.730 MeV 3H Ion
300
Bragg Ionization Curves in Boron Bragg Ionization Curves in LiF
Residual Energy in Boron
Ion Penetration Distance, 10B Film(in microns)
0 1 2 3 4
Tran
smitt
ed E
nerg
y (k
eV)
0
200
400
600
800
1000
1200
1400
1600
1800
20001.470 MeVα− Particle
1.777 MeV α− Particle
840 keV7Li Ion
1.015 MeV 7Li Ion
Residual Energy in LiF
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
Channel Number0 100 200 300 400 500 600 700 800
Cou
nts p
er C
hann
el
0
250
500
750
1000
1250
1500
1750
20001.1 µm 10B35 µm 6LiF3.0%
efficiency4.6%
efficiency
10B-coated devices require less material for optimum performance. 6LiF-coated devices have improved gamma ray discrimination.
10B or 6LiF Film Thickness (microns)
0 5 10 15 20 25 30 35 40
Perc
ent T
herm
al N
eutro
n D
etec
tion
Effic
ienc
y
0
1
2
3
4
5
Orthogonal FrontIrradiation
Orthogonal BackIrradiation
(LLD = 300 keV)
10B
6LiF
With the LLD set at 300 keV equivalent, maximum efficiencies range from 4% to 4.6% depending on the film and the irradiation direction. Hence, both 10B and 6LiF thin film devices have similar performance.
Thin-Film-Coated Device Concept
Main Design Considerations: 1. Neutron reactive backfill materials 2. Geometric pattern design 3. Substrate material
Basic Microstructure Detector Design
Improvement: Microstructures Semiconductor Neutron Detectors
D.S. McGregor, R.T. Klann, H.K. Gersch, E. Ariesanti, J.D. Sanders, and B. VanDerElzen, Conf. Rec. of the IEEE Nucl. Sci. Symp., San Diego, California, Nov. 4-9, 2001.
3% eff → 3.3% eff
Boron-10
Micro-Structured Semiconductor Neutron Detector
Trench Hole Pillar
We calculate intrinsic efficiency for a normally incident beam of thermal (2200 m-s-1) neutrons.
J.K. Shultis, D.S. McGregor, Nucl. Instrum. and Meth. A 606(2009) pp. 608-636.
Energy (MeV)0.001 0.01 0.1 1
Cro
ss S
ectio
n (b
/ato
m)
10-2
10-1
100
101
102
103
104
105
106
PhotoelectricCompton ScatteringPair ProductionTotal
CS = PE at 58 keV
Silicon Photon Cross Sections
at 465 keV, CEmax = 300 keV
at 686 keV, CEmax = 500 keV
For Si, the cross over for Compton scattering to dominate interactions above photoelectric is at approximately 60 keV. We usually set the lower level discriminator at or above 5 times this value (> 300 keV) to reduce gamma ray background.
Photoelectrons or Compton electrons with energies above 65 keV have transit lengths in Si >40 microns, a dimension larger than the lateral dimensions of the 6LiF filled trench devices!
Gamma-Ray Background
Micro-Structured Devices (Boron-Filled, Cell Dimension = 4 Microns)
LLD Setting (MeV)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Intri
nsic
Effi
cien
cy (p
er c
ent)
0
2
4
6
8
10
12
14
16
18
20
B RodsSi PillarsB Trenches
Cell Dimension = 4 micronsFeature Ratio = 50%Feature Depth = 10 microns
LLD Setting (MeV)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Intri
nsic
Effi
cien
cy (p
er c
ent)
0
5
10
15
20
25
30
35
40
B RodsSi PillarsB Trenches
Cell Dimension = 4 micronsFeature Ratio = 50%Feature Depth = 40 microns
Energy (MeV)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Rel
ativ
e N
umbe
r of C
ount
s
100
101
102
103
104
10B RodsSi Pillars10B Trenches
Energy (MeV)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Rel
ativ
e N
umbe
r of C
ount
s
100
101
102
103
104
10B RodsSi Pillars10B Trenches
10 micron deep features 40 micron deep features
pillar
trench
hole
pillar
trench
hole
J.K. Shultis, D.S. McGregor, Nucl. Instrum. and Meth. A 606(2009) pp. 608-636.
LLD Setting (MeV)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Intri
nsic
Effi
cien
cy (p
erce
nt)
0
5
10
15
20
LiF RodsSi PillarsLiF Trenches
Cell Dimension = 25 micronsFeature Ratio = 50%Feature Depth = 90 microns
LLD Setting (MeV)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Intri
nsic
Effi
cien
cy (p
erce
nt)
0
5
10
15
20
25
30
LiF RodsSi PillarsLiF Trenches
Cell Dimension = 25 micronsFeature Ratio = 50%Feature Depth = 175 microns
Energy (MeV)
0 1 2 3 4 5
Rel
ativ
e N
umbe
r of C
ount
s
100
101
102
103
6LiF RodsSi Pillars6LiF Trenches
Energy (MeV)
0 1 2 3 4 5
Rel
ativ
e N
umbe
r of C
ount
s
100
101
102
103
6LiF RodsSi Pillars6LiF Trenches
90 micron deep features 175 micron deep features
pillar
trench
hole
pillar
trench
hole
Micro-Structured Devices (LiF-Filled, Cell Dimension = 40 Microns)
J.K. Shultis, D.S. McGregor, Nucl. Instrum. and Meth. A 606(2009) pp. 608-636.
6LiF-Filled Trench Design
The design can yield thermal neutron intrinsic detection efficiencies exceeding 30%.
6LiF Trench Device Obverse Irradiation(300 micron deep trench)
Cell Dimension (microns)20 30 40 50 60 70 80 90 100 110
Per
cent
The
rmal
Neu
tron
Det
ectio
n E
ffien
cy
5
10
15
20
25
30
35
40
No cap10 microns20 microns30 microns40 microns50 microns
Trench Width is50% of Cell Dimension
6LiF Trench Device Reverse Irradiation(300 micron deep trench)
Cell Dimension (microns)20 30 40 50 60 70 80 90 100 110
Per
cent
The
rmal
Neu
tron
Det
ectio
n E
ffici
ency
10
15
20
25
30
35
40No cap10 microns20 microns30microns40 microns50 microns
Trench Width is50% of Cell Dimension
J.K. Shultis and D.S. McGregor, IEEE Trans. Nucl.Sci., NS-53 (2006) pp. 1659-1665.
Sidewall Width 10 um 12 um 14 um 16 um 18 um 20 umTrench Width 30 um 28 um 26 um 24 um 22 um 20 um
Total Eff. 36.33% 35.29% 34.05% 32.61% 30.98% 29.19%0.3 MeV LLD 34.04% 33.27% 32.27% 31.09% 29.66% 28.07%0.5 MeV LLD 32.29% 31.94% 31.13% 30.12% 28.82% 27.36%
14
Microstructured Semiconductor Neutron Detectors 6LiF backfilled
Efficiency Calculations
Calculated efficiencies for 6LiF-filled trench detectors as a function of cell fraction and perforation depth. For common etched features with a 40 micron cell width, over 30% efficiency can be reached with devices >350 microns deep. Unequal T/W ratios of 0.7 allows for over 35% efficiency.
J.K. Shultis, D.S. McGregor, Nucl. Instrum. and Meth. A 606(2009) pp. 608-636.
LLD = 300 keV
Efficiency Calculations
Calculated efficiencies for 6LiF-filled trench detectors as a function of cell fraction and perforation depth. For common etched features with a 40 micron cell width, over 30% efficiency can be reached with devices >350 microns deep. Unequal T/W ratios of 0.7 allows for over 35% efficiency. For aggressive features (14 micron trenches, 6 micron semiconductor fins) – > 35% efficiency is reached for 350 micron deep features and 47% efficiency is reached for 350 micron deep features. Sandwiched detectors with 10 micron fins and trenches can exceed 70% efficiency!
J.K. Shultis, D.S. McGregor, Nucl. Instrum. and Meth. A 606(2009) pp. 608-636.
LLD = 300 keV
Fabrication is performed with common VLSI processing methods and equipment.
MSND Fabrication
• Benefits – Better Uniformity Across Large Wafers
– This Leads to Uniform Responses From Each Device in an Array!
– Batch Wafer Processing (No Limit!) – Less Mechanical Damage than ICP RIE
• 3 Different perforation designs
– Straight Trench – Chevron Trench – Rhombus Hole/Pillar
Anisotropic Chemical (KOH) Wet Etching of (110) Si
Wet Etching of (110) Si
Wet Etching of (110) Si
• 3 Different perforation designs – Straight Trench – Chevron Trench – Rhombus Hole/Pillar
• Benefits – Batch Wafer Processing – Less Mechanical Damage than ICP RIE – Easier Fabrication of Advanced
Designs
KOH Etched
Conformal Diode Fabrication Process
• Diffusion In Holes – Covers sensitive surfaces
• Consumes Damage and Contamination – Easier to fabricate
100 μm Conformal Diode Leakage Current Density: 0.1μA / cm2
Dopant Introduction
• PIN Diodes are fabrication by the controlled introduction of n-type and p-type dopants
• Isolation is achieved by the growth of thick field oxides
KOH Etched
Cavity Backfilling of Microstructured Devices
LiF nano-powder Production and Backfilling The LiF granules form to nano-sided tiny cubic crystals through a heat treatment process, and can pack firmly into the microstructured cavities.
6LiF nanomaterial
Cavity Backfilling of Microstructured Devices
The LiF nanopowder is suspended in an solvent with an ultrasonic vibrator.
Cavity Backfilling of Microstructured Devices
The LiF nanopowder is compacted into the microstructures with a centrifuge.
43.4 µm
492 µm deep
34.5 µm trench
25.7 µm fin
Diode Characterization •Diodes are tested for leakage current and capacitance prior to mounting.
– < 10 nA cm-2 at an operational bias of 0 to -3V. – < 150 pF at an operational bias of 0 to -3V.
MSND Characterization
26
MSND Characterization
Detector
Neutron Testing •Diffracted thermal neutron beam at KSU TRIGA Mark II Nuclear Reactor
• Reactor Power – 0 to 500 kW • Thermal (0.0253eV) Neutron Flux: 1.72 x 102 {n cm-2 s-1 kW-1} • Calibrated against 3He-Gas Detector
Gamma-ray Sensitivity •137Cs source
• γ-ray Energy: 662 keV • 1 meter from DSMSND • Assay: 68.27 mCi • Exposure: 21.8 mR hr
-1
• 0.08 γ-ray µs-1 (per 4-cm2 area)
Kansas State University Efficiency Measurement Standard Method
Kansas State University Efficiency Measurement Standard Method
29
MSND Characterization
Neutron Efficiency of 4-cm2 MSND Detector • 4 cm2 MSND, 440-µm deep trenches, 10-µs charge integration time.
• 30.1 ± 0.5% at a 650 keV LLD with normal beam incidence.
• 37.6 ± 0.7% at a 650 keV LLD with 45 deg. beam incidence.
neutron converter material
semiconductorvolume
uniform parallel neutron beam
30
MSND Incident Angle
neutron converter material
semiconductorvolume
uniform parallel neutron beam
MSND Angular Efficiency Comparisons
Stacked Perforation Designs
Pulse height spectrum taken with a 6LiF-filled microstructured semiconductor neutron detector formed from two devices. The microstructures were 250 microns deep. Intrinsic thermal neutron detection efficiency was measured to be >42%.
Channel Number0 20 40 60 80 100 120 140
Rec
orde
d C
ount
s
102
103
104
105
106
Cd Shutter OpenCd Shutter Closed
4 cm2 Stacked Straight Trench Microstructure Design (200 micron deep Trenches)
200 um Deep Microstructure Stacked 4 cm2 Dual MSND Device
1
10
100
1000
10000
100000
0 25 50 75 100 125 150 175 200
Exp
erim
enta
l Cou
nts
Channel
New Design : 10 µs
New Design Background
Cs Response137
Stacked Perforation Design: 10 µs preAmp integration time
Pulse height spectrum taken with a 6LiF-filled microstructured semiconductor neutron detector formed from stacked 1cm2 devices. The microstructures were 250 microns deep. Intrinsic thermal neutron detection efficiency was measured for the new longer integration time and found to be 42.0 ± 0.25% at a 300 keV LLD.
34
DSMSND Future Work
Opposing PIN Design - Higher efficiency - Reduced neutron streaming
Adjacent PIN Design - Faster response
Opposing/Adjacent PIN Design - Higher efficiency - Reduced neutron streaming - Faster response
Dual-Sided Etched MSND Devices
• Diode fabrication – Devices are fabricated exactly as single-sided
devices. – Capable of batch processing. – Devices of similar dimensions have similar
theoretical maximum detection efficiencies – IHD Design is capable of +72% intrinsic
detection efficiency.
Opposing DSMSND Design
Interdigitated DSMSND Design
D.S. McGregor and R.T. Klann, patent US-6545281; allowed April 8, 2003. D.S. McGregor, R.T. Klann, patent US-7164138; allowed January 16, 2007.
DSMSNDs
• Ultra-high efficiency applications.
• Similar to Dual-Stacked MSND devices.
• Ultra-fast response applications.
• High charge-collection efficiency
LLD = 300 keVTr./Unit Cell 20 um 40 60 80 100
0.9 36.59% 35.54% 32.60% 26.54% 22.60%0.8 64.82% 52.28% 39.33% 32.23% 27.65%0.7 70.54% 57.05% 44.36% 36.20% 30.52%0.6 72.14% 60.03% 48.16% 38.37% 31.23%0.5 72.81% 61.25% 49.90% 38.96% 31.58%0.4 72.61% 60.65% 48.89% 39.13% 31.92%0.3 71.48% 58.29% 45.83% 37.66% 31.92%0.2 66.39% 54.09% 41.42% 34.36% 29.80%0.1 39.17% 38.08% 35.22% 29.25% 25.38%
Unit-Cell Width500-um-deep trenches, backfilled with LiF
36
DSMSND vs. MSND
DSMSND Efficiency Comparisons •Optimization of theoretical efficiencies occurs at 475-µm deep, 13/7-µm wide trenches with 20-µm pitch.
37
DSMSND Angular Efficiency Comparisons
DSMSND vs. MSND
MSNDs are now commercially available through Radiation Detection Technologies, Inc. (RDT)
Standard 2 cm x 2 cm 30% efficient devices and High-density arrayed devices are available, as well as custom detector configurations.
137F Ward Hall Mechanical and Nuclear Engineering Department
Kansas State University Manhattan, KS 66531
mcgregor@ksu.edu
http://www.mne.ksu.edu/research/centers/SMARTlab
Kansas State University SMART Laboratory
The presented research was funded in part by DTRA contract HDTRA1-12-C-0004
Additional Slides for Discussions
Reaction product ranges and residual energy in pure boron and LiF
Ion Penetration Distance, 10B Film(in microns)
0 1 2 3 4
Tran
smitt
ed E
nerg
y (k
eV)
0
200
400
600
800
1000
1200
1400
1600
1800
20001.470 MeVα− Particle
1.777 MeV α− Particle
840 keV7Li Ion
1.015 MeV 7Li Ion
Ion Penetration Distance, 6LiF Film(in microns)
0 4 8 12 16 20 24 28 32
Tran
smitt
ed E
nerg
y (k
eV)
0
500
1000
1500
2000
2500
3000
2.050 MeVα - Particle
2.730 MeV 3H Ion
300
1. 10B has higher macroscopic cross section, hence features need only be 60 microns deep to achieve 3 mean free path lengths.
2. Etch features must be on the order of 2-3 microns or less to reduce energy self absorption.
3. Energies are lower; LLD can not be set very high without losing significant counts.
4. Both alpha and Li ion contribute to electrical signal (3:1 ratio), causing more “wall effect” problems.
1. 6LiF has lower macroscopic cross section, hence features must be >350 microns deep to achieve 2 mean free path lengths.
2. Etch features can be larger, on the order of must be on the order of 28 – 32 microns or less without energy self absorption problems.
3. Energies are higher; LLD can be set high without losing significant counts.
4. Triton contributes much more to signal than alpha particle (7:1) – less wall effect issues.
“Sandwich” Designs
Double Outward Design
Requires that the applied voltage extend the depletion region entirely across the detector!
Double Inward Design
Difficult to manufacture!
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
“Sandwich” Designs
10B FilmThickness DF for Each Device (microns)
0 1 2 3 4 5 6
Perc
ent T
herm
al N
eutr
on
Det
ectio
n Ef
ficie
ncy
0
2
4
6
8
10
Double Inward DevicesDouble Outward Devices
(LLD = 300 keV)
6LiF Film Thickness DFfor Each Device (microns)
0 5 10 15 20 25 30 35 40
Perc
ent T
herm
al N
eutr
on
Det
ectio
n E
ffici
ency
0123456789
10
Double Inward DevicesDouble Outward Devices
(LLD = 300 keV)
6Li Film Thickness DFfor Each Device (microns)
0 20 40 60 80 100 120 140 160
Perc
ent T
herm
al N
eutr
on
Det
ectio
n Ef
ficie
ncy
0
5
10
15
20
25
30
Double Inward DevicesDouble Outward Devices
(LLD = 300 keV)
Double-inward design has highest efficiency.
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
Channel Number0 100 200 300 400 500 600 700 800
Cou
nts p
er C
hann
el
0
1000
2000
3000
4000
5000 1.1 µm 10B/ 60 µm 6Li1.1 µm 10B/ 30 µm 6LiF1.1 µm 10B35 µm 6LiF
6.3% efficiency
11.6% efficiency
3.0% efficiency
4.6% efficiency
Channel Number0 100 200 300 400 500 600 700 800 900 1000
Cou
nts p
er C
hann
el
0
500
1000
1500
2000
2500
3000
3500
4000 4 µm 10B with via holesSandwich - 4 µm 10B with via holes 30 µm 6LiF
3.9% efficiency
13.0% efficiency
Layered and Stacked Detectors
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
Au contact
Encapsulate
10B coating
6LiF Film Thickness (microns)
0 5 10 15 20 25 30 35
Ther
mal
Neu
tron
Det
ectio
n Ef
ficie
ncy
(per
cent
)0
5
10
15
20
25
30
3515 detectors
10 detectors
5 detectors
2 detectors
1 detector
Front Irradiation300 keV LLD
10B-coated and stacked devices.
10B Film Thickness (microns)
0 1 2 3 4 5
Ther
mal
Neu
tron
Det
ectio
n Ef
ficie
ncy
(per
cent
)
0
5
10
15
20
25
30
3515 detectors
10 detectors
5 detectors
2 detectors
1 detector
Front Irradiation300 keV LLD
6LiF-coated and stacked devices.
Why not just stack a bunch of detectors together? -“Stacked” Designs -
D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H. Yang, and R.T. Klann, Nuclear Instrum. and Meth., A500 (2003) p. 272.
Micro-Structured Devices
Cell Length and Width
Cavity Diameter
Cavu
tyD
epth
Cap Depth
Semiconductor
Neutron Reactive Material
1. Ion ranges and energy deposition determined by TRIM (from SRIM 2003). 2. Empirical curve fitting performed by TableCurve (Jandel 1998). 3. Thermal neutron beam is perpendicular to device surface 4. Monte Carlo approach used to simulate randomized neutron absorption
and reaction ion trajectories. 5. We assume 70% packing of material in the structures. 6. After millions of histories, efficiency is determined
by the number of events depositing energy above the lower level discriminator setting.
6LiF Rod Device Reverse Irradiation(300 micron deep holes)
Cell Dimension (microns)20 30 40 50 60 70 80 90 100 110
Per
cent
The
rmal
Neu
tron
Det
ectio
nEffi
cien
cy
10
12
14
16
18
20
22
24
26No Cap10 microns20 microns30 microns40 microns
Hole Diameter is60% of Cell Dimension
6LiF Rod Device Obverse Irradiation(300 micron deep holes)
Cell Dimension (microns)20 30 40 50 60 70 80 90 100 110
Per
cent
The
rmal
Neu
tron
Det
ectio
n E
ffici
ency
10
12
14
16
18
20
22
24
26No Cap10 microns20 microns30 microns40 microns
Hole Diameter is60% of Cell Dimension
J.K. Shultis and D.S. McGregor, IEEE Trans. Nucl.Sci., NS-53 (2006) pp. 1659-1665.
Effective intrinsic efficiency: the intrinsic efficiency at various irradiation angles normalized to the common irradiation direction and detector cross sectional area – or normalized to the highest interaction rate.
( )coscos
( ) 0.5 1 1 expc s ;oF F
Fp F p F
F
D DS D F D LL Lθ
θθ
− Σ ≈ + − − ≤ Σ
( ) cosco
( ) 0.5 exp 1 1 exp 1 ;s cos
cos F F Fp F p F
F
D L LS D F D LLθ
θ θθ
−Σ − −Σ ≈ + − − ≥ Σ
cosY θY
Y W>>Typically
W
MSND Characterization
Class 100 Cleanroom
Thin-Film Detectors
What are the choices? 1. The 10B(n,α)7Li reaction – inexpensive, good s, short ranges Q = 2.34 MeV (94%) – 1.47 MeV a, 840 MeV Li ion Q = 2.78 MeV (6%) – 1.78 MeV a, 1.02 MeV Li ion sth = 3840 barns 2. The 6Li(n,t)4He reaction – inexpensive, lower s, longer ranges Q = 4.78 MeV (100%) – 2.05 MeV α, 2.7 MeV 3H ion sth = 940 barns 3. The 157Gd(n,γ)158Gd reaction – expensive, high s, short ranges Energetic conversion electrons, emits only low energies between 70 keV- 220 keV (low particle yield) sth = 250,000 barns
Summary
1. Models should take into account accepted and realistic neutron detector calibration methods. - A model’s usefulness is diminished if the results can not be tested! 2. Efficiency tests should be performed with accepted characterization procedures – best performed following the original definition of cross section when possible. 3. A calibrated standard neutron detector should be used as a witness detector for efficiency calibrations. 4. Detectors with 1/v cross sections can be easily calibrated if the neutron intensity is constrained within the 1/v energy region. 5. Detectors using non-1/v reactions must be corrected for neutron energy. It is best to use a monoenergetic beam to reduce uncertainty.
Boron Filled Micro-Structured Devices
2 microns
Native oxide 50 nm
Thin pn contact diffusion > 150 nm
Dead region is 200 nm thick and consumes 36% of the volume. Probability of absorbing reaction products severely decreased.
“pillar”
“fin” Dead region is 200 nm thick and consumes 20% of the volume. Probability of absorbing reaction products decreased.
2 microns
“hole”
Dead region is 200 nm thick and consumes 10.75% of the volume. Minimal decrease in probability of absorbing reaction products.
6LiF Filled Micro-Structured Devices
Native oxide 50 nm
Thin pn contact diffusion > 150 nm
Dead region is 200 nm thick and consumes 4% of the volume. Very little decrease in volume.
“fin”
10 microns
“hole”
Dead region is 200 nm thick and consumes 2% of the volume. Very little decrease in volume.
10 microns
( ) ( ) ( )r rdF n N v v d dσ= v V v V
Why does this work?
Consider the laboratory system, where neutrons approach the nuclei with velocity
( ) rdI n v d= v v r rv = v
r = −v v V
To the capture nuclei, the neutrons compose a differential beam of intensity
which is equal to the relative velocity between the neutrons and nuclei.
where
and interact with the nuclei at a rate of
in interactions per cm3 s-1. The total interaction rate is
( ) ( ) ( )r rF n N v v d dσ= ∫∫ v V v V
Kansas State University Standard Method Calibration
McGregor et al., Nucl. Instrum. Meth., A608 (2009) 125.
Lamarsh, Nuclear Reactor Theory (Addison-Wesley, Reading, 1966)
Why does this work?
For materials with a 1/v cross section, such as 10B, 6Li, and 3He,
00( ) ( ) r
a r a rr
vv vv
σ σ=
0 0( )a aF E nv= ∑
Where is an arbitrary relative speed and is the corresponding cross section. Therefore,
0rv 0( )a rvσ
00
0 0
( ) ( ) ( )
( ) ( ) ( )
( )
a r r
ra r r
r
a r r
F n N v v d d
vn N v v d dv
N v nv
σ
σ
σ
=
=
=
∫∫∫∫
v V v V
v V v V
where v0 is an arbitrary lab speed and E0 is the corresponding energy.
McGregor et al., Nucl. Instrum. Meth., A608 (2009) 125.
Kansas State University Standard Method Calibration
Why does this work?
0 0( )a aF E nv= ∑
What this means is that the interaction rate for 1/v materials is independent of neutron energy, provided that the neutrons are in the 1/v range. If the neutrons are not exactly 2200 m s-1 neutrons, the detector interaction rate, and measured efficiency, are the same as if they were!
What are the weaknesses?
1. If non- 1/v materials are also attenuating the beam, the measurement may have error.
Using detector containers with low neutron absorption reduces the effect of non-1/v
absorbers (the steel 3He tube has thin walls).
2. If there is neutron contamination from epithermal or fast neutrons, the measurement may have error.
The diffracted beam significantly reduces neutrons in the non-1/v region.
Kansas State University Standard Method Calibration
Large MSND Detectors
Am-241 100 sec, Probe at Corner, Source at Opposite Corner
0
1000
2000
3000
4000
5000
6000
0 20 40 60 80 100 120 140 160 180 200
Channel
Cou
nts
0 Volts2 Volts4 Volts6 Volts8 Volts10 Volts12 Volts
Pulse tests from an 241Am alpha particle source.
Microstructured Semiconductor Neutron Detectors (MSNDs) •Mass-producible
– Dozen per 4” wafer, dozens of wafers per batch. •Same Adaptability
– Compact, rugged low-voltage operation, inexpensive. •Greater Neutron Absorption
– A single 500μm MSND absorbs >52% of incident flux. •Greater Neutron Efficiency (>45%)
Microstructured Semiconductor Neutron Detectors
Increased energy deposition from reaction products. Increased neutron absorption.
• Stacked 250 micron deep trench MSND devices • Straight Trench IHD Conformal Detector
• Neutron Detection Eff. = 36% • γ-Rejection Ratio > 3.0 x 106 {n/γ}; LLD = 22 ≈ 450 keV
Neutron/Gamma-Ray Rejection
Intrinsic efficiency: for a parallel beam, the intrinsic efficiency is the ratio of counts registered to radiation quanta passing through the detector.
( )( ) IAC
AreaDetectorCurrentNeutronRateCount
i ==ε
MSND Characterization
Effective intrinsic efficiency: the intrinsic efficiency at various irradiation angles normalized to the common irradiation direction and detector cross sectional area.
Typical dimensions (face perpendicular to neutrons): 10B films: 0.5 – 2.5 microns thick 6LiF films: 2 – 20 microns thick Substrate: 300 – 350 microns thick Area: 0.25 – 4 cm2
MSND Characterization
Kansas State University Standard Method Calibration
2exp exps s He Het toutd
in
ITI
− Σ −Σ= =
22 exp s sts
sin
ITI
− Σ= =
exp He Hetoutg
s
ITI
−Σ= =
1/2
1 exp s st ss
in
ITI
−Σ = =
( )1/2
exp 1 exp 1s s He Het t s outHe
in s
I II I
ε −Σ −Σ = − = −
McGregor et al., Nucl. Instrum. Meth., A608 (2009) 125.
=
HeHe cts
ctsdetdet εε
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