J.O. Newton

ANGULAR DISTRIBUTIONS OF GAMMA RAYS PRODUCED IN REACTIONS WITH HEAVY IONS.

Abstract Angular distributions of the stretched E2 gamma-ray transitions cascading from rotational or vibrational states, populated by heavy-ion, x n reactions, have been measured. The reactions were induced by 4 He, 11 B, 14 N and 19 F ions on targets in the mass range from 159 to 186. In one case the variation of anisotropy with bombarding energy was measured. Large anisotropies were observed indicating the strong alignments of the decaying states. The problems involved in a theoretical calculation of the alignments are discussed. The implications of the strong angular distributions for nuclear structure measurements with these reactions are considered.

Rotational bands in the light odd-mass Tl nuclei

Abstract The odd-mass nuclei 191–197 Tl have been formed in a variety of (HI, x n) reactions and studied by in-beam spectroscopic techniques. The decay of the high-spin Pb isomers, also formed in (HI, x n) reactions, leading to 193–197 Tl has also been investigated. Evidence is presented for the existence of levels of the previously proposed 9 2 − [505] rotational bands in the nuclei. Tentative evidence is given for the existence of oblate rotational bands, based on the 13 2 + [606] state, in 191–195 Tl.

Energy levels in 183Re from the 181Ta(4He, 2nγ)183Re reaction

Abstract Gamma rays and conversion electrons following the 181 Ta ( 4 He, 2n γ) 183 Re reaction have been studied with Ge(Li) detectors and with an electron spectrometer. Precise energy measurements and measurements on excitation functions, angular distributions and gamma-gamma coincidences have been performed. Evidence is given for rotational members of the 5 2 + [402] ground state band and for the 9 2 − [514] band up to 23 2 + and 21 2 − respectively. Both bands require a term of alternating sign when the energies of the states are expressed as power series in I ( I + 1). The quantity ( g K - g R )/ Q 0 was found to be constant up the bands within the experimental error. It is proposed that some gamma rays originate from the 9 2 − , 13 2 − and 17 2 − states of the 1 2 − [541] band. There is evidence that this band is strongly Coriolis mixed.

Possible oblate shape of 92− isomer in 199Tl

Abstract Low-lying 9 2 − isomeric states have been previously observed in the neutron-deficient oddmass thallium nuclei from mass 201 to 193. But there has as yet been no satisfactory explanation for the existence of such states, as the closest single-particle state is the h 9 2 orbital some 4 MeV higher in energy. We have studied the transitions and states leading to this isomeric level in 199Tl by in-beam spectroscopic techniques. Among the most prominent states observed above the isomeric level are an 11 2 − , 13 2 − , and possibly a 15 2 − state. Very similar levels have been found in the other light odd-mass thallium nuclei. Combining a pairing-energy correction suggested by Blomqvist with the idea that the observed negative parity levels form a rotational band based on the 9 2 − [505] Nilsson state in a nucleus with oblate deformation seems to explain all the information presently available on these levels.

Gamma-ray de-excitation of compound-nucleus-reaction products

Abstract The data currently available on the gamma-ray de-excitation of compound-nucleus-reaction products are summarized. They are shown to be consistent with the de-excitation process outlined by Grover and co-workers, when this is extended to include admixed collective bands. It appears that the presence or absence of an energy gap at a given angular momentum may play a crucial role in the gamma-ray cascade, thereby providing a source of information on nuclear pairing in states with large angular momentum.

COULOMB EXCITATION OF 235U

Abstract The Coulomb excitation of 235 U has been studied using 4 He, 16 O, and 40 Ar projectiles. Rotational bands at 921 keV and 638 keV (tentative) have been characterized as the K 0 + 2 and K 0 −2 γ-vibrational bands, respectively, and a somewhat tentative band at 1053 keV has been assigned as the β-vibrational band based on the ground state. Two bands at 633 and 822 keV are identified as the 5 2 − [752] and 9 2 − [734] Nilsson states, respectively. The large B (E2) value for exciting these bands indicates that they are strongly Coriolis coupled to the ground state band, 7 2 − [743] . A three-parameter Coriolis calculation, involving all eight components of the j 15 2 shell-model orbital, can account adequately for all the rotational energies and transition probabilities observed in these three bands.

Feeding times in (HI, xnγ) reactions

Abstract Feeding times, T f , for population of the quasi-rotational bands in a number of doubly even nuclei formed in ( d HI, xn ) reactions have been measured with the recoil-distance Doppler-shift technique. Values of 11±3, 5±2.5, 3±3 and 12 −4 +2.5 ps were obtained for T f the 166, 168, 170 Hf and 178 Os nuclei populated by the 150,152,154 Sm( 20 Ne,4n) and 154 Sm( 28 Si,4n) reactions, respectively. In all cases the fraction of slow-feeding component was less than a few percent.

Experimental study of Yb nuclei at high angular momentum

Abstract Methods have been developed to study the continuum γ-ray spectrum following heavy-ion compound-nucleus reactions. In the Yb nuclei studied these spectra were found to depend mainly on the input angular momentum, and not on the target-projectile system. The gross features of the spectra are a high-energy tail of ≈ 4 transitions corresponding to the statistical cascade and a lower-energy bump composed of many unresolved collective E2 transitions. Several methods were devised to obtain moment-of-inertia values from the detailed features of this bump for angular momenta as high as 50 ħ.

Evidence for reduced neutron pairing correlations in 165Yb

Abstract Three rotational sequences in 165Yb have been extended to high spins by using the 130Te(40Ar, 5n) and 150Nd(20Ne, 5n) reactions. Evidence is presented for a reduction of the neutron pairing correlations at the highest rotational frequencies (hω > 0.40 MeV), but no quantitative measure of this reduction can be made. There appears to be a conflict with the expectations of simple CSM calculations.

Shell effects at high spin in the γ-continuum from Te evaporation residues☆

Abstract The continuum γ-ray spectra following 110 Pd ( 12 C , x n γ) Te and 82 Se ( 40 Ar , x n γ) Te reactions at several bombarding energies have been measured. As in earlier studies, the spectra show three main components: (i) discrete transitions in the ground band, (ii) a yrast “bump”, and (iii) a high-energy statistical tail. In addition, there appears to be a minimum in the spectrum before the yrast bump. The existence of this minimum and of the two peaks of discrete γ - ray transitions indicate a non-rotational structure for the nuclei at low spin (≦30) before changing at higher spin to the rotational behavior suggested by the bump itself. Theoretical calculations including shell effects do show this type of behavior.