R. J. McDonald

Systematics of complex fragment emission in niobium-induced reactions

Abstract Complex fragments of 3 Z ⪅35 have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be, 12 C and 27 Al at bombarding energies of E / A =11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.

Emission of complex fragments from highly excited systems produced in 93Nb+9Be and 27Al reactions at EA = 25.4 and 30.3 MeV

Abstract Complex fragments with atomic numbers intermediate between that of the target and projectile have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be and 27 Al at bombarding energies of E A = 25.4 and 30.3 MeV. Experimental results from inclusive and coincidence measurements are presented and are used to characterize these fragments as the statistical, binary-decay products of highly excited compound nuclei formed in fusion-like reactions.

COMPLEX FRAGMENT EMISSION AT 50 MeV/u: COMPOUND NUCLEI FOREVER?

Abstract Fragments with 12⩽ Z ⩽35 have been detected in the reaction of 50 MeV/u 139 La on 12 C and are shown to be produced solely by the binary decay of a compound nucleaus formed in an incomplete fusion reaction. The source velocity, CM velocities, cross sections, and angular distributions extracted from the inclusive data are entirely consistent with this mechanism. The binary coincidence data confirm this interpretation and can be quantitatively accounted for by the singles data.

Superdeformed bands in Nd nuclei

Abstract Superdeformed bands were found in 134, 136 Nd. Their dynamic moments of inertia are about as large as those of the known superdeformed bands in 135 Nd and 132 Ce, corresponding to β ≈ 0.5. A high probability for staying in band is measured for the discrete superdeformed bands in 134, 136 Nd, in contrast to the unresolved superdeformed ridges.

Loss of collectivity along the yrast line in 158Er

Abstract Lifetimes of yrast states from spin 14 through 40 have been measured in 158Er. The Doppler-shift recoil-distance method was used in conjunction with the multi-Ge detector array HERA, and γγ coincidences were analyzed. The collectivity of the nucleus drops from 200 W.u. to 110 W.u. in the region of the second backbend. By spin 38 it has decreased further to 25 W.u., consistent with a band termination.

New structures at high spin in 159Er

Abstract The nucleus 159 Er was studied with the HERA γ-ray spectrometer. Three rotational bands were extended to higher spins, and three new bands were found. It is shown that specific orbitals rather strongly influence the high-spin behavior, some states remaining collective (somewhat triaxial), others evolving towards a non-collective oblate structure. One of the new bands is a higher- K band, similar to those found in heavier nuclei.

Evidence for spin fluctuations in the deep-inelastic reaction 165Ho + 165Ho at 8.5 MeVamu☆

Abstract Both the magnitude and alignment of the transferred angular momentum in the reaction 165Ho + 165Ho have been measured as a function of Q-value via continuum γ-ray multiplicity and anisotropy techniques. The spin transfer and the continuum γ-ray anisotropy increase throughout the quasi-elastic region. The spin transfer as a function of Q-value saturates at ~ 35ħ/fragment, the anisotropy peaks at a value of ~2 and then decreases to near unity for the largest Q-values. The observed anisotropies are in good agreement with predictions from an equilibrium statistical model in which thermal excitation of angular-momentum-bearing collective modes and neutron evaporation give rise to in-plane components of the angular momentum.

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.

Complex fragments from excited actinide nuclei. A new test of the finite range model

Complex fragments ranging in charge from 7less than or equal toZless than or equal to45 have been detected in binary coincidence following the reaction of 8.4 MeV/u/sup 232/Th+/sup 12/C, and are shown to arise from the binary decay of a /sup 244/Cm compound nucleus. This work confirms earlier radiochemical observations of very light fragments in the fission fragment mass distribution, establishes their binary character, and interprets their yield in terms of finite range potential energy barriers. 15 refs., 3 figs.

Excitation energy division in the 9 MeV/nucleon 56Fe+165Ho reaction

Abstract The centroids and widths of the excitation energy distribution for damped fragments produced in the 56 Fe + 165 Ho reaction at 9 MeV/nucleon have been determined. There exists a dependence of excitation energy division between the reaction fragments on energy loss and nucleon transfer.