E.K. Warburton

StrongE1transitions inBe9,Be11, andC13

The Doppler shift attenuation method has been used to make two measurements of the mean life of the /sup 11/Be 320-keV level. The results are 150 +- 30 fs from the /sup 9/Be(t,p..gamma..) /sup 11/Be reaction and 168 +- 17 fs from the /sup 3/H(/sup 9/Be,p..gamma..)/sup 11/Be reaction. The adopted value of 166 +- 15 fs, from the present and previous measurements, corresponds to an extremely strong E1 transition of 0.36 +- 0.03 W.u. The adopted energy of the ground state transition from this and previous measurements is 320.04 +- 0.10 keV. It is shown that the magnitude of this E1 transition can be understood on the basis of shell-model calculations only if realistic single-particle wave functions are used. A similar theoretical treatment is given for strong E1 transitions in /sup 9/Be and /sup 13/C.

Coulomb excitation of sd shell nuclei: A self-contained set of B(E2) values and lifetime measurements☆

Abstract Thick targets of Ne, Na, Mg, Al and Si were bombarded by beams of 28, 29 Si, 31 P, 32, 33, 34 S, and 35, 37 Cl ions. From the Coulomb excitation of both projectile and targets an interrelated and self-contained set of B (E2) values was obtained for the low-lying levels of stable nuclei in the range 20 ≦ A ≦ 37. The γ-rays from the decay of the levels formed in Coulomb excitation were observed at 0 to the beam. The γ-ray intensities yielded the B (E2) values, while the γ-ray lineshapes provided lifetimes for the levels via the Doppler-shift attenuation method. For the even- A nuclei, therefore, B (E2) values were extracted for the 2 + first-excited state by two independent methods while for the odd-A nuclei, B (M1) and B (E2) values were determined for several levels. The B (E2) values obtained via the two methods were found to be in excellent agreement. The B (M1) and B (E2) values are compared to recent shell model calculations.

The beta-decay of 8He

Abstract A many-level, many-channel R -matrix formula is used to fit the measured 8 He β-decay data, including the half-life, branching ratios and delayed neutron and triton energy spectra. Satisfactory fits are found with contributions from four 1 + levels of 8 Li, at 0.98, 3.08, 5.15 and 9.06 MeV. The 32 reduced width amplitudes are taken from a shell-model calculation, with only two requiring adjustment. Gamow-Teller matrix elements obtained from the fits are in reasonable agreement with shell-model values.

Large-basis shell-model treatment of A = 16

Abstract A recently constructed shell-model interaction is used to calculate wave functions for the 0+ states of 16O in a six-shell (0+2+4) h ω model space. Several different methods of dealing with problems arising from truncation of the model space at 4ħω are described. The preferred is to lower the 4ħω components by the same energy shift as occurs for the ground state when changing from 0ħω to (0+2+4) h ω . The strong role of the SU3(20) component of the interaction is described. It is shown that a clear and sensitive test of the wave functions of the ground state and 6049 keV deformed state is the 16N 2− unique first-forbidden decay rates to them. Good agreement is found for these observables.

Reorientation measurements using the doppler-shift method: static quadrupole moments of 20Ne, 22Ne, 24Mg, 26Mg and 28Si

Abstract The static quadrupole moments of the first excited Iπ = 2+ states in 20Ne, 22Ne, 24Mg, 26Mg and 28Si were measured via the reorientation effect observed in Coulomb excitation with beams of 32S, 34S and 37Cl ions. The quadrupole moments were deduced from the Dopplerbroadened γ-ray lineshapes observed at 0° with respect to the beam by means of a Ge(Li) detector. The results are: Q( 20 Ne , 2 + ) = −0.23±0.08 e· b , Q( 22 Ne , 2 + ) = −0.18±0.04 e· b , Q( 24 Mg , 2 + ) = −0.24±0.06 e· b , Q( 26 Mg , 2 + ) = −0.16±0.04 e· b (−0.12±0.04 e· b ), and Q( 28 Si , 2 + ) = +0.17±0.05 e· b . Two values are quoted for 26Mg corresponding to the two possible signs of an interference term arising from virtual excitations through the second excited 2+ state of 26Mg. A detailed description of the experimental technique and analysis is given, including the procedure whereby the semi-classical analysis of experimental lineshapes is modified to accord with a full quantal treatment of Coulomb excitation. Perturbing effects due to virtual E1 excitations and deorientation effects are calculated and found to be small.

Yrast decays in 87,89Zr, 87,88,89,90Y and 86Sr from 74,76Ge(18O,xn,yp,zαγ) reactions

Gamma-ray data from 74,76Ge+18O fusion-evaporation reactions are analysed to provide data on yrast decay schemes in 87,89Zr, 87,88,89,90Y and 86Sr. The data include excitation functions, angular distributions, gamma - gamma coincidences, linear polarisation measurements and lifetimes deduced from both Doppler shift attenuation and recoil distance measurements. Shell-model calculations are presented for 88,90Y and 86Sr.

Properties of 13B and 20Na: The second-class current problem

Abstract The half-lives of 13 B and 20 Na are measured: t 1 2 ( 13 B ) = 17.33 ± 0.17 ms ; t 1 2 ( 20 Na ) = 442 ± 5 ms . The mass difference is measured: 20 Na− 20 Ne = 13.892 ± 0.007 MeV. Combined with other data these measurements lead to ( ft ) + /( ft ) − = 1.166 ± 0.026 for A = 13 and 1.054 ± 0.023 for A = 20. If these numbers were to be directly interpreted in terms of secondclass currents we should find for the induced tensor coupling constant: g IT (13) = (2.6 ± 0.5) × 10 −3 ; g IT (20) = (1.5 ± 0.6) × 10 −3 .

Unique second- and third-forbidden β decay

Abstract The unique second-forbidden β-decay matrix elements of 10Be → 10B, 22Na → 22Ne and 26Al → 26Mg are calculated using both the shell model and the unified model. The unique third-forbidden β-decay matrix elements of 40K → 40Ca and 40K → 40Ar are calculated using the shell model. All final states are ground states except for 26Mg, in which case the decay is to the 2+ first= and second-excited states. The four second-forbidden transitions are rather well described by the Nilsson form of the unified model. There is some evidence in the decay of 10Be and 22Na for collective suppression of the β matrix elements analogous to the collective enhancement of E2 matrix elements. The 10Be → 10B matrix element is also reproduced by the Cohen-Kurath shell-model wave functions. Shell-model calculations that only assume 2s 1 2 and 1d 5 2 configurations are less successful for 22Na → 22Ne and 26Al → 26Mg. For the spherical shell-model calculation of the 40K decays, agreement cannot be reached if only the 1d 3 2 and 1f 7 2 shells are considered. Considerably better agreement for 40K → 40Ca is achieved if the j space is increased to include other 1p-1h terms in the 40K wave function. It is argued that agreement cannot be reached for the 40K → 40Ar rate without including terms from other shells (within 1ħω of 1d 3 2 and 1f 7 2 ) in the dominant two-particle—two-hole wave function of 40Ar, and that this phenomenon, for both 40K → 40Ca and 40K → 40Ar, follows from a general feature of n-forbidden unique β decay (with n odd) analogous to the inhibition of ground-state E1 transitions from low-lying T = |TZ| + 1 states and the associated appearance of an E1 giant resonance.

Band structure of 184Ir

Abstract States of 184Ir, excited through several (HI, xn) reactions were studied using in-beam γ-ray and conversion electron spectroscopy techniques. A new high-spin level scheme, comprising three different rotational bands is presented. The negative-parity ΔI = 1 ground-state band is the most complete example of the semidecoupled \ gp h 9 2 ⊗ v i 13 2 structure, in a prolate deformed situation, known to date. The other two bands are the doubly decoupled \ gph 9 2 ⊗ v 1 2 − [521] and a ΔI = 1, \ gp 5 2 + [402]⊗v i 13 2 based structure of the compressed type.

Hindrance phenomena in unique first- and third-forbidden β-decay

Abstract The characteristic hindrance of all unique n -forbidden beta decays (with n odd) among light nuclei ( A ⩽ 50) can be traced to correlations introduced by the repulsive T = 1 particle-hole force. These correlations are examined in perturbation theory. The results explain the experimentally observed hindrance, and in a large proportion of the cases, good numerical agreement is also obtained.