Search

The mass and energy distributions for fusion products corresponding to 128.9 MeV $^{32}\mathrm{S}$ on $^{12}\mathrm{C}$, $^{25}\mathrm{Mg}$, and $^{40}\mathrm{Ca}$ targets has been measured at 8\ifmmode^\circ\else\textdegree\fi{} lab using a time-of-flight spectrometer. The heaviest product observed with significant cross section for each system has a mass five units less than that of the corresponding compound system. Lighter dominant mass groups are separated by approximately three mass units. Such mass systematics can be understood in terms of neutron, proton, and $\ensuremath{\alpha}$-particle competition in the deexcitation of the compound system and are reproduced qualitatively by particle evaporation calculations.

Prominent intermediate structure correlations are suggested between /sup 12/C+/sup 12/C elastic and reaction channel excitation functions over a wide range of energies above the Coulomb barrier. The associated resonant states in /sup 24/Mg would have large partial widths to elastic, inelastic, and ..cap alpha..+/sup 20/Ne* channels.

Fusion cross sections for the reaction pairs /sup 12/C+/sup 14/N and /sup 10/B+/sup 16/O (leading to /sup 26/Al) and /sup 16/O+/sup 16/O and /sup 12/C+/sup 20/Ne (leading to /sup 32/S) are examined to extract critical angular momenta. At low energies entrance channel effects are present, whereas at higher energies the results are qualitatively consistent with a common limiting angular momentum characteristic of the compound nucleus. Comparison with angular-momentum-dependent level densities suggests that the fusion cross sections at intermediate energy are sensitive to the availability of a sufficient level density in the compound nucleus. A model calculation employing the criterion that the level spacing of a particular angular momentum be comparable to or smaller than the level widths is used to calculate fusion cross sections. The results are in qualitative agreement with experiment, accounting for both the magnitude of the cross sections and the periodicity of the gross structure in /sup 16/O+/sup 16/O.

Data have been obtained for the /sup 35/Cl + /sup 27/Al system at E/sub Cl/(lab) = 170 MeV in which the energy and mass of one particle is measured in coincidence with the energy of a second particle. These coincidence measurements indicate a two-body final state with kinetic energies corresponding to the Coulomb separation energy of the two fragments. The mass distribution of the two correlated fragments is symmetric about half the mass of the compound system. Such data is characteristic of a fission-like process. Similar fission-like cross sections are not observed in data obtained for this system at incident energies up to 145.6 MeV. The competition between particle evaporation and fission, assuming a rotating liquid drop fission barrier, has been calculated using the code ALICE. Such calculations are consistent with the present data. 4 figures.

Evaporation residues of reactions between /sup 24/ /sup 25/ /sup 26/Mg, /sup 27/Al, and /sup 28/Si as targets and 100--170-MeV /sup 35/Cl and 130 MeV /sup 32/S beams and also between /sup 28/Si and 117.5 MeV /sup 28/Si ions were measured at small forward angles by using a time-of-flight telescope. The mass distributions of the products show a complex structure, which is shown to vary smoothly with beam energy and target mass. A dominant decay mode of the compound nucleus can be assigned to each residue mass by using the shape of the angular distribution. An evaporation model calculation predicts qualitatively the main features of the mass distributions.

The /sup 12/C+ /sup 16/O resonance structure in the /sup 28/Si nucleus is examined by means of the /sup 12/C(/sup 16/O,..cap alpha..)/sup 2r/Mg reaction excitation functions in the energy range E/sub c.m./ = 26.6 to 42.9 MeV in 430 keV steps at theta/sub lab/ = 7.5/sup 0/. We could identify 64 discrete states in /sup 24/Mg up to 31.7 MeV of excitation energy. The excitation functions show abundant structure over the entire energy range. The summed excitation functions, which tend to average out statistical fluctuations, show pronounced intermediate structure enhancement in the cross section at E/sub c.m./approx. =29.5, 32.2, and 35 MeV and indicate the presence of a smaller peak at 37.3 MeV. The widths of these structures are about 1 MeV, which is intermediate between the value expected from ion-ion potential resonances and statistical fluctuations. The nonstatistical character of these structures is reinforced by some statistical tests and by the correlations in energy and width found in several exit channels. Our data also suggest a possible structural relationship between the /sup 28/Si resonances and certain /sup 24/Mg final states.

A study has been made of the ..gamma..-ray deexcitation following deeply inelastic collisions between 100 MeV /sup 16/O and /sup 27/Al. An experimental apparatus including a gas proportional particle telescope, a Ge(Li) detector and three NaI(T1) detectors was used. All possible particle-..gamma.. and particle-multiple-..gamma.. coincidences for projectilelike fragments with Z> or =3 were recorded. Yields and anisotropies of several discrete transitions were measufed, as well as the mean and width of the gross multiplicity distributin as a function of detected Z and reaction inelasticity. Estimates of average ..gamma..-ray transition energy, average energy carried by ..gamma..-ray deexcitation and the degree of side feeding in observed ..gamma.. cascades were made. Observed anisotropies are consistent with considerable fragment spin alignment being present. Values of mean ..gamma..-ray multiplicity never exceed simple sticking model predictions; particle emission effects are seen to be important. The summed radiative strength ofobserved discrete line transitions is found to be less than unity, sometimes considerably so, for most deeply inelastic collisions. A short discussion is given of insights obtained on the entry state distribution for ..gamma..-ray emission and of the results of statistical model calculations of the relative yields of observed final states.

A heavy-ion time-of-flight spectrometer has been brought into operation at the Brookhaven National Laboratory (BNL) MP Tandem-Van-de-Graaff facility. The start detector consists of a very thin plastic scintillation detector and the stop detector is a commercial surface-barrier detector. The design and operation of the spectrometry are discussed. An overall time resolution of ∼0.3 ns is found to be routinely attainable under realistic experimental conditions. Plans to include a magnetic quadropole lens in the flight path as well as the ultimate application of the spectrometer are discussed.

Excitation functions for the reaction /sup 12/C(/sup 16/O, ..cap alpha..)/sup 24/Mg have been measured from E/sub lab/(/sup 16/O) = 62 to 110 MeV using a counter telescope at theta/sub lab/ = 7.5/sup 0/. Selective population of relatively few states at very high excitation energies in /sup 24/Mg(E/sub x/(/sup 24/Mg) > 20 MeV) is seen. A possible correspondence is found between these states and the narrow resonances reported in /sup 12/C + /sup 12/C reactions. In addition, a possible correspondence between averaged yields in /sup 12/C(/sup 16/O, ..cap alpha..)/sup 24/Mg and gross structure seen in serveral /sup 12/C + /sup 12/C reaction channels is cited.

Prominent gross and intermediate width structures are observed in the /sup 12/C+/sup 12/C, /sup 12/C+/sup 12/C((2/sup +/), /sup 1/C((2/sup +/)+ /sup 12/C((2/sup +/), /sup 8/Be+/sup 16/O, and /sup 8/Be+/sup 16/O/sup +/(3/sup -/, O/sup +/) decay channels following /sup 24/Mg* population via the /sup 12/C(/sup 16/O,..cap alpha..)/sup 24/Mg reaction at E/sub c.m./ = 33 MeV. Evidence that the /sup 12/C(/sup 16/O,..cap alpha..)/sup 24/Mg reaction populates states in /sup 24/Mg which are associated with /sup 12/C+ /sup 12/C resonances is presented in the form of correlation analyses between the ..cap alpha..+ /sup 12/C+ /sup 12/C three-body spectra and previously measured /sup 12/C+ /sup 12/C elastic and inelastic excitation functions. Direct determination of /sup 12/C+ /sup 12/C widths from these measurements is obscured by a background of other strong transitions which appear to be present in the /sup 12/C(/sup 16/O,..cap alpha..)/sup 24/Mg singles spectrum.