Paul H. Stelson

On the “distribution of barriers” interpretation of heavy-ion fusion

Abstract The significance of the interpretation of heavy-ion fusion cross sections in terms of a distribution of potential barriers is discussed. The smoothing due to the quantal barrier penetration is shown to replace a set of discrete barriers by an effective continuous distribution. It is shown how this smoothed distribution may be obtained rather directly from the measured cross sections at near-and sub-barrier energies.

Neutron flow between nuclei as the principal enhancement mechanism in heavy-ion subbarrier fusion☆

Abstract We have studied the distributions of barriers required to fit experimental heavy-ion fusion cross sections at energies in the barrier region. Best fits were generally obtained with distributions which were flat, broad, and characterized by a sharp cutoff value at the low-energy end. The cutoff barrier is correlated with the separation energies of the valence neutrons and can be associated with the distance at which merged potentials just allow neutrons to flow between the nuclei. These features suggest that fusion initiated by neutron flow is the principal enhancement mechanism and that coupling to collective states plays a secondary role.

Elastic scattering of 16O by 16O

Abstract The cross section for 16 O+ 16 O elastic scattering at 90° c.m. was measured from 35 to 88 MeV c.m. Pronounced structure was observed, as strong as seen earlier in the 20–30 MeV region. Several optical-model potentials derived from data at lower energies fail to predict the observed structure.

Population of high spin states in 22Na by means of the 10B(16O, α) reaction☆

Abstract Strong selectivity has been observed in the population of states in 22 Na up to high excitation energies. Angular distributions were measured, and a comparison with Hauser-Feshbach calculations allow us to select strong candidates for the high-spin members of the K π = 3 + , T = 0 and K π = 0 + , T = 0 bands in 22 Na.

The Holifield Heavy Ion Research Facility

The Holifield Heavy Ion Research Facility has been in routine operation since July 1982. Beams have been provided using both the tandem accelerator alone and a coupled mode in which the Oak Ridge Isochronous Cyclotron is used as an energy booster for tandem beams. The coupled mode has proved to be especially effective and has allowed us to provide a wide range of energetic beams for scheduled experiments. In this report we discuss our operational experience and recent development activities.

Tests of compressed geometry NEC acceleration tubes

Abstract Tests have been performed in the 3 MV Pelletron test machine at NEC on a compressed geometry tube which increases the insulating length of the tube by eliminating the heated electrode assemblies (∼ 2.5 cm thick) at the end of each tube section. Some insert electrodes are changed to provide some trapping of secondary ions. The geometry tested provided an 18% increase in live ceramic in the tube. The compressed geometry tube allowed a terminal voltage of 3.55 MV on the 3 MV column at normal gradients of 30.3 kV/tube gap. The tube was also conditioned to more than 4 MV and remained stable in voltage with few sparks and with low X-ray levels for days at about 4 MV. This same performance could be achieved with or without arc discharge cleaning.

Hydrogen arc discharge cleaning of accelerator tubes

Abstract Accelerator tubes manufactured by NEC were cleaned by the hydrogen arc discharge method and tested on the NEC 3 MV test accelerator. We will discuss the results of this testing program. Generally, we confirm the favorable experience previously obtained by Korschinek et al. [1]. The familiar microdischarges exhibited by normal tubes are largely eliminated in arc discharge cleaned tubes. Thus, the arc discharge process has the same observable effect as voltage conditioning. This result suggests that the hydrogen discharge is effective in removing carbon and hydrocarbons from the surface of the accelerator tube.

Arc discharge conditioning test on the oak ridge 25URC accelerator

Abstract A hydrogen arc discharge has been run in the top five units (fifteen 20 cm tube sections) of the ORNL tandem accelerator to test the effectiveness of this method of cleaning the high voltage electrodes. The discharge was maintained in both the high- and low-energy tubes for a period of six hours. The arc current in each tube was 4.0 A and the hydrogen pressure was nominally 100 mTorr. The arc discharge does not appear to have significantly affected the operating voltage of the top five units of the accelerator. However, the voltage conditioning behavior of the tested units is markedly different.

A Multi-Accelerator System for Heavy Ions

The accelerators for the new National Heavy-Ion Laboratory (NHL) being planned at Oak Ridge will provide beams of all ions from helium to uranium with energies ranging from 100 MeV/u for light ions to 10 MeV/u for the heaviest ions accelerated. The heart of the proposed accelerator system is a large 4 sector isochronous cyclotron of the separated sector type with an energy rating of 440 q2/A MeV. Ions are to be injected into the large cyclotron from either a large tandem electrostatic acclerator or from the Oak Ridge Isochronous Cyclotron (ORIC). The ORIC will generally suffice as the injector up to the mass 160-180 range, beyond which use of the tandem is necessary. Beam intensities will range from 1013 ions/sec for 100 MeV/u oxygen to ~2 × 1012/sec for 10 MeV/u uranium ions. The beam emittance is expected to be less than 10 mm-mrad and the energy spread less than 0.1%. The large cyclotron uses auxiliary RF accelerating cavities operating at twice the main RF frequency to flattop the effective accelerating voltage waveform to thus increase the phase acceptance and reduce the energy spread of the beams.

The ORIC as a Heavy‐Ion Injector for a separated Sector Cyclotron

The characteristics of the Oak Ridge Isochronous Cyclotron (ORIC) for the injection of heavy‐ions into a separated sector cyclotron have been studied and found to be excellent. With a carbon foil between the accelerators the output energy of the ORIC is sufficient to give a stripped ion beam at least twice the charge state of the ORICs extracted beam up to mass 200. This allows an ideal magnetic field match to a separated sector cyclotron with a magnet fraction of 0.58. The 37‐MeV 40Ar4+ beam from the ORIC was used to test the lifetime of 20‐μg/cm2 carbon foils. It was found that the stripping characteristics were unchanged after a total transmission of 5 × 1017 particles per cm2. Results of the measurement of the ORIC beam loss as a function of pressure in the cyclotron tank and beam line are also presented.