ETD system

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Tesi etd-06242014-111803

Thesis type
Tesi di laurea magistrale
TLD efficiency calculations with heavy ions
Corso di studi
relatore Prof.ssa Rosso, Valeria
relatore Prof. Durante, Marco
Parole chiave
  • TLD
Data inizio appello
Riassunto analitico
Thermoluminescence dosimeters (TLDs) are solid state detectors widely <br>used in conventional<br>radiation detection and dose verification.<br>The development of ion beam cancer therapy, and the research in <br>radioprotection in space, stimulated the use of the TLDs for heavy ions <br>dosimetry. <br>The main advantages of this kind of detector, compared, e.g., to ionization <br>chambers, are the small dimensions, ease of handling, no interference on the <br>radiation field and the usability in solid state phantoms. <br>However the response of TLDs with dose is non-linear. It can be supralinear <br>and is affected by saturation effects as well.<br>The response of these detectors when irradiated with particle beams depends <br>also strongly on the quality of the radiation.<br>For this reason, in order to use TLDs with particle beams, and specifically <br>to get a prediction of their response in a treatment plan, a model that can <br>reproduce the behavior of these detectors in different conditions is needed.<br><br>In literature, several models describing the TLDs behavior already exist <br>and in this work we start briefly introducing some of them. In particular, we <br>focus on an extension of `local effect model&#39; (LEM).<br>This model stems from an amorphous track structure model and assumes<br>the knowledge of both the radial dose distribution around heavy ion<br>trajectories and the detector response to reference radiation. <br>Even though the LEM was originally developed for predicting the <br>response of biological systems following ion irradiation, it can also be <br>extended for efficiency calculations of solid state detectors, such as TLD.<br><br>In this context a new, simple and completely analytical algorithm for the <br>calculation of the efficiency dependence on ion charge Z and energy E has been <br>developed.<br>The response of the whole detector has been evaluated starting from the <br>response to a single ion of the beam. <br>The dose contributions coming from neighbouring tracks are assumed to add <br>up linearly. <br>This approach is realistic in a low dose approximation, but we nevertheless <br>analized its limits of validity.<br>Its main advantage is that, being fully analytical, it is <br>computationally fast and can be efficiently integrated in treatment <br>planning verification tools.<br>Furthermore, it is robust against modifications of the radial dose <br>distribution of a single ion in the detector, as well as to different detector <br>response models as we critically evaluated in our work.<br><br>The calculated values of the efficiency have been compared with experimental <br>data, as well as to other calculated values provided by different approaches.<br>Moreover, after implementing our model data in the treatment <br>planning code TRiP98, we performed signal calculations on macroscopic target <br>irradiated with an extended Carbon ion field. Also these results were compared <br>with recent experimental measurements as well as with alternative calculations.<br> The results, both for pure efficiency calculations and for their propagation <br>in macroscopic dose response prediction, achieve a level of accuracy which is <br>comparable to previous calculations, while needing a much lighter computational <br>effort. <br>