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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.