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%.     

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.     

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.     

Determination of counting losses and pile-up rejection in ionizing radiation spectrometry

The spectrometric system for ionizing radiation measurement with pile-up rejection and counting losses correction has been described. The results for HpGe, Ge(Li), Si(Li) and surface barrier detectors have been presented. The total count rate ranged from 500 to 10⁵ cps and different radioisotopes have been used. The counting losses correction accuracy has been within ±1% with tenfold reduction of background from pile-up pulses. The possibility of the system application for radiation intensity measurement of the mixture of short- and longlived radioisotopes has been discussed.     

Radiochemical determination of lead-210 in environmental water samples using Cerenkov counting

A relatively simple method has been developed for the determination of²¹⁰Pb via its -emitting daughter,²¹⁰Bi. Lead-210 was separated from interfering elements as lead sulphate. The precipitate was dissolved in an alkaline solution of EDTA and the Cerenkov signal produced by the build-up of²¹⁰Bi was counted 30 days after storage using tritium channel of a liquid scintillation counter. Cerenkov counting efficiency was found to be approximately 20%. A lower limit of detection of 5.1 mBq/1 (based on 3 of the background with 500 minute counting time) was achieved. Chemical recoveries in the range of 70–100% were determined gravimetrically. Interference associated with currently used methods is avoided. Data from from both spiked samples and natural samples are presented.     

Liquid scintillation and Cherenkov counting characteristics of109Cd

When¹⁰⁹Cd is placed in a liquid scintillation counter, 146±1 counts are obtained for every 100¹⁰⁹Cd nuclides which undergo decay. Thus, the average efficiency for each of the two steps in the decay process is 73%. In examining the distribution of energy among the windows available, it was noted that the distribution pattern is quite similar to that of¹⁴C. An average Cherenkov counting efficiency, in water, was found to be 0.0258±0.0005%. Both 0.010M K₂Cr₂O₇ and CHCl₃ quench the count rate, particularly in the lower energy channel, where count rates are suppressed by up to a factor of 20.     

Dead-time correction of counting losses in the γ-spectrometry of short-lived nuclides

A dead-time correction system, based on CAMAC-modules is developed for the -ray spectrometry of short-lived radionuclides. The linearity of the method is realized up to about 10⁴ cps.     

Low level determination of thallium in biological and environmental reference materials by RNAA using several counting methods

A new RNAA procedure was developed capable of low level determination of thallium in biological and environmental samples. After high fluence neutron irradiation in a nuclear reactor, wet ashing of samples and T1(I) separation by solvent extraction with sodium diethyldithiocarbamate at pH 13, several types of counting were employed to compare their detection limits and to utilize the self-validation principle of NAA. The following measurement modes were used: High efficiency counting of -rays of²⁰²T1 and Hg X-rays produced on decay of²⁰⁴T1 using a well-type HPGe detector, combined ^– ray and ^–-counting of²⁰⁴T1 with the aid of a HPGe planar detector, and liquid scintillation counting and counting of Cerenkov radiation of ^–-particles of²⁰⁴T1. The lowest detection limit of 0.034 ng of T1 was achieved on liquid scintillation counting of²⁰⁴T1. The method was applied for the analysis of biological NIST SRMs 1515, 1573a, 1577b and environmental NIST SRM 1633a. Good agreement was found between the thallium certified value in SRM 1633a and values determined in this work by all counting modes. For SRM 1573a, results in agreement were obtained by two counting modes, while counting of Hg X-rays of²⁰⁴T1 was only used for SRMs 1515 and 1577b.     

Determination of Np,Pu and Am in high level radioactive waste with extraction-liquid scintillation counting

A new method for the determination of transuranium elements, Np, Pu and Am with extraction-liquid scintillation counting has been studied systematically. Procedures for the separation of Pu and Am by HDEHP-TRPO extraction and for the separation of Np by TTA-TiOA extraction have been developed, by which the recovery of Np, Pu and Am is 97%, 99% and 99%, respectively, and the decontamination factors for the major fission products (⁹⁰Sr,¹³⁷Cs etc.) are 10⁴–10⁶. Pulse shape discrimination (PSD) technique has been introduced to liquid scintillation counting, by which the counting efficiency of -activity is >99% and the rejection of -counts is >99.95%. This new method, combining extraction and pulse shape discrimination with liquid scintillation technique, has been successfully applied to the assay of Np, Pu and Am in high level radioactive waste.     

Precise Ge(Li) spectrometry at high and variable counting rate

New circuits are presented to determine precisely the counting losses suffered in the entire gamma-ray spectrometer and to allow automatic correction for them even in the case of time-dependent counting rates as encountered in the measurement of short-lived radioisotopes. Experimental proof is given that the proposed circuitry allows accurate quantitative measurements in gamma-ray spectrometry. With counting rates up to 20,000 cps losses amount to less than 1.5%.     

Direct determination of32P or36Cl in tissue solubilizer solutions by Cherenkov-scintillation counting

Phosphorus-32 and chlorine-36 radioactivity was measured directly in commercially available tissue solubilizers using a liquid scintillation counter. Various wavelength-shifting compounds: β-naphthol, 4-methylumbelliferone, 7-amino-1,3-naphtalenedisulfonic acid, 2-hydroxy-3,6-naphthalenedisulfonic acid, anthranilic acid and salicylic acid were investigated to assess their suitability for the improvement of counting efficiency. Salicylic acid was selected which is fairly stable in alkaline solutions of tissue solubilizers and remarkably improves counting efficiency up to 90% for both nuclides. 0.1 g of soft tissues or blood can be solubilized with 1 cm³ of tissue solubilizer containing 2 g/dm³ of salicylic acid directly in 6 cm³ scintillation minivials. The sample channels ratio method for colour quench correction was found to be satisfactory.     

Trends in instrumentation for activation analysis of short-lived nuclides

This paper reports on the last year's two major activities of our nuclear instrumentation group it the field of high rate and high resolution gamma spectrometry which were mainly devoted to the needs of activation analysis of short-lived nuclides. The first of the projects was the completion of a state-of-the-art spectrometry system for very high counting rates which has been installed at the fast inrradiation and transport facility of the TRIGA reactor and now is the main instrument for the short-lived work of our radiochemistry group. Based on a laboratory-designed gated integrator pulse processing system and equipped with an Ortec Gamma-X detector of 20% relative efficiency with cooled FET and transistor reset preamplifier, it exhibits a basic resolution of 2.3 keV at 1332 keV which at a counting rate of 1.1 million cps of⁶⁰Co degrades to 3.4 keV. An essential feature of the system is a novel quantitative pileup rejector of the pulse counting type which has been specially designed for high rejection efficiency and at the same time, for the reliable exemption of valid events, and thus is a necessary prerequisite for quantitative real-time correction of counting losses by means of the Virtual Pulse Generator method. The second project included the successful implementation of the novel Preloaded Filter Technique (applied for patent), a new method for high resolution and high throughput processing of nuclear detector signals which, in contrast to conventional techniques, does not rely on a fixed pulse processing time per event which up to now was the main reason for pulse pileup and limited throughput, but, at the latest, terminates the filtering process of an individual event at the instant of arrival of the next event which results in optimized throughput and, at the same time, in a self-adapting, counting rate dependent shaping time. To that aim, the delta-noise filter of the system must be preloaded with the best estimate of the final result of the filtering process which is simply the unfiltered signal amplitude, to make sure that at the instant of termination of the filtering process the output of the filter deviates from the final value not more than by the decaying noise amplitude. Complemented by counting rate dependent step-noise filtering, this technique made possible the creation of a spectrometry system for all purposes which at low to medium counting rates is comparable to the best of the semi-Gaussian amplifiers and at high counting rates to the gated integrator. An experimental implementation of the Preloaded Filter combined with an Ortec Gamma-X detector of 15% relative efficiency resulted in a basic resolution of 1.9 keV at 1332 keV at a counting rate of 5000 cps slowly degrading to 3.2 keV at a counting rate of 650 000 cps of⁶⁰Co.     

Liquid scintillation counting of low activity63Ni

A simple and versatile method was developed to detect low specific-activity⁶³Ni with a liquid scintillation counter. High counting efficiency was obtained for a sample containing up to 220 mg of nickel using a mixture of a violet solution of nickel ammine complex and an emulsifier liquid scintillator. The detection limit was 0.06 Bq g^–1 Ni /1.6 pCi g^–1 Ni/ assuming the counting for 1000 min and 95% confidence level.     

Measurement of63Ni in highly radioactive Hanford waste by liquid scintillation counting

Characterization of high-activity radioactive waste stored in underground tanks at Hanford requires determining⁶³Ni (100 y). This low-energy -emitter (E_max 67 keV) must be separated with a high degree of radiochemical purity large amounts of other fission and activation products. The method to be discussed involves multiple precipitation steps with several holdback carriers added, followed by precipitations with dimethylglyoxime, ion exchange, and electrodeposition. The⁵⁹Ni activity is determined by low-energy photon spectrometry. The sample is then stripped from the counting disk with HNO₃, converted to the chloride form, and the⁶³Ni -spectrum is measured with high efficiency by liquid scintillation counting.Pacific Northwest Laboratory is operated for the U.S. Department of Energy under Contract DE-AC06-76RLO-1830.     

Colour quenching corrections on the measurement of 90Sr through Cerenkov counting

The determination of ⁹⁰Sr through the Cerenkov radiation emitted by its descendant ⁹⁰Y is a well-known method and firmly established in literature. Nevertheless, in order to obtain an accurate result based on a Cerenkov measurement, the experimental work must be extremely rigorous because the efficiency of Cerenkov counting is especially sensitive to the presence of colour. Any traces of colour in the sample produce a decrease in the number of photons detected in the photomultipliers and, therefore, this might cause a diminution in Cerenkov counting efficiency. It is essential not only to detect the effect of colour quenching in the sample but also to correct the decrease in counting efficiency. For this reason, colour quenching correction curves versus counting efficiency are usually done when measuring through Cerenkov counting. One of the most widely used techniques to evaluate colour quenching in these measurements is the channel ratio method, which consists of the measurement of the shift of the spectrum measuring the ratio of counts in two different windows. The selection of the windows for the application of the corrections might have an influence on the quality of the fitting parameters of the correction curves efficiency versus colour quenching degree and hence on the final ⁹⁰Sr result. This work is focused on the calculation of the counting efficiency decrease using the channel ratio method and on obtaining the best fitting correction curve. For this purpose, empirical curves obtained through artificial quenchers have been studied and the results have been tested in real samples. Additionally, given that the Packard Tri-Carb 3170 TR/SL liquid scintillation counter is a novel detector for use in Cerenkov counting, the previous calibration of the Tri-Carb 3170 TR/SL detector, necessary for the measurement of ⁹⁰Sr, is included.     

Recent progress and application of low level liquid scintillation counting

The use of an ultra low level liquid scintillation counter with extremely low background, MCA technique, storage and software evaluation of pulse height spectra has given very low LLD's, the possibility of - and -spectrometry to some extent and time saving optimization of counting conditions. Quick and very simple but yet accurate analytical methods could be worked out worked out for environmental measurements: Environmental levels of tritium,¹⁴C in several assimilation products,²²²Rn and²²⁶Ra in water without any sample pretreatment. Suggestions for possible further applications in environmenial monitoring and low level counting are given.     

Simultaneous determination of lead-210, gross beta and gross alpha activities of air filters by pulse shape discrimination liquid scintillation counting

A chemical procedure for transferring deposited solid matter from a cellulose filter into the liquid scintillation cocktail has been described. The influence of chemical and color quenching on alpha and beta detection efficiency, as well as on misclassification of beta and alpha pulses was corrected by an external standard method. Under the chosen pulse shape discrimination level (PSD), the alpha and beta detection efficiencies were above 85% and spillovers of alpha and beta pulses were below 10% and 2% respectively. Determination limits for samples containing up to 200 mg of mineral matter were 0.015 mBq m^–3 for alpha, 0.055 mBq.m^–3 for²¹⁰Pb and 0.055 mBq.m^–3 for beta activity (counting time 12000 s and volume of filtered air 1000 m³). The method has been applied for routine monitoring of²¹⁰Pb as well as for gross alpha and beta activities of longer-living radionuclides (T_1/2.>11 hrs) in suspended air matter.     

A software counting loss correction method for neutron activation analysis with short-lived radionuclides

A method has been developed for the correction of counting losses in NAA for the case of a mixture of short-lived radionuclides. It is applicable to systems with Ge detectors and Wilkinson or successive approximation ADC's and will correct losses from pulse pileup and ADC dead time up to 90%. The losses are modeled as a constant plus time-dependent terms expressed as a fourth order polynomial function of the count rates of the short-lived radionuclides. The correction factors are calculated iteratively using the peak areas of the short-lived radionuclides in the spectrum and the average losses as given by the difference between the live time and true time clocks of the MCA. To calibrate the system a measurement is performed for each short-lived nuclide. In a test where the dead time varied from 70% at the start of the measurement to 13% at the end, the measured activities were corrected with an accuracy of 1%.     

Determination of one-dimensional distribution of14C-labelled compounds

Three methods of determination of one-dimensional distribution of¹⁴C-labelled compounds are compared: measurement with position-sensitive detector /LB 282/511/, liquid scintillation counting /LSC/, and autoradiography with densitometric quantification. For a mutual comparison of these methods samples of blood serum were used, in which the fractional esterification rate /FER/ was determined by means of LCAT Test. The agreement of the methods of FER determination on the basis of the measurement with LB 282/511 or LSC was checked. In the case of autoradiography a correction for non-linearity between the blackening and the number of -particles striking the area unit was necessary.     

Correction factors required for quantitative large volume INAA

Extension of current INAA practice to accommodate large volume samples, of the order of 0.1 to 1 m³, requires consideration of the need for correction factors that are usually negligible for most small sample analyses. Direct INAA of large samples can be advantageous for materials that are not homogeneous such as contaminated soil, industrial raw materials or solid wastes. Large sample INAA can be adapted for the screening of such materials in situations when representative small samples would be difficult or tedious to obtain and analyze. However, appropriatecorrections are required to take into account neutron flux attenuation and gradients within large samples as well as absorption losses of emerging -rays and extended counting geometry. In this work, thermal neutron flux attenuation over several cm within a SLOWPOKE reflector site was measured, using monitors, to amount to 0.84 of incident flux and the quantitative sample self-shielding factors for cylindrical C₁₂H₂₂O₁₁ samples, were modeled within ±3% through adaptation of published values. Gamma-ray attenuation and variation in counting geometry over sample dimensions were experimentally determined and compared to calculated correction factors. Their computation was based on sample-detector geometry and utilized linear -attenuation coefficients of sample matrix and air. Similarly, agreement of large sample SRM's measured concentrations with certified values was within <5%, thus validating the use of these methods and their future extension to large volume sample INAA.