H.B. Mak

A Heavy Water Detector to Resolve the Solar Neutrino Problem

Abstract The observation of the following three reactions: νed→ppe; νxe→νxe; and νxd→νxpn (where νx is any left- handed neutrino) in a heavy water Cerenkov detector which is being designed, allows the solar 8B ve flux, spectrum and direction to be measured. In addition, the total solar 8 Bν c neutrino flux, direction and integral spectrum, independent of neutrino flavor, may be determined to provide several independent methods to resolve the solar neutrino problem.

Measurement of the absolute cross section of the 3He(4He, γ)7Be reaction at Ec.m. = 525 keV

Abstract The cross section for the radiative capture reaction 3 He( 4 He, γ) 7 Be has been measured at 525 keV in the centre-of-mass by detection of prompt capture γ- rays. The targets were 3 He-implanted Nb foils that allowed us to circumvent the experimental difficulties inherent in the use of extended gas cells for absolute measurements. The results give an inferred zero-energy cross-section factor of S 34 (0) = 0.47 ± 0.04 keV · b. The present result is compared with results from previous capture γ-ray yield and 7 Be-activity methods of measuring the cross-section factor.

Parity mixing in 21Ne; evidence for weak neutral currents in nuclei

The parity nonconserving circular polarization of γ-rays from the 2.789 → 0.0 MeV transition in ^(21)Ne is found to be (0.8 ± 1.4) × 10^(−3), which corresponds to a parity mixing matrix element │〈H_W〉│ = 0.009 ± 0.016 eV between the two members of the 2.8 MeV doublet. This matrix element can be combined with parity mixing matrix elements measured in ^(18)F and ^(19)F to predict isoscalar and isovector weak nucleon-nucleon coupling constants in excellent agreement with theoretical “best” values obtained in a Weinberg-Salam model calculation.

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.

Confirmation of the l-forbidden Gamow-Teller decay branch of 39Ca

Abstract A search for Gamow-Teller decay branches of 39Ca to excited states in 39K was conducted with a large HPGe detector which observed the decays of 39Ca samples delivered by a helium-jet transport system. A pair of plastic scintillators in front of the HPGe detector were used to reject events due to bremsstrahlung from the 39Ca positrons. With this setup, weak γ-rays could be observed with a degree of sensitivity not previously achieved. The l- forbidden 1 d 3 2 → 2 s 1 2 transition to the first excited state in 39K, was observed with a branching ratio of (2.50 ± 0.27) × 10−5. This result agrees with but is more accurate than the only previous measurement. Thus, the ratio of the l-forbidden Gamow-Teller and isovector M1 matrix elements in mass-39 nuclei continues to disagree with theoretical values obtained by Towner and Khanna from calculations involving core-polarization and meson-exchange currents. This leaves very few possibilities to explain the discrepancy between theory and experiment.

The l-forbidden m1 decay of the 2522 keV level in 39K

Abstract The mean lifetime of the first excited 1 2 + level in 39 K has been measured to be τ = 93 ± 13 fs by high recoil velocity DSAM. The level was excited by bombarding a 4 He-implanted Ta target with a 100 MeV 39 K beam. The lifetime was determined by fitting the Doppler-broadened lineshape of the 2522 keV gamma-ray transition to the 3 2 + ground state. Together with the measured B (E2), the B (M 1; d 3 2 → S 1 2 ) for this l -forbidden M1 transition was determined to be 0.0100 ±0.0027 μ 2 N . A theoretical calculation, taking into account core polarization, delta excitation and mesonexchange current contributions, underestimates the B ( M )1) by more than a factor of six. The experiment also measured the lifetimes of the 3019 keV and 3883 keV levels to be 20 ±4 fs and 11 ± 6 fs, respectively.

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.

Lifetime measurements of levels in 26Si and the ratio of neutron to proton transition amplitudes for mirror nuclei

Abstract The mean lifetimes of the 1796, 2784, 3333 and 4138 keV levels of 26 Si have been measured using the Doppler shift attenuation method. The ratios of neutron to proton transition matrix elements determined from these data and additional data from mirror transitions in 26 Mg are compared with recent data from inelastic pion scattering and with shell model calculations. The isoscalar strenghts are compared with α-scattering data.

Isospin-forbidden particle decays in light nuclei: (IV). Total width of the lowest T = 2 level of 24Mg

The lowest Jπ = 0+, T = 2 level of 24Mg has been observed as a narrow resonance in the 23Na(p, p0)23Na, 23Na(p, α0)20Ne and 23Na(p, α1)20Ne (1.63 MeV) reactions at Ep = 3.91 MeV. The total width of the level has been determined to be 300 eV < Γ < 700 eV. Trends in the reduced widths for isospin-forbidden particle decays of T = 2 levels for A ≦ 44 are discussed.