Investigation of neutron-induced reaction at the Goethe University Frankfurt

TL;DR

This paper reports initial neutron activation measurements at Goethe University Frankfurt using a Van-de-Graaff accelerator and discusses future experimental plans.

Contribution

It provides the first results of neutron-induced reactions at Goethe University Frankfurt and outlines future measurement plans.

Findings

Initial neutron activation data obtained

Use of 7Li(p,n) reaction for neutron production

Future measurement plans outlined

Abstract

We present first results and plans for future neutron activation measurements at the Goethe University Frankfurt. The measurements were performed at the Van-de-Graaff accelerator employing the 7Li(p,n) reaction.

Full text

1 Introduction

Neutron-induced reactions are relevant for many astrophysical scenarios [1]. The involved isotopes can be stable as during the s-process or radioactive as during the p-, i- or r-process. The different scenarios are characterized by different temperatures and neutron densities. Direct measurements of the relevant cross section are therefore ideally performed for many different energies. The most general method is the time-of-flight method, which typically requires large samples of isotopically enriched material, intense neutron sources and sophisticated detectors.

The activation method alleviates many of these costly requirements at the cost of integral measurements. The Van de Graaff accelerator at the Goethe University Frankfurt provides unpulsed proton beams of up to 20 $\mu$A in the energy regime between 1.5 and 2.5 MeV. This is ideally suited for the production of neutrons via the 7Li(p,n) reaction. Many different energy spectra can be produced depending on the proton energy, the thickness of the lithium layer and the position of the irradiated sample.

2 Activation technique

To obtain stellar cross sections from an activation experiment, the neutron spectrum should ideally correspond to the thermal spectrum at the respective $s$-process site [2]. The 7Li(p,n)7Be reaction, which represents the most prolific neutron source at low energy accelerators fulfills this requirement almost perfectly [3, 4, 5]. Adjusting the proton energy at $E_{p}=1912$ keV, 30 keV above the reaction threshold, yields a neutron spectrum with an energy dependence close to a Maxwellian distribution corresponding to $k_{B}T=25$ keV almost perfectly mimicking the situation during He shell flashes in AGB stars. A typical activation setup is depicted in Fig. 1.

The neutron spectrum can be significantly altered, if the proton energy, the proton-energy distribution, the thickness of the lithium target, or the angular coverage of the neutron field by the sample is modified [6, 7]. The neutron flux is typically determined using a reference sample of the same shape up- and downstream of the sample.

3 First measurements

The upcoming neutron facility FRANZ in Frankfurt (Germany) [8] will be based on the 7Li(p,n) reaction to produce neutrons by upgrading the proton source as well as high current lithium targets. In a first step, the Van-de-Graaff accelerator at the department of physics at the Goethe University Frankfurt was used to perform measurements of neutron-induced reactions.

3.1 Detection of $\gamma$-activities

We performed activations of natural samples of aluminum, potassium chloride, gallium and potassium bromide, see Figs 2-4. While recent activation measurements for the isotopes 27Al [9], 41K [10], and 79,81Br [11] exist, no activation-based data are available for 37Cl, 69,71Ga as well as for the population of the isomeric state of 82Br. The preliminary results shown in Figs 2-4 indicate that the activations were successful and we anticipate final results with 5-10% uncertainty.

3.2 Detection of $\alpha$-activities

A new detection system for charged particles was successfully tested for the first time, see Fig. 5. This setup allows the investigation of neutron-induced reactions with charged particles in the exit channel. This includes (n,$\alpha$), (n,p), and (n,fission) reactions. A first test was performed on the 7Li(n,$\gamma$)8Li($\beta^{-}$)8Be($\alpha$)$\alpha$ reaction. This reaction was investigated in the past with ionization chambers, [12]. For this purpose, the proton beam from the accelerator was periodically directed onto the neutron production target for 2 s and then deflected onto a beam stop for 2 s to observe the 0.8 s decay of 8Li followed by the prompt $\alpha$-decay of 8Be.

4 Outlook

In the near-term future we plan to perform activation measurements with different neutron spectra to constrain the stellar reaction rates at different temperatures. As soon as the RFQ-based accelerator is operational, we will perform activation as well as time-of-flight measurements on radioactive isotopes.

Acknowledgments

This research has received funding from the European Research Council under the European Unions’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 615126 and from the Deutsche Forschungsgemeinschaft RE-3461/4-1 and RE 3461/4-1.