Ch. Theisen

SAPhIR: a fission-fragment detector

SAPhIR is the acronym for S_aclay A_quitaine P_h_otovoltaic cells for I_somer R_esearch. It consists of solar cells, used for fission-fragment detection. It is a collaboration between 3 laboratories: CEA Saclay, CENBG Bordeaux and CEA Bruyeres le Châtel. The coupling of a highly efficient fission-fragment detector like SAPhIR with EUROBALL will provide new insights in the study of very deformed nuclear matter and in the spectroscopy of neutron-rich nuclei.

Investigation of 246Fm : in‐beam spectroscopy at the limits

The structure of 246Fm has been investigated using in‐beam γ‐ray spectroscopy. The experiment was performed at the University of Jyvaskyla using JUROGAM 2 coupled to RITU and GREAT. The 246Fm nuclei were produced using a 186 MeV beam of 40Ar impinging on a target of 208Pb. JUROGAM 2 was fully instrumented with TNT2D digital acquisition cards. The use of digital acquisition cards and a rotating target allowed for unprecedented beam intensities up to 71 particle‐nanoamperes for prompt γ‐ray spectroscopy. With all these major advances for spectroscopy a rotational band is observed for the first time in 246Fm and discussed here.

Gamma‐ray Spectroscopy Of Very Neutron‐Deficient Bi Isotopes

Prompt and delayed γ rays from in 191,193Bi have been identified using the recoil‐decay tagging (RDT), isomer tagging, and recoil gating techniques, resulting in extensive level schemes for both nuclei. Excitation energies of the isomeric 13/2+ states have been established and rotational bands based on them have been observed. The nearly spherical 9/2− ground‐state bands appear to be crossed by more deformed low‐lying structures. TRS calculations have been performed and B(M1)/B(E2) ratios have been extracted for the observed strongly‐coupled bands. In 193Bi, a 3 μs isomer has been interpreted as a h9/2 proton coupled to the 12+ state of the 192Pb core.

Excited states in the heavy nuclide 254No

In-beam γ-ray spectroscopy of the excited states in the heavy nuclide 254No have been studied in the reaction 208Pb(48Ca,2n)254No. The techniques of recoil-gating and recoil-decay-tagging were needed due to the dominant fission background. Prompt γ-rays were detected with a Ge detector array, consisting of four clover detectors in close geometry, and a gas-filled recoil separator (RITU) was used for detecting recoils and their α-decays. The observed six γ-rays were associated with E2-transitions in the ground state rotational band of 254No. The value β2=0.27±0.03 was extracted for the quadrupole deformation from the extrapolated 2+ excitation energy.

In‐beam spectroscopy of 254No

Evidence for the presence of non‐yrast states in 254No has been observed in two different in‐beam experiments carried out at the University of Jyvaskyla. Firstly an in‐beam conversion electron spectroscopic study was performed with the SACRED spectrometer, revealing a distribution consisting of high‐multiplicity electron cascades. This provides an indirect proof of the presence of high K‐bands in 254No. In a more recent in‐beam γ‐ray spectroscopic measurement, evidence for the decay of non‐yrast states in 254No has been observed for the first time. It is speculated that the observation of high‐energy γ rays is due to the decay of a K = 3 band‐head state.

Recoil-isomer tagging techniques at RITU

Techniques have been developed to study isomeric states in nuclei with the use of RITU (gas filled separator) at the University of Jyvaskyla. The first was the recoil-isomer tagging technique initially, utilised by D.M. Cullen to study the K π = 8− isomeric state in 138Gd [1]. The juro-sphere array was employed in conjunction with ritu and a focal plane array which consisted of several Compton-suppressed Germanium detectors, placed in close geometry around a multi wire proportional counter (mwpc) and a silicon strip detector used for the implantation of recoiling nuclei. This technique correlates prompt and delayed γ-ray transitions across isomeric states and identifies the lifetime of the isomer. The main constraint of this technique is the timing electronics. Only isomers with a half-life greater than the flight time through RITU(~ 300ns) and less than the collection time of the tdc’s (32 μs) can be identified. This time range and hence technique is ideal for the region of mass A = 140 near proton drip line nuclei which have small production cross-sections in comparison to their more neutron rich neighbours and lack charged particle emission suitable for recoil-decay-tagging. Subsequent experiments to ref.

Spectroscopy of very neutron-deficient 187,189Bi isotopes

Shape coexistence is well known to occur in nuclei, in particular near closed shells [1], where particle-hole excitations across the shell gap can create deformed intruder states. In the neutron-deficient lead isotopes (Z = 82), deformed structures appear at low excitation energy. The isotope 188Pb [2] shows for example a triple shape coexistence with oblate and prolate excited 0+ states that compete with the spherical ground state. The study of the odd-proton single-particle excitations in Bi isotopes allows to obtain information on the orbitals involved in the different shapes observed in this mass region.

γ-Ray Studies of Induced Fission 12C+238U

A fission fragment detector, SAPhIR, using photovoltaic cells has been developed and used with the multidetector array EUROBALL to select fission fragments produced in the 12C+238U eaction. These fission fragments, belonging to different regions of the mass chart, are very neutron rich.Their structure and, in particular, isomers identified in the range of 20ns up to 2μs, have been studied.

Indication of triple-shape coexistence in 188Pb

By using conversion electron and γ-ray spectroscopy combined with a recoil tagging method, excited states in 188Pb has been investigated. The existence of two excited low-lying 0+ states have been confirmed giving a new energy for the 03+ state, which are interpreted as evidence for a triple-shape coexistence in 188Pb.