Design and optimization of a proportional counter for the absolute determination of low-energy x-ray emission rates.

We recently presented the construction of a proportional counter for the absolute measurement of low-energy x-ray emission rates. Its main features were presented in a previous study, along with the testing of its performance as an absolute technique. Full details on its design, construction, and characterization processes are given in this paper. First, the analytical calculations and Monte Carlo simulations that led to the choice of optimal dimensions will be presented, followed by the study of the charge creation and the modeling of the electric field that confirmed that the criteria for a reproducible charge multiplication are met. Afterward, details on the electronics and gas control system will be provided. The dimensional measurements for the accurate determination of the solid angle will be described in detail. Special emphasis will be placed on the production of the detector window, which was made of reduced graphene oxide instead of beryllium. Some tests on the operation of the counter will be presented, namely, the measurement of spectra of several radionuclides that emit x-rays between 3 keV and 9 keV, as well as the fluorescence spectrum of chlorine, at 2.6 keV. A brief explanation of the determination of emission rates will be given, including the approach adopted for the treatment of the escape-peak area. The emission rate measured with the proportional counter was converted into activity concentration using the emission probabilities from the bibliography, in order to compare the results with those obtained by liquid scintillation counting, and both results were found to be in good agreement.

Absolute determination of low-energy X-ray emission rates with a proportional counter.

A defined-solid-angle proportional counter has been designed for the absolute determination of X-ray emission rates between 2 keV and 10 keV. The main features of the counter are described in this work, and spectra of two radionuclides with low-energy X-ray emissions are presented. The activity concentration of an 55Fe solution was determined by measuring the emission rate of three samples and using tabulated emission probabilities. This activity concentration is compared with the value obtained by liquid scintillation counting, and an excellent agreement has been found.

Alpha-particle emission probabilities of 231Pa derived from first semiconductor spectrometric measurements.

The nuclide 231Pa is a member of the 235U decay chain. It is a complex alpha emitter with 25 identified alpha emissions. Formerly published alpha-particle emission probabilities were derived from measurements taken with magnetic spectrometers. This work presents the first measurements made with semiconductor detectors. High-resolution alpha-particle spectrometry was carried out at CIEMAT and JRC using ion-implanted planar silicon detectors. Alpha-particle emission probabilities of 23 transitions were derived from deconvolutions of the spectra. For the major lines, uncertainties are lower than 1%, a significant improvement to existing data. The new data set will allow a more accurate evaluation of the decay scheme of 231Pa.

Determination of activity meter settings for the PET nuclides 44Sc and 89Zr.

Radionuclide activity meters depend on "calibration setting numbers" that are specific for a certain combination of radionuclide, volume and geometry. This work describes their determination for two PET emerging nuclides (44Sc and 89Zr), three commercial activity meters (CAPINTEC CRC15 BETA, ATOMLAB BIODEX 500 and VEENSTRA VDC 405) and two geometries: 1 mL plastic syringes and 5 mL glass vials. For 44Sc, only values for the dial setting for 5 mL ampoules were reported previously and its use for the assayed geometries would lead to significant differences. For 89Zr more data were available in the literature, and results are provided for new geometries. All results rely on the absolute standardisation in activity of solutions of both radionuclides.

Measurement of absolute γ-ray emission probabilities in the decay of 235U.

Accurate measurements were performed of the photon emission probabilities following the α decay of 235U to 231Th. Sources of highly enriched 235U were characterised in terms of isotopic composition by mass spectrometry and their activities were standardised by means of alpha-particle counting at a low defined solid angle. The standardised sources were subsequently measured by high-resolution γ-ray spectrometry with calibrated high-purity germanium detectors to determine the photon emission probabilities. Four laboratories participated in this work and reported emission probabilities for 33 γ-ray lines. Most of them agree with previously published evaluated data. In addition, new values are proposed for γ-lines which have been measured only once in the past.

Standardisation and half-life of 89Zr.

The nuclide 89Zr is being tested for the labelling of compounds with long blood circulation times. It decays by beta plus emission (22.8%) and by electron capture (77.2%) to 89Y. Its half-life has been determined by following the decay rate with two measurement systems; an Ionisation Chamber and an HPGe detector. The combination of six results gives a value of T1/2 = 78.333 (38) h, slightly lower than the DDEP recommended value of 78.42 (13) h. This radionuclide has also been standardised by liquid scintillation counting, 4πγ counting and coincidence techniques.

Study of the 243Am decay.

Gamma-ray emission probabilities for 243Am were published in 1996. Three new gamma transitions were then found: 46.84, 98.36 and 102.02keV. In the last evaluation by the Decay Data Evaluation Project, the emission probabilities of 46.84 and 102.02keV transitions were not included. Alpha-gamma coincidence measurements have been now used to check and revise the values for these two transitions and their emission probabilities. The emissions of the 31.13, 43.53, 74.66, 86.71 and 141.89keV transitions were also studied.

Concept design of a time-of-flight spectrometer for the measurement of the energy of alpha particles.

The knowledge of the energies of the alpha particles emitted in the radioactive decay of a nuclide is a key factor in the construction of its decay scheme. Virtually all existing data are based on a few absolute measurements made by magnetic spectrometry (MS), to which most other MS measurements are traced. An alternative solution would be the use of time-of-flight detectors. This paper discusses the main aspects to be considered in the design of such detectors, and the performances that could be reasonably expected. Based on the concepts discussed here, it is estimated that an energy resolution about 2.5keV may be attainable with a good quality source.

Direct measurement of alpha emission probabilities in the decay of 226Ra.

High-resolution alpha-particle spectrometry was performed to determine the main alpha-particle emission probabilities in the decay of 226Ra. Thin, homogeneous sources were prepared by electrodeposition on stainless steel disks. Alpha spectra with an energy resolution of 20keV were obtained in three laboratories and analysed with different deconvolution algorithms. In two set-ups, a magnet system was used to deflect conversion electrons to avoid their coincidental detection with the alpha particles. Spectra taken at close range without a magnet system yielded biased results which cannot be fully compensated by statistical corrections for coincidence summing. The derived emission probabilities of the three main alpha decays are 94.07 (1)%, 5.93 (1)%, and 0.0059 (15)%, respectively. They are in excellent agreement with calculated values derived from the P(γ+ce) decay scheme balance, which solves the existing discrepancy problem with two previous direct measurements published in literature.

Standardisation and precise determination of the half-life of (44)Sc.

The half-life of the positron-emitter (44)Sc has been determined by following the decay rate with two measurement systems; an Ionisation Chamber and a HPGe detector. The combination of seven results gives a value of T1/2=4.042 (25)h, about 2% higher than the recommended value of T1/2=3.97 (4)h (Browne, 2011) and with a lower uncertainty. This radionuclide has also been standardised by coincidence counting, and liquid scintillation counting techniques. A (44)Ti/(44)Sc generator developed at CIEMAT was used to obtain the (44)Sc solutions used in all measurements.

Preparation and characterisation of a (226)Ra spiked slag as reference material for radioactive control of steelworks.

One of the issues of the European Research Project MetroMetal is to develop reference materials in order to provide SI-traceable radioactivity monitoring in foundries. For this purpose, a protocol for preparing a set of identical standard slag samples, containing known activity concentrations of (226)Ra, has been developed. This paper describes the preparation of the raw material, the characterisation in terms of its mineralogical, chemical and radiological features, the spiking procedure and the homogeneity testing of the spiked samples.

100 years of radionuclide metrology.

The discipline of radionuclide metrology at national standards institutes started in 1913 with the certification by Curie, Rutherford and Meyer of the first primary standards of radium. In early years, radium was a valuable commodity and the aim of the standards was largely to facilitate trade. The focus later changed to providing standards for the new wide range of radionuclides, so that radioactivity could be used for healthcare and industrial applications while minimising the risk to patients, workers and the environment. National measurement institutes responded to the changing demands by developing new techniques for realising primary standards of radioactivity. Looking ahead, there are likely to be demands for standards for new radionuclides used in nuclear medicine, an expansion of the scope of the field into quantitative imaging to facilitate accurate patient dosimetry for nuclear medicine, and an increasing need for accurate standards for radioactive waste management and nuclear forensics.

High-resolution alpha-particle spectrometry of ²³8U.

The alpha-particle emission probabilities associated with the three main alpha transitions of (238)U were measured by high-resolution alpha-particle spectrometry. Highly enriched (238)U material was used and its isotopic composition characterised by mass spectrometry. Source production through electrodeposition was optimised to reconcile conflicting demands for good spectral resolution and statistical precision. Measurements were performed at IRMM and CIEMAT for 1-2 years in three different set-ups. A new magnet system was put into use to largely eliminate true coincidence effects with low-energy conversion electrons. Finally the accuracy and precision of the relative emission probabilities for the three transitions - 77.01 (10)%, 22.92 (10)% and 0.068 (10)%, respectively - have been improved significantly.

Radioactive waste management: review on clearance levels and acceptance criteria legislation, requirements and standards.

In 2011 the joint research project Metrology for Radioactive Waste Management (MetroRWM)(1) of the European Metrology Research Programme (EMRP) started with a total duration of three years. Within this project, new metrological resources for the assessment of radioactive waste, including their calibration with new reference materials traceable to national standards will be developed. This paper gives a review on national, European and international strategies as basis for science-based metrological requirements in clearance and acceptance of radioactive waste.

Alpha-particle emission probabilities in the decay of 240Pu.

Sources of enriched (240)Pu were prepared by vacuum evaporation on quartz substrates. High-resolution alpha-particle spectrometry of (240)Pu was performed with high statistical accuracy using silicon detectors and with low statistical accuracy using a bolometer. The alpha-particle emission probabilities of six transitions were derived from the spectra and compared with literature values. Additionally, some alpha-particle emission probabilities were derived from gamma-ray intensity measurements with a high-purity germanium detector. The alpha-particle emission probabilities of the three main transitions at 5168.1, 5123.6 and 5021.2 keV were derived from seven aggregate spectra analysed with five different fit functions and the results were compatible with evaluated data. Two additional weak peaks at 4863.5 and 4492.0 keV were fitted separately, using the exponential of a polynomial function to represent the underlying tailing of the larger peaks. The peak at 4655 keV could not be detected by alpha-particle spectrometry, while gamma-ray spectrometry confirms that its intensity is much lower than expected from literature.

Defined solid-angle counter with variable geometry.

We describe a defined solid-angle counter for the standardization of radioactive sources of alpha-particle emitters. It has been built with the aim of combining good counting efficiencies, low uncertainties and flexibility of operation. The distance between source and detector can be changed in a continuous way with a precision guide and a ball screw from 8 to 19cm, which correspond to counting efficiencies between 0.023 and 0.004 for small size sources. A linear encoder allows the accurate determination of the source position. Alpha spectra of the sources are measured with an implanted silicon detector with an active area of 2000mm(2). Uncertainties, excluding counting statistics, are below 0.1%.

Comparison of calculated spectra for the interaction of photons in a liquid scintillator. Example of 54Mn 835 keV emission.

The CIEMAT/NIST and TDCR methods in liquid scintillation counting, initially developed for the activity standardization of pure-beta radionuclides, have been extended to the standardization of electron capture and beta-gamma radionuclides. Both methods require the calculation of the energy spectrum absorbed by the liquid scintillator. For radionuclides emitting X-rays or gamma-rays, when the energy is greater than a few tens of keV the Compton interaction is important and the absorption is not total. In this case, the spectrum absorbed by the scintillator must be calculated using analytical or stochastic models. An illustration of this problem is the standardization of 54Mn, which is a radionuclide decaying by electron capture. The gamma transition, very weakly converted, leads to the emission of an 835 keV photon. The calculation of the detection efficiency of this radionuclide requires the calculation of the energy spectrum transferred to the scintillator after the absorption of the gamma ray and the associated probability of absorption. The validity of the method is thus dependent on the correct calculation of the energy transferred to the scintillator. In order to compare the calculation results obtained using various calculation tools, and to provide the metrology community with some information on the choice of these tools, the LS working group of the ICRM organised a comparison of the calculated absorbed spectra for the 835 keV photon of 54Mn. The result is the spectrum of the energy absorbed by the scintillator per emission of an 835 keV gamma ray. This exercise was proposed for a standard 20 ml LS glass vial and for LS cocktail volumes of 10 and 15 ml. The calculation was done for two different cocktails: toluene and a widely used commercial cocktail, Ultima Gold. The paper describes the results obtained by nine participants using a total of 12 calculation codes.

Standardization of 32P/33P and 204Tl by liquid scintillation counting.

Two solutions, one of 32P with a significant contribution of 33P, and another one of 204Tl were standardized by liquid scintillation counting in the frame of two intercomparison exercises organized by the BIPM in 2002. For the 32P solution, the counting efficiencies for 32P and 33P were obtained by the CIEMAT/NIST method. The individual contributions of 32P and 33P were determined by fitting a set of measurements to a function of the half-lives and counting efficiencies of both component nuclides. 204Tl was also standardized by the CIEMAT/NIST method using two counters, three scintillators, HiSafe 3, Insta-Gel Plus and Ultima-Gold, and two different kinds of samples. A total of 480 samples were measured over 45 days with results in full agreement within the uncertainties considered. The efficiency values are also compared to those used in the previous 204Tl intercomparison (1997).

Standardization of 134Cs by three methods.

The nuclide 34Cs decays by beta-emission followed by gamma-deexcitation to 134Ba with a half-life T(1/2) = 2.065 a. It has been standardized by three methods: liquid scintillation counting (LSC), 4pi beta-gamma coincidence counting and 4pi gamma counting. In the LSC measurements, the CIEMAT/NIST method was used to calculate the efficiency. For the coincidence measurements, a conventional 4pi beta (proportional counter)-gamma(NaI) system was used. For the 4pi gamma standardization, a well-type Nal(Tl) detector was modeled with the Monte Carlo package PENELOPE, and the counting efficiency obtained by calculation. Results of the three methods agree within 0.65%.