Abstract
In microdosimetry, lineal energies y are calculated from energy depositions ϵ inside the microdosimeter divided by the mean chord length, whose value is based on geometrical assumptions on both the detector and the radiation field. This work presents an innovative two-stages hybrid detector (HDM: hybrid detector for microdosimetry) composed by a tissue equivalent proportional counter and a silicon tracker made of 4 low gain avalanche diode. This design provides a direct measurement of energy deposition in tissue as well as particles tracking with a submillimeter lateral spatial resolution. The data collected by the detector allow to obtain the real track length traversed by each particle in the tissue equivalent proportional counter and thus estimates microdosimetry spectra without the mean chord length approximation. Using Geant4 toolkit, we investigated HDM performances in terms of detection and tracking efficiencies when placed in water and exposed to protons and carbon ions in the therapeutic energy range. The results indicate that the mean chord length approximation underestimate particles with short track, which often are characterized by a high energy deposition and thus can be biologically relevant. Tracking efficiency depends on the low gain avalanche diode configurations: 34 strips sensors have a higher detection efficiency but lower spatial resolution than 71 strips sensors. Further studies will be performed both with Geant4 and experimentally to optimize the detector design on the bases of the radiation field of interest.The main purpose of HDM is to improve the assessment of the radiation biological effectiveness via microdosimetric measurements, exploiting a new definition of the lineal energy (yT), defined as the energy deposition ϵ inside the microdosimeter divided by the real track length of the particle.
Highlights
Microdosimetry was developed to study the effect of radiation on cells
Tissue Equivalent Proportional Counter tissue equivalent proportional counters (TEPCs) have two main advantages compared to other microdosimeters: 1) the sensitive volume is confined in a macroscopic region of a well defined size and 2) the energy deposition is directly measured in tissue and does not require a conversion
In detail: panels A, B of Figure 3 and panels A, B, C, D of Figure 4 illustrate the kinetic energy distributions of all particles entering the TEPC, with and without the contribution from the primary ions; the track distributions of all the particles are plotted in panel C for protons and in panel E for carbons, with the mean chord length of 8.47 mm marked with a dashed red line; panels D for protons and F for carbons contain a comparison between the microdosimetric spectra calculated with the mean chord length approximation (yd(y)) or the real track length (yT d(yT ))
Summary
Microdosimetry was developed to study the effect of radiation on cells. Measuring the energy loss in a microscopic volume called for the development of new detection techniques. There are two types of microdosimeters: tissue equivalent proportional counters (TEPCs) and semiconductor-based detectors. The latter category includes silicon detectors based on different technologies (telescope detectors, silicon on insulator detectors, arrays of cylindrical p-n junctions with internal amplification [2, 3]) and diamond microdosimeters which are under study for their radiation hardness and tissue equivalence [4]. The TEPC detection system is based on the fact that the detection gas parameters (e.g., composition and density) are adjusted to match the stopping power of the desired tissue equivalent volume
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