29 research outputs found
Spin-parities of sub-threshold resonances in the F(p, )O reaction
The F(p, )O reaction is key to determining the F
abundance in classical novae. However, the cross section for this reaction has
large uncertainties at low energies largely caused by interference effects.
Here, we resolve a longstanding issue with unknown spin-parities of
sub-threshold states in Ne that reduces these uncertainties. The
Ne(He, He)Ne neutron pick-up reaction was used to
populate Ne excited states, focusing on the energy region of
astrophysical interest ( 6 - 7 MeV). The experiment was performed at
the Triangle Universities Nuclear Laboratory using the high resolution Enge
split-pole magnetic spectrograph. Spins and parities were found for states in
the astrophysical energy range. In particular, the state at 6.133 MeV
(E keV) was found to have spin and parity of
and we confirm the existence of an unresolved doublet close to 6.288 MeV
(E keV) with J = and a high-spin
state. Using these results, we demonstrate a significant factor of two decrease
in the reaction rate uncertainties at nova temperatures.Comment: 15 pages, 6 figures, accepted in Phys. Rev. C. Corrected typos and
reference
Study of the 25Mg(d,p)26Mg reaction to constrain the 25Al(p,γ )26Si resonant reaction rates in nova burning conditions
The rate of the Al(p, )Si reaction is one of the few key remaining nuclear uncertainties required for predicting the production of the cosmic -ray emitter Al in explosive burning in novae. This reaction rate is dominated by three key resonances (, and ) in Si. Only the resonance strength has been directly constrained by experiment. A high resolution measurement of the Mg(d, p) reaction was used to determine spectroscopic factors for analog states in the mirror nucleus, Mg. A first spectroscopic factor value is reported for the state at 6.256 MeV, and a strict upper limit is set on the value for the state at 5.691 MeV, that is incompatible with an earlier (He, He) study. These results are used to estimate proton partial widths, and resonance strengths of analog states in Si contributing to the Al(p, )Si reaction rate in nova burning conditions
Production of 26Al in stellar hydrogen-burning environments: spectroscopic properties of states in 27Si
Model predictions of the amount of the radioisotope 26Al produced in
hydrogen-burning environments require reliable estimates of the thermonuclear
rates for the 26gAl(p,{\gamma})27Si and 26mAl(p,{\gamma})27Si reactions. These
rates depend upon the spectroscopic properties of states in 27Si within about 1
MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the
28Si(3He,{\alpha})27Si reaction at 25 MeV using a high-resolution
quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with
a transfer reaction, we have constrained J{\pi} values for states in 27Si over
Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few
important cases, we generally confirm the energies and spin-parity assignments
reported in a recent {\gamma}-ray spectroscopy study. The magnitudes of neutron
spectroscopic factors determined from shell-model calculations are in
reasonable agreement with our experimental values extracted using this
reaction.Comment: accepted for publication in Phys. Rev.
Spin-parities of sub-threshold resonances in the F(p, )O reaction
The F(p, )O reaction is key to determining the F abundance in classical novae. However, the cross section for this reaction has large uncertainties at low energies largely caused by interference effects. Here, we resolve a longstanding issue with unknown spin-parities of sub-threshold states in Ne that reduces these uncertainties. The Ne(He, He)Ne neutron pick-up reaction was used to populate Ne excited states, focusing on the energy region of astrophysical interest ( 6 - 7 MeV). The experiment was performed at the Triangle Universities Nuclear Laboratory using the high resolution Enge split-pole magnetic spectrograph. Spins and parities were found for states in the astrophysical energy range. In particular, the state at 6.133 MeV (E keV) was found to have spin and parity of and we confirm the existence of an unresolved doublet close to 6.288 MeV (E keV) with J = and a high-spin state. Using these results, we demonstrate a significant factor of two decrease in the reaction rate uncertainties at nova temperatures
Structure of S30 with S32(p,t)S30 and the thermonuclear P29(p,γ)S30 reaction rate
The structure of proton unbound S30 states is important for determining the P29(p,γ)S30 reaction rate, which influences explosive hydrogen burning in classical novae and type I x-ray bursts. The reaction rate in this temperature regime had been previously predicted to be dominated by two low-lying, unobserved, Jπ= 3+ and 2+ resonances above the proton threshold in S30. To search for these levels, the structure of S30 was studied using the S32(p,t)S30 transfer reaction with a magnetic spectrograph. We have confirmed a previous detection of a state near 4700 keV, which had tentatively been assigned Jπ=3+. We have also discovered a new state at 4814(3) keV, which is a strong candidate for the other important resonance (Jπ=2+). The new P29(p,γ)S30 reaction rate is up to 4-20 times larger than previously determined rates over the relevant temperature range. The uncertainty in the reaction rate due to uncertainties in the resonance energies has been significantly reduced. © 2010 The American Physical Society
Nuclear structure of 30S and its implications for nucleosynthesis in classical novae
The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature
range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude
due to the uncertain location of two previously unobserved 3+ and 2+ resonances
in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of
silicon isotopes synthesized in novae, which are relevant for the
identification of presolar grains of putative nova origin, were uncertain by a
factor of 3. To investigate the level structure of 30S above the proton
threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam
gamma-ray spectroscopy experiments were performed. Differential cross sections
of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born
approximation calculations were performed to constrain the spin-parity
assignments of the observed levels. An energy level scheme was deduced from
gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction.
Spin-parity assignments based on measurements of gamma-ray angular
distributions and gamma-gamma directional correlation from oriented nuclei were
made for most of the observed levels of 30S. As a result, the resonance
energies corresponding to the excited states in 4.5 MeV - 6 MeV region,
including the two astrophysically important states predicted previously, are
measured with significantly better precision than before. The uncertainty in
the rate of the 29P(p,gamma)30S reaction is substantially reduced over the
temperature range of interest. Finally, the influence of this rate on the
abundance ratios of silicon isotopes synthesized in novae are obtained via 1D
hydrodynamic nova simulations.Comment: 22 pages, 12 figure
Experimental investigation of the 30S(α, p) thermonuclear reaction in x-ray bursts
We performed the first measurement of 30 S+α resonant elastic scattering to experimentally examine the 30 S(α, p) stellar reaction rate in type I x-ray bursts. These bursts are the most frequent thermonuclear explosions in the galaxy, resulting from thermonuclear runaway on the surface of accreting neutron star binaries. The 30 S(α, p) reaction plays a critical role in burst models, yet very little is known about the compound nucleus 34 Ar at these energies nor the reaction rate itself. We performed a measurement of alpha elastic scattering with a radioactive beam of 30 S to experimentally probe the entrance channel. Utilizing a gaseous active target system and silicon detector array, we extracted the excitation function from 1.8 to 5.5 MeV near 160° in the center-of-mass frame. The experimental data were analyzed with an R -Matrix calculation, and we discovered several new resonances and extracted their quantum properties (resonance energy, width, spin, and parity). Finally, we calculated the narrow resonant thermonuclear reaction rate of 30 S(α, p) for these new resonances
Ne 21 energy levels approaching the α -particle threshold
Background: Nuclei around Ne20 exhibit an interplay of different excitations caused by different aspects of nuclear structure, including single-particle and multiparticle configurations and collective rotations. One-nucleon transfer reactions selectively probe single-particle structures in these nuclei. These nuclei are also important to astrophysics, with a number of important reactions proceeding through this mass region. Purpose: Energy levels approaching the α-particle threshold in Ne21 are of importance to nuclear structure. The Ne20(d,p)Ne21 reaction was measured and the corresponding spectroscopic nuclear information was extracted. Method: States in Ne21 were populated using the Ne20(d,p)Ne21 reaction in forward kinematics. Protons were identified in the Triangle Universities Nuclear Laboratory (TUNL) Enge split-pole spectrograph and angular distributions were extracted. Spin-party assignments were made and neutron partial widths were determined based on distorted-wave Born approximation (DWBA) analysis. Results: Several new energy levels were observed at energies of 7176, 7235, 7250, and 7337 keV, and spin-parities are reported which generally agree with previous results where literature was available. Spin and parity assignments are reported for several energy levels along with estimated neutron widths for those states above the neutron threshold (Sn=6761keV). Conclusions: Results from this study are placed in context with a review of the available literature on all known states in this energy region of Ne21
Constraining nova observables: direct measurements of resonance strengths in 33S(p,\gamma)34Cl
The 33S(p,\gamma)34Cl reaction is important for constraining predictions of
certain isotopic abundances in oxygen-neon novae. Models currently predict as
much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This
overproduction factor may, however, vary by orders of magnitude due to
uncertainties in the 33S(p,\gamma)34Cl reaction rate at nova peak temperatures.
Depending on this rate, 33S could potentially be used as a diagnostic tool for
classifying certain types of presolar grains. Better knowledge of the
33S(p,\gamma)34Cl rate would also aid in interpreting nova observations over
the S-Ca mass region and contribute to the firm establishment of the maximum
endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance
which is affected by this reaction has been proposed as a thermometer to study
the peak temperatures of novae. Previously, the 33S(p,\gamma)34Cl reaction rate
had only been studied directly down to resonance energies of 432 keV. However,
for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both
below the 432 keV resonance and within the Gamow window that could play a
dominant role. Direct measurements of the resonance strengths of these states
were performed using the DRAGON recoil separator at TRIUMF. Additionally two
new states within this energy region are reported. Several hydrodynamic
simulations have been performed, using all available experimental information
for the 33S(p,\gamma)34Cl rate, to explore the impact of the remaining
uncertainty in this rate on nucleosynthesis in nova explosions. These
calculations give a range of ~ 20-150 for the expected 33S overproduction
factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae.Comment: 12 pages, 8 figures, Accepted for publication in Physical Review