P. H. Regan

The GREAT spectrometer

Abstract The GREAT spectrometer is designed to measure the decay properties of reaction products transported to the focal plane of a recoil separator. GREAT comprises a system of silicon, germanium and gas detectors optimised for detecting the arrival of the reaction products and correlating with any subsequent radioactive decay involving the emission of protons, α particles, β particles, γ rays, X-rays or conversion electrons. GREAT can either be employed as a sensitive stand-alone device for decay measurements at the focal plane, or used to provide a selective tag for prompt conversion electrons or γ rays measured with arrays of detectors deployed at the target position. A new concept of triggerless data acquisition (total data readout) has also been developed as part of the GREAT project, which circumvents the problems and limitations of common dead time in conventional data acquisition systems.

The GREAT triggerless total data readout method

Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment.

NEW ISOTOPES AND ISOMERS PRODUCED BY THE FRAGMENTATION OF 238U AT 1000 MEV/NUCLEON

Abstract Seven previously unobserved neutron-rich isotopes ( 209 Hg, 210 Hg, 211 Tl, 212 Tl, 218 Bi, 219 Po and 220 Po) have been identified among the fragmentation products of a 1000 MeV/nucleon 238 U beam incident on a beryllium target. The γ -ray decays of 9 known and 4 new μ s-isomers in nuclei around 208 Pb were also observed. Production cross sections and probabilities for populating isomeric states in the fragmentation reaction were determined and compared with a simple model based on the statistical abrasion-ablation approach.

Superallowed Gamow-Teller decay of the doubly magic nucleus 100Sn

The shell structure of atomic nuclei is associated with ‘magic numbers’ and originates in the nearly independent motion of neutrons and protons in a mean potential generated by all nucleons. During β+-decay, a proton transforms into a neutron in a previously not fully occupied orbital, emitting a positron–neutrino pair with either parallel or antiparallel spins, in a Gamow–Teller or Fermi transition, respectively. The transition probability, or strength, of a Gamow–Teller transition depends sensitively on the underlying shell structure and is usually distributed among many states in the neighbouring nucleus. Here we report measurements of the half-life and decay energy for the decay of 100Sn, the heaviest doubly magic nucleus with equal numbers of protons and neutrons. In the β-decay of 100Sn, a large fraction of the strength is observable because of the large decay energy. We determine the largest Gamow–Teller strength so far measured in allowed nuclear β-decay, establishing the ‘superallowed’ nature of this Gamow–Teller transition. The large strength and the low-energy states in the daughter nucleus, 100In, are well reproduced by modern, large-scale shell model calculations.

Discovery and cross-section measurement of neutron-rich isotopes in the element range from neodymium to platinum with the FRS

Abstract:Using the high-resolution performance of the fragment separator FRS at GSI we have discovered 60 new neutron-rich isotopes in the atomic number range of 60⩽Z⩽78. The new isotopes were unambiguously identified in reactions with a

First observation of a collective dipole rotational band in the A∼200 mass region

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O,5n) 198 Pb reaction at beam energies of 92 and 98 MeV. A highly regular rotational ΔI =1 band has been found. Angular correlation measurements confirm the transitions to be dipoles. Assuming M1 multipolarity for these transitions the experimental data are interpreted in terms of a π([505] 9 2 − ⊗[606] 13 2 + ) K π =11 − ⊗ν( i 13 2 ) 2 configuration of oblate shape. This is the first such collective oblate structure identified in the A ∼200 mass region.

K-forbidden transitions from multi-quasiparticle states

Abstract Four-quasiparticle, K π = 12 + states in 172 Hf and 178 W, with half-lives less than 2 ns, are found to decay to their respective K π = 0 + ground-state bands, in competition with transitions to intermediate- K , two-quasiparticle structures. All decay transitions are weakly hindered. When taken together with other K -isomer decay rates, an overall trend of decreasing hindrance with increasing excitation energy is evident. Estimates based on density-of-states considerations approximately reproduce the trend of the data, but also indicate the need to include other K -mixing effects.

Collective oblate dipole rotational bands in 198Pb

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O, 5n) 198 Pb reaction at beam energies of 92 and 98 MeV. Five collective rotational cascades of ΔI = 1 transitions have been found. Four are highly regular, one much more irregular. The structures are incorporated into a level scheme which extends up to approximately spin 32ħ and an excitation energy of about 10 MeV. Angular correlation measurements confirm the dipole character of the interband transitions. Their M1 multipolarity is inferred, and from this supposition the experimental data are interpreted in terms of oblate high- K two quasiproton configurations coupled to aligned neutron excitations. This interpretation is extended to include other ΔI = 1 oblate structures observed in 194–201 Pb. It is shown that the pattern of observed moments of inertia can be understood in the simple unpaired picture involving neutron i 13 2 excitations. The identical bands observed are interpreted in terms of the normal-parity weakly-coupled singlet orbital.

Competing T=0 and T=1 structures in the N=Z nucleus 6231Ga

Abstract The low-lying levels in the odd-odd N = Z nucleus 62 Ga have been identified for the first time. These data reveal a cascade of stretched-E2 transitions based on a T =0, 1 + bandhead which decays directly to the T =1, 0 + ground state. The observed levels are interpreted in the context of theshell model, using as a basis, the pf 5/2 g 9/2 orbits with a 56 Ni core.

Determination of the natural radioactivity levels in north west of Dukhan, Qatar using high-resolution gamma-ray spectrometry ☆

This study is aimed at the determination of the activity concentrations of naturally occuring and technologically enhanced levels of radiation in 34 representative soil samples that have been collected from an inshore oil field area which was found to have, in a previous study, the highest observed value of 226Ra concentration among 129 soil samples. The activity concentrations of 238U and 226Ra have been inferred from gamma-ray transitions associated with their decay progenies and measured using a hyper-pure germanium detector. Details of the sample preparation and the gamma-ray spectroscopic analysis techniques are presented, together with the values of the activity concentrations associated with the naturally occuring radionuclide chains for all the samples collected from NW Dukhan. Discrete-line, gamma-ray energy transitions from spectral lines ranging in energy from ∼100 keV up to 2.6 MeV have been associated with characteristic decays of the various decay products within the 235.8U and 232Th radioactive decay chains. These data have been analyzed, under the assumption of secular equilibrium for the U and Th decay chains. Details of the sample preparation and the gamma-ray spectroscopic analysis techniques are presented. The weighted mean value of the activity concentrations of 226Ra in one of the samples was found to be around a factor of 2 higher than the values obtained in the previous study and approximately a factor of 10 higher than the accepted worldwide average value of 35 Bq/kg. The weighted mean values of the activity concentrations of 232Th and 40K were also deduced and found to be within the worldwide average values of 30 and 400 Bq/kg, respectively. Our previous study reported a value of 201.9±1.5Stat.±13Syst.Bq/kg for 226Ra in one sample and further investigation in the current work determined a measured value for 226Ra of 342.00±1.9Stat.±25Syst.Bq/kg in a sample taken from the same locality. This is significantly higher than all the other investigated soil samples in the current and previous works. Notably, the Th levels in the same sample are within the worldwide average expectations, implying that the increased 226Ra concentration arises from TENORM processes.