Compton-suppressed Ge Research Articles

Transition strengths and multiple band structure in 82Sr.

High-spin states in $^{82}\mathrm{Sr}$ were studied using the $^{56}\mathrm{Fe}$${(}^{29}$Si,2pn) reaction at 95 MeV with a thick 78 mg/${\mathrm{cm}}^{2}$ natural Fe (92% $^{56}\mathrm{Fe}$) target. Prompt \ensuremath{\gamma}-\ensuremath{\gamma} coincidences were measured using three Compton-suppressed Ge detectors at 90\ifmmode^\circ\else\textdegree\fi{} relative to the beam and two at 30\ifmmode^\circ\else\textdegree\fi{}. Directional correlation of oriented nuclei ratios were determined from the 30\ifmmode^\circ\else\textdegree\fi{}-- 90\ifmmode^\circ\else\textdegree\fi{} coincidences to assist in assigning spin changes. Nine new states were found, along with 38 new transitions. The Doppler-shift attenuation method was used to determine the mean lifetimes of 22 states in $^{82}\mathrm{Sr}$. The continuum side feeding times into 10 states were determined by comparing the decay line shapes in coincidence with transitions above and below the state under study. The average transition quadrupole moment of 2.8(2) e b in the ground state band of $^{82}\mathrm{Sr}$ would correspond to a relatively large axial quadrupole deformation of ${\mathrm{\ensuremath{\beta}}}_{2}$=0.33, in agreement with theoretical predictions. The excited ${2}^{+}$ band has a higher moment of inertia but a lower transition quadrupole moment of 2.4(2) e b. In the negative-parity bands the quadrupole moment falls from about 3 to 2 e b with increasing spin.

Lifetimes of yrast states in 110Cd

Lifetimes or lifetime limits of 20 yrast levels in 110Cd have been measured using the recoil-distance method and the NORDBALL array of Compton-suppressed Ge detectors. From the reduced transition probabilities competing rotational and vibrational behaviour of 110Cd is revealed. Also non-collective states, associated with proton configurations, have been observed.

Band crossing observed in neutron-rich Pd isotopes via spontaneous fission of 252Cf.

High spin excited states in neutron-rich nuclei $^{112,114,116}\mathrm{Pd}$ have been investigated by measuring prompt \ensuremath{\gamma} rays emitted from a $^{252}\mathrm{Cf}$ spontaneous fission source. Two different measurements were performed. First, \ensuremath{\gamma}-\ensuremath{\gamma} coincidence data, necessary to determine the decay schemes, were obtained by using an array of 20 Compton-suppressed Ge detectors. Second, Z identification and the enhancement of a desired neutron channel were carried out using a x-n-\ensuremath{\gamma} multiplicity spectrometer. Ground state bands have been extended to ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{+}$ for $^{112}\mathrm{Pd}$, and ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${12}^{+}$ for $^{114}\mathrm{Pd}$ and $^{116}\mathrm{Pd}$. Band crossings were observed in all three isotopes. It was found from cranked shell model calculations that these backbends are due to the alignment of two ${\mathit{g}}_{9/2}$ protons. The measured energy levels were also compared with predictions of the interacting boson model.

Nuclear structure of odd-oddY82

Excited states in $^{82}\mathrm{Y}$ have been investigated via the $^{58}\mathrm{Ni}$${(}^{28}$Si,3pn${)}^{82}$Y and $^{56}\mathrm{Fe}$${(}^{29}$Si,p2n${)}^{82}$Y reactions using beams provided by the Florida State University Tandem-LINAC with energies of 95 to 135 MeV. Particle-\ensuremath{\gamma} coincidences with a full-sphere charged-particle detector and an excitation curve were used to assign \ensuremath{\gamma} rays to $^{82}\mathrm{Y}$. The $^{56}\mathrm{Fe}$${(}^{29}$Si,p2n${)}^{82}$Y reaction at 95 MeV was used to measure \ensuremath{\gamma}-\ensuremath{\gamma} coincidences with five Compton-suppressed Ge detectors. Lifetimes in the ns and ps ranges were measured using direct timing and the Doppler-shift attenuation method. A level scheme with 39 states was constructed up to tentative spins and parities of (${15}^{\mathrm{\ensuremath{-}}}$) and (${21}^{+}$). In the positive-parity yrast band a phase reversal was observed in the signature splitting at I=10\ensuremath{\Elzxh}. The B(M1) strengths in this band alternate with signature between about 0.1 and 1.0 ${\mathrm{\ensuremath{\mu}}}_{\mathit{N}}^{2}$. The B(E2) strengths of about 90 Weisskopf units are in good agreement with those predicted by Hartree-Fock-Bogolyubov calculations. Particle-rotor calculations indicate a prolate or triaxial core shape for the positive-parity band.

Systematic study of the mixed ground-state and “intruder” bands in 110, 112, 114Cd

The ground-state and the “intruder” bands in 112Cd were investigated by means of neutron capture and 110Pd(α, 2n) reaction spectroscopy. Measurements of γ-rays, angular distributions and γγ-coincidences were performed using Compton-suppressed Ge detectors. Our experimental results, together with published data on 110Cd and 114Cd, were interpreted systematically in the framework of IBA-2 configuration-mixing calculations. The phenomenological model parameters are compared with microscopic predictions based on the OAI mapping procedure.

High-spin structure of 167W and 168W

Excited states in 167W and 168W, populated in the 142 Nd( 30 Si, x n) reaction, were investigated at high spins using the ESSA30 array with 29 Compton-suppressed Ge detectors. The level schemes are extended to higher spins and several new rotational bands are identified. The results are discussed in comparison with existing data on the neighbouring N = 93 and 94 isotones and on the Z = 74W isotopes. A theoretical analysis is presented within the framework of a pairing-self-consistent cranked shell model using shape parameters obtained from total-routhian-surface calculations.

Shape-driving effects in 193Tl from the spectroscopy of yrast and near-yrast states

The nucleus 193Tl has been populated with the 160Gd( 37Cl,4n) reaction and its level scheme has been extended up to spin 41 2 h ̷ by using an array of 12 Compton-suppressed Ge detectors. Near-oblate band structures arising from a 9 − 2 state and markedly different, single-particle-like, structures above a 13 + 2 state have been established. The two structures are consistent with total routhian surface calculations indicating the existence of two minima in the total routhian surface of the nucleus which survive separately up to the observed spins. Details of the rotational-like part are discussed in terms of the cranked shell model, while the single-particle-like features of the nucleus are compared with non-collective structures in neighboring nuclei, in particular 191Hg.

High spin level structure of 63 143 Eu80

The level scheme of143Eu has been extended to I=75/2 in an experiment with the NORDBALL Compton-suppressed Ge detector array and the110Pd(37Cl, 4n) reaction. Most of the scheme shows irregular structure of multiparticle excitations. A strongly populated straight cascade of more than 10 stretched E2 transitions suggests the onset of collectivity.

Quantitative studies of continuum gamma-ray correlations in 170Hf.

We have studied the two-dimensional energy correlation of the continuum \ensuremath{\gamma} rays in $^{170}\mathrm{Hf}$ produced in a heavy-ion fusion reaction. The \ensuremath{\gamma} rays were measured with an array of Compton-suppressed Ge detectors and the Spin Spectrometer. A quantitative analysis has been carried out on the properties of the peaks and dips in the correlation spectra. The results indicate the existence of at least two widths in the \ensuremath{\gamma}-ray energy distribution: a narrow width of about 20 keV, most likely originating from decay along different discrete bands near the yrast line, and a wide width of about 200 keV from mixed states at higher excitation energy. Theoretical calculations including the effect of level mixing and fluctuations in rotational frequency and moment of inertia give results in general agreement with the experiment.

A study of high spin states in the transitional nucleus90Mo

The reaction58Ni(36Ar, 4p)90Mo has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particleδE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation inγγ- and particle-γγ-coincidence mode. An additional Ge detector was placed at 162‡ to the beam direction to provide information on DCO ratios. The level scheme of90Mo has been extended up to an excitation energy of about 12 MeV and probable spin 23ħ. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton 1f 5/2, 2p 3/2, 2p 1/2, and 1g 9/2 orbits and the neutron 2p 1/2, 1g 9/2, 1g 7/2, 2d 5/2, 2d 3/2, and 3s 1/2 orbits with some truncation were made for states above 9ħ and the predicted structure of these states is discussed.

Sidebands in 219Ra.

Excited states in $^{219}\mathrm{Ra}$ were studied by means of the $^{208}\mathrm{Pb}$${(}^{14}$C, 3n) reaction at 65 MeV. The investigation was made with 12 Compton-suppressed Ge detectors and a multicounter array. Transitions in $^{219}\mathrm{Ra}$ were selected by requiring appropriate conditions on fold and sum energy. The proposed level scheme shows two sidebands with alternating parity structure decaying to the ground-state band. It is consistent with a recent description of $^{219}\mathrm{Ra}$ in the framework of the reflection-asymmetric mean-field approach, which predicts a K=1/2 ground-state band and two low-lying bands with quantum numbers K=3/2 and K=5/2.

First observation of high-spin states in 167Ta and 168Ta

Excited states in 167Ta and 168Ta, populated in the 142Nd(30Si, pxn) reaction, were investigated using the ESSA30 array with 29 Compton-suppressed Ge detectors. Several rotational decay sequences are identified and for the first time a high-spin level scheme is established for these nuclei. The results for 167Ta are discussed in comparison with existing data in the neighbouring nuclei and with cranked shell model calculations. The possible intrinsic structure and its consequences for the yrast band in 168Ta are also briefly discussed.

Alignment effects in odd-mass Cs isotopes: Spectroscopy of 125Cs.

Several rotational bands have been populated in the Z=55 nucleus $^{125}\mathrm{Cs}$ using the fusion evaporation reaction $^{110}\mathrm{Pd}$${(}^{19}$F,4n), at a beam energy of 74 MeV. The resultant \ensuremath{\gamma} rays were detected using standard \ensuremath{\gamma}-ray spectroscopic techniques with an array of six Compton-suppressed Ge detectors and a fourteen-element bismuth-germanate ball. The low-lying yrast structure is found to be dominated by the unique parity \ensuremath{\pi}${\mathit{h}}_{11/2}$ orbital, manifest as a decoupled band with a large signature splitting. A crossing occurs in this band at \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$=0.42 MeV, which is interpreted as [\ensuremath{\nu}${\mathit{h}}_{11/2}$${]}^{2}$ pair alignment. In addition, a strongly coupled negative-parity structure with large B(M1)/B(E2) ratios is observed feeding both signatures of the \ensuremath{\pi}${\mathit{h}}_{11/2}$ band. Bands built on the normal-parity \ensuremath{\pi}${\mathit{g}}_{7/2}$ and \ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$ orbitals have also been confirmed and extended. These structures are compared to cranked-shell-model calculations and the systematics of the region.

High spin states in131Ce

Gamma ray spectra from the117Sn(18O,4n)131 Ce reaction have been studied with the NORDBALL array of 15 Compton-suppressed Ge detectors. States up to I=51/2 ħ, E≈8 MeV are populated. Observed bands are interpreted in terms of quasiparticle configurations.

Spectroscopy of195Hg and196Hg

High-spin states in the oblate-deformed nuclei195Hg and196Hg were populated in the198Pt(α, xn) reaction and investigated by in-beamγ-ray spectroscopy using six Compton-suppressed Ge detectors. The level schemes of both nuclei are extended considerably. The level structure is interpreted within the framework of the cranked shell model. Good agreement is obtained between model predictions and experimental results.

In-beam spectroscopy and microscopic calculations in the A = 60−80 region

High spin states in 68Ge and 72Se have been studied via 46Ti(68 MeV 25Mg, 2p) 68Ge and 47Ti(95 MeV 28Si, 2pn) 72Se reactions with the Holifield heavy ion tandem. Gamma-gamma coincidence experiments were carried out in the spin spectrometer and the close geometry Compton-suppressed Ge ball. In 68Ge the three known bands built on 8 + states were found to exhibit crossing transitions, and other new bands were discovered to feed into them. In 72Se only a single positive-parity yrast band is seen to have high spins. Detailed microscopic model calculations have been carried out for 68Ge and 72Se.

Deformation and signature splitting in 74Br.

The high-spin states of $^{74}\mathrm{Br}$ were studied via the $^{48}\mathrm{Ti}$${(}^{32}$S,\ensuremath{\alpha}pn${)}^{74}$Br and the $^{58}\mathrm{Ni}$${(}^{19}$F,2pn${)}^{74}$Br fusion-evaporation reactions at energies of 105 and 62 MeV, respectively, using the Florida State University Tandem LINAC. \ensuremath{\gamma}-\ensuremath{\gamma} coincidence arrays were produced using four Compton-suppressed Ge detectors. Two independent groups of \ensuremath{\gamma} rays were studied--one of positive parity and one of negative parity, both of which appear to be based on an isomeric ${4}^{(+)}$ state. Lifetimes were measured using both the Doppler-shift attenuation and recoil-distance methods. A cranking-model analysis of the bands shows that the kinematic and dynamic moments of inertia for the bands are rather similar and fairly constant for high spins with values of (20--23)${\mathrm{\ensuremath{\Elzxh}}}^{2}$/MeV. A phase reversal occurs in the signature splitting in the yrast band around spin 9 which is related to the quasiparticle alignments. The lifetimes imply an average axial quadrupole deformation of 0.37 in the yrast band and 0.34 in the negative-parity band. These deformations are close to those predicted by Hartree-Fock-Bogolyubov cranking calculations.

Transition strengths and deformation in 77Kr.

High spin states in $^{77}\mathrm{Kr}$ were populated using the $^{48}\mathrm{Ti}$${(}^{32}$S,2pn${)}^{77}$Kr reaction with a 106 MeV $^{32}\mathrm{S}$ beam from the Florida State University Tandem-LINAC accelerator. Three Compton-suppressed Ge detectors at 95\ifmmode^\circ\else\textdegree\fi{} and one at 18\ifmmode^\circ\else\textdegree\fi{} were used to determine \ensuremath{\gamma}-\ensuremath{\gamma} coincidence relations, directional correlation ratios and Doppler-shift attenuation lifetimes. The level scheme for the positive-parity band agrees with previous studies up to spin 37/${2}^{+}$. New levels at 9913 keV (41/${2}^{+}$) and 11 760 keV (45/${2}^{+}$) have been identified. The negative-parity band was extended to a probable spin state of 27/${2}^{\mathrm{\ensuremath{-}}}$, with three new levels identified. The kinematic and dynamic moments of inertia in the positive-parity band converge to a rather constant value of (22--24) ${\mathrm{\ensuremath{\Elzxh}}}^{2}$/MeV. The transition quadrupole moments average about 2.6 e b in this band and decrease somewhat after the \ensuremath{\pi}${\mathit{g}}_{9/2}$ alignment. The magnetic transition strengths alternate with signature and increase to about a nuclear magneton after the alignment. The kinematic moment of inertia in the negative-parity band increases with rotational frequency and exhibits the beginnings of a band crossing at \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approxeq}0.55 MeV, while the transition quadrupole moments decrease steadily with frequency. Cranking model calculations with the Nilsson plus BCS potential were performed for the quasiparticle energies and electromagnetic transition rates for both the positive- and negative-parity bands. The experimental data were reproduced well. As for the positive-parity band, \ensuremath{\beta}(3qp)>\ensuremath{\beta}(1qp) has been shown to be necessary in addition to \ensuremath{\gamma}(3qp)>0 to reproduce the signature dependence of the energies and B(M1) values.

Spectroscopy of 160Hf

Excited states in 160Hf were investigated up to spins I≈20 h ̵ using the OSIRIS array of 12 Compton-suppressed Ge detectors and a calorimeter of 48 BGO detectors. The results are interpreted within the framework of the cranked shell model. The systematics of the band crossing frequencies in the Hf isotopes are discussed. An analysis of the high-spin states in terms of double routhians shows the reduction of pair correlations with increasing rotational frequencies. NUCLEAR REACTIONS 144Sm( 20Ne, xn), E= 105−120 MeV; 144Sm( 19F, xn), E=

Low-energy resonances in 25Mg(p, γ) 26Al, 26Mg(p, γ) 27Al and 27Al(p, γ) 28Si

Gamma-ray decay schemes have been measured with bare and Compton-suppressed Ge detectors at low-energy resonances ( E P < 340 keV) in the (p, γ) reactions on 25Mg, 26Mg and 27Al. Altogether 58 new decay branches have been observed and a new 26Mg(p, γ) 27Al resonance has been found at E P = 154.5 ± 1.0 keV. The new branchings lead to J π ; T determinations (or limitations) for two states in 26Al and four states in 28Si. The absolute strengths of the 25Mg(p, γ) 26Al and 26Mg(p, γ) 27Al resonances have also been obtained, and the uncertainties of the stellar rates, deduced from the available data for both reactions, are significantly reduced. Some astrophysical consequences are discussed.