Thesis (MSc)--Stellenbosch University, 2005. / Some digitised pages may appear illegible due to the condition of the original hard copy. / ENGLISH ABSTRACT: Airborne gamma-ray spectrometry is used for exploration and environmental monitoring over large areas. Such datasets of 7-ray are acquired using large 16l NaI(T1) detector
crystals mounted in an aircraft. The response of these detectors to naturally occurring
radioactive isotopes is a function of detector volume, efficiency and resolution. This response
is modelled using the Monte Carlo method. A photon-electron transport computer
code, NIPET, is written to calculate the intrinsic efficiency and photofraction of these
large prismatic crystals. The intrinsic efficiency indicates the number of incident photons
that interact in some way with the detector resulting in a deposition of energy in the
crystal. The photofraction, on the other hand, indicates how many of these photons that
impinge on the detector are actually recorded with their full initial energy. This study
presents explicity calculated photofraction values for 16l NaI(Tl) crystals. Comparison of
these new values, calculated with this computer program, with that of published values
for small 3" x 3" crystals shows good agreement. However, for large crystals, the calculated
values differ from those in the literature by almost 15%. The study also finds that
for the gamma-ray energy range between 0.2 - 3.0 MeV, these detectors have an intrinsic
efficiency above 90%, whereas the photofraction values range from 80% for low energy
photons to above 40% for high energy photons in the case of large crystals. / AFRIKAANSE OPSOMMING: Lug gamma-straal spektroskopie is nuttig vir eksplorasie en omgewings ondersoeke oor
groot areas. Metings word tipies gedoen met 16l NaI(T1) detektors wat monteer is in ’n
vliegtuig. Die respons van hierdie detektors ten opsigte van natuurlike radioaktiewe isotope
is ’n funksie van detektorvolume, doeltreffendheid en oplosvermoë. Hierdie respons
word gemodeleer deur gebruik te maak van ’n Monte Carlo tegniek. ’n Rekenaarprogram,
NIPET, is geskryf om die intrinsieke doeltreffendheid en fotofraksie vir hierdie groot prismatiese
kristalle te bereken. Die intrinsieke doeltreffendheid dui die breukdeel van die
invallende fotone aan wat ’n interaksie ondergaan binne die detektor kristal en sodoende
energie deponeer. Die fotofraksie, daarenteen, dui die breukdeel invallende fotone aan
wat absorbeer word deur die kristal met hul voile invallende energie. In hierdie tesis
word eksplisiete fotofraksiewaardes vir 16l NaI(T1) kristale bereken. Goeie ooreenstemming
word gevind tussen fotofraksie waardes bereken met hierdie program en die waardes
gepubliseer in die literatuur vir klein 3" x 3" kristalle. Vir groter kristalle is daar egter
swakker ooreenstemming. Berekende fotofraksie waardes verskil met 15% van die waardes
in die literatuur. Daar word ook bevind dat vir gamma-energie tussen 0.2 - 3.0 MeV, die
intrinsieke doeltreffendheid vir hierdie tipe detektors bo 90% is en dat fotofraksiewaardes
wissel tussen 80% vir lae energie tot bo 40% vir hoë foton energie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50441 |
| Date | 03 1900 |
| Creators | Titus, Nortin P. |
| Contributors | Cowley, A. A., Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 82 p. : ill. |
| Rights | Stellenbosch University |
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