Neutron activations for lower s-process temperatures

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Introduction
Heavy elements beyond iron are mainly synthesised by neutron capture reactions. The slow neutron capture process (s-process) accounts for about 50 %. The corresponding neutron energies in the different astrophysical sites range from a few to about hundred keV [1,2]. By using the activation technique, neutron capture reactions have been studied over the last decades at k B T = 25 keV. With a modified approach, we measured neutron capture cross sections for lower s-process temperatures. With ring-shaped samples, a specific range of neutron emission angles is covered such that the neutron energy distribution corresponds to a Maxwell-Boltzmann distribution of k B T = 6 keV. We did first proof-of-principle measurements using nickel and tantalum samples. The neutron yield is significantly higher than in previous approaches using the 18 O(p,n) reaction as a neutron source [3]. While the neutron yield using the 18 O(p,n) approach is about 10 −4 lower than for the standard 25 keV setting, our approach reduces the neutron yield only by a factor of 10 −2 .

Activations for lower s-process temperatures
In the activation method, neutrons are produced by protons impinging on a Lithium target. The sample of interest is sandwiched by gold foils, which act as monitors for the neutron flux (Fig.1). The neutrons are produced via the reaction 7 Li(p,n). A lithium glass detector is placed behind the samples. The neutron flux fluctuations are monitored using the lithium glass detector. The working horse method for the last decades delivers quasi stellar neutrons at k B T = 25 keV for proton energies of E P = 1912 keV, if the sample covers the whole neutron cone. The simulation of the emitted neutrons was done using PINO [4] (Fig.2, left panel). The radioactive reaction product in the sample was characterized by γ-spectroscopy. Our new method relies on a partial angular coverage of the neutron cone. We simulated different geometries varying position and size of ring-shaped samples as well as the proton energy.  Using a proton beam energy of E P = 1914 keV and a ring-shaped sample placed 3.2 mm after the lithium target with an inner radius of 6 mm and an outer radius of 10 mm covers the neutron cone such, that the resulting neutron spectrum closely resembles a Maxwellian distribution of k B T = 6 keV (Fig.1, right panel).

Proof of principle
We measured the reaction 64 Ni(n,γ) 65 Ni at the energy of k B T = 6 keV and k B T = 25 keV. We identify the produced radioactive nuclei using γ-spectroscopy with two Broad Energy Germanium (BEGe) detectors in head-to-head geometry. Fig.3 shows a spectrum of the ring measurements. The left panel shows the identification of the produced radioactive 65 Ni using the γ-emission lines at E γ = 1482 keV and 1115 keV. Preliminary results are listed in Tab. 1.
As a second proof-of-principle, we performed a measurement of the reaction 181 Ta(n,γ) 182 Ta and 181 Ta(n,γ) 182 Ta m . The identification of the radioactive 182 Ta nuclei as well as its isomeric state 182 Ta m was done using γ-spectroscopy. Fig.3 shows the measurement for the ground state of 182 Ta (middle panel) and for the isomeric state of 182 Ta (right panel). The identification of the ground state 182 Ta was done using the γ-emission lines at E γ = 1121 keV, 1221 keV and 1189 keV. The isomeric state could be resolved by the γ-emission lines at E γ = 147 keV, 172 keV and 185 keV. The preliminary cross sections are given in Tab. 1.

Summary
To perform activation measurements at lower energies, it is possible to vary the angular coverage of the neutron cone. This can be achieved using ring-shaped of the samples. We  64 Ni was determined to σ k B T =6 keV = 12 mb which fits quit well to the KADO-NIS value σ k B T =5 keV =11.8 mb [5]. The neutron capture on 181 Ta into the ground state of 182 Ta was preliminary determined to σ k B T =6 keV = 2136 mb compared to KADONIS with σ k B T =5 keV = 2765 mb [5]. The neutron yield is reduced by a factor of 100 compared to the standard 25 keV setting, but is still a factor of 100 higher than previous attempts using the 18 O(p,n) reaction.