F.W.N. De Boer

An in-beam Ge(Li)-NaI(Tl) Compton suppression spectrometer

Abstract A Compton suppression spectrometer used for in-beam gamma-ray spectroscopy is described. The system consists of a large NaI(Tl)-crystal (O 25.4 cm, 25.4 cm long) and a large-volume closed-end Ge(Li) detector. The overall suppression factor is about 11, with an optimum of 14.5 at low energies. Examples are given of the use at proton- and π − -beams.

A study of the rotational side-bands in 162Dy

Abstract Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.

Intensities of γ-rays in the decay of 166mHo

Abstract New values are given for relative γ-ray intensities in the decay of 166m Ho, that can be used for intensity calibration.

A systematic investigation of the (α, 2nγ) reaction on medium-heavy nuclei

Abstract Exclusive neutron spectra and angular distributions have been measured for 28–35 MeV (α, 2nγ) reactions on various nuclei in the 80 ≦ A ≦ 210 region. Pre-equilibrium processes dominate the 35 MeV (α, 2nγ) reaction mechanism in much of this region. Analysis of systematic variation in the neutron spectrum parameters shows that the reaction mechanism is strongly correlated with the target neutron excess parameter ( N − Z / A . Analysis of the γ-decay of the entry states shows that well-defined incident angular momentum windows exist for the pre-etjuilibrium (α, 2nγ) reaction. These features are discussed in terms of various models for the reaction mechanism.

A study of the 84Kr(α, 2nγ)86Sr reaction

Abstract Decay properties of levels of 86 Sr populated by the 28 MeV 84 Kr(α, 2n γ ) reaction have been studied by in-beam γ-ray spectroscopy. The observation of several new levels with J ≧ 6 allows a detailed comparison of the 2p-2n structures of 86 Sr with those of neighboring N = 48 nuclei. The results are discussed in terms of the weak-coupling model.

The level structure of 90Mo

Abstract High-spin states of the N = 48 nucleus 90Mo have been studied using the 33 MeV 90Zr(3He, 3nγ) reaction. A previously unknown level structure above the 8+ isomer and several new lower-lying levels have been identified. The results are discussed in terms of shell-model calculations which allow four protons in the 2 p 1 2 and 1 g 9 2 subshells and two neutron holes in the 1 f 5 2 , 2 p 3 2 , 2 p 1 2 , or 1 g 9 2 orbitals.

Gamma-ray spectroscopy of87Y

The level structure of87Y has been studied using in-beam γ-ray spectroscopic methods. Many previously unknown states have been observed. Members of theπ(g9/2)ν(g9/2)−2 multiplet are identified. The results are discussed in terms of the weak-coupling model.

Search for resonant Bhabha scattering around an invariant mass of 1.8 MeV

Abstract A search was made for resonant Bhabha scattering of positrons on atomic electrons using two Mini-Orange spectrometers and relatively thick targets of low Z. No resonances were found within (σ res Γ) limits of ≅20 to 40 barn eV for lifetimes in the 10 −10 −10 −12 s range of an intermadiate particle propagating freely in the target material.

A study of 80,82,84Sr by (α, 2nγ) reactions

Abstract The AKr(α, 2nγ)A+2Sr reactions have been studied by in-beam γ-ray spectroscopy for A = 78, 80, and 82. States with spins up to 10+ in 82,84Sr and 6+ in 80Sr are identified. The Sr nuclei undergo a transition from spherical to deformed ground states as neutrons are removed from the N = 50 shell. The systematics of these nuclei are compared with calculations based upon the shell model and interacting boson model.

Decay pathways and entry-state population in (α, xnγ) and (3He, xnγ) reactions on 150Nd

Abstract Exclusive neutron spectra and angular distributions have been measured by n-γ coincidence techniques for (α, x n γ) and ( 3 He , x n γ) reactions on 150 Nd in the 25–44 MeV range. The results show that pre-equilibrium processes dominate low-energy light-ion induced reactions at certain beam energies. Analysis of the yrast γ-ray cascades in the residual nuclei shows that a well-defined minimum entry-state spin exists in final nuclei populated by pre-equilibrium processes. This in turn implies the existence of a minimum classical impact parameter for these reactions. These results are discussed in terms of phenomenological models of the reaction mechanisms.