D.K. McDaniels

The elastic scattering of intermediate energy protons from 40Ca and 208Pb

Abstract Elastic scattering of protons from 40 Ca and 208 Pb has been studied using the MRS facility at TRIUMF. Cross section and analyzing power ( A y ) angular distributions to 50° in the center of mass have been measured for incident proton energies of 200, 300, 400 and 500 MeV. Comparisons of the experimental results are made with calculations based on relativistic and non-relativistic phenomenological models, and with relativistic and non-relativistic microscopic models. The differences between the results of the non-relativistic and relativistic models manifest themselves in the spin observables. They are found to be smaller in the phenomenological approach than in the microscopic one and they depend on energy.

Decay of 75Se

Abstract The gamma-ray spectrum of 75Se has been studied with a high-resolution Ge(Li) spectrometer. Intensities (indicated in parentheses) of 12 lines and the energy of three lines above 401 keV have been measured: 66.05 keV (1.64±0.05), 96.74 keV (5.33±0.16), 121.12 keV (27.8±0.8), 135.99 keV (94.9±2.0), 198.60 keV (2.28±0.05), 264.62 keV (100), 279.57 keV (43.0±0.9), 304.0 keV (2.39±0.05), 400.7 keV (22.3±0.5), 419.6±0.5 keV [(3.22±0.06) × 10−2], 572.0±0.5 keV [(6.36±0.13) × 10−2] and 617.7±0.5 keV [(7.77±0.15) × 10−3]. Total K X-ray intensity and capture branching ratios have been determined; (5.8±1.8) % of all decays are found to lead to the ground state of 75As. The K-capture probability to the 401 keV level has been measured as PK/Ptotal = 0.885±0.018, in close agreement with the theoretical value of 0.88 computed after the theory of Brysk and Rose with allowance for electron exchange according to Bahcall.

Radiative capture of fast neutrons by 89Y and 140Ce

The γ-ray spectra from the reactions 89Y(n, γ)90Y and 140Ce(n, γ)141Ce have been measured in the neutron energy range of 6.2–15.6 MeV. The pulse-height spectra were recorded with NaI(Tl) spectrometers and time-of-flight techniques were used to improve signal-to-background ratio. Capture cross sections were determined for γ-ray transitions to the two 2d52 levels at 0 and 203 keV of 90Y and to the 2f72 ground state of 141Ce as well as integrated cross sections to bound states in these nuclei. The observed γ-ray spectra and partial radiative capture cross sections were compared with predictions of the direct-semidirect capture theory. The resonance behaviour with neutron energy of both the ground-state and integrated partial capture cross sections shows the validity of the semidirect model for 89Y and 140Ce in the region of neutron energy encompassing the giant-dipole resonance. The observed symmetry of the cross sections about the peak of the resonance argues strongly for the complex form of the particle-vibration coupling interaction. A detailed comparison of the predictions of the DSD model using the complex coupling interaction shows that the capture cross sections are relatively insensitive to the real part of the interaction.

Coulomb excitation of 67Zn and core-quasiparticle coupling in f52 nuclei

Abstract A qualitative explanation of the low-lying levels of g 9 2 shell nuclei using a recent theoretical improvement upon quasiparticle-phonon coupling theories has motivated the investigation of 67Zn, which belongs in the f 5 2 neutron shell. A Coulomb excitation experiment was performed with 3.2 to 4.86 MeV α-particles. De-excitation γ-rays were observed with a 45 cm3 Ge(Li) detector. In addition to excitation of previously known states at 184.6 keV ( 3 2 − ) and 880.0 keV ( 5 2 − ) , strong excitation of states at 815.2 keV ( 5 2 − , 7 2 − ) and 870.9 keV ( 3 2 − , 5 2 − , 7 2 − ) was observed. The latter state also appears to have been observed in high resolution (d, p) stripping work. Angular distribution data were taken at Eα = 4.0 and 4.86 MeV, and were used to obtain branching ratios, reduced upward transition probabilities, mixing ratios and spin assignments. The B(E2)↑ associated with the 67Zn levels populated in this experiment are (in units of e2 · b2) 184.6 keV (0.0190 ± 0.0014), 393.6 keV (0.00049 ± 0.00003), 815.2 keV (0.029 ± 0.002), 870.9 keV (0.04 ± 0.02), and 888.0 keV (0.0086 ± 0.0006). An extensive diagonalization using three single-quasiparticle states ( f 5 2 , p 3 2 , and P 1 2 ) in 67Zn, each coupled to a core phonon, was carried out and detailed predictions were made for level energies, reduced transition probabilities, magnetic dipole and electric quadrupole moments, relative γ-ray intensities, and mixing ratios. Generally, the predictions are in good agreement with 67Zn experimental data. Data for other f 5 2 shell nuclei lend further qualitative support to a quasiparticle-phonon coupling picture for odd-A nuclei in the f 5 2 shell. It is suggested that an extra 3 2 − low-lying level present in several f 5 2 shell nuclei has characteristics similar to those of the three-quasiparticle “intruder” state described by Kisslinger.

Excitation of giant resonance modes in 90Zr and 120Sn by 200 MeV protons

Abstract Giant resonances in 90Zr and 120Sn have been studied using inelastic scattering of 200 MeV protons. The isovector giant dipole and isoscalar giant quadrupole and giant octupole (L=3,3 h ω) resonances are clearly observed. An upper limit of ≈5% is placed on the 2ħω hexadecapole resonance peak. For 90Zr peak is observed which is consistent with recently reported M1 strength.

Coulomb excitation of 73Ge and quasiparticle-phonon coupling theories

Abstract The excitation energies and γ-ray decay modes of several low-lying states in the nucleus 73Ge have been studied using Coulomb excitation with 2.6–4.0 MeV α-particles on an enriched 73Ge target to test the predictions of quasiparticle-phonon coupling theories, in particular to test if there are any high-spin states in the low-energy spectrum. Gamma-rays were observed with a 60 cm3 Ge(Li) detector. Thick-target γ-ray yields were utilized to establish Coulomb excited levels, to clarify the γ-ray decay of 73Ge, and to obtain reduced upward transition probabilities, B(E2)↑, associated with the levels indicated as follows: 68.6 keV (0.073 ±0.007), 499.0 keV (0.0091±0.0005) and 825.6 keV (0.077±0.004). Angular distributions of the de-excitation γ-rays from the Coulomb excited levels were measured and various spin possibilities for each level were eliminated by the χ2 technique. The following spinparity assignments are consistent with our γ-ray angular distribution measurements: 68.6 keV ( 5 2 + , 7 2 + , 9 2 + ), 499.0 keV ( 7 2 + ), and 825.6 ( 7 2 + , 13 2 + ). Branching ratios were extracted from the γ-ray angular distribution measurements. The eB(E2)↑ values for the first 2+ excited states of 72Ge(0.18), 74Ge(0.29), and 76Ge(0.27) were also obtained. The results for 73Ge are compared with the predictions of several quasiparticle-phonon coupling theories and some suggestions are made to improve agreement between experiment and theory.

Validity of collective model DWBA analysis for intermediate energy proton scattering to low-lying states of 208Pb

Abstract New measurements of inelastic proton scattering to low-lying collective states of 208 Pb at 200 and 400 MeV are reported. Deformation lengths ( δ H = βR ) extracted from angular distributions for the 3 − (2.614), 5 − 1 (3.198 MeV), 5 − 2 (3.709 MeV), 2 + (4.086 MeV) and 4 + (4.324 MeV) states are in good accord with values extracted at other incident proton energies. The fact that the deformation lengths for these levels are independent of incident proton energy within experimental uncertainty provides further support of the validity of the collective DWBA for medium energy proton scattering to strongly excited states. Advantage is taken of this to extract more precise values for the ratio of neutron to proton multipole matrix elements for both the low-lying states and the giant quadrupole resonance at 10.6 MeV.

A surface-delta description of analyzing power measurements for collective states of Ni and Zn isotopes☆

Abstract Analyzing powers and differential cross sections for the inelastic scattering of 15 MeV polarized protons to the stronger collective states of 58, 60 Ni and 64 Zn have been measured. The data were compared to detailed microscopic reaction calculations using a central plus spin-dependent effective interaction. The nuclear structure wave functions used were obtained from a quasiparticle random-phase appoximation calculation using a spin-dependent surface delta interaction and a basis set containing both neutron and proton configurations. The overall quality of the fits strongly suggests that the surface-delta wave functions provide a good representation for spherical nuclei in this intermediate mass range. The effects of exchange and further modifications to the microscopic effective interaction are discussed.

Relativistic medium effects for 208Pb(p,p′) at intermediate energies

Abstract Analyzing power and spectral data for the inclusive quasifree ( p ,p ′) reaction on 208Pb at 290 MeV are presented. Cross sections were measured over an angular range of 4°–26° and for excitation energies up to 160 MeV. The free surface response model provides a good description of the shape of the continuum. At the quasifree peak the large angle Ay(θ) data drop below the free nucleon values. This difference is a possible indication of relativistic medium effects.

Elastic and inelastic scattering of 280 and 489 MeV protons from 58Ni

Abstract Nonrelativistic and relativistic collective deformed potential models have been used to study the excitation of seven states in 58Ni by 280 and 489 MeV unpolarized protons. Optical model potentials were determined at each energy by fitting elastic scattering data using both of the models. Deformation lengths were also obtained using both models with no dependence on incident energy observed for either model. The average value of the hadronic deformation lengths along with the charge deformation lengths were used to calculate neutron to proton multipole matrix elements ( M n M p ). Five transitions, 2+1, 1.45 MeV; 2+3, 3.04 MeV; 2+4, 3.26 MeV; 4+1, 2.46 MeV; and the 4+5, 4.75 MeV state were determined to be isoscalar, i.e. M n M p = N Z . The 4+2, 3.63 MeV state was observed to be a proton-like transition with M n M p = 0.43 ± 0.18. At small scattering angles, the relativistic model gave consistently larger cross sections than the nonrelativistic model. This effect was much larger at 280 MeV than at 489 MeV.