Thomas Davinson

Louvain-Edinburgh Detector Array (LEDA): a silicon detector array for use with radioactive nuclear beams

We discuss the design and implementation of the Louvain-Edinburgh Detector Array: a charged particle detector array composed of silicon-strip detectors which is used for the study of nuclear astrophysics and nuclear physics using radioactive nuclear beams at the Louvain-1a-Neuve radioactive nuclear beam facility

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

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.

Performance of the Recoil Mass Spectrometer and its Detector Systems at the Holifield Radioactive Ion Beam Facility

The recently commissioned Recoil Mass Spectrometer (RMS) at the Holifield Radioactive Ion Beam Facility (HRIBF) is described. Consisting of a momentum separator followed by an E-D-E Rochester-type mass spectrometer, the RMS is the centerpiece of the nuclear structure endstation at the HRIBF. Designed to transport ions with rigidities near K = 100, the RMS has acceptances of +/- 10% in energy and +/- 4.9% in mass-to-charge ratio. Recent experimental results are used to illustrate the detection capabilities of the RMS, which is compatible with many detectors and devices

Accelerated radioactive beams from REX-ISOLDE

In 2001 the linear accelerator of the Radioactive beam EXperiment (REX-ISOLDE) delivered for the first time accelerated radioactive ion beams, at a beam energy of 2 MeV/u. REX-ISOLDE uses the method of charge-state breeding, in order to enhance the charge state of the ions before injection into the LINAC. Radioactive singly-charged ions from the on-line mass separator ISOLDE are first accumulated in a Penning trap, then charge bred to an A/q < 4.5 in an electron beam ion source (EBIS) and finally accelerated in a LINAC from 5 keV/u to energies between 0.8 and 2.2 MeV/u. Dedicated measurements with REXTRAP, the transfer line and the EBIS have been carried out in conjunction with the first commissioning of the accelerator. Thus the properties of the different elements could be determined for further optimization of the system. In two test beam times in 2001 stable and radioactive Na isotopes (Na-23-Na-26) have been accelerated and transmitted to a preliminary target station. There Ni-58- and Be-9- and H-2-targets have been used to study exited states via Coulomb excitation and neutron transfer reactions. One MINIBALL triple cluster detector was used together with a double sided silicon strip detector to detect scattered particles in coincidence with gamma-rays. The aim was to study the operation of the detector under realistic conditions with gamma-background from the beta-decay of the radioactive ions and from the cavities. Recently for efficient detection eight tripple Ge-detectors of MINIBALL and a double sided silicon strip detector have been installed. We will present the first results obtained in the commissioning experiments and will give an overview of realistic beam parameters for future experiments to be started in the spring 2002.

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.

Proton radioactivity from highly deformed nuclei.

Proton emission half-lives are calculated within the DWBA formalism for {sup 131}Eu and {sup 141}Ho assuming permanent quadruple deformation. The decay rates are consistent with a decay from either [411 3/2] or [413 5/2] Nilsson states for {sup 131}Eu and [523 7/2] Nilsson state for {sup 141}Ho.

A double-sided silicon strip detector system for proton radioactivity studies

Abstract A new double-sided silicon strip detector has been developed to study the phenomenon of ground-state proton radioactivity. Highly proton-rich fusion-evaporation reaction products are velocity and mass separated using the Daresbury recoil separator before being implanted into the strip detector located at the separator focal plane. The double-sided strip detector has 48 strips per face with a pitch of 335 μm. Front and back strips are orthogonal providing an effective pixel area of 0.09 mm2, enabling correlations between implanted ions and subsequent decays to be clearly established. Test results obtained using the reactions 58 Ni + 92 Mo → 150 Yb ∗ and 58 Ni + 54 Fe → 112 Xe ∗ are reported. In the latter commisioning experiment the proton decay of 109I was unambiguously established using the correlated decay sequence 109I→p108Te→α104Sn.

Experimental determination of the 7Be+p scattering lengths

The Be-7 + p elastic cross section has been measured at the Centre de Recherches du Cyclotron RIB facility at Louvain-la-Neuve in the c.m. energy region from 0.3 to 0.75 MeV by bombarding a proton-rich target with a radioactive 7Be beam. The recoil protons have been detected in the angular range theta(c.m.) = 120.2degrees-131.1degrees and theta(c.m.) = 156.6degrees-170.2degrees using the LEDA system. From a R-matrix analysis of the cross section data. we obtain the energy and the width of the I resonance (E-X = 0.77 MeV). The iota = 0 scattering lengths a(01) = 25 +/- 9 fm (channel spin I = 1) and a(02) = -7 +/- 3 fm (channel spin I = 2) have been deduced. They are compared to values expected from charge-symmetry properties. Implications on the low energy S-factor of the Be-7(p,gamma)B-8 reaction are discussed

Information about the 12C(α,γ)16O reaction from the β-delayed proton decay of 17Ne

We are studying the β-delayed proton decay of 17Ne with the goal of determining the E2 part of the 12C(α,γ)16O cross section at energies relevant to helium burning in stars. We have determined branching ratios for proton and α-decay for states in 17F from 8.08 to 11.193 MeV. In addition, we have observed the break-up of the isobaric analogue state (IAS) at 11.193 MeV into three particles via two channels: proton decay to the 9.59 MeV state in 16O which breaks up into an α-particle plus 12C; and α-decay to the 2.365 MeV state in 13N which breaks up into a proton plus 12C. This is the first reported observation of the decay of the IAS to the 1− state in 16O at 9.59 MeV.