Reactor production of promethium-147

Highlights

• The method described in this paper defines a selective means to prepare large quantities of ¹⁴⁷Pm.

• Pm-147 can be produced via β^– decay of ¹⁴⁷Nd produced via ¹⁴⁶Nd[n,γ]¹⁴⁷Nd(t_1/2 = 10.98 d, β^–)¹⁴⁷Pm nuclear reaction.

• Separation of ¹⁴⁷Pm from mg amount of Nd target via extraction chromatography is described.

• For 24-d irradiation at ORNL-HFIR, ¹⁴⁷Pm yield reaches a maximum value of 101.8 MBq/mg (2.75 mCi/mg) at 60 d post EOB.

• Because of large neutron capture cross-sections of ¹⁴⁷Pm, the yield of ¹⁴⁷Pm does not significantly increase with longer irradiation.

Abstract

In this paper, we describe the ¹⁴⁷Pm production yields and level of impurities from several targets that consisted of milligram quantities of highly enriched ¹⁴⁶Nd oxide irradiated at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory for durations ranging from 24 to 180 h. A comparison between theoretical and experimental data are also presented, and attempts were made to empirically evaluate the neutron capture cross-sections of 41.3-d ^148mPm and 5.4-d ^148gPm. For a one-cycle irradiation (~24 days), ¹⁴⁷Pm yield reaches a maximum value of 101.8 MBq/mg (2.75 mCi/mg) at 60 days after the end of bombardment. Because of large neutron capture cross-sections of ¹⁴⁷Pm, the yield of ¹⁴⁷Pm does not significantly increase with longer irradiation. Our estimates of the thermal neutron capture cross-section and resonance integral for ¹⁴⁶Nd at 1.48 ± 0.05 b and 2.56 ± 0.25 b, respectively, were consistent with the reported values. The effective neutron capture cross-section of ¹⁴⁷Pm to ^148mPm was 53.3 ± 2.7 b–a factor of 2 lower than the 98.7 ± 6.5 b calculated from reported cross-sections. The measured σ_eff to the ground state (5.37-d ^148gPm) was 82.0 ± 4.1 b; ~34% lower than the value of 139 ± 10 b calculated from reported cross-sections. In this work, we also describe the development of a chemical process based on extraction and ion-exchange chromatography for separation of ¹⁴⁷Pm from milligram quantities of ¹⁴⁶Nd and other impurities. Sequential separation of Pm from the Nd target and from other radioisotopic impurities (¹⁵³Gd and ^154&155Eu, ¹⁹²Ir, and ⁶⁰Co) was achieved using a LN extraction resin in HCl media followed by further purification of Pm from ⁶⁰Co and ¹⁹²Ir using a low cross-linking cation exchange resin. Based on these data, we estimated that two rounds of purification under our experimental conditions can provide a mass separation factor of >10⁴ between Pm and Nd. Our data indicate that curie quantities of ¹⁴⁷Pm with suitable chemical and radioisotopic purity for applications in beta voltaic batteries can be produced by irradiating gram quantities of highly enriched ¹⁴⁶Nd in the flux trap of HFIR for one cycle.

Introduction

There is growing interest in the availability of low energy beta emitters for variety of important applications in defense, homeland security, and industry. These applications include beta batteries and other low-power, long-lived power sources including pacemakers, luminescence devices, electron capture detectors, and thickness gauges (Lieberman et al., 2007, Roberts and Van Tuyl, 1965, Walko et al., 1991). Light produced from luminescence devices is used directly in visual devices to produce signals that require dependable operation, and it is used indirectly in photocells converting the energy into an electric current. Low-energy beta emitters also show promise as a heat source and auxiliary power for satellites and space probes in a multitude of future NASA scientific missions. The list of potential radioisotopes for the above applications is quite limited and includes ³H (t_1/2 = 12.3 y), ⁶³Ni (t_1/2 = 100 y), ¹⁴⁷Pm (t_1/2 = 2.62 y), ¹⁵⁵Eu (t_1/2 = 4.76 y), and ¹⁷¹Tm (t_1/2 = 1.92 y). As these radionuclides are all are neutron-rich, the Oak Ridge National Laboratory (ORNL) High Flux Isotope Reactor (HFIR) represents a unique and powerful resource for their production. At present, ³H and ⁶³Ni are the only radioisotopes from the above list that are routinely produced. For applications in micro-devices, however, a much higher specific activity is required, which only can be provided by ¹⁴⁷Pm,¹⁵⁵Eu, and ¹⁷¹Tm, which have much shorter half-lives than ³H and ⁶³Ni. Although rather large quantities of ¹⁴⁷Pm can be produced from ²³⁵U fission [Pressly 1960], ¹⁵⁵Eu and ¹⁷¹Tm can only be synthesized at a large scale by neutron capture reactions on stable ¹⁵⁴Sm and ¹⁷⁰Er, following the β^– decay of the short-lived intermediate radionuclides ¹⁵⁴Sm(22.8%)[n,γ]¹⁵⁵Sm(t_1/2 = 22.8 m, β^–)¹⁵⁵Eu and ¹⁷⁰Er(14.9%)[n,γ]¹⁷¹Er(t_1/2 = 7.52 h, β^–)¹⁷¹Tm, respectively.

Promethium-147 β⁻ decays with a half-life of 2.6 years, and 99.994% of the time, this decay feeds to the ground state of ¹⁴⁷Sm, and consequently the decay of ¹⁴⁷Pm is followed by emission of an extremely weak γ-ray at 121.64 keV with an intensity of only 2.85 × 10⁻³%. The maximum energy of the β⁻ particles from ¹⁴⁷Pm is 224.5 keV, with an average energy of ~62 keV (Firestone and Shirley, 1996, Brown et al., 1986).

Traditionally, ¹⁴⁷Pm has been isolated in large amounts from fission products from nuclear fuel processing (the yield from ²³⁵U fission to isobar 147 is 2.232%, International Atomic Energy Agency, 2006). Until the 1970s, ORNL had a large inventory of ¹⁴⁷Pm that had been isolated from fission products at the Hanford nuclear facility in Washington state (Pressly et al., 1960). Since nuclear fuel is no longer processed in the United States, the ORNL inventory has long been exhausted, and there is currently no domestic source of significant amounts of ¹⁴⁷Pm (¹⁴⁷Pm is currently imported from the Russian Federation). However, similar to ¹⁵⁵Eu and ¹⁷¹Tm, ¹⁴⁷Pm can be also produced via β^– decay of a short-lived predecessor, in this case ¹⁴⁷Nd (t_1/2 = 10.98 d), which is produced by irradiating enriched ¹⁴⁶Nd in a nuclear reactor via the ¹⁴⁶Nd[n,γ]¹⁴⁷Nd(t_1/2 = 10.98 d, β^–)¹⁴⁷Pm nuclear reaction (Jarrett and Van Tuyl, 1970). The approach requires separation of microgram quantities of Pm from grams of Nd target (Knapp et al., 2008).

In this paper, we describe the production yields and level of impurities from several targets that consisted of milligram quantities of highly enriched ¹⁴⁶Nd as oxide irradiated at the ORNL HFIR for durations ranging from 24 to 180 h. We also report new values for the ¹⁴⁷Pm burnup cross-sections using a highly purified sample of ¹⁴⁷Pm as the target material. A comparison between theoretical and experimental data are also presented, illustrating that large neutron capture cross-sections of ¹⁴⁷Pm limit the amount of ¹⁴⁷Pm which can be produced even at the highest neutron flux available at the HFIR. Further, attempts are made to empirically evaluate the neutron capture cross-sections of 41.3-d ^148mPm and 5.4-d ^148gPm. In this work, we also describe the development of the chemical process based on extraction and ion-exchange chromatography for separation of ¹⁴⁷Pm from milligram quantities of ¹⁴⁶Nd targets and other impurities. The method described defines a selective means to prepare large quantities of ¹⁴⁷Pm suitable for applications in beta voltaic batteries. The level of radioisotopic impurities in the final product are also briefly described.