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Reduction of the secondary ion background in measured track structure data in nanometric volumes

20.12.2019

In previous examinations of the track structure in nanometric volumes, the comparison of frequency distributions, measured using the PTB nanodosemeter, with simulated frequency distributions of ion cluster sizes showed the following. For small ion clusters with a large relative frequency, there was a very good agreement between the measurement and the simulation. This was in contrast to large ionization clusters, where significant deviations were observed between the measurement and the simulation; a fact that was true in the case of all investigated combinations of the target gas and the radiation quality.

By scattering ionized target gas molecules in the ion transport system, secondary electrons are generated which are, in turn, able to ionize neutral target gas molecules in the transport system. The background of secondary ions produced in this way is superimposed by the primary ions, and both “ion types” cannot be separated from each other. To correct the secondary ion background, the measured frequency distributions were unfolded to minimize the effect of this background [1].

Since the generation of secondary ions depends on the operating parameters of the PTB nanodosemeter, the influence of these operating parameters on the secondary ion background and the detection probability of the instrument were systematically investigated. These investigations were undertaken with the aim of minimizing the secondary ion background while at the same time maintaining the spatial distribution of the detection probability. At the end of this process, a new set of operating voltages for the ion transport optics had been developed (the ion transport optics extracts the target gas ions from the target volume where they are generated, and transports them to the detector). In this new set of operating voltages, the voltages at the electrodes of the ion optics have been reduced to one third of the previous voltage values [2].

Fig. 1: Comparison of the measured frequency distributions (EXP) of ionization cluster sizes (with the old (HV) and new (LV) operating parameters) with the corresponding simulations (MC) in a target gas of 1.2 mbar C3H8 and with alpha particles from a 241Am nuclide source. Left: frequency distributions for particle tracks passing the target volume centrally (= 0 mm). Right: frequency distributions for particle tracks passing the target volume at a distance of = 4 mm from the central axis of the target volume.

Figure 1 shows the comparison of measured frequency distributions (EXP) of ionization cluster sizes (with the old (HV) and new (LV) operating parameters) with the corresponding simulations (MC) in a target gas of 1.2 mbar C3H8 and with alpha particles from a 241Am nuclide source. The two graphs display the frequency distributions for particle tracks of alpha particles passing the target volume centrally (left, d = 0 mm) and at a certain distance from the central axis of the target volume (right, d = 4 mm). The frequency distribution for d = 0 mm measured with the new low voltages shows, for cluster sizes ⪆ 20, a significantly steeper drop towards larger cluster sizes than the distribution measured with the old, high voltages. It also agrees better with the simulation. However, the secondary ion background has not completely disappeared, as indicated by the plateau for cluster sizes ⪆ 50. For measured frequency distributions where the primary ion passes the target volume at a large distance, the reduction of the secondary ion background is much less pronounced. This has been shown by measurements with the new and old voltages for d = 4 mm. Furthermore, the agreement with simulations is not as good as for = 0 mm, which also indicates a remaining “residual secondary ion background”.

All in all, the new operating parameters of the PTB nanodosemeter have led to a significantly reduced secondary ion background.

Literature

(1)   G. Hilgers, M.U. Bug, H. Rabus: Unfolding the background of secondary ions in measured nanodosimetric ionisation cluster size distributions; JINST 14 P03023 (2019).

(2)   G. Hilgers, H. Rabus: Reducing the background of secondary ions in an ion-counting nanodosimeter; JINST 14 P07012 (2019).

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