K.W. Allen

Radiative decays of unbound high spin states in 24Mg: (I). Positive parity states

Abstract The 20Ne(α, γ)24Mg reaction has been used to measure radiative decay rates for known high spin levels at 12.86(6+), 13.44(6+) and 14.15(8+) MeV in 24Mg, and for previously unidentified levels at 12.00, 13.04 and 14.08 MeV. Gamma-ray angular distributions indicate that the 12.00 and 14.08 MeV levels are probably 6+ levels, while the 13.04 MeV level is the lowest 0+ T = 1 state. A new level at 11.005 MeV, populated in the radiative decay of the 14.08 MeV level, is most likely the lowest 5+ T = 1 state in 24Mg. Alpha-particle widths have also been measured for several of these levels and for the known 6+ level at 13.85 MeV using the 20Ne(α, α′γ)20Ne reaction and previously determined branching ratios. The results for both radiative and particle decay widths are compared with recent shell-model calculations performed within a full (sd)8 basis.

A cryogenically pumped gas target for the study of angular distributions of gamma rays following radiative capture

Abstract A cryogenically pumped gas target, suitable for the study of angular distributions of gamma rays following radiative capture, has been constructed. A correction procedure has been developed to take into account the finite extent of the gamma emitting region within the target. The system has been tested in a study of the angular distribution of gamma rays from the well known 2 + , T =1 state in 20 Ne at 10.27 MeV, excited as a resonance in the reaction 16 O(α, γ) 20 Ne.

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.

Measurements of the radiative widths of the first T = 1, 2+ state of 20Ne and their relationship to analog β-decays

Abstract The radiative widths for decays of the 20 Ne T = 1, 2 + (10.27 MeV) state were measured by resonance α-capture in the reaction 16 O(α, γ) 20 Ne. A special windowless gas-cell target yielded a low-background spectrum enabling six γ-branches to be observed with a Ge(Li) detector. The six branches correspond to decays from the 10.27 MeV level to the following levels: 2 + (7.83 MeV), 2 + (7.42 MeV), 3 − (5.62 MeV), 2 − (4.97 MeV), 2 + (1.63 MeV) and 0 + (g.s.). The branching ratios and radiative widths Γ γ to these levels are: 7.83 MeV [(0.22 ± 0.06)%, 0.008 ± 0.002 eV], 7.42 MeV [(6.9 ± 0.4)%, 0.31 ± 0.04 eV], 5.62 MeV [(2.1 ± 0.2)%, 0.097 ± 0.014 eV], 4.97 MeV [(1.3 ± 0.1)%, 0.060 ± 0.008 eV], 1.63 MeV [(88.9 ± 0.5)%, 4.08 ± 0.43 eV] and 0.0 MeV [(0.64 ± 0.14)%, 0.029 ± 0.008 eV]. The radiative widths to the 1.63 MeV and 7.42 MeV levels are used to determine the CVC predictions of the weak magnetism form factors and their effects on certain β-decay observables are evaluated.

The radiative width of the 8+ member of the Kπ = 2+ band in 24Mg

Abstract The radiative width of the 14.1 MeV, 8 + , K π = 2 + level in 24 Mg has been measured using the 20 Ne (α, γ) 24 Mg reaction and a differentially cryopumped gas target. The result, Γ γ = 85 ± 23 meV, implies E2 strengths of 9.1 ± 2.4 and 0.8 ± 0.2 W.u. for the transitions to the 9.53 (6 + , K π = 2 + ) and 8.11 (6 + , K π = 0 + ) MeV levels, respectively. These are in excellent agreement with the predictions of recent large basis shell model calculations.

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.

High-spin negative parity states in 19F

Abstract Levels at 7.17, 8.29, 8.96 and 9.88 MeV in 19 F have been assigned spin and parity 11 2 − , 13 2 − , 11 2 − and 11 2 − , respectively, from resonance strength and γ-ray angular distribution measurements employing the 15 N(α,γ) 19 F reaction. An earlier assignment of 11 2 + to the 8.96 MeV level is incorrect. The measured properties of the 11 2 − states are compared with the results of both SU (3) shell model and cluster model calculations.

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.