K. S. Toth

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

Precision mass difference measurements in light rubidium and krypton isotopes utilizing beta-endpoints

Abstract A technique for precise determination of β - endpoints with an intrinsic germanium detector has been developed and checked with 27 Si and 58 Cu sources; Q EC values of 5272 ± 26, 8063 ± 42 keV have been obtained for mass-separated samples of 77 Rb, 75 Rb and 77 Kt, respectively. Mass surface systematics in this region have been examined and found to be consistent.

A Monte Carlo study of the response of a germanium detector to electrons and positrons

Abstract A Monte Carlo code has been developed which predicts electron and positron response functions of a planar intrinsic germanium (Ge) detector. The operational flow chart for the main program is given and the function of each subroutine is discussed. The characteristics of the response functions are studied by considering effects due to bremsstrahlung, annihilation radiation and the total electron transit distance. It is found that electron spectra are distorted primarily near the full energy peak by bremsstrahlung while positron spectra are seriously distorted by both annihilation radiation and bremsstrahlung. Total transit distances calculated here are found to be in good agreement with those calculated using the Bethe-Bloch formula. The effects of bremsstrahlung, annihilation radiation and total stopping distance, calculated independently in the complete code, are then used to construct an abbreviated version which reduces the computer time by a factor of 16. Sample computations are presented for a 16 mm diameter by 7 mm thick, planar detector. The calculated response functions reproduce experimental spectra accurately; they are also used to analyze positron spectra and obtain endpoint energies.

A microscopic description of inelastic 12C scattering from 208Pb

Abstract Heavy-ion inelastic scattering is described microscopically by using an effective nucleon-nucleon interaction and RPA hole-particle wave functions. The relative cross sections for different multipoles can be sensitive to the range of the interaction. The strength is determined by fitting elastic scattering. The model is used to analyze 98 MeV 12C ions exciting 208Pb. The range required is shorter than that for the bare interaction between nucleons. Collective model fits are also presented for comparison.

The Oak Ridge Radioactive Ion Beam Facility

Abstract The Oak Ridge RIB Facility has been approved and is under construction. The general layout and capabilities of this facility are described, as is the possibility of studying nuclei near the 100 Sn double closed shell, new regions of exotic nuclear shapes, proton halo nuclei, and rp-process nucleosynthesis.

The α-decay rate of 188Pb

Abstract As part of our investigation of the reported anomalous α-decay rates of even-even lead nuclei with A ≦ 192 we have measured the α-decay branching ratio of 188 Pb. The isotope was produced in the 180 W( 16 O, 8n) reactions, and its decay properties were investigated with the use of the Oak Ridge on-line isotope separator facility. Gamma-ray and α-particle spectra were measured with the detectors placed in calibrated geometries. From these measurements the 188 Pb α-decay branching ratio was determined to be (22 ± 7)%, a value about seven times greater than that available in the literature. This result, together with our recent data for 190 Pb and 192 Pb, is discussed within the context of α-decay-rate systematics in the lead region.

On the reactions of protons with 231Pa and 232Th

Abstract Yields of α-radioactive products from reactions induced in 231 Pa targets by protons from 35 to 63 MeV were measured by recoil-collection techniques. The results, as similarly observed by Lefort and co-workers for the reactions of 232 Th+p at energies ≦ 85 MeV, indicate that the yields of reactions involving charged-particle emission are comparable to or larger than those involving only neutron emission. In addition, the experimental yield curves exhibit high-energy tails. Results for both the 231 Pa and 232 Th targets are compared with the predictions of nuclearreaction calculations that take into account the competition between fission and particle emission. Bearing in mind the assumptions inherent in the calculations and the fact that no parameters were fitted to the data, we found that the compound-nucleus model, with fission, did not account for the experimentally observed trends. The intra-nuclear cascade model, including fission competition in the compound-nuclear de-excitation phase, on the other hand, predicted excitation functions that were reasonably consistent with the results for (p, x n) and (p, p x n) reactions. Neither model was successful in accounting for the (p, α3n) data. In the case of 231 Pa results, it was found that recoil-range effects had to be included in the nuclear-reaction calculations.

Excitation energies of the h112 and d32 neutron states in 145Gd and 147Dy

A new activity is identified as 147Dym and the d32 and h112 neutron states are found to be at 27 and 749 keV in 145Gd and 72 and 751 keV in 147Dy. These energies confirm Silverbergs prediction that for N = 81 nuclei the h112−d32 spliting should reach a maximum at 139Ce.

Masses and proton separation energies obtained from Q and Q measurements

For many nuclei beyond the proton drip line in the Z>72, N>82 region, both proton and α emission are energetically allowed. In the case of some proton emitters, there are α-decay chains emanating from both parent and daughter nuclei. This means that if the mass excess of one member of an α-decay chain is known, then the mass excesses for all members of both chains can be obtained. In addition, proton separation energies may be derived for nuclei in the α-decay chain of the proton emitter. The method of time- and space-correlations also allows the identification of isomeric states in these nuclei. As an example, a large number of mass excesses and proton separation energies for ground and metastable states have been derived from Qα and Qp values obtained from the proton emitters 165,166,167Ir, 171Au, 177Tl, and their daughters.

The use of a velocity filter in α-decay studies of microsecond radioactivities

Abstract A new method of studying microsecond radioactivities of evaporation residues produced by heavy-ion reactions is described. Forward recoiling products were separated from the beam particles by using a velocity filter and implanted into a Si(Au) detector. Their subsequent α decays were then studied with the same detector.