Reducing the background of secondary ions in an ion-counting nanodosimeter

Abstract

In previous investigations using the PTB Ion Counter nanodosimeter, significant deviations for large cluster sizes were found in the comparison between measured and simulated data of ionisation cluster size distributions. These deviations could be explained quantitatively using a simple parametric model for a background of secondary ions, which are produced within the transport system of the target gas ions to the ion detector. To correct for these secondary ions, unfolding procedures were applied to the measured data to remove the effects of this background. However, the parameters of the model describing the background of secondary ions strongly depend on operational parameters of the nanodosimeter, and any change in these operational parameters requires a new characterisation of the model's parameters. In this work, the influence of the nanodosimeter's operational parameters on the background of secondary ions and the collection efficiency of the detected ions was therefore systematically investigated. At the end of this process operational conditions were found where the background of secondary ions is reduced substantially while the effect on the spatial distribution of the collection efficiency is still acceptable. For some beam geometries of particular relevance for radiation biology, cluster size distributions measured with these new operational parameters can be used directly so that the unfolding procedures, and consequently the characterisation of the secondary ion background with respect to the parameters required by the corresponding model used in the unfolding, can be omitted.

Characterisation of the PTB ion counter nanodosimeter's target volume and its equivalent size in terms of liquid H2O

“…2 (Fig. 2 from [18]). The interaction region, which is located between the electrodes of a plane parallel-plate capacitor, is filled with the target gas at a pressure in the order of 1 mbar.…”

“…The spatial distribution of p, which has rotational symmetry around the central axis of the extraction aperture, decreases with increasing distance z from the extraction aperture and with increasing radial distance r from the symmetry axis (Fig. 7 in [18]). The integral of p(r,z,td) over the drift time td is the "full window" extraction efficiency ηfw(r,h), where the vertical coordinate h introduced in the previous sections relates to z via h = z -z0, where z0 denotes the height above the lower electrode of a particle trajectory striking the PSD centrally.…”

“…The PSD is not pixel-based, but rather covered with resistive layers on the front and back sides of the silicon chip that are contacted along the "horizontal" and the "vertical" axes, respectively. Position detection in both resistive layers works according to the charge division principle (for details see [18]). Thus, in data processing virtual rectangular pixels of arbitrary size can be configured by choosing independently the dimensions for the two directions.…”

“…Thus, in data processing virtual rectangular pixels of arbitrary size can be configured by choosing independently the dimensions for the two directions. The uncertainties associated with the measurement of the ICSDs and the imaging of the target volume due to the imaging properties of the PSD have been discussed in [18,21].…”

“…2/20 (Fig. 1 from [18]). The ionisation cluster size produced in the target is a stochastic quantity that results from the superposition of the pattern of ionisations within a particle track and the geometrical characteristics of the target volume.…”