ETD system

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Tesi etd-07202016-153637


Thesis type
Tesi di dottorato di ricerca
Author
PALLA, LARA
email address
lara.palla@virgilio.it
URN
etd-07202016-153637
Title
Controlled dose irradiation with pulsed beams of different ion species
Settore scientifico disciplinare
FIS/07
Corso di studi
FISICA
Commissione
tutor Prof.ssa Rosso, Valeria
tutor Dott. Taccetti, Francesco
Parole chiave
  • micro-channel plate
  • thermoluminescence
  • thermoluminescence dating
  • tandem accelerator
  • pulsed beamline
  • controlled irradiations
  • fluence detector
  • ion fluence
Data inizio appello
22/08/2016;
Consultabilità
completa
Riassunto analitico
The possibility to carry out nuclear physics experiments using precisely controlled ion beams is an ever increasing request. This thesis describes the development and the optimization of an experimental setup that allows monitoring “on-line” the planned irradiations. The work has been carried out at the pulsed beam facility of INFN LABEC laboratory, where a Tandem accelerator is present. The developed fluence control system has been successfully employed with bunches made of different ionspecies in different irradiation configurations. The use of a pulsed beam gives the<br> advantage to span over a wide dose range: from very low dose levels (≤ 1 mGy) up to about 1 kGy, that corresponds to a ion fluence in the range 10^(5)-10^(11) cm^(−2). The bunch repetition frequency and the average number of ions forming the bunches can be varied depending on the dose that must be delivered. In case a large dose is required, the measurement time can be shortened by working with bunches made of tens or hundreds of ions with a bunch rate up to about 1 kHz. On the contrary, very small dose levels become possible if bunches made of a few ions are used (down to single particle bunches) with a bunch frequency down to 1 Hz or even manual single shot. Regarding the possible projectile ions for irradiations, protons with energies ranging from a few hundreds of keV up to 6 MeV can potentially be employed and also heavy ions (such as Li, C, O, Si) with energies up to 1-1.5 MeV per nucleon. The developed control system is also able to control the fluence during alpha irradiations; however this last type of irradiation requires a specific ion source and will be available in the close future, after an upgrade of the Tandem facility.<br>An on-line fluence control system has to count the number of ions hitting the target sample thus giving the possibility to calculate the ion fluence with high accuracy. Besides, real time monitoring of the particle fluence can compensate for possible changes of the beam intensity (usually reductions in the long term).<br>At the basis of the developed fluence control system there is a fluence detector based on a thin Al foil; the foil is positioned along the beam path and is crossed by the ions before reaching the sample. The idea is to exploit the physical process of electron emission that takes place when the ion bunch crosses the conductive foil. A Micro-Channel Plate (MCP) multiplier detects the emitted electrons. The response of the foil-MCP assembly is calibrated by means of a silicon detector positioned downstream the foil. After the calibration the sample replaces the silicon detector and is irradiated with the desired number of bunches; then the delivered dose is calculated. Tests show that the system determines the ion fluence with a precision of the order of 1% or better.<br>One of the main applications of this monitoring system is in the field of cultural heritage for studies related to the refinements of the technique of Thermoluminescence dating. Thermoluminescence (TL) is the light emission (mainly visible) that takes place during the heating of particular solid materials that have been irradiated. By measuring the intensity of the luminescence light and by applying a specific calibration procedure it is possible to employ TL materials as accurate dosimeters. Quartz andfeldspars belong to TL materials and are natural mineral constituents of clay. TL dating is a well established technique that is used to determine ages of archaeological (mainly ancient pottery and ceramics) or geological materials (ocean sediments, volcanic events, calcareous deposits, meteorites) and to authenticate historical artefacts.<br>Accurate dating requires the knowledge of the light response of quartz to different kinds of incoming radiation (different also in energy) and its dependence on the dose rate absorption. In fact the luminescence response and the luminescence efficiency (emitted light intensity) of the material after irradiation with heavy particles, is significantly different with respect to gamma or beta irradiation. This is related to radiation damage caused in the material by heavy particles. γ radiation and β and α particles, from the decay chains of U, Th and K present in the clay itself and in the surrounding<br>soil, are the main contributors to the environmental radioactivity, and they show a wide energy spectrum. The developed fluence control system, now operative at LABEC, is a powerful tool for all these ongoing studies and also to deeply investigate the properties of the TL materials. Some preliminary measurements with TL samples have been carried out and confirm the reliability of this system.<br>
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