Multi-layer ²³5UF4-6LiF-Au targets for high-resolution fission fragment measurements.

Multi-layer (235)UF4-(6)LiF-Au targets have been produced by vacuum deposition on thin polyimide foils with an areal density, measured by spectrophotometry, of about 33µgcm(-2). The foils were first covered with an Au-layer and then, with a second layer of (6)LiF, both by vapour deposition. The (235)UF4 layer was prepared by fluoride sublimation. Each deposited mass was characterized separately by means of differential weighing for the Au and (6)LiF layers and by low-geometry alpha-particle counting for the (235)UF4 layer. The atomic abundances of the uranium base material have been measured by thermal ionization mass spectrometry. The targets were prepared for measuring fission-fragment emission yields with high mass-resolution.

Preparation of thick uranium layers on aluminium and stainless steel backings.

The methods of electrodeposition and "molecular plating" were studied for the production of uranium targets with an areal density up to 0.6 mg cm(-2) on aluminium and up to 1.5 mg cm(-2) on stainless steel backings from aqueous and organic electrolytes. For characterisation of the deposited material, gamma-ray spectrometry, alpha-particle spectrometry, X-ray fluorescence, X-ray powder diffraction, scanning electron microscopy and autoradiography were applied.

Measurement of the 109Cd half-life.

The half-life of (109)Cd was measured by following the decay of sources from a radiochemically pure solution with two different measuring systems: an ionisation chamber and a high-purity germanium (HPGe) detector. The measurements were performed over a period of 3.6 years, i.e. about 2.8 half-lives of (109)Cd. The resulting half-life values and detailed uncertainty budgets (k=1) are presented for both systems. The result obtained with the ionisation chamber, 462.36 (33) days, and the one obtained with the HPGe detector, 461.92 (76) days, are mutually consistent. The weighted mean of our measured values, T(1/2)((109)Cd)=462.29 (30) days, is consistent with the currently recommended values of 461.4 (12) days (Schönfeld and Dersch, 2004; IAEA, 2007) and 462.0 (3) days (Xiaolong et al., 2010). From a set of selected experimental values published after 1970, a "partially weighted mean" (Pommé and Spasova, 2008) of T(1/2)((109)Cd)=462.36 (39) days was calculated. More measurements are needed to resolve the discrepancies among literature data and to reduce the final uncertainty on the (109)Cd half-life.

Measurement of the (54)Mn half-life.

The half-life of (54)Mn was measured by following the decay of sources from a radiochemically pure solution using three different measuring systems: an ionisation chamber, a high-purity germanium (HPGe) detector and two 7.5 cm (diameter) × 7.5 cm (height) NaI(Tl) scintillation detectors in opposite position. The measurements were performed over a period of 3 years, i.e. about 3.5 half-lives of (54)Mn. The resulting half-life values and detailed uncertainty budgets are presented for the three measuring systems. The half-life obtained with the ionisation chamber, 312.32 (9) days, is consistent with but more precise than the ones obtained with the HPGe detector, 311.9 (5) days and the NaI(Tl) detectors, 311.9 (6) days, respectively. Our final half-life value of 312.32 (9) days is rather consistent with the currently recommended values of 312.29 (26) (IAEA, 2007) and 312.13 (3) days (Helmer and Schönfeld, 2004), even though the uncertainty of the latter may be underestimated. From a partially weighted mean (Pommé and Spasova, 2008) of selected experimental values published after 1970, a new best estimate of T(1/2)((54)Mn)=312.20 (8) days was calculated.

International exercise on 124Sb activity measurements.

An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories. In order to identify possible biases, the participants were asked to use all possible activity measurement methods available in their laboratory and then to determine their reference value for comparison. Thus, measurement results from 4pibeta-gamma coincidence/anti-coincidence counting, CIEMAT/NIST liquid-scintillation counting, 4pigamma counting with well-type ionization chambers and well-type crystal detectors were given. The results are compared and show a maximum discrepancy of about 1.6%: possible explanations are proposed.

International exercise on 124Sb photon emission intensities determination.

An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories, six of which sent results for photon emission intensities both in absolute and in relative terms. From these results and including previous published values, a consistent decay scheme was worked out, proving that problems in activity measurements have not been due to decay scheme data.

Half-life measurement of 124Sb.

The half-life of (124)Sb was determined experimentally by following the decay of a source from a radiopure solution with a Centronic IG12 ionisation chamber. Thousands of measurements were performed over a period of 358 days, i.e. about six half-life periods. However, the data analysis was restricted to the first 221 days, in order to limit the dominant uncertainty component associated with the hypothetical possibility of a systematic error on background subtraction. The resulting value for the (124)Sb half-life, 60.212 (11) days, is found to be in very good agreement with published values, but carries a lower uncertainty. Major uncertainty contributions pertain to possible systematic errors in background correction, long-term changes in source-detector geometry and medium- and long-term instability of the instrument. Additional measurements were performed with a high-purity germanium detector to confirm the above value.

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.

alpha-Particle and gamma-ray spectrometry of a plutonium solution for impurity determination.

A highly enriched (240)Pu solution was measured by alpha-particle and gamma-ray spectrometry to determine other radionuclides present in the material as impurities. Low activities of (238)Pu, (241)Am, (243)Cm and (244)Cm were determined by measuring thin sources, made from the original solution, in a high-resolution alpha-particle spectrometer. The sources were prepared by evaporating the plutonium solution on quartz plates in a vacuum chamber. From the ingrowth of (241)Am in the original solution, the amount of (241)Pu could be calculated. After radiochemical separation of (241)Am, the plutonium was measured by high-efficiency alpha-particle spectrometry to determine the amount of (238)Pu. The enriched (240)Pu material was also measured by high-resolution gamma-ray spectrometry, using two different HPGe detectors to determine the impurities of (239)Pu and (241)Am. The preparation of the sources and the measurement methods are described and discussed. The measured impurities, given in % of the (240)Pu activity, are compared with the values on the certificate.

EC intercomparisons for laboratories monitoring environmental radioactivity.

International measurement comparisons are organised regularly for EU laboratories involved in monitoring radioactivity, with emphasis on meeting routine measurement conditions. Using the recent comparison of 137Cs in air filters as an example, the whole cycle is described: establishment of traceable reference values, spiking of individual filters for the comparison and their quality assurance, treatment and measurement of filters in the participating laboratories and evaluation of comparison results. The treatment of an individual result, deviating widely from the reference value, is discussed. Monte-Carlo simulations allow to estimate the maximum errors possibly made due to a non-suitable measurement geometry.

Half-life measurement of 55Fe.

The half-life of 55Fe was determined experimentally and compared with literature values. A solid 55Fe source was prepared by electrodeposition and repeatedly measured in an X-ray detector at a constant, low solid angle. In total, more than six hundred measurements were performed over a period of ten months, corresponding to one third of the half-life. The experimental method and the corresponding uncertainty budget are presented. The result of 1005.2+/-1.4 days is slightly higher than the current recommended values.

Aspects of sample preparation for the determination of actinoids in soil.

In the frame of the re-certification measurements of the IAEA-375 soil reference material the activity concentration of four actinoids (U, Th, Pu and Am) was determined. The method is based on microwave digestion or aqua regia leaching, separation by extraction chromatography and alpha-particle counting source preparation by co-precipitation or electro-deposition. This article describes some of the problems encountered during the validation of the sample preparation method and their solutions.

Characterisation of reference materials for radioactivity with assigned values traceable to the SI units.

The Institute for Reference Materials and Measurements participates in an IAEA co-ordinated project to upgrade Analytical Quality Control Services intercomparison materials to reference materials. The principal objective is to ensure that assigned property values are traceable to the International System of Units. General principles and practical mechanisms for achieving this traceability are described.

Standardisation of low-activity actinide solutions by alpha-particle counting at a defined solid angle.

In the frame of an IAEA co-ordinated research project 232U, 236Pu and 229Th tracers were standardised by alpha-particle counting at a defined solid angle. The aim of this project was to assign property values that are traceable to the International System of Units (SI). This article depicts the standardisation of the tracers, elaborating on problems related to low activity and interference from daughter nuclides. The activity concentrations of 232U, 236Pu and 229Th were found to be, respectively (734+/-4), (142.0+/-0.9) and (528.2+/-2.6) Bq g(-1), at reference date (1 January 2003).

Study of alpha-particle emission probabilities and energies in the decay of 240Pu.

In the frame of EUROMET Project No 325 "Analysis of Plutonium Alpha-Particle Spectra", the isotopic composition of a 238, 239, 240Pu mixture was measured by mass spectrometry and by high-resolution alpha-particle spectrometry. The latter revealed a discrepancy with published data of the alpha-particle emission probabilities (Palpha) and the energies (Ealpha) of the two major peaks of 240Pu. New values have been derived: Palpha1 = 0.7256 (6), Palpha2 = 0.2735 (7) and Ealpha1 = 5168.54 (14)keV, Ealpha2 = 5124.10 (15)keV, relative to reference peaks of 238Pu and 239Pu.

Concept for an off-line gain stabilisation method.

Conceptual ideas are presented for an off-line gain stabilisation method for spectrometry, in particular for alpha-particle spectrometry at low count rate. The method involves list mode storage of individual energy and time stamp data pairs. The 'Stieltjes integral' of measured spectra with respect to a reference spectrum is proposed as an indicator for gain instability. 'Exponentially moving averages' of the latter show the gain shift as a function of time. With this information, the data are relocated stochastically on a point-by-point basis.

Experimental verification of the half-life of 65Zn.

The half-life of 65Zn was determined experimentally and compared with literature values. Two ampoules of a 65Zn solution were measured at regular intervals in two ionisation chambers over a period corresponding to nearly two half-lives. The result of 243.8+/-0.3 days is in agreement with the value of 244.0+/-0.2 days obtained by De Roost et al. (Z. Phys. 250 (1975) 395) at the same institute (previously called CBNM) in 1972. Moreover, this half-life is supported by most of the other measurements reported in the literature.

Self-absorption correction in standardisation of 204Tl.

In the frame of the CCRI(II) (Consultative Committee for Ionizing Radiation at the BIPM) working group on 204Tl, dedicated to investigate the problem and explain the reasons for the discrepancy between methods in standardisation of 204Tl, IRMM was assigned the task of studying the influence of self-absorption in solid sources of 204Tl. This was investigated by measuring the activity of quantitative solid sources of different carrier content, prepared by employing a special source drying technique, developed at IRMM. The activity of the solid sources was determined using a 4pi pressurised proportional counter. The self-absorption was calculated using the activity determined from liquid scintillation spectrometry, unaffected by self-absorption, as the reference value. After correction for foil absorption and non-detected X-rays, the self-absorption showed a linear relation to the logarithm of the dry mass of the source. For a typical source, the self-absorption correction for 204Tl was found to be as high as 8%. This article describes a way to minimise and correct for self-absorption in solid sources of 204Tl or nuclides with similar decay properties.

Standardization and decay data of 237Np.

This paper reports contributions from participants in the EUROMET project (No. 416) which was entitled "237Np research into problems relating to purification, characterization and standardization". Primary standardizations were made by the defined low solid angle, coincidence, 4pi alpha, 2pi alpha and liquid scintillation counting methods. Secondary standardizations were made with calibrated gamma-ray spectrometers. Absolute X-ray, gamma-ray and alpha-particle emission probabilities were also determined. The results for the successful conclusion of both primary and secondary standardization are presented together with the values for alpha-particle and gamma-ray emission probabilities determined in this exercise. Several significant inconsistencies remain with the gamma-ray emission probabilities, and these are highlighted.

Standardisation of 89Sr using three different methods.

Three independent methods were used to standardise a 89Sr solution within the frame of an international comparison organised by BIPM/CCRI(II): 4pi liquid scintillation (LS) spectrometry, based on the CIEMAT/NIST efficiency tracing method, and direct activity measurements with a windowless 4pi CsI(Tl)-sandwich spectrometer and a 4pi pressurised gas proportional counter. Quantitative solid point-like sources were prepared paying special attention during the source drying phase. The impurities in the original solution were measured and corrected for in the results of all three methods. The activity concentration of 89Sr was found to be 26.21 +/- 0.08 kBq g(-1). A new half-life value for 89Sr of 50.61 +/- 0.05 days was determined from the LS measurements. The measurement methods, including the impurity measurements, are described and the results compared. considering the advantages and disadvantages of each method.