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Radiation Technology Articles
Spectroscopy on Thick HgI2 Detectors: A Comparison Between Planar and Pixelated Electrodes
Thick mercuric iodide (HgI2 ) detectors
are investigated as potential room temperature gamma-ray spectrometers. By
using pixelated anodes the induced charge on the electrode is dependent
mainly on electron movement and is almost
independent of the depth of interaction. Moreover, by reading out the
planar cathode signal simultaneously, the depth of interaction can be
determined and any effects of electron charge loss can be corrected. By
combining these two methods (pixelated anodes and depth sensing), the
resolution from 1 cm thick HgI2 devices can be improved to 1.4% FWHM when
using a Cs-137 point source. These results were obtained using a modest
electric field (2500 V/cm) and relatively short shaping times (4-16 ms)
for HgI2 . A comparison between conventional planar readout and single
polarity charge sensing techniques with wide band-gap semiconductors is
discussed.
more...
Spectral Performance of Mercuric Iodide Gamma Ray Detectors at Elevated Temperatures
The effects of elevated temperatures
on the spectral performance of a planar mercuric iodide (HgI2) gamma ray detector were
evaluated at 25oC, 35oC, 45oC and 55oC using two test isotopes,
137Cs at 662 keV and 241Am at 59 keV. Spectral analysis was
used to determine the spectral parameters (i.e. %FWHM of the full energy
peak, the peak channel position and the peak to background ratio).
Spectral performance degraded slightly with increasing test temperatures,
but recovered on returning to ambient conditions. The results demonstrate
that temperature excursions up to 55oC minimally degrade the spectral
performance of mercuric iodide detectors.
more...
Observation of High-Pressure Phase Transition in Xenon Gas
Investigating thermal neutron absorption in Xe +3% 3He gas mixture, we
have observed a phase transition in the gas at 0.18 g/cm3 density and at
room temperature.
More...
HgI2 Detector with a Virtual Frisch Grid
We investigated the performance of a
3 x 3x 6 mm3 HgI2 detector with a virtual
Frisch grid. A metal shield, separated from the material with a thin layer
of dielectric, is deposited on the crystal side surfaces, forming the
virtual grid. Detectors with this geometry can be used for fabrication of
large-area high-efficiency detector arrays that are currently required for
nuclear medicine and radioactive material monitoring. We obtained a
moderate energy resolution of ~5% FWHM at 511 keV for the tested detector,
which can be explained by the fluctuation of the charge loss due to the
electron trapping by the small number but geometrically large defects
existing inside the HgI2
material.
more...
Evaluation of Mercuric Iodide Detectors for SNM Monitoring
Detectors made from single crystals of
mercuric iodide are used in increasing numbers for the detection and
analysis of gamma radiation. The large density and high atomic numbers of
the constituent elements of mercuric iodide result in a high efficiency
compared with other solid state detectors. The material has a very high
threshold for damage by neutrons and charged particles, and its high resistivity makes it possible to fabricate large area detectors with low
noise levels at ambient temperatures. The technology to fabricate these
detectors makes it possible to manufacture large volume counters and
sensitive spectrometers with a resolution of 3% FWHM or less at the gamma
ray energy of 662 keV. They have been operated between –20 to 60 degrees
Celsius, and the performance is stable over periods of years. These
properties make the mercuric iodide detectors ideally suited for the
routine monitoring of stored materials, hold-up measurements, and in
unattended systems.
more...
Dual-Anode High-Pressure Xenon Cylindrical Ionization Chamber
A new approach to design of high-pressure
xenon cylindrical ionization chambers is investigated. The dual-anode is used instead of a single anode
surrounded with a shielding grid in the conventional design. Two coplanar anode wires are stretched
near the axis of the chamber with a sensitive volume of 9 cm in diameter and 20 cm in
length. Both the wires are kept at the same ground potential and DC-coupled to charge-sensitive
preamplifiers. For the majority of the interactions, only one of the wires collects the
electrons produced by ionizing particles. Both the wires detect the charge induced by uncollected
positive ions. The difference between the signals read out from the wires is proportional
to the charge of collected electrons. Absence of the grid makes the detector more
robust, less sensitive to vibrations, and inexpensive. The first experimental results are
compared to Monte Carlo simulations. The optimal chamber design is discussed.
more...
Comparison of Several Detector Technologies for Measurement of Special Nuclear Materials
Gamma spectral analysis of
measurements made on Special Nuclear Materials (SNM) provides
identification and sometimes quantification of the SNM present. Different
radiation detection technologies have been used for measurement of SNM. Selection of one of these for a particular
application depends on the information that can be obtained using that detector and on the support needed to
use the detection technology, especially if it is to be applied at sites where limited support is available. In
some cases, detector selection may be determined by a need to limit the information available from
measurements due to international treaty requirements. There are many studies of system capabilities in SNM
measurement. Most of these used either scintillator or high-resolution (HPGe) detectors embedded in a
unique electronics and software combination. Their respective developers claimed good results for their
instrument. Unfortunately, the ‘packaging’ often obscured the question of what can be obtained from
various detector technologies. While instrumentation and analysis is important, a study comparing only detector characteristics
would serve as a starting point for development of systems for SNM measurement. Our
detector study consisted of measurements on shielded and unshielded high-enriched uranium and
Plutonium sources using detectors: NaI(Tl), Cadmium Zinc Telluride (CZT), Mercuric Iodide (HgI2),
and two HPGe detectors, one with liquid nitrogen cooling and one with electromechanical cooling. Measurements
were made simultaneously using all detectors. As expected, all detectors are suitable for SNM
measurements in cases where one radionuclide, such as
235U, is the dominant constituent, but only the high-resolution detectors
are suitable for isotopic composition determinations using ‘standard’ spectroscopic analysis.
Room temperature semiconductor detectors (CZT, HgI2)
can provide limited isotopic information. Development of improved analysis algorithms for room-temperature semiconductor detectors would be
desirable because of their attractive characteristics: small sizes and minimal support requirements.
more...
Unattended Radiation Sensor Systems for Remote Terrestrial Applications and Nuclear Nonproliferation
The design of instrumentation for
remote sensing presents special requirements in the areas of power consumption, long-term stability, and compactness. At
the same time, the high sensitivity and resolution of the devices needs to be preserved. This
paper will describe several instruments suitable for remote sensing developed under the sponsorship
of the Defense Threat Reduction Agency (DTRA). The first is a system consisting of a
mechanical cryocooler coupled with a high-purity germanium (HPGe) detector. The system is portable
and can be operated for extended periods of time at remote locations without servicing. The
second is a hand-held radiation intensity meter with high sensitivity that can operate for
several months on two small batteries. Intensity signals above a set limit can be transmitted to a
central monitoring station by cable or radio transmission. The third is a small module incorporating
one or more high resolution mercuric iodide detectors and front end electronics. This
unit can be operated using standard electronic systems, or it can be connected to a separately
designed, pocket-size module that can provide power to any detector system and can process detector
signals . It incorporates a shaping amplifier, a multichannel analyzer, and gated integrator
electronics to process the slow signal pulses generated by room temperature solid state detectors. The
fourth is a high pressure xenon (HPXe) ionization chamber filled with very pure xenon gas at high
pressure, so that the efficiency and spectral resolution are increased above the normally
available gas-filled tubes. The performance of these systems will be described and discussed.
more...
Development of a Portable Monitor
Constellation Technology recently marketed a portable portal monitor for use by emergency response personnel
and others needing a temporary means to screen people for contamination. The monitor consists of two-detector
panels and frame hardware to hold the
panels erect. Parts are designed so that one person can move and set up
the monitor relatively quickly. The presentation traces the
development of the monitor from the original prototype delivered to Health
Canada. Detector, electronics, and firmware selections are discussed on a
basis of building a practical and sensitive instrument that meets customer
needs. Testing, both environmental and radionuclide detection, was a key
part of the development process. Decisions during the design process
necessarily involve cost and performance tradeoffs. It is essential that
these tradeoffs impact the monitor’s performance as little as possible
while keeping the cost within reason. Test results show the performance of
the "final" product.
more...
High-Pressure Xenon Detectors for Applications in Portal Safeguard Systems and for Monitoring Nuclear Waste
Constellation Technology Corporation has developed a family of high-pressure xenon ionization chambers (HPXe) with
mass of working medium between 100 g and 2 kg. The detectors demonstrate a good energy resolution (below 3% FWHM at
662 keV for 1 kg detector) that is close to the resolution of room temperature semiconductor detectors. The detectors
employ relatively simple design, robust construction, and are available in a variety of pipe-like configurations of up
to 1 m long. The sensitive area of HPXe may cover a few square feet and detect gamma rays with efficiency close to
solid state detectors. Both in experimental modeling and computer simulations, HPXe has shown a good potential for
detecting kilogram amounts of nuclear materials passing through the vehicle portal of a facility designated to store
special nuclear materials. HPXe detectors have been tested at hard radiation conditions typical for a space orbital
station. There is a positive experience of using HPXe at 120oC with a specially designed preamplifier. There is no
temperature and radiation degradation, as well as no charge trapping in xenon itself. Radiation stability of the
working medium (Xe) is practically unlimited. All these features make HPXe detectors ideal candidates for monitoring
nuclear waste tanks and other nuclear installations with hard radiation conditions.
more...
High-Pressure Helium-3 Scintillation Position-Sensitive
Thermal Neutron Detector
The objective of this study is to demonstrate
the feasibility of constructing a compact neutron detector that is sensitive to thermal and epithermal neutrons
and has high rejection efficiency relative to gamma-ray background. A two-channel
high-pressure 3He scintillation detector is considered for the detection of neutrons in
coincidence mode. The detector consists of two large avalanche photodiodes viewing a gas
volume filled with pressurized 3He. Experiments with the detector demonstrate
high efficiency to neutrons and high rejection ability to gamma rays. Position sensitivity
of the detector to thermal neutrons is demonstrated and confirmed with computer
simulations.
more...
Application of High-Pressure Xenon Detectors in Portal Safeguard Systems and for Monitoring Nuclear Waste
Constellation Technology Corporation has
developed a family of high-pressure xenon ionization chambers (HPXe) with working medium masses between 100 g and 2 kg. The
detectors demonstrate a good energy resolution (below 3% FWHM at 662 keV for 1 kg
detector) that is close to the resolution of room temperature semiconductor detectors
with a few grams of working mass. The detectors employ relatively a simple design, robust
construction and are available in a variety of pipe-like configurations up to 1.5 m long.
The sensitive area of HPXe may cover a few square feet and detect gamma rays with an efficiency
close to solid state detectors. Both in experimental modeling and computer simulations it is
shown that HPXe has a good potential for detecting kilogram amounts of nuclear materials
passing through the vehicle portal of a facility designated to store special nuclear materials.
HPXe detectors have been tested at hard radiation conditions typical for an orbital space station.
There are positive experiences of using HPXe at 120oC with specially designed preamplifiers.
There is no temperature or radiation degradation as well as no charge trapping in xenon itself. The
radiation stability of the working medium is practically unlimited. All these features make HPXe detectors
ideal candidates for monitoring nuclear waste tanks and other installations with hard
radiation conditions.
more...
Recent Advances in Development of Mercuric Iodide Radiation Detectors
The performance of mercuric iodide radiation detectors has
improved during the past two years to the point where they can be
generally applied in instruments for the measurement and spectral analysis
of nuclear radiation. This is due to systematic optimization of the
processes to prepare the material and to fabricate the detectors. This
paper will review these accomplishments in terms of spectral resolution,
long-term stability and elevated temperature results. New detector
arrangements and electronic systems, which have been developed to match
the properties of the detectors, will be described.
more...
Limitations of the Hecht Equation Encountered in
Measuring Mu Tau Products in Mercuric Iodide
While attempting to measure µt
products in HgI2 using the pulse rise-time method, a
method based on the Hecht equation, fundamental limitations of the Hecht equation were revealed. These
limitations restrict the equations utility, even for the case for which it was originally developed, i.e. when carrier trapping
occurs without de-trapping. The Hecht equation purportedly analytically characterizes the charge collection process under uniform
field and permanent, charged carrier trapping conditions. Since de-trapping occurs in HgI2,
particularly in high spectral performance HgI2 detectors, it was recognized
that limitations would be encountered in applying the Hecht equation.
more...
Mercuric Iodide X-Ray and Gamma Ray Detectors for Astronomy
The recent technological
developments and availability of mercuric iodide detectors have made their
application for astronomy a realistic prospect. Mercuric iodide, because of its high
resistivity and high density, can be used in a variety of astronomy instrumentation where high spectral resolution, low noise
levels, stability of performance, resistance to damage by charged particles and overall ruggedness are of critical importance.
X-ray detectors with areas of 12 to 100 mm square and 1 mm thickness have absorption efficiencies approaching 100% up to 60 keV.
The spectral resolution of these detectors ranges from 400 eV to 600 eV at 5.9 keV, depending on their area, and the
electronic noise threshold is less than 1.0 keV. Gamma ray detectors can be fabricated with dimensions of 25 mm x 25 mm x 3
mm. The spectral resolution of these detectors is less than 4% FWHM at energies of 662 keV. Because of the high atomic numbers
of the constituent elements of the mercuric iodide, the full energy peak efficiency is higher than for any other
available solid-state detector that makes measurements up to 10 MeV a possibility. The operation of gamma ray detectors has been
evaluated over a temperature range of -20 through +55 degrees Celsius, with only a very small shift in full energy peak
observed over this temperature range. In combination with Cesium Iodide scintillators, mercuric iodide detectors with 25 mm
diameter dimensions can be used as photodetectors to replace bulky and fragile photomultiplier tubes. The spectral
resolution of these detectors is less than 7% FWHM at 662 keV and the quantum efficiency is larger than 80 % over the whole area of
the detector. Details of the properties and performance of the mercuric iodide detectors will be presented and discussed.
more...
The Long-Term Spectral Stability of HgI2 Gamma-Ray Detectors
An evaluation of the spectral
performance of eight planar mercuric iodide (HgI2) gamma-ray detectors under continuous bias voltage for a duration of up to 2000 hours has demonstrated the high
degree of long-term stability of mercuric iodide as a radiation detector material. Spectral parameters determined in this
evaluation include the %FWHM, the peak-to-valley and peak-to-background ratios, the gain stability of the full energy peak,
and the preamplifier offset voltages. Isotopes with three distinct energies were used for these measurements: 137 Cs (662 keV),
57 Co (122 keV) and 241 Am (59 keV). The spectra were analyzed and spectral parameters were generated using Robwin©, a spectral analysis program
developed by Constellation Technology. Robwin performs simultaneous
non-linear fitting of several key elements of the spectrum, emphasizing
the continuum for the entire spectrum, the photopeak response function of
all lines in the spectrum, the relative intrinsic efficiency of the detector and the photopeak resolution width.
more...
Application of Mercuric Iodide Detectors to the Monitoring and Evaluation of
Stored Special Nuclear Materials
Mercuric iodide is a very promising material
for the monitoring of stored nuclear materials that can be characterized by the energies and relative intensities of the
gamma ray spectra that the materials produce. The high density and high average atomic number of
mercuric iodide results in higher efficiency than other solid-state detectors such as CdTe, CZT
or HPGe. In addition, the high resistivity of the material at room temperature makes it possible
to produce relatively large detectors with very low noise levels.
Recent technological advances have made it possible to routinely
fabricate detector structures of different dimensions tailored to the energy ranges to be investigated.
Typical designs have a nominal energy operating range of 30 - 1300 keV with spectral
resolution of 2% or better at 662 keV. This resolution can be improved by the use of advanced electronics
and/or tailored detector design. These detectors operate over a temperature range of less than
zero to greater than fifty degrees Celsius and have stable performance over long operating
lifetimes. The stability, resolution, efficiency, and radiation hardness of these detectors make
them ideally suited to unattended monitoring systems either as radiation counters or
spectrometers.
The detectors are packaged together with a preamplifier in a small,
lightweight and rugged Mercury ModuleTM that can be used individually or as part of
an integrated array of detectors. Several modules can be connected to a common signal processing system
so that several locations can be monitored using one spectrum analysis and monitoring
system.
Spectral data of the detectors will be presented and the electronic
layout of the Mercury Module will be shown. Performance
measurements on several representative materials will be shown. Plans for an extended monitoring system to be used in
large storage vaults will be presented and discussed.
more...
Recent Advances in Mercuric Iodide Detector Fabrication and Instrument Development
Mercuric Iodide is a preferred
candidate material for truly room-temperature radiation detectors because
of its large electronic bandgap (2.1 eV) and the high atomic number of its
constituent elements, which results in a high photopeak efficiency. The spectroscopic performance of the detectors is
determined by the electronic transport properties of the material which depends on the purity and the structural homogeneity of the
single crystals from which the detectors are fabricated. Recent advances in purification and crystal growth have made it
possible to fabricate routinely large, stable gamma ray and X-ray detectors and counters.
The gamma ray detectors (25mm x 25mm x 3mm) with a spectral resolution
better than 4% at Cs137 are combined with a preamplifier and voltage filter in a stand-alone module which minimizes
microphonic noise. This module can be incorporated in a portable spectrometer instrument with a total weight of less than
4 kg (batteries included). The counter detectors (25 mm x 25mm x 5mm) are incorporated in a pocket-size radiation intensity
counter with high sensitivity.
more...
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