D. J. Morrissey

A search for superheavy elements with half-lives between a few minutes and several hundred days, produced in the 48Ca+248Cm reaction

Abstract The results of recent unsuccessful attempts to synthesize and identify superheavy elements at the Lawrence Berkeley Laboratory SuperHILAC are described. A thin 248Cm target was irradiated with 48Ca ions at an average energy in the target of 255 MeV. Direct counting of thin recoil foils for short-lived spontaneous fission activity was done. Two long irradiations were also made and radiochemical group separations for the superheavy elements were carried out. In the first radiochemical experiment, a thin superheavy element sample was prepared and has been continuously counted for spontaneous fission events in a dual surface barrier coincidence counter. In the second experiment, two superheavy element fractions were obtained and were counted for spontaneous fission events; the two fractions contained those elements that co-precipitated with copper sulfide from either an acid or basic solution.

Angular momentum transfer and partition in the deep-inelastic reaction 664 MeV 84Kr + natAg

Abstract In- and out-of-plane angular distributions have been measured for sequential α-decay from target-like fragments produced in fully relaxed heavy-ion collisions. At angles equal to or larger than the target-recoil direction, the α-particle energy spectra are evaporation-like and the in-plane angular distributions are consistent with isotropy in the rest frame of the target recoil. The out-of-plane distributions exhibit an anisotropy of approximately two. Fragment spins were extracted from these distributions as a function of mass asymmetry. These spins are in agreement with those obtained from a simultaneous γ-ray multiplicity measurement. Both the fragment kinetic energies and intrinsic spins are consistent with rigid rotation of an intermediate complex consisting of two substantially deformed spheroids in near proximity.

Measurement of the product mass distributions from heavy-ion-induced nuclear reactions: I: Gamma-ray spectrometric product identification

A system for the measurement of the product mass yield distributions from heavy-ion-induced nuclear reactions has been developed. This system is based on the gamma-ray spectrometric identification of product radioactivities in the irradiated target materials or in separated chemical fractions derived from the target. Gamma-ray spectra are collected as a function of time and all peaks and peak areas are identified and measured with a modified version of SAMPO. Decay curves for all gamma-rays observed in two or more spectra are constructed by the computer code TAU1. A current table of all the known gamma-ray transitions is then used to match a known gamma-ray energy and half-life to each measured decay curve. This is done interactively with the computer code TAU2 and a Tektronix graphics terminal. Examples of the various options are given. Nuclear reaction cross-sections are calculated on weighted average of all the observed gamma-rays for each product nuclide after the identifications have been screened for duplicate or erroneous identifications and for self-consistency.

Intrinsic fragment spins generated in the reactions of 20Ne with 197Au and 238U at 12.6 MeV/nucleon

Abstract The average magnitude and alignment of the intrinsic spin of the heavy partner from the reaction of 252 MeV 20 Ne with 197 Au and 238 U were determined as a function of Q -value. These spin values were extracted from sequential fission angular distributions obtained in coincidence with projectile-like products. For all Q -values a large out-of-plane anisotropy was observed, while for large negative Q -values an in-plane anisotropy was observed. A very large entrance-channel mass-asymmetry was chosen to provide a stringent test of equilibrium statistical model predictions for the spin alignment. The importance of determining the direction of the line-of-centers of the dinuclear system at scission is discussed. Large values of P ZZ were deduced for all Q - values. P XY was observed to be positive in the quasielastic region and negative in the deep-inelastic region. The extracted alignment data are compared to equilibrium statistical model calculations.

FURTHER STUDIES OF LARGE COLLISION RESIDUES IN RELATIVISTIC HEAVY ION REACTIONS WITH HEAVY NUCLEI

Abstract Target residue mass and charge distributions have been measured radiochemically for the reaction of 25.2 GeV 12 C ions with Au and Pb. Enhanced product yields for A = 140−170 are found that are not present in reactions of Pb with GeV protons. The shape of the mass distribution in this region is used to define a pre-equilibrium product mass distribution.

Implications of the target residue mass and charge distributions in the interaction of 8.0 GeV20Ne with181Ta

Target residue mass and charge distributions have been measured radioanalytically for the reaction of 8.0 GeV20Ne ions with Ta. These distributions are very similar to those distributions observed in the reaction of Ta with protons of equivalent total energy rather than equivalent velocity. Detailed comparisons of the production cross sections for near target residues with abrasion-ablation model calculations show these products to have an excitation energy of ∼75 MeV.

Angular momentum, statistical equilibrium and sequential fission in very asymmetric systems

The in-and out-of-plane angular distributions for fission fragments in coincidence with projectile-like products from the reaction of 252 MeV20Ne with197Au and238U have been measured. The results are compared to a statistical model which has successfully explainedγ-ray anisotropies from a heavy symmetric system. The agreement is rather good after proper consideration of the direction of the line-of-centers at contact.

Lowered fusion cross section in the quadruply magic heavy-ion system 48Ca + 208Pb

Abstract The results of radioanalytical mass yield distribution studies of the reaction of 48 Ca with thick 208 Pb targets at the effective laboratory energies of 255 and 300 MeV are reported. Complete fusion cross sections are found to be significantly lower than those found for 40 Ar induced reactions with non-magic targets apparently showing the effect of the projectile and target nuclear structure on such cross sections.

Large contribution of deep inelastic processes to reactions of40Ar and48Ca with238U

Differential recoil range distributions have been measured for heavy-reaction products ranging from Te(Z=52) to quasielastic transfer products near the charge and mass of the targets for the reactions of 276 MeV48Ca+238U, 237MeV and 250 MeV40Ar+238U, and 259 MeV40Ar+197Au. The measured recoil range distributions for the40Ar+197Au reaction agree with range distributions calculated from the known projectile-like fragment angular distributions for this reaction. The angular distributions of recoil products formed in the uranium target reactions are deduced and show that the products in the75Re to83Bi region have backward peaked angular distributions characteristic of deep inelastic reactions. The heavy product angular distributions smoothly vary from a (1/sinθ) shape to an exponential shaped backward peak as the atomic number of the product increases from 52 to 83. The trend in the deduced angular distributions for those elements for which recoil range distributions were determined in the40Ar+197Au reaction and the 250 MeV40Ar+238U reaction is similar, suggesting that just as for the Ar+Au system the composite system for the uranium target reaction is also not fully equilibrated along the mass asymmetry coordinate. These conclusions show that the fraction of the total reaction cross section resulting in complete fusion must be re-evaluated for the40Ar+238U reaction and similar heavy-target reactions.

Angular-momentum transfer and alignment in deep-inelastic reactions for nearly symmetric heavy-ion systems

Abstract The magnitude and alignment of the spin transferred to the fragments in the deep-inelastic reactions of 8.5 MeV/nucleon 165Ho on 176Yb, 148Sm, and natAg were investigated using continuum γ-ray multiplicity and anisotropy techniques. The detection system consisted of a highly redundant arrangement of particle and γ-ray detectors and a γ-ray multiplicity filter. By using suitable reduced quantities, we show that for the most negative Q-values the multiplicity data are consistent with rigid-rotation of the intermediate dinuclear complex. The anisotropy data are compared to an equilibrium statistical model calculation. The sensitivity of the calculation to different assumptions concerning the composition of the γ-ray spectra is investigated. The magnitude and alignment of the spin imparted to the individual fragments as a function of Q-value are extracted for the three reactions.