Perforated Semiconductor Neutron Detector Modules for Detection of

Perforated Semiconductor Neutron Detector Modules for
Detection of Spontaneous Fission Neutrons
Douglas S. McGregor, Steven L. Bellinger, David Bruno, Shawn Cowley,
Melanie Elazegui, Walter J. McNeil, Eric Patterson, Troy Unruh
SMART Laboratories, Mechanical and Nuclear Engineering, Kansas State University,
Manhattan, KS 66506
Ph: 785-532-5284, Email: mcgregor@ksu.edu
Clell J. Solomon, J. Kenneth Shultis
Mechanical and Nuclear Engineering, Kansas State University, Manhattan, KS 66506
Blake B. Rice
Nuclear Regulatory Commission, Rockville, MD 20852
Abstract— Compact neutron detectors are being
designed and tested for use as low-power real-time
personnel dosimeters and for remote neutron sensing. The
neutron detectors are pin diodes that are mass produced
from high-purity Si wafers. Each detector has thousands of
circular perforations etched vertically into the device. The
perforations are backfilled with 6 LiF to make the pin diodes
sensitive to thermal neutrons. The prototype devices
delivered over 3.8% thermal neutron detection efficiency
while operating on only 15 volts. The highest efficiency
devices thus far have delivered over 12% thermal neutron
detection efficiency. Devices moderated with high density
polyethylene (HDPE) can be used for fast neutron detection.
Compact packages with or without remote readout
electronics are under construction and characterization.
C
I. INTRODUCTION
OATED semiconductor diodes have been studied as
neutron detectors for many decades [1]. The basic
configuration consists of a common Schottky barrier or pn
junction diode, upon which a neutron reactive coating, such
as 10 B or 6 LiF, has been applied. Such devices are compact,
easily produced, and have low power requirements.
However, they are restricted to low thermal neutron
detection efficiencies, typically no greater than 4.5%
intrinsic efficiency, due to reaction product energy selfabsorption
in the neutron sensitive coating [2].
In the present work, the coating is based on the 6 Li(n,t) 4 He
reaction. When thermal neutrons are absorbed in 6 Li, a 2.73
MeV triton and a 2.05 MeV alpha particle are ejected in
opposite directions. The reaction products from the
6 Li(n,t) 4 He reaction are more energetic than those of the
10 B(n,α) 7 Li or 157 Gd(n,γ) 158 Gd reactions and, hence, are
much easier to detect and discriminate from background
radiations.
6 Li has a relatively large microscopic thermal
neutron absorption cross section of 940 b, although it is less
than 157 Gd and 10 B. The absorption cross section for 6 Li
follows a 1/v dependence [3,4]. Although devices have been
fabricated with enriched 6 Li metal as the converter film [2],
pure Li is highly reactive and difficult to prevent from
decomposing, even when using encapsulates. It is the stable
compound LiF that is used more often. The mass density of
6 LiF is 2.54 g cm -3 , and the resulting macroscopic thermal
neutron absorption cross section is 57.5 cm -1 .
The advantages of coated diodes as neutron detectors
include compact size, a low power requirement, low cost
VLSI mass production methodology, and ruggedness. Yet,
since basic planar thin-film coated diode detectors can only
achieve practical maximum thermal neutron detection
efficiencies of approximately 4.5% [2], they have
experienced only modest utilization as neutron radiation
detectors.
Recent advances with inductively-coupled-plasma reactiveion-etching
(ICP-RIE) using high-aspect ratio deep etching
(HARDE) techniques have allowed for unique perforated
neutron detector structures to be realized [6]. In such a
configuration, the diode is permeated with perforations
which are backfilled with neutron reactive materials, such as
10 B and 6 LiF [6]. As a result, the intrinsic thermal neutron
detection efficiencies of perforated diodes can be increased
above 30%, more than 6 times that of a common planar thin
film coated diode [7]. It was shown in previous work that
perforated devices incorporating 6 LiF as the converter can
achieve higher intrinsic thermal neutron detection
efficiencies than devices with 10 B as the converter [6].

Figure 1 - Perforations are etched into the Si substrate with
ICP-RIE. Shown are holes etched into Si, each 30 microns
in diameter and 20 microns deep on a center-to-center pitch
of 60 microns. A protective oxide has been grown in and
around the perimeter of the holes.
Figure 2 - 30 µm diameter and 100 µm deep perforations in
Si that have been completely backfilled with 6 LiF by using
the low pressure condensation method.
Figure 3 - The fabrication process developed to produce
the perforated detectors was designed for mass
production.
(Fig. 1). The hole pitch is 60 microns center-to-center. The
active area of the detector is 25 mm 2 with the device
diameter being 5.6 mm and die dimensions being 7 mm x 7
mm. The perforations in the first generation devices were
backfilled by low pressure condensation deposition [8]. The
wafer and 6 LiF powder are placed inside a vacuum furnace.
The perforated substrate is mounted to a cold plate inside
the vacuum furnace. The 6 LiF powder is evaporated and
transported with a carrier gas to the perforated substrate
where it condenses inside the miniature perforations (Fig. 2).
Presently, over 60 detectors can be produced per 3 inch
diameter wafer, as shown in Fig. 3. The details of the
detector processing steps can be found elsewhere [6,8].
Basic Module Design
First-generation perforated detectors were coupled to a
simple counting system with a low cost, low power, and a
small transimpedance and gain amplifier combination. The
amplifiers and a detection diode are located in a sealed 4 pin
Hence, of the three main materials commonly used for thinfilm
coated thermal neutron detectors, the following work
concentrates only on devices constructed with 6 LiF as the
converter material.
Detector Dimensions
II. DETECTOR DESIGN
The perforated detectors were produced as Si pin diodes.
The diode fabrication processes were carried out on 3-inch
diameter float zone, double-side polished, >5 kΩ-cm, n-type
Si wafers approximately 325 µm thick. The devices have 30
micron diameter holes that are typically between 20 to 100
microns deep. The devices have an insulating coating inside
the holes to reduce problems of excessive leakage current
Figure 4 - Shown is a 6 LiF filled and coated perforated
detector, with a total active diameter of 5.6 mm, mounted
alongside the preamplifier/amplifier circuit.

pin crystal carrier for noise isolation.
Figure 5 - The perforated detectors operate on minimal
voltage. Shown are the internal parts to the module, with
the detector, preamplifier/amplifier, power supply and
digital display. The entire package is powered by 3 AAA
batteries.
(14 pin footprint) package widely used in the manufacture of
crystal oscillators (Fig. 4). The wire bonds between the
diode and the transimpedance amplifier are also potted in an
electronics grade epoxy to ensure stability. Once packaged,
the detector platform is well shielded from external light,
electrical noise, and humidity. The detector module (Fig. 5)
supplies power for the amplifier and perforated detector bias
(nominally around 15 volts), and returns a positive polarity
pulse output from neutron interactions between 70 and 130
mV with a pulse width of approximately 4 µs (depending on
the individual diode). The module requires an operating
voltage between 2.7 to 5 volts at 150 µA. The module bias
supply uses a charge pump circuit with an external
adjustment potentiometer to supply between 10 and 20 volts
of positive detector bias. It is based on a LT1615-1 DC/DC
step up converter. The bias supply is also contained in a 4
Pulse Width (µs)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
90
0 5 10 15 20
Bias Voltage (volts)
Pulse Width
Pulse Height
140
130
120
110
100
Pulse Height (mV)
Figure 6 - Detector output pulses as a function of applied
bias. The pulse height reaches a maximum at 10 volts
reverse bias, whereas the pulse width continues to decrease
beyond 18 volts reverse bias.
Pulses exiting the amplifier circuit are converted into a
logic-level pulse for the counting circuits. An adjustable
voltage level is set with a potentiometer and connected to
the negative input of a low power comparator. The pulse is
connected to the non-inverting input of the comparator with
an r/c filter to remove a small dc offset introduced by the
amplification stage. This allows the device to “discriminate”
pulse heights, as well as filter out spurious noise. The
prototype arrangement operates on 3 AAA cell batteries (4.5
volts), and uses a low power commercial counter to display
neutron induced counts. The unit is constructed using
surface mount components and techniques, and is housed in
an aluminum box. Module testing was initially performed
with an 241 Am alpha-particle source. The aluminum housing
has a hole drilled above the perforated detector covered by
an aluminized mylar window. The hole allows for the 5.5
MeV alpha particles to enter the detector and produce
pulses.
The device modules were tested with an 241 Am alpha
particle source for pulse width and pulse height as a function
of bias voltage. The pulse widths, on average, leveled off at
approximately 3.7 µs beyond 15 volts bias. The pulse height
leveled off in magnitude at approximately 130 mV for
voltages exceeding 10 volts bias (Fig. 6). Since the depletion
region need extend only beyond the holes to produce full
charge collection, the voltage needed for optimum operation
is actually less than needed for full depletion, observed to be
35 volts from CV measurements.
III. NEUTRON MEASUREMENTS
The devices were tested in a tangential thermal beamport at
the Kansas State University TRIGA Mark II Nuclear
Reactor Facility. Each detector was tested in the same
electronics box under identical conditions. The electronics
box and detector were aligned in the beam port using an
indexing holder. A thin film coated calibration detector,
which had a coating of only 1400 angstroms of 95%
enriched 6 LiF, was used to determine the average thermal
flux at a reactor power of 225 kW, which yielded an average
calibration flux unit of 1.1 n cm -2 s -1 W -1 ± 0.75%. Hence, at
a reactor power of 200 W, the average slow flux at the
indexed detector test location was 220 ± 1.65 n cm -2 s -1 as
compared to the calibration detector.
Figure 7 shows a calculated efficiency for a perforated diode
with 30 microns diameter 20 micron deep holes backfilled
with 6 LiF. The spectral features and predicted efficiency
were determined using the model described in the literature
[7]. Distinct features include the 2.73 MeV single particle
limit (from the triton) and the dual particle continuum,

Relative Counts Tallied
10 5 2.73 MeV limit
10 4
10 3
10 2
10 1
2.05 MeV limit
Monte Carlo simulation
Hole diameters = 30 microns
Hole depths = 20 microns
Hole pitch = 60 microns
Cap thickness = 10 microns
10 7 histories
Maximum efficiency = 3.59%
dual particle
continuum
0 1 2 3 4 5
Energy Deposited (MeV)
Recorded Counts
10 5 2.73 MeV limit
10 4
10 3
10 2
10 1
lower level
discriminator
limit
0 1 2 3 4 5
Scaled Energy (MeV)
dual particle
continuum
Hole diameters = 30 microns
Hole depths = 20 microns
Hole pitch = 60 microns
Cap thickness = 10 microns
Efficiency = 3.8%
pulse
pileup
Figure 7 - Calculated efficiency and spectral features for
a perforated detector with 30 µm diameter 20 µm deep
holes filled with 6 LiF.
which will appear only if the perforated device is
functioning properly. By comparison, Fig. 8 shows a
spectrum for a perforated device with 30 micron diameter
and 20 micron deep holes backfilled with 6 LiF. The
measured thermal neutron detection efficiency was 3.8%,
which compares well to the modeled result. Further, the 2.73
Figure 8 - Measured efficiency and spectral features for a
perforated detector with 30 µm diameter 20 µm deep
holes filled with 6 LiF.
MeV single particle limit and the dual particle continuum
are clearly present. Devices with holes 100 microns deep
have yielded efficiencies above 12.5%.
It is anticipated that the neutron detectors can be used for
fast neutron detection for remote sensing. In such a case, the
modules can be packaged in high density polyethylene
(HDPE) to scatter fast neutrons down to slow energies
where the perforated detectors have higher detection
efficiency. A device of this sort can be stationed in locations
capable of accumulating counts from a fast neutron source
over a long period of time.
Figure 9 shows a prototype perforated neutron detector
platform inserted between two HDPE disks, each being 2
inches in diameter and two inches thick. The entire device is
approximately the same size of a common 12 oz beverage
can. The preamplifier, amplifier and charge pump are
installed in the base of the HDPE moderator. Figure 10
shows the response to fast neutrons from a 252 Cf source at
distances ranging from 0.5 m to 2 m. The detector in the
package was a low thermal neutron detection efficiency
(2%) perforated diode. There is a noticeable difference in
the count rates as a function of HDPE cylinder orientation
for detector to source distances less than 2 m. Higher count
rates were observed with the HDPE cylinder oriented
perpendicular to the neutron source as opposed to parallel.
IV. REMOTE CAPABILITY
Figure 9 - Perforated detectors with miniature
preamplifiers packaged in high density polyethylene
(HDPE).
The detector packages are being outfitted with commercial
radio transmitter technology for remote applications. The
wireless detector network is constructed of two major
pieces. The first and most important piece is the detector
board. The second piece is the wireless communications
module.

8000
7000
Perpendicular
Parallel
Counts per Hour
6000
5000
4000
3000
2000
1000
0
60 80 100 120 140 160 180 200
Distance from Source(cm)
Figure 10 - Count rate of a 2% efficient perforated
semiconductor neutron detector packaged in high density
polyethylene (HDPE) as a function of distance from a
252 Cf source.
The remote detector board combines three basic components
needed to detect a neutron, as with the basic module, those
being signal generation, bias supply, and digital converter.
The same detector platform as shown in Fig. 4 is used. The
bias supply is a simple adjustable capacitor charge pump. It
is also contained within a crystal oscillator package, but it
requires an external inductor to reduce power supply noise.
After a neutron is detected, the pulse is passed through a
high pass filter to remove any dc offsets. Then the pulse is
converted to a digital signal using a simple comparator
circuit. The resulting digital pulse is connected to an
interrupt line on the wireless module via a 51 pin connector
on the underside of the detector board.
The wireless module chosen for this prototype is
manufactured by Crossbow, named a MICAz. This module
conforms to the new IEEE 802.15.4 ZigBee standard for low
power wireless sensor networks, and operates in the 2.4
GHz band. Some attractive features of ZigBee networks are
low power requirements, large network sizes, and small
processing and memory requirements. A full discussion of
the ZigBee protocol can be found elsewhere [9].
One of the more attractive features of the ZigBee protocol is
the mesh network. Any module or “mote” can act as a router
or base station. As long as any individual mote is within
range of another mote, messages can be chained back to the
base station. According to the datasheet for the MICAz
series of motes [10], the outdoor range is 75 – 100 m, and
the indoor range is from 20 – 30 m, assuming a ½ wave
dipole antenna and line of sight (for outdoor range). The
lower frequency (900, 430 and 315MHz) MICA2 modules
extend outdoor range to around 300 m. The combined
detector board and remote transmitter is shown in Fig. 11.
Figure 11 – Detector board, with the detector package
and charge pump, mounted atop the remote transmitter
module and battery package.
Another attraction of the MICAz mote is the built in
Atmega128 processor. The processor allows pulse counting
from each detector, in which the counts are transmitted to a
base computer via the mesh network. The Atmega128
processor also has a built in A/D converter for future
applications. The simple software for the detector system
utilizes an interrupt vector for the external interrupt pin to a
subroutine which increments a counter. The mote sends the
accumulated count back to the base station in regular
intervals. If a large number of events are counted before the
timed communication interval, the mote will report back to
the base.
V. CONCLUSIONS
Si-based perforated diode detectors have been fabricated
and demonstrated in compact low-power packages. The
shallow hole device yielded 3.8% thermal neutron detection
efficiency. Future generation devices will be constructed
from diodes with much deeper perforations and a tighter
perforation matrix. Both changes will increase the intrinsic
thermal neutron detection efficiency, with calculations and
models indicating that efficiencies above 30% can be
reached [7].
We plan on extending our existing platform to include three
detectors – one fast neutron, one thermal neutron, and a
gamma ray detector (most likely CZT). The two neutron
detectors are simple to interface to the processor since they
only produce pulses. The gamma ray detector also provides
pulses, but the pulse height information will need to be
stored as well. We can accomplish this by using a sample
and hold circuit along with a discriminator circuit like those
used in the neutron detectors. Once a pulse is detected, the
sample and hold circuit will capture the peak voltage and the
microprocessor will convert the peak into a number. The

number will be used as an index to an array that will be
incremented each time a pulse of that specific height is
detected.
VI. ACKNOWLEDGEMENTS
This work was supported under DTRA contract DTRA-01-
03-C-0051, National Science Foundation IMR-MIP grant
No. 0412208, and US Department of Energy NEER Grant
DE-FG07-04ID14599.
VII. REFERENCES
[1] Neutron Detectors, International Atomic Energy
Agency, Bibliographical Series, No. 18, Vienna, 1966.
[2] D.S. McGregor, M.D. Hammig, H.K. Gersch, Y-H.
Yang, and R.T. Klann, “Design Considerations for
Thin Film Coated Semiconductor Thermal Neutron
Detectors, Part I: Basics Regarding Alpha Particle
Emitting Neutron Reactive Films,” Nuclear Instruments
and Methods A500, 272-308, 2003.
[3] D.I. Garber, R.R. Kinsey, BNL 325: Neutron Cross
Sections, 3rd ed., Vol. 2, Curves, Brookhaven National
Laboratory, Upton, 1976.
[4] V. McLane, C.L. Dunford, P.F. Rose, in: Neutron
Cross Sections, Vol.2, Academic Press, San Diego,
1988.
[5] D.S. McGregor, R.T. Klann, H.K. Gersch, E. Ariesanti,
J.D. Sanders, and B. Van Der Elzen, “New Surface
Morphology for Low Stress Thin-Film-Coated Thermal-
Neutron Detectors,” IEEE Trans. Nuclear Science 49,
1999-2004, 2002.
[6] D.S. McGregor, S. Bellinger, D. Bruno, W. Dunn, W.J.
McNeil, E. Patterson, B.B. Rice, J.K. Shultis,
“Perforated Diode Neutron Detector Modules
Fabricated from High-Purity Silicon,” Radiation
Physics and Chemistry, 2007, in press.
[7] J.K. Shultis and D.S. McGregor, “Efficiencies of
Coated and Perforated Semiconductor Neutron
Detectors,” IEEE Trans. Nuclear Science NS-53,
1659-1665, 2006.
[8] D.S. McGregor, S. Bellinger, W.J. McNeil, E.
Patterson, B.B. Rice, J.K. Shultis, “Non-Streaming
High-Efficiency Perforated Semiconductor Neutron
Detectors, Methods of Making Same and Measuring
Wand Utilizing Same, US patent pending, 2007.
[9] www.zigbee.com, March, 2007.
[10] www.xbow.com/Products/Product_pdf_files/Wireless_p
df/MICAz_Datasheet.pdf, March, 2007.