Detection limits for some transuranic nuclides using low-energy photon spectrometry

Abstract

During the past 40 years, artificially produced transuranium nuclides have been introduced in the aquatic systems as consequences of atmospheric fallout from nuclear testing and of controlled release from nuclear reprocessing plants; the study of the behaviour of long-lived transuranic elements in oceanic media is of great importance from the projected development breeder technology. Usually, picocurie amounts of these isotopes are mainly determined in environmental sample by α spectrometry after tedious radiochemical separations. Otherwise, we tested [1] the availability of non-destructive low-energy photon spectrometry to measure activity levels of fission products after a nuclear test; in actual experiments, we attempted to give the sensitivities attained by this method for transuranic elements. The activities were measured with a X-ray spectrometer which consists to a 200 mm 2 area, 10 mm depth planar HPGe detector; the pulses given by the linear amplifier were analysed on a 4096 channels MCA. The energy resolution (FWHM) obtained with this system was better than 180 eV at Fe K α line. The samples used for counting were: 1. standard activity sources of 241Am, 243Am and 244Cm provided by L.M.R.I. or by I.A.E.A. 2. brown algae Fucus vesiculosus coded AGI/1: intercalibration sample provided by the International Laboratory Of Marine Radioactivity (I.A.E.A.). 3. many surface sediments from Nord Cotentin (France). Standard solutions of transuranic elements were electroplated on stainless disks using the Talvitie's method [2]; as concerns solid sample, accurately weighted quantities were packed in thin plastic containers. The samples were counted for time intervals between 50 and 360 ks. The absolute efficiency of the HPGe system was plotted against energy using counting runs with well known 152Eu, 133Ba and 241Am sources. The Minium Detectable Activity (MDA) attained by low energy photon or X-ray spectrometry is related to the branching ratio Γ = Nγ/Nα or X + N x/N α of the isotope under consideration. The MDA (in pCi) is calculated from the formula: MDA = 27.02N p γ·ϵ·t with t = counting time (s); ϵ = absolute detection efficiency. The net peak area N p and the subtracted background area N γ were related by: N p 3 ̆ √N b. The Table gives the MDA obtained in this method: 241Am 243Am 244Cm 237U 59.54 1.2 74.67 0.7 42.84 15.3 59.54 1.2 NpL β 2.5 NpL α 2.6 NpL β 2.5 E(keV) MDA E(keV) MDA E(keV) MDA E(keV) MDA