H.G. Thomas

Encapsulated Ge detectors: Development and first tests

Abstract This paper discusses the development and the realisation of an encapsulated Ge detector. The properties of a first prototype detector are; relative efficiency e rel = 21.4%; energy resolution: Δ E γ(1.33MeV) = 2.10 keV and Δ E γ(122 keV) = 1.10 keV. The encapsulation technology offers advantages; several encapsulated Ge detectors can be mounted closely packed in a cryostat, all electronic parts of the first preamplifier stage are accessible and induced radiation damage of the Ge crystal can be annealed in a standard furnace.

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.

Development of a composite Ge detector for EUROBALL

Abstract The aim of the EUROBALL project is the development of γ-ray detectors with substantially improved qualities, and the realization of large solid angle arrays, in particular of 4π spectrometers employing these detectors. This article especially focusses on the development of a Cluster detector consisting of seven large hexagonal tapered and encapsulated Ge detectors which are surrounded by a common BGO anti-Compton shield. First results of the properties of a hexagonal tapered Ge detector and of an encapsulated Ge detector are presented.

Development of segmented Ge detectors for future γ-ray arrays

The EUROBALL Cluster detector is composed of seven encapsulated Ge detectors in a common cryostat with a total volume of 2000 ccm of HP Gemanium. The development and the performance of the Cluster detector is summarized. Up to six Cluster detectors were used in pre-EUROBALL experiments at the S-DALINAC Darmstadt, at the tandem-postaccelerator facility of the MPI-K Heidelberg and at the UNILAC at GSI. Examples of these experiments - the excitation of dipole modes with (γ, γ′)-reactions and the first coincidence spectroscopy of the N=Z-nucleus 68Se with a CLUSTER Cube - are discussed. The development of segmented encapsulated Ge detectors for a MINIBALL at the radioactive beam facility REX-ISOLDE has been launched. The status of the project is presented.

The response of the Euroball Cluster detector to γ-radiation up to 10 MeV

The Euroball Cluster detector consists of seven closely packed hexagonal encapsulated germanium detectors which are mounted in a common cryostat and surrounded by a BGO suppression shield. By adding back the Compton scattered events between the seven germanium detectors the detector system behaves similar as a single germanium detector with a volume of 2000 cm3. The response of this very large germanium detector to γ-rays up to 10 MeV has been measured using a short lived 24Al source produced at the Cologne FN Tandem accelerator. At 10 MeV the efficiency of the Cluster detector is nearly one order of magnitude larger than that of a 30% germanium detector.

Fine structure of the E1 response in 140Ce below the particle threshold

Abstract The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

Delayed g9/2 Alignment in the N=Z Nucleus 72Kr

The structure of Kr-72 has been investigated at GASP through the Ca-40(Ca-40,2 alpha) reaction at a beam energy of 160 MeV using the 4 pi ISIS Si-ball for reaction channel selection. The level scheme has been extended up to an excitation energy of approximate to 8.5 MeV. The spins and parities of the observed levels are assigned tentatively. The observed band shows the predicted change from oblate to prolate shape. The four quasi-particle g(9/2) alignment is found to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of standard mean-field calculations and may reflect either additional correlations in the T = 0 pairing channel or coupling to vibrational degrees of freedom or both

Two-phonon J = 1 states in even-mass Te isotopes with A = 122–130☆

Abstract Excited states of the nuclei 122,126,130 Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

Two Compton polarimeter constructions for modern standard γ-spectroscopy

Abstract Two types of Compton polarimeters for γ-linear polarization measurements are presented. The first one consists of standard Ge-detectors and unites an easy handling and high grade of universality with high polarization sensitivity in the γ-ray energy range 180–2500 keV. In this device, a low efficiency Ge-crystal scatterer is used with a special diameter-to-length ratio which optimizes the probability for Compton scattering. In addition, the background in the summed energy spectra is significantly reduced by requiring coincident γ-ray energies to obey the Compton scattering formula. The second device fits the need of accessory units which can be used with the new generation of multi-detector arrays. It is based on a segmented planar Ge-crystal surrounded by an anti-Compton shield and possesses a high coincidence efficiency in the 50–1000 keV range. For both spectrometers we performed calibration measurements to obtain the sensitivity Q and the coincidence efficiency e c . From these values we deduced the “figure of merit” F = Q 2 e c which gives a kind of quality description for polarimeters.

Change of the dipole strength distributions between the neighbouring γ-soft nuclei 194Pt and 196Pt☆

Abstract A nuclear resonance fluorescence experiment with two highly efficient EUROBALL Cluster detectors has been performed on the γ -soft nucleus 194 Pt. Dipole excitations were observed between 2 and 4 MeV excitation energy. They are tentatively interpreted as the main fragments of the scissors mode based on the measured excitation strengths and a comparison to microscopic calculations in the framework of the quasiparticle random phase approximation (QRPA). The data indicate large differences to the neighbouring isotope 196 Pt: a doubling of the observed dipole strength and a shift of the energy centroid by about 600 keV. None of the currently available models is able to reproduce these features consistently in both nuclei.