Philip Woods

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

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.

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.

Long range absorption in the scattering of 6He on 208Pb and 197Au at 27 MeV

Quasi-elastic scattering of 6He at E_lab=27 MeV from 197Au has been measured in the angular range of 6-72 degrees in the laboratory system employing LEDA and LAMP detection systems. These data, along with previously analysed data of 6He + 208Pb at the same energy, are analyzed using Optical Model calculations. The role of Coulomb dipole polarizability has been investigated. Large imaginary diffuseness parameters are required to fit the data. This result is an evidence for long range absorption mechanisms in 6He induced reactions. Comment: 10 pages, 10 figures, minor corrections. To appear in Nucl. Phys. A

Proton radioactivity from 146Tm. The completion of a sequence of four odd-odd proton emitters

Abstract Proton emission has been observed from the new isotope 146 Tm. Two transitions have been identified at energies of 1119±5 keV and 1189±5 keV corresponding to Q -values of 1127±5 keV and 1197±5 keV respectively. The measured half-lives of 235±27 ms and 72±23 ms are well reproduced by simple WKB calculations assuming proton emission from h 11 2 orbitals. The proton transitions are assigned to a 10 + isomeric state and a low lying 5 − or 6 − level.

New limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate from direct measurements using radioactive beams

Inexplosive stellar hydrogen burning, the hot CNO cycles and the rp-process are mainly linked by the reaction sequence O-15(alpha,gamma) Ne-19(p,gamma)Na-20. Using intense Ne-19 radioactive beams, both the Ne-19(p,gamma) and the Ne-19(d,n) reaction have been studied. Upper and lower limits for the Ne-19(p,gamma) reaction rate have been deduced, allowing to conclude that the O-15(alpha,gamma) reaction is most likely the bottleneck reaction.

First Measurement of Several β-Delayed Neutron Emitting Isotopes Beyond N=126

The β-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with β-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb, and Bi in the mass region N≳126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the β-decay strength distribution. This provides important constraints on global theoretical models currently used in r-process nucleosynthesis.