Rp-process Path Research Articles

New 26P(p, γ)27S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

Accurate nuclear reaction rates for 26P(p, γ)27S are pivotal for a comprehensive understanding of the rp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of 26P(p, γ)27S because of the lack of nuclear mass and energy level structure information for 27S. We reevaluate this reaction rate using the experimentally constrained 27S mass, together with the shell model predicted level structure. It is found that the 26P(p, γ)27S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances at E = 1.104, 1.597, and 1.777 MeV above the proton threshold in 27S. The new rate is overall smaller than the other previous rates from the Hauser–Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new 27S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in the rp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of 27S/26P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path 26P(p, γ)27S(β +,ν)27P is not as important as previously thought for producing 27P. The adoption of the new reaction rates for 26P(p, γ)27S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements.

Proton emission study as a guide to astrophysical rp process

Proton emitters play an important role in deciding the path of the astrophysical rapid proton capture (rp) process. The lifetime of these nuclei depends on several factors, like the deformation, angular momentum of the emitted proton, residual interaction between valence proton and neutron (especially in case of odd-odd nuclei) and so on. Therefore, it is worth to investigate the structure of proton emitters to understand the rp process path. However, due to lack of data in this exotic region, the theoretical models should be robust and the dependence on the free parameters should be minimal. In this direction, we have developed the first microscopic approach to study the triaxially deformed odd-odd proton emitters. The application of the developed approach to 108I, a recently observed proton emitter to investigate the end cycle of the rp process, is discussed.

Stellar reaction rate of 55Ni(p, γ)56Cu in Type I X-ray bursts

$$^{56}\hbox {Cu}$$ is close to the waiting-point nucleus $$^{56}\hbox {Ni}$$ and lies on the rapid proton capture (rp) process path in Type I X-ray bursts (XRBs). In this work, we obtained a revised thermonuclear reaction rate of $$^{55}\hbox {Ni}(\hbox {p},\gamma )^{56}\hbox {Cu}$$ in the temperature region relevant to XRBs. This rate was recalculated based on the recent experimental level structure in $$^{56}\hbox {Cu}$$ , the recently measured proton separation energy of $$S_\text {p}$$ = 579.8(7.1) keV, together with shell-model calculation, and the mirror nuclear structure in $$^{56}\hbox {Co}$$ . The associated uncertainties in the rates were estimated by a Monte Carlo method. Our revised rate is significantly different from the recent results, which were partially based on experimental results; in addition, we found that a result in a previous work was incorrect. We recommend our revised rate to be incorporated in the future astrophysical network calculations.

Mass measurement of Fe51 for the determination of the Fe51(p,γ)Co52 reaction rate

Background: The $^{51}\mathrm{Fe}(p,\ensuremath{\gamma})^{52}\mathrm{Co}$ reaction lies along the main rp-process path leading up to the $^{56}\mathrm{Ni}$ waiting point. The uncertainty in the reaction $Q$ value, which determines the equilibrium between the forward proton-capture and reverse photodisintegration $^{52}\mathrm{Co}(\ensuremath{\gamma},p)^{51}\mathrm{Fe}$ reaction, contributes to considerable uncertainty in the reaction rate in the temperature range of interest for Type I x-ray bursts and thus to an $\ensuremath{\approx}10%$ uncertainty in burst ashes lighter than $A=56$.Purpose: With a recent Penning trap mass measurement of $^{52}\mathrm{Co}$ reducing the uncertainty on its mass to 6.6 keV [Nesterenko et al., J. Phys. G 44, 065103 (2017)], the dominant source of uncertainty in the reaction $Q$ value is now the mass of $^{51}\mathrm{Fe}$, reported in the 2016 atomic mass evaluation to a precision of 9 keV [Wang et al., Chin. Phys. C 41, 030003 (2017)]. A new, high-precision Penning trap mass measurement of $^{51}\mathrm{Fe}$ was performed to allow the determination of an improved precision $Q$ value and thus new reaction rates.Method: $^{51}\mathrm{Fe}$ was produced using projectile fragmentation at the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory, and separated using the A1900 fragment separator. The resulting secondary beam was then thermalized in the beam stopping area before a mass measurement was performed using the LEBIT 9.4T Penning trap mass spectrometer.Results: The new mass excess, $\mathrm{ME}=\ensuremath{-}40189.2(1.6)$ keV, is sixfold more precise than the current AME value, and $1.6\ensuremath{\sigma}$ less negative. This value was used to calculate a new proton separation energy for $^{52}\mathrm{Co}$ of 1431(7) keV. New excitation levels were then calculated for $^{52}\mathrm{Co}$ using the nushellx code with the GXPF1A interaction, and a new reaction rate and burst ash composition was calculated.Conclusions: With a new measured $Q$ value, the uncertainty on the $^{51}\mathrm{Fe}(p,\ensuremath{\gamma})$ reaction rate is dominated by the poorly measured $^{52}\mathrm{Co}$ level structure. Reducing this uncertainty would allow a more precise rate calculation and a better determination of the mass abundances in the burst ashes.

Exotic phenomena in medium mass N ≃ Z nuclei within the beyond-mean-field approach

Recent studies on properties of proton-rich nuclei in the A = 70 mass region brought insights into fundamental symmetries and interactions and produced characteristics required by the astrophysical scenarios on the rp-process path revealing coexistence phenomena which dominate the structure and dynamics of these exotic nuclei. Isospin mixing and related phenomena like Coulomb energy differences, mirror energy differences, triplet energy differences as well as superallowed Fermi beta decay in isovector triplets have been investigated. The interplay between isospin-symmetry-breaking and shape-coexistence effects on isospin-related phenomena in nuclei as well as weak interaction processes under terrestrial and stellar x-ray burst environment have been explored in the frame of the beyond-mean-field complex Excited Vampir variational model. We used an adequate model space and the effective interaction was derived from a G-matrix starting from the charge-dependent Bonn CD potential. Results concerning the influence of shape coexistence and mixing on the structure and β-decay properties of exotic nuclei will be reviewed.

Isospin-symmetry breaking and shape coexistence in A∼70 analogs

Properties of proton-rich nuclei in the A∼70 region could bring insights into fundamental symmetries and interactions and may have relevance for the astrophysical scenarios on the rp-process path. Recent results concerning the interplay between isospin-symmetrybreaking and shape-coexistence effects on isospin-related phenomena in A=70 and A=74 isovector triplets obtained within the beyond-mean-field complex Excited Vampir variational model using an effective interaction obtained from a G-matrix based on the charge-dependent Bonn CD potential in a relatively large model space are presented.

Nucleosynthesis of proton-rich nuclei. Experimental results on the rp-process

We report in this study the nuclear properties of proton-rich isotopes located along the rp-process path. The experiments have recently been performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The level properties above the proton separation energy of the nuclei 30S, 36K and 37Ca were measured with precision of < 10 keV. This will allow a reduction in the determination of the astrophysical (p,γ) reaction rate under rp-process conditions.

New experimental efforts along the rp-process path

The level structure just above the proton threshold of the nucleus 30S has been studied using the neutron removal process on fast radioactive beams at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. In this work we provide a description of the experimental setup. The present status of the analysis is also discussed.

Precision mass measurements of radioactive nuclei at JYFLTRAP

The Penning trap mass spectrometer JYFLTRAP was used to measure the atomic masses of radioactive nuclei with an uncertainty better than 10 keV. The atomic masses of the neutron-deficient nuclei around the N = Z line were measured to improve the understanding of the rp-process path and the SbSnTe cycle. Furthermore, the masses of the neutron-rich gallium (Z = 31) to palladium (Z = 46) nuclei have been measured. The physics impacts on the nuclear structure and the r-process paths are reviewed. A better understanding of the nuclear deformation is presented by studying the pairing energy around A = 100.

Improving the nuclear physics input along the rp-process path

The level structure of 30 S was studied at the NSCL by using neutron removal reactions with a radioactive 31 S beam. The γ -decay from excited states in 30 S was measured in a Ge-detector array. The results discussed for this work will reduce the uncertainties in the determination of the astrophysical 29 P(p, γ ) 30 S reaction rate under rp -process conditions.

Investigating the rp-process with the Canadian Penning trap mass spectrometer

The Canadian Penning trap (CPT) mass spectrometer at the Argonne National Laboratory makes precise mass measurements of nuclides with short half-lives. Since the previous ENAM conference, many significant modifications to the apparatus were implemented to improve both the precision and efficiency of measurement, and now more than 60 radioactive isotopes have been measured with half-lives as short as one second and with a precision ( Δm/m) approaching 10-8. The CPT mass measurement program has concentrated so far on nuclides of importance to astrophysics. In particular, measurements have been obtained of isotopes along the rp-process path, in which energy is released from a series of rapid proton-capture reactions. An X-ray burst is one possible site for the rp-process mechanism which involves the accretion of hydrogen and helium from one star onto the surface of its neutron star binary companion. Mass measurements are required as key inputs to network calculations used to describe the rp-process in terms of the abundances of the nuclides produced, the light-curve profile of the X-ray bursts, and the energy produced. This paper will present the precise mass measurements made along the rp-process path with particular emphasis on the “waiting-point” nuclides 68Se and 64Ge.

Nuclear structure of the exotic mass region along the rp process path

Isomeric states in the nuclei along the rapid proton capture process path are studied by the projected shell model. Emphasis is given to two waiting point nuclei 68Se and 72Kr that are characterized by shape coexistence. Energy surface calculations indicate that the ground state of these nuclei corresponds to an oblate-deformed minimum, while the lowest state at the prolate-deformed minimum can be considered as a shape isomer. The impact of these isomer states on isotopic abundance in x-ray bursts is studied in a multi-mass-zone x-ray burst model by assuming an upper-lower limit approach.

Precise mass measurements of astrophysical interest made with the Canadian Penning trap mass spectrometer

The processes responsible for the creation of elements more massive than iron are not well understood. Possible production mechanisms involve the rapid capture of protons (rp-process) or the rapid capture of neutrons (r-process), which are thought to occur in explosive astrophysical events such as novae, x-ray bursts, and supernovae. Mass measurements of the nuclides involved with uncertainties on the order of 100 keV or better are critical to determine the process ‘paths’, the energy output of the events, and the resulting nuclide abundances. Particularly important are the masses of ‘waiting-point’ nuclides along the rp-process path where the process stalls until the subsequent β decay of the nuclides. This paper will discuss the precise mass measurements made of isotopes along the rp-process and r-process paths using the Canadian Penning Trap mass spectrometer, including the mass of the critical waiting-point nuclide 68 Se.

Masses along the rp-process path and large scale surveys on Cu, Ni and Ga with ISOLTRAP

Mass measurements with exceptionally low uncertainty can be performed with the Penning trap mass spectrometer ISOLTRAP. Out of the large number of recent measurements two regions of the chart of nuclei have been selected for this article. The astrophysically interesting N = Z nuclei above Z = 32 and neutron-rich copper, nickel and gallium isotopes have been investigated. The relative mass uncertainty obtained is generally ⩽ 1 ⋅ 10−7.

New approach for measuring properties of rp-process nuclei.

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture (rp) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the gamma-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, 33Ar, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated 32Cl(p,gamma)33Ar rate that is critical to the modeling of the rp process. This approach has the potential to measure key properties of almost all interesting nuclei on the rp-process path.

Models for Type I X‐Ray Bursts with Improved Nuclear Physics

Multizone models of Type I X-ray bursts are presented that use an adaptive nuclear reaction network of unprecedented size, up to 1300 isotopes, for energy generation and include the most recent measurements and estimates of critical nuclear physics. Convection and radiation transport are included in calculations that carefully follow the changing composition in the accreted layer, both during the bursts themselves and in their ashes. Sequences of bursts, up to 15 in one case, are followed for two choices of accretion rate and metallicity, up to the point at which a limit cycle equilibrium is established. For (M)over dot=1.75x10(-9) M-circle dot yr(-1) (and (M)over dot=3.5x10(-10) M-circle dot yr(-1), for low metallicity), combined hydrogen-helium flashes occur. These bursts have light curves with slow rise times (seconds) and long tails. The rise times, shapes, and tails of these light curves are sensitive to the efficiency of nuclear burning at various waiting points along the rp-process path, and these sensitivities are explored. Each displays ``compositional inertia`` in that its properties are sensitive to the fact that accretion occurs onto the ashes of previous bursts that contain leftover hydrogen, helium, and CNO nuclei. This acts to reduce the sensitivity of burst properties to metallicity. Only the first anomalous burst in one model produces nuclei as heavy as A=100. For the present choice of nuclear physics and accretion rates, other bursts and models make chiefly nuclei with Aapproximate to64. The amount of carbon remaining after hydrogen-helium bursts is typically less than or similar to1 and decreases further as the ashes are periodically heated by subsequent bursts. For (M)over dot=3.5x10(-10) M-circle dot yr(-1) and solar metallicity, bursts are ignited in a hydrogen-free helium layer. At the base of this layer, up to 90 to carbon prior to the unstable ignition of the helium shell. These helium-ignited bursts have (1) briefer, brighter light curves with shorter tails, (2) very rapid rise times (>0.1 s), and (3) ashes lighter than the iron group.

Mass measurements of70Se,71Se,72Br,and73Br

The path and termination point for the rapid proton-capture (rp) process above Ni-56 is uncertain due to a lack of knowledge of nuclear properties, especially masses, near the proton drip line. To address this need we have begun a program to measure masses along the rp-process path in the A similar to 60-80 region using beta-gamma coincidence spectroscopy, Q(EC) values derived from beta (+) spectrum end points were used to calculate mass excess values for Se-70, Se-71, Br-72, Br-73. The results are compared to predictions of mass models and to estimates based on systematic trends in this region.

Beta decay properties of 67,68Se and the astrophysical rp-process path.

The $^{67}\mathrm{Se}$ and $^{68}\mathrm{Se}$ isotopes near the proton drip line were produced by spallation of zirconium with 600 MeV protons at CERN. Beta decay properties were determined for the first time using the selectivity of molecular ion beams at the on-line mass separator ISOLDE. Chemically selective production was obtained through separation of the molecular ion ${\mathrm{COSe}}^{+}$. The half-lives of $^{67}\mathrm{Se}$ and $^{68}\mathrm{Se}$ were measured as 107\ifmmode\pm\else\textpm\fi{}35 ms and 35.5\ifmmode\pm\else\textpm\fi{}0.7 s respectively. From \ensuremath{\beta}-\ensuremath{\gamma} and \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, and conversion-electron measurements, a decay scheme was obtained for $^{68}\mathrm{Se}$. For the $^{68}\mathrm{As}$ ground state and several excited states, spin and parity are unambiguously determined from \ensuremath{\beta}-decay rates and electromagnetic multipolarity assignments. In addition to the strong Fermi branch between the mirror ground states of $^{67}\mathrm{Se}$ and $^{67}\mathrm{As}$ [${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=(5/2${)}^{\mathrm{\ensuremath{-}}}$,${\mathit{T}}_{\mathit{z}}$=\ifmmode\pm\else\textpm\fi{}1/2], a GT branch to an excited state in $^{67}\mathrm{As}$ is observed. Implications of $^{67}\mathrm{Se}$ and $^{68}\mathrm{Se}$ properties in the rp-process path are considered.

Halflife measurements of the rp-process nuclei 61Ga, 63Ge, and 65As

Using the A1200 beam analysis device at the National Superconducting Cyclotron Laboratory, we have measured the halflives of several nuclei along the rp-process path near the proton-drip line. Halflife results for 61Ga, 63Ge, and 65As (0.15±0.03 s, 95 −25 +23 ms, and 0.19 −0.07 +0.11 s, respectively) and their implications for the rp-process are presented.