Search

High-spin states in 197Bi were studied with the AFRODITE γ-ray array at iThemba LABS using the 181Ta( 22Ne, 6n) reaction at a beam energy of 125 MeV. A new shears band was found and linked to the low-lying states in 197Bi. Its dynamic moment of inertia, \( \Im^{{(2)}}_{}\), is considerably larger than the \( \Im^{{(2)}}_{}\) of the shears bands in the neighbouring Pb isotopes. This is probably a result of the involvement of an additional high-Kh9/2 proton orbital.

The 106, 108Rh and 110, 112Ag nuclei have been produced as fission fragments following the fusion reaction 28Si + 176Yb at 145 MeV bombarding energy and studied with the Eurogam2 array. The yrast high-spin states of these four odd-odd nuclei, which are observed for the first time, consist of rotational bands in which the odd proton occupies the πg 9/2 subshell and the odd neutron the νh 11/2 subshell. Their behaviour as a function of spin values does not vary with the number of neutrons: as observed in the odd-N neighbouring nuclei, the motion of the odd neutron remains decoupled from the motion of the core, from N = 61 to N = 65. Moreover, the staggering observed in the yrast bands of odd-odd isotopes is strongly reduced as compared to the large values displayed by the rotational bands built on the πg 9/2 subshell in the odd-A Rh and Ag isotopes. The results of particle-rotor calculations indicate that this reduction is related to a change of the core deformation.

The 104,106,108Ru nuclei have been produced as fission fragments following the fusion reaction 28Si + 176Yb at 145 MeV bombarding energy; prompt gamma rays emitted in the reaction were detected using the Eurogam II array. Their high-spin level schemes have been obtained, particularly several side bands have been extended or newly observed. Experimental results are discussed in the framework of both collective model and the rotating mean-field approach in which the Gogny force is the sole input. These calculations provide very good descriptions of both low-spin and high-spin structures observed in the neutron-rich even-A Ru isotopes.

A brief introductory survey of gamma-ray detector arrays and in-beam electron spectrometers developed during the last three lustra is followed by a broad discussion of the general requirements for single and multiple in-beam e-gamma spectrometers. A detailed analysis is made of a few important tools for reducing the low-energy electron background (i.e. simultaneous E-Bp selection, true and random coincidence rates, EM static fields around the target region). This survey and discussion are illustrated by the results of several in-beam e-gamma measurements dealing with nuclear spectroscogic investigations of a few medium and heavy nuclei. (C) 1999 Elsevier Science B.V. All rights reserved.

Three superdeformed bands have been observed in ${}^{191}$Au using the EUROGAM II multidetector array. The previously known yrast superdeformed band has been extended up to very high rotational frequencies and two new excited bands have been assigned to this nucleus. These bands behave as signature partners and display characteristics which suggest they could be based either on the $\ensuremath{\pi}$ [532]3/2 orbital alone or on that orbital plus the $\ensuremath{\pi}$ [530]1/2 orbital, which become the $[\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{431}]$3/2,1/2 doublet in the pseudospin formalism.

The decay-out spectrum of the superdeformed yrast band in {sup 192}Hg comprises a quasicontinuum, from which about 50 weak discrete transitions of energy between 1 and 3.2MeV are resolved. The fluctuations of the one-dimensional quasicontinuum spectrum are studied with the fluctuation analysis method, which shows that of the order of a few thousand different transitions are available in the first step of the decay-out cascades. The experimental effective number of transitions is compared to schematic theoretical calculations. {copyright} {ital 1996 The American Physical Society.}

To describe magnetic resonance (MR) imaging findings in acute infectious cellulitis and assess its value for the diagnosis of severe necrotizing forms.Spin-echo (SE) T1- and T2-weighted imaging was performed in 36 patients with acute infectious cellulitis. T1-weighted SE images obtained after injection of a paramagnetic contrast agent were also obtained when an abscess was suspected on precontrast images. Sixteen patients underwent surgical débridement, along with fascial and muscle biopsy.Distinct MR imaging features were found in patients with necrotizing soft-tissue infections, that is, hyperintense signal on T2-weighted images at the deep fasciae, poorly defined areas of hyperintense signal on T2-weighted images within muscles, and peripheral enhancement on contrast material-enhanced T1-weighted images. In nonnecrotizing cellulitis, signal intensity abnormalities were seen only in the subcutaneous fat.The precise extent of acute cellulitis and the presence of necrotizing soft-tissue infections can be determined with MR imaging, particularly on T2-weighted images.

Two excited superdeformed bands observed in , which have an identical energy relationship with the even-even core, are suggested to be two members of the pseudo-spin doublet .

The DIAMANT‐AFRODITE combination has been used to investigate incomplete fusions reactions via the 13C+170Er entrance channel. The intensity of 176Hf (populated via the α 3n exit channel) is ∼8% of 178W (populated via 5n evaporation) which is ∼8 times stronger than that expected from complete fusion. Moreover, 2αxn exit channels leading to Yb nuclei are observed with intensities that are ∼30‐to‐40% of 176Hf, for which no yield is expected from complete fusion. A comparison of the intensities from the two‐α‐ and one‐α‐gated data is consistent with fragmentation of the 13C beam into (α‐α‐α‐n) which suggests that the population of Yb nuclei results from fusion (or “massive transfer”) of one the break‐up α‐particles. A campaign of measurements is scheduled for late 2007 with further investigations planned for 2008, including the continuation of the study of superdeformation in 32S.

Lifetimes of high‐spin states have been measured for 142Gd and 134Nd with EU‐ROBALL IV and AFRODITE, respectively, using DSAM. From calculations in the cranked Nilsson‐Strutinsky model it was concluded that at the high‐spin states of the (+,0)1 band, 142Gd represents a triaxial nucleus rotating around the longest principal axis.

High resolution spectra from fast proton radiative capture in {sup 208}Pb are presented for three energies of the incident protons: 14.8, 15.7, and 16.9 MeV. The collected spectra were analyzed with emphasis on the region of capture to unbound states in order to search for possible structure and to study the competition between two basically different phenomena: the statistical process versus the fluctuation effects in radiative capture to unbound final states. In the energy region of incoming protons considered in our experiment, the statistical processes modeled with a Hauser-Feshbach calculation dominate. The spectra of primary {gamma} rays, coincident with the low energy one from the decay of the f{sub 7/2} single-particle state, show a prominent structure which is predominantly of g{sub 9/2} character. The calculations of continuum spectra based on the extended direct-semidirect model and the Hauser-Feshbach statistical approach successfully reproduce the experimental data.

The high-spin normally deformed and superdeformed states of ${}^{194}\mathrm{Hg}$ have been populated in an exit channel of the ${}^{16}{\mathrm{O}+}^{186}\mathrm{W}$ reaction at 110 MeV bombarding energy. The Doppler shift analysis of the energies of the \ensuremath{\gamma} transitions de-exciting both normally deformed and superdeformed states in ${}^{194}\mathrm{Hg}$ indicates a lower recoil velocity of the ${}^{194}\mathrm{Hg}$ nucleus compared to that of the ${}^{196}\mathrm{Pb}$ nucleus obtained from the $6n$ exit channel of the complete-fusion compound nucleus. This suggests that incomplete fusion plays the main role in the population of normally deformed and superdeformed states of ${}^{194}\mathrm{Hg}$ in this reaction.

In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section σ cap by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to σ cap. For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate σ cap. Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding σ cap through use of an evaporation-model code that takes account of the FF degree of freedom. Some earlier results on σ cap obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the “extra-push” phenomenon discussed. One may also try to obtain σ cap by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the 86Kr + 208Pb system at iThemba LABS in South Africa will be presented.

The Chessboard section of the DIAMANT CsI detector array has been merged into the AFRODITE γ‐ray spectrometer acquisition system. The details of the data acquisition merging is explained together with how consistency is maintained and ensured between the two distinct systems.

The “Chessboard” section of the DIAMANT charged‐particle array has been coupled with the AFRODITE γ‐ray spectrometer at the iThemba Laboratory for Accelerator Based Sciences. Charged‐particle‐γ‐ray coincidence data were recorded during the bombardment of a 176Yb target with a 13C beam at an energy of 90 MeV. The purpose of the investigation was to study the population of metastable states in hafium nuclei via incomplete fusion reactions in which the beam breaks up due to its α‐cluster character. Of note was the observation of the band based on the Kπ = 16+, T1/2 = 31 year isomer in 178Hf to its 19+ member. Also, decays from the high‐K isomeric states in 174Yb and 176Yb. which were populated via 3αxn channels, indicative of complete break‐up of the 13C beam.

In a commissioning measurement, the “Chessboard” section of the DIAMANT charged‐particle array has been coupled with the AFRODITE γ‐ray spectrometer at the iThemba Laboratory for Accelerator Based Sciences. Two data‐sets were obtained following the bombardment of a 170Er target with a 13C beam at energies of 80 and 70 MeV, respectively. Offline analysis has thus far enabled the extension of a number of rotational bands associated with high‐K intrinsic states in 176Hf. Also, the A = 172, 173 and 174 stable isotopes of ytterbium were populated via 2αxn channels with strengths of ∼30‐to‐40% of the αxn yields. This, together with the comparative weakness of the pxn channels, is consistent with incomplete fusion as the dominant reaction mechanism responsible for the α‐particle emission.

Excited states in $^{191}$Pt have been populated using the reaction $^{186}$W + $^{11}$B at 85MeV beam energy. Gamma-ray coincidences were measured using the EUROGAM-II spectrometer array. The level scheme has been extended considerably. Evidence for favoured structures of highly triaxial shape with $\gamma \sim$ -90° coexisting with oblate structures has been obtained by comparison with total Routhian surface and cranked shell model calculations. The presence of proton excitations in both positive- and negative-parity states has been confirmed.

The level scheme of Au-190 was extended up to high spin using gamma spectroscopy with the Eurogam-II array and internal conversion measurements with the electron- gamma spectrometer of Orsay. Sever ...

The level scheme of Au-191 was considerably extended to high spin using gamma spectroscopy with the Eurogam-II array and internal conversion measurements with the Orsay electron spectrometer. Several bands that appear as strongly coupled were associated with low- Omega configurations in Au-191 . Using total routhian surface and cranked shell model calculations it was shown that these bands have a rotation-aligned nature and are most likely generated by the nonaxiallity of the nucleus induced by the odd proton. The predictions for the presence of such bands in Au-191 and in the oblate nuclei in the Pt-Au-Hg region found excellent agreement with the experimental data, as well as the predictions for their alignments and band-crossing frequencies in Au-191 .

This paper reports on a test experiment to investigate the viability of using in‐beam gamma‐ray spectroscopy with projectile fragmentation reactions to explore the structure of heavy, neutron‐rich nuclei. The experiment used a ‘thin target’ fragmentation reaction with a 30 MeV per nucleon 12C beam on a 30 mg/cm2 self‐supporting gold target. The initial analysis has identified a number of previously known discrete cascades in a variety of in‐beam reaction products. The decay of products has also been investigated and an initial comparison of their relative yields with predictions from the EPAX parameterisation begun. The relevance and future viability of this method as a complimentary technique to fragmentation‐isomer studies to investigate the near‐yrast properties of heavy, neutron‐rich nuclei is discussed.

Excited states in Ta-164 were populated in the Nd-142(Al-27,5n)Ta-164 reaction and investigated using the AFRODITE spectrometer Arran. The yrast rotational decay sequence Lip to spin 21(-) is identified for the first time and assigned to the pi[514]9/2(-) x nu[660]1/2(+) configuration. A less intense. excited hand is also identified. Low-spin signature inversion is observed in the yrast band, and is compared with the extended total Routhian surface calculations. It is likely that quadrupole pairing plays a role in generating the anomalous signature splitting, but an additional mechanism apart front the assumed mean field and quadrupole pairing interaction may be required to account for the experimental data.

Excited isomeric states ${I}^{\ensuremath{\pi}}{=20}^{+}$ have been identified for the first time in odd-odd ${}^{190,192}\mathrm{Au}$ nuclei. Two independent measurements of their half-lives were performed, using the recoil shadow anisotropy method with the Eurogam-II data, and using the time distribution of K-electron lines from the Orsay electron-spectrometer data. Half-lives of $7.0\ifmmode\pm\else\textpm\fi{}0.3$ and $5.4\ifmmode\pm\else\textpm\fi{}0.3$ ns were obtained for these isomers in ${}^{190}\mathrm{Au}$ and ${}^{192}\mathrm{Au},$ respectively, by the latter measurements. The $\ensuremath{\pi}{h}_{11/2}^{\ensuremath{-}1}\ensuremath{\bigotimes}\ensuremath{\nu}{i}_{13/2}^{\ensuremath{-}2}{h}_{9/2}^{\ensuremath{-}1}$ configuration was assigned to these isomeric states. A shape transition towards highly triaxial deformation with $\ensuremath{\gamma}\ensuremath{\sim}\ensuremath{-}90\ifmmode^\circ\else\textdegree\fi{}$ is suggested by total Routhian surface calculations, which could be the cause for their relatively hindered decay.

Excited states in ${}^{165}\mathrm{Ta}$ were populated in the ${}^{142}\mathrm{Nd}{(}^{27}\mathrm{Al}{,4n)}^{165}\mathrm{Ta}$ and ${}^{141}\mathrm{Pr}{(}^{28}\mathrm{Si}{,4n)}^{165}\mathrm{Ta}$ reactions and investigated using the AFRODITE array. The yrast rotational decay sequence up to spin ${53/2}^{\ensuremath{-}}$ is identified and assigned to the $[514]{9/2}^{\ensuremath{-}}$ configuration. The nuclear shape is investigated using total Routhian surface calculations. The experimental results are discussed in relation to existing data in the neighboring Ta isotopes and results from cranked shell model calculations. Unexpectedly large signature splitting, for a high-$\ensuremath{\Omega}$ configuration, is observed in the yrast band. Further discrepancies are observed between theoretical and experimental values for the band crossing frequency and signature splitting of the $B(M1)/B(E2)$ ratios. The possibility that these discrepancies are a consequence of a large deviation from an axially symmetric nuclear shape is investigated.

High-spin states in 113-116Cd have been investigated with the fusion-fission reaction 28Si +176Yb at 145 > MeV. The experiment has been performed with the Eurogam2 array. New rotational bands based on two quasi-particle states have been observed for even-even cadmium isotopes. A new level scheme based on the 11/2− isomeric state is proposed for 115Cd and the one of 113Cd has been extended to spin (31/2−). The decoupled bands identified in both odd-A nuclei are interpreted as being built upon a low-Ωh11/2 quasi-neutron configuration. Microscopic Hartree-Fock + BCS calculations confirm the prolate deformation in this mass region especially for the odd cadmium isotopes.

The low-spin collective excitations of even-even Pd nuclei in the A ≈ 110 mass region indicate a structural transition from an anharmonic vibrator to a gamma-soft rotor. This observation is supported by recent calculations of the potential energy surfaces using different types of mean field approaches. In particular, prolate-to-oblate shape transition has been predicted at 111Pd [1]. Experimental evidence for a shape change or shape coexistence can be obtained by high-spin studies of these transitional nuclei. Band crossings in the odd-N Pd nuclei are especially important to understand the origin of the alignments. No band structures built on the low-lying states were known in heavier odd Pd nuclei (A≥109). High-spin states in these nuclei cannot be populated using fusion-evaporation reactions. The heavier neutron-rich 112,114,116Pd nuclei have been studied from spontaneous fission of 252Cf [2]. In this work the less neutron-rich Pd nuclei have been produced as fission fragments following the fusion reactions 28Si + 176Yb at 145 MeV. The 28Si beam was provided by the Vivitron accelerator in Strasbourg. Gamma rays were recorded with the EUROGAM2 array [3]. The level schemes were constructed by examining γ- spectra extracted from two and three dimensional matrices.

The present work reports on a study of high spin states in 190Pt On the basis of TRS calculation, an interpretation is proposed for the new structures as well as for the previously known 10- isomer.

About eighty nuclei in the A≈100 mass region have been observed as fission fragments following the fusion evaporation 28,30Si+176Yb at 145 MeV bombarding energy. These nuclei have been identified from their γ-ray cascades detected with the EUROGAM2 array. The level schemes of several stable or neutron-rich nuclei have been extended to higher spins. From coincidences between transitions in complementary fragments, γ-rays de-exciting new high-spin states of hitherto unexplored isotopes can be assigned and some aspects of the fission mechanism can be studied.

The DIAMANT light charged-particle detector will be coupled with the AFRODITE γ-ray spectrometer at iThemba LABS under a bilateral agreement between South Africa and Hungary. As a first step, the standalone 'Chessboard' section of DIAMANT has been integrated with AFRODITE in order to study incomplete fusion reactions instigated by beams of 13C ions incident on targets of 170Er and 176Yb. A fuller implementation of DIAMANT with AFRODITE will take place in due course.

The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194 Pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194 Pb is established at 4877 ± 1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the SD band are also deduced assuming that the SD states have even spin and positive parity.

The Recoil Shadow Anisotropy Method for measuring nanosecond lifetimes is being implemented on the AFRODITE array and four new isomers were found in 198,200Bi, 164Ta and 162Lu.

The EUROGAMarray has been used to investigate the decay out of the yrast superdeformed (SD) band in 194Pb. Eigth single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy (4877 ± 1.5 keV) and the spin (6+) of the lowest observed SD state in 194Pb are established.

High-spin states in 113–116Cd have been investigated with the heavy-ion induced fission reaction 28Si + 176Yb at 165 MeV. The experiment was performed with the Eurogam II array. New rotational bands based on two quasi-particle states have been observed for even-even cadmium isotopes. A new level scheme based on the 11/2– isomeric state is proposed for 115Cd and the one of 113Cd has been extended to spin (31/2–). Hartree–Fock calculations for even-even isotopes show a prolate deformation in this mass region. The decoupled bands identified in both odd nuclei are interpreted as being built upon a low-Ω11/2 quasi-neutron configuration.

High-energy γ rays produced by the 208 Pb ( p , γ) reaction were measured with the AFRODITE germanium detector array. The use of Ge instead of NaI detectors improved the energy resolution by more than a factor of 10 in the γ -ray energy range between 10 and 19 MeV. Proton captures into different single particles states in 209 Bi were thus clearly separated, most notably for the i 13/2 high-spin state. The measured excitation functions are in very good agreement with the predictions of the direct–semi-direct model, including captures to the i 13/2 state, for which the previous agreement had been poor.