T P Morrison

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.

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.

Experimental investigation of the structure of 124Ce

Gamma-ray transitions have been observed for the first time in 124Ce using the Daresbury Recoil Separator. The excitation energy of the first excited state, 142 keV, implies a deformation epsilon 2 approximately=0.31. This value confirms the trend to higher deformation for the more neutron-deficient cerium isotopes, but it is larger than that predicted by recent calculations.

An on-line computer controlled time digitizer for half-life measurements

Abstract A novel time digitizer has been commissioned to facilitate the measurement of isometric lifetimes or the measurement of the lifetimes of nuclear reaction products from μs upwards. The time digitizer has been used in conjunction with a beam chopper on the Liverpool University EN tandem Van de Graaff accelerator to measure and store the time intervals between the beam off markers and every subsequent γ-ray occuring in the beam off period. The device has been interfaced as a pseudo ADC into the Liverpool on-line computer system such that the beam on and off periods and time dispersion can be set from the on-line display and are then used for spectrum labelling. Exponential decay spectra of 1000 channels or more may be quickly built up on magnetic tape for every γ-ray contained in a Ge(Li) spectrum and the decay of selected γ-rays or sections of the spectrum may be monitored via the on-line display. The system has been tested in the range 20 μs to 20 s against accurately measured isometric half-lives in 38 K, 24 Na and 77 Se. Further measurements have been carried out for several other cases where there has been disagreement in the literature.

A level scheme for 220Ra

The nucleus 220Ra has been populated by the reaction 18O+208Pb at a beam energy of 83 MeV. The positive-parity members of the yrast band have been identified to Jpi =12+ and the negative-parity members from Jpi =5- to Jpi =11-. The alternating positive- and negative-parity levels are strongly coupled by E1 transitions. The sequence of levels observed and the large ratio B(E1)/B(E2) suggest that 220Ra has a stable octupole deformation.

Quasi-band structure in 64Zn

The reactions 61Ni( alpha ,n gamma )64Zn and 56Fe(11B,2pn gamma )64Zn have been used to make spin assignments of Jpi =6+,6+ and 7- to the levels in 64Zn at 3994, 4237 and 4636 keV respectively. Lifetime measurements of 175+or-50 fs and 190+or-60 fs for the two Jpi =6+ levels yield E2 transition strengths of 23+or-5 Wu for the 61+ to 41+ transition and 37-9+17 Wu for the 62+ to 42+ transition. These results indicate the presence of two quasi-bands in 64Zn.

Gamma-ray studies in 64Zn: levels above 3.1 MeV

The 61Ni ( alpha ,n gamma ) 64Zn reaction has been studied 8.6 MeV and 9.35 MeV bombarding energies together with the 56Fe(11B, p2n gamma )64Zn reaction at 30 MeV bombarding energy. The gamma -ray decay scheme of 64Zn has been deduced up to 5.7 MeV excitation by means of gamma - gamma coincidence experiments and n- gamma coincidence experiments. Spins and parities have been deduced for most of the high-spin states between 3.1 and 5.7 MeV excitation by means of gamma -ray angular distribution measurements together with gamma -ray linear polarisation measurements. The lifetimes of levels have been measured by DSAM enabling electromagnetic transition rates to be determined for many transitions. The results have been compared with shell-model, cluster-vibration model and interacting boson approximation calculations.

Lifetime and linear polarisation measurements with the 80Kr(α,nγ)83Sr and 82Kr(α,nγ )85Sr reactions

Mean lifetimes of 83Sr, 84Sr and 85Sr levels have been measured by DSAM. The reactions used were (?,n?) and (?, 2n?) with solid krypton targets. Angular distribution and linear polarisation measurements yielded several multipole mixing ratios and J? assignments. The J? of the 4.9 h isomer in 85Y is shown to be 2/9+.

Experimental upper limit for a γ branch from the 236U shape isomer

Delayed gamma -particle coincidences following the reaction 235U(d,p)236U at Ed=11 MeV were recorded in order to search for a gamma decay of the 236U( tau =165 ns) shape isomer. An experimental upper limit was determined for the branching ratio ranging from Gamma gamma / Gamma f<4.5 at Egamma =0.5 MeV to gamma gamma / Gamma f<1.5 at Egamma =2.5 MeV for a single gamma cascade. This result for 236U appears to be at variance with the observed gamma branching intensity of the 238U shape isomer.

Gamma ray spectroscopy in 64Zn-levels below 3.1 MeV in excitation

States in 64Zn were populated using the 61Ni( alpha ,n gamma )64Zn reaction at a beam energy of 7.8 MeV. Excitation energies, gamma ray branching ratios, angular distributions, linear polarisations and centroid shifts were measured for levels in 64Zn below 3.1 MeV in excitation. Electromagnetic multipole mixing ratios were determined for many of the gamma ray transitions. New spin and parity assignments of Jpi =3+ and 2+ have been made for the levels at 2979 keV and 3005 keV, whilst the spins and parities of the 2737, 2794 and 3078 keV levels are confirmed at 4+, 2+ and 4+ respectively. The spin and parity of the 3093 keV level has been restricted to either 3+ or 2+.