H.C. Evans

Coincidence measurement of the C-12 (alpha, gamma) O-16 cross section at low energies

The E1 capture cross section for the reaction 12C(α,γ)16O to the 16O ground state has been measured for Ec.m.=1.29–3.00 MeV. A γ-ray–recoil-particle coincidence technique has been used to obtain essentially background-free reaction yields. A three-level R-matrix analysis of the E1 S factor leads to an allowed range for SE1 at 300 keV of 0.00–0.14 MeV b. A model-dependent R-matrix analysis and a hybrid R-matrix analysis result in values of 0.14 and 0.08 MeV b, respectively, for SE1(300).

The decay of 20Na: Measurements of isospin mixing and the weak vector coupling constant as well as other new decay data

Abstract Kinematic shifts of α particles from the β + delayed α decay of 20 Na have been measured as a function of the energy of the coincident β + particles. Beta-neutrino-alpha triple-correlation coefficients were determined for six β + transitions from 20 Na( J π , T = 2 + , 1) to excited 2 + states in 20 Ne; ft values were also deduced. For the superallowed 2 + → 2 + transition to the T = 1 analogue state in 20 Ne the results yielded a measurement of the vector weak coupling constant, G V /( c ) 3 = (1.103±0.028) × 10 −5 GeV −2 . For the other five (allowed Gamow-Teller) transitions, limits were established for the vector (Fermi) component and, from them, limits deduced on isospin mixing amplitudes for the daughter states in 20 Ne. Weak, previously unknown, β-transitions were assigned to the decay of 20 Na based on the observation of new α groups and one new γ-transition.

12C( alpha, gamma )16O cross sections at stellar energies.

The excitation function of the C12(α,γ)16O reaction and the angular distribution of its γ rays were measured at nine center-of-mass energies ranging from E=1.37 to 2.98 MeV. These measurements allowed the separation of the E1 and E2 contributions and their extrapolation to the region of astrophysical interest. The analysis of all the available E1 cross sections with the K-matrix method and with a three-level R-matrix method yields a consistent prediction of 79±16 keV b for the E1 S factor at 300 keV. The E2 contribution at the same energy is 36±6 keV b from a cluster model analysis of the present data. The best estimate of the total S factor at 300 keV is 120 keV b, and it is unlikely that it will fall outside the range of 80–160 keV b.

The half-lives of 26mAl, 34Cl, 38mK: Precision measurements with isotope-separated samples

Abstract The half-lives of 26m Al, 34 Cl and 38m K have been measured with isotopically pure samples prepared with an on-line isotope separator. Emitted β + particles were observed with a large-solid-angle, high-efficiency, low-background gas proportional counter, from which signals were multiscaled. The counting electronics had a well-defined non-extendable dead time. Each sample decay curve was individually corrected for dead time before being summed with other accumulated data, no pile-up corrections being necessary. The analysis procedure was tested on hypothetical data. The effect of contaminant isobars on the resultant half-lives has been investigated. Our half-life results are 26m Al : 6346.2 ± 2.6 ms, 34 Cl: 1527.7 ± 2.2 ms and 38m K: 924.2 ± 0.3 ms. The quote uncertainties include — for the first time — explicit provision for all possible contaminants.

Explosive hydrogen burning of 31P

Abstract Proton threshold states in 36 Ar have been studied via the reactions 35 Cl( 3 He, d) 36 Ar, 32 S( 6 Li, d) 36 Ar, 32 S(α, γ) 36 Ar, 35 Cl(p, γ) 36 Ar and 35 Cl(p, α) 32 S to investigate their influence on a possible SC1 reaction cycle in explosive hydrogen burning. Three new states in 36 Ar have been observed in the ( 3 He, d) reaction at E x = 8806, 8887 and 8923 keV. Deuteron angular distributions were measured for 14 states near the 35 Cl+p threshold and were analyzed with DWBA calculations. Values of transferred orbital angular momenta, spectroscopic factors and proton partial widths were determined. Gamma-ray spectra have been measured at ten (p, γ) resonances. Three new resonances were observed at E R = 311, 416 and 627 keV, corresponding to 36 Ar states at E x = 8806, 8909 and 9117 keV, respectively. Excitation and resonance energies, γ-ray branching ratios and resonance strengths are presented. The astrophysical implications of our results for explosive hydrogen burning of 35 Cl are discussed.

The alpha widths of the 5603, 5605 and 5668 keV states in 18F

Abstract A differentially pumped gas target has been used for high resolution studies of the 14 N( α , α ) 14 N differential cross section in the region of 5650 keV excitation energy in 18 F. Interference anomalies in the excitation functions were observed at energies corresponding to levels at 5603, 5605 and 5668 keV in 18 F. The data were analysed using the R -matrix formalism, and the level parameters of the resonances were determined to be Γ α = 42.8 ± 1.6 eV, l α = 0; Γ α = 32.0 ± 2.1 eV , l α = 1 ; and Γ α = 130 ± 5 eV, l α = 1 for the 5603, 5605 and 5668 keV states respectively. The isospin mixing between the 5605 and 5668 keV levels and the possibility of studying the parity mixing between the 5603 and 5605 keV levels are discussed.

The 18O(p, α)15N cross section at low energies

Abstract The 18 O(p, α) 15 N reaction cross section has been measured over the energy range 661 keV > E c.m > 223 keV. The S -function was extrapolated to energies of astrophysical interest using the R -matrix theory. The S -factor, S 0 , is estimated to be 46 MeV · b which is a factor of 3 larger than the value used in a recent tabulation of nuclear reaction rates. The effects of broad levels near the proton threshold are discussed.

Isospin-forbidden particle decays in light nuclei

Abstract The lowest T = 3 2 level of 21 Ne has been observed as an isospin-forbidden resonance in the 17 O(α,n) 20 Ne(1.63 MeV) reaction at an incident energy of 1862.2±2.6 keV, corresponding to an excitation energy of 8856.7±2.2 keV in 21 Ne. Total and partial decay widths were determined as follows: Γ = 2.8±0.5 keV; Γ n (0.0)/ Γ Γ α Γ n (1.63)/ Γ = 0.42±0.16 eV. No resonant structure was observed for the second T = 3 2 level in 21 Ne and upper limits of 0.4 eV and 1.0 eV were determined for Γ α Γ n (0.0)/ Γ and Γ α Γ n (1.63)/ Γ , respectively. All available information on partial widths for isospin-forbidden nucleon decay of T = 3 2 levels from mass-9 to mass-41 have been compiled. Trends observable in these data are discussed.

Perturbed angular distribution in 53Mn following resonant proton capture

Abstract The g -factor of the 378 keV ( 5 2 − ) level in 53 Ma has been measured to be 1.30±0.12 by an integral perturbed angular correlation method following a low energy recoil implantation into Ni by the 52 Cr(p.γ), 53 Mn reaction. The result is consistent with a predominantly ( πf 7 2 ) −3 configuration for the state.

The direct determination of the masses of unstable atoms with the chalk river on-line isotope separator

Abstract A new technique has been developed to measure the spacing of atomic mass doublets of radioactive isotopes directly with an on-line isotope separator. It relies not on ion detection but on observation of the specific radioactive signature of the isotopes under study. Consequently, line shapes and centroids can be determined, free of interference and with great accuracy, even if the corresponding beams strongly overlap or if they are contaminated by unwanted isobars or isomers. In particular, it is of no consequence if one or both members of the doublet are masked by stable background peaks. Doublets are peak matched as in a conventional mass spectrometer. The technique has been evaluated with beams of radioactive nuclides whose masses are known independently. Based on careful calibrations, two new mass values have been obtained: 72Br, 71 936 340 ± 430 μu and 63Ga, 62 939 570 ± 150 μu.