NUCLEON-INDUCED EXCITATION OF COLLECTIVE BANDS IN CARBON-12 AND THE APPLICATION TO NEUTRON DOSIMETRY AT E(N) > 20 MEV

This work involves the measurement and analysis of neutron elastic and inelastic scattering cross section from ('12)C at incident neutron energies between 20 and 26 MeV. These data cover an energy range that has not previously been investigated with neutrons and where the direct interaction mechanism should be dominant. These energies are also of particular interest in the fields of radiation protection and radio-therapy. The objectives of the present work are to analyze excitation of the measured collective states in ('12)C and to develop an energy dependent optical model potential that can be used to calculate quantities of importance to neutron dosimetry at all energies between 20 and 100 MeV.; The present differential cross sections were obtained by the time-of-flight technique using the beam swinger facility of Ohio University. The required neutrons were produced by the T(d,n) reaction with the incident deuteron beam produced by the Tandem Van de Graaff accelerator. A scattering sample of 24.16 gm of graphite formed in a 2.54 cm long, 2.54 cm diameter right cylinder was used at each neutron energy. The neutron detectors, a matched array of seven 20.32 cm diameter, 10.16 cm thick NE213 liquid scintillator optically coupled to RCA4522 photomultiplier tubes, were placed at 13 m away from the sample. The measurements were performed between 27 and 40 different angles from 10('(DEGREES)) to 160('(DEGREES)). In addition to these experiments, a separate series of experiments was performed using a large sample (47.45 gm) at 22 and 24 MeV to investigate the weakly excited states above the 3(,1)('-) (9.641 MeV) level in ('12)C. Differential cross sections for nine excited states in the first 15 MeV excitation of ('12)C were resolved and measured. New rotation-vibration formalisms were developed to be used with the code ECIS79 in order to analyze the collective motions of ('12)C.; Excellent agreement between experimental data and theoretical calculation is obtained for (a) the ground state rotational band, (b) 0(,2)('+) (7.655 MeV) state using (beta)-vibration plus breathing mode, (c) the 1(,1)('-) (10.84 MeV) and 2(,1)('-) (11.83 MeV) states using K('(pi)) = 1('-) octupole vibration, and (d) the 3(,1)('-) (9.64 MeV) and 4(,1)('-) (13.35 MeV) states using K('(pi)) = 3('-) octupole vibration. An energy dependent optical model potential has been obtained that describes the present neutron scattering data, differential proton scattering data of higher energies and total neutron cross sections between 20 and 100 MeV. The present work has significantly extended the library of differential neutron cross sections for ('12)C and has brought unity to the theoretical description of scattering for this deformed, light nucleus.