Ch. Gund

MINIBALL A Ge detector array for radioactive ion beam facilities

Abstract Dedicated Ge-detector arrays are being developed for the investigation of rare γ decays with low γ-ray multiplicity at the upcoming radioactive ion beam facilities. These arrays are optimized for the high full-energy peak efficiency and angular resolution of the γ-ray detection needed for a proper Doppler correction of the γ-rays emitted by fast recoiling nuclei. MINIBALL will consist of 40 six-fold segmented, encapsulated Ge detectors which are clustered in eight cryostats with three detectors each and four cryostats with four detectors, respectively. The individual components - the six-fold segmented Ge detector, the cryostats, the fast preamplifier, the digital pulse-processing electronics and the mechanical frame - and their properties are described. The results of test measurements with the first MINIBALL cluster detector using a 137 Cs source and the in-beam reaction D( 37 Cl, n) 38 Ar are presented. It is shown that from pulse-shape analysis of the events within a detector segment the effective granularity of the MINIBALL array can be enhanced from 240 to ∼ 4000. The specifications of MINIBALL are compiled on the basis of experimental data. First results with a 12-fold segmented, encapsulated detector are discussed with respect to the feasibility of future γ-ray tracking arrays.

The REX-ISOLDE project

The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE.

Physics with Ge-Miniball Arrays

For the measurement of rare gamma decays with low gamma-ray multiplicities (M-gamma < 15) dedicated Ge-detector arrays are being developed. They are optimized with respect to the full-energy peak efficiency rather than the resolving power. These so-called miniball arrays will pre dominantly be used at radioactive ion beam facilities. The detector arrays will have a high granularity using segmented Ge-detectors to provide a sufficient angular resolution of the gamma rays which is needed for proper Doppler-shift corrections and small Doppler-broadening. We will describe shortly the MINIBALL presently developed by a German, Belgian, and Italian collaboration and the wide range of physics to be addressed with this new detector system.

A position sensitive parallel plate avalanche counter for single particle and current readout

A parallel plate avalanche counter (PPAC) has been developed to serve as a radioactive beam monitor detector for the REX-ISOLDE project at CERN. The PPAC has a compact design with an active diameter of 4 cm, a low e!ective thickness of &1 mg/cm2, a spatial resolution in the x and y directions of 1.6 mm, an applicability for all heavy ions with Z52, and a large dynamical range in counting rates of up to 109 s~1 due to single particle or current readout, respectively. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx; 25.60.!t

The MINIBALL array

Abstract MINIBALL a highly efficient, position sensitive γ -spectrometer is under construction for upcoming radioactive-beam experiments. In contrast to high spin arrays the total full energy peak efficiency and not the resolving power is optimized with a factor of two larger efficiency for γ -ray energies up to 10 MeV and for low to medium γ -ray multiplicity events. Cluster detectors, incorporating three or four Ge crystals, will be employed to provide high flexibility and to accomodate the various demands of the experiments.

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

Abstract:As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 106 particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on 2H and 9Be targets at REX-ISOLDE energies of 2.2 MeV . A are confirmed.

Miniball: A Gamma-Ray Spectrometer With Position-Sensitive Ge Detectors For Nuclear Structure Studies At REX-ISOLDE

Miniball is a dedicated Ge detector array which has been developed for the investigation of rare γ decays at the new radioactive beam facility REX‐ISOLDE [1, 2] at CERN. The array is optimised for high full‐energy peak efficiency and for high granularity needed to perform Doppler corrections of γ‐rays emitted by fast moving nuclei. Miniball will finally consist of 40 six‐fold segmented, encapsulated detectors which are clustered in eight cryostats with three detectors each and four cryostats with four detectors, respectively. It is shown that from an analysis of the pulse shapes and of the amplitudes of the mirror charges in the adjacent segments the effective granularity of Miniball can be enhanced from 240 to ≈ 4000. The properties of Miniball are compiled on the basis of experimental data. Examples of the first data measured with Miniball are presented.

Investigations of 159-163Dy using Incomplete Fusion Reactions

The isotopes 159–163Dy have been investigated using the incomplete fusion reactions 7Li → 158,160Gd at beam energies of 8 MeV/u. The γ-rays were detected in the GASP spectrometer in coincidence with the fast charged particles detected in the ISIS silicon ball. The level schemes of all five isotopes could be amply extended. The most important result was the observation of both the ground state and Stockholm bands in 162Dy up to high spin covering the band crossing region. The crossing in this nucleus is shifted to a higher frequency in comparison to the lighter Dy isotopes. The interaction strength between the two bands could be determined with high precision and with this result, a full oscillation of the interaction strength from one node to the next within an isotopic chain could be observed for the first time.

RES-ISOLDE - Post-accelerated radioactive beams at CERN-ISOLDE

The ISOLDE RIB-facility at CERN has today been producing a vast range of radioactive beams since more than 30 years. The low-energy beams of ISOLDE will be complemented by a post-accelerator, REX-ISOLDE, currently being assembled. In order to convert the pseudo-DC, singly-charged beam from the ISOLDE mass separators into a cooled and bunched beam at higher charge states, a novel scheme of trapping, cooling, and charge-state breeding has been devised, using a linear Penning trap and an Electron Beam Ion Source (EBIS). This allows for subsequent acceleration by a short, cost-effective LINAC consisting of an RFQ, an IH-structure and three seven-gap resonators, reaching 0.8–2.2 MeV/u. The installation of REX-ISOLDE is well underway and the first post-accelerated radioactive beams are expected to be obtained during late 2000.

New insights in the first backbending region of heavy Dy isotopes

SummaryThe7Li-induced incomplete fusion reaction158/160Gd(7Li,p(d,t)xn) (beam energy 56 MeV) has been used to investigate in detail the first backbending region of the heavy stable Dy nuclei160/162Dy. In162Dy, the measured low interaction strength between the ground band and the Stockholm band of |Vgs| ≈ 17 keV confirms the predicted existence of a node in the interaction strength for nuclei with neutron number betweenN = 96 andN = 98, which has not been accessed earlier. The crossing frequency between the ground band and the Stockholm band in162Dy was measured to be hwc = 320 keV, which is about 40 keV higher than the expected value of hwc = 280 keV known from other nuclei in this mass region. In160Dy, an yrare band could be established for the first time in a nucleus with a very large interaction strength. From fits to level energies as well as from branching ratios, an interaction strength of |Vgs| ≈ 220 keV was extracted.