Two aspects of applied nuclear physics have been studied in this thesis; Monte Carlo simulations for high-sensitivity monitoring of radioactive xenon and high-accuracy neutron-proton scattering measurements for neutron physics applications and fundamental physics. The Monte Carlo simulations have been performed for two systems for detection of radioactive xenon, using the MCNP code. These systems, designed for monitoring of violations of the Comprehensive Nuclear-Test-Ban Treaty, are based on coincident detection of electrons and gamma rays, emitted in beta decay of xenon nuclides produced in nuclear weapons explosions. In general, the simulations describe test data well, and the deviations from experimental data are understood. The neutron-proton scattering measurements have been performed by measuring the differential np scattering cross section at 96 MeV in the angular range θc.m.= 20° – 76°. Together with an earlier data set at the same energy, covering the angles θc.m.= 74° – 180°, a new data set has been formed in the angular range θc.m.= 20° – 180°. This extended data set has been normalised to the experimental total np cross section, resulting in a renormalisation of the earlier data of 0.7 %, which is well within the stated normalisation uncertainty for that experiment. The results on forward np scattering are in reasonable agreement with theory models and partial wave analyses and have been compared with data from the literature.

Developing radiation delivery systems for optimisation of absorbed dose to the target without normal tissue toxicity requires advanced calculations for transport of radiation. In this thesis absorbed dose and fluence in different radiotherapy applications were calculated by using Monte Carlo (MC) simulations. In paper I-III external neutron activation of gadolinium (Gd) for intravascular brachytherapy (GdNCB) and tumour therapy (GdNCT) was investigated. MC codes MCNP and GEANT4 were compared. MCNP was chosen for neutron capture reaction calculations. Gd neutron capture reaction includes both very short range (Auger electrons) and long range (IC electrons and gamma) products. In GdNCB the high-energetic gamma gives an almost flat absorbed dose delivery pattern, up to 4 mm around the stent. Dose distribution at the edges and inside the stent may prevent stent edge and in-stent restenosis. For GdNCT the absorbed dose from prompt gamma will dominate over the dose from IC and Auger electrons in an in vivo situation. The absorbed dose from IC electrons will enhance the total absorbed dose in the tumours and contribute to the cell killing. In paper IV a model for calculation of inter-cluster cross-fire radiation dose from β-emitting radionuclides in a breast cancer model was developed. GEANT4 was used for obtaining absorbed dose. The dose internally in cells binding the isotope (self-dose) increased with decreasing β-energy except for the radionuclides with substantial amounts of conversion electrons and Auger electrons. An effective therapy approach may be a combination of radionuclides where the high self-dose from nuclides with low β-energy should be combined with the inter-cell cluster cross-fire dose from high energy β-particles. In paper V MC simulations using correlated sampling together with importance sampling were used to calculate spectra perturbations in detector volumes caused by the detector silicon chip and its encapsulation. Penelope and EGSnrc were used and yielded similar results. The low energy part of the electron spectrum increased but to a less extent if the silicon detector was encapsulated in low z-materials.