P.B. Treacy

Nuclear levels near thresholds

Abstract Cross sections for reactions proceeding through unstable states of nuclei may be expressed in terms of a density-of-states function. A widely-used form ϱ (1) of this function is compared with a recently suggested form ϱ (2) and it is shown that cross sections involving levels of the nuclei Be 8 , Be 9 and B 9 which have been interpreted in terms of ϱ (2) can be satisfactorily explained on the basis of ϱ (1) . Certain inconsistencies in the use of ϱ (2) are pointed out, in addition to a property which makes it unsuitable for general use. From the form of ϱ (1) it is concluded that an anomalous peak, or “ghost”, may be associated with an isolated level sufficiently close to a threshold, and that such a ghost could be mistakenly identified as a separate level. A discussion of ghosts is given, and related to levels in some light nuclei.

The B11 (p, α) Be8 reaction and C12 states between 15 and 20 MeV

Detailed angular distributions were measured of alpha particles from the B/sup 11/(p, alpha )Be/sup 8/ reaction for proton energies between 0.7 and 6.0 Mev. Previous spin and parity assign ments of 0/sup +/ and 2/sup +/ for levels at 17.77 and 18.34 Mev in C/sup 12/ are confirmed, and partial proton and alpha particle widths deduced for both levels. A level at 20.63 Mev, of width 27 kev, is assigned 3/sup -/. The behavior of the cross section for proton energies below 2 Mev is discussed in terms of effects of levels below the proton threshold in C/sup 12/. (auth)

12C-α elastic scattering and states of 16O between 9.16 and 12.11 MeV

Abstract Elastic scattering of alpha particles from 12 C has been measured for 16 O excitation energies between 9.26 and 12.11 MeV. The results have been interpreted using a real phase-shift analysis and parameters from single-level fits have been assigned for broad 0 + , 1 − and 3 − levels of 16 O.

The 6Li(d, α)4He reaction and states of 8Be between 22 and 32 MeV

Abstract Excitation functions and angular distributions for the 6 Li(d, α) 4 He reaction have been studied with beam energies between 3 and 12 MeV. Evidence is given for participation of 8 Be states at 27.5 MeV (4 − ) and near 30 MeV (6 + ).

Levels of N13 near 7 MeV excitation (II)

Abstract Excitation functions and angular distributions of protons inelastically scattered by carbon have been measured for beam energies between 5.2 and 6.6 MeV in order to give information on the levels of N13 near 7 MeV excitation. An analysis is made of these data, together with previously measured angular distributions of the 4.43 MeV γ-rays emitted following the inelastic scattering. Good agreement with these results can be obtained by assuming contributions from a small number of interfering levels of N13. Levels at 7.16 and 7.38 MeV are found to have spins and parities 7 2 + and 5 2 − or 7 2 − , respectively, which differ from assignments previously accepted. Partial widths are obtained for these levels and for the 3 2 + level at 6.88 MeV. The 5 2 + level at 6.38 MeV gives important contributions to the inelastic scattering cross section in the region studied, and small but significant contributions can come from the 1 2 − ground state and the 3 2 − level at 3.51 MeV.

Magnetic properties of F19 in the 197 keV nuclear level

Abstract Gamma rays are observed from the de-excitation of the second excited state of F19 produced by inelastic proton scattering. It is found that anisotropy of this radiation is quenched by a uniform magnetic field applied normal to the proton beam, and a measurement of this effect yields the following value for the nuclear spectroscopic splitting factor: gI = 1.8±0.4. The degree of anisotropy cannot be influenced by a field applied parallel to the proton beam in the case of solid NaF and MnF2 targets, but with a gas target of CF4 such a field restores the undisturbed distribution. This effect is analysed with the aid of energy loss data and implies as an order of magnitude a hyperfine structure constant a = 0.2×10−18 erg for the free F19 in its ground P 3 2 state.

On complex angular momentum in nuclear reactions

Abstract The scattering amplitude and reaction line shapes in a spinless charged-particle channel, are analysed in terms of poles in angular momentum. The case of 4He plus 4He is considered as an example of the one-pole approximation. Practical difficulties are considered in regard to lack of unitarity, wrong threshold behaviour and ambiguity of background, in the representation. A modified representation is proposed, from which these difficulties have been eliminated.

9Be(3He, α) reactions to the 16.63 and 16.92 MeV states of 8Be

Abstract Coincidences have been observed between successively-emitted α-particles from the 9 Be( 3 He, α) 8 Be(α) reaction at a laboratory bombarding energy of 3 MeV. The reaction has been shown to proceed predominantly by successive transitions via states in 8 Be. The observed angular correlations have been interpreted to show that the spins and parities of the 16.63 and 16.92 MeV states of 8 Be are both 2 + .

The lowest T = 2 state at 5.989 MeV in 28Al

Abstract Comparison of the ratio of yields of the 26 Mg( 3 He, p) 28 Al and 27 Al(d, p) 28 Al transitions shows a strong enhancement for transitions to the 5.989 MeV state of 28 A1. This and the ( 3 He, p) angular distribution, support an assignment T = 2 for this level.

A remeasurement of the 12C(α, γ0) excitation function in the vicinity of the 12.44 and 13.1 MeV levels of 16O

Abstract The excitation function of the 12 C(α, γ 0 ) reaction at θ = 90° has been remeasured for bombarding energies between 6.5 and 8.5 MeV. The measurement was made to resolve discrepancies apparent in earlier measurements relating to the absolute cross section, the location of the lower 1 − resonance near 7.05 MeV ( 16 O excitation energy 12.44 MeV) and to the relative peak cross sections of this resonance and a second 1 − resonance at 7.88 MeV.