Precise atomic masses of neutron-rich Br and Rb nuclei close to the r-process path

The Penning trap mass spectrometer JYFLTRAP, coupled to the Ion-Guide Isotope Separator On-Line (IGISOL) facility at Jyv?skyl?, was employed to measure the atomic masses of neutron-rich ^85-92Br and ^94-97Rb isotopes with a typical accuracy less than 10keV. Discrepancies with the older data are discussed. Comparison to different mass models is presented. Details of nuclear structure, shell and subshell closures are investigated by studying the two-neutron separation energy and the shell gap energy.     

First hint on a change of the 210Po alpha-decay half-life in the metal Cu

In the metal Cu cooled to T = 12K the α-decay half-life of ²¹⁰Po nuclides, located inside the metal, was measured to be shorter by 6.3±1.4% compared to that at room temperature.     

High-precision masses of 29-33Mg and the N = 20 shell “closure”

High-precision mass measurements have been performed on the exotic magnesium isotopes ^29-33Mg using the MISTRAL radiofrequency spectrometer, especially suited for very short-lived nuclides. This method, combined with the powerful tool of resonant laser ionization at ISOLDE, has provided a significant reduction of uncertainty for the masses of the most exotic Mg isotopes: a relative error of 7×10^-7 was achieved for the weakly produced ³³Mg that has a half-life of only 90ms. Moreover, the mass of ³³Mg is found to change by over 250keV. Verifying and minimizing binding energy uncertainties in this region of the nuclear chart is important for understanding the lack of binding energy that is normally associated with magic numbers.     

Alpha decay of the new isotopes 188,189Po

New neutron-deficient isotopes ^188,189Po have been produced in the complete fusion reaction of ⁵²Cr ions with a ¹⁴²Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In ¹⁸⁹Po a ground state to ground state α decay with E_α1= 7540(20) keV, T_1/2= 5(1) ms and two fine structure α-decays at E_α2= 7264(15) keV and E_α3= 7316(15) keV have been observed. In ¹⁸⁸Po (T_1/2= 400⁺²⁰⁰ ₋₁₅₀μs) a ground state to ground state α decay at E_α= 7915(25) keV and a fine structure α decay at E_α= 7350(40) keV have been found. Improved data on the α-decay of ¹⁸⁹Bi were obtained. Received: 15 October 1999     

Mass measurements of neutron-deficient nuclides close to A = 80 with a Penning trap

The masses of ^{80, 81, 82, 83}Y, ^{83, 84, 85, 86, 88}Zr and ^{85, 86, 87, 88}Nb have been measured with a typical precision of 7keV by using the Penning trap setup at IGISOL. The mass of ⁸⁴Zr has been measured for the first time. These precise mass measurements have improved S_p and Q_EC values for astrophysically important nuclides.     

New states in heavy Cd isotopes and evidence for weakening of the N = 82 shell structure

A chemically selective laser ion source has been used in a β-decay study of heavy Ag isotopes into even-even Cd nuclides. Gamma-spectroscopic techniques in time-resolving event-by-event and multiscaling modes have permitted the identification of the first 2⁺ and 4⁺ levels in ¹²⁶Cd₇₈, ¹²⁸Cd₈₀, and tentatively the 2⁺ state in ¹³⁰Cd₈₂. From a comparison of these new states in ₄₈Cd with the E(2⁺) and E(4⁺)/E(2⁺) level systematics of ₄₆Pd and ₅₂Te isotopes and several recent model predictions, possible evidence for a weakening of the spherical N = 82 neutron-shell below double-magic ¹³²Sn is obtained. Received: 13 July 2000 / Accepted: 12 October 2000     

Observation of M3 isomeric transition from <Superscript>156m</Superscript>Pm through the βdecay of <Superscript>156</Superscript>Nd

An M3 transition in a doubly odd nucleus of ¹⁵⁶Pm was identified by internal conversion electron measurement through the β-decay of ¹⁵⁶Nd which was separated from the fission products of ²³⁵U using the on-line mass separator KUR-ISOL. The isomeric state at 150.3keV de-excites to the ground state with the M3 transition, and the spin-parity is considered to be 1^-. Nilsson configurations are also discussed on the basis of the systematics.     

High-spin states in odd-odd 168Lu

High-spin states in the odd-odd ¹⁶⁸Lu nucleus, populated in the ¹⁵⁴Sm(¹⁹F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg _7/2[404]7/2⁺⊗νi _13/2[642]5/2⁺ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh _11/2[514]9/2⁻⊗νi _13/2[642]5/2⁺ and πh _9/2[541]1/2⁻⊗νi _13/2[642]5/2⁺ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one. Received: 15 June 1998 / Revised version: 11 January 1999     

First decay scheme of 113Tc and identification of 113Ru m

Very neutron–rich fission products of the mass chain A=113 obtained from the IGISOL on–line mass separator have been investigated by γγ coincidence techniques and γ-spectra multiscaling. Gamma–rays following β–decay of ¹¹³Tc have been observed for the first time and a new 0.5 s isomeric state has been found in ¹¹³Ru. Received: 10 February 1998     

Shape coexistence near the double-midshell nucleus 111Rh

The decay of ¹¹¹Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of ¹¹¹Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at ¹⁰⁹Rh₆₄, with two neutrons less than ¹¹¹Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. Received: 25 June 1997 / Revised version: 10 September 1997     

Penning trap assisted decay spectroscopy of neutron-rich 115Ru

Exotic, neutron-rich ¹¹¹Mo and ¹¹⁵Ru nuclei, produced in proton-induced fission of ²³⁸U target, were separated with the IGISOL mass separator. The separator was coupled to the JYFLTRAP Penning trap to select the ions of a single, desired element out of the isobaric IGISOL beam. Monoisotopic samples of ¹¹⁵Ru and ¹¹¹Mo ions were observed with a microchannel plate detector after the trap or were implanted on a catcher foil for gamma- and beta-ray coincidence spectroscopy. In spite of short data taking time new gamma transitions were identified in the beta decay of very neutron-rich ¹¹⁵Ru.     

Breaking of the SU(4) limit for the Gamow-Teller strength in N ∼ Z nuclei

We have performed large-scale shell model calculations of the Gamow-Teller strength distributions in N ∼ Z pf -shell nuclei. These calculations were motivated by the experimental attempts to measure the low-lying GT strength for the even-even N = Z + 2 or N = Z - 2 nuclei ⁴⁶Ti , ⁵⁰Cr , ⁵⁴Fe and ⁶²Ge , where a sizable low-energy GT strength could be interpreted as reminiscence of SU(4) symmetry; in the limit of exact SU(4) symmetry the GT_- strength would be concentrated in a single transition to the lowest T = 0, J = 1⁺ state in the daughter. We confirm that the SU(4) symmetry is strongly broken by the spin-orbit interaction and by increasing neutron excess.     

High spin structure of 117I

High-spin states in ¹¹⁷I have been studied via the ¹⁰³Rh(¹⁸O, 4n) reaction at a beam energy of 85 MeV. Many deformed rotational bands built on the proton h_11/2, g_7/2, and g_9/2 orbitals have been identified. Among them, an unfavoured rotational band and a quasi-gamma band based on the h_11/2 state have been newly observed. Moreover, positive-parity states above I^π= 35/2⁺ reported in the previous work have been rearranged. Several energetically favoured states have been assigned to noncollective oblate states based on various quasiparticle configurations with β₂≈ 0.18–0.19 and γ= 60°. Received: 1 March 1999     

Production of heavy neutron-rich nuclei “south" of lead

The production of heavy neutron-rich nuclei is investigated using cold-fragmentation reactions. More than 25 new heavy neutron-rich nuclei were produced in the fragmentation of ²⁰⁸Pb projectiles at 1 A GeV impinging a beryllium target. The precise measurement of their production cross sections enables to benchmark the predictive power of different model calculations and make reliable predictions of the production rates expected in future radioactive beam facilities.     

Half-lives of platinum isotopes from photoactivation

The half-lives of the platinum isotopes ^189,191,197Pt and of the 4-day isomer in ^195mPt have been measured with high precision using the photoactivation technique. The γ-ray activity was counted over several half-lives with a high-purity germanium detector. The measured half-lives are: T_1/2(¹⁸⁹Pt) = 12.1 ± 1.5 h; T_1/2(¹⁹¹Pt) = 2.862 ± 0.007 d; T_1/2(¹⁹⁷Pt) = 19.96 ± 0.05 h; T_1/2(^195mPt) = 4.0104 ± 0.0047 d. Received: 13 September 1999     

Measurement of the electric quadrupole moments of 26–29Na

The nuclear electric quadrupole moments of the isotopes ²⁶Na, ²⁷Na, ²⁸Na and ²⁹Na were measured by -NMR spectroscopy in single crystals of LiNbO₃ and NaNO₃. High degrees of nuclear polarization were produced by optical pumping of the sodium atoms in a fast beam with a collinear laser beam. The polarized nuclei were implanted into the crystals and NMR signals were observed in the -decay asymmetries. Preparatory measurements also yielded improved values for the magnetic moments of ^27-31Na and confirmed the spin I=3/2 for ³¹Na. The results are discussed in comparison with large-basis shell model calculations. Received: 1 February 2000 / Accepted: 3 April 2000     

Recoil distance half-life measurements of the excited states in 145Sm

A recoil distance method was used to measure half-lives of the excited states of ₁₄₅Sm. The reaction used was ₁₃₉La(₁₀B, 4n)₁₄₅Sm. A plunger system was used. Half-lives were determined for two excited states for the first time. The yrast 27/2₊ state was found to have a half-life of 1.1 ± 0.2 ns corresponding to the retardation of 3.1 × 10₋₄ comparing with the single particle estimate of M1. The excitation energy of this state was well reproduced by the shell model calculation having a mixed configuration of [π{h^11/2(g^7/2)₋₂ (d^5/2)₋₁}¹⁰⁻, νf^7/2] + [π{h^11/2(g^7/2)₋₁}⁹⁻,νh^9/2]. Another retarded E1 transition was also found in a decay of a 21/2₊ state. Its retardation was 1.6 × 10₋₄ comparing with the single particle value. Received: 9 September 1997 / Revised version: 12 June 1998     

β-decay half-lives of very neutron-rich isotopes of elements from Ti to Ni

The unknown β-decay half-lives of 22 isotopes far off stability (5 < T _Z < 10) in the region from Ti to Ni were measured at GSI, Darmstadt. The nuclei were produced in a fragmentation reaction of 500 A.MeV ⁸⁶Kr-projectile impinging on a thick Be target. The isotopes of interest were separated and identified with the fragment separator, FRS, by a combination of Bρ,Z, and ToF techniques. An additional range separation was performed by a selective implantation into granular detectors. The spatial and time correlations of the implant with the consecutively detected β-particles were used to determine the unknown half-lives. For nuclei far off stability, β-decay chains were measured and analyzed as well, leading to an even more reliable evaluation of the lifetimes. The large discrepancies found between the measured and the theoretical values emphasize that most recent theoretical work is not an improvement over calculations made almost a decade ago. Received: 1 October 1997     

Quadrupole moments of superdeformed bands in 193Tl

Lifetimes of states in the two strongest superdeformed (SD) bands in ¹⁹³Tl were measured using the Doppler-shift attenuation method. The reaction ¹⁷⁶Yb(²³Na,6n)¹⁹³Tl at a beam energy of 129 MeV was used and γ-rays were detected by the Gammasphere array. Quadrupole moments of 18.3(10) eb and 17.4(10) eb were extracted for SD bands 1 and 2, respectively, using the fractional Doppler-shifts of the SD transitions. The previously reported linking transitions of these SD bands to normal deformed near yrast levels could not be confirmed. No other candidates for linking transitions could be established. Received: 27 April 1999