R. D. Page

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.

Nuclear isomers in superheavy elements as stepping stones towards the island of stability

A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the ‘island of stability’; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons—the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.

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.

Search for 28O and study of neutron-rich nuclei near the N = 20 shell closure

Abstract A search for 28 O with a 78 AMeV beam of the neutron-rich isotope 36 S has been performed for the first time. Evidence for the unbound character of 28 O was obtained. In the same experiment the half-lives of the very neutron-rich isotopes 27,29 F and 30 Ne were measured and those for 28,29 Ne and 30,31 Na reexamined. The results are compared to shell-model predictions and conclusions drawn regarding the extent of the region of deformation around N = 20.

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.

Early onset of ground state deformation in neutron deficient polonium isotopes

In-source resonant ionization laser spectroscopy of the even-A polonium isotopes (192-210,216,218)Po has been performed using the 6p(3)7s (5)S(2) to 6p(3)7p (5)P(2) (λ=843.38  nm) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in (200-210)Po with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.

Orbital dependent nucleonic pairing in the lightest known isotopes of tin

By studying the (109)Xe→(105)Te→(101)Sn superallowed α-decay chain, we observe low-lying states in (101)Sn, the one-neutron system outside doubly magic (100)Sn. We find that the spins of the ground state (J=7/2) and first excited state (J=5/2) in (101)Sn are reversed with respect to the traditional level ordering postulated for (103)Sn and the heavier tin isotopes. Through simple arguments and state-of-the-art shell-model calculations we explain this unexpected switch in terms of a transition from the single-particle regime to the collective mode in which orbital-dependent pairing correlations dominate.

Production of antihydrogen at reduced magnetic field for anti-atom trapping

We have demonstrated production of antihydrogen in a 1 T solenoidal magnetic field. This field strength is significantly smaller than that used in the first generation experiments ATHENA (3 T) and ATRAP (5 T). The motivation for using a smaller magnetic field is to facilitate trapping of antihydrogen atoms in a neutral atom trap surrounding the production region. We report the results of measurements with the Antihydrogen Laser PHysics Apparatus (ALPHA) device, which can capture and cool antiprotons at 3 T, and then mix the antiprotons with positrons at 1 T. We infer antihydrogen production from the time structure of antiproton annihilations during mixing, using mixing with heated positrons as the null experiment, as demonstrated in ATHENA. Implications for antihydrogen trapping are discussed.

In-beam spectroscopy using the JYFL gas-filled magnetic recoil separator RITU

The techniques of recoil-gating and recoil-decay tagging have been employed at Jyvaskyla to perform in-beam γ-ray and electron spectroscopy studies of heavy nuclei. The JUROSPHERE γ-ray array and the SACRED electron spectrometer have been placed at the target position of the JYFL gas-filled recoil separator recoil ion transport unit (RITU). The RITU separator has been used to collect the recoils of interest and separate them from beam particles and fission products. At the focal plane a detector system consisting of time-of-flight and implantation detectors has been used for further event identification. The method and some highlights from the results in the lead region close to the proton drip line and in the transuranium region will be presented and discussed.

Identification of excited states in 167Os and 168Os: shape coexistence at extreme neutron deficiency

Excited states in the very neutron-deficient isotopes Os-167 and Os-168 have been observed using the reaction Sn-112(Ni-58, 2pxn). The JUROSPHERE gamma -ray spectrometer array was used in conjuncti ...