[sup 30]S Beam Development and X-ray Bursts

Abstract

Over the past three years, we have worked on developing a well-characterized 30 S radioactive beam to be used in a future experiment aiming to directly measure to extrapolate the 30 S(α,p) stellar reaction rate within the Gamow window of Type I X-ray bursts. The importance of the 30 S(α,p) reaction to X-ray bursts is discussed. Given the astrophysical motivation, the successful results of and challenges involved in the production of a low-energy 30 S beam are detailed. Finally, an overview of our future plans regarding this on-going project are presented.The 30 S(α,p) reaction is a significant link in the αp-process, which competes with the rpprocess in Type I X-ray bursts (XRBs) [1], but the reaction rate is virtually unconstrained by experimental data. Not only are there no data directly measuring the 30 S(α,p) cross section in the literature (at any energy), but there are no conclusive experimental reports on the nuclear structure of the compound nucleus 34 Ar above the α-threshold with regard to any possible α-resonances. Hydrodynamic models of XRBs indicate that variation of the theoretical reaction rate has significant consequences.XRBs are understood to result from thermonuclear runaway in the hydrogen-and helium-rich accreted envelopes on the electron-degenerate surfaces of neutron star binary systems [2]. Accretion ensures a steady flow of fresh material will spread around the surface of the neutron star only to be buried by the continuous pile-up of more matter. Equilibrium between a fierce gravitational pressure and the constant energy release of the β -limited CNO cycle is broken by a thin-shell instability, triggering the onset of explosive nucleosynthesis [3,4] . Although powerful, these bursts do not disrupt the binary star system, hence X-ray bursters exhibit recurring episodes with hourly, daily, or more extended regularity, making them the "most common thermonuclear explosions in the universe" [5]. Computer simulations reproduce the energy release, burst profiles (rise time, peak wavelength, and decay curve), and the recurrence time-scales of these astrophysical phenomena.

Measurement of the 30 S+α system for type I X-ray bursts

“…The main impurity was 29 P 15+ , which has a production cross section around ∼ 10 2 mb/sr [11], about two orders of magnitude higher than 30 S under these conditions (∼ 1 mb/sr) [12]. Further details on the RI beam production and required development were reported previously [13,14,15,16].…”

30S(\(\alpha \), p) Thermonuclear Reaction Rate from Experimental Level Structure of 34Ar

“…The 30 S beam was produced with a typical purity of 28%, an intensity of 8 × 10 3 pps, and separated at 4.0 MeV/u [7]. We injected 1.6 × 10 9 30 S ions during the main measurement over 2 days into an active target system filled with He+CO 2 gas [8].…”

Experimental investigation of the30S(α, p) thermonuclear reaction in x-ray bursts

“…The radioactive 30 S beam was produced in-flight via the 3 He( 28 Si, 30 S)n reaction using the Center for Nuclear Study (CNS) low-energy radioactive ion beam separator (CRIB) [10,11], where we bombarded a cryogenically-cooled gas cell [12] of 3 He at 1.72 mg · cm −2 with a 28 Si beam at 7.3 MeV/u [13]. The 30 S beam arrived on target at 1.6 MeV/u, an average of 30% purity, and with an intensity of 10 4 particles per second.…”