JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by Weizmann Institute, Israel, in 2004

Guy Garty

Supervisor: Amos Breskin

Development of ion-counting nanodosimetry and evaluation of its relevance to radiation biology

 Keywords:

• Gaseous detectors
• Dosimetry concepts and apparatus
• Microdosimetry and nanodosimetry
• Models and simulations

 Abstract:
 The goal of our research is the development of novel concepts and tools for the precise evaluation of the ionization track structure, induced by charged particles traversing a sparse gaseous medium. The nanodosimeter is based on counting single radiation-induced ions formed within small volumes of low density gas simulating condensed matter of million times smaller dimensions; it enables, for the first time, the modeling of the interaction of radiation with condensed mater on a nanometer scale, relevant for the understanding of radiation damage to tissue, at the DNA scale.

 Within this work we have designed, constructed and tested two nanodosimeters. The nanodosimeters were mounted at accelerator beams (both at the Weizmann institute of Science and at the Loma Linda University-Medical Center in California) and used for measuring the ionization clusters induced by radiation fields spanning 4 orders of magnitude in average ionization density (LET values of 0.4 keV/μm to 700 keV/μm). Up to an LET value of 26 keV/μm, we have reliably measured cluster size distributions in conditions equivalent to the irradiation of DNA in vitro. The measured ion cluster size distributions were validated by extensive simulations of primary and secondary interactions in the gas, ion transport and counting.

 To complement these measurements, the final effect of radiation on DNA was also quantified by irradiating plasmid DNA. We have measured the formation of single and double strand breaks, as well as clustered lesions containing a combination of strand breaks and base damages, in irradiated DNA.

 While both types of measurements yield important data to their respective fields, it is only through a correlation of both measurements, that it is possible to model the phenomena of radiation-induced mutagenesis and cell death, which are induced by large ionization clusters. In this project, we present a basic model, which predicts the measured yields of clustered DNA lesions, based on cluster size distributions within a gas model, measured under equivalent conditions. To the best of our knowledge, this is the first time that such a comparison between the physical energy deposition and the biological endpoints, becomes possible.