A.N. James

Microsecond mass separation of heavy compound nucleus residues using the Daresbury recoil separator

Abstract The recoil separator at the Daresbury Laboratory is described. An efficient charged particle beam transport system is combined with Wien filters to select heavy ions within a small velocity range at angles around 0° which have been produced by heavy ion induced reactions. The system separates and refocusses particles with a given mass to charge ( A Q ) ratio. Resolution in A Q is 1 in 300. The time taken to determine the atomic mass A and atomic number Z of an individual nucleus passing through the recoil separator is about 10−6 s. This is fast enough for phenomena associated with the compound nucleus decay to be electronically tagged according to the nuclear species produced. An example using γ-ray detectors around the compound nucleus formation target is given.

Construction and use of a three Ge(Li) Compton polarimeter

Abstract A three Ge(Li) Compton polarimeter has been constructed, and its sensitivity to γ-ray linear polarisation determined in the energy range 0.4 to 4.4 MeV. An example is given describing its application to resolve a spin ambiguity for the 4868 keV level in 33 S populated by the reaction 30 Si(α,n) 33 S at an α-particle bombarding energy of 9.8 MeV.

A double-sided silicon strip detector system for proton radioactivity studies

Abstract A new double-sided silicon strip detector has been developed to study the phenomenon of ground-state proton radioactivity. Highly proton-rich fusion-evaporation reaction products are velocity and mass separated using the Daresbury recoil separator before being implanted into the strip detector located at the separator focal plane. The double-sided strip detector has 48 strips per face with a pitch of 335 μm. Front and back strips are orthogonal providing an effective pixel area of 0.09 mm2, enabling correlations between implanted ions and subsequent decays to be clearly established. Test results obtained using the reactions 58 Ni + 92 Mo → 150 Yb ∗ and 58 Ni + 54 Fe → 112 Xe ∗ are reported. In the latter commisioning experiment the proton decay of 109I was unambiguously established using the correlated decay sequence 109I→p108Te→α104Sn.

The statistical analysis of γ-ray angular correlation experiments

Abstract The statistical analysis of γ-ray angular correlation experiments is discussed with the emphasis on experiments where the reaction mechanism does not lead to exactly known alignment. If a model of the reaction producing the alignment is made then it can be used to estimate the most probable value of the alignment and the confidence in that prediction. The inclusion of such estimates in the analysis is described. A procedure for including many types of relevant data into the analysis is described. Particular emphasis is given to the general technique of estimating the error to quote in association with a mixing ratio measurement. A cautious recipe for the estimate is suggested which follows the procedures of linear least squares regression analysis.

Evidence for shape coexistence in the N=Z nucleus 7236Kr36

Abstract Gamma rays associated with the decay of states in the N = Z nucleus 72 Kr have been identified following the 16 O( 58 Ni, 2n) 72 Kr reaction at a mean beam energy of 170 MeV. Identification was made using the Daresbury Recoil Separator. The first excited state was found to be at 709.1 ± 0.3 keV and to be populated with a cross section of 60 ± 25 υ b. The pattern of gamma rays associated with 72 Kr indicates the co-existence of nuclear shapes.

High-spin states in the mirror nuclei 49Cr and 49Mn

Abstract The γ spectra of the neutron-deficient nuclei 49 Mn and 49 Cr have been observed using the reactions 12 C( 40 Ca, p2n) 49 Mn and 12 C( 40 Ca,2pn) 49 Cr at E (Ca)=160 MeV.Radiation from 49 Mn and 49 Cr was identified by detecting recoil ions in a 0° mass separator. Data from γ-γ, recoil-γ and recoil-γ-γ coincidence spectra and recoil-γ angular correlations allowed the level scheme of 49 25 Mn 24 to be deduced up to 6 MeV (19/2 − ).The levels of the mirror nucleus 49 24 Cr 25 were observed to over 10 MeV (27/2 − ). Relative Coulomb energies for the mirror states were found to have a smooth J dependence. The rotation-like level spacing at low spin is interrupted at J =19/2 − , and this is interpreted as an alignment effect.

The response of an isobutane filled ion chamber to heavy ions

A gridded ion chamber with its anode split to record both total energy and rate of energy loss has been used with isobutane gas to detect ions of seventeen elements (17⩽Z1⩽92) in the velocity range 2<(137v/c)<6. The rate of energy loss and the ionisation collection efficiency were found to be independent of pressure over the range 15–50 Torr. The effective charge of the ion moving in the gas is observed to oscillate with Z1. This variation and its effect on the resolution of the rate of energy loss signal are discussed. The effective charge of the moving ion in the ion chamber window (solid polypropylene) does not oscillate with Z1 but probably depends on the ion charge before entering the window.

GDR decay in hot Hg isotopes. An approach to improve the sensitivity to GDR decay following fusion reactions

Abstract The giant dipole resonance (GDR) decay from the compound nucleus 190Hg∗ at E ∗ ≈ 52 MeV is studied in coincidence with mass separated evaporation residues. It is demonstrated that the selection of decay channels with few-neutron emission, combined with a precise matching of the input excitation energy enhances the experimental sensitivity to the details of the GDR strength function by a factor of ten or more.

Mirror nuclei at high spin in the f72 shell

New results for the Coulomb energy differences in the A=47 mirror nuclei 47Cr and 47V for yrast states up to spin 312ℏ are presented and shown to agree with earlier predictions made within a deformed, crancked shell model approach. The observed total difference in Coulomb displacement energies at low and high angular momenta is also explained within the spherical shell model. The first example of alignment in an N=Z nucleus, 48Cr, is reported.

Proton radioactivity from 146Tm. The completion of a sequence of four odd-odd proton emitters

Abstract Proton emission has been observed from the new isotope 146 Tm. Two transitions have been identified at energies of 1119±5 keV and 1189±5 keV corresponding to Q -values of 1127±5 keV and 1197±5 keV respectively. The measured half-lives of 235±27 ms and 72±23 ms are well reproduced by simple WKB calculations assuming proton emission from h 11 2 orbitals. The proton transitions are assigned to a 10 + isomeric state and a low lying 5 − or 6 − level.