Indirect determination of Li via ⁷⁴Ge(n,γ)^75mGe activation reaction induced by neutrons from ⁷Li(p,n)⁷Be reaction

Abstract

An indirect method to determine Li by ⁷⁴Ge(n,γ)^75mGe activation reaction induced in a high purity Ge (detector) crystal by neutrons from the ⁷Li(p,n)⁷Be reaction in a typical particle-induced γ-ray emission (PIGE) spectroscopy experimental set-up is described. Performed with proton beams of energies in excess of 1.88 MeV, the threshold energy (E_th) of the ⁷Li(p,n)⁷Be reaction, the determination involves the activity measurement of ^75mGe isotope that has a half-life of 47.7 s and decays with the emission of 139 keV γ-rays. Rapidity, selectivity and sensitivity down to ppm levels are the attractive features of the method. It is a suitable alternative to ⁷Li(p,p′γ)⁷Li reaction based PIGE technique in the analyses of matrices that contain light elements such as Be, B, F, Na and Al in significant proportions. Interferences can arise from elements, for example V and Ti, that have E_th ⩽ 1.88 MeV for (p,n) reaction. In the case of elements such as Cu, Mo which have with E_th > 1.88 MeV, the incident proton beam energy can be judiciously selected to avoid or minimize an interference. The method, under optimized irradiation conditions, does not entail a risk of neutron stimulated degradation of the performance of the detector. Besides analytical purposes, the measurement of the ^75mGe activity can serve as a powerful tool to monitor even low (∼25 n/cm² s) thermal neutron fluxes.

Introduction

Proton induced nuclear reactions namely ⁷Li(p,p′γ)⁷Li, ⁷Li(p,γ)⁸Be and ⁷Li(p,α)⁴He are widely used for the analysis of Li in materials [1]. Among these, ⁷Li(p,p′γ)⁷Li is often the reaction of choice for the determination of the element by the particle induced γ-ray emission (PIGE) technique. The analysis is generally performed using a 1–4 MeV proton beam and involves the measurement of the 478 keV prompt γ-rays by a high purity germanium (HPGe) detector. The reaction has a high cross-section and is largely free from interferences: except B [¹⁰B(p,αγ)⁷Be → ⁷Li] and Mn [⁵⁵Mn(p,nγ)⁵⁵Fe] no other element interferes in the analysis. As a result, it enables a sensitive determination of Li, in many cases down to ppm levels.

The reactions ⁷Li(p,γ)⁸Be (Q = 17.3 MeV) and ⁷Li(p,α)⁴He (Q = 17.3 MeV), on the other hand, are mainly used for depth profile measurements [2], [3]. The typical depth resolutions of these reactions are about 0.15 and 1 μm, respectively. ⁷Li(p,n)⁷Be (Q = −1.644 MeV) is another important reaction induced by protons. It is a well-studied nuclear reaction, particularly as a source of (monoenergetic) neutrons. However, it has been used only on few occasions for the determination of Li. In one such application, Sellschop et al. utilized the reaction in the analysis of diamond [4]. They reported a detection sensitivity of about 2 ppm but did not provide any experimental or analytical details. In yet another application, Sippel and Glover analysed sedimentary rocks using a BF₃ counter for neutron detection and achieved a detection sensitivity of about 100 ppm [5]. Meanwhile, it is important to note that ⁷Be formed in the reaction decays by electron capture (EC) (t_1/2 = 53.3 d) emitting 478 keV γ-rays. The activity of ⁷Be produced under irradiation conditions generally employed in the analysis of Li by PIGE is very low, and therefore the 478 keV γ-rays detected during such measurements are overwhelmingly prompt. The reaction ⁷Li(p,n)⁷Be → ⁷Li, however, is extensively used for the determination of Li by charged particle activation analysis (CPAA) [6].

In this paper we present a novel approach for the determination of Li which combines the ⁷Li(p,n)⁷Be reaction and neutron activation analysis (NAA) involving the ⁷⁴Ge(n,γ)^75mGe reaction. In a typical PIGE experimental set up, neutrons produced in a thick, Li containing target as a consequence of the ⁷Li(p,n)⁷Be reaction, initiate the ⁷⁴Ge(n,γ)^75mGe reaction in a HPGe detector. ^75mGe (t_1/2 = 47.7 s) emits 139 keV γ-rays which are measured by the detector thereby facilitating the determination of Li. The report includes studies on interferences, compares the results with those obtained by PIGE involving the ⁷Li(p,p′γ)⁷Li reaction and shows that the ⁷⁴Ge(n,γ)^75mGe reaction occurring in the HPGe detector can be advantageously utilized to monitor even low fluxes of neutrons as well.