Precise measurements of the thick target neutron yields of the 7Li(p,n) reaction

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Abstract

Thick target neutron yield of the 7Li(p,n)7Be reaction was measured in the proton energy range from 1.95 to 2.3 MeV by determining induced activity of the 7Be. A HPGe detector was used to detect the 478 keV gamma-rays emitted through 7Be decay. A series of irradiations with nominal proton energies of 1.95, 2.0, 2.1, 2.2, and 2.3 MeV were carried out. In an independent experiment, raw neutron spectra were collected by a 3He ion chamber for the same series of proton energies. From the raw neutron spectra, it was noted, that the effective proton energies were lower than the nominal by 50–58 keV. After corrections for the proton energy offsets were applied, the measured neutron yields matched the analytically calculated yields within 20%. Long term stability of neutron yield was tested at two nominal proton energies, 2.1 and 1.95 MeV over an experimental period of one year. The results show that the yield at 2.1 MeV was stable within rmse variation coefficient of 4.7% and remained consistent even when the lithium target was replaced, whereas at 1.95 MeV, the maximum fluctuations reached a factor of 10.

Introduction

Due to relatively low threshold energy, high neutron yield, and soft neutron spectrum, 7Li(p,n) neutron sources have been used in various applications [1], [2], [3], [4]. As target material, pure metal lithium has mostly been employed when maximizing neutron yield is a priority. Although metal lithium target is the best way to achieve the highest neutron yield for a given proton energy and current, there are many factors requiring careful handling and operation. Because lithium has a relatively low melting temperature (180 °C), the target cooling system must have a sufficient heat removal capacity so that the proton beam can be delivered at the desired operating conditions of proton energy and current.

Another practical difficulty stems from the chemical instability of lithium. Due to its strong oxidation potential, lithium metal quickly creates an oxidation layer on the surface when exposed to air. Although target preparation and installation is generally carried out under a noble gas environment, there may always be a risk of temporal air exposure. The oxide layer can lead to a decrease in total neutron yield compared to the yield from a pure metal target.

The second perturbation is that carbon deposits can accumulate on the surface of the lithium target, which is unavoidable when oil diffusion vacuum pumps are used in the proton beam line. The carbon deposition lowers the neutron production rate by making the incident proton beam partially lose its energy in the carbon layer prior to reaching lithium.

Analytically, neutron yield from thick lithium target is calculated as [5], [6], [7]d2YdΩdEn=nLiqe(dσpndΩ)CMSdΩCMSdΩdEpdEn1S(Ep),where nLi is the number density of 7Li, q is the total proton charge incident on lithium target, e is the elementary charge, (dσpn/dΩ)CMS is the differential cross-section for the 7Li(p,n) reaction in the center of mass system, Ep and En denote proton and neutron energies, respectively, and finally S(Ep) represents the proton stopping power in lithium. However, many groups reported experimental thick target 7Li(p,n) neutron yield data and found significant discrepancies between analytically calculated and experimental yields [8], [9], [10]. The analytically calculated neutron yields in all these studies assumed a pure metal lithium target and no attempts were made to correct for the perturbation effects listed above. The corrections for these effects are difficult because the quantitative information on oxide and carbon layers is hard to estimate. For the neutron yield near threshold, there is another factor to be taken into account. The 7Li(p,n) cross-section in this region changes rapidly, therefore the proton energy stability plays an important role. To account for large discrepancies (30% to a factor of 2) between the calculated and experimental yields, Aslam et al. [8] attributed them to the oxidation effect while Lee et al. [9] suggest insufficient LiF target thickness.

In this study, as a simple way of correcting all these perturbation effects, the effective incident proton energy was determined from the measured maximum neutron energy. The neutron spectrum was acquired using a 3He ion chamber. The measurements were conducted for five different proton energies from 1.95 to 2.3 MeV, so that the effect of finite proton energy spread can be compared for regions where the yield varies rapidly with proton energy versus regions where the yield is relatively slowly varying.

Section snippets

Accelerator and associated equipment

The accelerator based neutron source used in this study consists of the 3.0 MV Van de Graaf accelerator (model KN3000, HVEC, USA), the analyzing magnet bending the proton beam by 50°, and the lithium target assembly, as shown in Fig. 1. Total proton charge delivered to the target is measured by the current integrator (model 1000, Brookhaven Instruments Corporation, USA) connected to the target assembly. The accuracy of the current integrator specified by the manufacturer is typically 0.02%. A

Cross-calibration of the MGS-1 Eu standard with the locally produced Eu source.

Two sets of measurements at three distances: 25, 33, and 55 cm from the HPGe detector head were taken (Table 1). For determination of the 152Eu activity, five peaks were analyzed: 344.3, 411.1, 778.9, 964.1, and 1408.0 keV. For the 154Eu radioisotope, only the 1274.5 keV peak was used.

Very good linearity between peak counting rates indicates that the source positioning accuracy was sufficient. Since the statistical uncertainty is negligible, the most significant cause of error for the strong Eu

Conclusions

Total neutron yield of the 7Li(p,n) reaction was measured in the proton energy range from 1.95 to 2.3 MeV in order to explain the discrepancy between measured and calculated yields reported by many groups [8], [9], [10]. Using a 3He ion chamber it was found that the maximum neutron energies in raw spectra are consistently lower than expected from analytical calculations. A simple graphical method was applied to each incident proton energy spectra to estimate the difference. Based on the

Acknowledgements

This work has received support from the Natural Sciences and Engineering Research Council of Canada through research grant to S.H.B.

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