L.K. Fifield

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 + .

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.

Precision Doppler-shift attenuation lifetime measurements in 28Si and 31P

The mean lives of three excited states have been measured by bombarding an implanted 4He target with a 28Si beam and observing the (particle coincident) gamma -ray line-shapes at 0 degrees . The results are: (in 28Si) 1778 keV level, tau m=697+or-39 fs; (in 31P) 1266 keV level, tau m=761+or-47 fs, 2234 keV level, tau m=391+or-25 fs. The last lifetime is of interest in attempts to calibrate the Doppler-shift attenuation method at low initial recoil velocities.

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.

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.

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.

The second 132+ state in 19F

Abstract A cryogenically pumped gas target has been used to study resonances in the 15 N(α, γ) 19 F reaction for E α = 5.1 to 8.6 Mev. Gamma-ray intensity and angular distribution measurements for a resonance at E α = 8.105 Mev, corresponding to a state at 10.411 MeV in 19 F, restrict the spin and parity of the state to 11 2 + or 13 2 + . Comparison with shell model calculations indicate that the state is a strong candidate for the second 13 2 + (2s, 1d) 3 level in 19 F.