High-resolution alpha-particle spectrometry of 238U
Introduction
The IAEA CRP F42006 (Coordinated Research Activity on “Updated Decay Data Library for Actinides”) expressed the need for more accurate values of alpha-particle emission probabilities for 238U (Kellet et al., 2011, Kellet, 2012). This task was taken on as part of a work package on improvement of decay data in the JRP ENG08 “Metrology for New Generation Nuclear Power Plants—MetroFission” (Johansson et al., 2011). Due to its extremely low specific activity, 238U is difficult to measure by high-resolution alpha-particle spectrometry. The main difficulty is to combine good spectral resolution, which requires thin sources, with good statistical precision, which requires a relatively high count rate and therefore sufficiently thick sources.
The alpha-particle energy spectrum shows two major peaks (at 4151 keV and 4198 keV) and a small one at lower energy (4038 keV). Only two measurements of 238U alpha-emission probabilities have been reported (Kocharov et al., 1959, García-Toraño, 2000), the most recent values – Pα,0=77.54 (50)%, Pα,1=22.33 (50)% and Pα,2=0.13 (3)% – being recommended by evaluators (Chisté and Bé, 2006). The challenge of the current work was to improve the quality of the existing decay data through better isotopic purity of source material, spectral resolution, statistical precision and more accurate correction for true coincidence effects between alpha-particles and conversion electrons. This cocktail of conflicting demands called for metrological solutions with respect to source preparation, detector stability, optimisation of measurement geometry and suppression of conversion electron signals.
Highly enriched 238U material was obtained and its isotopic composition characterised by mass spectrometry. Extensive investigation went into source preparation by electrodeposition, involving many free parameters such as type of electrolyte, shape and rotation of electrode, surface quality of backing, source radius and thickness (Jobbágy et al., 2011, Jobbágy et al., 2013). High-resolution alpha-particle spectrometry measurements were performed for 1–2 years in three different set-ups, with a different focus on count rate and resolution. A new magnet system was developed (Paepen et al., 2014) and used to prevent conversion electrons from being detected in coincidence with the alpha particles. Finally the accuracy and precision of the relative emission probabilities for the three transitions could be improved significantly. In this paper, the measurements and results are described.
Section snippets
Source preparation
The applied source production procedure is a modification of the electrodeposition method by Dos Santos et al. (2004). Jobbágy et al., 2011, Jobbágy et al., 2013 have described in detail the different source electrodeposition parameters tested at IRMM and CIEMAT to reconcile the inherently opposing requirements of high resolution and sufficient count rate needed to determine the alpha emission probabilities more accurately. Three different electrolytes were tested with respect to attainable
Alpha peak fit
The least-squares fitting software ALPACA (CIEMAT) (García-Toraño, 1996) and ALPHA (IRMM) (Pommé and Sibbens, 2008) were used to deconvolute the 238U alpha spectra. In Fig. 1, Fig. 2, Fig. 3, measured 238U alpha spectra and their spectral deconvolution are shown for three configurations. In all the spectra, the three main alpha-peaks of 238U at 4038 (5) keV, 4151 (5) keV and 4198 (3) keV are easily identified and well separated. The energy resolution and peak-to-valley ratio are noticeably better
Discussion
The uncorrected emission probabilities obtained in configuration CIEMAT1 (G=2%, no magnet) are in good agreement with the previously published measurement results obtained by García-Toraño (2000) in similar conditions in the same laboratory. This time, better statistical precision was achieved, leading to a lower amplitude of the small peak, Pα,2. The new emission probabilities are also consistent with the results obtained with a similar configuration at IRMM (IRMM1, G=1.7%, no magnet).
The
Conclusions
The alpha-particle emission probabilities of 238U have been measured with high-resolution alpha-particle spectrometry. Sources were prepared of highly enriched 238U material by electrodeposition with 18–32 cm diameter and up to 26 μg/cm2 thickness. This was the result of a compromise between spectral resolution and counting rate.
The measured emission probabilities, shown in Table 1, agree within uncertainties with the previously published data but are significantly more precise. The new data are
Acknowledgement
The research leading to these results has received funding from the European Union on the basis of Decision No 912/2009/EC.
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