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Particle track reconstruction in the measurement of the ionization clusters distribution with the aid of an ion-counting nanodosimeter


In the field of nanodosimetry, a cooperation between Department 6.6 and the Loma Linda University Medical Center (LLUMC, San Bernardino, USA) has existed for several years. The aims of the cooperation are, among others, the further development of the nanodosimeter developed at the Weizmann Institute of Science (Rehovot, Israel) (of which two prototypes exist: one at PTB and one at LUMC), and its characterization in different radiation fields with the aid of the ad-hoc Monte Carlo (MC) program developed in Department 6.6. Last year, the nanodosimeter at LLUMC was upgraded with a detector which detects the input and the output coordinates of the primary particles. This upgrading allowed the track structure of alpha particles and protons to be determined for the first time with a good spatial resolution and was also used to measure the ionization cluster distribution for primary electrons. A first experiment was performed with a Sr-90/Y-90 source (Figure 1) which emits an electron spectrum with a maximum energy of 2.3 MeV. The primary beam was aligned with distances of 0 nm, 6 nm and 21 nm from the central axis of the measuring volume and the results were compared with MC simulations.

Figure 1 : Experimental set-up of the LLUMC nanodosimeter with schematic representation of a track structure for ions (filled circles) and for electrons (open circles).

First, the evaluation of the results for primary electrons did not show agreement between the measured and the calculated ionization cluster distribution (Figure 2 a). The reason for this was that (i) the geometry for the simulation (1.5 cm of measurement gas between the source and the measuring volume) did not correspond to the geometry of the experiment (27 cm of measuring gas) and that (ii) the current detector system is not yet able to completely take the particle track structure of the electrons into account. Due to the - compared to ions - much larger scattering and wider angle distribution for electrons, the simulation of the complete length of the gas chamber is very important for primary electrons.

In Figure 2 b, the reconstruction of the particle track has been integrated into the analysis. As can be seen, the experiment and the simulation show a good agreement.

Figure 2 : Ionization cluster distribution for Sr-90/Y-90. Experimental data (symbols) compared to (a) MC (unbroken lines) without complete transport of the electrons in the gas chamber and (b) with complete transport and consideration of the particle track reconstruction. The symbols show the cluster distributions the primary beam aligned for distances of 0 nm (), 6 nm () and 21 nm () from the central axis of the measuring volume.

In summary, these results show how important it is to upgrade the ion-counting nanodosimeter with a system suitable for the tracing of the particle track and the triggering of the electrons.


  1. Garty, G.; Shchemelinin, S.; Breskin, A.; Chechik, R.; Assaf, G.; Orion, I.; Bashkirov, V.; Schulte, R.; Grosswendt, B.:
    The performance of a novel ion-counting nanodosimeter.
     Nucl. Instr. & Meth A 491, 212-235 (2002).