Hydrogen burning of 24, 25, 26Mg in explosive carbon burning

The absolute resonance strengths of the ^24–26Mg(p, γ)^25–27Al reactions have been studied. The absolute values S_c.m.=1.03 ± 0.11, 2.3 ± 0.2 and 2.2 ± 0.2 eV for the frequently used standard resonances at E_p=823, 434 and 454 keV in the reactions ^24–26Mg(p, γ)^25–27Al, respectively, have been determined. In addition, absolute values for use as standards were determined for the 684 keV ²⁵Mg(p, γ)²⁶Al and 840 and 1966 keV ²⁶Mg(p, γ)²⁷Al resonances. New relative values were determined for the strengths of the if²⁵Mg(p, γ)²⁶Al resonances at ifE_{p = 0.3–1.7 MeV} and the ²⁶Mg(p, γ)²⁷Al resonances at E_p = 0.3–2.1 MeV. The resonance energies and total widths obtained in the present work are also given. The branching ratios and angular distributions of the γ-rays used in the absolute strength determinations were obtained in the same set-up as the absolute strength measurement. Using the revised resonance strengths, the astrophysical reaction rates of hydrogen burning of ^24–26Mg in explosive carbon burning in the stellar temperature region of T = (0.1–5.0) × 10⁹ K are compared with Hauser-Feshbach calculations. The present reaction rates are also discussed in the frame of the MgAl cycle.     

Total strength of the magnetic dipole resonance in <Superscript>27</Superscript>Al

The γ decay of the resonance-like structure observed in the ²⁶Mg(pγ)²⁷ Al reaction in the energy range E _p = 0.8–3.0 MeV of accelerated protons has been investigated. The M1 resonance on the ground and excited states of ²⁷Al with E* = 844 and 1014 keV is identified. The total strength of the M1 resonance on the ground state of this nucleus is determined. The position and total strength of this resonance are explained taking into account pairing forces.     

Proton threshold states in 26Al

The energy levels of ²⁶Al between E_x = 6.3 and 6.5 MeV, corresponding to proton threshold energies in the ²⁵Mg + p reaction from $\text{E}{}_{\mathrm{<mtext>p</mtext>}}\text{= 0}\text{to}\text{200}\text{keV}\text{, have been investigated using the reactions}{}^{27}\text{Al(su3}$He, α)²⁶Al and ²⁴Mg(³He, p)²⁶Al. Despite early work reporting a doublet at E_x = 6346 keV and E_x = 6362 keV, most subsequent work reported a single state with conflicting spin and parity assignments. Our work clearly establishes the presence of the doublet and resolves the conflicts. We find that there are six states in this excitation energy region, including a new state at E_x = 6410 ± 5 keV. The l-values leading to possible spin and parity assignments for all these states have been made using DWBA. We conclude however that only three of these six states may contribute to the production of ²⁶Al by the ²⁵Mg(p, γ) reaction in the MgAl cycle and the new structure of ²⁶Al reported here can substantially increase the production rate of ²⁶Al in stellar reactions.     

T=1 states in26Al

The resonance reaction²⁵Mg(p, γ)²⁶Al in the energy rangeE _p =300–400 keV was used to populate high-lying bound states in the self-conjugate nucleus²⁶Al. The existence of three resonances atE _p =304, 316 and 388 keV was verified, the spins of the ones at 316 and 388 keV were found to be 3^? and 2⁺, respectively. The spin of the bound level at 4,547 keV level was fixed to beJ ^π=2⁺. Several strong isovector Ml transitions were observed, which led toT=1 assignments for the levels atE _x =4,191, 4,547, 4,599, 5,141 and 5,542 keV. The results for excitation energies and values ofJ ^π,T together with previous experimental and theoretical data on²⁶Al and the neighboring |T _z |=1 nuclei²⁶Mg and²⁶Si are discussed in the framework of the isospin model.     

Measurement of the 15N(α, γ)19F resonances at Ec.m. = 536 keV and 542 keV

The E_c.m. = 536 keV and E_c.m. = 542 keV resonances in the ¹⁵N(α, γ)¹⁹F reaction have been studied using a thick Ti¹⁵N target, determining ωγ = (9.7 ± 2.0) × 10⁻⁵ eV and ωγ < 1 × 10⁻⁵ eV for these two resonances, respectively. For the 542 keV resonance this value is substantially smaller than had been assumed in earlier model calculations, and this reduces the rate of the analog ¹⁵O(α, γ)¹⁹Ne reaction (connecting the Hot CNO cycle and the rp-process) by a factor of ≈ 3 for temperatures in the region of T₉ = 1.     

Low energy resonances in21Ne(p,γ)22Na examined with a high energy resolution ion beam

TheE _R=126, 272 and 291 keV resonances in the²¹Ne(p, γ)²²Na reaction have been investigated with a high-energy-resolution ion beam. TheE _R=272 keV resonance was found to consist of two states separated by (888+5) eV, where the lower (higher) energy member is a high-spin (low-spin) state. All four resonances have widths less than a few eV, which is an improvement of nearly two orders of magnitude below previously reported limits. The influence of atomic effects on the determination of the correct value for the resonance energy is examined.     

Resonance strength measurements in the 24Mg(p, γ) 25Al reaction

The strength ωγ of the E_p = 823 keV resonance in ²⁴Mg(p, γ) ²⁵Al was determined to be ωγ = 490 ± 70 me V, by comparison with the Rutherford cross section measured in the energy region of E_p = 600–800 keV. The strength of other low-lying resonances at E_p = 224, 419, 1483, 1623, 1654 and 2010 keV were determined relative to this new value.     

Derp 3/2-Analogzustand in51V

In an investigation ofT=7/2 analogue states in⁵¹V the⁵⁰Ti(p, γ) excitation curve has been measured for proton bombarding energies 1280–1480 keV and 2340–2660 keV. From the (p, γ) resonances 29 new virtual levels in the region of 9316–9510 keV excitation energy in⁵¹V were determined. The strong resonance atE _p=1 371 keV has been identified as the isobaric analogue state of the⁵¹Ti ground state by determining spin and parity of this resonance to be 3/2^?. There is no evidence for a strong analogue resonance in⁵¹V corresponding to the 1.16 MeV _{p 1/2} state in⁵¹Ti. The γ-decay of the _{p 3/2} analogue state has been studied by measuring branching ratios and angular distributions of primary γ-transitions with a Ge(Li) detector.M1E2 mixing ratios have been determined for these transitions. The total width of the resonance for γ-decay is found to be Γ_γ=1.6±0.4 eV. New bound levels in⁵¹V have been introduced at 3576, 4651 and 4661 keV excitation energy. TheJ ^π values of the 3085, 4770, and 4863 keV states are determined to be 5/2^?, 5/2^?, 3/2^?, respectively. The analogue-antianalogueM1 transition strength is found to be considerably reduced compared to the situation ins-d shell nuclei.     

Search for low-energy resonances in25Mg(p,γ)26Al

The reaction²⁵Mg(p,γ)²⁶Al has been investigated atE _p (lab)=80-350keV. The existence of the resonance atE _p =317keV has been verified and new resonances have been found atE _p =198, 255 and 304 keV. TheE _p = 304keV resonance represents a new compound state in²⁶A1. Information on branching ratios,Ωγ values, total widths andJ ^π assignments for the observed resonances as well as upper limits on themy strengths for the expected resonances atE _p =97 and 134 keV are given. The astrophysical aspects of the data are discussed in the light of the renewed interest in the precise location of²⁶A1 nucleosynthesis.     

Low-energy behavior of the 3He(α, γ)7Be cross section

Cross sections for the ³He(α, γ)⁷Be reaction have been measured at several energies from E_c.m. = 165 to 1169 keV by counting prompt γ-rays from a windowless, differentially pumped, recirculating, ³He gas target. The cross-section factor S₃₄(E_c.m.) and branching ratio γ₁/γ₀ were determined at each energy. Cross sections were also measured at E_c.m. = 947 and 1255 keV by counting the γ-rays from the ⁷Be produced in a ³He gas cell with a Ni entrance foil. Combining the results of these two independent experiments yields a zero-energy intercept for the cross-section factor of S₃₄(0) = 0.53 ± 0.03 keV · b. The relationship between these measurements and several theoretical calculations, and the import of the extrapolated cross section for the solar-neutrino problem are discussed.     

The Jπ = 3− doublet at Ex = 6241 keV in 18F: Isospin mixing

The resonance previously observed at E_x = 6241 keV in the reactions ¹⁷O(p, γ)¹⁸F and ¹⁴N(α, γ)¹⁸F has been found to be a closely separated doublet (ΔE_x = 2.09±0.04 keV). Resonance strengths in ¹⁷O(p, γ)¹⁸F, ¹⁷O(p, α)¹⁴N and ¹⁴N(α, γ)¹⁸ have been measured and partial widths for the resonance states have been obtained. The doublet was further investigated by means of the ⁴He(¹⁴N, α)¹⁴N reaction using the differentially pumped gas target at CRNL. Analysis of these data using R-matrix theory yielded J^π = 3^? for both states and more precise values of the α-particle widths (Γ_α(lower) = 133 ±4 eV; Γ_α(upper) = 137 ±4 eV). The reduced α-particle widths and the E1 and M1 transition strengths indicate that the doublet arises from the nearly complete mixing of T = 0 and T = 1 eigenstates which in the absence of an isospin breaking interaction would have been separated by less than 40 eV. The α-particle scattering experiments included the J^π = 1⁺ resonance at E_x = 6257 keV and yielded the new values of Γ_α = 580±12 eV and Γ_p = 25⁺³⁵_?25 eV.     

Search for resonances in 12C(9Be, α)17O

Excitation functions at 7° (lab) have been measured from E_c.m. = 5.1 to 11.4 MeV in approximately 114 keV steps for 15 groups of final states in ¹⁷O populated by the ¹²C(⁹Be, α) reaction. Statistical tests have been used to locate possible non-statistical structure in the excitation functions. Possible anomalies were found near E_c.m. = 6.3, 7.5, 8.9 and 9.7 MeV. Angular distributions were measured at E_c.m. = 9.20, 9.71 and 10.23 MeV for the three lowest excited states in ¹⁷O. The data have been compared with Hauser-Feshbach calculations in addition to the following reaction mechanisms: compound plus a single resonance, compound plus interfering resonances and compound plus direct reactions.     

The structure of27Al

Gamma decay modes and spin assignments of levels in²⁷Al have been investigated by proton-γ-ray angular correlation measurements in the²⁴Mg(α, pγ) reaction at 14.2, 15 and 15.6 MeV bombarding energy and byγ-ray angular distribution measurements on the E_p=1820, 2114, 2293 and 2574 keV resonances of the²⁶Mg(p, γ) reaction. Unique spin assignments were obtained as follows: I=3/2 for the 5827 keV state, I=5/2 for the 6115, 6465, 6765, 7577, 7721, 8097, 8136, 8324 keV states, I=7/2 for the 5433, 6533, 7413, 8037, 8442, 8586 keV states, I=9/2 for the 5418, 6512, 6713, 7997 keV states, I=11/2 for the 5500, 6948, 7400, 8396 keV states. The level scheme and electromagnetic properties of levels are compared with the results of shell model calculations which use the complete configuration space of the 0d_5/2-1s_1/2-0d_3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent and extends into the region of high level density above 7 MeV excitation energy. The total B(M1↑) below 8.5 MeV excitation energy is evaluated, using published resonance fluorescence and (e, e′) data, and quenching relative to the free-nucleon predictions is reexamined. Evidence in favour of a prolate ground state deformation of²⁷Al and implications of this work for the astrophysically interesting²⁶Al(p, γ) reaction are discussed.     

Low-energy fusion cross sections of D + D and D + 3He reactions

The D(d, n)³He and D(d, p)T reactions have been investigated at E_c.m = 2.98 to 162.5 keV and the ³He(d, p)⁴He reaction at E_c.m. = 6.95 to 141.8 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems of the extended and quasi-point supersonic jet type. The measurement of absolute cross sections, angular distributions and excitation functions is reported. The data extend into the thermal energy region of future fusion reactors. The results for the D(d, n)³He and D(d, p)T reactions are in good agreement with previous work, while substantial differences are found for ³He(d, p)⁴He.     

Energy levels of 37K

Angular distributions have been measured for six ³⁶Ar(p,γ)³⁷K resonances in the E_p = 0.9−3.0 MeV energy region. These experiments yield spin- and parity assignments to seven excited states of ³⁷K, γ-ray branching ratios for each state, together with the multipolarity mixing ratios of most transitions. Lifetimes have been deduced from the same data with the DSA method.Excitation energies have been determined with a precision up to 20 eV. A resonance- and excitation-energy measurement at the E_p = 918keV resonance yields a ³⁶Ar(p, γ)³⁷K Q-value of Q = 1857.57 ± 0.09 keV.The ³⁶Ar(p, p₁γ) yield curve, measured in the E_p = 2.6–3.3 MeV region, yields additional information on resonance and excitation energies of ³⁷K states with a typical precision of 0.6 keV.The new data on ³⁷K levels allow a detailed comparison of the level schemes of the mirror nuclei ³⁷Ar and ³⁷K. The present experimental data agree quite well with the results of previous shell-model calculations.     

Total width of the lowest T = 2 state in 24Mg

By analyzing thick-target excitation functions of the $\text{E}{}_{\mathrm{<mtext>p</mtext>}}\text{= 3906}\text{keV}{}^{23}\text{Na}\text{(}\text{p}\text{, γ)}$ resonance we have determined Γ_c.m ≦ 530⁺⁴⁰_?70 eV for the width of the lowest T = 2 state of ²⁴Mg. The beam energy resolution function was measured using a narrow ²⁷Al(p, γ) resonance at E_p = 3671 keV, for which we obtain Γ_c.m. = 180 ±50 eV.     

The neutron decay of the resonance in the 12C + 16O system at Ec.m. = 19.7 MeV

The reaction ¹²C(¹⁶O, n)²⁷Si has been studied at nine incident energies in the range of E_cm = 19.39 to 22.6 MeV. At the known 19.7 MeV resonance two states at 15.37 and 17.04 MeV in ²⁷Si, probably with high spins, were strongly excited. The width of the resonance in these states is consistent with the known width of about 400 keV (c.m.). No resonating states were found above this resonance.     

Investigation of the D(<Superscript>3</Superscript>He, <Emphasis Type="Italic">p</Emphasis>)<Superscript>4</Superscript>He Reaction in the Astrophysical Energy Region of 18–30 keV

The D(³He, p)⁴He reaction is first investigated in a solid target of deuterated zirconium (ZrD) in the ³He⁺ ion energy range E_He = 18–30 (E = 7.2?12.0keV keV in the center-of-mass system) with a step of 2 keV. The electron screening potential U_e = (617.8 ± 154.7) eV and the D(³He, p)⁴He reaction enhancement factors are experimentally determined in the given energy range. The measured electron screening potential is six times higher than in gaseous targets. This can be due to the ZrD lattice effects, which have not been studied either theoretically or experimentally so far. The D(³He, p)⁴He reaction has been investigated at the pulsed plasma Hall accelerator (Tomsk).     

High-spin states in27Al

The²⁴Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the²⁷Al levels atE _x =5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theE _x =7399 keV level and a limitτ<20 fs for the levels atE _x =7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in²⁷Al were obtained by a study of the²⁶Mg(p, γ) reaction at the E_p=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theE _x =5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atE _x =6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the²⁴Mg(α, p) and²⁶Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.     

The 18O(p, α)15N reaction at stellar energies

The ¹⁸O(p, α)¹⁵N reaction has been investigated in the energy range E_p = 72–935 keV. The three known resonances above E_p = 620 keV have been confirmed and four new resonances have been found below E_p = 340 keV. All observed resonances correspond to known compound states in ¹⁹F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 10⁹ K. The energy averaged astrophysical reaction rates are compared with predictions.