The telecentric zoom lens system (ZLS) has proven to be invaluable in flash x-ray field operations and recent successful
experiments pertaining to stockpile stewardship. The ZLS contains 11 custom-manufactured lenses, a turning mirror
(pellicle), and an x-ray-to-visible-light converting scintillator. Images are recorded on a fully characterized CCD. All
hardware is supported by computerized, programmable, electro-mechanical mounts and alignment apparatus. Seven
different glass material types varying in chemical stoichiometry comprise the 11 ZLS lenses. All lenses within the ZLS
are out of the path of direct x-ray radiation during normal operation. However, any unshielded scattered x-ray radiation
can result in energy deposition into the lenses, which may generate some scintillating light that can couple into the CCD.
This extra light may contribute to a decrease in signal-to-noise ratio (SNR) and lower the overall fidelity of the
radiograph images. An estimate of the scintillation generation and sensitivities for each of the seven types of glass used
as lenses in the ZLS is presented. This report also includes estimates of the total observed background decoupling that
each of the lens material types contribute.
We are investigating scintillator performance in radiographic imaging systems at x-ray endpoint energies of 0.4 and 2.3 MeV in single-pulse x-ray machines. The effect of scene magnification and geometric setup will be examined along with differences between the detector response of radiation and optical scatter. Previous discussion has reviewed energy absorption and efficiency of various imaging scintillators with a 2.3 MeV x-ray source. The focal point of our study is to characterize scintillator blur to refine system models. Typical detector geometries utilize thin tiled LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) assembled in a composite mosaic. Properties of individual tiles are being studied to understand system resolution effects present in the experimental setup. Comparison of two different experiments with different geometric configurations is examined. Results are then compared to different scene magnifications generated in a Monte-Carlo simulation.
A networked gamma radiation detection system with directional sensitivity and energy spectral data acquisition capability is being developed by the National Security Technologies, LLC, Remote Sensing Laboratory to support the close and intense tactical engagement of law enforcement who carry out counterterrorism missions. In the proposed design, three clusters of 2″ × 4″ × 16″ sodium iodide crystals (4 each) with digiBASE-E (for list mode data collection) would be placed on the passenger side of a minivan. To enhance localization and facilitate rapid identification of isotopes, advanced smart real-time localization and radioisotope identification algorithms like WAVRAD (wavelet-assisted variance reduction for anomaly detection) and NSCRAD (nuisance-rejection spectral comparison ratio anomaly detection) will be incorporated. We will test a collection of algorithms and analysis that centers on the problem of radiation detection with a distributed sensor network. We will study the basic characteristics of a radiation sensor network and focus on the trade-offs between false positive alarm rates, true positive alarm rates, and time to detect multiple radiation sources in a large area. Empirical and simulation analyses of critical system parameters, such as number of sensors, sensor placement, and sensor response functions, will be examined. This networked system will provide an integrated radiation detection architecture and framework with (i) a large nationally recognized search database equivalent that would help generate a common operational picture in a major radiological crisis; (ii) a robust reach back connectivity for search data to be evaluated by home teams; and, finally, (iii) a possibility of integrating search data from multi-agency responders.
The Remote Sensing Laboratory (RSL) is developing a tactical, networked radiation detection system that will be agile, reconfigurable, and capable of rapid threat assessment with high degree of fidelity and certainty. Our design is driven by the needs of users such as law enforcement personnel who must make decisions by evaluating threat signatures in urban settings. The most efficient tool available to identify the nature of the threat object is real-time gamma spectroscopic analysis, as it is fast and has a very low probability of producing false positive alarm conditions. Urban radiological searches are inherently challenged by the rapid and large spatial variation of background gamma radiation, the presence of benign radioactive materials in terms of the normally occurring radioactive materials (NORM), and shielded and/or masked threat sources. Multiple spectral anomaly detection algorithms have been developed by national laboratories and commercial vendors. For example, the Gamma Detector Response and Analysis Software (GADRAS) a one-dimensional deterministic radiation transport software capable of calculating gamma ray spectra using physics-based detector response functions was developed at Sandia National Laboratories. The nuisance-rejection spectral comparison ratio anomaly detection algorithm (or NSCRAD), developed at Pacific Northwest National Laboratory, uses spectral comparison ratios to detect deviation from benign medical and NORM radiation source and can work in spite of strong presence of NORM and or medical sources. RSL has developed its own wavelet-based gamma energy spectral anomaly detection algorithm called WAVRAD. Test results and relative merits of these different algorithms will be discussed and demonstrated.
We have investigated scintillator efficiency for MeV radiographic imaging. This paper discusses the modeled detection efficiency and measured brightness of a number of scintillator materials. An optical imaging camera records images of scintillator emission excited by a pulsed x-ray machine. The efficiency of various thicknesses of monolithic LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) are being studied to understand brightness and resolution trade-offs compared with a range of micro-columnar CsI:Tl (thallium-doped cesium iodide) scintillator screens. The micro-columnar scintillator structure apparently provides an optical gain mechanism that results in brighter signals from thinner samples. The trade-offs for brightness versus resolution in monolithic scintillators is straightforward. For higher-energy x-rays, thicker materials generally produce brighter signal due to x-ray absorption and the optical emission properties of the material. However, as scintillator thickness is increased, detector blur begins to dominate imaging system resolution due to the volume image generated in the scintillator thickness and the depth of field of the imaging system. We employ a telecentric optical relay lens to image the scintillator onto a recording CCD camera. The telecentric lens helps provide sharp focus through thicker-volume emitting scintillators. Stray light from scintillator emission can also affect the image scene contrast. We have applied an optical light scatter model to the imaging system to minimize scatter sources and maximize scene contrasts.
Neutron counting using large arrays of pressurized 3He proportional counters from an aerial system or in a maritime
environment suffers from the background counts from the primary cosmic neutrons and secondary neutrons caused
by cosmic ray‒induced mechanisms like spallation and charge-exchange reaction. This paper reports the work
performed at the Remote Sensing Laboratory–Andrews (RSL-A) and results obtained when using two different
methods to reduce the cosmic neutron background in real time. Both methods used shielding materials with a high
concentration (up to 30% by weight) of neutron-absorbing materials, such as natural boron, to remove the lowenergy
neutron flux from the cosmic background as the first step of the background reduction process. Our first
method was to design, prototype, and test an up-looking plastic scintillator (BC-400, manufactured by Saint Gobain
Corporation) to tag the cosmic neutrons and then create a logic pulse of a fixed time duration (~120 μs) to block the
data taken by the neutron counter (pressurized 3He tubes running in a proportional counter mode). The second
method examined the time correlation between the arrival of two successive neutron signals to the counting array
and calculated the excess of variance (Feynman variance Y2F)1 in the neutron count distribution from Poisson
distribution. The dilution of this variance from cosmic background values ideally would signal the presence of manmade
neutrons.2 The first method has been technically successful in tagging the neutrons in the cosmic-ray flux and
preventing them from being counted in the 3He tube array by electronic veto—field measurement work shows the
efficiency of the electronic veto counter to be about 87%. The second method has successfully derived an empirical
relationship between the percentile non-cosmic component in a neutron flux and the Y2F of the measured neutron
count distribution. By using shielding materials alone, approximately 55% of the neutron flux from man-made
sources like 252Cf or Am-Be was removed.
The National Security Technologies, LLC, Remote Sensing Laboratory has recently used an array of six smallfootprint
(1-inch diameter by 3-inch long) cylindrical crystals of thallium-doped sodium iodide scintillators to obtain
angular information from discrete gamma ray–emitting point sources. Obtaining angular information in a near-field
measurement for a field-deployed gamma sensor is a requirement for radiological emergency work. Three of the
sensors sit at the vertices of a 2-inch isosceles triangle, while the other three sit on the circumference of a 3-inchradius
circle centered in this triangle. This configuration exploits occlusion of sensors, correlation from Compton
scattering within a detector array, and covariance spectroscopy, a spectral coincidence technique.
Careful placement and orientation of individual detectors with reference to other detectors in an array can provide
improved angular resolution for determining the source position by occlusion mechanism. By evaluating the values
of, and the uncertainties in, the photopeak areas, efficiencies, branching ratio, peak area correction factors, and the
correlations between these quantities, one can determine the precise activity of a particular radioisotope from a
mixture of radioisotopes that have overlapping photopeaks that are ordinarily hard to deconvolve. The spectral
coincidence technique, often known as covariance spectroscopy, examines the correlations and fluctuations in data
that contain valuable information about radiation sources, transport media, and detection systems. Covariance
spectroscopy enhances radionuclide identification techniques, provides directional information, and makes weaker
gamma-ray emission—normally undetectable by common spectroscopic analysis—detectable. A series of
experimental results using the concept of covariance spectroscopy are presented.
Experimental work performed in the area of neutron detector development at the Remote Sensing Laboratory-Andrews Operations (RSL-AO) sponsored by the U.S. Department of Energy, National Nuclear Security Administration (NNSA) in recent months is presented in this article. Ten pressurized He-3 tubes with an inch of annular polyethylene moderator around them are arranged to make an isosceles triangle with three fold symmetry. MCNPX simulation code has been used to obtain the angular response function of the detector array using a bare Cf-252 source. The neutron counts profile shows possibility of determining the angle of a near - field neutron source with adequate angular resolution for emergency response.
Mobile detection of kilogram quantities of special nuclear materials (SNM) during maritime transportation is a
challenging problem for the U.S. Department of Homeland Security. Counting neutrons emitted by the SNM and
partitioning them from background neutrons of multiple origins is the most effective passive means of detecting the
SNM. Unfortunately, neutron detection, counting, and partitioning in a maritime environment is complex due to the
presence of spallation neutrons (commonly known as "ship effect") and to the complicated nature of the neutron
scattering in that environment. This work studied the possibilities of building a prototype neutron detector using boron-
10 (10B) as the converter in a novel form factor called "straws" that would address the above problem by examining
multiplicity distributions of neutrons originating from a fissioning source. Currently, commercially manufactured fission
meters (FM) are available that separate cosmic neutrons from non-cosmic neutrons and quantitatively determine the
strength of a fissioning source; however, these FMs use 3He, which is becoming increasingly difficult to procure; also
the size and weight of a commercial FM is not conducive to manual neutron detection operations in a maritime
environment. The current project may provide a near-term solution to the crisis that has arisen from the global scarcity of
3He by offering a viable alternative to the FM. The prototype detector provides a large-area, efficient, lightweight, more
granular neutron responsive detection surface (to facilitate imaging) to ease the application of the new FMs.
In the area of nuclear radiological emergency response and preparedness applications, interest in
neutron detection stems from several factors. Unlike gamma rays, which are abundant in nature and
present serious difficulties in differentiating a signal from a changing background, whose values are
location specific, neutrons are rare and nearly homogenous in spatial distribution. Additionally, many
special nuclear materials (SNM) emit neutrons either directly by spontaneous fission or produce
neutrons indirectly through (α, n) reactions in nearby light elements. Also of importance in detection
scenarios is the fact that neutrons are not easily attenuated. Typically neutron detection is done by
simply counting the low energy thermal neutrons by employing pressurized helium tubes operated at
high voltages. Not much emphasis is put on determining the energy of the incident neutrons. However,
critical information can be obtained by analyzing the neutron energy given off from radioactive
materials. In the detection of an SNM, neutron energy information from an unknown source can be of
We have modeled, designed, and prototyped multi-element neutron energy spectrometers that contain
three to five pressurized helium tubes of dimensions 2" diam. x 10" in length. Each individual helium
tube has a specific amount of high density plastic neutron moderators to slow down the incident
energetic neutrons to an accurately estimated energy. A typical spectrometer is a set of moderator
cylinders surrounding detectors that have high efficiency for detecting thermal neutrons. The larger the
moderator, the higher the energy of incident neutrons for which the detector assembly has matched
detection efficiency. If all the detectors are exposed to the same radiation field and the efficiency as a
function of energy (response function) of each of the detectors is known, the neutron energy spectrum
can be determined from the detector count rates.
Monte Carlo simulation results of response function calculations for different arrays of helium tubes
with varying amount of moderators will be shown. Experimental evidence of effectiveness of a set of
moderated helium tubes to measure the hardness of the incident neutrons will be demonstrated.
High-current pulsed bremsstrahlung X-ray sources with endpoint energies in the 100 keV to 1 MeV regime are commonly used to radiograph dynamic events. Knowledge of the output spectra can assist in both improving these sources and in analyzing the imagery. Consequently, we have begun developing a spectrometer for this regime, which we refer to as the Collimated Step-wedge Spectrometer (CSS). It is based on a set of severl input channels, each comprising a collimator, X-ray filter, and scintillator. The scintillation light from all channels is recorded in a single from of a high-S/N CCD camera. Knowledge of the filters' attentuation and the scintillator's spectral response should allow unfolding an X-ray spectrum. An Initial test was performed with a bremstrahlung source of 300 keV endpoint energy, but with a non-optimal filter set.
High power pulsed x-ray sources (XRS) are common place for many radiographic and plasma physics applications. Developing such XRS’s for particular applications require accurate end-point voltage and dose measurements to adequately characterize and model these devices.
A simple mathematical relationship, which yields end-point energy results as function of measured peak voltage and time response, has been extracted from the data sheets and specifications of a proprietary manufactured Silicon x-ray detector (XRD). The model takes into account the fractional energy absorbed, response time, linear absorption coefficients (including both photo-electric and Compton incoherent interactions), physical geometry, and transfer function of the biasing circuit.