When a nuclear explosion occurs certain radionuclides are emitted, notably xenon. Due to the fact that xenon is a noble gas, it is hard to contain and can therefore be detected far from the explosion site. There are four isotopes of xenon that are of interest in the detection of a nuclear explosion: 131mXe, 133mXe, 133Xe and 135Xe. By constantly measuring the amount of these isotopes in the air, changes in the concentration in an indication that a nuclear explosion has occurred. In this thesis a detector was modelled in GEANT4 and focuses on one kind of noble gas detector: SAUNA - the Swedish Automatic Unit for Noble gas Acquisition. SAUNA uses the coincidence technique in order to determine the concentration of xenon there is in the air. By using the coincidence technique, it is possible to reduce the impact of the background radiation and therefore increase the efficiency of the detector. 133Xe has a coincidence when it first undergoes beta decay, with an endpoint energy of 346 keV, and then emits a 80 keV gamma particle. 135Xe has also a dual coincidence, a beta decay with an endpoint energy of 910 keV together with a 250 keV gamma-ray. However both these isotopes have a triple coincidence decay that also can be exploited: for 133Xe, a beta particle with endpoint energy of 346 keV, a 30 keV X-ray and a 45 keV conversion electron, while for 135Xe there is instead of the gamma particle a 30 keV X-ray and a 214keV conversion electron that can be emitted together with the beta particle. The 30 keV X-ray together with the beta particle for 133Xe can also be used as a dual coincidence, in that case the conversion electron is ignored. For 133Xe, when a beta particle, a 45 keV conversion electron, and a 30 keV X-ray are emitted, the model was able to detect all three particles in 69.2% ± 0.1 of the cases. However, when only the particles with a detected energy within a 5 keV interval of their generated energies are considered to be in coincidence, then for 133Xe triple coincidence occurs in 22.9% ± 0.2 of the cases. For 135Xe the model was able to detect the triple coincidence (between a beta, 214 keV CE and 30 keV X-ray) in 63.5% ± 0.1 of the cases. This work shows that adding another particle in a coincidence reduces the chance to detect the coincidence. The positive effect of adding another particle in a coincidence is that the minimum detectable concentration of xenon should be smaller. The goal for future detectors should be to make it possible for the detector to take advantage of the triple coincidences but at the same time be also able to use the dual coincidences.