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We are developing a positive ion detector with single ion sensitivity for radiation track structure characterization [1]. The device combines the operational principles of thick gas electron multipliers (THGEM) [2], working in reverse polarity, and resistive plate chambers (RPC). As shown in figure 1, ions produced in a low pressure gas volume drift towards a THGEM-like structure made of a single side-clad dielectric plate provided with holes of millimetric dimensions. On the other side of the plate, a high resistivity cathode connected to a negative high voltage is used to generate a high restricted field across the hole structure. Positive ions focused into the holes are accelerated and produce ion-impact ionization of the working gas, which develops into a controlled discharge. The discharge is confined in time due to the high volume resistivity of the cathode and in space by walls of the holes in the dielectric plate. Because of the high electron gain, the signal induced in the readout electrode can be acquired with standard electronics. Moreover, using a 2D readout strip configuration, coordinates of the holes generating the signal can be reconstructed, which, together with a drift time measurement, allows a 3D reconstruction of charged particle tracks formed in the working gas. This device has possible applications in radiobiology. The spatial distribution of ionization events within biological targets of nanometric dimensions (e.g., DNA) is strongly correlated with the biological effectiveness of ionizing radiation. By comparing the track structure of different radiation qualities, the difference in their biological effect at equal absorbed doses can be studied. The use of a working gas pressure of the order of a few millibars allows experimental simulation of the radiation interaction with nanoscopic biological targets by expanding condensed matter dimensions up to a million times. Monte Carlo simulations show that by using tissue equivalent gases and appropriate volume scaling factors equivalent ionization spatial distribution can be obtained in gas and water [3]. Initial measurements with a detector prototype filled with propane gas revealed low ion detection efficiency, of the order of a few percent [4]. A thorough detector characterization showed that both low ion-impact ionization probability and long detector dead time due to the long recovery of the electric field in the hole after the discharge impacted the detector performance. In order to tackle the first issue, a study of detector efficiency as a function of dielectric plate thickness was performed. Measurements with detectors made of a few holes in polystyrene plates ranging from 3 to 10 mm in thickness showed that efficiency increased with hole height. Different cathode materials were tested in order to reduce the detector dead time. When a semiconductive glass cathode with volume resistivity of the order of 10ˆ12Ω⋅cm was used rather than a standard glass cathode, the detector efficiency improved. Additional effort has been made minimizing spurious discharges that increase dead time. In particular, in the latest detector design the cathode is excluded from the low pressure environment, and the PCB board manufacturing has been commissioned to the CERN PCB workshop in order to minimize manufacturing imperfections of holes and readout electrodes. Further, the possibility of using a slightly conductive glass GEM to avoid charge-up effects and decrease dead time is under study. We conclude that, while preliminary results are promising, additional work is necessary to improve the ion detector performance in order to build a versatile 3D radiation track structure detector. References: 1. Bashkirov V. A., Hurley R. F. and Schulte R. W. A novel detector for 2D ion detection in low-pressure gas and its applications. In: NSS/MIC Conference Record, IEEE, pp. 694–698, (2009). 2. Chechik R., Breskin A., Mörmann D. and Shalem C. Thick GEM-like hole multipliers: properties and possible applications. Nucl. Instr. Meth. A535, 303 (2004). 3. Grosswendt B. Nanodosimetry, from radiation physics to radiation biology. Radiat. Prot. Dosim. 115, 1–9 (2005). 4. Casiraghi M., Bashkirov V., Hurley F. and Schulte R. A novel approach to study radiation track structure with nanometer-equivalent resolution. Eur. Phys J. D 68, 111 (2014). ---------- Figure 1: Schematic representation of the single ion detector design. From top to bottom: copper anode providing the drift field (Ed) through the sensitive volume, PCB board with 2D hole-array and readout electrode, high-resistivity cathode providing the accelerating field (Ea) in the holes.
