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30S RI Beam Production and X-ray Bursts

The present work reports the results of 30S radioactive beam development for a future experiment directly measuring data to extrapolate the 30S(alpha,p) stellar reaction rate in Type I X-ray bursts, a phenomena where nuclear explosions occur repeatedly on the surface of accreting neutron stars. We produce the radioactive ion 30S via the 3He(28Si,30S)n reaction, by bombarding a cryogenically cooled target of 3He at 400 Torr and 80 K with 28Si beams of 6.9 and 7.54 MeV/u. In order to perform a successful future experiment which allows us to calculate the stellar 30S(alpha, p) reaction rate, Hauser-Feshbach calculations indicate we require a 30S beam of ~10^5 particles per second at ~32 MeV. Based on our recent beam development experiments in 2006 and 2008, it is believed that such a beam may be fabricated in 2009 according to the results presented. We plan to measure the 4He(30S,p) cross-section at astrophysical energies in 2009, and some brief remarks on the planned (alpha,p) technique are also elucidated.


Motivation
The 30 S(α,p) 33 Cl reaction is a significant link in the αp process, which competes with the rp process on accreting neutron stars [ILI99,SCH99,PAR08], but there is no experimental data to date. X-ray bursts (XRBs) with multiple peaks in their bolometric luminosity are explained by Fisker, Thielemann & Wiescher as multiple releases of nuclear energy, separated by a nuclear waiting point [FIS04]. Fisker, Thielemann & Wiescher's 2004 model reproduces these doublepeaks for lower accretion rates (∼ 8 × 10 −10 M year −1 ) of solar material [AND89] in low mass X-ray binaries (LMXBs), resulting in helium shell flashes. In all XRBs, pre-burst burning takes place via the β -limited hot CNO cycle at T 9 = 0.1 − 0.2 K (T 9 = 10 9 K). A thin-shell instability leads to explosive nucleosynthesis [VAN74,HAN75], leading to break-out from the HCNO cycles, and the rp process commences primarily on 19 Ne and 21 Na seed nuclei [WIE99] at T 9 = 0.4 − 1.3 K [WOO04,FIS08]. The αp process, which does not rely on slow β -decays, begins to compete with the rp process, increasing the energy generation rate by as much as three orders of magnitude in one second [WAL81].
XRBs exhibit recurrent bursts with frequencies typically ranging from hours to days, and there are some 100 known X-ray bursters. However, three systems have exhibited multiple peaks in their bolometric luminosities [SZT85,VAN86,PEN89,KUU02], which constrain the models of XRBs, although there have been a plethora of proposed explanations since the were first observed. Fisker, Thielemann & Wiescher recently proposed a nuclear waiting point at 30 S, because the modeled burning is impeded from further (p,γ) captures by (γ,p) reactions, and by varying the Hauser Feshbach statistical reaction rate of 30 S(α,p) by a factor of 100, Fisker et al. produce a different burst profile where the double peak structure is notably diminished [FIS04]. We will perform a direct experimental measurement of the 4 He( 30 S,p) cross section, which requires a low energy (E beam = 32 MeV) 30 S RI beam of intensity ∼ 10 5 pps and high purity. We plan to scan the energy region corresponding to the Gamow window from 1-2 GK (1.4 MeV ≤ E cm ≤ 3.8 MeV).
α-capture reactions in the αp process proceed through T z = (N−Z) 2 = −1 compound nuclei, and cross sections on nuclei with T z = ±1 near A∼20 are shown to be dominated by natural parity α-cluster resonances [BRA99a, BRA99b, GRO02, DAB03, APR05]. Similar behavior is expected at higher mass regions [ANG99], casting doubt on the results of Hauser Feshbach statistical modeling in such cases [FIS04]. With the exception of 4 He( 18 Ne,p) 21 Na [BRA99a,BRA99b,GRO02], there is no direct (α,p) experimental data involving radioactive nuclei. These experimental data on 18 Ne(α,p) are taken mostly at energies higher than those found in XRBs, resulting in a small overlap with the Gamow window. Charged particle reactions are inhibited by the Coulomb barrier, and the cross sections of typical astrophysical reactions range from below picobarns to the microbarn range [WIE99]. The unlikely reaction probabilities require extremely low levels of detector background coupled with long stretches of nuclear beam time in order to get good reaction statistics. With respect to radioactive nuclei interacting with alpha particles, the foil windows often used for gaseous helium targets pollute the spectra with scattering events and cause energy straggling in the beam and ejecta, and radioactive beam time is a highly sought after commodity which limits the time available for individual experiments. To optimize use of RI beam time, indirect methods are critical for locating possible resonances, but there is at present only one known level (no spin/parity assignment) in 34 Ar at the relevant astrophysical energies (7.54 MeV ≤ E x ≤ 9.55 MeV).

30 S Beam Production
We successfully separated out the proton-rich isotope 30 S, after bombarding a cryogenic, windowed 3 He gas cell with a stable 28 Si beam using the 3 He( 28 Si, 30 S)n reaction. We tested two primary beams: 28 Si 9+ (6.9 MeV/u and 100 pnA); 28 Si 10+ (7.54 MeV/u and 10 pnA); both beams were extracted from the CNS HyperECR ion source and accelerated by the AVF cyclotron of the RIKEN Nishina Center. We tested three production target gas pressures up to 400 Torr, finding 30 S yield was maximized at the highest pressure we tested. We conducted two day RI beam development runs in both 2006 and 2008 at the CRIB facility [KUB02, YAN05] (see Figure 1), operated by the University of Tokyo and located at RIKEN. The cocktail beam is sent through two magnetic dipoles to an achromatic focal point F2, where we identify different species in the cocktail beam by radiofrequency (RF) time-of-flight (T OF) and energy E. We also tested a 1.5 µm mylar degrader at the dispersive focal plane.
To measure the 4 He( 30 S,p) 33 Cl cross section, we will bombard a thick 4 He gas cell with a mono-energetic beam of 30 S. We will gate on 30 S beam ions in RF and T OF and detect reaction protons downstream. We will scan the Gamow window for T 9 = 1 − 2 K → E beam = 11.3 − 32.3 MeV on target. We then plan to reconstruct event-by-event 4 He( 30 S,p) reactions, by tracking 30 S beam ions in two delay-line PPACs, measuring corresponding recoil protons with an array of ∆E − E silicon telescopes at various angles. We tested three different 4 He gas cells and detector setups with beams of 14 O, 21 Na, and 28 Si. We plan to measure the 4 He( 30 S,p) cross section in 2009.

Results
All our 30 S RI beam results were improved with use of 28 Si 10+ (7.54 MeV/u) primary beam. 30 S 16+ achieved 30% purity, 30 MeV, and 500 pps per 10 pnA at F3. 30 S 14+ achieved 0.8% purity, 32 MeV, and 1.2 ×10 4 pps per 10 pnA at F3. We determined that the RF-T OF spectrum of 30 S 15+ is contaminated by the leaky beam 28 Si 14+ due to similar charge-to-mass ratios, although we did not attempt to separate these two ions with a thin degrader at the momentum dispersive focal plane F1. Figure 2 shows 30 S 16+ is easy to distinguish, but the intensity is far below the desired 10 5 pps, however, the WF transmission was an astonishing 80%. Figure 3 shows we separate 30 S 14+ in the RF vs. T OF spectrum at ∼ 10 4 pps per 10 pnA, but due to limited time, we were not able to optimize the purity. When we attempted the use of a 1.5 µm aluminized mylar degrader at F1, the intensity on-target decreased by 80%, which can be improved with better optics tuning.
In 2008, we also tested the charge-state distribution of 28 Si beam ions in carbon compared to Havar foil, to test if adding a carbon stripper foil after our Havar-windowed production target increases the yield of highly charged states of 30 S. Although we attempted to measure the chargestate distributions directly with 30 S in May 2008, the primary beam current was fluctuating greatly due to instability of the HyperECR, and we were unable to properly normalize the data for comparison. However, for a 28 Si beam of 3.4 MeV/u (comparable to the 30 S energy achieving maximum transmission intensity at this point in the beam-line), it was found that 28 Si 14+ transmission through carbon foil is an order of magnitude higher than for Havar foil, 28 Si 13+ transmission was a factor of 5 greater for carbon compared to Havar, and 28 Si 12+ transmission was less than a factor of 2 greater for carbon compared to Havar. Thus, it may be possible to preferentially populate higher charge states of 30 S by use of a carbon stripper foil.
In the future, we hope to achieve the requisite 10 5 pps by extracting 28 Si 10+ at a higher rate allowing for intensities of up to 30 pnA at F0, and accelerate the beam for a larger number of turns in the AVF cyclotron up to 8.2 MeV/u. By comparing the 30 S intensity results from the ∼ 0.5 MeV/u energy increase from Decemeber 2006 to May 2008, it is concluded that a 0.5 MeV/u energy increase in this range corresponds to nearly an order of magnitude increase in the 30 S production yield. The production gas target has successfully held up to 700 Torr, so we may test if RI beam yield continues to increase with pressure. By combining these various improvements, we expect to achieve a 30 S intensity above 10 5 pps in the future.
This work is made possible through the CNS and RIKEN collaboration and funding from the National Science and Engineering Research Council of Canada.