J. A. Caggiano

40Ca(α,γ)44Ti and the production of 44Ti in supernovae

The nuclide 44Ti is predicted to be produced in significant quantities in core-collapse supernovae, and indeed it has been observed in the supernova remnant Cassiopeia-A by space-based γ-ray telescopes. The main production of 44Ti takes place in the α-rich freeze-out phase deep inside the supernova. The key reactions governing the 44Ti abundance have been identified in an earlier sensitivity study. Using the recoil mass spectrometer DRAGON at the TRIUMF-ISAC facility in Vancouver, Canada, we measured the main production reaction 40Ca(α,γ)44Ti, resulting in an increased reaction rate compared to the rate derived from previous prompt γ-ray studies, which is commonly used in supernova models. The uncertainty of the 44Ti production is now dominated by the rate of reactions with short-lived nuclides around 44Ti, namely 45V(p,γ)46Cr, 44Ti(α,p)47V and 44Ti(α,γ)48Cr. The sensitivity of these reactions on the 44Ti production has been revisited.

Absolute determination of the 22Na(p,γ)23Mg reaction rate in novae

Gamma-ray telescopes in orbit around the earth are searching for evidence of the elusive radionuclide 22Na produced in novae. Previously published uncertainties in the dominant destructive reaction, 22Na(p,γ)23Mg, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beam line, which included beam rastering and cold vacuum protection of the 22Na implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ∼20 C bombardment as a result of protective layers. We avoided the nneed to know the absolute stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for Ep = 213, 288, 454, and 610 keV are stronger by factors of 2.4–3.2 than previously reported. Upper limits have been placed on proposed resonances at 198, 209, and 232 keV. These substantially reduce the uncertainty nin the reaction rate. We have re-evaluated the 22Na(p,γ) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to 22Na destruction in novae. Hydrodynamic nsimulations including our rate indicate that the expected abundance of 22Na ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.

Absolute determination of theNa22(p,γ)Mg23reaction rate in novae

Gamma-ray telescopes in orbit around the earth are searching for evidence of the elusive radionuclide 22Na produced in novae. Previously published uncertainties in the dominant destructive reaction, 22Na(p,γ)23Mg, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beam line, which included beam rastering and cold vacuum protection of the 22Na implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ∼20 C bombardment as a result of protective layers. We avoided the nneed to know the absolute stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for Ep = 213, 288, 454, and 610 keV are stronger by factors of 2.4–3.2 than previously reported. Upper limits have been placed on proposed resonances at 198, 209, and 232 keV. These substantially reduce the uncertainty nin the reaction rate. We have re-evaluated the 22Na(p,γ) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to 22Na destruction in novae. Hydrodynamic nsimulations including our rate indicate that the expected abundance of 22Na ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.

Mass measurements of

J. A. Clark1,2,3, A. Parikh1, F. Buchinger4, J. A. Caggiano1, J. E. Crawford4, C. Deibel1, J. P. Greene3, S. Gulick4, J. C. Hardy5, A. A. Hecht3,6, J. K. P. Lee4, A. F. Levand3, R. Lewis1, B. F. Lundgren3, P. D. Parker1, G. Savard3, N. D. Scielzo3, K. S. Sharma2, I. Tanihata3, W. Trimble3, J. C. Wang2,3, Y. Wang2,3, C. Wrede1, and Z. Zhou3 1Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520, USA 2Department of Physics and Astronomy, University of Manitob a, Winnipeg, MB R3T 2N2, Canada 3Physics Division, Argonne National Laboratory, Argonne, I L 60439, USA 4 Department of Physics, McGill University, Montreal, PQ H3A 2T8, Canada 5Cyclotron Institute, Texas A&M University, College Statio n, TX 77843-3366, USA 6Department of Chemistry, University of Maryland, College P ark, MD 20742, USA

^{22}

The radionuclide 22Na is a potential astronomical observable that is expected to be produced in classical novae in quantities that depend on the thermonuclear rate of the 22Na(p,γ)23Mg reaction. We have measured the strengths of low-energy 22Na(p,γ)23Mg resonances directly and absolutely using a radioactive 22Na target. We find the strengths of resonances at Ep=213, 288, 454, and 610 keV to be higher than previous measurements by factors of 2.4-3.2, and we exclude important contributions to the rate from proposed resonances at Ep=198, 209, and 232 keV. The 22Na abundances expected in the ejecta of classical novae are reduced by a factor of ≈2.

Mg and

Two inconsistent sets of 24Al excitation-energy measurements have been used to determ ine esonance energies for the 23Mg(p,γ)24Al reaction. This discrepancy results in a factor of five vari ation in the calculated thermonuclear 23Mg(p,γ)24Al reaction rate at T = 0.25 GK, and presents a challenge to an imminent radioactive ion-beam measuremen t of this reaction that will rely on precisely known resonance energies. We have measured the 24Mg(3He,t)24Al reaction using a 30MeV 3He beam from the tandem Van de Graaff accelerator at Yale Univ ersity’s Wright Nuclear Structure Laboratory. The Yale Enge magnetic spectrograph was used to momentum-analyze reaction products; a position-sensitive ionization drift chamber backed by a scintillator at the focal plane was used to identify tritons and measure the exci tation energies of corresponding states in24Al. We find good general agreement with one of the two previous set of measurements and determine an energy of Ec.m. = 474(6) keV for what is thought to be the most important23Mg(p,γ)24Al resonance astrophysically [the previous measurements y ielded values of Ec.m. = 499(5) and 458(10) keV]. A more precise thermonuclear 23Mg(p,γ)24Al rate will help to constrain the determination of nuclear flow out of the NeNa cycle, and production of A ≥ 20 nuclides, in explosive hydrogen burning over a temperature range 0.2 < T < 1.0 GK.

^{26}

In the explosive astrophysical environment of a type I x-ray burst, the low Q value of 293(50) keV for proton capture on {sup 30}S induces a (p,{gamma})({gamma},p) equilibrium that may lead to a waiting point in the rapid proton capture (rp) process at {sup 30}S. The excitation energies of the first and candidate second T=3/2 levels in {sup 31}S were recently determined to an uncertainty of 2 keV by measuring triton spectra and t-p angular correlations from the {sup 31}P({sup 3}He,t){sup 31}S*(p){sup 30}P reaction. By using this new information together with existing experimental information on the first T=3/2, A=31 isobaric multiplet and the isobaric multiplet mass equation, the {sup 30}S(p,{gamma}){sup 31}ClQ value is predicted to be 284(7) keV. Similarly, by using the second T=3/2 multiplet, the energy of the dominant resonance in the thermonuclear {sup 30}S(p,{gamma}){sup 31}Cl reaction is tentatively predicted to be E{sub c.m.}=453(8) keV and this supports a {sup 31}Ar {beta}{sup +}-delayed proton-decay observation of this resonance at E{sub c.m.}=461(15) keV. These substantial reductions in the uncertainties in the thermonuclear {sup 30}S(p,{gamma}){sup 31}Cl reaction rate and Q value constrain the region of temperature-density-composition parameter space where the {sup 30}S(p,{gamma})({gamma},p) equilibrium and the {sup 30}S waiting point may bemorexa0» active.«xa0less

Si via

By measuring the {sup 31}P({sup 3}He,t){sup 31}S, {sup 31}P({sup 3}He,t){sup 31}S*(p){sup 30}P, and {sup 32}S(d,t){sup 31}S reactions, the level scheme of {sup 31}S has been refined and extended up to E{sub x}=9.5 MeV. A total of 17 new levels, and 5 tentative new levels, have been measured. In addition, 5 tentatively known levels have been confirmed. The uncertainties in the excitation energies of many known {sup 31}S levels have been reduced substantially. Spin constraints have been made for 8 proton-unbound levels by measuring 18 triton-proton angular correlations from the {sup 31}P({sup 3}He,t){sup 31}S*(p){sup 30}P reaction. Finite proton-decay branching ratios (including discrimination between decays to the ground state and first two excited states of {sup 30}P) have been measured for 38 levels, and upper limits have been set for 3 additional levels. The lowest isospin T=3/2 level has been observed, and candidates for the second and third T=3/2 levels have been identified. The new experimental information on {sup 30}P+p resonance parameters has been used together with data from previous measurements to calculate the thermonuclear, resonant {sup 30}P(p,{gamma}){sup 31}S reaction rate over three orders of magnitude in temperature: 0.01<T<10 GK. Good agreement is found with estimates based on Hauser-Feshbach statistical models overmorexa0» the range 0.08<T<10 GK, but differences are found with rates previously estimated using the experimental information at hand.«xa0less

(p,t)

Using recent data we reduce the systematic uncertainty in our measurement [Phys. Rev. C 76, 065803 (2007)] of the excitation energy of the second level above the proton threshold in {sup 24}Al and find it to be 2523(3) keV, a factor of two improvement over our previously reported value of 2524(6) keV.

reactions and Penning traps

The short-lived nuclide 44 Ti is an important nuclide for the understanding of explosive nucleosynthesis. The main production reaction, 40 Ca(α, γ) 44 Ti, has been studied in inverse kinematics with the recoil mass spectrometer DRAGON located at the TRIUMF-ISAC facility in Vancouver, Canada. The temperature range relevant for α-rich freeze-out during a core-collapse supernova has been covered entirely with a 40 Ca beam of 0.60 to 1.15 MeV/nucleon. All relevant quantities for the calculation of the astrophysical reaction rate have been measured directly. Because of many previously undiscovered resonances, the reaction rate derived from the energy dependent 44 Ti yield is higher than the one based on previous prompt γ-ray studies commonly used in supernova models. The presented new rate results in an increased 44 Ti production in supemovae.