E.F. Garman

Radiative decays of unbound high-spin states in 24Mg (II)

Abstract Known negative-parity levels at 11.59(5 − ), 12.44 (7 − ), 13.06(5 − ), and 13.76(5 − ) MeV in 24 Mg have been studied as resonances in the 20 Ne(α, γ) 24 Mg reaction, and their radiative and α-particle decay widths deduced. The 12.44 MeV level is identified as the 7 − member of a K π = 0 − band by virtue of a strongly enhanced E2 γ-ray transition to the 10.03 MeV 5 − level. The 13.06 MeV level exhibits a strongly enhanced γ-ray transition to a probable 3 − level at 10.33 MeV, indicating the existence of a new negative-parity rotational band in 24 Mg. Known 4 + levels at 12.05, 12.12, 12.16, 12.51 and 13.05 MeV have also been studied as 20 Ne(α, γ) 24 Mg resonances in a search for the 4 + member of a proposed excited K π = 0 + band via an enhanced E2 transition to the 8.65 MeV 2 + level. No such transition was observed, but the 12.05 MeV level was identified as the second 4 + T = 1 level in 24 Mg. In addition, γ-ray decay schemes and angular distributions provided the following information on the spins and parities of three sharp resonances near 14 MeV in 24 Mg: 14.10, (2, 4) + ; 14.146, 4 + (3 − , 5 − ) and 14.32, 4 + .

Decay of the lowest T = 2 state in 40Ca

Abstract The lowest 0+; T = 2 state in 40Ca has been excited as a resonance in 36Ar(α,γ)40Ca at Ex = 11988±2 keV; гp1 has been determined and limits placed on Γ, Γp2, Γp3 and Γα1. Transitions to 1+;T = 1 levels at 10321.0 ± 1.6 keV and 9868.0 ± 1.7 keV were observed and their strengths measured. Ground-state M1 strength from 1+;T = 1 levels is inferred.

Yrast 8+ and 9+ states of 22Na

Abstract Gamma-decaying high-spin states of 22Na populated in the 12C(14N, α) reaction were studied, using a recoil-coincidence technique to measure values of Γ γ Γ and an α-γ coincidence arrangement to measure lifetimes and γ-decay branches. Good candidates have been found for the yrast 8+ and 9+ levels at excitation energies of 8572±3 and 9813±3 keV, respectively. Their properties are compared with those predicted by the shell model and test these calculations in the region where band termination is expected.

Evidence for isovector parity mixing in the 11.264 MeV 1+ T = 1 state in 20Ne

Abstract A search has been carried out for the 1 + T = 1 level at 11.264 MeV in 20 Ne formed as a parity-forbidden resonance in the 16 O(α,γ) 20 Ne reaction. The yield of 11.26 MeV γ-rays was measured as a function of α-particle energy over an interval of 8 keV centred on the expected resonance energy of the 1 + T = 1 level. An energy and intensity standard was provided by the nearby 1 − T = 1 resonance at an excitation energy of 11.275 MeV in 20 Ne. A significant improvement in the fit to the measured yield curve was obtained by including a sharp resonance at the expected energy of the 1 + T = 1 level. From the analysis it is concluded that the 1 + T = 1 level has been observed with 90% confidence. The resonance strength extracted from the fit implies a parity-forbidden a-particle width of 42(+-20)× 10 −6 eV for the 1 + T = 1 level. The corresponding parity non-conserving matrix element linking the 1 + T = 1 level with the 1 + T = 0 state at 11.23 MeV therefore lies within the, range 0.8 eV ⩽ 〈 V PNC Δ T = 1〉 ⩽ 2.6 eV.

States in 21Ne via the 12C(13C, αγ)21Ne reaction

Abstract Many gamma-decaying levels up to 8.25 MeV in 21 Ne have been populated via the 12 C ( 13 C , α) 21 Ne reaction at E 13 c = 19.7 McV. Gamma-rays de-exciting these levels were detected in coincidence with α-particles, making it possible to study the γ-decay of individual levels in isolation. A variant of the Doppler shift attenuation method due to Branford and Wright was used to measure the lifetimes of most of the states observed. The relevance of the information obtained to hand structure in 21 Ne is discussed. Shell-model calculations are also presented for positive-parity states, and compared with the available experimental information.

Alpha widths in 19F from a study of resonances in 15N(α0, α0)15N

Abstract The 15 N( α 0 , α 0 ) 15 N reaction in a windowless gas target has been used to study nine states in 19 F between E x = 8.288 and 10.411 MeV. Resonances were identified by their γ-decay schemes, and yield curves of elastically scattered α-particles were collected at three angles. R matrix theory was used to analyse the yield curves, and Γ α , and where not previously known J and π, were extracted. The partial widths are compared with cluster model theory.

Measurement of the alpha particle decay width of the first 0+, T = 2 state in 40Ca

Abstract The alpha decay width and total width of the first 0 + , T = 2 state in 40 Ca have been measured by the elastic scattering of α-particles from 36 Ar. The results are Γ α 0 = 80 ± 10 eV and Γ = 81 ± 10 eV.

A high spin γ-decaying state at 16.9 MeV in 24Mg

Abstract States with excitation energy > 15 MeV populated in the reaction 12C(16O, αγ)24Mg have been studied using a total energy γ-ray spectrometer for E lab ( 16 O ) between 40 and 67 MeV. A previously unknown state has been identified at 16.904 ± 0.003 MeV with Γγ/Γ=0.13±0.02, mean lifetime

Measurement of a highly retarded α-particle decay in 20Ne

Abstract The partial widths for α-particle and γ-ray decay of the second 2 + T = 1 level in 20 Ne at 12.22 MeV have been measured. The reaction 19 F( 3 He, d) 20 Ne was employed, with deuterons detected in singles and in coincidence with γ-rays from the decay of the 12.22 MeV level. The coincidence efficiency was determined using the 100% γ-decay of the 10.89 MeV 3 + T = 1 level. The ratio of the γ-decay width to the total width of the 12.22 MeV level was measured to be 0.88 ±0.05, which combined with a recent 16 O (α, γ) 20 Ne measurement yields a very small α-particle width of 0.32±0.06 eV. This result is also of relevance to the radiative decay of the 18.43 MeV 2 + T = 2 level in 20 Ne, and confirms a large discrepancy between the measured γ-decay width of the 2 + T = 2 level and the predictions of the shell model.

Lifetime of the 1.04 MeV 0+, T = 1 state in 18F

Abstract The mean life of the 1.04 MeV 0 + , T = 1 state in 18 F has been measured as 2.2±0.6 fs using a refinement of the Doppler-shift attenuation method. The reaction 3 He( 16 O, p) 18 F at a mean beam energy of 26.6 MeV was used to populate the 1.04 MeV level with a recoil velocity v c = 4.4 % . The targets were 3 He implanted in foils of magnesium, aluminium, tantalum and gold. The result is compared with shell-model calculations and its relevance to the search for parity non-conservation in the system comprising the 1.04 MeV level and the 1.08 MeV 0 − , T = 0 level in 18 F is discussed.