ISNAP Seminars - Abstracts 2008 Fall

Topic: Nucleosynthesis Modes in the High-Entropy-Wind of Core-Collapse Supernovae

Khalil Farouqi (University of Chicago, IL) / Oct. 27, 2008

In an attempt to constrain the astrophysical conditions for the nucleosynthesis of the classical r-process elements beyond Fe, large-scale dynamical network calculations within the model of an adiabatically expanding high-entropy wind (HEW) of type II supernovae (SN II) were performed.
A simple superposition of several entropy-components (S) with model-inherent weightings results in an excellent reproduction of the overall Solar-System (SS) isotopic r-process residuals, as well as the more recent observations of elemental abundances of metal-poor, r-process-rich halo stars in the early Galaxy.
For the heavy r-process elements beyond Sn, the HEW predicts a robust abundance pattern up to the Th, U r-chronometer region. For the lighter neutron-capture region, an S-dependent superposition of (i) a normal á-component directly producing stable nuclei, including s-only isotopes, and (ii) a component from a neutron-rich á-freeze out followed by the rapid re-capture of â-delayed neutrons emitted from the far-unstable seed nuclei can occur.
In agreement with several recent halo-star observations in the 60 < A < 100 region, the HEW confirms a Z-dependent non-correlation, respectively partial correlation with the heavier “main” r-process elements.

Topic: Reaction Rates for Dense Matter Burning

Mary Beard (University of Notre Dame) / Sept. 15, 2008

An interesting question in nuclear astrophysics is the fate of post X-ray burst matter. Once an accreting neutron star has undergone thermonuclear runaway, the ashes of the bursts are forced down into the crust of the neutron star. With increasing density a volume element of matter undergoes a number of electron captures and neutron emissions, resulting in low mass neutron rich nuclei. At sufficiently high densities pycnonuclear reactions can set in.
We recently formulated a single phenomenological expression for the calculation of pycnonuclear reaction rates which is not only valid in mixed component plasma, but can also be extended to cover the five stellar burning regimes. Here we use this expression in concert with a dynamic reaction network code to model the neutron star crust composition at high densities.

Topic: JINA REACLIB in Type-I X-ray Bursts

Richard H. Cyburt (NSCL, Michigan State University) / Sept. 8, 2008

Nuclear astrophysics requires knowledge of experiments, observations and models. Of particular importance is nuclear physics input, in the form of thermonuclear reaction rates, weak decay rates and nuclear masses. This interchange between theory and experiment and observations is quite prevalent in studies of the explosions on neutron star surfaces, known as X-ray bursts. The Joint Institute for Nuclear Astrophysics (JINA) REACLIB Project, is an ongoing effort to maintain the most current and accurate library of thermonuclear reaction rates and ground state weak decay rates.
A consequence of updating rates, is that old rates are kept as optional rate versions for users. We have just finished a round of updating and have created 2 new snapshot libraries, V0.5 and V1.0. Both libraries contain the newest experimental-based rates and differ only in the statistical model reaction rates adopted. I will discuss the biggest changes to these libraries and explore the impact they have on X-ray burst predictions. Our studies can be used by experimentalists/observers/theorists to improve our understanding of these bursts.

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Topic: Studies on nuclear structure on and away from the stability line

Maitreyee Saha Sarkar (Nuclear and Atomic Physics Division, Saha Institute of Nuclear Physics, Kolkata, INDIA) / Aug. 18, 2008

The seminar will be aimed to discuss some of our recent work. We have performed experimental and theoretical investigations in two regions of the nuclear chart: stable nuclei with N ˜ Z near ⁴⁰Ca and highly neutron-rich ones above ¹³²Sn. In the recent times, India has acquired quite a few Clover detectors to build an Indian National Gamma Array (INGA). We have used earlier campaigns of this array for gamma spectroscopic studies. Characterisation of Clover detector also forms a part of our study. Apart from discussing our different work in brief, I shall highlight our new results on neutron-rich isotopes of semi-magic Sn and discuss the experimental characterisation of Clover detectors.
I shall start with our theoretical work in neutron-rich ¹³²Sn region. Large basis un-truncated shell model (SM) calculations have been done for the A = 138 neutron-rich nuclei above the ¹³²Sn core using two (1+2) - body nuclear Hamiltonians, viz., realistic CWG and empirical SMPN. Calculated binding energies (with SMPN only), excitation spectra, and B(E²) values are nicely reproduced by both the interactions for even-even A = 138 isobars for which experimental data are available. But the two theoretical results differ dramatically for ¹³⁸Sn, a nucleus on the r-process path. CWG predicts nearly constant energies of the first 2⁺ states for the even-even Sn isotopes above the ¹³²Sn core, normally expected for semi-magic nuclei. But SMPN predicts a remarkable new feature: decreasing first E (2⁺), E(4⁺) energies with increasing neutron number. The predicted energies for the Sn isotopes fit in the systematics for the E (2⁺) energies of their isotones with Z > 50. This observation provides very useful ingredients for reviewing the problems related to the nuclear astrophysics in general.
The second part of my talk deals with the high-energy (till 11 MeV) characterisation of the Compton suppressed Clover detector. This work has been done as a prelude to our experimental study of mass 40 region using a Clover array. Typical characteristics of a Clover detector like addback factor, hit pattern distribution have been determined. Relative efficiency and the ratios of escape peaks to the full energy peak of Clover in add and addback modes have been compared with those of a standard HPGe detector. In this connection I shall also discuss our preliminary work on the improvement of the background characteristics of normal HPGe and Clover detectors at the low energies.

Topic: Study of Nuclear Weak Response by Using Strong Interaction

Professor Yoshitaka Fujita ( Osaka University, Japan ) / Aug. 5, 2008

Gamow-Teller (GT) transition is one of the most popular nuclear weak processes of spin-isospin type. It is of interest not only in the study of nuclear physics, but also in astrophysics; it plays important roles, for examples, in supernova-explosion or nuclear synthesis. Relatively limited information can directly be obtained through the study of weak processes, such as B decay or neutrino induced reactions. However, it was found that (p, n) charge-exchange reactions at intermediate incoming energies (E > 100 > MeV) and at 0° could selectively excite GT transitions, that extended the region of excitation energy of the study. With one-order-of-magnitude improvement of the energy resolution in (³He,t) measurements at 140 MeV/nucleon, fine structures of GT excitations, even those of GT giant resonances, can now be studied. Determination of GT transition strengths for pf-shell nuclei with astrophysical interest is discussed. We show that quantum number "isospin" plays important roles in such studies.

Topic: Measurements of Nuclear Astrophysics Reactions in CIAE

Professor W.P. Liu ( CIAE, China Institute of Atomic Energy, China ) / Aug. 4, 2008

To account for the short half and extremely low reaction cross section, novel indirect approach is often the only solution. One of such approach is using direct reaction which involves same proton or neutron transfer as in radiation capture, by using the beams of low energy unstable nuclei. This technique uses DWBA analysis of experimental angular distribution to extract asymptotic normalization constants or nuclear spectroscopic factors. Then this radical contribution is inserted to capture rates calculations. This approach is tested to be reliable with the precision mainly limited by the ambiguity of optical potentials.
This talk will summarize the nuclear astrophysical studies using the unstable ion beam facility GIRAFFE in CIAE, by indirect measurements. We measured the angular distributions for some single proton or neutron transfer reactions, such as ⁷Be(d,n)⁸B, ¹¹C(d,n)¹²N, ⁸Li(d,p)⁹Li etc, in inverse kinematics, and derived the astrophysical S-factors or reaction rates of relevant (p,r) and (n,r) reactions by asymptotic normalization coefficient, spectroscopic factor, and R-matrix approach at astrophysical relevant energies.

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