Study of and reactions with a Multi-Sampling Ionization Chamber
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 13C16O and 22Ne25Mg reactions which are the main sources of neutrons in the slow neutron-capture process (s-process). Moreover, many 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 process [2], which is a reaction sequence of and reactions including the 14O17F, 18Ne21Na, 22Mg25Al, 26Si29P, 30S33Cl and 34Ar37K reactions thought to occur in X-ray bursts, i.e., thermonuclear explosions on the surface of accreting neutron stars. The reaction path in the 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 reactions have been studied directly in the past, and so for most cases the astrophysical reaction rates of the process were estimated by experiments measuring their time-inverse 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 and 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 and reactions. In order to verify the experimental technique we have chosen to measure the 17O20Ne reaction in a previously studied excitation energy range. We have also remeasured the 23Na26Mg and 23Na26Al 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 8Li11B reaction [6], [7]. At the energies used in these experiments, however, the 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 and reactions can be separated and measured simultaneously. For this, we use the 23Na+4He system in an energy range where both the and 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 and reaction products. Section 3 presents the comparison of the 17O 20Ne reaction measured with MUSIC and the data obtained by traditional techniques in normal kinematics. The 17O 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 23Na26Mg and 23Na26Al 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 and 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 and 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 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 reactions were detected in a graphite sphere neutron detector [12]. Due to a negative Q value of −5.656 MeV, the 17O20F reaction is energetically forbidden in this energy
The 23Na26Mg and 23Na26Al reactions
The 23Na(26Mg and 23Na(26Al reactions are important for our understanding of the 26Al production in massive stars () [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 and 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 and 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
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.
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