Thesis Defence: Thermal Measurements of Medical Isotope Targets

Grace Dehnel, supervised by Dr. Andrew Jirasek, will defend their thesis titled “Thermal Measurements of Medical Isotope Targets” in partial fulfillment of the requirements for the degree of Master of Science in Medical Physics.

An abstract for Grace Dehnel’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

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ABSTRACT

Medical isotopes are used in medicine to diagnose or treat various diseases, such as cancer and heart disease. These isotopes are used in radiopharmaceuticals that are needed to perform Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT) scans, as well as for the treatment of cancer using Targeted Radionuclide Therapy (TRT).

This thesis focuses on establishing a reliable method for temperature measurement in cyclotron medical isotope production targets using fibre Bragg gratings (FBGs). The research addresses the gap between theoretical and experimental radioisotopic yields, which partly comes from insufficient understanding of the thermal conditions within targets during irradiation.

A preliminary study was performed to establish temperature bounds in solid targets using low melting-point metals. The results from these experiments established a refined helium heat transfer coefficient (875±105 W/m2K) using ANSYS thermal modelling software with energy deposition input from FLUKA, a Monte Carlo particle transport simulation package. After this process was completed, a sensitivity analysis was performed since it was noted that minor changes in target setup affected the simulated temperature distribution.

The primary experiments, performed using FBG sensors in liquid (water) targets at TRIUMF’s 13 MeV cyclotron, demonstrated successful correlation between the wavelength change and beam current adjustments. In early irradiations, boiling conditions were detected when the FBG wavelengths plateaued even as the beam current increased, at which point the temperature was calculated to be within ±5 °C of 100 °C. A significant finding was that the FBG sensors require a radiation annealing process (determined to be 2830±30 μA·min outside the beam path and 1920 ± 20 μA·min directly in the beam path) before providing stable measurements in the radioactive environment.

From these results, it can be concluded that while FBG sensors can perform temperature measurements in radioactive environments, they must undergo annealing prior to making reliable measurements.

This research provides empirical data and lays the groundwork for obtaining additional data necessary for validating existing target models that presently rely on simulated input. These models are crucial for optimizing target design that will contribute to improving the efficiency of cyclotron-based radioisotope production.