Ag Drentje

QMG/2, A MAGNETIC SPECTROGRAPH FOR NUCLEAR-RESEARCH

Abstract A new magnetic spectrograph, the QMG/2, has been designed for the Kernfysisch Versneller Instituut, Groningen. It is similar to the Q3D spectrographs in use at several nuclear physics laboratories but includes some new features. Among these are a second multipole, a straight focal plane, constant dispersion, constant angle of incidence (45°) on the detector, and a device called a “snake” used to adjust mechanically the effective shape of the field boundaries after manufacture.

A versatile detection system for a broad range magnetic spectrograph

The focal plane detector for the QMG/2 magnetic spectrograph of the Kernfysisch Versneller Instituut is described. It consists of two-dimensional position sensitive proportional detectors and scintillation detectors. The properties of the components of the set-up and of the system as a whole in conjunction with the associated electronics and software are presented. The system allows easy optimization of spectrograph focusing and correction of kinematic effects. The position resolution is about 1 mm for relatively highly ionizing particles (50 MeV α-particles), and slightly worse for low-ionizing particles (50 MeV protons).

A study of the isoscalar giant quadrupole resonance and other collective states in 16O excited by inelastic alpha scattering at Eα = 104 MeV

Abstract The 16O(α,α′)16O reaction was studied at Eα = 104 MeV in the region of the giant resonances (GR). A prominent structure which could be resolved into various components is observed between Ex = 17 and 25 MeV. The components of the structure above Ex = 20 MeV vary in shape with angle indicating the existence of various multipolarities. DWBA calculations were performed with the collective form factor prescription. For l = 0, 1 and 3, DWBA calculations were also performed using microscopic wave functions. The forward angular behaviour of the structure between Ex = 23 and 24 MeV is found to be consistent with l = 0 transfer, while for other components of the structure above E x = 20 MeV , l≧3 contributions could not be excluded. The fraction of the isoscalar quadrupole energy weighted sum rule (EWSR) exhausted between Ex = 17 and 25 MeV is 40−10%+20%. Our extracted isoscalar quadrupole strength distribution agrees qualitatively with theoretical results. Many states of well known spin and parity are excited below Ex = 17 MeV. Satisfactory fits could be obtained for the natural parity states with DWBA using the collective form factor prescription except for the 2+ state at 9.85 MeV and 4+ state at 10.35 MeV and the 1−, T = 0 states. For the 2+ and 4+ states, coupled channel calculations (CC) describe the experimental angular distributions correctly, while for the 11− states we obtain good fits using a microscopic wave function description of these states.

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.

Experimental determination of monopole strength in 40Ca between 10 and 20 MeV excitation energy

The distribution of isoscalar monopole strength in 40Ca was determined from singles 40Ca(α, α′) and from coincident 40Ca(α,α′α0) data at Eα =120 MeV and scattering angles from 0° to 3°. Between 10.5 and 15.7 MeV excitation energy (23±5)% of the EO EWSR was located from the singles and from the coincidence measurements independently, while (30 ±6)% of the EO EWSR was found in the interval 10.5–20 MeV from the singles data only.

SELECTIVE SINGLE-ELECTRON CAPTURE INTO (N,L) SUBSHELLS IN SLOW COLLISIONS OF C-6+, N-6+, O-6+ AND NE-6+ WITH HE, H-2 AND AR

By means of VUV photon spectroscopy absolute (n, l)-subshell-selective electron capture cross sections sigma nl have been determined for collisions of C6+, N6+, O6+, Ne6+ with He; C6+, N6+, O6+ with H2 and O6+ with Ar, in the velocity range 0.15-0.5 au. These cross sections depend strongly on the collision velocity, in contrast with total cross sections for electron capture and cross sections for capture into different n shells. One n shell is predominantly populated. The comparison of sigma nl( nu ) for different ions with the same ionic charge suggests that the structure of the ionic core is not very important for the l distribution, as long as the energy levels of the ionic species under consideration are almost hydrogen-like. The comparison of results for O6+-H2, Ar shows that the binding energy of the target is much more important than its internal structure for the resultant shape of sigma nl( nu ).

CHARGE-DISTRIBUTION OF MULTIPLY CHARGED IONS SCATTERED FROM TUNGSTEN

The authors have measured the charge state distribution of 20 keV multiply charged Ne, Ar and Kr ions scattered from a polycrystalline tungsten target. For primary charge states up to 8+ only singly and doubly charged reflected particles are observed. However, when the primary charge state exceeds 8+, scattering into triply charged ions does occur and the shapes of the respective energy spectra also change. An explanation is given in terms of autoionisation processes occurring when the scattered particle leaves the surface in a highly excited state.

Measurement of metastable fractions in multiply charged ion beams by ion excitation in core-conserving electron capture

Conservation of the electronic configuration of ground-state and metastable ions in the capture of an electron from Li(2s) into excited states has been utilised to measure metastable ion beam fractions for multiply charged ions. Results are presented for beams of C2+, N2+, N3+ and O4+ respectively, which have been produced by means of an ECR ion source.

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.

Giant Quadrupole Resonance in 24,26Mg : A Comparison of Inelastic-Scattering and α-Capture Experiments

The distribution of giant quadrupole resonance strength in /sup 24,26/Mg obtained from inelastic ..cap alpha..- and proton-scattering experiments and ..cap alpha..-capture experiments have now been shown to be compatible. The results show that the giant quadrupole resonance strength in /sup 24/Mg and /sup 26/Mg is fragmented into several states or clusters of states and suggest that the giant quadrupole resonance states have very different ..cap alpha../sub 0/-decay strength.