The excitation of isomeric states by accelerator neutrons from the 7Li(p, n)7Be reaction and their application in selective activation analysis

https://doi.org/10.1016/S0168-583X(97)00979-8Get rights and content

Abstract

The 7Li(p, n)7Be reaction has proved to be useful in selective neutron activation analysis via the production of nuclear isomers. This is due to the relatively high yield and narrow energy spread of the neutrons that can be produced with a thin lithium target. By a suitable choice of proton energy and target thickness, neutrons have been produced that are below the energy region for the excitation of threshold reactions such as 28Si(n, p)28Al and above the region where neutron capture reactions like 27Al(n, γ)28Al have large cross sections. These neutrons have been used for the selective excitation of nuclear isomeric states by inelastic scattering. Numerical methods were used to predict the resultant neutron spectra for different values of the lithium target thickness at different proton energies. The isomeric state of gold was excited by neutrons from the above reaction at proton energies between 4 and 10 MeV. The possible interfering reactions from silicon and aluminium, present in many ores, silicates and rock samples, have also been examined. The variation of the normalised peak areas, as a function of the proton energy incident on lithium targets of thicknesses of 0.8 and 2.8 mm is also presented here.

Introduction

In the early years of neutron physics, neutrons for nuclear reactions were obtained from reactions like 9Be(α, n)12C and 9Be(γ, n)8Be, where the alpha particles and the gamma rays, obtained from natural radioactive decays, interacted with the target nuclei. However, these sources suffered from disadvantages such as low neutron yields and a broad neutron energy spread. Neutrons produced from fission processes in nuclear reactors have also proved to be useful in activation analysis. In most cases activation analysis by reactor neutrons utilises an intense flux, but the neutron spectrum is not suitable for selective activation analysis.

With the advent of charged particle accelerators it became possible to produce nearly monoenergetic neutrons through reactions induced by using protons, deuterons, tritons and alpha particles on light and medium weight nuclei. For a number of elements, nuclear isomers can be excited by the inelastic scattering of intermediate energy neutrons. The use of such neutrons for activation analysis via isomer excitation is convenient because the cross sections for neutron capture and most threshold reactions are relatively low in this region and the neutron energy can be controlled so as to avoid interference from these processes. The 7Li(p, n)7Be has been very useful for such purposes [1]. The fluxes from reactions of this type are much lower than those in reactors, but they have the following advantages: (i) a virtual absence of thermal neutrons and a reduced gamma ray fluence; (ii) a better defined neutron spectrum; (iii) they require a much smaller installation, and (iv) the safety requirements are less problematic.

Section 2describes the evaluation of neutron yields and spectra from charged particle reactions, particularly the 7Li(p, n)7Be reaction. This is followed by the experimental procedure in Section 3. Section 4presents the results and discussion, and Section 5concludes this paper.

Section snippets

Evaluation of the neutron yields

(i) The stopping power and the range of protons in lithium. In considering the production of neutrons by the proton bombardment of lithium, it is convenient to calculate the lithium stopping power in order to evaluate the proton range inside the lithium target. The theoretical value of the stopping power can be obtained from the quantum mechanical calculation of the collision process, which is given by the Bethe formula [2]:dEdx=e24πε024πz2NAmc2β2Aln2mc2β2IlnI−β2−β2,where m is the electron

Experimental procedure

The proton beams were provided by the EN Tandem van de Graaff accelerator of the Schonland Research Centre at the University of the Witwatersrand. The average current output was about 1 μA. A beam collimator with a diameter of 1 cm and lithium foils with thicknesses of 0.8 and 2.8 mm were used. The samples were transported from the irradiation room to the counting table and vice versa by an automated pneumatic transfer system. A decay time of 15 s was allowed for all the samples. Table 1 shows

Results and discussion

The response functions of the normalised peak areas are shown in Fig. 2. The responses of the interfering (n, p) reactions from silicon and aluminium have been compared with that of the gold in the gold ore. The results show that an incident proton energy in the region of 6.5 MeV corresponds to a maximum isomer yield for the 0.8 mm thick target. The 2.8 mm target shows a somewhat consistent increase in the 197mAu yield with proton energy. The interferences from the aluminium and the silicon

Conclusion

The above results suggest that accelerators are useful tools for the selective activation analysis of large samples that contain elements with low-lying isomeric nuclear states using neutrons in the intermediate energy region, where the interferences from threshold and capture reactions are relatively small. With accelerators it is easy to control the reaction parameters and thus the resultant neutron spectrum. By using a suitable combination of lithium thickness and incident proton energy, a

Acknowledgements

The authors would like to thank J.U.M. Beer, A.H. Andeweg, Prof. H. Annegarn, I.D. McQuen, M. Rebak, and B.R. Kala for their help and support in this work.

References (6)

  • J.I.W Watterson et al.

    Nucl. Instr. and Meth. B

    (1990)
  • C.G Clayton et al.

    Int. J. Appl. Radiat. Isot.

    (1985)
  • G Calvi et al.

    Nucl. Phys.

    (1962)
There are more references available in the full text version of this article.

Cited by (0)

View full text