Study of (α,p) and (α,n) reactions with a Multi-Sampling Ionization Chamber

https://doi.org/10.1016/j.nima.2017.03.060Get rights and content

Abstract

A large number of (α,p) and (α,n) reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed direct measurements of the previously measured reactions 17O (α,n) 20Ne, 23Na (α,p) 26Mg, and 23Na (α,n)26Al. These reactions were investigated in inverse kinematics using 4He gas in the detector to study the excitation functions in the energy range of about 2–6 MeV in the center of mass. We found good agreement between the cross sections of the 17O (α,n) 20Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the 23Na (α,p) 26Mg and 23Na (α,n) 26Al reactions.

Introduction

Helium, the second most abundant element in the universe, plays an important role in nuclear astrophysics through a variety of α-particle induced reactions. Examples are the 13C(α,n)16O and 22Ne(α,n)25Mg reactions which are the main sources of neutrons in the slow neutron-capture process (s-process). Moreover, many (α,n) reactions have been found to be relevant for the nucleosynthesis of light nuclei in the rapid neutron-capture process (r-process) in neutrino-driven winds [1]. Other examples of α-induced reactions occur in the so-called (α,p) process [2], which is a reaction sequence of (α,p) and (p,γ) reactions including the 14O(α,p)17F, 18Ne(α,p)21Na, 22Mg(α,p)25Al, 26Si(α,p)29P, 30S(α,p)33Cl and 34Ar(α,p)37K reactions thought to occur in X-ray bursts, i.e., thermonuclear explosions on the surface of accreting neutron stars. The reaction path in the (α,p) process proceeds through a region of β-unstable nuclei located on the proton-rich side of the valley of stability and therefore requires experiments with radioactive ion beams done in inverse kinematics. Very few of these (α,p) reactions have been studied directly in the past, and so for most cases the astrophysical reaction rates of the (α,p) process were estimated by experiments measuring their time-inverse (p,α) reactions which, however, only probes the ground-state to ground-state transition.

For systems involving stable nuclei, the experiments can use thin targets which are bombarded by high-intensity beams of α-particles. The outgoing protons and neutrons can be detected in arrays of particle detectors, covering a large fraction of the angular distributions. After integrating the angular distribution the experiment is then repeated at a slightly different energy resulting in an excitation function covering the energy range of interest. This method can be quite time consuming due to the large number of energy changes required. An advantage is the good energy resolution that can be obtained by using thin targets, which can give information about the resonant structure in the compound system. While resonances play a role in nuclear astrophysics, the astrophysical reaction rates in quiescent and explosive stellar environments are calculated with energy-averaged cross sections. This information can be obtained by activation, i.e. by bombarding stacks of thicker target foils with beams of α-particles and stopping the short-ranged radioactive reaction products in the target or in separate catcher foils. This technique works only for unstable reaction products with sufficiently long half-lives and appropriate decay properties [3]. While this method can measure an excitation function in an experiment with one beam energy, it requires targets with good homogeneity and well-known thicknesses and the knowledge of the energy loss of the beams in the target material.

The goal of this paper is to discuss the use of a MUlti-Sampling Ionization Chamber (MUSIC) detector to simultaneously measure (α,p) and (α,n) reactions of astrophysical interest. MUSIC has a segmented anode that allows the investigation of a large part of an excitation function with a single measurement [4], [5], thus offering the opportunity to study the cross sections of astrophysically relevant (α,p) and (α,n) reactions. In order to verify the experimental technique we have chosen to measure the 17O(α,n)20Ne reaction in a previously studied excitation energy range. We have also remeasured the 23Na(α,p)26Mg and 23Na(α,n)26Al reactions which are important for the production of 26Al in massive stars.

A similar detector based on a proportional counter has been used in the past for a study of the 8Li(α,n)11B reaction [6], [7]. At the energies used in these experiments, however, the (α,p) channel leading to Be particles (Z=4) was energetically forbidden and, thus, for this relatively light system the separation between the 8Li beam (Z=3) and the 11B reaction products (Z=5) was quite large, simplifying the identification of the particles of interest. One of the goals of our study is to demonstrate that the reaction products of the (α,p) and (α,n) reactions can be separated and measured simultaneously. For this, we use the 23Na+4He system in an energy range where both the (α,p) and (α,n) reaction channels are energetically allowed.

This paper is organized as follows. In Section 2 we give a brief description of the MUSIC detector and discrimination of the (α,p) and (α,n) reaction products. Section 3 presents the comparison of the 17O (α,n)20Ne reaction measured with MUSIC and the data obtained by traditional techniques in normal kinematics. The 17O (α,n)20Ne reaction has a sufficiently large negative Q-value (Q=−5.656 MeV) so that it does not contribute in the energy range studied in the experiment. Section 4 presents the study of the 23Na(α,p)26Mg and 23Na(α,n)26Al reactions and gives the results in an energy range where both reactions contribute. A summary and an outlook for future experiments is presented in Section 5.

Section snippets

The Multi-Sampling Ionization Chamber (MUSIC)

The MUlti-Sampling Ionization Chamber (MUSIC) has been previously used for measurements of fusion reactions involving stable and radioactive nuclei [4]. We now explore its applicability to measure (α,p) and (α,n) reactions. Since a more detailed description of the detector has already been given in a separate publication [5] we will only summarize the main operation principles here and focus on the analysis of the (α,p) and (α,n) measurements. A cross section of the detector is shown in Fig. 1

The 17O(α,n)20Ne reaction

As a first test case of MUSIC we have investigated the 17O+4He system. The 17O (α,n)20Ne reaction has been studied in Ref. [11] by bombarding a thin 17O target (equivalent to an energy loss of ∼35 keV for 1 MeV α particles) with α-particle beams in the c.m. energy range of 0.75–4.3 MeV. The outgoing neutrons from the (α,n) reactions were detected in a graphite sphere neutron detector [12]. Due to a negative Q value of −5.656 MeV, the 17O(α,p)20F reaction is energetically forbidden in this energy

The 23Na(α,p)26Mg and 23Na(α,n)26Al reactions

The 23Na(α,p)26Mg and 23Na(α,n)26Al reactions are important for our understanding of the 26Al production in massive stars (M>8M) [17]. Although the physics behind the study of these reactions is interesting, in this work we will focus on the experimental technique only. A second level of complexity is added since both (α,p) and (α,n) channels are open. Therefore the aim of this work was to demonstrate that the MUSIC detector is able to separate and measure these reactions simultaneously. The

Summary

We have described a method to measure excitation functions of angle- and excitation-energy integrated cross sections of (α,p) and (α,n) reactions which are of interest to nuclear astrophysics. Making use of a multi-sampling ionization chamber MUSIC and the advantages of inverse kinematics, the reaction products of the two reactions can be detected with close to 100% detection efficiency.

The application of the active target system MUSIC to α-particle induced reactions was tested for the 17O(α,n)

Acknowledgments

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under contract number DE-AC02-06CH11357. The authors J. L. and D. S. G. also acknowledge the support by the U.S. Department of Energy, Office of Science, Office of Nuclear Science, under Award No. DE-FG02-96ER40978. This research used resources of ANL's ATLAS facility, which is a DOE Office of Science User Facility.

References (19)

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

Cited by (23)

  • Classification of events from α-induced reactions in the MUSIC detector via statistical and ML methods

    2024, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
  • ATHENA: An active target detector for the measurement of total cross sections

    2023, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
  • Improving the characterization of fusion in a MuSIC detector by spatial localization

    2022, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
  • The Encore active target detector: A Multi-Sampling Ionization Chamber

    2021, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
  • MuSIC@Indiana: An effective tool for accurate measurement of fusion with low-intensity radioactive beams

    2021, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
View all citing articles on Scopus
View full text