A Vanderwoude

THE DECAY OF ISOSCALAR GIANT-RESONANCES IN MG-24 AND CA-40

Abstract In this paper we present data on the charged-particle decay of the isoscalar 2+ strength between 10 and 20 MeV excitation energy (Ex) in 24Mg and 40Ca. The isoscalar strength was excited by inelastic scattering of 120MeV α-particles at 14° and 12.5° for 24Mg and 40Ca, respectively. The charged particles originating from the decay were detected in coincidence with the α′ particles at several angles in the scattering plane. Jπ assignments of the decaying states were made on the basis of the angular correlation pattern of the α0 decay to the ground state of 20Ne and 36Ar, respectively, using a DWBA calculation for the m-state population of the decaying state. For 40Ca, about 40% of the E2 EWSR is found to be located in the interval Ex = 13.5 ± 1.5 MeV, which is similar to what has been found from previous inelastic scattering experiments at Ex = 18 ± 2 MeV, but much more than such experiments located in the region Ex = 12–15 MeV. The difference for the region Ex A similar conclusion can be drawn from the behaviour of the forward-backward asymmetry in the angular correlations of the decay particles as a function of the excitation energy FBA(Ex). For 40Ca, FBA(Ex) for all decay channels increases smoothly on the average once Ex is above a well-defined threshold, which is due to the onset of knock-out processes. For 24Mg, however, the FBA(Ex) for the α0 shows a large fluctuation as a function of Ex, indicating an interference process between semi-direct decay and knock-out processes.

Four-nucleon pickup from 64Zn and 64Ni

The 64Zn and 64Ni(d, 6Li) reactions have been studied at 55 MeV bombarding energy. Eight final states in 60Ni and eight in 60Fe were identified and angular distributions were measured for these transitions from 8° to 35° in the lab system. A zero range, cluster model DWBA analysis is presented and several L-assignments are made. The L = 0 transitions both from the present work and from (6Li, d) data in the literature, are discussed in terms of the isovector pairing model. A brief comparison of the 64Zn(d, 6Li) results with previous (6Li, d), (t, p) and (p, t) data leading to 60Ni is also presented.

SATURATION OF THE WIDTH OF THE GIANT-DIPOLE RESONANCE AT HIGH-EXCITATION ENERGIES

Abstract Gamma-ray energy spectra have been measured for the 16 O + 118 Sn reaction at incident beam energies of 200 and 280 MeV. Comparing the experimentally obtained spectra with statistical-model calculations shows that the width of the isovector giant dipole resonance saturates beyond excitation energies of 165 MeV in the compound nucleus.

O-16 (ALPHA, P) F-19 REACTION AT EALPHA = 40 MEV

The 16O(α, p)19F reaction has been studied at Eα = 40 MeV. In this reaction high-spin states at high excitation energy are selectively excited. A study of positive parity states ranging from Jπ = 12+ to 132+ shows that the reaction can be described in terms of a direct transfer of a three-nucleon cluster with internal quantum numbers l = 0 and s = 12. An analysis in terms of a zero-range DWBA calculation accounts rather well for the shape of the angular distributions. The relative spectroscopic factors S for the transitions to the Jπ = 12+, 32+, 52+, 72+ and 92+ states have been compared with the intensity of the (λ, μ) = (6, 0) component in the SU(3) wave functions and are found to be in excellent agreement with calculated values. For the assumed Jπ = 112+ state at Ex = 9.90 MeV and the Jπ = 132+ states at Ex = 4.65 and 10.42 MeV the S-values are about three times larger than calculated. This might be due to small (sd)(fp)2 admixtures in the wave functions for these states. The analysis of the transitions to the negative parity states indicates that the shape of their angular distributions is sensitive to whether the transferred three-nucleon cluster originates from a p(sd)2 or a (sd)2(fp) coupling. Two negative parity states at Ex = 8.9 and 12.8 MeV are rather strongly populated. They could be the lower spin members of the weak-coupling doublets arising from the coupling of a p12. hole to the 20Ne Jπ = 6+ and 8+ states, respectively. No evidence is found for the excitation of the high-spin members. This is in agreement with the cluster model of Buck et al. which takes into account the spin-orbit potential of the triton cluster.

γ-decay of the deeply-bound hole states in 101Pd, 105Pd, 107Pd, 111Sn, 117Sn observed in the (3He, αγ) reaction at 70 MeV

Abstract The γ-decay of deep-hole states in 101, 105, 107 Pd was studied via the ( 3 He, αγ) reaction at E 3 he = 70 MeV and supplemented by data from 112, 118 Sn targets to investigate the deep-hole spreading mechanism. The γ-decay pattern for the g 9 2 deep-hole state shows a strong dependence on the spreading width: if the deep-hole state is observed as a sharp peak, it mainly decays to the low-lying 7 2 + state by a spin-flip M1 transition with a large M1-E2 mixing ratio; if the deep-hole state is observed as a broad bump, it decays statistically indicating the complete spreading of the hole strength over the underlying states; if the deep-hole state is observed with a structure intermediate between a sharp peak and broad bump, its γ-decay shows both decay patterns. A sharp peak at E x = 2.396 MeV in 101 Pd which carries a large fraction of the g 9 2 hole strength ( C 2 S = 2.0) was found to be a single state having a width of less than 2.5 keV. For the spin-flip M1 transition the destructive interference between the g 9 2 component and the coupled components of the deep-hole state was found in heavily spread states. A quasiparticle-plus-rotor (QPR) model was applied to calculate the fragmentation in the doorway stage for the g 9 2 neutron deep-hole state in the Pd isotopes. A reasonable agreement between the calculation and the experimental results was obtained for the strength fragmentation, for the nucleus 101 Pd. However, the large M1-E2 mixing ratio experimentally observed was not reproduced.

Two-nucleon T = 0 transfer reactions in the 0p shell

Abstract The 16 O(d, α) 14 N, 14 N(d, α) 12 C and 12 C(d, α) 10 B reactions at E d = 40MeV and the 12 C(α d)1 14 N at E α = 55 MeV were investigated. A total of seventeen transitions are analysed in terms of one-step, zero-range DWBA calculations, using the two-particle coefficients of fractional parentage obtained from the Cohen-Kurath Op shell wave functions. For most transitions, fair agreement is obtained between experiment and calculation, possible exceptions being the transition to the E x = 4.43 MeV, J π = 2 + state in 12 C and to the E x = 2.15 MeV, J π = 1 + state in 10 B, for which the calculations predict too much L = 0 strength. Where possible, a comparison with previous (p, 3 He) results is made. In 14 N a state at E x = 11.04 MeV was observed for which the values ( J π ; T ) = (3 + ; 0) are suggested. In 12 C we found, in addition to the well known T = 0 states, two relatively sharp T = 0 states at E x = 19.50 ± 0.10 and 20.55 ± 0.10 MeV. The shape and strength of the angular distribution for the transitions to these states can be approximately accounted for by the calculations, although no one-to-one correspondence between observed and predicted levels could be established.

GDR DISSIPATION AND NUCLEAR SHAPE IN HOT FAST-ROTATING DY NUCLEI

Abstract The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations ( β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β eq . Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈 β ∼0.35.

STUDY OF ISOVECTOR CHARGE-EXCHANGE EXCITATIONS AND THEIR DECAY VIA THE O-16(HE-3, TPBAR) REACTION

Abstract The nucleus 16F was studied via the 16O(3He, t) reaction at 81 MeV. Differential cross sections for many states were obtained and interpreted with DWBA calculations, using microscopic wave functions and an effective projectile-nucleon interaction. Proton decay to several states in 15O was observed and angular correlations for protons in coincidence with tritons detected at θ = 0° were measured. Several spin-parity assignments have been made. The distribution of isovector ΔL = 1 strength could be deduced. The analog of the giant dipole resonance ( E x ⋍ 9.5 MeV ) is strongly excited. The magnetic quadrupole strength has two strong components, one low, at Ex = 0.424 MeV, and one high, at E x ⋍ 7.5 MeV . Evidence is given for a proportionality between cross section and M2 strength for transitions to Jπ = 2− states, which possibly make the (3He, t) reaction a suitable tool for determining quantitatively isovector M2 (or Bij) strengths.

ON THE EXCITATION OF ISOVECTOR DIPOLE STRENGTH BY INELASTIC PROTON-SCATTERING IN THE GIANT-RESONANCE REGION IN LIGHT-NUCLEI

Abstract A detailed comparison between inelastic α and p scattering in the giant resonance region of 24, 26 Mg, 28 Si and 40 Ca shows that there is no evidence for ΔT = 1, E1 excitation in the (p, p′) spectra. This is consistent with DWBA calculations using a recently obtained isovector interaction potential.

Neutron decay of the isobaric analog state in 208Bi

The isospin-forbidden neutron decay of the isobaric analog state in 208Bi has been measured, following its excitation via the 208Pb(3He, t)“*Bi reaction at 61.2 MeV. In contrast to the proton decay, which is direct, the neutron decay spectrum has a statistical shape. Its branching ratio is determined to be (37 f 3)%. The results suggest that the damping of the isospin impurity is fast. Therefore, neutron decay results mostly from the final stage of the damping process and does not contribute to the width of the IAS.