B. Elbek

Coulomb excitation of vibrational states in deformed even nuclei

Abstract Low-lying states in deformed even rare-earth nuclei have been studied by Coulomb excitation with 43.5 MeV oxygen ions. Gamma rays from thin oxide targets (≈0.3 mg/cm 2 ) or thin metal targets (0.5–1.2 mg/cm 2 ) were observed in coincidence with back-scattered ions. The yields were normalized on the intensity of the scattered particles. Double groups of gamma rays ascribed to the 2 +′ → 0 + and the 2 +′ → 2 + transitions from K = 2 states were observed in Nd 150 (1.07 MeV), Sm 152 (1.09 MeV), Sm 154 (1.44 MeV), Gd 156 , Gd 158 , Gd 160 (1.02 MeV), Dy 160 , Dy 162 (0.89 MeV), Dy 164 (0.77 MeV), Er 164 (0.86 MeV), Er 166 , Er 168 , Er 170 (0.93 MeV) and Yb 176 (1.27 MeV), where the energies in the parentheses are given for 2 +′ states not observed earlier. Gamma rays ascribed to the 0 +″ → 2 + , 2 +″ → 0 + and 2 +″ → 2 + transitions from K = 0 states were observed in Nd 150 (0.69 MeV), Sm 152 , and Gd 154 (0.68 MeV), where the energies are for the 0 +″ states. Gamma rays in Sm 154 and Gd 156 suggest K = 0 states at 1.10 and 1.06 MeV, respectively. The B (E2 values determined from the yields range from 2 to 8 single-particle units, being roughly inversely proportional to the excitation energies for the levels. The theory of multiple Coulomb excitation for the different bands was in qualitative agreement with the experimental observations.

Inelastic scattering from even rare earth isotopes

Abstract The Coulomb excitation cross sections for the first excited state of 15 even-mass nuclei in the region of the rare earths have been determined from measurements on inelastically scattered protons and deuterons. From the cross sections, reduced E2-transition probabilities are obtained, in general in good agreement with those derived from the directly measured lifetimes of the nuclear states. The intrinsic quadrupole moments determined from the transition probabilities show, for each element, a regular variation with the mass number, but discontinuities occur when passing from one element to another. This behaviour appears to be correlated with the variation in the moment of inertia for the same nuclei. The individual intrinsic quadrupole moments of odd-mass nuclei in the same region often deviate from the moments of the neighbouring even isotopes, but on the average the quadrupole moments of odd-mass nuclei are almost the same as those for even nuclei.

Inelastic scattering from odd-mass rare earths

Abstract The reduced electric quadrupole transition probabilities and excitation energies of the accessible low lying states in Eu151, Eu153, Tb159, Ho165, Er167 and Tm169 have been measured by Coulomb excitation with protons or deuterons having an initial energy of about 4.5 MeV. The measurements were performed on the groups of inelastically scattered particles by means of a magnetic spectrograph, a method particularly suited for the determination of the absolute transition probability. The results are discussed in terms of the rotational model.

Collective vibrational states in even Gd nuclei

Abstract The energy levels of the even nuclei of gadolinium have been investigated by inelastic scattering of 12 MeV deuterons. The deuteron spectra were recorded in a magnetic spectograph with an overall energy resolution of ≈ 8 keV. On the basis of semi-empirical rules governing the intensities and angular distributions of the inelastics groups, collective octuple and quadrupole pole vibrations were localized. As a rule, two or three octupole state wereidentified in each nucleus below an excitation energy of approximately 2.5 MeV. In the same region, the collective γ-vibrations and, in some cases, β-vibrations have been identified. Only few other groups were present.

Population of rotational states by proton induced reactions

Abstract Spectra of conversion electrons from targets of Tb 159 , Ho 165 , Tm 169 and Lu 175 bombarded with 8–12 MeV protons were studied by means of a magnetic spectrometer. At the lower bombarding energies mostly electron lines from rotational states populated by (p, n) reactions were seen. On the basis of the electron lines from proton bombardment of Ho 165 a partial level scheme for Er 165 was obtained. At higher bombarding energies the (p, 2n) reaction dominates and leads to states in even nuclei. Ground-state rotational bands in Dy 158 , Er 164 , Yb 168 and Hf 174 were populated up to the 8 + state, in Er 164 , possibly the 10 + state was seen. The observed intensities of the rotational transitions can be fairly well accounted for by a simple statistical calculation.

VIBRATIONAL STATES OF DEFORMED NUCLEI POPULATED BY THE (p,2n) REACTION

Abstract Spectra of prompt internal conversion electrons with energies up to 2 MeV were obtained from targets of 159Tb, 165Ho, 169Tm, 175Lu and 181Ta bombarded with 12 MeV protons. Gamma-electron coincidence spectra were also measured. Most of the observed lines in the region around 1 MeV have been ascribed to decays from vibrational bands populated by the (p, 2n) reaction. Quadrupole vibrations with K = 2 have been found in 158Dy (945 keV), 164Er (857 keV) and 168Yb (986 keV). Bands with K = 0 were found in 158Dy (991 keV), 164Er (1238 keV), 168Yb (1156, 1197 and 1543 keV), 174Hf (827 keV) and 180W (908 keV). The K = 0 assignment was made on the basis of an E0 component in the decay. The rotational bands were populated up to levels with spin six or more. The spacing of energy levels in the vibrational bands indicates an increase in the moment of inertia as a function of the spin similar to that of the ground-state bands. In the gamma bands there is also a small relative displacement of even and odd spin levels. Mising of the excited and ground-state bands has been calculated from the E2 branching ratios, but accounts for only a small fraction of the energy depression in the ground state rotational bands.

INELASTIC SCATTERING FROM HAFNIUM AND WOLFRAM ISOTOPES

Abstract The reduced electric quadrupole transition probabilities and the excitation energies of the accessible low lying states in the stable isotopes of Hf and W (except the rare isotope W180) have been measured by Coulomb excitation. The measurements were performed on inelastically scattered protons and deuterons recorded in a heavy particle spectrograph. The results are discussed in terms of the rotational model. The measured transition probabilities are compared with those derived from lifetime measurements.

Collective vibrational states in even dysprosium nuclei

Abstract The energy levels of the even dysprosium nuclei have been investigated by inelastic scattering of 12 MeV deutrons. A magnetic spectrograph was used to record the deutron spectra with an over-all energy resolution of ≈ 8 KeV. Below an excitation energy of approximately 2.5 MeV, collective octupole states, collective γ-vibrations and in some cases β-vibrations have been identified. The identifications were based on semi-empirical rules governing intensities and angular distributions of the inelastic groups. For each nucleus, most of the octupole strenght was found in a single state at approximately 1300 keV excitation. There is some evidence for this state to have K π = 2 − .

Collective vibrational states in even erbium nuclei

Abstract The cross sections for inelastic scattering of 12 MeV deuterons from all the even Er isotopes have been measured at 90° and 125° by means of a magnetic spectrograph. The 2 +′ and 4 +′ states in the gamma vibrational bands were observed in all nuclei, but only in a few cases the K = 0 β-vibrations were observed. Several octupole vibrational states were identified in each nucleus below an excitation energy of approximately 2.3 MeV. The reduced transition probabilities B (E2) and B (E3) were determined from the observed inelastic cross sections.

ROTATIONAL LEVELS POPULATED BY INELASTIC DEUTERON SCATTERING.

Abstract The cross sections for inelastic deuteron scattering to rotational states of doubly-even nuclei in the rare-earth region have been measured at a deuteron energy of 12 MeV and a scattering angle of 125°. The population of the 4 + states is considerably less regular than expected from the observed population of the 2 + states. This is taken as an indication for a direct E4 transition to these states. The estimated maximum hexadecapole deformation parameter β 4 is about 0.09.