D.G. Burke

The (3He, α) reaction on Yb isotopes with possible observations of indirect excitations

Abstract Angular distributions of ( 3 He, α) reactions have been measured with 28 MeV 3 He on targets of 172 Yb, 174 Yb and 176 Yb. The alpha spectra were analysed with a magnetic spectrograph and levels up to excitation energies of 2 MeV were studied with a resolution (FWHM) of 25 keV. Distorted wave Born approximation (DWBA) calculations were carried out using optical-model parameters which had been found suitable in the lead region. The angular distributions obtained from these calculations were consistent with those measured for strongly populated states and the DWBA results have been used to extract relative spectroscopic factors for a number of levels. The transitions with l ≈ 6 yielded the largest cross sections so a comparison of the present results with earlier (d, t) data resulted in the identification of a number of new high spin states. Tentative assignments of the 5 2 − [523] and 5 2 + [642] Nilsson orbitals have been made in each of the three final nuclei, although in most cases there is evidence for strong Coriolis mixing of the j = 13 2 states. There may also be fragmentation of the singleparticle states, as predicted by Soloviev and his coworkers. In each of the final nuclei studied, a state was observed at the expected position of the j = 17 2 member of the 7 2 + [633] band. The data indicate that this state is not populated by a direct l = 8 neutron transfer. Further deviations from the simple direct reaction process are found in a comparison of (d, t) and ( 3 He, α) cross sections for the 5 2 and 7 2 members of the 5 2 − [512] band in 173 Yb.

The rotational structure of 227Ra

Abstract The level structure of 227Ra has been studied using the (n, γ), (d, p) and ( t , d) reactions and the β− decay of 227Fr. A model-independent level scheme was established including 28 levels below 1500 keV. Cross sections and excitation energies have been measured for 72 levels below 2.5 MeV and analysing powers have been measured for 25 levels below 1.4 MeV. The level structure is interpreted in terms of the Nilsson model. The ground-state configuration is 3 2 + [631↑] and the first excited band [633↓] starts at 1.7 keV. Several octupole vibrations coupled to single-particle configurations are tentatively assigned above 280 keV. The neutron binding energy was determined to be 4561.41 ± 0.27 keV. The half-life of 227Fr was measured to be 148 ± 2 s.

Studies of proton states in 173Lu, 175Lu and 177Lu with (3He, d) and (α, t) reactions

Abstract Proton particle states in the odd lutetium isotopes 173Lu, 175Lu and 177Lu have been studied using the (3He, d) and (α, t) reactions. The beams of 28 MeV 3He and 30 MeV α-particles were provided by the University of Rochester model MP tandem Van de Graaff accelerator. The reaction products were analysed with an Enge-type broad-range magnetic spectrograph and detected with photographic emulsions. The experimental peak widths (FWHM) were 15–20 keV and the spectra were studied up to excitation energies of 2 MeV in each final nucleus. The present results confirm earlier assignments for the low-lying 7 2 + [404] , 5 2 + [402] , 1 2 − [541] and 9 3 + [514] bands. Higher spin rotational members have been observed for several of these previously known bands. A number of new intrinsic states have been found and interpreted in terms of the Nilsson model. These include the 9 2 − [514] orbital in 173Lu and the 1 2 − [541] orbital in 177Lu. The 1 2 − [530] band, and probably the 3 2 − [532] band have been identified at excitation energies above 1 MeV in all three isotopes. Relative spectroscopic factors were extracted with the aid of DWBA calculations. These spectroscopic factors showed evidence for appreciable Coriolis mixing in some cases, particularly between the 1 2 − [541] and 3 2 − [532] orbitals.

Single-proton states in 151Pm

Abstract The 152 Sm ( t , α) 151 Pm reaction was studied using 17 MeV polarized tritons from the tandem Van de Graaff accelerator at the Los Alamos Scientific Laboratory. The α-particles were analyzed using a Q3D magnetic spectrometer and detected with a helical-cathode position-sensitive counter. The overall resolution was ∼ 18 keV FWHM. Measurements of the 150 Nd( 3 He, d) 151 Pm reaction were made using 24 MeV 3 He beams from the McMaster University tandem accelerator. The deuteron spectra were analyzed with a magnetic spectrograph using photographic emulsions for detectors, yielding a resolution of ∼ 13 keV FWHM. By comparing the measured angular distributions of ( t , α ) and ( 3 He, d) cross sections and ( t , α ) analyzing powers with DWBA predictions it was possible to assign spins and parities to many levels. The present results confirm earlier assignments of rotational bands based on the low-lying 5 2 + [413], 5 2 − [532], 3 2 + [411] and 1 2 + [420] orbitals. In addition, states at higher excitation have now been assigned to the 1 2 + [411] and 7 2 + [404] orbitals, and members of the 3 2 + [422], 5 2 + [402], 3 2 − [541] and 7 2 − [523] bands are tentatively proposed. The spectroscopic strengths can be explained reasonably well by the Nilsson model when pairing and Coriolis mixing effects are included.

Proton states in the N = 88 nuclei 149Pm, 151Eu and 153Tb populated in the (τ, d) and (α, t) reactions

Abstract The (τ, d) and (α, t) reaction on targets of 148Nd, 150Sm and 152Gd have been studied, using beams of 24 MeV 3He and 27 MeV 4He from the McMaster University FN tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions. The (α, t) spectra were measured at two angles for each target, and the (τ, d) reactions were studied at 8 or 9 angles. The l-values for a number of low-spin states were determined from the (τ, d) angular distributions, and ratios of the (α, t) and (τ, d) cross sections were used to obtain l-values for several other states. There are some striking similarities in the observed structures of the three final nuclei, 149Pm, 151Eu and 153Tb. In each case there are low-lying strongly populated 11 2 − states and a higher lying l = 5 level somewhat below 1 MeV of excitation energy. Several states (10 in 149Pm, 17 in 151Eu and 8 in 153Tb) appear to be populated via l = 2 transitions, and there are strongly excited 1 2 + levels at ≧ 1 MeV of excitation energy in each case. Of particular interest is a 7 2 − state located ≦ 50 keV above the lowest 11 2 − state in each nuclide. The relatively strong populations of these 7 2 − levels in the present experiments are contrary to expectations based on the simple shell model as there are no f 7 2 states in the 50

A study of levels in 153Eu and 155Eu by the (3He, d) reaction

Abstract The ( 3 He, d) reaction has been used with targets of 152 Sm and 154 Sm to study levels in 153 Eu and 155 Eu. The 28 MeV 3 He beam was obtained from the University of Rochester model MP tandem Van de Graaff accelerator and the deuteron reaction products were analysed with a broad range magnetic spectrograph. The rotational bands based on the low-lying 5 2 + [413] , 5 2 − [532] , and 3 2 + [411] states had been previously assigned in both nuclei. The present results confirmed these assignments and, in addition, located some of the higher spin members for these bands. The previous assignments of the 1 2 + [411] orbital at 0.636 MeV in 153 Eu and 0.768 MeV in 155 Eu have been proven conclusively to be false.

Studies of 158, 160, 162, 164, 166Dy levels with the (t, p) reaction

Abstract Angular distributions of the 156, 158, 160, 162, 164 Dy(t, p) reactions have been measured using 17 MeV tritons from the McMaster University Tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions, resulting in overall peak widths of 15–20 keV (FWHM). Levels populated with L =0 transitions were identified from the unambiguous angular distributions, and at least one previously-unknown I π = 0 + state was found in each nuclide studied. New I π =0 + levels were found at energies of 1269, 1549, 1743, and 2000 keV in 158 Dy, 1457 and 1709 keV in 160 Dy, 2126 keV in 162 Dy, 1774 keV in 164 Dy, and 1149 keV in 166 Dy. Also, the previously proposed 0 + assignment for the 1655.4 keV level in 164 Dy has been confirmed. For the neutron-rich nuclide 166 Dy there was previously no information on excited states listed in the Nuclear Data Sheets, and many levels have been located in the present study, including the gamma band and an excited K π =0 + band. The 1457 keV 0 + state in 160 Dy is a possible candidate for the bandhead of the previously reported S-band. The fraction of the L =0 strength which feeds excited states is unsually high in several cases, and particularly for 164 Dy, in which the total L =0 strength to excited levels is ∼35% of that for the ground state. This can be explained qualitatively in terms of a “sub-shell closure” corresponding to the gap in the Nilsson diagram at N =98, and supports the earlier explanation of large L =0 strengths to excited states in the 161 Dy(t, p) 163 Dy reaction as being due to this gap. Most of the L π =0 + excited states observed in previous (p, t) studies were not populated in the present work, thus ruling out a pairing vibrational interpretation for these levels.

Empirical evidence for coriolis coupling of octupole states in 172Yb

The nucleus 172Yb has been studied using the (3He, α) and (d, t) reactions on targets of 173Yb. Beams of 12 MeV deuterons and 24 MeV 3He particles were obtained from the McMaster FN Tandem Van de Graaff accelerator and the reaction products were analysed with a magnetic spectrograph. The (3He, α) reaction was particularly useful in identifying states formed by coupling the 72+ [633] transferred neutron to the 52− [512] target ground state. The Kπ = 1− coupling of these orbitals makes up a large fraction of the Kπ = 1− octupole vibration, which was found to be strongly perturbed by Coriolis coupling with other octupole states. The most striking feature is that Coriolis coupling with the Kπ = 0− octupole band, which has only odd-spin rotational members, produces a significant odd-even shift in the rotational spacing of the Kπ = 1− band. These results are in excellent agreement with the predictions of Neerg.ard and Vogel. The observation of the Kπ = 6∼, 72+[633] + 52−[512] rotational band up to spin I = 9 confirmed the previous assignment for the bandhead of this configuration to the isomeric state at 1551 keV. The Kπ = 3+ and Kπ = 8+, case112− [505] ± case52−[512], two-neutron configurations have also been identified in the present work. In addition, several other new assignments have been tentatively proposed.

The (t, p) reaction across the shape transitional Gd nuclei

Abstract The 152 Gd(t, p) 154 Gd reaction has been studied using 15 MeV tritons from the McMaster University tandem accelerator facility. The reaction products were analyzed with a magnetic spectrograph. Distinctive L = 0 angular distributions indicated that four 0 + states were populated, the first three with comparable strengths and the fourth with a cross section about an order of magnitude smaller. The 2 + 3 state at 1418 keV, previously interpreted as the 2 + member of the two-phonon beta-vibrational band, has been populated as strongly as the 2 + member of the ground-state band. The 0 + 3 and 2 + 3 states in 154 Gd are analogous to the previously observed “spherical” states coexisting in the “deformed” 152 Sm isotone.

Two-quasineutron states in 168Er populated by the 167Er(d, p) and 167Er(t, d) reactions

Abstract The nuclear structure of 168 Er has been studied with the 167 Er(d, p) 168 Er and 167 Er(t, d) 168 Er reactions, using 12 MeV deuterons and 15 MeV tritons from the McMaster tandem Van de Graaff accelerator. The reaction products were analyzed with an Enge split-pole magnetic spectrograph and detected with photographic emulsions. Angular distributions were obtained for levels up to ∼2.5 MeV excitation, with typical resolutions of ∼9 keV and ∼11 keV (FHWM) for the (d, p) and (t, d) reactions, respectively. Since the I, K π values for all levels up to ∼2 MeV were known from previous ( n, γ ) studies, the main contribution of this study was to determine the admixtures of specific two-quasineutron configurations to the various bands. Earlier {7/2 + [633]±1/2 − [521]} assignments for the 1094.0, K π = 4 − and 1541.5 keV, K π = 3 − bands have been confirmed, although the full {7/2 + [633]+1/2 − [521]} strength is not observed in the 1094.0 keV band. The K π = 1 − octupole band at 1358.8 keV has a dominant {7/2 + [633] − 5/2 − [512]} component, as predicted by the Soloviev model. The K π = 6 − , {7/2 + [633] + 5/2 − [512]} configuration has been assigned in this work to the 1773.2 keV level. The K π = 4 − band at 2059.9 keV is found to contain ∼35% of the {7/2 + [633] + 1/2 − [510]} strength, and a ∼25% admixture of the {7/2 + [633] − 1/2 − [510]} configuration is tentatively assigned in the K π = 3 − band at 1828.0 keV. Relative cross sections for members of the 168 Er ground state band suggest the presence of a mixed wave function for the 167 Er target ground state. The K π = 0 + , 1217.1 keV band exhausts the {7/2 + [633] − 7/2 + [633]} strength which does not go to the ground band, while the K π = 0 + , 1422.0 keV band was not observed. The Soloviev model is the only one which has made quantitative predictions that can be compared with the present results. In general, it is quite successful in explaining the experimental data.