Search

The {sup 70,72}Ge({ital n},{ital p}){sup 70,72}Ga reactions have been measured at a beam energy of 200 MeV in order to study the behavior of Gamow-Teller (GT) strength near the {ital N}=40 subshell closure. The GT quenching factor extracted for {sup 70}Ge is consistent with results for {sup 54}Fe and with calculations done on the quasiparticle random-phase-approximation model if they are renormalized by a factor of about 0.6. The quasiparticle random-phase approximation predicts a drastic reduction in concentrated GT strength from {sup 70}Ge to {sup 72}Ge which is also seen in the data. These results are relevant to models of supernovae.

Cross sections for the 48 Ti(n, p) reaction have been measured at angles of 0°, 6°, and 12° at an energy of 200 MeV. The measurements are compared with results of DWIA calculations to obtain estimates of transition strengths for L = 0, 1, and ⩾2 up to an excitation energy of 25 MeV. Gamow-Teller strength ( L = 0) is peaked between 3 and 4 MeV excitation energy, with a significant distribution extending to about 12 MeV. The L = 1 strength is found mainly between 6 and 20 MeV while the cross section for transitions with L ⩾2 increases from 10 MeV to the upper limit of the measurements. The distribution of Gamow-Teller strength is in poor agreement with theoretical distributions used to calculate the lifetime for double beta decay of 48 Ca.

Cross-sections and recoil polarizations for the reactions gamma + p --> K^+ + Lambda and gamma + p --> K^+ + Sigma^0 have been measured with high statistics and with good angular coverage for center-of-mass energies between 1.6 and 2.3 GeV. In the K^+Lambda channel we confirm a structure near W=1.9 GeV at backward kaon angles, but our data shows a more complex s- and u- channel resonance structure than previously seen. This structure is present at forward and backward angles but not central angles, and its position and width change with angle, indicating that more than one resonance is playing a role. Rising back-angle cross sections at higher energies and large positive polarization at backward angles are consistent with sizable s- or u-channel contributions. None of the model calculations we present can consistently explain these aspects of the data., 5 pages, 3 figures, submitted to Physical Review Letters

The fission of nuclei with $159\ensuremath{\le}A\ensuremath{\le}232$ induced by the bombardment of 19.1 to 44.5 MeV $^{3}\mathrm{He}$ ions has been measured using solid-state detectors with time-of-flight measurements. Analysis with statistical fission theory, including precompound nucleon emission, indicates fission barriers which decrease only slightly relative to the liquid-drop model values with decreasing $Z$, approaching about 90% of the liquid-drop model barrier for $A\ensuremath{\approx}160$. These results are in contrast with measurements at higher angular momenta which indicate much lower fission barriers (60 to 70% of the liquid-drop model) for this mass region. The angular correlations indicate complete, or slightly greater than complete momentum transfer to the compound system. This is in opposition to that observed using heavier projectiles where incomplete momentum transfer is probable.NUCLEAR REACTIONS, FISSION $^{159}\mathrm{Tb}$, $^{165}\mathrm{Ho}$, $^{169}\mathrm{Tm}$, $^{\mathrm{nat}}\mathrm{Ta}$, $^{\mathrm{nat}}\mathrm{Re}$, $^{194,195}\mathrm{Pt}$, $^{197}\mathrm{Au}$, $^{202}\mathrm{Hg}$, $^{205}\mathrm{Tl}$, $^{206,208}\mathrm{Pb}$, $^{\mathrm{nat}}\mathrm{Bi}$, $^{\mathrm{nat}}\mathrm{Th}$($^{3}\mathrm{He}$,f), $E(^{3}\mathrm{He})=19.1 \mathrm{to} 44.5$ MeV; measured ${\ensuremath{\sigma}}_{F}(\ensuremath{\theta}, E)$, angular correlations, coincident fragment kinetic energies; deduced relative mass division, total kinetic energy release, statistical-model fission parameters.

Differential cross sections and analyzing powers for the /sup 7/Li(p-arrow-right,..pi../sup +- /) reactions leading to discrete states of /sup 8/B and /sup 8/Li, as well as to the continuum, are presented at T/sub p/ = 280, 354, and 489 MeV. These results are compared with previously published results at 200 and 250 MeV. At fixed four-momentum transfers squared in the region 0.6>t>0.4 GeV/sup 2//c/sup 2/, the energy dependence of the (p,..pi../sup +/) and (p,..pi../sup -/) differential cross sections is quite similar, in sharp contrast to earlier results from the /sup 13/C(p,..pi../sup +- /) reactions. The observed variations in the (p,..pi../sup -/) energy dependence are presumed to reflect differences in the relative importance of resonant and nonresonant amplitudes in the (p,..pi../sup -/) reaction mechanism. The analyzing powers of the (p-arrow-right,..pi../sup -/) and (p-arrow-right,..pi../sup +/) reactions are qualitatively very similar except for a difference in sign.

Rotational sidebands in 166 Er were observed using the 24 MeV 164 Dy(α, 2nγ) reactions. The ground-state band was observed up to spin 16 + and does not backbend. A strong backbend is, however, observed in a K π = ( O + ) sideband, indicating that the 12 + state of the previously unknown S-band is at 2656 keV. The γ-band shows significant rotational alignment above I = 10 + . Levels of at least two negative-parity bands, one of which is primarily the K π = 2 − octupole vibration, are also observed.

The interactions between a K/sup ..pi../ = 5/sup -/ aligned two-quasineutron band and two octupole bands have been observed in /sup 164/Er. Band crossings occur in both even- and odd-spin sequences. The members of all three bands are observed from the bandheads (L/sup ..pi../ = 1/sup -/, 2/sup -/, and 5/sup -/) to about 12/sup -/.

Proton-induced pion production to the continuum was investigated via the reactions /sup 13/C(p,..pi../sup -/)X, /sup 13/C(p,..pi../sup +/)X, and /sup 16/O(p-arrow-right,..pi../sup +/)X at bombarding proton energies of 250, 354, and 489 MeV, in the kinematic region where the pion has nearly its maximum allowable kinetic energy. Results from the /sup 16/O(p-arrow-right,..pi../sup +/)X reaction indicate that at small missing momenta the analyzing powers can be adequately described by a quasi-free p-arrow-rightp..-->..d..pi../sup +/ process, but as the missing momentum is increased this description becomes less accurate. The energy dependence of the cross section for the inclusive (p,..pi../sup +/) reactions is very similar to that of previously published exclusive (p,..pi../sup +/) data from the same target nuclei. The double differential cross sections for the ..pi../sup -/ data, however, exhibit an energy dependence different from that of the exclusive (p,..pi../sup -/) reaction.

Inclusive measurements of the linear momentum transfer distribution and exclusive studies of light-charged particles as a function of linear momentum transfer have been performed for the reaction of 150-MeV /sup 6/Li ions with /sup 238/U. Linear momentum transfer properties and an upper limit for the complete fusion cross section for this system were deduced from the folding angle between binary fission events and found to be consistent with data for heavier complex projectiles. The exclusive light-charged-particle data demonstrate the existence of fission following the transfer of all possible subdivisions of the /sup 6/Li projectile, as well as inelastic scattering. Absorptive breakup involving /sup 4/He-/sup 2/H cluster structure is found to be the dominant reaction mechanism. In addition, substantial yields of energetic protons associated with full linear momentum transfer are observed in the backward hemisphere. However, the temperature inferred from these spectra is nearly three times as large as that for a fully equilibrated compound nucleus, suggesting a nonequilibrium source for these particles.

Angular correlations between binary fission fragments have been measured in coincidence with light charged particles for 64, 129, 199, and 269 MeV {sup 3}He-induced reactions on a {sup 238}U target. Linear momentum transfer and missing linear momentum have been determined for a broad range of light-charged-particle energies {ital E}{sub LCP} and emission angles {theta}{sub LCP}. Average momentum transfer increases from a fraction of beam momentum for small detection angles to nearly full momentum transfer at backward angles. The missing momentum depends primarily on available energy'' {ital E}{sub av}=E{sub beam}{minus}E{sub LCP}, but neither on beam energy {ital E}{sub beam} nor light particle detection angle. Backward proton and deuteron emission is incompatible with statistical evaporation following complete or incomplete fusion.

The 166Er(3He, 3nγ) and 164Er(α, 2nγ) reactions were used to populate rotational side-bands in the N = 96 nucleus 166Yb. The aligned v( i 13 2 ) 2 S-band was observed from Iπ = 8+to 12+, while the β- and γ-vibrational bands were observed from the bandheads to Iπ = 10+and 12+, respectively. Negative-parity bands with Kπ = 0−, (2−) and 5− were also observed. Band crossings and interaction effects are seen in both the positive- and negative-parity excitations below about h ω = 0.25 MeV , the frequency at which the first yrast backbend occurs.

Decay properties of /sup 194,195,196/Pb were studied using singles and coincident ..gamma..-ray spectroscopy. Sources were produced by the 95 MeV /sup 197/Au(/sup 6/Li,xn) reactions. No direct evidence for the decay of a low-spin isomer of /sup 195/Pb was observed. Several new levels were observed in /sup 195,196/Tl, and are interpreted in terms of a rotational model.

Differential cross section measurements have been made for 250 MeV proton inelastic scattering from /sup 28/Si. Hadronic deformation lengths for the states at 1.78 (2/sup +/), 4.62 (4/sup +/), 6.8 (3/sup -/ and 4/sup +/), 9.70 (5/sup -/), and 10.2 MeV (3/sup -/) have been extracted using a first-order vibration model distorted-wave Born approximation description. Deformation lengths for the 2/sup +/ and 4/sup +/ states are found to be independent of proton energy from 40 to 500 MeV. Cross sections for the excitation of the giant resonance region are found to be highly fragmented between 16 to 25 MeV. This region of excitation is found to contain an energy weighted sum rule depletion of 70% for the isovector giant dipole resonance, 26% for the isoscalar giant quadrupole resonance, and 5% for the isoscalar hexadecapole resonance. No structure was observed in the inelastic continuum region of 25 to 45 MeV of excitation energy.

Inclusive polarized proton-nucleus scattering on 208 Pb has been studied at 290 and 500 MeV. The measurements cover a broad range (0–250 MeV) of excitation energy over the 4°–27° angular range. The measured analyzing powers and cross sections for the continuum were compared with recent relativistic and nonrelativistic distorted-wave impulse approximation calculations. The cross section in the continuum region is found to be dominated by single-step quasi-free scattering. Differences of up to 40% between the analyzing powers measured at the quasi-free peak as opposed to free NN values strongly support the relativistic impulse approximation predictions for the quasi-free region, and indicate the presence of relativistic medium effects. Analyzing powers and cross sections of several low-lying states were compared with both nonrelativistic and relativistic distorted-wave Born approximation predictions.

Differential cross sections and analyzing powers are presented for the /sup 16/O(p-arrow-right,..pi../sup +/)/sup 17/O* reaction at T/sub p/ = 250, 354, and 489 MeV and compared with preliminary 200 MeV data as well as previously published data taken at other energies above and below the ..delta../sub 1232/ resonance region. The differential cross sections for each final state exhibit an energy dependence similar to that of the pp..-->..d..pi../sup +/ reaction at equivalent center of mass energy and four-momentum transfer, as has been seen previously for (p,..pi../sup +/) reactions with other target nuclei. The shape of the analyzing power angular distributions varies with the /sup 17/O/sup */ nuclear final state. Some states exhibit broad similarities with that of the p-arrow-rightp..-->..d..pi../sup +/ reaction, while others are markedly different.

Fission of /sup 238/U, /sup 209/Bi and /sup 197/Au induced by both positive and negative pion beams at incident energies of 60 to 100 MeV is examined. Energy and mass spectra and angular correlations of coincident fission fragments are compared to theoretical calculations and to other data. Excitation energies of the fissioning systems in excess of 100 MeV are deduced, implying that the fission examined in this study is caused primarily by true absorption of the pion. The masses of the coincident fission fragments are found to have a wider distribution than those from fissioning systems of similar excitation energy induced by nucleon beams. The fission cross sections are close to the reaction cross sections for /sup 238/U and scale with the fissility for the other nuclei studied.

The scattering of protons off individual nucleons in the nucleus, known as ‘quasi‐free’ scattering, gives information on the modification of the nuclear force inside the nuclear medium. In order to extract this information, we need to know the nucleon‐nucleon (N‐N) interaction in free space, and the propagation of the proton through the nucleus (from elastic scattering). Direct comparison of the analyzing powers and polarizations for quasi‐free scattering as opposed to N‐N scattering show large (40%) differences. Conventional (non‐relativistic) calculations are unable to reproduce this effect. Relativistic calculations, which use large, opposing vector and scalar potentials reproduce this effect for both isoscalar and isovector inclusive reactions and for exclusive (p, 2p) reactions.

New measurements of inelastic proton scattering to low-lying states of /sup 208/Pb at 200 and 400 MeV are reported. Deformation lengths extracted from angular distributions for the 3/sup -/ (2.614 MeV), 5/sub 1//sup -/ (3.198 MeV), 5/sub 2//sup -/ (3.209 MeV), 2/sup +/ (4.086 MeV) and 4/sup +/ (4.324 MeV) states are in good accord with values extracted at other incident proton energies. The fact that the deformation lengths are independent of incident proton energy within experimental uncertainty provides support for the validity of the collective DWBA for medium energy proton scattering to strongly excited states. Advantage is taken of this to extract statistically more precise values of the ratio of neutron to proton multipole matrix elements (M/sub n//M/sub p/). Different methods of determining the appropriate average value of M/sub n//M/sub p/ are discussed. 3 refs., 1 tab.