H.A. Roth

The EUROBALL neutron wall - design and performance tests of neutron detectors

The mechanical design of the EUROBALL neutron wall and neutron detectors, and their performance measured with a Cm-246,Cm-248 fission source are described. The array consists of 15 pseudohexaconical detector units subdivided into three, 149 mm high, hermetically separated segments and a smaller central pentagonal unit subdivided into five segments. The detectors are filled with Bicron BC501A liquid scintillator. Each section of the hexaconical detectors is viewed by a 130 mm diameter Philips XP4512PA photomultiplier while the sections of pentagonal detectors are viewed by Philips XP4312B PMTs. The tests of n-gamma discrimination performed by zero-crossing and time-of-flight methods show a full separation of gamma- and neutron events down to 50 keV recoil electron energy. These tests demonstrate the excellent timing properties of the detectors and an average time resolution of 1.56 ns. The factors determining the efficiency of neutron detectors are discussed. The total efficiency for the full array for a symmetric fusion-evaporation reaction is predicted to be 0.30

A 2π neutron and γ-ray multiplicity filter for the NORDBALL detection system

Abstract A description of the 2π neutron and γ-ray multiplicity filter designed as a part of the NORDBALL detection system is given. The filter consists of ten hexagon and six pentagon shaped detectors filled with BC501 liquid scintillator. Individual detector properties as well as system behaviour including neutron scattering are discussed and test results are presented. The properties of the n-γ pulse shape discriminators used for the system were studied. The discriminators, based on the zero-crossing principle, showed a very efficient separation in the 100:1 dynamic range of neutrons and γ-rays starting from 70 keV of recoil electron energy. Results from a use of the filter in a study of the 28 Si + 58 Ni reaction are presented.

Cd-98(48)50: The two-proton-hole spectrum in Sn-100(50)50

Excited states in {sup 98}Cd, two proton holes from {sup 100}Sn, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the ({pi}g{sub 9/2}){sup {minus}2} two-proton-hole spectrum below a T{sub 1/2}=0.48(16) {mu}s isomer is deduced and compared to shell-model predictions. A tentative I{sup {pi}}=(8{sup +}) assignment, as suggested by systematics, yields a strongly reduced B(E2,8{sup +}{r_arrow}6{sup +})=0.44({sup +20}{sub {minus}10}) W.u., corresponding to an effective proton charge of e{sub {pi}}=0.85({sup +20}{sub {minus}10}) e , which is at variance with existing theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}Excited states in Cd-98, two proton holes from Sn-100, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the (pi g(9/2))(-2) two-proton-hole spectrum below a T-1/2 = 0.48(16) mu s isomer is deduced and compared to shell-model predictions. A tentative I-pi = (8(+)) assignment, as suggested by systematics, yields a strongly reduced B(E2,8(+) --> 6(+)) = 0.44((+20)(-10)) W.u., corresponding to an effective proton charge of e(pi) = 0.85((+20)(-10))e, which is at variance with existing theoretical predictions.

Experimental determination of cross-talk between neutron detectors

Neutron cross-talk measurements have been performed with 14.1 MeV neutrons for organic scintillator detectors with n-gamma discrimination capability. The probability for cross-talk between two 156 mm thick neutron detectors with a hexagonal cross section (effective diameter 167 mm) placed 0.33 m apart was found to be (2.5+/-1.1) 10(-4). A similar measurement between two 100 mm thick, cylindrically shaped detectors with 50 mm diameter and at 75 mm distance from each other yielded a cross-talk probability of (2.5+/-0.4) 10(-3). The measured cross-talk probabilities are in agreement with values from a Monte Carlo calculation.

Proton-neutron interaction at N≅Z — First observation of the Tz=1 nucleus 46 94 Pd48 in-beam

The neutron deficient nucleus94Pd was identified and studied for the first time by in-beam spectroscopy. An Iπ=(14+) isomer with t1/2=0.8 (2) μs was observed in a recoil catcher setup inside the multi-detector γ-array OSIRIS. Filter detectors for neutrons and charged particles were used to identify the (2p2n) exit channel of the reaction58Ni+40Ca, populated with only 0.06% of the total evaporation residue cross section. The structure of the isomer is discussed within the frame work of shell model calculations in the (p1/2,g9/2) model space with emphasis on the g92/2 T=0, I=1,9 ”pairing” two-body matrix elements.

High-spin studies of the neutron deficient nuclei In-103, In-105, In-107, and In-109

High-spin states of the isotopes In-103,In-105,In-107,In-109 have been investigated using in-beam gamma-ray spectroscopic methods. Results from three different experiments are presented. Targets of Fe-54, Cr-50, and Mo-92 were bombarded by a 270 and 261 MeV Ni-58 beam and by a 95 MeV F-19 beam, respectively. Reaction channel separation was achieved with a charged-particle detector array and in the first two experiments also with a 1 pi neutron detector system. As a result of these experiments the level schemes of In-103,In-105,In-107,In-109 were significantly extended. Excited states of these odd-A indium isotopes are discussed within the framework of the nuclear shell model and the hole-core coupling scheme. The systematics of excited states of light odd-A indium isotopes is also discussed

In-beam gamma-ray spectroscopy of Cd-102

Neutron deficient nuclei close to 100Sn have been studied using the NORDBALL Ge-detector array together with ancillary particle detectors. Evaporation residues from the compound nucleus 108Te were identified with charged particle and neutron detectors. In this paper a considerable extension of the level scheme of the nucleus 102Cd is presented. The strongest cascade of the new level scheme reveals an irregular sequence of dipole transitions above Iπ = 10+ extending up to spin 17. A strongly populated rather regular side band consisting of four quadrupole transitions ranging from spin 9 to spin 17 was also discovered. This band was tentatively assigned negative parity. Shell model calculations were performed and a very good agreement with the experimental results was found. The excited states could successfully be interpreted as neutron particle and proton hole excitations with respect to the doubly closed core 50100Sn50.

Band structures of nuclei around N, Z = 40 selected by charged particle detection

Abstract High-lying and high spin states of nuclei around N,Z=40 have studied using the ( 58 Ni + 28 Si) and ( 58 Ni + 32 S) reactions and NORDBALL Ge-Array equipped with a Silicon Ball for light charged particle detection. Simultaneously complementary experiment have been carried out using in-beam and β decay measurements to establish lifetimes and spin-party values of low-lying and ground states at Kyushu University. Some of results are reported.

Multiparticle-hole states of high spin in N < 50, A ≈ 90 nuclei: 1. The transitional nucleus 4389Tc46

Abstract High spin states in the nucleus 89 Tc have been studied via the fusion evaporation reaction 58 Ni( 40 Ca,2αp) 89 Tc at 180 MeV beam energy. The NORDBALL γ-ray spectrometer equipped with auxiliary detectors for light particle selection was used to measure γγ- and particle-γγ coin-cidences. Some 60 transitions were placed into a level scheme comprising 38 levels reaching up to 9.2 MeV excitation energy and a possible spin of I = 45/2 h . The level scheme is compared to those of neighbouring nuclei and interpreted in terms of the spherical shell model. The calculations were performed with different sets of parameters within a restricted π(p 1 2 ) , π(g 9 2 ) , ν(p 1 2 ) and ν(g 9 2 ) configuration space. States above 2.3 MeV excitation energy are well reproduced by shell model calculations based on an empirical residual interaction, whereas collective excitations are suggested to contribute to the wave functions of lower lying states.

First Identification and Shell Model Structure of 92Rh

Excited states in theN=47 nucleus92Rh were populated via the fusion evaporation reaction58Ni(40Ca,αpn)92Rh at 180 MeV beam energy. A level scheme reaching up to about 7 MeV and probable spins of (19+) and (21−) was established and interpreted with the shell model in the (p1/2,g9/2) model space.