Neutron-induced charged-particle reaction studies in nuclear astrophysics with a Micromegas based gaseous detector

. Neutron-induced charged-particle reaction studies on various unstable nuclei play an important role in understanding various nucleosynthesis processes occurring in explosive astrophysical scenarios. We are pursuing a novel experimental approach to study neutron-induced charge particle cross-sections for various unstable nuclei at e ﬀ ective temperatures of 1.5-3.5 GK using the 7 Li( p , n ) 7 Be reaction as a neutron source with three orders of magnitude higher neutron intensities with respect to currently available neutron time-of-ﬂight facilities. We plan to perform our experiments with a 10-µ A proton beam at the Physikalisch Technische Bundesanstalt facility (PTB, Germany), with a Mi-cromegas based gaseous detector being developed.


Introduction
Neutron-induced charged-particle reaction studies on various unstable nuclei are imperative for a complete understanding of the nucleosynthesis occurring in many astrophysical sites and events. This information is used as input for network calculations that provide predictions on the expected abundances [1][2][3]. Comparing these predictions to the observed abundances, may, for example, provide an indication either on the accuracy of the models, the contribution of different astrophysical events to the galactic isotopic content, or the frequency of specific events such as supernovae.
A complete understanding of supernovae nucleosynthesis requires data on thousands of nuclear reaction rates of different types, most of them not yet experimentally accessible, at temperatures of 1.5-3.5 GK. Sensitivity studies have pointed out several (n, p) and (n, α) reactions to have relatively high impact on the final isotopic abundances [3][4][5][6][7]. Experimental cross section data on (n, p) and (n, α) reactions at the relevant energies for explosive nucleosynthesis is scarce or non-existent for most isotopes of interest. One reason is the need for an intense neutron source at energies of 10-2000 keV, corresponding to a Maxwellian flux distribution at temperatures of 1.5-3.5 GK. Currently, such experiments are performed using the neutron Time-of-Flight (TOF) technique. Today's state-of-the-art neutron TOF facility, produces neutrons at an intensity of about 10 6 n/s at the relevant energy range [8]. Another difficulty is the target size. Even for stable isotopes, the limited range of the outgoing charged particle impose a strong limit on the effective target size. For unstable isotopes the target size is even a bigger problem, both in terms of production and target handling. Note that the activation approach is not feasible for most isotopes of interest, which naturally decay via electron capture or β + , which are indistinguishable from the (n, p) reaction by activation measurements. Our novel experimental approach to study this cross-section measurements at stellar temperatures using 7 Li(p, n) 7 Be reaction as a neutron source with three-four orders of magnitude higher neutron intensities as compared to TOF facilities is described in detailed in a recent article [9]. We will use this method to study (n, p) and (n, α) cross-section at explosive stellar temperature, for various nuclei for which experimental data is poor or nonexistent. We plan to conduct our experiments at PTB, Germany, with a neutron intensity on the order of 10 9 n/s on the target, a three orders of magnitudes above existing neutron TOF facilities with a gaseous detector being developed as described in detail ahead.

Experimental details and plans
We aim to measure the neutron-induced charge-particle reaction at neutron energies, 1 -2000 keV. We plan to conduct measurements important to supernovae nucleosynthesis with life times on the order of 10 5 years or above, which we consider feasible with the current experimental setup. Neutrons are produced via the 7 Li(p, n) 7 Be reaction with a monoenergetic proton beam at various energies from 1.9 MeV -3.6 MeV in a step of 0.1 MeV. The target of interest will be placed in a gaseous detector, and the outgoing charged particles are detected by ionization.  7 Be reaction, which further impinges on a target place inside the Micromegas-based gaseous detector. The detector is located 2 cm downstream of the neutron source. Fig. 2 (a) shows the calculated neutron spectra at the target, for a thick lithium target, and 5 cm 2 target located 2 cm downstream the lithium target at proton energies from 1.9 MeV to 3.6 MeV and Fig. 2 (b) shows the total neutron intensity going through the sample as a function of proton energy. The neutron spectra were calculated using SimLiT [10]. Reaction products from the neutron-induced reaction are detected inside the gas-filled detector.

The detector
The detector is cylindrical with a cathode, equipotential rings, and a segmented Micromegas detector. The sample will be placed in the center of the cathode inside the chamber facing the Micromegas. Such a design of the detection system provides a large solid angle for chargeparticle detection. The separation between the cathode and the Micromegas is 18 cm, and the active volume diameter is 16 cm. The segmentation scheme of the Micromegas board is shown in the right panel of Fig. 1.

Fill gas
We carried out detailed analysis for various gas mixtures for the detector being developed for neutron-induced charge particle reaction experiments. Fig. 3 shows electron drift velocity as a function of the field for various gas mixtures obtained using the Magboltz program [11]. We plan to use Ne(90 ) and CF 4 (10 ) gas mixtures in the pressure range of 3 -4 atmosphere as detection gas. The Ne/CF 4 gas mixture is chosen as the detection gas as mixtures containing argon and hydrogen had to be omitted to avoid background from neutron scattering and 41 Ar. The addition of CF 4 gas has many desirable merits viz; high drift velocity, helpful for fast detectors and reduced sensitivity to neutron background compared to hydrogenated molecules [12]. Pure CF 4 gas can also be used at low pressure but may have one demerit of lower gain as compared to Ne/CF 4 gas [12].

Read-out plane
The anode plane will be a segmented Micromegas detector. It is divided into nine circular rings with a central pad ring of diameter 3 cm, second, third, fourth, fifth, sixth, seventh, and eighth rings of diameter, 4.5 cm, 6 cm, 7.5 cm, 9 cm, 10.5 cm, 12 cm, 14 cm respectively, and ninth ring of diameter 16 cm. These outer rings are further segmented into four sections, thus giving 33 segmented pads on the anode plane, as shown in the right panel of Fig.1. The central pad faces the target; hence it will detect any heavy charged particle emitted from the reaction. It will serve as a trigger to exclude most of the background events. The outermost ring, shown in red, will serve as a veto pad to exclude events that do not originate in the detector. The purpose for the rest of the segmentation is to allow particle identification based on ∆E-E technique, and to reduce the background rates on each individual pad as discussed ahead.

Geant4 simulation and Background
One of the significant challenges of this experiment is the intense expected background. In order to investigate the expected background level and improve the design of the detector, we carried out a Geant4 [13] simulation of the designed experimental setup. The primary background sources are neutron and γ radiation. The neutron spectrum will mainly produce low-energy background due to hadron scattering in the gas. Similarly, an intense 478-keV γ radiation from the 7 Li(p, p´γ) reaction will cause low-energy background due to Compton scattering in the gas. This background can be reduced by adding a 1.6 cm lead shielding between the source and the detector. Other γ radiations background are the 14.6 and 17.6 MeV γ-rays from the 7 Li(p, γ) reaction. Although they are of lower intensity and have a low probability of interacting with the gas, they still contribute via (γ, p) and (γ, α) reactions. The β + decays from the target itself do not contribute much to the background. Fig. 4 (a) shows a graphical illustration of Geant4 simulation in which γ-rays being bombarded on detector. Fig. 4 (b) shows the estimated background from all sources as calculated by Geant simulation for 3.6 MeV incident proton energy on Li along with expected yields and energies from the 26 Al(n, p 1 ) channel, which is the most important channel for the total 26 Al(n, p) rate. The broad proton peak is due to the wide neutron energy distribution. The spectrum presented in Fig. 3 was obtained for 10 10 primary particle events. Another issue is the background rate. Although it is possible to define a high threshold for this experiment and reduce the DAQ load, the actual ionization rate in the gas might hinder the operation of the detector. One of the goals of the segmentation on the anode plane we chose is to reduce the rate on a specific pad to a reasonable level. The simulation of the experimental setup implies that the proposed experiment is feasible despite the high background level.

Experiments
We plan to carry out, 26 Al(n, p) 26 Mg, 26 Al(n, α) 23 Na [t 1/2 =7.2 x 10 5 Years], and 40 K(n, p) 40 Ar, 40 K(n, α) 37 Cl [t 1/2 =1.25 x 10 9 Years] cross-section measurements. For the 26 Al case, the target thickness is limited by the availability of the unstable isotope and its price.  Considering a 100 nCi target and 10-µA proton beam, and using TENDL-2019 cross section evaluation [14], a rate of 10 3 -10 4 detections per hour is calculated for the protons and the αparticles. For 40 K case, considering 0.25 mg/cm2 target thickness and assuming 10-µA proton beam, a rate of 10 4 -10 5 detections per hour is expected for both protons and α-particles.

Summary
Measurements of neutron-induced cross sections on unstable nuclei at explosive stellar temperatures require high-intensity neutron fluxes at the relevant energies. We plan to use the 7 Li(p, n) 7 Be reaction with varying proton energies to produce a set of cross section measurements that can be combined with proper weights to obtain Maxwellian-Averaged Cross-Section (MACS) values at temperatures of 1.5-3.5 GK. Further work is being pursued with regard to detector building, testing and commissioning for the first experiment.