Investigation of components for a nanodosimetric particle track detector

Nanodosimetry has already been used in the “Radiation Effects” Department for many years as a method to establish physical and metrologically realizable quantities for the effect of ionizing radiation on biological tissue. The quantities are derived from the so–called ionization cluster size distribution which is currently represented by the nanodosemeter of PTB [1]. Due to the complex topology of DNA, current studies in the field of radiation biology show that the spatial correlation of such cluster damage plays an essential role in the range of a few nanometers to some 10 nanometers (nm).

To also be able to measure this correlation metrologically (and to simultaneously increase the sensitivity by a much larger measurement volume), a so–called Track Imaging Nanodosemeter (TINA) is being developed. TINA is able to measure the size and spatial distribution of ionization clusters, the so–called track structure, with some 10 nm resolution in a cubic volume with an edge length of 200 nm to 300 nm. This instrument consists of a gas–filled cylindrical time projection chamber (TPC) in which the electrons of the ionization track of a passing ionizing particle are drifted in a homogeneous electric field to a gas amplification structure where they are amplified. In a next step, they can be recorded by suitable readout electronics and be further processed in a computer.

Figure 1 shows the prototype of the TINA–TPC (here still without the final readout electronics), as it was built to optimize the amplification structures and as it is currently being studied. As an amplification element, a multilayer “Thick–GEM” [2] was built and tested in different configurations. The SPIDR system with the Timepix 3 sensor, that was developed at NIKHEF in Amsterdam, was selected as the readout electronics [3]. Besides the sensor head with its 4 Timepix ASICs that are arranged in a square, it has a fast digital readout system. The sensor head consists of a matrix of 512 x 512 charge–sensitive pixel elements, each with an edge length of 55 µm. For each charge pulse, the location, the amount of charge and the arrival time are registered for the relevant pixel(s), with a time resolution of up to 1.6 ns. From the pixel address and the time measurement, the location of the initial ionization event can be determined in all 3 spatial coordinates within the TPC.

Fig. 1: Schematic representation and photo of the TINA time projection chamber. Here still without the Timepix 3 sensor chip that is required for the three-dimensional spatially resolved readout that will later replace the pickup electrode that is shown in the schematic diagram.

A special challenge of this detector principle is the question of how the expensive and extremely sensitive Timepix sensor can be protected from occasional electrical breakdowns in the gas amplification structure. For this purpose, the method of the dielectrically quenched discharge was investigated during which the readout chip is separated from the sensitive volume of the TPC via a highly resistive layer [4]. Electric discharges would be limited by the highly resistive layer to a level that is not critical for the Timepix–ASIC. However, a sufficiently long–term stable and high–voltage–resistant material with a required specific volume resistivity of 10⁹ – 10¹¹ Ω cm could not yet be found.

Parallel tests with the Timepix–ASIC have shown that also purely capacitively coupled (differentiated) charge pulses are registered by the chip. This was utilized to implement a complete galvanic separation between the gas amplification structure and the Timepix charge sensor. To ensure that the capacitively coupled signal could be localized by the Timepix, the anode of the gas amplifier was made by using a resistive layer with a specific surface resistance of approx. 2 MΩ [4]. Currently, the TPC is being tested in a long–term test under realistic operating parameters, and the Timepix 3 charge sensor will be installed when this test has been successfully completed.

Literature

(1)   V. Conte, A. Selva, P. Colautti, G. Hilgers, H. Rabus, Track structure characterization and its link to radiobiology, Radiation Measurements 106 (2017) 506

(2)   M. Cortesi et al, Multi-layer Thick Gas Electron Multiplier (M-THGEM): a new MPDG structure for high-gain operation at low-pressure; Review of Scientific Instruments 88, 013303 (2017)

(3)   B. v. d. Heijden, J. Visser, M. v. Beuzekom, H. Boterenbrood, S. Kulis, B. Munneke, F. Schreuder; SPIDR, a general-purpose readout system for pixel ASICs, Journal of Instrumentation 12 C02040 (2017)

(4)   F. Immel, Untersuchung von Signalverstärkerstrukturen und Entwicklung des Datenaufnahmesystems für einen nanodosimetrischen Teilchenspurdetektor Masterarbeit im Studiengang Elektrotechnik, angefertigt an der PTB und vorgelegt an der Fachhochschule Ostfalia, Wolfenbüttel, Mai 2019

(5)   V. Dangendorf et al; Detectors for time-of-flight fast-neutron radiography; Nucl. Instrum. and Methods A542 (2005) 197