Mja Devoigt

High-spin neutron excitations in neutron-deficient even-mass Pb isotopes

Abstract High-spin states in 194, 196 pb have been investigated using the 188 Os( 12 C, χn) 194, 196 Pb and the 198 Hg(α, 6n) 196 Pb reactions. The experiments included γ-ray excitation function, γ-γ coincidence, γ-∑γ coincidence, γ-ray angular distribution, lifetime and conversion electron measurements. The level schemes of 194 Pb and 196 Pb have been extended up to states with J π = 15 − ( E = 3955 keV) and J π = 20 + ( E = 5444 keV), respectively. All levels except two new isomers which have most likely spin and parity J π = 10 − ( 194 Pb ) and J π = 11 − ( 196 Pb ) can be interpreted as neutron excitations. The excitation energies of levels with spin J ⩽ 12 are well accounted for by the one-broken-pair model including 1p1h excitations. The B (E2, 12 + −10 + ) values reflect the systematic trend for a pure ( i 13 2 ) 2 configuration. Levels with spin 12 J i 13 2 neutron holes are coupled to core excitations in the A + 2 nucleus and also by calculations performed within the two-broken-pair model. Within this framework the J π = 20 + state observed in 196 Pb has a pure ( i 13 2 ) 4 configuration. The two new isomers have most likely a proton 2p2h structure. The excitation energy of the J π = 11 − isomer in 196 Pb agrees with that predicted for a deformed 1 2 + [404] − 29 2 − [514] 13 2 + [606] 11 − state.

PROTON PARTICLE-HOLE STATES AND COLLECTIVE EXCITATIONS IN PB-196

Abstract A collective band built on the Jπ = 0+ intruder state in 196Pb has been identified by measuring coincidences between γ-rays following the 198Hg(α,6n)196Pb reactions and conversion electrons corresponding to the E002+ → 01+ transition. The band-structure is disturbed by a Jπ = 8+ isomer (T 1 2 = 50 (15) ns ) . This newly found isomer and the Jπ = 11− isomer observed previously in this nucleus are interpreted as proton 2p-2h states too. Evidence for the proton 2p-2h nature of the Jπ = 11− isomer is obtained from the measured g-factor g = 0.96 (8).

Rotational bands in 181Pt

Abstract The 144 Nd( 40 Ar, 3n) 181 Pt reaction has been used to study rotational bands by means of an extensive set of gamma spectroscopic measurements. The bands 1 2 − [521], 5 2 − [512], 7 2 − [514] a 9 2 + [624] are identified up to maximum spin of ( 53 2 ). A sharp bandcrossing is observed at a rotational energy of h ω = 0.28 MeV in the 7 2 − [514] bands. The crossings are discussed in terms of possible v i 13 2 and πh 9 2 alignments. Exceptionally strong M1 transitions are observed above the crossing.

K-SHELL IONIZATION OF DY ATOMS PRODUCED IN (ALPHA,CHI-N) AND (C-12,CHI-N) NUCLEAR-REACTIONS

Abstract The K-shell ionization cross sections, σ K Z , are determined for Dy atoms produced in bombardments of several Gd targets with α-particles in the energy range 47–130 MeV and of 148, 150 Nd targets with 12 C ions at 65 and 90 MeV. The determination is based on the measured intensity ratios of the characteristic K X-rays of the product atoms to those of the target elements. Analysis of the simultaneously measured discrete in-beam γ-ray spectra allows one to determine ionization cross sections for Dy product atoms, σ Z K ( der ), derived as a sum of the products of the γ-ray intensities times the corresponding K-shell internal conversion coefficients. The measured and the total derived values are found to be identical to within the experimental errors. The partial ionization yields per reaction are analyzed as functions of the average angular momentum left in each exit channel. Strong dependence of these yields on the nuclear structure is found. Moreover, the variation of the yields with the angular momentum is also strongly structure dependent. Properties of the quasicontinuum γ-radiation deexciting the reaction residues are considered. Upper intensity limits are determined for the low-energy magnetic dipole components in this radiation.

Band-crossing Phenomena in Hf-167,Hf-168 and W-170,W-171

Abstract High-spin states in 167,168Hf and 170, 171W have been excited by the reactions 159 Tb ( 14 N , x n ) 173 − x Hf and 155 Gd ( 20 Ne , x n ) 175 − x W . The yrast bands have been observed up to J π = 49 2 + in 167Hf, Jπ = 28+ in 168Hf, J π = 45 2 + in 171W and Jπ = 22+ in 170W. Both even-even nuclei display a strong backbend around Jπ = 14+, whereas 167Hf shows an upbend at ω ∼- 0.33 MeV and 171W exhibits a progressive gain in aligned angular momentum above ω ∼- 0.26 MeV. At even higher rotational frequencies, bridges have been found in the central valleys of the γ-γ correlation matrices at ω ∼- 0.42 and 0.52 MeV for the Hf isotopes and ω ∼- 0.45 and 0.47 MeV for the W isotopes. Deduced moments of inertia for the Hf isotopes using the correlation data show a smooth increase up to about the rigid-sphere value at ω ∼- 0.5 MeV. The data are satisfactorily accounted for by cranked shell-model calculations. In particular, a qualitative interpretation is given for the experimentally observed systematic difference in the strength of the interaction potential for the N = 95 and N = 97 isotopes of Hf and W.

BACKBENDING IN HF-167 AND HF-168 AND THE BAND-CROSSING PICTURE

Abstract Using two Compton-suppression spectrometers in an Eγ−Eγ coincidence experiment, the yrast bands in 167,168Hf were extended up to 49 2 and 28 ħ, respectively. The i 13 2 positive parity band in 167Hf experiences backbending at a higher frequency than the first backbending in 168Hf, and no second backbending is obseerved in 168Hf. New information is threby obtained on the nature and interaction strength of the crossing bands in the vicinity of N = 96.

STUDY OF QUASI-CONTINUUM GAMMA-RAY DEEXCITATION OF DY-152-161 NUCLEI POPULATED IN ALPHA-INDUCED AND C-12-INDUCED REACTIONS

Abstract The influence of the reaction mechanism and of the nuclear structure at high angular momentum on the γ-ray deexcitation process is studied for Dy nuclei produced in a (40–80 MeV) and 12 C (80–110 MeV) induced reactions on 154, 155, 158, 160 Gd and 146, 150 Nd targets, respectively. The quasi-continuum (q.c.) γ-ray spectra for selected exit channels are decomposed into collective and statistical parts for which the γ-ray energies, the multiplicities and partly the multi-polarities are determined. Large differences in the constitution of the q.c. γ-ray cascades and in the population strengths of high-spin yrast states in 152 Dy and 156 Dy are attributed to nuclear structure effects at high angular momentum. This constitution and the centroids of the γ-ray entry regions in the energy versus angular-momentum plane are also evaluated as functions of beam energy and projectile. The γ-ray multiplicities and both coordinates of the entry region centroids increase initially with bombarding energy and then level off or even decrease at the highest energies. The observed increase is reproduced by a statistical-decay model, but at the higher bombarding energies strong deviations occur which are discussed in view of the reaction mechanisms, including incomplete fusion and non-equilibrium neutron emission, particularly absorptive breakup.

Band structures in 98Ru and 99Ru

Abstract The level schemes of 98, 99Ru were studied with the reactions 98Mo(α, 3nγ) and 98Mo(α, 4nγ) at Eα = 35 to 55 MeV, using a large variety of in-beam γ-ray detection techniques and conversion-electron measurements. A search for the 3− state was carried out with the reaction 98Ru(p, p′). The ground-state band of 98Ru was excited up to Jπ = (12)+ and a negative-parity band up to (15)−. New levels in 98Ru were found at Ex = 2285 (Jπ = 4+), 2435 (Jπ = (3−, 4+)), 2671, 3540, 4224, 4847, 4915 (Jπ = (12)+), 4989 (Jπ = (12+)), 5521 (Jπ = (13)−), 5889, 6591 (Jπ = (15)−), and 7621 keV. New unambiguous spin and parity assignments were made for the levels at Ex = 2014 and 3852 keV, as Jπ = 3+ and 9−, respectively. New levels in 99Ru were found at Ex = 1976, 2021 ( J π = ( 15 2 + ) ), 2393, 2401 ( J π = ( 17 2 + ) ), 2875 (π = (+)), 3037, 3201 ( J π = ( 23 2 ) − ), 3460 ( J = ( 17 2 ) ), 3484 ( J π = ( 21 2 + ) ), 3985, 4224 ( J π = ( 27 2 − ) ), and 5359 keV. The 1070 keV, J π = 11 2 − level in 99Ru has a half-life of 2.8 ns. A strongly excited negative-parity band is built on this level. A positive-parity band based on the ground state was excited up to J π = ( 21 2 + ) . The level schemes are well reproduced by the interacting boson model in the vibrational limit.

Energy and angular momentum transfers in equilibrium and pre-equilibrium 158Gd(α, xn) reactions

Neutron time-of-flight spectra and γ-ray multiplicities were measured in coincidence with discrete γ-rays for specific exit channels in the 158Gd(α, xn) reactions at Eα = 70 MeV. The neutron energy spectra and angular distributions were analysed in terms of equilibrium and non-equilibrium decay processes. Significant emission of non-equilibrated neutrons was found, amounting to about 40 %, 25 % and 15 % of the total neutron emission in the 4n, 5n and 6n exit channels, respectively. The corresponding average kinetic energies in this precompound phase were around 12, 8 and 4.5 MeV. The angular momentum carried away by the neutrons was found to be rather constant for all exit channels and on the average about 3ħ units. It is found that the total energy and angular momentum removed by the neutrons and γ-rays agrees within the experimental errors with the calculated values for the compound nucleus.