A N James

A 385 mev measurement of (p,2p) cross-sections in medium-weight and heavy nuclei

Abstract Measurements of differential cross sections for (p, 2p) reaction at an incident proton energy of 385 MeV are interpreted in terms of a distorted momentum distribution for the struck proton in the original target nucleus. The target nuclei studied were 40 Ca, 45 Sc, 59 Co, 58 Ni, 120 Sn, 208 Pb and 209 Bi. In all but the heaviest nuclei, the distorted momentum distribution varies with binding energy of the proton in such a way as to invite a simple shell-model interpretation. This simple interpretation indicates that the binding energy of the Is protons does not increase with mass number after it has reached a value near 55 MeV.

Quasi-elastic proton-alpha collisions in carbon

Abstract The possibility of quasi-elastic p-α collisions in carbon nuclei has been investigated experimentally by looking for correlated proton-alpha coincidences when C12 is bombarded by 150 MeV protons. A peak was found in the angular correlation centred about the kinematic angle for free p-α collisions and with a width, which when interpreted as arising from the α-cluster momentum before impact, gives momenta of about the same size as are observed in high-energy (p, 2p) reactions. Under the circumstances of the experiment the probability of finding an α-cluster in the C12 nucleus was found to be 0.30−0.11+0.23 using a plane wave impulse approximation to integrate over the angular correlation. This figure is considered to be large since no allowance has been made for the strong nuclear absorption effects expected. The detailed results pose certain problems for the α-cluster model of the C12 ground state.

A measurement of the 12C(p, 2p) reaction at 385 MeV with good momentum resolution☆

Abstract A method of measuring absolute values of (p, 2p) reaction cross sections at 385 MeV has been developed which uses wide range magnetic spectrometers. The measurements made on 12 C(p, 2p) provide new evidence supporting the interpretation of the cross section in terms of a spectator model. This interpretation gives an estimate of the momentum distribution of the protons bound in 12 C and the results show that the effect of distortion of the ingoing and outgoing proton waves is to reduce the magnitude measured for the momentum distribution but not to change its shape. We find the magnitude of the peak of the p state distorted momentum distribution to be 0.27±0.03 fm 3 , a result not in agreement with previous measurements. The resolution in momentum space is sufficient to resolve the minimum of the p state distorted momentum distribution and we find that there is no conflict with the theoretical predictions of the depth of this minimum.

Lifetimes and decays of energy levels in 30Si and 30P

Abstract The mean lifetimes of 15 levels in 30 Si and of 7 levels in 30 P have been measured by the Doppler-shift attenuation method. Limits on the lifetimes of a further 8 levels in 30 Si and of the 3018 keV level in 30 P have been obtained. Branching ratios and excitation energies have been measured in both nuclei. The levels were populated by the reactions 27 Al(α, p) 30 Si and 27 Al(α, n) 30 P at α-particle bombarding energies of 5.0, 6.3 and 8.0 MeV. Theγ-rays were detected with a Ge(Li)-NaI(Tl) escape suppressed and pair escape spectrometer.

Decay modes and lifetimes of levels in 39K below 7.3 MeV excitation

Abstract Levels in 39K have been populated using the (p, pγ) reaction and the decays of levels below 7.3 MeV have been established by a particle-gamma coincidence experiment. The mean lifetimes of 24 levels have been determined in a singles experiment using the Doppler shift attenuation method. Limits on the lifetimes of a further 10 levels in 39K have been obtained. Discussion of the properties of levels in 39K is given.

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.

Lifetimes and decay modes of energy levels in 34S

Abstract The excitation energies, branching ratios, and mean lifetimes of the low-lying levels in 34 S were measured. The lifetimes were measured by the Doppler shift attenuation method using the 31 P(α, p) 34 S reaction at a bombarding energy of 7.0 MeV. The γ-rays were detected at six angles with a Ge(Li)-NaI(Tl) anti-Compton and pair spectrometer. The mean lifetimes, in fs, were found to be: 2127 keV, 440 ± 50; 3303 keV, 218±30; 3915 keV, 1600±130; 4075 keV,

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.

Gamma-ray spectroscopy experiments for levels in 39K below 8.4 MeV excitation

The gamma-ray spectroscopy of sixty excited states in 39K below an excitation energy of 8.4 MeV has been studied. The data were obtained mainly from the 36Ar+ alpha reaction at beam energies between 7.5 and 16.0 MeV, the remaining data coming from the 24Mg+18O and 28Si+16O reactions, both at beam energies of 42 MeV. 13 new levels were established in 39K and several previously unobserved branches were revealed between the previously known levels. Spins, parities, mixing ratios and gamma-ray branching ratios were determined from angular distribution and linear polarisation measurements using the 36Ar+ alpha reaction at several different beam energies. Mean lifetimes were deduced using both the Doppler-shift attenuation method (with the 36Ar+ alpha reaction) and the recoil-distance method (with the 28Si+16O reaction). The results have allowed transition strengths to be calculated for the decays between many of the levels. The results are discussed in terms of both the shell model and the simple weak coupling model.

Lifetime measurements in 40K

The Doppler-shift attenuation method has been used to measure the lifetimes of 19 levels in 40K and four levels in 40Ar. Limits have also been set on the lifetimes of other levels in 40K. The measurements were made using the 37Cl(α, n)40K and 37Cl(α, p)40Ar reactions. Enriched targets of lead chloride and barium chloride were used. The lifetimes obtained with different stopping materials and different recoil velocities were compared where possible and were found to be consistent. A γγ coincidence experiment was performed to elucidate the decay schemes in 40K and 40Ar. The negative-parity states of 40K have γ-ray decay schemes consistent with simple shell-model calculations for d32−1f72 and d32−1p32 configurations. More information is needed before the 2p-2h and 4p-4h positive-parity states can be classified.