Quantitative gamma-ray spectroscopy of <Superscript>237–239</Superscript>Np

Accurate assays of radioactive materials by gamma-ray spectroscopy depend on many factors, the most obvious being accurate gamma-ray intensities. We have used radiochemical methods to prepare pure samples of actinides. One portion of each of these samples was used to measure disintegration rates (generally by liquid scintillation counting), and the remainder was used to obtain count rates of specific gamma-rays in a well-characterized HPGe spectrometer. These methods are illustrated by results from studies of neptunium isotopes 237, 238, and 239.     

An activation analysis system for short-lived radioisotopes including automatic dead-time corrections with a microcomputer

A system based on an IBM-PC microcomputer coupled to a Canberra Series 80 multichannel analyser has been developed for activation analysis with short-lived radioisotopes. The data transfer program can store up to 77 gamma-ray spectra on a floppy disk. A spectrum analysis program, DVC, has been written to determine peak areas interactively, to correct the counting losses, and to calculate elemental concentrations.     

Maintaining Accuracy in Gamma-Ray Spectrometry at High Count Rates

Gamma-ray spectrometry losses through pulse processing dead time and pile-up are best assayed with an external pulse technique. In this work, the virtual pulse generator technique as implemented commercially with the Westphal loss free counting (LFC) module is set up and tested with four high resolution gamma-ray spectrometers. Dual source calibration and decaying source techniques are used in the evaluation of the accuracy of the correction technique. Results demonstrate the reliability of the LFC with a standardized conventional pulse processing system. The accurate correction during high rate counting, including during rapid decay of short lived activities, has been the basis for highly precise determinations in reference materials studies.     

High resolution gamma-spectroscopy well logging system

A Gamma Spectroscopy Logging System (GSLS) has been developed to study sub-surface radionuclide contamination. The absolute counting efficiencies of the GSLS detectors were determined using cylindrical reference sources. More complex borehole geometries were modeled using commercially available shielding software and correction factors were developed based on relative gamma-ray fluence rates. Examination of varying porosity and moisture content showed that as porisity increases, and as the formation saturation ratio decreases, relative gamma-ray fluence rates increase linearly for all energies. Correction factors for iron and water cylindrical shields were found to agree well with correction factors determined during previous studies allowing for the development of correction factors for type-304 stainless steel and low-carbon steel casings. Regression analyses of correction factor data produced equations for determining correction factors applicable to spectral gamma-ray well logs acquired under non-standard borehole conditions.     

Loss-free counting with a digital spectrometer and a software program

The computer program LFREE was written to do loss-free counting with a digital spectrometer. It runs in parallel with the normal data acquisition software and corrects the counting losses once per second without interrupting data acquisition. The spectrometer's live time clock is used to measure the live time fraction. Tests showed that losses are accurately corrected at variable count rates which cause dead times as high as 80%. For half-lives of the order of 10 seconds, the accuracy is limited by the response of the live time clock to very rapidly changing count rates.     

An automated gamma-ray counting system for fast neutron activation analysis

An automated gamma-ray counting system was designed and built for use in counting long-lived nuclides (T>hr) produced during analysis by fast neutron activation. The system is mechanically simple, vet interfaced to sophisticated control and counting equipment for completely automated counting.     

Real-time correction of counting losses in nuclear pulse spectroscopy

Reviewing the current status of real-time correction of counting losses in nuclear pulse spectroscopy, the pileup problem is identified as the last question not resolved satisfactorily up to now. Correction of pileup losses in provided, at least in principle, by the classical pulse generator method, however, severe limitations in test frequency prohibit its application to real-time correction of counting losses. A solution is offered by the novel principle of the virtual pulse generator which obviates the shortcomings of the classical method simply by not introducing pulses into the spectroscopy system. Instead, the probability for pileup-free pulse processing is determined by suitable tests of the system status at arbitrarily high test frequencies. After a discussion of the principles of the new method and its application to a real-time correction system experimental evidence is provided for the complete correction of counting losses of more than 98% under conditions of stationary as well as variable counting rates up to the limit of stable operation of the underlying spectroscopy system which is 800 000 c/s for an experimental high-rate gamma spectrometer.     

A comparative study of some nuclear methods for235U/238U isotopic ratios determination

In the present work, a comparative study is made among nuclear methods for²³⁵U/²³⁸U ratios determination: activation analysis followed by high-resolution gamma-ray spectrometry, delayed neutron counting, passive gamma-ray and alpha spectrometry. Activation analysis followed by high-resolution gamma-ray spectrometry yielded a relative standard deviation down to 0.1% and a relative error of about 1% for standards of uranium enriched to 14%. Passive gamma-ray spectrometry using Ge(Li) detectors yielded a relative error down to 0.1% for enriched uranium and values even lower for the standard deviation. Passive gamma-ray spectrometry using Low Energy Photon Detector (LEPD) yielded a precision of 0.2% and a still better accuracy for enriched standards. In the case of alpha spectrometry, a relative error down to 0.5% and a precision of about 1% were obtained, also for enriched uranium standards. Delayed neutron counting allowed a relative standard deviation of about 7% and a relative error of about 2%, for standards of depleted uranium.     

Improved radioxenon gamma-spectrometry counting system and its efficiency calibration: Monte Carlo simulation and experimental results at enriched xenon counting environment

To give satisfactory efficiency both for X- and gamma-ray photon, an improved counting system has been developed in CTBT Canadian radioxenon laboratory. The counting system consists of a BEGe detector coupled with a thin carbon fiber window counting cell, that can perform a reliable and efficient radioxenon measurement. A semi-empirical calibration procedure was adopted, which is a combination of experimental measurement and mathematical simulation. Mathematical calibration tool is Monte Carlo simulation software named VGSL. Advanced gamma-spectrum analysis software, named Aatami, was used for gamma-ray peak shape fitting and X-ray multiplets deconvolution. The calculated full energy peak efficiency curve covers from 30 to 700 keV and agrees well with experimental data points within 2%. The efficiency curve can provide radioxenon analysis both for X-rays and gamma-rays with high quality. The efficiency distortion near xenon k-absorption edge of 35 keV, which is caused by high concentrated xenon in the counting cell, is also discussed.     

The perfection of loss-free counting

Pileup losses in nuclear pulse spectrometry also depend on energy as lower energies produce narrower pulses which in turn have better chances to avoid pulse pileup. Consequently, in our present system individual energy-dependent pileup correction factors are calculated for all events, making it what very probably may be called the first perfect implementation of Loss-Free Counting. Temporal response and quantitative performance of the new system are tested over the whole range of counting rates (up to 10⁶ c/s) and counting losses (up to 99%) by means of short-lived isomeric transitions and a fast rabbit system.     

Uranium assay of fuel rods by passive gamma-ray spectrometry

The spontaneous gamma-rays characteristic of uranium isotopes can be detected and measured in order to identify the isotopic composition of uranium and to assay its total amount in various objects and materials. In order to test these methods of passive gammaray spectrometry in practice, the²³⁵U-enrichment of a known fuel rod was determined by counting 186 keV gammas from²³⁵U with Ge(Li) detectors of different volumes. The 1001 keV gammas characteristic to²³⁸U were counted too. Expected counting rates of 186 keV and 1001 keV gammas for WWER fuels are given and the suitability of passive gamma-ray spectrometry for the determination of²³⁵U-enrichment and for the assay of total uranium in reactor fuels are discussed based on these measurements.     

Cerenkov counting rates of radionuclides for activation analysis purposes

Cerenkov counting efficiencies for a large number of radionuclides formed by (n, γ) reactions and used in the field of activation analysis have been obtained by two different experimental procedures. A comparison between the Cerenkov counting technique and scintillation low-resolution gamma-ray spectrometry is also presented. Results are summarized in tables and briefly discussed.     

Anticosmic-shielded ultralow-background germanium detector systems for analysis of bulk environmental samples

A low-level gamma-ray counting system has been developed which reduces system background, relative to other typical low-background systems, by a factor of ten in the energy region below one MeV, and by as much as a factor of forty at higher energies. This germanium-diode gamma-ray spectrometer was constructed for a modest investment above that required for a conventional germanium detector. The techniques involved use: (1) materials of known radiopurity to surround the diode, (2) an active external anticosmic shield to reduce the background continuum due to interactions of cosmic particles with the detector and passive lead shielding, and (3) nitrogen exhausted from the cryogenic dewar to minimize the introduction of ubiquitous radon decay nuclei into the sample counting chamber. A novel method for handling samples prior to counting is presented. Also; some of the difficulties encountered in calibrating a system intended for bulk samples are discussed.Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.     

Validation of a loss-free counting system for neutron activation analysis with short-lived indicators

The use of loss-free counting systems makes possible the exact correction for pile-up and dead-time losses during counting of a mixture of short-lived radionuclides even at very high count rates. However, counting statistics cannot be calculated by taking into account only the Poisson distribution of the incoming -quanta, such as is done in existing computer programs for -spectrometry. At moderate count rates Müller statistic was found to account for the observed variability between duplicate countings; however, at higher count rates the variability of weighing factors was found to be significant in comparison with the Müller statistic. While counting statistics could not be correctly estimated for short-lived species, experiments showed excellent accuracy for initial dead times up to 90%.     

Optimized automated activation analysis

A combination of special techniques has been developed for optimization of experimental conditions in order to improve the analytical capability, to facilitate automation and to broaden the applicability of instrumental neutron activation analysis. The techniques used are: (1) compensation for the rapid radioactive decay of short-lived nuclides with the increase of the counting efficiency by automatic source movement to the detector during the counting period, to minimize count rate variations and to prolong the counting period, (2) repeated cyclic and cumulative activation to improve the counting statistics, (3) instrumental correction of counting losses at high and varying count rates by a loss-free counting system and (4) differentiation of the reactor neutron spectrum to enhance the counts from the nuclides of interest by reducing matrix interferences. By optimized combination and automation of these techniques significant improvement of the capability of instrumental neutron activation analysis can be achieved.     

Background of low-level gamma-ray spectrometer due to the atmospheric 220Rn and 222Rn

In connection with low-level gamma-ray counting of natural samples, background due to 220Rn- and 222Rn-daughters was monitored. The results obtained for 4 months showed that the background gamma-ray from 222Rn-daughters was more variable than that of 220Rn-daughters. An efficient air-conditioning was helpful to keep the background stable. It was practically equivalent to the use of N2-gas. Radiometric data for dust samples filtered from the laboratory air suggested that Rn-daughters tended to be removed by air-conditioning.     

Low-level radioactive waste measurement comparisons of three gamma-ray counting systems

Three different gamma-ray counting systems constructed by 1–3 HPGe detectors were used in this study to compare their system performance. One measurement scheme involved positioning a single HPGe detector on a movable cart with a 90° collimation angle to the observed item. The other two waste assay systems were configured with two or three HPGe detectors towards the sample drums, while the three-HPGe-detector counting system was in a shielded counter cavity. The measurement consistency of 38 low-level waste drums, system operating costs and acquisition times to achieve the same MDA for these counting systems were compared and discussed in this study.     

Radon Mapping of Soils in St. Elizabeth, Jamaica

Soil gas measurements of radon were made in St. Elizabeth, Jamaica using nuclear track etch detectors. The results were compared to gamma-ray spectroscopy measurements made in the laboratory on soil samples collected at the radon measurement sites. An assessment of the degree of disequilibrium of uranium and thorium was also made by comparison with neutron activation analysis, delayed neutron counting and equivalent uranium and thorium measurements. The results to date for equivalent uranium and radon show a strong correlation and indicate the possibility of soil radon mapping in Jamaica using gamma-ray spectroscopy. Three anomalous sites have been identified.     

Empirical method of counting losses correction in gamma-ray spectrometry at elevated counting rate

Empirical method of counting losses correction in -ray spectrometry at elevated /up to 1000 cps/ counting rate is suggested. Using experimental data it was found that a counting losses correction coefficient was a lineare function of true fractional deadtime of spectrometer. It was shown that counting losses in peak area of⁶⁰Co /1332 keV/ corrected by the empirical method did not exceed 1.2% with fractional dead-time up to 35%.