Nuclei in the vicinity of ‘island of inversion’ through the fusion reaction

The level structures of N ～ 19 nuclei such as ^32,34P have been investigated using the ¹⁸O(¹⁸O,xnyp) and ¹⁸O(¹⁶O,xnyp) reactions at an incident beam energy of about 34 MeV. The de-exciting γ-transitions were recorded using an array of clover detectors. These detectors have the dual advantage of higher efficiency at E _γ ≥ 2 MeV, and are capable of providing information on the linear polarization of the observed γ-transitions. These polarization measurements when coupled with the angular correlations help us to assign uniquely the spin parity for the observed levels. The experimental results have been compared with the predictions of the spherical shell model. The shell-model calculations are able to reproduce the observed energy levels to a reasonable degree. However, the observed transition probabilities are not reproduced by the calculations. Hence there is a need to re-visit these calculations using more detailed and microscopic effective interactions.     

Angular momentum transfer in 12C-, 20Ne- and 40Ar-induced fission

Angular momentum transfer in a variety of ¹²C-, ²⁰Ne- and ⁴⁰Ar-induced fission reactions has been investigated using γ-ray multiplicity techniques. Fission fragments were detected in coincidence using a pair of solid-state detectors. The fragment masses were deduced from the kinetic energies and emission angles using two-body kinematics. The γ-ray multiplicities (M_γ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, M_γ is nearly independent of the exit-channel mass asymmetry. The strongest dependence on mass is observed in the systems ¹⁵⁴sm + 240 MeV ⁴⁰Ar, where a minimum exists at symmetry, and ¹⁹⁷Au + 164 MeV ²⁰Ne, where nuclear structure effects are suggested by the data. For all the reactions the quantity M_γ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of M_γ generally appears to be larger than expected on the basis of rigid rotation, suggesting a spin enhancement effect. The data are compared with a simple model which assumes the statistical excitation of a variety of angular momentum bearing collective modes. Reasonable agreement is obtained with the experimental results. The roles of other collective effects, such as shape fluctuations and angular momentum fractionation, are also considered.     

In-beam study of 102Sn

Excited states in ¹⁰²Sn have been identified for the first time, in an in-beam γ-ray spectroscopic experiment. Two γ-ray transitions with energies 1472 and 497 keV following the decay of the seniority 6⁺ isomer with t_1/2 = 1.0(5) μs were unambiguously assigned to ¹⁰²Sn. Due to the very low cross section of about 2 μb for producing ¹⁰²Sn in the reaction ⁵⁰Cr(⁵⁸Ni,1α2n), a highly selective detector setup utilizing NORDBALL ancillary detectors and a recoil catcher device was used. High γ-ray detection efficiency was achieved with two EUROBALL Ge cluster detectors.     

In-beam study of102Sn

Excited states in¹⁰²Sn have been identified for the first time, in an in-beamγ-ray spectroscopic experiment. Twoγ-ray transitions with energies 1472 and 497 keV following the decay of the seniority 6⁺ isomer with t_1/2=1.0(5)μs were unambiguously assigned to¹⁰²Sn. Due to the very low cross section of about 2µb for producing¹⁰²Sn in the reaction⁵⁰Cr(⁵⁸Ni,1α2n), a highly selective detector setup utilizing NORDBALL ancillary detectors and a recoil catcher device was used. Highγ-ray detection efficiency was achieved with two EUROBALL Ge cluster detectors.     

Evidence for nuclear shape coexistence in180Hg

Theγ decay in the radiative fusion reaction⁹⁰Zr+⁹⁰Zr→¹⁸⁰Hg+xγ has been observed in an array of NaI detectors. States up to 6⁺ in the yrast sequence of¹⁸⁰Hg are tentatively assigned and suggest the coexistence of weakly oblate and strongly prolate nuclear shapes. The difference in potential energy between the two inferred shapes has dropped to about 200 keV, continuing the downward trend observed in the heavier even isotopes^188–182Hg.     

Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV ¹⁶O + ¹⁷⁶Yb and ²⁰⁸Pb and 104-MeV ⁴He + ¹⁸⁸Os and ²⁰⁹Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10^?21 s.     

Decay of149Gd and directional correlations of gamma cascades in149Eu

The decay of¹⁴⁹Gd(9.4d) to the levels of¹⁴⁹Eu has been studied using large volume high resolution Ge(Li) and NaI(Tl) detectors,γ-γ coincidence and directional correlation experiments have been performed. Newγ-rays at energies 131.9, 278.6, 384.7, 415.7, 599.2, 726.5, 734.7, 956.4 and 1,231.1 keV in¹⁴⁹Eu have been observed. Three new levels in¹⁴⁹Eu have been placed at 956.4, 1,082.0 and 1,231.1 keV. Spin-parity assignments have been made to some of the levels in¹⁴⁹Eu. Multipolarity assignments have been made for most of the electromagnetic transitions from the conversion coefficients obtained through the knowledge of ourγ-transition intensities and the published conversion-electron intensities. Based on the newγ-transitions observed in the singles and coincidence spectra, a revised level scheme has been proposed for¹⁴⁹Eu and compared with previously published level schemes. Radioactivity¹⁴⁹Gd; measuredT _1/2,T _γ,γγ coin, γγ(θ), I.C.C.,¹⁴⁹Eu deduced levels,J,γ-mixing,π. Enriched targets, Ge(Li) detectors.     

Study of nuclei far from stability with the AYE-Ball array

The coupling of a Compton-suppressed Ge (CsGe) detector array to a recoil mass separator (RMS) has seen limited use in the past due to the low efficiency for measuring recoil-γ ray coincidences (< 0.1%).With the building of new generation recoil separators and gamma-ray arrays, a substantial increase in detection efficiency has been achieved. This allows for the opportunity to measure excited states in nuclei with cross-sections approaching 100 nb. In this paper, results from the coupling of a modest array of CsGe detectors (AYE-Ball) with a recoil separator (FMA) will be presented.     

A rotational band in82Zr

The neutron deficient nucleus⁸²Zr has been studied through the reaction⁵⁸Ni(²⁸Si, 2p 2n)⁸²Zr with a calculated cross section of 0.3% of the total fusion cross section atE(²⁸Si)=128 MeV. Gamma rays from excited states of⁸²Zr were measured and identified using the NORDBALL escape-suppression array at the Niels Bohr Institute. Reaction channel selection was performed by use of neutron and charged particle detector arrays in conjunction with the germanium detectors. A rotational band in⁸²Zr has been identified with spins up to 20 ? by analysis of reaction channel selectedγ-γ coincidence matrices. Band crossings were identified at frequencies 0.57 and 0.69 MeV.     

Directional correlations of gamma cascades in153Gd

TheEC-decay of¹⁵³Tb to the levels of¹⁵³Gd has been studied in singles and coincidence mode using high resolution x-ray andγ-ray Ge(Li) detectors. The results, in general, are in agreement with the recent findings of Tuurnalaet al. showing a very high density of low-spin states in¹⁵³Gd. The results of theγγ directional correlation experiment using two Ge(Li) detectors together with the I.C.C. data have been used in assigning the spin and parity of 5/2^?, 5/2^?, 3/2^?, 3/2⁺, 5/2⁺, 3/2⁺ and 3/2⁺ to the 41.6, 109.8, 129.1, 212.0, 303.5, 945.6 and 1102.0 keV states respectively. Multipolarity assignments to majority of theγ-transitions have been made from the I.C.C. data obtained through the knowledge of ourγ-ray intensities and the published conversion-electron intensities. From the multipole characters of theγ-transitions, parity assignments have been made to seven states in¹⁵³Gd. Our results have been discussed and compared with that of Tuurnalaet al.     

The Decay of130La to levels in130Ba

Theγ-radiation associated with the β⁺ decay of¹³⁰La to levels in¹³⁰Ba is studied. The level scheme is established on the basis of theγ-γ coincidence measurements emploing two Ge(Li) detectors. The available experimental data are compared to collective model calculations withγ-soft P.E.S. andγ-dependent inertial functions. To reproduce levels energyB _ββ value twice as large as other mass parameters is needed.Radioactivity: ¹³⁰La [from¹³⁰Ba(p, n),E _p=10MeV]; measuredE _γ,I _γ,γγ-coin.¹³⁰Ba deduced levels,J, π, logft. Enriched target, Ge(Li) detectors.     

High-spin states in the T z = −1/2 nucleus 55Ni

High-spin states of the isospin T _z = ? 1/2 nucleus ⁵⁵Ni have been identified for the first time by means of the reaction ²⁸Si(³⁶Ar,2αn) at 143 MeV beam energy. The Gammasphere array together with ancillary detectors was used to detect γ rays in coincidence with evaporated light particles. The level scheme of ⁵⁵Ni comprising four transitions is compared to its mirror partner ⁵⁵Co and shell-model calculations in the fp shell.     

Decay of90m,g Rb

The decay of mass separated⁹⁰Rb has been studied usingβ,γ and conversion electron spectrometers in single and coincidence modes. The half-lives of the ground state and the isomeric level have been confirmed to be 162± 3 and 258±4 s, respectively. Theγ-rays from the decays of ground state and metastable state have been identified by measuring the decay of individual photopeaks using Ge(Li) detectors. The multipolarity of the 106.6 keV isomeric transition has been determined to beM3. Separate decay schemes of the ground and metastable states are presented. The levels of⁹⁰Sr are discussed in terms of the available experimental and theoretical information. Radioactivity.^90m,gRb [from²³⁸U(p, f)]; measuredT _1/2,E _γ,I _γ,γ—γ coin,I _ce,cc,E _β deduced logft,⁹⁰Sr levels, mass separated⁹⁰Rb.     

Tentative level diagram of146Pm below 1.1 MeV

The level scheme of the doubly odd nucleus¹⁴⁶Pm was investigated by means of in-beam spectroscopy. The¹⁴⁶Nd(p, n) reaction in the 3.5–8 MeV proton energy interval on enriched targets has been used to populate the exited states of¹⁴⁶Pm. High resolutionγ-ray spectra using Ge(Li) and hyperpure Ge detectors and conversion electrons with a mini-orange spectrometer were measured. Withγ-exitation functions andnγ-coincidences usingnγ-discriminated NE231 neutron detectors transitions in¹⁴⁶Pm were identified,γγ-coincidences gave information aboutγ-cascades. With these data a level scheme of 38 levels below 1.1 MeV exitation energy has been deduced including 73 transitions. For 12 transitions experimental conversion coefficients have been evaluated, which agree with the expected values forM1/E2-transitions.     

Magnetic moments of excited baryons

The explanation of the nucleon’s resonance spectrum is a long standing challenge. Magnetic moments of resonances are a valuable testing ground for baryon structure calculations. We have shown in a former experiment that the extraction of the Δ⁺(1232) magnetic dipole moment is feasible via the measurement and analysis of the reaction γp→π⁰γ^′p, in which the photon γ^′ carries the desired information. Such an experiment has been repeated with high precision using the Crystal Ball and TAPS detectors at the MAMI accelerator facility. Preliminary results will be presented. Furthermore, the perspectives for accessing the magnetic moment of the S₁₁(1535) resonance in radiative η photoproduction γp→ηγ^′p will be discussed.     

Band structures in73Se

Excited states of⁷³Se have been investigated up to spin, 21/2 using techniques of in-beamγ-ray spectroscopy in connection with the⁷⁰Ge(α, n) reaction. Mean lifetimes of 12 levels have been determined applying Doppler-shift andγ-RF-methods. Five different bands have been identified that reflect a variety of different excitation modes. The decoupled 9/2⁺ band is likely to correspond to an oblate deformation while the 5/2⁺ band is interpreted as a strongly coupled prolate band built on the Nilsson configuration [422] 5/2⁺. The 3/2^? band is a strongly coupled band built on the [301] 3/2^} configuration.Nuclear reactions:⁷⁰Ge(α,n),E=14, 16, 18, 19, 20MeV; measuredE _γ,I _γ,σ(E _γ,θ),γγ-coin, linear polarization, DSA,γ(t).⁷⁵Se deduced levels,I, π, τ, δ(E2/M1), B(σλ). Enriched targets, Ge detectors.     

Gamma-gamma directional correlations and coincidence studies in154Gd

The intensities, placements andE2/M1 mixing ratios of transitions in the decay of¹⁵⁴Eu have been carefully studied to provide accurate data for microscopic calculations. Coincidence relationships in the decay of¹⁵⁴Eu have been studied extensively with a multiparameterγ-γ coincidence system with two large volume Ge(Li) detectors. Spectra in coincidence with twenty energy gates were analyzed. Twenty-nine new coincidence relationships were established and confirmed most, but not all, of several levels previously assigned by energy fits only. From an analysis of coincidence spectra and singles spectra with a 18% efficiency Ge(Li) detector new information on the gamma-ray intensities were obtained. Precise values of theE2/M1 mixing ratios of transitions from the gamma- and beta-vibrational bands to the g.s. band have been determined fromγ-γ directional correlation measurements with a NaI(Tl)-Ge(Li) detector coincidence system. Mixing ratios were obtained for a number of other transitions including those fromK ^π =0^? and 2⁺ bands from direct and skipped cascade correlations.     

Studies of γ-ray transitions in171Yb

The decay of¹⁷¹ Lu(t_1/2=8.2 day) to the levels of¹⁷¹Yb has been studied using large volume high resolution Ge(Li), x-ray Ge(Li) and NaI(Tl) detectors.γ-γ coincidence and directional correlation experiments have been performed. Spin-parity of 5/2^?, 7/2⁺, 5/2^?, 9/2⁺, 7/2^?, 7/2^?, 11/2⁺, 9/2^?, 9/2^? and 5/2^? have been assigned to the 75.87, 95.26, 122.37, 167.62, 208.00, 230.61, 259.02, 835.21, 948.37 and 1024.87 keV levels respectively. Several new coincidence relations have been observed in the present work. Two newγ-transitions at 141.45 and 821.30 keV observed and two levels at 766 and 944 keV been proposed. Multipolarity assignments have been made for most of theγ-transitions from theK-shell internal conversion coefficients obtained using the presentγ-ray intensities and the published conversion electron intensities. A revised level scheme for¹⁷¹Yb has been proposed and compared with published data. Radioactivity¹⁷¹Lu; measuredT _1/2,E _γ ,I _γ ,γγ coin,γγ (θ), I.C.C.,¹⁷¹Yb deduced levels,J, γ-mixing,π. Enriched targets, Ge(Li) detectors.     

Thermal neutron captureγ-ray spectroscopy of59Ni and61Ni

Theγ-radiation emitted after thermal neutron capture in isotopically enriched⁵⁸Ni and⁶⁰Ni was measured at the ILL high flux reactor by means of Ge/NaI detectors operated in Compton suppression and pair spectrometer mode. The neutron binding energies were determined asB _n (⁵⁹Ni)=8999.15(23) keV and B_n(⁶¹Ni)=7820.07(20) keV; some 95% of the totalγ-ray fluxes through^59,61Ni were assigned. Theγ-ray strength functions of the primary transitions and the level densities are discussed.