Development of
Thermal Neutron Detection via
Boron-Rich Heterodiode Sensors.
Alan Briggs, Robert Venn
Cambridge Microfab Ltd
Alan Owens
ESTEC Noordwijk
2011
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Microfab heterodiode neutron sensors
-20082 wire devices diced from an 18 * 18mm silicon tile
Die size is 6*6mm
Neutron sensitive area is also 6* 6mm
But, electrode diameter is 1,2,3, or 4mm
-2011A 2 wire device in a plastic package (shown without lid).
A 9*9 mm die with a 50mm2 active area .
Neutron sensitive area identical to electrode area
Silicon Substrate
2
Detecting region 50mm2
© Cambridge Microfab Ltd 2011
The 10B – neutron reaction
The isotope 10B has a high thermal neutron cross section
( 3840 barns cf. 5333 barns 3He)
–
10B
& 11B occur naturally in the ratio 1:4 but we now use a boron-enriched source
with >99% 10B
Essentially the 10B(n,α)7Li reaction can be written:
10B
+
1n
7Li
(1.015 MeV) + α (1.777 MeV) [Reaction Q value = 2.792 MeV]
7Li
(0.840 MeV) + α(1.470 MeV) + γ(0.48 MeV) [Q value = 2.310 MeV]
The two possible capture reactions have probabilities of 6% and 94% respectively
An alpha particle and a lithium ion result from either reaction path
As these high-energy particles pass through a semiconductor they interact to
dissipate energy and generate free charge carriers.
–
3
(i.e. > 100,000 electron/hole pairs per event).
To derive a signal from the device we must separate these free charge carriers
before they recombine (using an electric field).
© Cambridge Microfab Ltd 2011
Cambridge Microfab heterodiode neutron detector schematic
– the neutron reactive region forms part of the diode.
neutron reaction producing
high energy alpha and
lithium ion products
upper electrode structure
Neutron converter
10B-rich p-type
semiconductor layer
Li ion
neutron
alpha
N-type silicon substrate
completes the heterodiode
structure
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lower electrode
structure
© Cambridge Microfab Ltd 2011
General features of semiconductor detectors
(all types of radiation).
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Semiconductor detectors typically operate using a reverse-biased
p-n junction diode configuration to minimize “dark” current (i.e.
current/signal in the absence of excitation).
Detection takes place in the vicinity of the p-n junction where
there is a high-field region that is depleted of free charge carriers
(known as the “Depletion Region”).
There is a capacitance associated with the Depletion Region and
the response time of the detector varies inversely with its
capacitance.
In semiconductor detector terms our current 50mm2 neutron
detector is a large-area device (with a larger area the associated
capacitance begins to be inconveniently large).
You might fabricate a larger-area detector by tiling individual
devices together.
© Cambridge Microfab Ltd 2011
Typical device Current-Voltage characteristic 2011
Leakage <5 nA for 50mm2
device
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© Cambridge Microfab Ltd 2011
Typical device Current-Voltage characteristic 2008
Lit and dim traces from the same two 4mm
devices – reverse detail
tile3C2bright
tile3C2dim
tile4C2bright
tile4C2 dim
0.9 µA => 72nA mm-2
1.7µA => 135nA mm-2
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© Cambridge Microfab Ltd 2011
Developments since 2008
• Improved absorber layer quality allows large-area devices
• Mesa etch so neutron-converter area matches electrode area
• x5 greater efficiency using 10B enriched absorber
-2008Top electrode sits on the p type neutron converter
The converter film extends to the edge of the die
Top electrode area is smaller than the converter area
-2012The converter film has been “mesa” etched
There is no converter outside the electrode perimeter
natural isotopic composition absorber
10B
enriched absorber
Top electrode
p type neutron absorber
Silicon substrate
Bottom electrode
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© Cambridge Microfab Ltd 2011
Benefit of mesa etching
-2011RC product is roughly constant to all areas of the
active region.
Electrode area accurately defines the active area.
Event rise times are all similar.
Efficiency <= predicted 7.5% maximum
-2008RC product depends on where in the active region
a neutron reaction occurs.
Electrode area does not define the active area!
Variable rise time events.
Apparent efficiency greater than predicted.
Variable RC region
depleted region with very high resistivity
behaves as a capacitor
Boundary of neutron reactive layer
N type silicon substrate
Lower electrode
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© Cambridge Microfab Ltd 2011
Detector with preamplifier in housing
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© Cambridge Microfab Ltd 2011
Output from detector mounted in housing
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Output from charge
sensitive pre-amp.
© Cambridge Microfab Ltd 2011
“Semi-direct” detection at present
P-type side of the junction is not depleted so reaction products
must penetrate to the n-side to register a signal.
But no “dead space” between converter and detection region.
No signal from this
capture event
Neutron converter
10B-rich P-type
semiconductor layer
N-type silicon
substrate
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Li ion
neutron
alpha
© Cambridge Microfab Ltd 2011
Microfab detector – Detectivity
•
•
•
•
•
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Current Microfab devices have a neutron capture
probability of ~18%
~40% of capture events give rise to a signal
Calculated efficiency (Monte Carlo simulation) ~7.5%
Measured efficiency ~3%
But devices are good for applications where high
sensitivity is not required.
© Cambridge Microfab Ltd 2011
Energy-resolved (MCA) detector data
Courtesy Jirí Vacík, Nuclear Physics Institute, Academy of
Sciences of the Czech Republic, Rež near Prague
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1.4x10
Detector: Venn
9
7
B(n,alpha) Li
nth
8
8.0x10
7
Li1
8
B convertor
10
10
9
1.0x10
DETECTOR
Counts / channel
1.2x10
6.0x10
α1
7
Li0
8
4.0x10
R331
8
2.0x10
T = 2:15:00 h
0
200
400
600
α0
800
1000
1200
1400
1600
Channel number
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© Cambridge Microfab Ltd 2011
Neutron Spectrum from NPL
10B
15
6%
7Li
(1.015 MeV) + α (1.777 MeV)
94%
7Li
(0.840 MeV) + α(1.470 MeV) + γ(0.48 MeV)
+ 1n © Cambridge Microfab Ltd 2011
Calibration with alpha source
Courtesy Carlos Granja, Institute of Experimental and Applied Physics, Czech
Technical University in Prague, CZ-12800 Prague 2, Czech Republic
Alpha data ex-housing
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© Cambridge Microfab Ltd 2011
Sensitivity to other radiation – Specificity
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The Microfab Heterodiode sensor needs to respond to respond to alphas (and Li
ions) with energies from 1.78MeV all the way down to the noise floor
The aluminium alloy sensor casing shields the sensor from alpha particles and IR
through to ~AlKα(
(1.49keV) but higher energy photons as well as neutrons can
penetrate the casing
As the photon energy rises the probability of its transmission through the case rises
but the probability of its absorption in a sensitive region of the device drops
Placing an ~ 35kBq 241Am source directly against on the plastic back of a sensor
gives a convenient test situation. One half of the 59.5keV gammas are transmitted
through the device
Our initial data suggests that at this energy typically ~0.02% of these give rise to a
signal - this is a value which we can alter by deliberate processing variations
Photon energy from a synchrotron source has been used to probe the response
uniformity, energy linearity and approximate noise floor of some test devices.
( Synchrotron sources give very high photon fluences)
© Cambridge Microfab Ltd 2011
Comparison with 3 He-based neutron detectors
Current Microfab devices have cross sectional area of 0.5cm2 and useable signal probability
up to 7.5% using a lower limit of detection of 100keV
–
–
3He
tubes tend to use a fill pressure so that neutron capture probability is 100% or more for
a neutron crossing their diameter and they give a useable signal for almost every neutron
reaction.
–
–
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As the sensor area is scaled up the device capacitance increases and the useable lld rises
Single sensors are close to planar but can be stacked to give higher detection probability
The “event signal” probability does not drop off as the device is scaled in size.
3He Sensors are typically spherical or cylindrical and are associated with a high voltage
3He
Microfab devices are rugged, compact, low-voltage, portable and relatively
inexpensive.
tubes remain preferred for detecting a low neutron flux over a large area with a single
channel of readout
© Cambridge Microfab Ltd 2011
Grateful acknowledgements
Work originally funded under ESA contracts
“Technical Assistance in the Development of a Solid-State Neutron detector
for Planetary Missions”
(18306/04/NL/HB & 22438/09/NL/AF)
Alan Owens, Johannes van der Biezen at ESA
Tone Peacock, Solve Andersson, formerly at ESA
Danny Palmer, Lukasz Jankowski, Jeannie Venn, at Cambridge Microfab Ltd
http://www.microfab.co.uk/
Graeme Taylor and David Thomas at NPL Teddington
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© Cambridge Microfab Ltd 2011
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© Cambridge Microfab Ltd 2011
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© Cambridge Microfab Ltd 2011
The sequence between neutron arrival and its reporting
(Moderation of neutron to thermal energy range)
10B absorbtion reaction
One or the other reaction product (in special cases both) gives
rise to hole-electron pair generation
Transient current pulse arrives at device electrode under
external or self bias field
Pre-amplification via a pulse amplifier or charge sensitive
amplifier, can be AC or DC coupled
Signal processing options after the preamplifier
–
–
–
Finally signal summary and collection options
–
-
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Simple edge detection via comparator
Classical Pulse Shaping
Digital Signal Processing
A simple counter
MCA eg slide 14
© Cambridge Microfab Ltd 2011
Most sensors have been tested with a charge sensitive
preamplifier designed by Solve Andersson
The CR110 charge sensitive preamplifier by Fred Olscher of Cremat, Inc. is an alternative with very
high gain and works well with our heterodiode neutron sensors. Shown here integrated into an
enclosure with sensor power and output connectors in an alloy housing 20*75*40mm
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© Cambridge Microfab Ltd 2011