The radiation dose in radiation therapy is mainly measured by ion chamber. The ion chamber measurement will not be accurate if there is not enough phantom material surrounding the ion chamber to provide the electron equilibrium condition. The lack of electron equilibrium will cause a reduction of dose. This may introduce problems in treatment planning. Because some planning algorithms cannot predict the reduction, they over estimate the dose in the region. Electron disequilibrium will happen when the radiation field size is too small or the density of irradiated material is too low to provide sufficient electrons going into the dose volume. The amount of tissue required to provide electron equilibrium in a 6MV photon beam by three methods: direct calculation from Klein-Nisina equation, measurement in low density material phantom and a Monte Carlo simulation is done to compare with the measurement, an indirect method from a planning algorithm which does not provide an accurate result under lateral electron disequilibrium. When the error starts to happen in such planning algorithm, we know that the electron equilibrium conditions does not exist. Only the 6MV photon beam is investigated. This is because in most cases, a 6MV small fields are used for head and neck (larynx cavity) and 6MV fields are commonly used for lung to minimise uncertainity due to lateral electron at higher energies. / Master of Science (Hons)

A heavily filtered fast neutron irradiation system (FNIS) was developed for a variety of applications, including the study of long-term health effects of fast neutrons by evaluating the biological mechanisms of damage in cultured cells and living animals such as rats or mice. This irradiation system includes an exposure cave made with a lead-bismuth alloy, a cave positioning system, a gamma and neutron monitoring system, a sample transfer system, and interchangeable filters. This system was installed in the irradiation cell of the Texas A&M University Nuclear Science Center Reactor (NSCR). By increasing the thickness of the lead-bismuth alloy, the neutron spectra were shifted into lower energies by the scattering interactions of fast neutrons with the alloy. It is possible, therefore, by changing the alloy thickness, to produce distinctly different dose weighted neutron spectra inside the exposure cave of the FNIS. The calculated neutron spectra showed close agreement with the results of activation foil measurements, unfolded by SAND-II close to the cell window. However, there was a considerable less agreement for locations far away from the cell window. Even though the magnitude of values such as neutron flux and tissue kerma rates in air differed, the weighted average neutron energies showed close agreement between the MCNP and SAND-II since the normalized neutron spectra were in a good agreement each other. A paired ion chamber system was constructed, one with a tissue equivalent plastic (A-150) and propane gas for total dose monitoring, and another with graphite and argon for photon dose monitoring. Using the pair of detectors, the neutron to gamma ratio can be inferred. With the 20 cm-thick FNIS, the absorbed dose rates of neutrons measured with the paired ion chamber method and calculated with the SAND-II results were 13.7 ?? 0.02 Gy/min and 15.5 Gy/min, respectively. The absorbed dose rate of photons and the gamma contribution to total dose were 6.7??10-1 ?? 1.3??10-1 Gy/min and 4.7%, respectively. However, the estimated gamma contribution to total dose varied between 3.6 % to 6.6 % as the assumed neutron sensitivity to the graphite detector was changed from 0.01 to 0.03.

A new generation of medical grade Cf-252 sources was developed in 2002 at the Oak Ridge National Laboratory (ORNL). The combination of small size and large activity of these Cf-252 sources makes them suitable to be used with the conventional high-dose-rate (HDR) remote afterloading systems for interstitial brachytherapy. A recent in-water calibration experiment showed that the measured gamma dose rates near the new source are slightly greater than the neutron dose rates; contradicting the well established neutron-to-gamma dose ratio of approximately 2:1 at locations near a Cf-252 brachytherapy source. Specifically, the MCNP-predicted gamma dose rate is a factor of two higher than the measured gamma dose rate at the distance of 1 cm, and the differences between the two results gradually diminish at distances farther away from the source. To resolve this discrepancy, we updated the source gamma spectrum by including in the ORIGEN-S data library the experimentally measured Cf-252 prompt gamma spectrum as well as the true Cf-252 spontaneous fission yield data to explicitly model delayed gamma emissions from fission products. We also investigated the bremsstrahlung x-rays produced by the beta particles emitted from fission-product decays. The results show that the discrepancy of gamma dose rates is mainly caused by the omission of the bremsstrahlung x-rays in the MCNP runs. By including the bremsstrahlung x-rays, the MCNP results show that the gamma dose rates near a new Cf-252 source agree well with the measured results and that the gamma dose rates are indeed greater than the neutron dose rates. The calibration experiment also showed discrepancies between the experimental and computational neutron dose profiles obtained. Specifically the MCNP-predicted neutron dose rates were ~25% higher than the measured neutron dose rates at all distances. In attempting to resolve this discrepancy the neutron emission rate was verified by the National Institute of Standards and Technology (NIST) and an experiment was performed to explore the effects of bias voltage on ion chamber charge collection. So far the discrepancies between the computational and experimental neutron dose profiles have not been resolved. Further study is needed to completely resolve this issue and some suggestions on how to move forward are given.

Weak nucleon nucleon couplings are largely unknown because of the involved theoretical and experimental challenges. Theoretically the topic is difficult due to the non-perturbative nature of the strong interaction, which makes calculations of the couplings challenging. Experimentally, the topic is difficult given that 1) the observables are determined by ratios between strong couplings and weak couplings which differ in size by seven orders of magnitude, and 2) theoretically clean and predictable measurements are almost always restricted to simple systems that do not allow for effects that enhance the size of the asymmetry. However parity violation (PV) can be used to separate out the weak part and thus studies of PV in hadronic systems could offer a unique probe of nucleon structure. The n-3He experiment at the Spallation Neutron Source was performed to measure the parity violating asymmetry of the recoil proton momentum kp with respect to the neutron spin in the reaction n + 3He ---> p + T + 764 keV. This asymmetry is sensitive to the isospin-conserving and isospin-changing (∆I = 0, 1, 2) parts of the Hadronic Weak Interaction (HWI), and is expected to be small (~10-7). The goal of this experiment was to determine this PV asymmetry with a statistical sensitivity of 2x10-8. We also measured the parity even nuclear asymmetry proportional to kp · σn x kn for the first time for verification of nuclear theory and for confirmation of the sensitivity of our experiment to the parity violating asymmetry.

Recently, the Oak Ridge National Laboratory (ORNL) successfully encapsulated a new generation of medical grade Cf-252 sources having intensities and size comparable to that of the widely used high-dose-rate (HDR) Ir-192 brachytherapy sources. Advent of the new sources, therefore, marked a new era for Cf-252-based neutron brachytherapy (NBT). As part of source calibration and characterization process, a study has been conducted at Georgia Tech lately on determining the neutron and gamma dose rates in water surrounding the new Cf-252 source. A Lucite-walled water phantom was built for this study. The neutron and gamma dose rates were determined both by ion chamber measurements and by Monte Carlo code MCNP. The results show that the measured neutron absorbed dose rates were approximately 25% lower than that predicted by MCNP for all dose positions in water, suggesting that the Cf-252 content of the new source is actually 25% lower than the ORNL's estimate. The measured gamma absorbed dose rates in water, on the contrary, are higher than that predicted by MCNP. The differences between the measured and MCNP-predicted gamma doses are not uniform for all dose positions; they are most pronounced (~a factor of two) at the distance of 1 cm, and fall to approximately 30% at distances 2 cm and beyond. These results suggest that the spectrum of gamma rays emitted from the new Cf-252 source may contain significantly more low-energy gamma rays than the previously published spectrum used in MCNP.

Tracercenter på Norrlands Universitetssjukhus producerar radiotracers. För att mäta aktiviteten hos de radioaktiva isotoperna används idag en jonkammare tillsammans med mjukvara byggt på Windows XP. Målet med det här examensarbetet är att bygga ny mjukvara som är kompatibelt med Windows 10. Mjukvaran ska tillhandahålla kalibreringsmöjligheter för isotoperna F-18, C-11 och Cs-137 samt kunna överföra uppmätt värde till datahanteringssystemet PETra.Med användandet av kodspråket Python och biblioteket TkInter har ett användargränsnitt och bakomliggande funktionskod utvecklats. Mjukvaran kommunicerar med både jonkammare och PETra seriellt via COM-Port. Linjäritetstester har gjorts för jonkammaren som påvisat god linjäritet. Detta resultat bekräftar mjukvarans tillförlitlighet med en liten felmarginal. Ytterligare tester behövs för att fastställa magnituden av felmarginalen. / Tracercenter at Norrlands University Hospital is a producer of radiotracers. To measure the activity of the radioactive isotopes an Ion Chamber combined with software built on Windows XP is used. The purpose of this thesis is to develop new software for the ion chamber compatible with Windows 10. The software has to provide a calibration function for the isotopes F-18, C-11 and Cs-137 aswell as be able to transfer measured activity to the laboratory information management system PETra.A graphical user interface together with backend code has been developed with Python and the library TkInter. The software communicates with both the ion chamber and PETra with a serial connection via COM-Port. Linearity tests have been made for the ion chamber which has shown that the output of the ion chamber itself is linear. This result confirms the software’s reliability with a slight margin of error. Further tests is needed to determine the magnitude of the margin of error.

At the BC Cancer Agency (BCCA), Vancouver Island Centre (VIC), the clinical verification of Intensity Modulated Radiation Therapy (IMRT) treatment plans involves comparing Portal Image (PI) -based three-dimensionally reconstructed (EPIDose) dose distributions to planned doses calculated using the Pencil Beam Convolution (PBC) algorithm. Discrepancies surpassing established action levels constitute failure. Since 2007, the failure rate of IMRT verification process had been increasing, reaching as high as 18.5% in 2009. A retrospective evaluation of clinical IMRT verification failures was conducted to identify causes and possible resolutions. Thirty clinical verification failures were identified. An equipment malfunction was discovered and subsequently repaired, and several failures were resolved in the process. Statistical uncertainty in measurement outcome was small in comparison to action levels and not considered significant to the production of failures. Still, over 50% of the redelivered plans were shown to consistently fail. A subgroup of consistent verification plans were compared to ion chamber point dose measurements. Relative to ion chamber measurements, EPIDose underestimated the dose while the dose calculation algorithm (PBC, Eclipse version 8.1.18) overestimated the same point dose. Comparisons of individual fields demonstrated that none were identifiably problematic; dose discrepancies were the result of minor but accumulating dose differences. Consistent verification failures were recalculated using two advanced dose calculation engines (the Anisotropic Analytical Algorithm and Monte Carlo). In general, verification metrics improved, and all failures were resolved. Three distinct indices of fluence modulation (or complexity) were shown to correlate with verification metrics. This indicated that deficiencies in both the leaf motion calculator and the PBC (Eclipse version 8.1.18) had likely contributed to the production of failures. In conclusion, clinical verification failures were resolved retrospectively by replacing faulty equipment and using more advanced methods of planned dose calculation, supporting the efficacy and continued use of PI-based three dimensional dose reconstruction for IMRT verification.