102Ag: In-beam gamma-ray spectroscopy

Gamma-rays associated with the decay of states in the ¹⁰²Ag nucleus have been identified, sixty-four of them for the first time following the reaction ⁵⁰Cr(⁵⁶Fe, 3pn)¹⁰²Ag at a mean energy of 195 MeV. Identification was made using an array of nine escape-suppressed Ge detectors coupled to the Daresbury Recoil Separator. Excited states in ¹⁰²Ag were identified using recoil-gamma and γγ coincidences. From the intensity balance and the coincidence data, a new set of levels was identified which may be interpreted as reminiscent of “three-quasiparticle” bands in neighboring odd-mass nuclei and similar to a four-quasi-particle band observed in ¹⁰⁶Ag.     

In-beam gamma-ray spectroscopy of 190, 197Po

γ -ray transitions in the neutron-deficient nuclei ^{190, 197}Po have been identified using the JUROGAM Ge detector array coupled to the RITU gas-filled separator and the GREAT spectrometer. The yrast band of ¹⁹⁰Po has been firmly established through γ -γ coincidences and extended up to a spin and parity of 14⁺. It displays similar behaviour to its isotones ¹⁸⁶Hg and ¹⁸⁸Pb above the 4⁺ level, thus confirming its prolate nature. In ¹⁹⁷Po the band built upon the 13/2⁺ isomer has been extended up to a spin and parity of 33/2⁺, while the non-yrast band has been observed for the first time. The behaviour of ¹⁹⁷Po is found to be similar to that of the nearby even-mass isotopes, which is consistent with the model in which the i _13/2 neutron is weakly coupled to the states in the even-even core.     

In-beam gamma-ray spectroscopy with exotic beams at the NSCL

Projectile fragmentation provides radioactive beams at intermediate velocities (v/c = 0.3-0.5) by physical means of fragment separation. With the development of position-sensitive photon detectors it has become possible to measure the energies and directions of photons emitted in-flight from such fast-moving exotic beams. This allows the reconstruction of the photons' energies emitted from an exotic projectile with high accuracy. It can be advantageous to employ photon detection in experiments with exotic beams since photons can traverse matter easily and their attenuation can be calculated. Experiments with standard luminosities can be carried out at intermediate beam energies with thick secondary targets (order of g/cm²) and very low incident beam rates (order of particle/s or less). Experimental success in this field is strongly correlated with the development of photon detectors such as position-sensitive scintillation detectors or segmented germanium detectors. In-beam gamma-ray spectroscopy of fast exotic beams has been successfully used at all projectile fragmentation facilities in intermediate-energy heavy-ion inelastic scattering experiments, knockout reactions and fragmentation reactions. Here, we focus on experimental results for neutron-rich exotic nuclei in the π(sd )-shell. Measurements and detector developments carried out at the NSCL at Michigan State University during the last four years are discussed. Received: 1 May 2001 / Accepted: 4 December 2001     

In-beam spectroscopy of231Pa

Information on energy levels and onE 2 andM 1 matrix elements in²³¹Pa has been obtained using conversion-electron and gamma-ray spectroscopy following the²³²Th(p, 2n)²³¹Pa reaction and Coulomb excitation of the radioactive target²³¹Pa by⁴He and³²S ions. The results are analyzed in the framework of the rotational model, applied to the rotational band built on the 1/2^?[530] Nilsson state whose 3/2^? member forms the ground state of this nucleus. The deviations of the level energies from the rigid-rotor values can be described by Coriolis couplings. The analysis of the Coulomb-excitation process shows that a constant set of rotational parameters Q₀, g_R, g_K, andb can fairly well account for the measured line intensities.     

In-beam spectroscopy of224Th

The ß-decay half-life of¹³⁰Cd has been measured and its importance as N=82 ‘waiting-point’ nucleus for astrophysical rapid neutron capture (r?) process scenarios is discussed.     

In-beam spectroscopy of 120Xe

Excited states of ¹²⁰Xe have been studied by in-beam γ-ray spectroscopy using the ¹¹⁰Cd(¹³C, 3n) and ¹¹¹Cd(¹²C, 3n) reactions. The yrast band was observed up to the 14⁺ level, showing backbending at the 12⁺ state. A γ-band and two new negative-parity bands were found. The low-lying levels of ¹²⁰Xe could be described by the IBA-1 hamiltonian plus a triaxial degree of freedom.     

In-beam spectroscopy of104Sn

The very neutron deficient nucleus 104Sn has been identified in in-beam spectroscopy using the reaction⁵⁰Cr(⁵⁸Ni, 2p2n) and neutron and charged particle multiplicity filter detectors. Excited states up to I ≈ 10 and E_x=4 MeV were observed and the level scheme is discussed in the frame work of the spherical shell model.     

In-beam gamma-ray and electron spectroscopy following the75As(p,n)75Se reaction

The level scheme of⁷⁵Se has been studied through the⁷⁵As (p, n) reaction at proton energies from 1.5 to 5.0 MeV.γ-ray and internal conversion electron measurements were made using NaI (T1) and Ge(Li) detectors and a six-gap electron spectrometer. A proportional counter and a thin window NaI(T1) detector were used to detectγ-rays with energies less than 30 keV. The level scheme has been established by observing the thresholds of variousγ-rays and byγ-γ and e^?-γ coincidence measurements. New levels at 133.0, 293.2, 790.0, 953.0, 1020.8, 1184.3, 1198.5 and 1258.2 keV not observed in earlier (p, n) studies have been established. Conversion coefficients of most of the low-lying transitions have been determined. Angular distributions of some of theγ-rays were also measured and compared with the statistical model calculations. DefiniteJ ^π assignments have been made to most of the low-lying levels. Life-times of the 112.1, 133.0, 286.7 and 293.2 keV levels have been measured to be 0.69±0.12, 5.3±0.6, 1.35±0.15 and 31±2 nsec respectively. The reduced transition probabilities for various low-lying transitions have been determined and compared with recent calculations. The 1/2^? and 9/2⁺ levels hitherto unknown in this nucleus has been identified. The structure of the low-lying levels is discussed in terms of the existing models.     

In-beam and decay spectroscopy of transfermium nuclei

In recent years the body of experimental data on nuclei with masses A?250 has increased dramatically. Nuclei that had been out of reach for experimental studies have now become available for study through a variety of approaches, both with in-beam spectroscopic methods and through spectroscopy following the decay of isomeric states or alpha decays at the focal plane of powerful separators. This article aims to collect the currently available experimental data on nuclei between Cm (Z=96) and Db (Z=105). The review of this data builds on the evaluations in the literature and focusses on those datasets obtained most recently.     

In-beamγ-ray spectroscopy of102In

Neutron deficient nuclei close to ¹⁰⁰Snhave been investigated in-beam by γ-ray spectroscopic methods using the NORDBALL detector array. A beam of 270 MeV ⁵⁸Niwas used to bombard a target of ⁵⁴Fe.Reaction channel separation was achieved with a 4π charged particle multidetector setup together with a 1π neutron detector wall placed in the forward direction. Excited states of ¹⁰²Inwere identified for the first time. The level scheme constructed from γ-γ-particle-coincidence and γ-γ-angular correlation analysis is presented. The structure of ¹⁰² In is discussed and compared to neighboring nuclei in the framework of the nuclear shell-model.     

In-beam spectroscopy of 253, 254No

In-beam conversion electron spectroscopy experiments have been performed on the transfermium nuclei ^{253, 254}No using the conversion electron spectrometer SACRED in nearly collinear geometry in conjunction with the gas-filled separator RITU at the University of Jyv?skyl?. The experimental setup is discussed and the spectra are compared to Monte Carlo simulations. The implications for the ground-state configuration of ²⁵³No are discussed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: rdh@ns.ph.liv.ac.uk RID="b" ID="b"Present address: GANIL, F-14021 Caen, France. RID="c" ID="c"Permanent address: IReS Strasbourg, IN2P3-CNRS, F-67037-Strasbourg, France. RID="d" ID="d"Present address: CEA/DIF DCRE/SDE/LDN F-91680 Bruyeres-le-Chatel. RID="e" ID="e"Present address: Daresbury Laboratory, Daresbury WA4 4AD, UK. RID="f" ID="f"Permanent address: IPN Lyon, IN2P3-CNRS, F-69037 Lyon, France.     

High spin states in116Te

The high spin states in¹¹⁶Te were populated in the Ru(¹⁹F,p4n) reaction at E_lab=90MeV and the subsequent deexcitation was studied using the γ-ray spectroscopic techniques. Ninteen new transitions have been added to extend the level scheme. A ΔI=2 rotational band is identified for the first time in an even-A Tellurium nucleus.     

High-resolution gamma-ray spectroscopy

To obtain the highest possible resolution in a measurement has always been one of the major challenges for experimental physicists because increased resolution generally results in progress. At the Institut Laue-Langevin, gamma rays emitted after neutron capture can be recorded with parts-permillion resolution. This is achieved by diffracting the gamma rays on highly perfect Sior Ge crystals. Precise measurement of the Bragg angles and the crystal lattice spacings permits the determination of wavelengths or energies. This outstanding resolving power allows the measurement of extremely small Doppler effects caused by the emission of primary gamma rays. These so-called gamma-ray-induced Doppler broadening measurements have given rise to applications in both nuclear and condensed matter physics.     

In-beam γ-ray spectroscopy on56Ni

Yrast levels in the doubly-magic nucleus⁵⁶Ni have been studied by in-beam gg-spectroscopy with the reaction⁵⁴Fe(α, 2n)⁵⁶Ni. A cascade of five γ -transitions is established. A shell model calculation of 2p2h, T=0 states, using empirical matrix elements, suggests the assignments 8⁺ and 10⁺ for the two highest levels.     

In-beam spectroscopy of 126Ce and 127Pr

High spin states in ₁₂₆Ce and ₁₂₇Pr were populated via heavy ion reactions with 182-MeV ₃₅Cl projectiles on a thick ₉₆Ru target. Prompt γ–γ coincidences were measured. New states in ₁₂₆Ce and ₁₂₇Pr were found. The results are discussed in the framework of the core–quasiparticle coupling model. Received: 16 June 1998     

In-beam conversion electron spectroscopy using the SACRED array

Conversion electron studies of medium-heavy to heavy nuclear mass systems are important where the internal conversion process begins to dominate over gamma-ray emission. The use of a segmented detector array sensitive to conversion electrons has been used to study multiple conversion electron cascades from nuclear transitions. The application of the Silicon Array for ConveRsion Electron Detection (SACRED) for in-beam measurements has successfully been implemented.     

In-beam Mössbauer spectroscopy at heavy-ion accelerators

The method of “in-beam Mössbauer spectroscopy (IBMS)” as presently performed at heavy-ion accelerators is presented and reviewed. Experimental aspects are outlined and special features of this technique in comparison with more conventional radioactive ion implantation are stressed. The review is centered on the fate of⁵⁷Fe atoms implanted in metals and semi-conductors in which very limited or vanishing solubility for Fe exists. Under these conditions Fe takes up to a rather large extent interstitial positions. These can be characterized by the Mössbauer parameters, in particular by the isomer shift. Already at rather low temperatures one observes in several materials a dynamic behaviour of the Fe interstitial which can be described in some cases as localized motion and in others as onset of long-range interstitial diffusion. Additional information on the dynamic behaviour and the electronic structure in some of the systems presented comes from Perturbed Angular Distribution of γ-rays (PAD). which also is an “in-beam” technique working with a 10⁺ isomeric state of⁵⁴Fe.     

In-beam γ-ray spectroscopy of the neutron deficient 100Ag

Conversion coefficients and conversion ratios of someγ-transitions in²⁰¹Tl and²⁰³Tl were measured by means of simultaneous spectroscopy ofγ-quanta and conversion electrons employing intensity calibrated semiconductor detectors. The experiments aimed at a precision as high as possible within the limits of the experimental methode used. Theγ-transitions were of the typeM 1(+E2). From the measured values the penetration parameters of the internal conversion for theM 1 components (λ) together with the multipol mixing ratios (δ²) were deduced. The necessary theoretical coefficients were taken from the tables of Hager and Seltzer. The results are for the ?-forbidden 331 keV transition in²⁰¹Tl,λ=+4.0±1.0,δ ²=1.78±0.16; for the ?-allowed 361 keV transition in²⁰¹Tl,λ=+0.5±0.5,δ ²=0.02±0.02; for the ?-forbidden 279 keV transition in²⁰³Tl,λ=+ 5.71±0.45,δ ²=1.55±0.08.     

In-beamγ-ray spectroscopy of the odd-odd nucleus144Tb

The level scheme of theN=79 nucleus¹⁴⁴Tb was investigated via in beamγ-ray spectroscopy using the¹¹²Sn (³⁵Cl,n2p),¹¹⁶Sn(³²S, 3np) and⁸⁹Y(⁵⁸Ni, 2np) reactions. States up toI～20 andE _x～5 MeV were established above the 6^? β ⁺ decaying isomer. Two new isomers with half-life of 0.67(6) µs and 2.8(3) µs were identified. The level scheme of¹⁴⁴Tb is interpreted in the framework of the spherical shell model with a few valence nucleons outside the¹⁴⁶Gd core.     

In-beam γ spectroscopy using DIC with a radioactive Ne beam

A study of deep-inelastic and multi-nucleon transfer reactions to populate neutron-rich O, Ne and F nuclei is here presented. The reaction under analysis employed a beam of radioactive ²⁴Ne at 7.9 AMeV, provided by the SPIRAL facility at Ganil, impingin on a ²⁰⁸Pb target. The reaction products have been detected in the VAMOS spectrometer in coincidence with gamma rays measured by the EXOGAM array. Preliminary results here presented show a selectivity in the population of states of different nature.