Hand-held Neutron Detector
Proliferation of the weapons of mass destruction such as nuclear weapons is a serious threat.
Prevention of their spread has reached a state of heightened urgency in recent years. One of the
ways to passively determine the presence of nuclear weapons is to detect and identify
characteristic signatures of highly enriched uranium and weapons grade plutonium. Neutrons
and gamma rays are two signatures of these materials. Gamma ray detection techniques are
useful because the presence of gamma rays of specific energies can confirm the presence of a
particular isotope. This technique however, has one significant limitation. In the presence of a
dense surrounding material such as lead, gamma ray attenuation can be significant. This can
mask the gamma ray signatures of these special nuclear materials (SNM). Neutrons, on the other
hand, easily penetrate dense and high atomic number materials. For heterogeneous or dense
materials such as samples of metals, oxides, and nuclear waste, gamma ray attenuation is too
high to permit accurate correction of the measured signal. Under these circumstances, passive
assay techniques based on neutron detection are preferable. When detected, neutrons directly
indicate the presence of spontaneously fissioning isotopes (plutonium and californium) and
induced fissions (uranium). Therefore, neutron detection is an important component of the
overall detection techniques used in identifying SNM.
In radioisotope identification devices to date, the neutron detection was readily achieved
using He-3 tubes. Unfortunately, in recent years the quantity of this gas is becoming limited,
therefore, new solutions are required for an efficient detection system that would allow neutron
detection with an ability to discriminate gamma ray events from neutron events. Gamma
discrimination is critical because gamma rays are common background in neutron detection
environment during SNM monitoring.
In this project we propose a handheld thermal neutron detector based on a Cs2LiYCl6:Ce
(CLYC) scintillator [Combes, van Loef], which is an ideal candidate for the task [Bessiere,
Glodo 08, Glodo 09]. CLYC offers (1) efficient thermal neutron detection (higher per-volume
than He-3); (2) excellent separation between gamma and neutron particles (better than 10-6); and
(3) gamma-ray energy resolution as good as 4% at 662 keV for dual mode (neutron and gamma)
detectors. The last property is very fortunate, since the majority of current handheld thermal
neutron detectors include a separate gamma detector. In most cases, in addition to neutron
counts the detection system should provide information about the dose rate and / or simple
isotope characterization based on four categories – NORM (Natural occurring radiation
materials), SNM (Special nuclear materials), Medical and Industrial Radio-nuclides. The good
gamma ray energy resolution of CLYC should guarantee an accurate energy compensated dose
rate and reliable characterization of gamma ray radiation. In the last couple of years, CLYC
manufacturing has progressed and 1 and 2 inch crystals are being routinely grown at RMD for
internal and government purposes [Higgins]. Crystals with diameter as large as 3 inch have been
grown. Moreover, the CLYC technology is currently being transferred by RMD to a commercial
setting (Hilger), where full scale manufacturing of these crystals will take place.The final goal of this effort is to develop a handheld thermal neutron detector utilizing CLYC
scintillators. In Phase I of the project we provided strong foundations for achieving this goal.
We have shown that CLYC works well with silicon photomultipliers (MPPC from Hamamatsu).
A CLYC/MPPC system provides a very compact device due to small size of this light detector.Built detectors showed clear neutron peaks (7% energy resolution), were capable of pulse shape
discrimination, and could easily provide dose equivalent information for gamma ray radiation.
Such combination works even if the crystal is in a form of a 1 in right cylinder, although pillar
type geometry was found to be optimal from the efficiency point of view. Due to their optimal
surface to volume ratio, our pillar detectors provided twice as many counts as a high pressure
He-3 tube per volume unit.
The objective of the Phase II effort is to design and construct prototypes of a compact
neutron detector based on the CLYC scintillator. The Phase II work will be based on the Phase I
experiments and results. It will focus on developing the detector and instrumentation technology
to achieve the project goal of designing and prototyping a compact handheld neutron detector.
The main areas of research and development will include (1) detector module optimization, such
as detector form factor, light readout, and interface; (2) study of the detector signal shape and
PSD performance as a function of the temperature and temperature stabilization of the system;
(3) electronic module design and prototype development. Our goal will be to develop and realize
a concept that can be expanded into multi-component systems, e.g. backpack implementation.
In this project we will collaborate with Dr. Sara Pozzi at the University of Michigan. She
will assist with the modeling and optimization of the neutron and gamma ray response of our
detectors. The optimization will include detector and moderator dimensions.
Small Business Information at Submission:
Radiation Monitoring Devices, Inc.
44 Hunt Street Watertown, MA -
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