H. Schaffner

A triplet of differently shaped spin-zero states in the atomic nucleus 186 Pb

Understanding the fundamental excitations of many-fermion systems is of significant current interest. In atomic nuclei with even numbers of neutrons and protons, the low-lying excitation spectrum is generally formed by nucleon pair breaking and nuclear vibrations or rotations. However, for certain numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change. The first experimental evidence for this phenomenon came from the observation of shape coexistence in 16O (ref. 4). Other unexpected examples came with the discovery of fission isomers and superdeformed nuclei. Here we find experimentally that the lowest three states in the energy spectrum of the neutron deficient nucleus 186Pb are spherical, oblate and prolate. The states are populated by the α-decay of a parent nucleus; to identify them, we combine knowledge of the particular features of this decay with sensitive measurement techniques (a highly efficient velocity filter with strong background reduction, and an extremely selective recoil-α-electron coincidence tagging method). The existence of this apparently unique shape triplet is permitted only by the specific conditions that are met around this particular nucleus.

A Position Sensitive

The performance of a position sensitive gamma-ray scintillator detector (PSD) is described. This PSD is based on a lutetium yttrium oxyorthosilicate (LYSO) crystal read out by a crossed-wire anode position sensitive photomultiplier tube (PSPMT). The main difference with respect to similar existing devices is the individual multi-anode readout (IMAR) approach that is followed here. This method allows to exploit better the intrinsic characteristics of the PSPMT, thus yielding better linearity, improved spatial resolution, and a larger field of view. The new detector is intended for the characterization of 3-D position sensitive germanium detectors.

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A new 3D HPGe detector scanner based on PET technique has been proposed and implemented. The scanning technology utilizes a 2D position sensitive detector array made of fibre scintillation detectors ordered in cone geometry and read out by position sensitive photomultipliers. This allows simultaneous scanning of numerous coordinate points. The third coordinate of the interaction point in the germanium volume to be scanned is obtained by measuring gamma-rays Compton scattered by 90deg using collimated scintillation detectors. The first results of the proposed scanning technology clearly demonstrate the feasibility to obtain high quality data from a germanium detector within a reasonable time employing a moderate 1 MBq activity gamma source.

-Ray Scintillator Detector With Enhanced Spatial Resolution, Linearity, and Field of View

A new multiparameter experiment on ternary fission of 252Cf has recently been successfully performed by using the CODIS2 detector system and two GSI segmented Super Clover Ge γ-ray detectors. The experimental setup and the main subjects of interest are briefly described.

The GSI HPGe detector scanner - a sophisticated device based on PET technique

Dynamics of the electron spin in the Photoelectric Effect was studied in time-reversal via the process of Radiative Recombination. For this the photorecombination into an excited state of an ion and its subsequent radiative decay was observed. Pairs of correlated photons were detected in conicidence. Such an observation is the first of its kind for the initial ion as heavy as bare uranium.

Spontaneous ternary fission of 252Cf: New experiment with GSI super clover detectors

Isomeric states in the semimagic Sn128-130 isotopes were populated in the fragmentation of a Xe-136 beam on a Be-9 target at an energy of 750 A.MeV. The decay of an isomeric state in Sn-128 at an excitation energy of 4098 keV has been observed. Its half live has been determined to be T-1/2 = 220(30) ns from the time distributions of the delayed gamma rays emitted in its decay. gamma gamma coincidence relations were analyzed in order to establish the decay pattern of the newly established state toward the known (7(-)) and (10(+)) isomers at excitation energies of 2092 and 2492 keV, respectively. Based on a comparison with results of state-of-the-art shell-model calculations the new isomeric state is proposed to have the nu h(11/2)(-3)d(3/2)(-1) configuration with the four neutron holes in Sn-132 maximally aligned to a total spin of I-pi = 15(-).

First observation of correlated photons emitted by heavy highly charged ions in the process of radiative recombination

The spin and configurational structure of excited states of Cd-127, the two-proton and three-neutron hole neighbor of Sn-132, has been studied. An isomeric state with a half-life of 17.5(3) mu s was populated in the fragmentation of a Xe-136 beam on a Be-9 target at a beam energy of 750 MeV/u. Time distributions of the delayed gamma transitions and gamma gamma coincidence relations were exploited to construct a decay scheme. The observed yrast (19/2)(+) isomer is proposed to have dominant configurations of nu(h(11/2)(-3))pi(g(9/2)(-1), p(1/2)(-1)), nu(h(11/2)(-2)d(3/2)(-1))pi(g(9/2)(-2)), and nu(h(11/2)(-2), s(1/2)(-1))pi(g(9/2)(-2)) and to decay by two competing stretched M2 and E3 transitions. Experimental results are compared with the isotone Sn-129. The new information provides input for the proton-neutron interaction and the evolution of neutron hole energies in nuclei around the doubly magic Sn-132 core. (Less)