Mohini W. Rawool-Sullivan

A method for correcting NaI spectra for attenuation losses in hand-held instrument applications

In this paper we present the gross-count material basis set (GC-MBS) method for estimating transmission losses in measurements of radionuclides using low-resolution gamma-ray detectors such as NaI or CdZnTe. The application to continuum-subtracted peaks measured with high-purity Ge detectors is also discussed. To illustrate the method, we apply it to the correction of spectra from enriched uranium that have been distorted by various intervening absorbers. We also examine the application to simultaneous determination of the uranium enrichment through an unknown absorber using an NaI detector.

An innovative method for extracting isotopic information from low-resolution gamma spectra

Abstract A method is described for the extraction of isotopic information from attenuated gamma-ray spectra using the gross-count material basis set (GC-MBS) model. This method solves for the isotopic composition of an unknown mixture of isotopes attenuated through an absorber of unknown material. For binary isotopic combinations the problem is nonlinear in only one variable and is easily solved using standard line optimization techniques. Results are presented for NaI spectrum analyses of various binary combinations of enriched uranium, depleted uranium, low burnup Pu, 137 Cs, and 133 Ba attenuated through a suite of absorbers ranging in Z from polyethylene through lead. The GC-MBS method results are compared to those computed using ordinary response function fitting and with a simple net peak area method. The GC-MBS method was found to be significantly more accurate than the other methods over the range of absorbers and isotopic blends studied.

A simple algorithm for estimation of source-to-detector distance in Compton imaging

Compton imaging is used to predict the location of gamma-emitting radiation sources. The X and Y coordinates of the source can be obtained using a back-projected image and a two-dimensional peak-finding algorithm. The emphasis of this work is to estimate the source-to-detector distance (Z). The algorithm presented uses the solid angle subtended by the reconstructed image at various source-to-detector distances. This algorithm was validated using both measured data from the prototype Compton imager (PCI) constructed at the Los Alamos National Laboratory and simulated data of the same imager. Results show this method can be applied successfully to estimate Z, and it provides a way of determining Z without prior knowledge of the source location. This method is faster than the methods that employ maximum likelihood method because it is based on simple back projections of Compton scatter data.

Estimation of obliquely scattered gamma-ray response functions in the gross-count tomographic gamma scanner (GC-TGS) method

It was recently shown that a logarithmic response-function technique based on the material basis set (MBS) formalism used in the tomographic gamma scanner (TGS) method allows gross spectra from NaI detectors to be used in both measuring MBS transmission corrections using external transmission sources and in applying the corrections to emission spectra to arrive at radionuclide mass estimates. In this work we have attempted to show that addition of the oblique scatter component can increase the accuracy of the GC-TGS measurements with both NaI and high-purity germanium detectors. This paper describes the formalism behind the GC-TGS method and the improvement achieved in the analysis by including the scatter component.

{pi}{sup +}+d{r_arrow}p+p reaction between 18 and 44 MeV

A study of the reaction {pi}{sup +}+d{r_arrow}p+p has been performed in the energy range of 18{endash}44 MeV. Total cross sections and differential cross sections at six angles have been measured at 15 energies with an energy increment of 1{endash}2 MeV. This is the most systematic data set in this energy range. No structure in the energy dependence of the cross section has been observed within the accuracy of this experiment. {copyright} {ital 1997} {ital The American Physical Society}

Extended radiation source imaging with a prototype Compton imager

This paper reports results from a prototype Compton imager (PCI) which consists of three planes of silicon pixel detectors as a scattering detector followed by an array of CsI(Tl) crystals as an absorbing detector. The CsI(Tl) array is mounted directly behind the silicon detectors. Simple back-projection algorithms are not sufficient to resolve extended shapes, but iterative algorithms provide the necessary de-convolution. List-mode maximum likelihood expectation maximization (LM-MLEM) is an iterative algorithm that reconstructs the most probable source distribution for a given data set. LM-MLEM attempts to reconstruct an image by finding successive approximations to the true source data. Each data set imaged is different, but the number of iterations required for convergence is typically 10-30 for the PCI. In this paper, reconstructed images of point and extended sources using measured PCI data are presented. Data are corroborated using GEANT4 simulations.

The LANL prototype Compton gamma-ray imager: design and image reconstruction techniques

Today we face serious threats from radiological dispersion devices, improvised nuclear devices, and unsecured nuclear materials. Detectors which are currently used to detect and characterize these radioactive materials (or the radioactive materials within larger assemblies) suffer from large background rates. In addition, these detectors have only minimal ability to localize the position of the source without the use of mechanical collimators - which reduce efficiency. Imaging detectors using the Compton scattering process have the potential to provide greatly improved sensitivity through their ability to reject off-source background by localizing the source. We are developing a prototype device using readily available detector systems to explore Compton imaging technology. Our aim is to build a proof-of-concept device to study the Compton technology and to benchmark simulation efforts that will guide development of larger, more efficient devices that would be needed for deployment in the field. Here we discuss the concept of our detector design and results from Monte Carlo simulations of our prototype detector. We present an extension of an imaging technique used for gamma-ray astronomy to near-field sources

Application of the gross-count material basis set (GC-MBS) method to the analysis of CdZnTe spectra

The gross-count material basis set model (GC-MBS) for response-function fitting is applied to CdZnTe spectra. This method has been developed to solve for the isotopic composition of an unknown mixture of radionuclides attenuated through absorbers of unknown material. Results are presented for source strength and isotopic composition measurements of binary combinations of highly-enriched uranium (HEU), depleted uranium (DU), Np-237, Ba-133, Cs137 and variable Pu burnups attenuated through a suite of absorbers ranging in Zeffective from polyethylene through lead. Comparisons are made between GC-MBS, net area method and simpler ordinary response-function methods.

Temperature effects on CdZnTe detector performance

CdZnTe detectors are potential replacements for traditional room temperature detectors such as NaI in many applications. One particularly suitable application could be their use in portable field instruments for isotopic identification. To fully exploit them for this purpose, however, their behavior in conditions likely to be encountered in the field must be fully characterized and understood. At Los Alamos National Laboratory, we are studying one of these conditions, the effect of temperature extremes, on CdZnTe detectors. In the case of a 1/spl times/1/spl times/1 cm/sup 3/ detector with a coplanar anode structure, we have found temporary degraded performance at elevated temperatures near 50/spl deg/C and a failure mode at temperatures below -20/spl deg/C. This paper discusses the performance of two CdZnTe crystals when exposed to temperature extremes.

Response of a sampling calorimeter to low energy pions, muons, and positrons

Abstract Measurements of the response of the participant calorimeter to 250–400 MeV/c π+, μ−, and e+ are described. The participant calorimeter is a Pb/Fe/scintillator sampling calorimeter with a novel wavelength shifting fiber optic readout which is used in experiment 814 at Brookhaven National Laboratory. The e+/π+ response ratio at 250–400 MeV/c is larger than it is at higher momenta. Previous measurements of the e/π response ratio with sampling calorimeters found that the value decreased as the particle energies were reduced below about 1 GeV. This difference is attributed to the different absorption probabilities for π+ and π− at low momentum.