Elsevier

Radiation Physics and Chemistry

Volume 140, November 2017, Pages 480-486
Radiation Physics and Chemistry

Progress on the chemical separation of fission fragments from 236Np produced by proton irradiation of natural uranium target

https://doi.org/10.1016/j.radphyschem.2017.02.004Get rights and content

Highlights

  • Development of a radiochemical separation scheme for separating 236gNp.

  • Measurements using α and high-resolution γ-ray spectrometry.

  • γ-ray spectroscopy used indicates presence of 236Np ground state.

  • Clear signature of 236Pu characteristic lines.

Abstract

The aim of the current work is to develop and validate a radiochemical separation scheme capable of separating both 236gNp and 236Pu from a uranium target of natural isotopic composition (~1 g uranium) and ~200 MBq of fission decay products. A target containing 1.2 g of UO2 was irradiated with a beam of 25 MeV protons with a typical beam current of 30 μA for 19 h in December 2013 at the University of Birmingham Cyclotron facility. Using literature values for the production cross-section for fusion of protons with uranium targets, we estimate that an upper limit of approximately 250 Bq of activity from the 236Np ground state was produced in this experiment. Using a radiochemical separation scheme, Np and Pu fractions were separated from the produced fission decay products, with analyses of the target-based final reaction products made using Inductively Couple Plasma Mass Spectrometry (ICP-MS) and high-resolution α particle and γ-ray spectrometry.

Introduction

The procedures used in radiochemical separations are rarely fully quantitative, thus it is necessary that an accurate chemical yield is determined for the radionuclide of interest. In order to carry this out, it is necessary to have a way of monitoring the chemical recovery, which can be accomplished by the use of a radiochemical yield tracer. The overall aim of the research is dedicated to producing an isotopically pure radioactive tracer for Np analysis. The choices available for a suitable Np tracer are constrained (Larijani et al., 2015) one can use a Pu yield tracer such as 236Pu (Jerome et al., 2014). However, with the complex chemistry of the actinide series, there is always the risk of speciation of Pu and Np during the analysis.

The radionuclide 236gNp has a ground state half-life of 1.55(8) ×105 years (Chechev and Kuzmenko, 2012a) and is of interest as a chemical yield tracer for the radiochemical and mass spectrometric analysis of 237Np (t½: 2.144(7) ×106 y; Chechev and Kuzmenko, 2010a), a significant long-lived waste product from nuclear reactor operation, arising from the α decay of 241Am (t½: 432.6(6) y; Chechev and Kuzmenko, 2010b). Various production mechanisms of 236Np involving charged-particle irradiation of uranium targets (235U, 236U or natU) have been studied, with the proton bombardment of natU showing the most promising results in terms of 236Np production yield and isotopic purity (Jerome et al., 2014). The metastable state, 236mNp (t½: 22.5(4) h; Chechev and Kuzmenko, 2012b) is also produced in these reactions. The metastable state and ground state of 236Np both decay to 236Pu (t½: 2.858(8) y; Browne and Tuli, 2006) and 236U (t½: 2.343(6) ×107 y; Luca, 2012). The 236Pu daughter subsequently decays to 232U (t½: 70.6(11) y; Pearce, 2008), both of which can be used as indirect signatures of the production of 236mNp. The primary challenge in the radiochemical separation of 236Np from the production target material is to isolate trace amounts of ground state 236Np (specific activity ~360 MBq g−1) from significant quantities of uranium from the target material and fission products produced during irradiation (Larijani et al., 2015).

The ultimate aim of the current work is to develop and validate a radiochemical separation scheme capable of separating both 236gNp and 236Pu from a uranium target of natural isotopic composition (~1 g uranium) and ~200 MBq fission products. In the current paper, results for the target and separated fractions are presented, which have been measured using α and high-resolution γ-ray spectrometry to confirm the production and decay of 236Np in either the ground state or metastable state, which consolidate our previous paper on this topic (Jerome et al., 2014). The mass distribution of the fission residues created during the target bombardment has been presented previously (Larijani et al., 2015). Results are also presented for the effectiveness of extraction chromatography (TEVA resin, Triskem International) for the major fission product elements using mixed stable-element standard solutions, with recoveries of the separated fractions measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

Section snippets

Target irradiation

A target containing 1.2 g of UO2 was irradiated with a beam of 25 MeV protons of typical beam current of 30 μA at the University of Birmingham Cyclotron facility. The target was produced by compacting natural uranium dioxide at 60 kN cm−2 between an aluminium target holder and a 0.025 mm titanium foil, which was then subjected to a proton irradiation for a total duration of 19 h over a period of 3 days (9–11th December 2013). Directly following the irradiation, the target was left to cool for

Gamma spectrometry

The LN2 cooled high purity germanium (HPGe) γ-ray spectrometer ‘Lancelot’ which is semi-planar HPGe detector with a carbon fibre detector window and a relative (to a 3''×3'' NaI(Tl) detector) efficiency of ~65% was used to perform a series of measurements of the prepared sample. The fractions were measured for a typical counting time of 250,000 s in September 2016, just under 3 years after the initial irradiation, resulting in the decay of the shorter-lived fission products that were measured in

Methodology for determination of the 236gNp content

The decay of the long-lived, high-spin coupling ground state in 236Np (236gNp) proceeds by both β decay to 236Pu and by electron capture to 236U (Browne and Tuli, 2006) with branching ratios of approximately 13% and 86.3% “respectively”. The metastable excited state in 236Np (236mNp) decays by β- and electron capture to 236Pu and 236U respectively. The branching ratios for these competing decay branches from 236mNp are listed as 50(3) % for each decay mode in the most recent nuclear data

Conclusions

A radiochemical scheme has been developed for the separation of Np and Pu from fission fragments, produced as part of the target irradiation of natural uranium. Our spectrometric analysis show that the chemical separation scheme adopted separates Pu from fission fragments and other α-particle emitting radionuclides, including the target uranium material. The results to identify the production of Np are more complicated. The γ-ray spectroscopy used shows the signature γ ray for the decay of the

Acknowledgement

This work is funded through the UK National Measurement Office (Grant no. 117925). P.H. Regan also acknowledges support from UK Science and Technology Facilities Council (STFC) grant ST/L005743/1.

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