Session Q12: Nuclear Astrophysics

Measurement of the Total Cross-Section for the $^{9}$Be($\gamma $,n$\alpha )\alpha $ reaction

The $^{9}$Be($\gamma $,n$\alpha )\alpha $ cross section is key to understanding isotopic abundances of nuclei produced during the r-process. The inverse reaction bridges the unstable mass gaps at A = 8 leading to $\alpha (\alpha $n,$\gamma )^{9}$Be($\alpha $,n)$^{12}$C and so on, producing seed nuclei for the r-process and setting the neutron-to-seed nucleus ratio that drives universal isotopic abundance predictions [Ref 1,2]. In order to make high precision measurements ($\pm $ 5{\%}) of the $^{9}$Be($\gamma $,n$\alpha )\alpha $ cross-section which includes narrow resonances, a tunable gamma ray beam with small $\Delta $E/E is required along with gamma and neutron detectors whose efficiencies are well known. We used TUNL's high intensity gamma ray source (HI$\gamma $S) to measure the cross sections for $^{9}$Be($\gamma $,n) in the energy range of 1.55 to 5.0 MeV with beam energy resolutions between 14 and 150 keV as determined by large Ge detector. The neutrons were detected using $^{3}$He proportional counter. The most recent experimental results as well as their astrophysical consequences will be presented. [Ref 1] B. Meyer \textit{et al.}, Astro J., \textbf{399} 656-664 (1992). [Ref 2] T. Kajino \textit{et al.}, Nuc. Phys. A, \textbf{704}, 165c-178c (2002)   

Search for a Broad 9.11 MeV $2^+$ in $^{12}C$*

Carbon is formed during stellar helium burning in the ``triple- alpha process''-- the $^8Be(\alpha,\gamma)^{12}C$ reaction that is mostly governed by the $0^+$ state at 7.654 MeV. At high temperatures (T $>$ 3 GK) higher lying states in $^{12}C$ may contribute. A broad $(\Gamma_\alpha = 560 \ keV, \Gamma_\gamma = 0.2 \ eV) \ 2^+$ state at 9.11 MeV in $^{12}C$ was included in the NACRE compilation following theoretical prediction for the $2^+$ member of the rotational band built on top of the $0^+ $ state at 7.654 MeV. It increases the production of carbon at temperatures in excess of 1 GK by up to a factor of 15. An Optical-Readout Time Projection Chamber (O-TPC) operating with $CO_2$ gas that is being used at the High Intensity $\gamma$-ray Source (HI$\gamma$S) at TUNL is ideally suited for a search of such a state via the identification of triple alpha events from the $^{12}C(\gamma,3\alpha)$ reaction. We have studied this reaction at $E_{\gamma}$ = 9.55, 10.54, 10.84 and 11.14 MeV. Only 4 triple alpha events were observed during a 10 hour long measurement at 9.55 MeV which are consistent with the known broad $1^-$ state at 10.84 MeV in $^{12}C$. Using the known cross section of the $^{16}O(\gamma,\alpha)^{12}C$ (= 4.2 $\mu $b) we place the upper limit of $\Gamma_{\gamma} (0^+_{g.s.} \rightarrow 2^+) \ <$ 1.28 meV at $E_\gamma = 9.55$ MeV, hence $B(E2: 2^+ \rightarrow 0^+_{g.s.}) \ < \ 4 \times 10^{-3}$ W.u.\\ $^*$Work supported by the U.S. Department of Energy grant No. DE- FG02-94ER40870 and DE-FG02-97ER41033.   

First Direct Measurement of the Resonant $^{17}$F(p,$\gamma$)$^{18}$Ne Reaction Rate

Of significant importance in astrophysical events like novae and x-ray bursts is the rate of the $^{17}$F(p,$\gamma$)$^{18}$Ne reaction, where it contributes to element synthesis and energy generation. It is crucial for the production of radioactive $^{18}$F in such scenarios, which is a target of gamma-ray telescopes like GLAST and INTEGRAL. However, the $^{17}$F(p,$\gamma$)$^{18}$Ne reaction rate had not been determined experimentally. A 3$^{+{}}$ state in $^{18}$Ne predicted to dominate the rate was found at 599.8 keV using the $^{17}$F(p,p)$^{17}$F reaction, but the resonance strength, which is directly related to the reaction rate, remained unknown. For the first time, the $^{17}$F(p,$\gamma$)$^{18}$Ne reaction has been measured directly with the Daresbury Recoil Separator, using a mixed beam of radioactive $^{17}$F and stable $^{17}$O from the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. A resonance strength and $\gamma$ partial width for the 599.8 keV resonance was deduced, as well as an upper limit on the direct capture cross section at an energy of 800 keV. These results have been used to calculate a new reaction rate in the temperature range of interest, and the preliminary implications of this new, faster rate will be discussed.   

Spin assignments of $^{22}$Mg levels through a $^{24}$Mg(p,t)$^{22}$Mg measurement

The $^{18}$Ne(alpha,p)$^{21}$Na reaction plays a crucial role in the (alpha,p) process, which leads to the rapid proton capture process in X-ray bursts. The reaction rate depends upon properties of $^{22}$Mg levels above the alpha threshold at 8.14 MeV. Despite recent studies of these levels, only the excitation energies are known for most with no constraints on the spins. We have studied the $^{24}$Mg(p,t)$^{22}$Mg reaction at the ORNL Holifield Radioactive Ion Beam Facility, and by measuring the angular distributions of outgoing tritons, we provide some of the experimental constraints on the spins of astrophysically-important $^{18}$Ne(alpha,p)$^{21}$Na resonances. Details of the experimental setup and results will be presented. * This work was supported in part by the US DOE and the NSF.   

First direct measurement of $^{23}$Mg(p,$\gamma$)$^{24}$Al with DRAGON

During explosive nucleosynthesis, the $^{23}$Mg(p,$\gamma$)$^{24}$Al capture reaction may function as a breakout from the NeNa to the MgAl cycles. Depending on the resonance strength and energy, such a breakout could substantially affect the production of $^{26}$Al and $^{22}$Na which have been detected by orbital satellite. This important reaction was directly studied at astrophysically relevant energies ($E_{lab} \simeq$ 490 keV/u) by the DRAGON collaboration during the summer and fall of 2008. However, due to limitations of the ISAC facility, the experiment was complicated by a $^{23}$Na contamination ranging from 2 to 5000 times more intense than the $^{23}$Mg component. To compensate, a new local time-of-flight system and a multi-segmented ion chamber were used for particle identification. This talk will present and discuss some details of the experiment as well as initial results.   

Direct Measurement of Low-Energy Resonances in $^{31}$P(p,$\alpha$)$^{28}$Si and $^{35}$Cl(p,$\alpha$)$^{32}$S

Reaction cycles in explosive hydrogen burning in novae and X-ray bursts influence both the energy generation and the processing of material to higher masses. The $^{31}$P(p,$\alpha$)$^{28}$Si and $^{35}$Cl(p,$\alpha$)$^{32}$S reactions are thought to lead to the formation of reaction cycles in the Si-Ar region, but the strength of these cycles depends on the (p,$\gamma$)/(p,$\alpha$) reaction rate ratio. Previous attempts to measure the strength of low-energy resonances in $^{32}$S and $^{36}$Ar have relied on indirect methods or resulted only in setting upper limits for a number of the resonances of interest. We have measured the strength of low-energy resonances in $^{32}$S and $^{36}$Ar at Oak Ridge National Laboratory by using stable $^{31}$P and $^{35}$Cl beams and a differentially pumped windowless hydrogen gas target to detect p-$\alpha$ coincidences in arrays of silicon strip detectors. Details of the experimental configuration and results will be presented. $^{*}$This work is supported in part by the U.S. DOE and NSF.   

Precision Mass Measurements of Heavy $^{252}$Cf Fission Fragments Near the Astrophysical r-Process Path

Precision mass measurements of species near the path of the astrophysical r-process---expected to occur in core-collapse supernovae or neutron star mergers---are vital to reduce the uncertainties in the relevant neutron separation energies given by mass models and the consequent predictions of nucleosynthesis yields. As part of an ongoing program, the Canadian Penning Trap mass spectrometer at Argonne National Laboratory is measuring the masses of fission products from a 150~$\mu$Ci $^{252}$Cf source placed inside a large-volume He gas catcher. Presented measurements include Pr, Nd, Pm, Sm, Eu, and Gd to N = 96, 97, 98, 99, 98, and 99 respectively with precisions near 15~keV/$c^2$, and our results differ from the AME 2003 by up to 390~keV/$c^2$. Measurements of many more neutron-rich isotopes will be made in 2009 at the CARIBU upgrade to the ATLAS accelerator at ANL, which is approaching operation.   

The $^{11}{\rm B}(p,\alpha)^{8}{\rm Be}^*$ Reaction at the 0.675 MeV Resonance

There is interest in using the $^{11}{\rm B}(p,\alpha)^{8}{\rm Be}^*$ reaction near the 0.675 MeV resonance in an aneutronic fusion reactor. A detailed model of the reactor requires knowledge of the angular and energy distribution of the outgoing $\alpha$ particles. The state-of-the-art model of the reaction near the 0.675 MeV resonance assumes a sequential process leading to one high energy $\alpha$ and a nearly flat continuum of energies for the remaining two ``secondary'' $\alpha$ particles. Singles and coincidence data taken at TUNL using silicon surface barrier detectors do not agree with this model. The authors propose a three-body reaction at the 0.675 MeV resonance in which two of the $\alpha$ particles equally share almost all of the center-of-mass energy described by a Breit-Wigner distribution and the third particle receives the remaining energy. This new reaction model is in good agreement with both the singles and coincidence data. Details of both model calculations will be presented and compared with the experimental results.   

Mapping Neutron Star Crusts with the Lightcurves of Quasi-Persistent Transients

The quiescent emission from neutron star transients with long accretion outbursts can inform us about the physics of dense matter. We construct models of the thermal relaxation of the neutron star crust following the end of an accretion outburst. In agreement with Shternin et al., we find that the thermal conductivity of the neutron star crust is high, consistent with a low impurity concentration. The lightcurve has the form of a broken power-law. The initial power-law decay gives a direct measure of the crust temperature profile in the outer crust. The time of the break, at hundreds of days post-outburst, corresponds to the thermal time where the crust transitions from a classical to a quantum crystal, close to neutron drip. We calculate in detail the constraints on the crust parameters of both KS 1731-260 and MXB 1659-29 from fitting their cooling lightcurves.