Low-cost activation analysis at small research reactors

A software implementation of a loss-free counting multichannel analyzer, storing immediately into the multimegabyte memory of a low-cost 486 or Pentium type PC, enables the real-time control of a rabbit system as well as the collection of up to 1000 pairs of simultaneously recorded loss-corrected and non-corrected spectra of 16 k channels each, in a true sequence without time gaps in between, at throughput rates of up to 200 kc/s.¹ Intended for activation analysis of short-lived isomeric transitions, the system renders possible peak to background optimizations and separations of lines with different half-lives without an a priori knowledge of sample composition by summing up appropriate numbers of spectra over appropriate intervals of time. By automatically adapting the noise filtering time to individual pulse intervals, the Preloaded Digital Filter (PLDF) combines low- to medium-rate resolutions comparable to those of high-quality Gaussian amplifiers with throughput rates of up to 100 kc/s, and high-rate resolutions superior to those of state-of-the-art gated integrator systems. In contrast to commercially available digital filters, the PLDF in its new implementation performs pulse shortening as well as pole zero cancellation in the analog domain. This not only results in a simpler digital core but also, for the first time, makes possible the use of a low-cost ADC in a spectrometric application.² A simple but highly effective rabbit system is made from an aluminum incore part, inexpensive plastic tubing and an industrial pressurized air generator. Large sample containers have a volume of 25 cm³. Smaller containers of 5 cm³ are automatically separated from a transport capsule. A transport time of below 0.5 s enables activation analysis of short-lived isomeric transitions. The combination of rabbit system, PLDF and software based multichannel analyzer provides a low-cost but powerful solution for NAA at Triga reactors in developing countries, a forthcoming research project of the IAEA, Vienna.     

Automatic activation analysis

Automatic activation analysis (AAA) is rendered possible by a unique neutron activation analysis facility for short-lived isomeric transitions based on a fast rabbit system with sample changer and sample separation, and an adaptive digital gamma-spectrometer for very high counting rates of up to 10⁶ cps. The system is controlled by a computer program performing irradiation control, neutron flux monitoring, and gamma-spectrometry with real-time correction of counting losses, spectra evaluation, nuclide identification and calculation of concentrations in a fully automatic procedure. As spectrometry is done by means of hundreds of sequentially measured pairs of concurrently recorded loss-corrected and non-corrected spectra, concentrations are derived from an optimally weighted average of all individual occurrences in this sequence of spectra which also enable the separation of isomeric transitions with coinciding energies but different half-lives such as ^116m2In (162.4 keV, T _1/2 = 2.2 s) and ^77mSe (162.2 keV, T _1/2 = 17.4 s). To clear up repeatedly voiced misconceptions concerning the errors of loss-free counting our findings of 1978 and 1981 are reiterated, namely that the counting error of a peak in a corrected spectrum may be derived consistently from the error of the same peak in the respective non-corrected spectrum and from the error of weighting factors in the corresponding region of interest, according to the principle of propagation of errors. Experimental proof is provided for conditions of stationary as well as rapidly varying counting rates and spectral shapes. To the memory of Vincent P. Guinn.     

Fast neutron activation analysis using short-lived radionuclides

Fast neutron activation analysis experiments were performed to investigate the analytical possibilities and prospective utilization of short-lived activation products. A rapid pneumatic transfer system for use with neutron generators has been installed and applied for detecting radionuclides with a half-life from 300 ms to 20 s. The transport time for samples of total mass of 1–4 g is between 130 and 160 ms for pressurized air of 0.1–0.4 MPa. The reproducibility of transport times is less than 2%. The employed method of correcting time-dependent counting losses is based on the virtual pulse generator principle. The measuring equipment consists of CAMAC modules and a special gating circuit. Typical time distributions of counting losses are presented. The same 14 elements were studied by the conventional activation method (single irradiation and single counting) by both a typical pneumatic transport system (run time 3 s) and the fast pneumatic transport facility. Furthermore, the influence of the cyclic activation technique on the elemental sensitivities was investigated.     

Determination of polonium-210 in phosphoric acid

Polonium-210 in phosphoric acid has been recognized as a significant source of alpha contamination of processed Si-wafers for memory devices of computer. In the present work, a convenient method was developed for the determination of trace²¹⁰Po in phosphoric acid of high purity. For the determination,²⁰⁹Po was used as a yield tracer. The present method consists of (1) addition of the tracer to 5 ml aliquot of phosphoric acid sample, (2) pH adjustment (to 2) of the sample solution to make up electrolytic solution, (3) electrodeposition for the simultaneous achievement of Po separation and preparation of counting source on stainless-steel disc, and (4) alpha-ray spectrometry. By the developed method, more than 95% of Po was separated from phosphoric acid sample onto counting disc. The minimum detectable radioactivity of²¹⁰Po in 5 ml of phosphoric acid was about 0.03 mBq by counting the electrodeposited alpha-activity for 10 days under a counting efficiency of ≈30%.     

Determination of some elements by epithermal neutron activation analysis for the Arctic aerosol

Summary We have determined nineteen trace elements in 685 aerosol filter samples collected during 1964-1978 in northern Finland by the Finnish Meteorological Institute. In this paper we present some procedures and results for very short (~25 s), short (~3-54 min), and medium (12-35 h) lived isotopes as determined by epithermal NAA in conjunction with and without Compton suppression. Elements with a I_γ/σthratio are favorable to be determined by epithermal NAA. Silver was determined by a one minute epithermal irradiation because of a very short ¹¹⁰Ag half-life. Antimony, arsenic, cobalt, bromine, indium, iodine, potassium, silicon, tin, tungsten, and zinc were determined by a ten minute epithermal irradiation. For silver determination, samples were counted without transferring the filter from the irradiated vial, however, for ten minute irradiation all samples were transferred to a non-irradiated vial and counted both in the normal and Compton mode by the HPGe gamma-spectrometry system with a decay time of about 10 minutes and counting time of 15 minutes. Each day a maximum of 16 samples were irradiated and immediately following the short counting, these samples were loaded into an automatic sample changer in sequence of irradiation and counted for an hour in both normal and Compton modes. This has proven to be an extremely cost effective measure thus reducing the need to employ long-lived NAA to analyze other elements such as Ag, Co, Sn and Zn and Ag for air pollution source receptor modeling.     

Advances in 14 MeV neutron activation analysis by means of a new intense neutron source

A new intense 14 MeV neutron generator with cylindrical acceleration structure has been put in operation at the GKSS Research Center Geesthacht. The sealed neutron tube is combined with a fast pneumatic rabbit system with particular capabilities for neutron activation analysis involving shortlived reaction products. The sample transfer time is less than 140 ms. The maximum neutron flux available for activation is 5.2·10¹⁰ n/cm²s. Theoretical sensitivity predictions made in a previous study have been verified for some important trace elements. As a first application, samples of freeze-dried suspended matter and fishes of the Elbe river were analyzed.     

Rapid and accurate determination of89/90Sr in radioactive samples by Cerenkov counting

An improved and rapid method for determination of⁹⁰Sr via its daughter nuclide,⁹⁰Y, in aqueous samples from the low-level radioactive wastes by Cerenkov counting was established. This technique is applicable to beta particles maximum energies greater than 0.263 MeV in aqueous solution. A comparison of⁹⁰Sr determination by Cerenkov counting and standard wet chemical separation techniques indicates a high degree of correlation and excellent agreement. For a 20 ml aqueous sample following the prescribed experimental conditions and a 60 min counting interval, the detection limit was 0.20 dpm/ml (0.10 pCi/ml), and the relative deviation is less then 5%.     

Measurement of High Energy Gross Beta and 40K by Cerenkov Counting in Liquid Scintillation Analyser

The Environmental Survey Laboratory is responsible for the monitoring of radioactivity due to natural and artificially produced radionuclides in a variety of samples available around the nuclear facilities at Tarapur. Standard methods of radiochemical separation and counting are followed to determine -emitters in various matrices. To make the measurement simple and detect lower levels of contamination in some of the matrices, a method was developed to measure the high energy gross -activity primarily due to ⁹⁰Y in borehole water and urine samples of occupational workers by Cerenkov counting in a liquid scintillation analyser. Cerenkov counting results of borehole samples were compared with the total gross -activity measured by a low background beta-counter. The combined measurements were used to evaluate ⁹⁰Sr and ¹³⁷Cs levels without chemical separation or gamma-spectrometry. Cerenkov counting technique was also utilised to evaluate ⁴⁰K in drinking water and intercomparison water samples. The paper presents the methodology and results of a few measurements using the technique.     

Low-level determination of plutonium by gamma and L X-ray spectroscopy

We have developed an analytical method for detection of²³⁹Pu in aqueous samples at concentrations as low as 10^–10M. This nuclear counting technique utilizes the uranium L X-rays, which follow the alpha-decay of plutonium. Because L X-rays are specific for the element and not for the individual isotope, the isotopic composition of the plutonium sample must be known. The counting efficiency in the 11–23 keV range is determined from a plutonium standard, and the concentration of the sample is then calculated from the L X-ray count and the isotopic composition. The total L X-ray count is corrected for possible contributions from other radionuclides present as impurities by measuring the low-energy gamma-spectrum for each contaminant to establish specific photon/X-ray ratios. The ratios are important when²⁴¹Pu and²⁴²Pu are measured, because the respective decay chain members produce non-U L X-rays. This new method can replace the use of labor-intensive radiochemical separation techniques and elaborate activation methods for analysis of²³⁹Pu in aqueous samples. It is also applicable for assaying plutonium in liquid wastes that pose possible hazards to the environment.     

A new highly efficient set-up for cyclic and pseudocyclic short time activation analysis with high count rates

A new counting geometry with a simple sample changer was constructed to enable cyclic and pseudocyclic short-time activation analysis. With the new system it is possible to cycle a sample, or successively an indefinite number of samples up to 20 times. The sample changer acts at the same time as sample catcher for two n-type HPGe detectors and can release the sample into a well-type HPGe detector. The new system enables the simultaneous counting of the irradiated samples by means of two endcap HPGe detectors, and subsequent counting by means of the well HPGe detector or both detector types. A well detector ensures a high counting efficiency which improves the sensitivity of a large number of short lived nuclides. Some standard reference materials (i.e., BCR-176, NIST SRM 1633b, IAEA-336, 335b, 335c) were prepared and analysed in replicates. The results indicate that up to 46 nuclides can be determined in BCR-176 if the samples are irradiated with and without the⁶LiD converter. An automatic evaluation programme was developed that determines the FWHM calibration parameters for each spectrum for accurate peak-area estimation at high count rates.     

14 MeV neutron reactions producing gamma-ray emitting nuclides with half-lives below 3 seconds

Fast neutron activation analysis (FNAA) experiments were performed using a Cockroft-Walton neutron generator and a fast pneumatic rabbit system with a sample transport time of 120 ms. With this facility cyclic activations of¹⁸O, Zr, Ba and Pb leading to the short-lived reaction products¹⁵C,^90mZr,^136mBa and^207mPb were investigated. Derived from these measurements the analytical sensitivities of the involved reactions will be discussed.     

Rapid determination of radiostrontium in seawater samples

A new method for the determination of radiostrontium in seawater samples has been developed at the Savannah River National Laboratory (SRNL) that allows rapid pre-concentration and separation of strontium and yttrium isotopes in seawater samples for measurement. The new SRNL method employs a novel and effective pre-concentration step that utilizes a blend of calcium phosphate with iron hydroxide to collect both strontium and yttrium rapidly from the seawater matrix with enhanced chemical yields. The pre-concentration steps, in combination with rapid Sr Resin and DGA Resin cartridge separation options using vacuum box technology, allow seawater samples up to 10 L to be analyzed. The total ⁸⁹Sr + ⁹⁰Sr activity may be determined by gas flow proportional counting and recounted after ingrowth of ⁹⁰Y to differentiate ⁸⁹Sr from ⁹⁰Sr. Gas flow proportional counting provides a lower method detection limit than liquid scintillation or Cerenkov counting and allows simultaneous counting of samples. Simultaneous counting allows for longer count times and lower method detection limits without handling very large aliquots of seawater. Seawater samples up to 6 L may be analyzed using Sr Resin for ⁸⁹Sr and ⁹⁰Sr with a minimum detectable activity (MDA) of 1–10 mBq/L, depending on count times. Seawater samples up to 10 L may be analyzed for ⁹⁰Sr using a DGA Resin method via collection and purification of ⁹⁰Y only. If ⁸⁹Sr and other fission products are present, then ⁹¹Y (beta energy 1.55 MeV, 58.5 day half-life) is also likely to be present. ⁹¹Y interferes with attempts to collect ⁹⁰Y directly from the seawater sample without initial purification of Sr isotopes first and ⁹⁰Y ingrowth. The DGA Resin option can be used to determine ⁹⁰Sr, and if ⁹¹Y is also present, an ingrowth option with using DGA Resin again to collect ⁹⁰Y can be performed. An MDA for ⁹⁰Sr of <1 mBq/L for an 8 h count may be obtained using 10 L seawater sample aliquots.     

Enrichment of trace anions from water with 2,2'-diaminodiethylamine cellulose filters

Cellulose filters with immobilized 2,2'-diaminodiethylamine (DEN) functional groups are studied for trace anion preconcentration from aqueous solution, with subsequent x-ray fluorescence measurements. For most oxoanions with a central metal atom, nearly quantitative collection can be achieved by 10-cm² DEN filters under the following optimized conditions: pH 3–6, filtration rate up to 0.5 ml cm^-2 min^-1, and sample volume up to 100 ml cm^-2. The collection yield is independent of the trace oxoanion concentration up to at least 1.5 μmol cm^-2. Although the DEN filter exhibits some selectivity towards oxoanions with a central metal atom, ionic strength affects the results; the collection efficiency is strongly depressed with salt (e.g. NaCl) concentrations above 0.01 M. The applicability of the DEN filter in anion collection is therefore limited to dilute solutions.     

An improved method for low-level222Rn determination in environmental waters by liquid scintillation counting with pulse shape analysis

A method in which²²²Rn is extracted from 0.5 1 water samples to 20 ml toluene is described. 10 ml toluene solution with extracted²²²Rn is directly added to a glass scintillation vial containing 10 ml of liquid naphthalene based scintillation cocktail. Apart from diluting by toluene, the final counting solution still has excellent properties in terms of: / separation by pulse shape analysis, detection efficiency and background in the -region. The detection limit of²²²Rn for 0.5 1 water samples was 1.5 mBq l^–1 (for 12,000 s count time). The concentration of²²²Rn in different environmental samples such as rain, tap and mineral waters as well as deep well waters were determined.     

Optimization of liquid scintillation counting techniques for the determination of carbon-14 in environmental samples

The goal of this work was to optimize the liquid scintillation counting techniques for the determination of¹⁴C in stack effluent gases and in environmental samples such as biological and air samples. Carbon-14 activities in most environmental samples were measured with the direct CO₂ absorption method. The highest figures of merit were found through the variation of Carbosorb E and Permafluor V ratio, and measurement windows. The best condition was an 1:1 volume ratio. Average 2.35 g of CO₂ was reproducibly absorbed in the 20 ml mixture within 40 minutes. The counting efficiency determined by repeated analysis of NIST oxalic acid standard and the background count rate were measured to be 58.8±1.4% and 1.88±0.06 cpm, respectively, in case of saturated solution. The correction curves of counting efficiency for partially saturated solutions and for saturated solutions with quenching were prepared, respectively. The overall uncertainty of the sample specific activity for near background levels was estimated to be about 7% for 4 hours counting at 95% confidence level. Stack effluent gas samples were measured by a gel suspension counting method. After precipitation of CO₂ in the form of BaCO₃, 140 mg of which was mixed with 6 ml H₂O and 12 ml of Instagel XF. The counting efficiency was measured to be 71.5±1.7% and the typical sensitivity of this technique was about 510 mBq/m³ for a 100 min count at a background count rate of 4.7 cpm. For the benzene counting method measurements were performed with a mixture of 3 ml benzene and 1 ml of scintillation cocktail (5 g of butyl-PBD in 100 ml of scintillation-grade toluene) in a low potassium 7 ml borosilicate glass vial. The counting efficiency and the background count rate were measured to be 64.3±1.0% and 0.51±0.05 cpm, respectively. The long-term stability of samples has been checked for all the counting techniques over a two week period, during which no apparent change in counting efficiency and background level was found.     

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.     

A sequential radiochemical procedure for isotopic analysis of uranium and thorium in soil

A sequential radiochemical procedure for isotopic analysis of uranium and thorium in soil has been developed. Analysis involves total dissolution of the samples to allow equilibration of the natural isotopes with added tracers, followed by radiochemical separation using anion exchange chromatography (BioRad AG 1–X8). Further separation and purification is performed employing solvent extraction techniques. Finally, the U and Th fractions are co-precipitated with lanthanum and cerium fluoride, respectively, and quantified by alpha-particle spectrometry. Overall chemical yields range from 60 to 90%. Under normal operating conditions and present counting set up, the minimum detectable concentration (MDC) is approximately 2 Bq/kg for soil samples. This is based on one gram aliquot of sample, 80% chemical yield, and 1000 minute counting with a detector having about 15% counting efficiency. The procedure has been successfully tested with Standard Reference Materials. Various soil samples were analyzed with high chemical yields and fine quality of alpha-spectra. Decontamination factor studies were performed to determine the extent of the carry over of²¹⁰Po,²²⁵Ac,²²⁶Ra, and²²⁹Th into U fraction and²¹⁰Po,²²⁵Ac,²²⁶Ra, and²³²U into Th fraction.     

Determination of Hg from Cu concentrates by X-ray fluorescence through preconcentration on polyurethane foam

A methodology was developed for the separation and determination of microamounts of mercury from copper concentrate samples by wavelength dispersive X-ray fluorescence (WDXRF) after solid-phase extraction of mercury from iodide medium using polyurethane foam (PUF). The best sorption conditions for the Hg-KI-PUF system were settled using X-ray fluorescence technique after collection of ground PUF on a filter paper by vacuum filtration and direct measurement of the intensity signal of the sorbed mercury on PUF. The main parameters of sorption such as iodide concentration, pH, shaking time and sample dilution effect were studied. The system shows rapid kinetic sorption and maximum X-ray intensity signal was achieved after shaking for 2 min a 0.01 mol l⁻¹ iodide solution containing microamounts of mercury in the pH range from 1.0 to 9.0. Effective sorption up to a volume of 0.9 l allows preconcentration of mercury. A linear fit up to 50 μg mercury was obtained by the plot of the initial mercury mass in the bulk solution (0.5 l) vs. its respective XRF intensity signal measurement on ground PUF after the sorption process. The calibration sensitivity, quantification and detection limits found were 9.09 CPS μg⁻¹, 9.0 and 2.7 μg, respectively. The sorption of many elements was also evaluated under the best conditions. High concentrations of Cu(II) and Fe(III) interfere seriously. Mercury-selective separation could be achieved using citrate or EDTA as masking agent; no interference due to copper matrix samples was observed in citrate medium. This methodology was evaluated by recovery for mercury determination in copper concentrate ore samples supplied by a mining industry and copper sulfate salts; the results were between 98% and 106%.     

An automatic NAA analyzer for short-lived nuclides

An automatic activation analyzer with sample changer, pneumatic transfer system and fast pulse counting with real time pulse pile-up and dead time compensation is described. Transfer times between 80 and 300 ms from irradiation position to measurement station can be obtained. Counting losses are corrected within 10% up to total count rates of 120 kc/s.     

Cesium sorption from concentrated acidic tank wastes using ammonium molybdophosphate-polyacrylonitrile composite sorbents

The radiological examination of water requires a rapid screening that permits the determination of gross alpha-activity in order to decide if further radiological analyses are necessary. The usual method of gross alpha-determination includes sample evaporation to dryness on a disk and counting using a ZnS(Ag) scintillation detector or a proportional counter. The determination of gross alpha-activity in sea water using this method has a number of drawbacks such as high mass attenuation, lack of homogeneity in the precipitate, and very small volume of sample. In this work, coprecipitation is proposed to enable a 500 ml sample of water to be analyzed. The minimum detectable activity (MDA) achieved was 0.004 Bq^.l^–1 for a counting time of only 4 hours. Weight variation, self-absorption factor, efficiency and blanks were studied. In order to test reproducibility of the method, several Spanish coastal sea water samples were analyzed by two different laboratories.