Call number: LD2668 .T4 1966 H116 / Master of Science

published_or_final_version / Physics / Master / Master of Philosophy

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A solid-state DE-E telescope beta spectrometer has been built and used to study beta-minus spectra. The system consists of a 15 mm x 500 mm$ sp2$ hyperpure germanium (HPGe) E-detector and a 300 $ mu$m x 200 mm$ sp2$ Si DE-detector with a special "keyhole" design. / The response function of the system has been determined experimentally using mono-energetic electron beams with incident energies ranging from 1 to 12 MeV. / The beta endpoint energies of 25 nuclei, namely, $ sp{32}$P, $ sp{88}$Rb, $ sp{91-99}$Rb, $ sp{91-96}$Sr, $ sp{99-100}$Sr, $ sp{92-96}$Y and $ sp{100}$Y have been measured. The Q-beta values and mass excesses for A = 91-100 have been determined. The results for $ sp{99}$Rb, $ sp{99-100}$Sr and $ sp{100}$Y have been obtained for the first time. / The beta strength functions for odd mass Rb isotopes, namely $ sp{91-99}$Rb have been measured for the first time from direct beta spectroscopy with a solid-state telescope system. The experimental results have been compared to calculations using the Brown-Bolsterli (B-B) model.

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Bibliography: p. 171-173.

This thesis contains an account of a measurement of the neutron energy spectrum from radiative pion capture in carbon, oxygen and calcium. The measurement was performed at six n-γ correlation angles for the case of carbon and oxygen, and seven n-γ correlation angles for the case of calcium. The observed energy spectra fall off approximately exponentially between 2 and 30 MeV. Evidence of resonance structure was observed at about 4 MeV in the energy spectra of carbon and oxygen. This is in agreement with a theoretical prediction of the giant multipole excitation model. The neutron yield versus n-γ correlation angles peaks at 180° for all three nuclei studied. This indicates the quasi-free mechanism also plays an important role in the radiative pion capture process. / Ph. D.

The use of organic scintillators is an established method for the measurement of neutron spectra above several hundred keV. Fast neutrons are detected largely by proton recoils in the scintillator resulting from neutron elastic scattering with hydrogen. This leads to a smeared rectangular pulse-height distribution for monoenergetic neutrons. The recoil proton distribution ranges in energy from zero to the incident neutron energy. In addition, the pulse-height distribution is further complicated by structure due to energy deposition from alpha particle recoils from interactions with carbon as well as carbon recoils themselves. In order to reconstruct the incident neutron spectrum, the pulse-height spectrum has to be deconvoluted (unfolded) using the computed or measured response of the scintillator to monoenergetic neutrons. In addition gamma rays, which are always present when neutrons are present, lead to Compton electron recoils in the scintillator. Fortunately, for certain organic scintillators, the electron recoil events can be separated from the heavier particle recoil events in turn to distinguish gamma-ray induced events from neutron-induced events. This is accomplished by using the risetime of the pulse from the organic scintillator seen in the photomultiplier tube as a decay of light. In this work, an organic scintillator detection system was assembled which includes neutron-gamma separation capabilities to store the neutron-induced and gamma-induced recoil spectra separately. An unfolding code was implemented to deconvolute the spectra into neutron and gamma energy spectra. In order to verify the performance of the system, a measurement of two reference neutron fields will be performed with the system, unmoderated Cf-252 and heavy-water moderated Cf-252. After the detection system has been verified, measurements will be made with an AmBe neutron source.

In the deep burn research of Very High Temperature Reactor (VHTR), it is desired to make an accurate estimation of absorption cross sections and absorption rates in burnable poison (BP) pins. However, in traditional methods, multi-group cross sections are generated from single bundle calculations with specular reflection boundary condition, in which the energy spectral effect in the core environment is not taken into account. This approximation introduces errors to the absorption cross sections especially for BPs neighboring reflectors and control rods. In order to correct the BP absorption cross sections in whole core diffusion calculations, energy spectrum reconstruction (ESR) methods have been developed to reconstruct the fine group spectrum (and in-core continuous energy spectrum). Then, using the reconstructed spectrum as boundary condition, a BP pin cell local transport calculation serves an imbedded module within the whole core diffusion code to iteratively correct the BP absorption cross sections for improved results. The ESR methods were tested in a 2D prismatic High Temperature Reactor (HTR) problem. The reconstructed fine-group spectra have shown good agreement with the reference spectra. Comparing with the cross sections calculated by single block calculation with specular reflection boundary conditions, the BP absorption cross sections are effectively improved by ESR methods. A preliminary study was also performed to extend the ESR methods to a 2D Pebble Bed Reactor (PBR) problem. The results demonstrate that the ESR can reproduce the energy spectra on the fuel-outer reflector interface accurately.