Nuclear astrophysics experiments: Reactions and elastic scattering

Experimental methods in nuclear astrophysics experiments with radioactive beams are described, and evaluated. The importance of performing the (p, p) elastic scattering in parallel to a (p, α) or a (p, γ) reaction is emphasized. Received: 1 May 2001 / Accepted: 4 December 2001     

Photo-induced nucleosynthesis: Current problems and experimental approaches

Photon-induced reactions play a key role in the nucleosynthesis of rare neutron-deficient p -nuclei. The paper focuses on , , and reactions which define the corresponding p -process path. The relation between stellar reaction rates and laboratory cross-sections is analyzed for photon-induced reactions and their inverse capture reactions to evaluate various experimental approaches. An improved version S _C(E) of the astrophysical S -factor is suggested which is based on the Coulomb wave functions. S _C(E) avoids the apparent energy dependence which is otherwise obtained for capture reactions on heavy nuclei. It is found that a special type of synchrotron radiation available at SPring-8 that mimics stellar blackbody radiation at billions of kelvins is a promising tool for future experiments. By using the blackbody synchrotron radiation, sufficient event rates for and reactions in the p -process path can be expected. These experiments will provide data to improve the nuclear parameters involved in the statistical model and thus reduce the uncertainties of nucleosynthesis calculations.     

Half-lives of rp-process waiting point nuclei

We give results of microscopic calculations for the half-lives of various proton-rich nuclei in the mass region A = 60-90, which are involved in the astrophysical rp-process, and which are needed as input parameters of numerical simulations in Nuclear Astrophysics. The microscopic formalism consists of a deformed QRPA approach that involves a self-consistent quasiparticle deformed Skyrme Hartree-Fock basis and residual spin-isospin separable forces in both the particle-hole and particle-particle channels. The strength of the particle-hole residual interaction is chosen to be consistent with the Skyrme effective force and mean-field basis, while that of the particle-particle is globally fixed to = 0.07 MeV after a judicious choice from comparison to experimental half-lives. We study and discuss the sensitivity of the half-lives to deformation and residual interactions.     

Measurement of the 24Mg(p, t)22Mg reaction for the states near the 21Na + p threshold

Differential cross-sections of the ²⁴Mg(p, t)²²Mg reaction were measured at 34.68 MeV for the states near the proton threshold at 5.502 MeV in ²²Mg. Among them, the new states at 5.962, 6.046, 6.246 and 6.323 MeV, which were reported previously, have been confirmed. Angular distributions for these states were analyzed by distorted-wave Born-approximation calculations to deduce the spins and parities. The angular distribution for the 5.714 MeV state, which is considered to be most crucial for the stellar reaction ²¹Na(p, γ)²²Mg, has been found to be consistent with J ^π = 2⁺ assignment. The 6.046 MeV state is newly assigned to have J ^π = 0⁺, and the 5.962 MeV state is tentatively assigned to have J ^π = (1^-). These two states will also play an important role for ²²Mg production in novae. Received: 7 March 2002 / Accepted: 7 May 2002     

A study of the proton resonant property in 22Mg by elastic scattering of 21Na + p and its astrophysical implication in the 18Ne(α, p)21Na reaction rate

Proton resonances in ²²Mg have been investigated by the resonant elastic scattering of ²¹ Na + p . The ²¹Na beam with a mean energy of 4.00 MeV/nucleon was separated by the CNS radioactive-ion-beam separator (CRIB) and bombarded a thick ( CH_{2 n} target. The energy spectra of recoiled protons were measured at scattering angles of θ_c.m. ≈ 172^° , 146^° , respectively. Several excited states observed before have been confirmed including two states (at 6.616, 6.796 MeV) observed at TRIUMF. A new state at 7.06 MeV has been observed, and another new one at 7.28 MeV is tentatively identified due to its low statistics. The proton resonant parameters were deduced from an R -matrix analysis of the differential cross-section data with a SAMMY-M6-BETA code. The astrophysical implication for the ¹⁸ Ne(α, p)²¹ Na reaction has been briefly discussed.     

Stars from birth to death: Laboratories for exotic nuclei?

Recent developments in theoretical model calculations for the synthesis of the chemical elements in stars are reviewed. Special emphasis is put on a discussion of various astrophysical sites, including the Sun and core collapse and thermonuclear supernovae. Results of numerical simulations are presented and discussed, together with new results concerning solar-system abundances as well as abundances observed in very metal-poor stars, in the context of searches for constraints on the still rather uncertain nuclear-physics data and astrophysical models. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: wfh@mpa-garching.mpg.de     

Potential resonant screening effects on stellar 12C + 12C reaction rates

The ¹²C + ¹²C fusion cross-sections show a resonant behavior down to the lowest energies accessible so far in the laboratory. If this tendency continues into the astrophysical energy range, the stellar ¹²C + ¹²C reaction rates have to be corrected for resonant screening effects, in addition to the conventional screening corrections. We estimate the resonant screening effects in the weak electron screening limit for hydrostatic burning and white-dwarf environments. Received: 24 May 2002 / Accepted: 31 July 2002 / Published online: 7 November 2002 RID="a" ID="a"e-mail: langanke@ifa.au.dk Communicated by J. ?yst?     

Multi-reflection time-of-flight mass spectrograph for short-lived radioactive ions

RIKEN’s new RI-Beam Factory (RIBF) will provide unprecedented access to neutron-rich nuclei of importance to r-process nucleosynthesis. We are constructing an advanced multi-reflection time-of-flight mass spectrograph to perform precision mass measurements of these nuclei. We discuss the device and compare its performance to that of the well-known Penning trap mass spectrometer.     

New low-energy RIB separator CRIB for nuclear astrophysics

An in-flight RIB separator at low energies, which is the first extensive separator at low energies, called CRIB, is just under construction at CNS. This consists of a double-achromatic magnetic separator, a window-less gas target, and a Wien filter. Some characteristics of the CRIB are described. Possible experimental plans are also discussed in our nuclear astrophysics project for the study of the explosive hydrogen burning process, especially on the onset mechanism. Received: 1 May 2001 / Accepted: 4 December 2001     

A first experimental approach to the 15O + α elastic scattering

The ¹⁵O(α,α)¹⁵O elastic scattering is investigated using a ¹⁵O radioactive beam and a He gas cell limited by Mylar windows. The width of a ¹⁹Ne state at an excitation energy of 5.35MeV is measured as Γ_α = 3.2±1.6keV, in agreement with charge symmetry estimate.     

Impurity nuclear physics

Using a germanium-detector array for hypernuclear γ spectroscopy (Hyperball), we measured B(E2) of the ⁷ _ΛLi hypernucleus and observed a significant shrinkage of the ⁶Li core induced by a Λ-particle. In this way, nuclear properties can be drastically changed by introducing a Λ-particle, which can be investigated by high-resolution hypernuclear γ spectroscopy. In the future neutron-rich hypernuclei will also be studied, where interesting modifications of nuclear structure by a Λ-particle are expected. Received: 1 May 2001 / Accepted: 4 December 2001     

In-beam study of 145Tb

The high-spin states of ¹⁴⁵Tb have been studied in the ¹¹⁸Sn(³²S, 1p4n) reaction at ³²S energies from 161 to 175 MeV using techniques of in-beam γ-ray spectroscopy. Measurements of γ-ray excitation functions and γ-γ-t coincidences were performed with 12 BGO(AC)HPGe detectors. Based on the measured results of γ-γ coincidences, γ-ray anisotropies and DCO ratios, a level scheme for ¹⁴⁵Tb was established for the first time. The observed excited states show typical irregular pattern in a spherical nucleus, and the low-lying levels have been interpreted qualitatively with a particle-core coupling. Received: 21 September 2001 / Accepted: 9 November 2001     

Beta decay rates for nuclei with 115<A<140 for r-process nucleosynthesis

For r-process nucleosynthesis the β-decay rates for a number of neutron-rich intermediate heavy nuclei are calculated. The model for the β-strength function is able to reproduce the observed half-lives quite well.     

Coulomb energy differences between isobaric analogue states in 70Br and 70Se

Gamma ray transitions de-exciting states in the N = Z nucleus ⁷⁰Br have been identified for the first time using the GASP and EUROBALL arrays coupled with ancillary detectors. The level scheme of ⁷⁰Br has been established by means of particle-gated γ-γ and γ-γ-γ coincidences. The Coulomb energy differences between isobaric analogue states in ⁷⁰Br and ⁷⁰Se show a deviation from the expected behaviour which could be related to dripline effects. Received: 18 July 2001 / Accepted: 25 September 2001     

Assessment of saddle-point-mass predictions for astrophysical applications

Using available experimental data on fission barriers and ground-state masses, a detailed study of the predictions of different models concerning the isospin dependence of saddle-point masses is performed. Evidence is found that several macroscopic models yield unrealistic saddle-point masses for very neutron-rich nuclei, which are relevant for the r-process nucleosynthesis.     

Experimental nuclear astrophysics with radioactive ion beams

Reactions involving unstable nuclei play an important role in many astrophysical sites. Radioactive ion beams provide a unique tool to investigate the structure of such unstable nuclei as well as the cross sections for many reactions of astrophysical relevance. This paper provides a brief survey of some recent results in experimental nuclear astrophysics with Radioactive Ion Beams, particularly for processes which take place during explosive hydrogen burning in novae and X-ray bursts. Some prospects for future studies at next generation facilities are also briefly discussed.     

Polarization observables in elastic electron deuteron scattering including parity- and time-reversal-violating contributions

The general formalism for polarization observables in elastic electron deuteron scattering is extended to incorporate parity- and time-reversal-violating contributions. Parity-violating effects arise from the interference of γ and Z exchange as well as from the hadronic sector via a small parity-violating component in the deuteron. In addition we have allowed for time-reversal-invariance-violating contributions in the hadronic sector. Formal expressions for the additional structure functions are derived, and their decomposition into the various multipole contributions are given explicitly. Received: 19 December 2000 / Accepted: 14 February 2001     

Isomeric state of 80Y and its role in the astrophysical rp-process

A careful investigation of the isomeric transition of the long-lived state at 228.5 keV excitation energy in ⁸⁰Y has been done. The HIGISOL facility at the Jyv?skyl? isochronous cyclotron has been used. We used the electron magnetic transporter to prepare an appropriate source and to measure the electron spectra in clean background conditions. The measured internal conversion coefficient α_K = 0.50±0.07 allows unambiguous 1^- identification for the 228.5 keV first excited isomeric state in ⁸⁰Y. With a “bare" half-life of 6.8±0.5 s found in this work, this state is strongly populated in the rp-process during X-ray bursts and has therefore to be taken into account in X-ray burst model calculations. However, because of the similarity of the β-decay half-lives of isomeric and ground states, we find a maximum reduction in the effective β-decay lifetime of ⁸⁰Y of only 17±2%. Our results pave the way for a future investigation of the impact of the isomeric state on the “effective" ⁸⁰Y proton capture rate. Received: 14 March 2001 / Accepted: 10 July 2001     

Nucleosynthesis in thermonuclear supernovae

We review our understanding of the nucleosynthesis that occurs in thermonuclear supernovae and their contribution to Galactic Chemical evolution. We discuss the prospects to improve the modeling of the nucleosynthesis within simulations of these events.     

High-spin states in the 96Tc nucleus

High-spin states in the ⁹⁶Tc nucleus have been studied with the reactions ⁸²Se(¹⁹F,5nγ) at 68 MeV and Zn(³⁶S,αpxn) at 130 MeV. Two γ-ray cascades (irregular bandlike structures) have been observed up to an excitation energy of about 10 MeV and spin 21-22?. Received: 19 January 2001 / Accepted: 30 March 2001