Study of thermally excited nuclei through <Emphasis Type="Italic">E</Emphasis>1 and <Emphasis Type="Italic">E</Emphasis>2 decay from collective modes

The nuclear system at the limit of excitation energy and angular momentum is here studied in the case of the superdeformed nucleus ¹⁴³Eu using -spectroscopy techniques. The data are based on a EUROBALL experiment using the reaction ³⁷Cl + ¹¹⁰Pd Eu + 4n. The influence of thermal energy on superdeformed configurations is investigated through the analysis of the quasi-continuum spectra formed by E2 transitions among states of excited rotational bands with energy extending up to 4-5 MeV above the yrast line. In particular, the effective lifetimes of the discrete rotational bands forming ridge structures in - coincidence matrices is measured by a Doppler Shift Attenuation Method. The deduced quadrupole deformation of 10 eb indicates that the nucleus maintains its collectivity with increasing excitation energy, supporting the superdeformed character of the excited nuclear rotation. The obtained number of superdeformed discrete bands forming the ridge structures is found in good agreement with microscopic cranked shell model calculations including the decay-out process into the lower deformation minimum. In addition, the nuclear properties at higher excitation energies are investigated through the E1 -decay of the giant dipole resonance (GDR). It is found that the intensity of the superdeformed yrast and excited bands increases by a factor of approximately 1.6 when a coincidence with a high-energy -ray is required, showing the importance of the E1 cooling in the feeding mechanism of the superdeformed states.Received: 2 December 2002, Published online: 2 March 2004PACS: 21.10.Tg Lifetimes - 21.10.Re Collective levels - 23.20.Lv transitions and level energies - 27.60. + j      

Isospin mixing in the 4<Superscript> + </Superscript> doublet of <Superscript>54</Superscript><Emphasis Type="Italic">Co</Emphasis>

The T = 1 admixture into the T = 0 member of a recently discovered isospin-doublet in ⁵⁴Co is obtained from the measured electromagnetic E2/M1 multipole mixing ratio, of the transition. Combining these data with shell model calculations for strong isovector M1 and isoscalar E2 electromagnetic matrix elements one obtains a value for the T = 1 admixture into the T = 0, 4 ⁺ state of . The corresponding mixing matrix element in the 4 ⁺ doublet is keV.Received: 31 October 2002, Published online: 9 March 2004PACS: 21.10.Hw Spin, parity, and isobaric spin - 21.60.Cs Shell model - 23.20.Gq Multipole mixing ratios     

Study of drip line nuclei through two-step fragmentation

We have studied the structure of light neutron-rich nuclei around N = 16 by employing the in-beam -ray spectroscopy technique using the fragmentation of secondary beams of ^25,26Ne, ^27,28Na and ^29,30Mg isotopes. This secondary-beam cocktail was obtained by the fragmentation of a ³⁶S beam at 77.5 MeV by the SISSI/GANIL facility. By a second-step fragmentation, we have measured -ray-residue coincidences in ^17-20C and ^23,24O and described the obtained levels in the framework of the shell model.Received: 31 October 2002, Published online: 24 February 2004PACS: 23.20.Lv transitions and level energies - 21.60.Cs Shell model - 27.20. + n - 27.30. + t      

Evidence for an isomer in 76 Ni

In the experiment performed at the LISE2000 spectrometer at GANIL neutron-rich nickel isotopes were studied by microsecond isomer spectroscopy. Evidence for an isomer in ⁷⁶Ni is found, consistently with the shell model prediction of an 8 ⁺ state of structure.Received: 24 October 2002, Published online: 17 February 2004PACS: 23.20.Lv transitions and level energies - 27.50. + e      

First observation of excited states in the <Superscript>110</Superscript>Mo nucleus

The ground-state band in ¹¹⁰Mo has been observed for the first time. The band, comprising six levels, has been populated in the spontaneous fission of ²⁴⁸Cm and studied by means of prompt -ray spectroscopy using the EUROGAM2 array. The ratio suggests that the deformation of ¹¹⁰Mo is smaller than that in ¹⁰⁸Mo but may stabilize at higher neutron number, where an oblate shape is expected. The new data suggests that the deformation of Sr and Zr isotopes decreases above neutron number N = 64.Received: 23 February 2004, Published online: 2 June 2004PACS: 23.20.Lv transitions and level energies - 21.60.Cs Shell model - 25.85.Ca Spontaneous fission - 27.60. + j      

Properties of <Superscript>152</Superscript><Emphasis Type="Italic">Gd</Emphasis> collective states

The decays of ¹⁵²Tb (T _1/2 = 17.5 h) and ¹⁵²Gd excited states have been investigated by the analysis of -rays and -coincidences measured with the use of high-resolution HPGe detectors. The source of ¹⁵²Tb was prepared by chromatographic isolation followed by electromagnetic separation from a tantalum target irradiated by an internal proton beam of the LNP JINR phasotron. New and more precise data on the -transitions and excited states of ¹⁵²Gd are reported. Using previously published data on internal-conversion electrons many transition multipolarities are suggested, some of them with E0 admixture. The ratio was found for a number of low-lying levels, for most of the levels their spins, parities, and are given. The excited levels of ¹⁵²Gd were deduced from the analysis of -coincidences. The experimental level energies and reduced transition probabilities are compared with the calculations by phenomenological formulae as well as in the frameworks of theoretical models.Received: 21 May 2002, Revised: 26 May 2003, Published online: 18 November 2003PACS: 29.30.Kv X- and gamma-ray spectroscopy - 23.20.-g Electromagnetic transitions - 21.10.-k Properties of nuclei; nuclear energy levels - 21.60.-n Nuclear structure models and methodsJ. Adam: On leave from Nuclear Physics Institute of ASCR, e, Czech Republic.V.S. Pronskikh: On leave from Saint Petersburg State Institute of Technology, Russia     

Systematics of deformed states around doubly-magic <Superscript>40</Superscript>Ca

The low-lying rotational bands of A = 36-48 nuclei are consistently explained by starting from the recently discovered, superdeformed intrinsic state of ³⁶Ar as the core, filling successively the first three Nilsson orbits above the Fermi border. The critical single-particle energies were obtained from experimental data as were the residual interactions in the parametrization of Brink and Kerman. Implicit are the rearrangement energies due to configuration-dependent equilibrium deformations. The binding energies of 20 experimental bandheads were used to derive the parameters while another 38 bandheads were subsequently predicted and identified almost completely. The Racavy expression reduced by 20% reproduces or predicts the values of the deformation parameter . The empirical Nilsson model amended by -vibrational and rotation-aligned bands accounts completely for the multi-particle excitations from the N = 2 into the N = 3 major shell which are not accessible by shell-model calculations. In the case of ⁴⁰Ca a spectrum of 42 states below E _x = 8 MeV is explained.PACS: 21.60.Ev Collective models - 21.10.Dr Binding energies and masses - 27.30. + t - 27.40. + z      

High-spin states in the vibrational nucleus 114 Cd

High-spin states of the neutron-rich vibrational nucleus ¹¹⁴Cd have been studied using the incomplete fusion reaction ¹¹⁰Pd(⁷Li, p2n) and the GASP spectrometer in conjunction with the ISIS Si ball. About 50 new states with excitation energies up to 7 MeV and angular momentum were observed and for many of them, spin and parity could be firmly assigned. The band-like stuctures in ¹¹⁴Cd are compared to the corresponding ones in the even-even neighbour ¹¹²Cd.Received: 31 October 2002, Published online: 17 February 2004PACS: 23.20.Lv transitions and level energies - 27.60. + j      

The yrast spectroscopy of neutron-rich nuclei produced in deep-inelastic processes

The highly sensitive GASP array at the INFN Legnaro Laboratory was used to study the -ray de-excitation of neutron-rich nuclei produced in the deep-inelastic processes which occur when 230 MeV ³⁶S ions interact with a target of ¹⁷⁶Yb. Yrast decay schemes were identified in over forty target-like fragments and in over twenty projectile-like fragments. Analysis of the data has resulted in extensions to the yrast decay sequences of the target-like species, ¹⁷⁶Hf, ¹⁶⁶Er, ¹⁷²Yb, and ¹⁵²Sm. New transitions have also been observed in a number of projectile-like species including ³⁴P and ⁴¹Cl. Experimental results are compared with the results of shell model calculations.Received: 29 October 2002, Published online: 17 February 2004PACS: 23.20.Lv Gamma transitions and level energies - 27.40. + z - 27.70. + q      

The <Emphasis Type="Italic">N</Emphasis> = <Emphasis Type="Italic">Z</Emphasis><Emphasis Type="Italic">f</Emphasis><Subscript>7/2</Subscript>-shell nuclei: Experimental highlights

During the last decade, as the experimental and computing means and techniques have rapidly evolved, the experimental investigation of the f_7/2-shell nuclei has gained renewed interest. TheN = Z nuclei studied with the GASP array range from ⁴⁴Ti to ⁵²Fe. The results extended the knowledge of their structure up to high spins and excitation energies, above band terminations, where the competition with the charged-particles emission was initially thought to obscure the possibility of gamma-ray spectroscopy investigation. The paper highlights some of the most outstanding properties of these nuclei such as the nuclear rotation and backbending effects, band termination states, yrast traps, non-natural parity bands, competition between T = 0 and T = 1 pn pairing modes.Received: 30 October 2002, Published online: 16 March 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 21.60.Cs Shell model - 23.20.Lv transitions and level energies - 27.40. + z C.A. Ur: On leave from NIPNE Bucharest, Romania     

Wobbling phonon excitations in strongly deformed triaxial nuclei

The wobbling mode is uniquely related to triaxiality and introduces a series of bands with increasing wobbling phonon number, . The pattern of -transitions between the wobbling excitations will be influenced by the presence of an aligned particle. Evidence for the wobbling mode was obtained recently, and even a two-phonon wobbling excitation has now been identified in ¹⁶³Lu. The similarity of the data in ¹⁶³Lu to new strongly deformed triaxial bands and connecting transitions in the neighbouring nuclei, ¹⁶⁵Lu and ¹⁶⁷Lu, establishes wobbling as a more general phenomenon in this region. The higher phonon wobbling excitations show some anharmonicity, but the characteristic large E2 strength is observed. The wobbling interpretation is based on the comparison of electromagnetic decay strength between bands to particle-rotor calculations in which an i_13/2 proton is coupled to a triaxial core.Received: 30 October 2002, Published online: 23 March 2004PACS: 27.70. + q - 23.20.-g Electromagnetic transitions - 21.10.Re Collective levels     

Hard-to-reach nuclei studied with deep-inelastic heavy-ion reactions

Spectroscopic studies performed with deep-inelastic heavy-ion reactions are reviewed for two regions of neutron-rich nuclei. The identification of isomers in nearly complete series of Sn isotopes and the resulting systematic of B(E2) values for isomeric transitions is presented and followed by the discussion of shell model states studied in neutron-rich Te isotopes including the new four neutron-hole isomers identified in ¹³⁰Te. Yrast spectroscopy studies of nuclei from the doubly magic ²⁰⁸Pb region are described by outlining the highest spin states observed in the ²⁰⁸Pb core nucleus. The E3 transitions observed abundantly in yrast decays are discussed within the framework of particle-octupole vibration coupling and the validity of a simple rule connecting energy shifts of octupole states built on one-particle states with the ones observed for two-particle states is demonstrated.Received: 15 January 2003, Published online: 23 March 2004PACS: 21.60.Cs Shell model - 23.20.Lv transitions and level energies - 27.60. + j - 27.80. + w      

Complete spectroscopy in high-spin cranking calculations

Exact and approximate quantum numbers of the cranked Nilsson-Strutinsky (CNS) formalism are exploited to calculate excited bands in fixed configurations with the energy of the individual bands minimized with respect to deformation for all spin values. The formalism is applied to ⁵⁹Cu, where all bands which appear important in the decay out of the superdeformed band are calculated.Received: 14 November 2002, Published online: 17 February 2004PACS: 21.10.Ma Level density - 21.60.Cs Shell model - 23.20.Lv transitions and level energies - 27.50. + e C. Andreoiu: Present address: Department of Physics, University of Guelph, Guelph, Ontario, Canada N1G 2W1.     

Cluster structure of <Superscript>16</Superscript>O

A folding potential describing the -scattering on ¹⁶O over a broad energy range 25.8-146.0 MeV is constructed on the basis of -like cluster and unclustered-nucleon configurations of ¹⁶O. The resulting potential does not need any renormalization to fit the angular distribution of elastic cross-sections. The effects of the repulsive part of - and -nucleon interactions are investigated. The analysis suggests that both the - repulsive potential and the unclustered nucleonic configuration in the target are important to describe the scattering data over a broad range of incident energies. The root-mean-square radius for the ¹⁶O nucleus is deduced.Received: 5 December 2002, Published online: 9 October 2003PACS: 25.55.Ci Elastic and inelastic scattering - 24.10.Ht Optical and diffraction models - 21.60.Gx Cluster models     

A new isomer in 136 Ba populated by deep inelastic collisions

Excited states in ¹³⁶Ba, populated in deep inelastic collisions by the interaction of 450 MeV ⁸²Se ions with a ¹³⁹La target, have been studied by means of in-beam -ray spectroscopy. A new isomer with has been identified at an excitation energy E_x = 3.357 MeV. The half-life was determined as ns. The extracted B(E2) value is much smaller than those in ¹³²Te and ¹³⁴Xe. This hindrance is investigated by a shell model calculation.Received: 22 September 2003, Revised: 11 November 2003, Published online: 6 April 2004PACS: 21.10.Tg Lifetimes - 23.20.Lv transitions and level energies - 27.60. + j      

Alpha-gamma decay studies of <Superscript>251, 253</Superscript>No and their daughter products <Superscript>247, 249</Superscript>Fm

The isotopes ²⁵¹No and ²⁵³No were produced in nuclear reactions ²⁰⁶Pb(⁴⁸Ca,3n)²⁵¹No and ²⁰⁷Pb (⁴⁸Ca,2n)²⁵³No. Radioactive decay of these isotopes and their daughter products has been investigated by means of - and - - spectroscopy. An isomeric state ^251m No, having a half-life of s and decaying by emission of particles of keV was identified. The measured decay data allowed for the construction of partial level schemes for ²⁵¹No, ^247,249Fm and ^243,245Cf and an extrapolation of energy systematics of single particle levels of N = 145, 147 and 149 isotones with even Z towards higher atomic numbers up to Z = 102.PACS: 23.60. + e decay - 27.90. + b - 21.10.-k Properties of nuclei; nuclear energy levels     

The ν9/2[404] orbital and the deformation in the A ∼ 100 region

A T _1/2 = 16(2)ns isomeric level, populated in the spontaneous fission of ²⁴⁸Cm, has been found in ¹⁰¹Zr at 941.8keV in a measurement of prompt -rays using the EUROGAM2 array. The level is interpreted as a K-isomer corresponding to the 9/2[404] neutron-hole excitation. The quadrupole moment, Q ₀ = 3.6(4)eb, deduced for the new band indicates a large deformation of , which is produced by a specific shape-coexistence mechanism. This is another observation of the 9/2[404] orbital in the mass region, confirming the presence of the effect reported first in our previous work. The 9/2[404] neutron-hole excitation is also confirmed in the ⁹⁷Sr nucleus and the deformation is determined for this configuration. The properties of the 9/2[404] neutron orbital and its influence on the onset of deformation in the region are discussed.PACS: 23.20.Lv transitions and level energies - 21.60.Cs Shell model - 25.85.Ca Spontaneous fission - 27.60. + j      

Delayed crossing in the \pi h_{9/2} 1/2-[541] band of 173 Lu

High-spin states in the odd-proton nucleus ¹⁷³Lu have been populated in a ¹⁷⁰Er(⁷Li,4n) reaction and the emitted -radiation was detected with the GASP array. The favoured and unfavoured sequences of the band have been considerably extended. The favoured sequence does not show any indication of a band crossing up to a rotational frequency of 0.45 MeV. This is the largest crossing frequency ever observed for 1/2^- [541] bands of odd-proton nuclei with and . Previous Cranked Shell Model (CSM) calculations underestimate the crossing frequency by 100 keV.Received: 19 May 2003, Published online: 11 November 2003PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 21.10.Re Collective levels - 23.20.Lv transitions and level energies - 27.70. + q      

High-resolution in-beam particle-gamma coincidence spectroscopy

A short summary of the present status of prompt particle decays in the mass region is presented. It includes recent results from a GAMMASPHERE experiment, which aimed at the combined high-resolution spectroscopy of light charged particles and -rays to allow for more detailed studies of known decays and the identification of new cases of discrete prompt proton and -particle emission from highly and superdeformed states.Received: 10 October 2002, Published online: 17 February 2004PACS: 23.50. + z Decay by proton emission - 23.60. + e decay - 27.40. + z - 27.50. + e      

Observation of bound excited states in 15B

The structure of the A/Z = 3 nucleus ¹⁵B has been investigated using the in-beam -spectroscopy technique with a fragmentation reaction of a ³⁶S beam on a ⁹Be target at 77.5 . The fragments were identified and selected by their energy loss and time of flight using the SPEG spectrograph. -ray energies and intensities have been measured in coincidence with the projectile-like fragments. From this information as well as from the -coincidence relationships a level scheme is proposed for ¹⁵B up to the neutron separation energy. The experimental results have been interpreted using shell model calculations in the psd valence space. Effects of the weakly bound nature of the valence neutrons have been observed.Received: 4 May 2004, Revised: 2 September 2004, Published online: 19 October 2004PACS: 25.70.Mn Projectile and target fragmentation - 23.20.Lv transitions and level energies - 27.20. + n - 21.60.Cs Shell model