4? Particle?Hole States in 16O

Abstract

4 -states of 16 0 at excitation energies of 17·79 and 19·80 MeV (T = 0) and 18·98 MeV (T = 1) are observed to be strongly excited in inelastic scattering of 135 MeV protons at momentum transfers of 200-500 MeV/c. The previous ambiguity in the spin and parity assignments for the 17·79 MeV state is removed.Excitation of stretched particle-hole 6-states has been observed (Adams et al. 1977) in the inelastic scattering of intermediate energy protons on 24Mg and 2sSi. The predominant configuration in these 6-states was assumed to be If 7 / 2 (1d s / 2 )-1, and the measured differential cross sections were successfully interpreted in terms of this configuration.In I p-shell nuclei the maximum angular momentum achievable by a single particle-hole excitation is 4 -, in a Id s / 2 (l P3/2) -1 excitation. Only two 4-states in 16 0 have been confirmed in transfer reactions (MairIe et al. 1978); a third state was observed but definite spin and parity assignments could not be made. The present work was undertaken in an attempt to observe these states in a (p, p') reaction at 135 MeV bombarding energy and to attempt to clarify the existence of a third 4-state.The incident beam of 135MeV protons, which was obtained from the Indiana University Cyclotron Facility (IUCF), bombarded a 10 mgcm -2 Mylar (C 10 H s 0 4 ) target. The carbon contribution was measured by bombardment of a polystyrene target ((CH)n) of similar thickness, and the oxygen contribution was obtained by subtraction of spectra.The inelastically scattered protons were detected with the QDDM magnetic spectrograph of the IUCF, to which the beam analysis system and beam line were dispersion matched. The excitation energy bite at the focal plane of the spectrograph was about 6t MeV, and the spectrograph was set to cover the excitation region from 14 to 20·5 MeV in 16 0. The focal plane detector system was composed ofa helical delay-line gas counter for position measurement (Officer et al. 1975), followed by two plastic scintillators. The differential energy loss of detected particles in the two scintillators was used in particle identification; a triple coincidence from the detectors was required. Overall resolution for most of the spectra was about 80 keV FWHM.

Complementary reaction analyses and the isospin mixing of the states in 16O

“…We place more reliability in the proton inelastic scattering results that we discuss next, especially since the separate (bound) proton and neutron amplitudes constructively interfere to define the cross sections. In this subsection, we present the results of analyses of 135 MeV inelastic scattering data [30], of 135 MeV (p, n) data (to the IAS 4 − state in 16 F) and of the analyzing power measured in that experiment [13]. The latter is of particular interest in that spin observables usually are quite sensitive to details in the calculations.…”

“…The third is classified as an isovector state whose analogue in 16 F has been seen in charge exchange (p, n) experiments [12,13,14]. Furthermore, electron scattering (transverse) form factors for two of those states have been measured and analyzed [6,7], π (±) inelastic scattering cross sections for energies in the ∆ resonance region have been studied [15], and the cross section from inelastic scattering and cross section and analyzing power from charge exchange scattering of 135 MeV protons have been reported [13,30].…”