Proton Beam Test Of A High Granularity Calorimeter For Proton Computed Tomography
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Radiation therapy is a non-invasive method of treating cancer. The purpose of it is to stop cancer cells from reproducing properly which will in turn cause the cancer cells to die out. This is achieved by having ionising radiation do enough damage to the DNA molecule in the cells so that they are unable to repair it. This leads to failure in the reproductive phase of the cell cycle and the cell dies. The traditional way to deliver radio therapy is by γ-rays in the form of Bremsstrahlung from an electron accelerator. The radiation is delivered from multiple angles and the beam is normally collimated to fit the projection of the tumour being radiated. The radiation therapy is usually planned according to information gained from CT- scans that map the insides of the patient. X-ray CT provides excellent information about the density distribution of the patient which in turn gives us information about his or her anatomy. This is then used to plan the dose delivery. However, a high-energy photon beam will deposit most of its energy in just up to a few cm of the body and therefore generally delivers a large integral dose to the body during treatment. Charged particles, like light ions, deliver their dose in a much more convenient fashion than photons. They deposit little energy at the start of their track and deliver more as they slow down, the energy deposition peaks as they come to a halt inside the patient. This energy deposition peak is known as the Bragg Peak. Particle therapy is a branch of radiation therapy that utilises charged particles instead of photons. The advantage of this is that by carefully positioning the Bragg Peak it is possible to deliver a large dose to the tumour while keeping the total dose to healthy tissue at a minimum. It is therefore very important to precisely estimate the range of a particle in a patient to make sure that the desired dose is delivered where it should be. Regular X-ray CT-scans provide excellent information of how an MeV photon beam will be attenuated by the patient body. This is not quite the case with charged particles as they interact with matter differently from photons. Calculating the stopping power for light ions based on data from X-ray CTs can result in range uncertainties of up to 1 cm. An intuitive way to solve this problem is to use charged particles as the basis for the CT-scan. Using protons, for example, gives us information of the stopping power directly without the need of conversion from some other modality. A proton-CT device will have to be able to accurately determine the energy and trajectory for each individual proton in order to reconstruct an image. This needs to be done at a rate high enough for it to be suitable in a clinical setting. One of the proposed concepts for a proton-CT detector is a high-granularity digital sampling calorimeter. Such a prototype has been constructed as a proposed future upgrade in the ALICE experiment. This prototype has been tested at the proton accelerator at KVI in Groeningen, Netherlands and in this thesis we will have a look at how it performs in a therapeutic particle beam. We will look at how the protons scatter as they go through the detector and how the liberated charges collected in the sensitive layers of the detector behave.
PublisherThe University of Bergen
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