R.A. Cunningham

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

A detection system for the study of alpha and proton radioactivity on the Daresbury recoil mass separator

Abstract A detection system has been constructed to measure the alpha and proton decays of short-lived (> μs) nuclear species lying in the proximity of the proton drip line. Proton-rich nuclei produced in fusion-evaporation reactions at angles around 0° are mass separated in flight using the Daresbury Recoil Mass Separator. A two-dimensionally position sensitive silicon surface barrier detector is used to measure evaporation residue implantations and decays. The position information is used to identify the mass of the parent nucleus and to correlate causally related events. The operation and performance of the detection system are described.

Band Structure in 79Y and the Question of T=O Pairing

Gamma rays in the N=Z + 1 nucleus Y-79 were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental setup consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma rays were found to form a strongly coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N=Z and N=Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T=0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. [S0556-2813(98)50612-7].

Evidence for the alpha decay of108I

A Residue Implantation Detection System has been used in conjunction with the Daresbury Recoil Mass Separator in a search for radioactivity from the unknown nuclides108I and105Te. In the reaction of 260 MeV58Ni ions with a54Fe target two new decay lines at energies of 3.730±0.025 MeV and 3.885±0.025 MeV corresponding to cross sections of approximately 0.3 μb and 0.4 μb respectively were identified in theA=108 region of the separators focal plane. These decay lines are tentatively assigned to the alpha decays of108I. No evidence for a proton decay branch of this nuclide could be found. In a second reaction in which a50Cr target was bombarded with 230 MeV58Ni ions, no alpha decay peak from105Te could be identified. Cross section dependent half life limits were determined from which correspondingQ-value limits were deduced.

Use of the64Ni(36S,34Si)66Zn reaction to measure the mass of34Si

The reaction64Ni(36S,34Si)66Zn atE=198 MeV has been used to measure the mass excess of34Si; a value of −19.971−0.044+0.037 MeV was obtained. This result which is in agreement with the previous value (−19.85±0.30 MeV) is in excellent agreement with recent shell-model mass predictions. The result is discussed within the context of the proposed region of deformation aroundZ=11,N=20.