Rapid Proton Capture Process Research Articles

Capture reactions into Borromean two-proton systems atrpwaiting points

We investigate even-even two-proton borromean systems at prominent intermediate heavy waiting points for the rapid proton capture process. The most likely single-particle levels are used to calculate three-body energy and structure as a function of proton-core resonance energy. We establish a linear dependence between two- and three-body energies with the same slope, but the absolute value slightly dependent on partial wave structure. Using these relations we predict low-lying excited states in the isotones following the critical waiting points. The capture rate for producing a borromean bound state is described both based on a full three-body calculation and on a very simple analytic rate expression for temperatures about $1-5$~GK. This rate is valid for both direct and sequential capture paths, and it only depends on the three-body resonance energy. As a result the relevant path of the radiative capture reactions can be determined. We present numerical results for $E1$ and $E2$ photon emission, and discuss occurrence preferences in general as well as relative sizes of these most likely processes. Finally, we present narrow estimated intervals for the proton capture rates relevant for the critical waiting points.

The $T_{z} = \pm 1 \to 0$ and $\pm 2 \to \pm 1$ Mirror Gamow--Teller Transitions in $pf$-shell Nuclei

Gamow–Teller (GT) transitions are the most common weak-interaction processes in the Universe. They play important roles in various processes of nucleosynthesis, for example, in the rapid proton-capture process (rp-process). In the pf-shell region, the rp-process runs through neutron-deficient nuclei with Tz = −2, −1, and 0 mainly by means of GT and Fermi transitions, where Tz is the z component of isospin T defined by Tz = (N − Z)∕2. Under the assumption of isospin symmetry, mirror nuclei with reversed Z and N numbers, and thus with opposite signs of Tz, have the same structure. Therefore, symmetry is also expected for the GT transitions starting from and ending up in mirror nuclei. We have been studying the Tz = −2 →−1 and −1 → 0 GT transitions in β decays, while those from stable Tz = +2 and +1 nuclei by means of hadronic (3He,t) charge-exchange (CE) reactions. The results from these studies are compared in order to examine the mirror-symmetry structure in nuclei. In addition, these results are combined for the better understanding of GT transitions in the pf-shell region.

Mass excess of 69Br and the rp process

The proton separation energy Sp of −786.07±11.49 keV has been evaluated for 69Br from a least squares fit of mass difference of analog states versus α/A1/3, where α is the average charge of the mirror nuclei and A is the mass number. The extracted Sp value is indicative of the rapid proton-capture process rp, and subsequent Type I X-ray bursts.

β Decay in the Region of Neutron-deficient 69,70,71Kr

Decay spectroscopy was performed for neutron-deficient nuclei ranging from zinc to krypton with isospin −32≤Tz≤0. Measurements of correlated β-delayed protons allowed us to determine the isobaric analog states fed from the decay of 65Se and 69Kr, constraining the spin of the 69Kr ground state. Preliminary results regarding the half lives for the Tz=−1 systems, relevant to the rapid proton capture (rp) process, are discussed.

Measurement of astrophysically important excitation energies of58Zn with GRETINA

The level structure of neutron-deficient 58 Zn has been extracted experimen- tally. This nucleus is important for the rapid proton-capture process. 58 Zn was produced by using a (d,n)-type transfer reaction on 57 Cu in inverse kinematics at beam energies of 75 MeV/u. Several -ray transitions have been identified. The experiment utilized the state-of-the-art GRETINA -ray energy tracking array in conjunction with the large- acceptance spectrometer S800 at NSCL. The excitation energies of the identified low- lying states in 58 Zn are important for constraining the 57 Cu(p, ) 58 Zn reaction rate under X-ray burst conditions.

IMPORTANCE OF Q-VALUES IN ASTROPHYSICAL RAPID PROTON CAPTURE PROCESS

The importance of measuring Q-values in rapid proton capture process has been investigated. The microscopic optical model, derived using a nucleon–nucleon interaction and densities from relativistic mean field (RMF) calculations, has been utilized to calculate the reaction rates. It has been observed that the Q-values involved in the reactions at waiting points at A = 60 and 64 are very important in determining the final abundance of the process. Some other Q-values also play a crucial role in the final abundance of nuclei near the end point of the process.

Reexamining theβdecay of53,54Ni,52,53Co,51Fe, and50Mn

The β decay of 53,54Ni, 52,53Co, 51Fe, and 50Mn was investigated via the fragmentation of a 58Ni primary beam with an energy of 68.6 MeV/u. The proton-γ coincidences of 53Ni β-delayed proton emission were observed. Based on the analysis of the proton-γ coincidence events, it was inferred that the previous assignment of the excitation energy for the isobaric analog state in 53Co may be problematic. The half-lives of these nuclei were obtained, in which the uncertainty of 52Co half-life was reduced by a factor of 3. The half-lives were evaluated and used as inputs of nucleosynthesis calculations of the rapid proton-capture process in an x-ray burst.Received 28 September 2012DOI:https://doi.org/10.1103/PhysRevC.87.024312©2013 American Physical Society

A model for rp-process within the (p,)-(,p) equilibrium approximation and its application

Many of the stable nuclei at the neutron-deficient side of the beta-stability valley cannot be synthesized in the neutron-capture processes. The origin of the so-called p-nuclei has been a long standing question in the nuclear astrophysics. The rapid-proton capture process (rp-process) proposed in the 1980s was one of the possible mechanisms to be responsible for some light p-nuclei. In this work, a model for rapid-proton capture process (rp-process), within the (p,)-(,p) equilibrium approximation, is established. In the framework of this model, the influence of astrophysical conditions (i.e., proton number density, temperature, and proton irradiation time) on the rp-abundance pattern is investigated. In addition, the recent-measured mass data of neutron-deficient atomic nuclei by using the storage ring mass spectrometry at IMP, Lanzhou, are employed in our calculation. It is found that the abundance at A=41 is increased by two orders of magnitude when using the new by determined mass of 41Ti, while the uncertainty of the computed abundance is reduced by almost two orders of magnitude.

ENDPOINT OF rp PROCESS USING RELATIVISTIC MEAN FIELD APPROACH AND A NEW MASS FORMULA

Densities from relativistic mean field calculations are applied to construct the optical potential and, hence calculate the endpoint of the rapid proton capture (rp) process. Mass values are taken from a new phenomenological mass formula. Endpoints are calculated for different temperature-density profiles of various X-ray bursters. We find that the rp process can produce significant quantities of nuclei upto around mass 95. Our results differ from existing works to some extent.

β-Delayed proton-decay study of 20Mg and its implications for the [formula omitted] breakout reaction in X-ray bursts

Under astrophysical conditions of high temperature and density, such as for example found in X-ray bursts, breakout can occur from the hot CNO cycles into the rapid proton capture process. A key breakout route is via the sequence O15(α,γ)Ne19(p,γ)Na20. The Ne19(p,γ)Na20 reaction rate is expected to be dominated by a single resonance at 457(3) keV. The identity of the resonance has been under discussion for a long time, with Jπ=1+ and 3+ assignments suggested. In this study of the β-delayed proton decay of 20Mg we report a new, significantly more stringent, upper limit on the β-decay branch to this state of 0.02% with a confidence level of 90%. This makes a 1+ assignment highly unlikely and favours a 3+ assignment for which no branch is expected to be observed. The 3+ state is predicted to have a significantly higher resonance strength, and to produce a proportionately higher Ne19(p,γ)Na20 reaction rate in X-ray burst conditions.

Half-lives of ground and isomeric states in 97Cd and the astrophysical origin of 96Ru

First experimental evidence for a high-spin isomer (25/2+) in 97Cd, a waiting point in the astrophysical rapid proton capture process, is presented. The data were obtained in β-decay studies at NSCL using the new RF Fragment Separator system and detecting β-delayed protons and β-delayed γ rays. Decays from ground and isomeric states were disentangled, and proton emission branches were determined for the first time. We find half-lives of 1.10(8) s and 3.8(2) s, and β-delayed proton emission branches of 12(2)% and 25(4)% were deduced for the ground and isomeric states, respectively. With these results, the nuclear data needed to determine an rp-process contribution to the unknown origin of solar 96Ru are in place. When the new data are included in astrophysical rp-process calculations, one finds that an rp-process origin of 96Ru is unlikely.

Stellar weak decay rates in neutron-deficient medium-mass nuclei

Weak decay rates under stellar density and temperature conditions holding at the rapid proton capture process are studied in neutron-deficient medium-mass waiting point nuclei extending from Ni up to Sn. Neighboring isotopes to these waiting point nuclei are also included in the analysis. The nuclear structure part of the problem is described within a deformed Skyrme Hartree-Fock + BCS + QRPA approach, which reproduces not only the beta-decay half-lives but also the available Gamow-Teller strength distributions, measured under terrestrial conditions. The various sensitivities of the decay rates to both density and temperature are discussed. In particular, we study the impact of contributions coming from thermally populated excited states in the parent nucleus, as well as the competition between beta decays and continuum electron captures.

Low-energy (p,γ) reactions in Ni and Cu nuclei using a microscopic optical model

Radiative capture reactions for low energy protons have been theoretically studied for Ni and Cu isotopes using the microscopic optical model. The optical potential has been obtained in the folding model using different microscopic interactions with the nuclear densities from Relativistic Mean Field calculations. The calculated total cross sections as well as the cross sections for individually low lying levels have been compared with measurements involving stable nuclear targets. Rates for the rapid proton capture process have been evaluated for astrophysically important reactions.

First mass measurement of short-lived nuclides at HIRFL-CSR

Recent commissioning of the HIRFL-CSR has demonstrated its ability to perform direct mass measurement for short-lived nuclides. Projectile fragments produced by the 78Kr ions at 481.88 MeV/u were separated with the new radioactive beam line in Lanzhou (RIBLL2), and injected into and stored in the experimental storage ring (CSRe). By operating the CSRe as an isochronous mass spectrometry, a typical mass resolution around 2.0×10−5 has been achieved. The masses for 63Ge, 65As and 67Se were measured for the first time. The measured masses are compared with theoretical predictions and the location of the proton drip-line for As isotopes is discussed. The implication of the 65As mass in the astrophysical rapid proton capture process has also been addressed.

Constraint on the astrophysicalNe18(α,p)Na21reaction rate through aMg24(p,t)Mg22measurement

The $^{18}\mathrm{Ne}(\ensuremath{\alpha},p)^{21}\mathrm{Na}$ reaction plays a crucial role in the $(\ensuremath{\alpha},p)$ process, which leads to the rapid proton capture process in x-ray bursts. The reaction rate depends upon properties of $^{22}\mathrm{Mg}$ levels above the $\ensuremath{\alpha}$ 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 $^{24}\mathrm{Mg}(p,t)^{22}\mathrm{Mg}$ reaction at the Oak Ridge National Laboratory (ORNL) Holifield Radioactive Ion Beam Facility (HRIBF), and by measuring the angular distributions of outgoing tritons, we provide some of the first experimental constraints on the spins of astrophysically important $^{18}\mathrm{Ne}(\ensuremath{\alpha},p)^{21}\mathrm{Na}$ resonances.

β-decay half-life of therp-process waiting-point nuclideMo84

A half-life of 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 s has been deduced for the ground-state $\ensuremath{\beta}$ decay of $^{84}\mathrm{Mo}$, more than $1\ensuremath{\sigma}$ shorter than the previously adopted value. $^{84}\mathrm{Mo}$ is an even-even $N=Z$ nucleus lying on the proton dripline, created during explosive hydrogen burning in type I x-ray bursts in the rapid proton capture ($\mathit{rp}$) process. The effect of the measured half-life on $\mathit{rp}$-process reaction flow is explored. Implications on theoretical treatments of nuclear deformation in $^{84}\mathrm{Mo}$ are also discussed.

The15O(α,γ)19Ne Reaction Rate and the Stability of Thermonuclear Burning on Accreting Neutron Stars

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of theaccretedmaterialintheatmosphereof theneutronstarleadstoathermonuclearexplosion,whichisobservedas an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized bya sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction 15 O(�; � ) 19 Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the 15 O(�; � ) 19 Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts

Astrophysical nuclear reactions and the break-out from the hot CNO cycles

Thermonuclear stellar explosions depend critically on the available fuel and the ignition through nuclear reactions. In this paper we intend to demonstrate the sensitive interplay between nuclear fuel supply and depletion by nuclear reaction processes at high temperature and densities. We demonstrate this on the example of X-ray bursts which are explained as thermonuclear explosions in the atmosphere of an accreting neutron star. These bursts occur periodically between hours and days depending on the accretion rate. The burst itself is triggered by the 15O(α,γ) 19Ne reaction regulating the flow between the β -limited hot CNO cycle and the rapid proton capture process. The 18Ne(α,p) 21Na reaction on the other hand provides a continuous processing flow of light 4He fuel towards heavier elements. The reaction rates of both α capture processes carry significant uncertainties. In this paper we report on the measurements of the critical nuclear parameters for the determination of the rate of the 15O(α,γ) 19Ne and the 18Ne(α,p) 21Na reactions. We demonstrate and analyze the impact of these experimental results on the burst pattern and burst periodicity for a range of accretion rates.

Mass measurements in the endpoint region of the rp-process at SHIPTRAP

The Penning-trap mass spectrometer SHIPTRAP was designed for precision mass measurements of radionuclides produced in fusion-evaporation reactions. The latest measurement campaign covered heavy nuclei (A > 90) related to the astrophysical rapid proton capture process. The masses of 34 neutron-deficient radionuclides have been measured since February 2006 with relative uncertainties between 5 × 10-8 and 1 × 10-7. Furthermore, the use of an octupolar RF excitation for the time-of-flight ion-cyclotron-resonance technique was investigated and an increase of the resolving power by a factor of ten was observed in agreement with simulations. This will allow to resolve isomeric states with excitation energies of a few 10 keV only.

The rp‐Process in Neutrino‐driven Winds

Recent hydrodynamic simulations of core-collapse supernovae with accurate neutrino transport suggest that the bulk of the early neutrino-heated ejecta is proton rich, in which the production of some interesting proton-rich nuclei is expected. As suggested in recent nucleosynthesis studies, the rapid proton-capture (rp) process takes place in such proton-rich environments by bypassing the waiting point nuclei with β+-lives of a few minutes via the faster capture of neutrons continuously supplied from the neutrino absorption by protons. In this study, the nucleosynthesis calculations are performed with a wide range of neutrino luminosities and electron fractions (Ye), using semianalytic models of proto-neutron-star winds. The masses of proto-neutron stars are taken to be 1.4 and 2.0 M☉, where the latter is regarded as the test for somewhat high-entropy winds (about a factor of 2). For Ye > 0.52, the neutrino-induced rp-process takes place in many wind trajectories, and p-nuclei up to A ~ 130 are synthesized in interesting amounts. However, 92Mo is somewhat underproduced compared to other p-nuclei with similar mass numbers. For 0.46 < Ye < 0.49, on the other hand, 92Mo is significantly enhanced by the nuclear flows in the vicinity of the abundant 90Zr that originates from the α-process at higher temperature. The nucleosynthetic yields are averaged over the ejected masses of winds, and further, over the Ye distribution predicted by a recent hydrodynamic simulation of a core-collapse supernova. Comparison of the Ye- and mass-averaged yields to the solar compositions implies that the neutrino-driven winds can potentially be the origin of light p-nuclei up to A ~ 110, including 92,94Mo and 96,98Ru, that cannot be explained by other astrophysical sites.