G.C. Phillips

The significance of the generalized density of states function for nuclear spectra

Abstract The cluster model of the nucleus suggests that three- (or more-)body decay may be treated as a time sequence of two-body interactions. The cross section for particle emission into a spectrum of energies may be calculated if it assumed that the reaction a + A → D ∗ → → b + C + c occurs as a sequence of two-body decays: D ∗ → B ∗ + b ; B ∗ → c + C . If B∗ is produced in a localized state of radius a and decays only via the single two-body channel c+C whose phase shift is known, then the cross section for emission of particles b is largely determined by a density of states factor ϱ = (1/π)d/dEB(δl(EB)+ϕl(a, EB)). This result is discussed and compared both to experiments and to other treatments of the same problem.

The scattering of alpha particles from C12

The elastic scattering of alpha particles from C12 is experimentally studied in the energy range 2.5 to 4.8 Mev using a precision, small-volume, gas scattering chamber. Excitation functions are measured at the center-of-mass (c.m.s.) angles 70 deg 7 , 90 deg 0 , 99 deg 15 , 109 deg 54 , 125 deg 16 , 140 deg 16 , 149 deg 27 , and 166 deg 35 , and angular distributions are measured at 2.820, 3.377, 3.989, and 4.640 Mev. These data are parameterized in terms of nuclear scattering phase shifts and the phase shifts have been analyzed using the single level dispersion theory. Three states in O16 are observed: a 1- state at 9.552 Mev, a 2+ state at 9.815 Mev, and a 4+ state at 10.329 Mev. These states are found to have c.m.s. reduced widths of 608, 1.07, and 195 kev respectively, with corresponding ratios to the Wigner limit of 85, 0.15, and 27%.

Three α-particle decay of C12

Abstract The reaction B11(p, 2α)He4 has been studied at proton bombarding energies of 2.0, 2.65, 3.25, 3.73, 4.00, 5.08 and 5.64 MeV. These energies excite several resonantstates in C12. The kinematics of the reaction were completely determined by detecting in coincidence two of the three final α-particles in solid-state detectors. This determination of the kinematics permitted discrimination between sequential two-body decay and direct three-body decay. The reaction mechanism was found to be sequential two-body decay involving the ground and the broad first-excited states of Be8, with events involving the latter showing strong interference effects. These interference effects were attributable to the identity of the three final particles and to the ambiguity of the order of their emission. Not more than 5 % of the experimental cross section could be attributed to the direct three-body decay mechanism; indeed, the contribution from the direct process is probably much smaller than the quoted 5 %. Hence, the conclusion must be that, for C12 with an excitation energy between 17 and 21 MeV, the decay mechanism is predominantly sequential two-body decay. Also the total cross section for this reaction is of the order of 100 to 200 mb; thus it is comparable to the geometric cross section. These two facts imply that, in this energy region, C12 has a strong cluster configuration of an α-particle plus a Be8 nucleus in either the ground or the first-excited state.

The scattering of protons from He3

Abstract The elastic scattering of protons from He3 has been investigated from 2.0 to 4.8 MeV with the Rice University Van de Graaff accelerator and a small-volume, gas scattering chamber. Nine excitation curves were measured for centre-of-mass angles between 39.23° and 166°, and angular distributions have been measured for proton energies of 2.01, 3.01, 3.99, and 4.54 MeV. The phase shifts derived from these experimental data and from the data obtained at other laboratories are compared with the theoretical calculations of Bransden and Robertson. These results tend to favour the Serber rather than the symmetrical type of exchange interaction for the nucleon-nucleon potential.

Measurement of (p, n) thresholds at tandem energies

Abstract The “counter ratio” technique of Bonner et al. for measuring neutron thresholds has been extended to bombarding energies greater than 6 MeV. Neutron background problems had previously precluded neutron threshold measurements with this technique at these energies. A system was developed which allowed the beam to be stopped 9.14 m from the thin foil target in a heavily shielded cave. No slits were used other than those of the analysing magnet, and they were shielded from the experimental area. A remotely rectractable plunger permitted accurate alignment of the beam by minimizing the beam current on a Ta disk with a 4.7 mm aperture. the slow counter consisted of 2 BF3 tubes imbedded in paraffin through which the beam passed. Improvement in background by a factor of 5 or better over stopping the beam in a 58Ni slug at the target was achieved. Thresholds were measured for various (p, n) reactions, and the results are given as follows according to the reaction (level in residual nucleus) threshold energy in MeV: 13C(p, n)13N(0) 3.235, (2)6.965; 27Al(p, n)27Si(0)5.800, (1) 6.611, (2) 6.793, (3) 8.046, (4) 8.549; 52Cr(p, n)52Mn (1) 5.975, (2) 6.149; 58Ni(p, n)58Cu (0) 9.509; 60Ni(p, n)60Cu (0) 7.025, (1) 7.092, (2) 7.325; 62Ni(p, n)62Cu (0) 4.812, (1) 4.852, (2) 5.109. Many resonances were observed in the various compound nuclei.

The scattering of He3 by He4

Abstract Cross sections for the elastic scattering of He 3 by He 4 have been measured for incident He 3 energies in the range 2.5 ≦ E He 3 ≦ 5.7 MeV (lab system) using a Van de Graaff accelerator and a differentially pumped gas scattering chamber. Excitation functions were measured at centre-of-mass angles 54°44′, 63°27′, 73°57′, 90°2′, 104°38′, 106°36′, 116°37′, 125°19′ and 140°50′. Angular distributions were measured at E He 3 = 2.467, 3.600, 4.761 and 5.703 MeV. The scattering phase shifts were obtained from these data. The general trend of the S-wave phase shift with energy is similar to that for hard-sphere scattering with a radius R = 2.8 fm, although there is some indication of a systematic departure from this. The variation of the δ 1 + phase shift with energy is consistent with the effect of the Be 7 ground state, calculated using single-level dispersion theory. However, the behaviour of the δ 1 − phase shift cannot be explained by the effect of only the Be 7 first excited state. The presence of the second excited state in Be 7 explains the behaviour of the δ 3 + phase shift fairly well. The parameters used for this state were J π = 7 2 − , R = 4.4 fm , characteristic energy = 3.56 MeV , reduced width = 0.7 MeV .

A TWO-BODY CLUSTER MODEL OF THE NUCLEUS

Calculations are presented for coupled Schrodinger equations which develop the detailed behavior predicted by the two-body cluster model described in a preceding paper. The results show that: 1) narrow resonances in the scattering cross section may be quantitatively described by this model, 2) the cross section in the neighborhood of the threshold of another channel shows the Wigner cusp, and 3) the configuration mixing of bound states is a necessary consequence of the model. As an example, the model is used as a means of parameterizing the lowest states in C^(13). This method allows a fit to the scattering phase shift and the level structure, at the same time making a definite prediction of the fractional parentage of the bound state.

Three-body break-up in 7Li(d, nα)4He and 7Li(d, αα)n reactions

Abstract The reaction 7 Li(d, n α) 4 He has been studied detecting neutron-alpha coincidences. The energies of the detected particles were determined by means of time-of-flight measurement for neutrons and a silicon, surface-barrier detector for alphas. Simultaneously alpha-alpha coincidences from 7 Li(d, αα)n were detected using two solid-state detectors. The measurements were performed at bombarding energies from 2.6 to 4.0 MeV in steps of 0.2 MeV. In addition the reaction 7 Li(d, αα)n was studied separately at 4.0 and 9.0 MeV for several angular settings of the two solid-state detectors. The most important reaction mechanism is found to be sequential decay through intermediate 5 He and 8 Be states.

Phase shift analysis of the elastic scattering of protons from oxygen

Abstract The elastic scattering of protons by O16 has been investigated in the range of bombarding energies from 3.5 MeV to 5.75 MeV; twelve excitation functions and four angular distributions have been measured. A phase shift analysis of the cross section data shows the existence of a broad (Γ lab = 0.220 MeV ) 3 2 − state at 4.60 MeV in F17 and a very broad (Γ lab = 1.0 MeV ) 3 5 + state at 4.97 MeV in F17. In addition the analysis indicates the possible existence of states with assignments 3 2 − , 7 2 − and 3 2 + at F17 excitations of 5.47, 5.67, and 5.84 MeV, respectively. The 3 2 − and 3 2 + states at 4.60 MeV and 4.97 MeV are shown to have a fairly large probability of being found in the two-body configuration p+O16.

The Cr52(d, pγ)Cr53 reaction☆

Abstract Energy levels and gamma decay schemes for the nucleus Cr53 have been observed in the reaction Cr52(d, pγ)Cr53 using particle-gamma coincidence techniques. Levels have been observed at 0.56, 1.01, 1.29, 1.54, 1.97, 2.32, 2.69, 3.19, 3.60 and 3.70 MeV. Tentative spin and parity assignments have been made for a number of these levels by combining the gamma decay results with the results of other experiments, particularly stripping analysis.