Leonello Servoli

The HASPIDE project of INFN exploits hydrogenated amorphous silicon (a-Si:H) as a sensitive material for innovative detectors, mainly focused on dosimetry both for standard and FLASH radiotherapy, but also fo other applications. The detector, deposited on a thin and flexible substrate of polyimide, has been developed with different contacts configurations during past years, giving rise to the necessity to compare them in order to determine which configuration is best, for a specific application. In particular, three types of contacts are under exam in this work: the first one is a classical n-i-p junction, where a thick layer of intrinsic a-Si:H material is sandwiched between n- and p-doped layers of hydrogenated amorphous silicon; the second configuration is a charge-selective contacts device (CSC), where the doped regions are replaced with metal oxides which selectively enhance the mobility of one charge-carrier, blocking the other and the last one is the hybrid type, in which the intrinsic region is coupled with a n-doped layer on the bottom and a hole selective metal oxide on the top. This work aims to give a comparison in the performance of three devices differing only in the contact configuration, in terms of dark current analysis and sensitivity to X-ray beams, so that the characteristics of each one can be analyzed and exploited.

Interventional Radiology (IR) exposes medical staff to scattered X-rays, making active real-time personnel dosimetry highly desirable. In this work, the ARX3A0 CMOS image sensor was investigated as a candidate sensitive element for active personnel dosimetry in IR. The sensor was characterized through laboratory and clinical measurements in order to assess its baseline stability, electronic noise, spectroscopic response in a controlled low energy range, dose rate response and time resolved behavior under realistic scattered radiation fields.In laboratory conditions, dark measurements were performed to evaluate pedestal stability and noise as a function of the main acquisition parameters. A fluorescence calibration using Cu, Pb and Mo targets was carried out in the 8-20 keV range to verify the linearity of the reconstructed charge response under controlled conditions. Dose rate measurements were then performed by comparison with a reference dosimeter under controlled X-ray irradiation. The results show stable dark behavior, low electronic noise and a linear charge response within the investigated fluorescence energy range. An approximately linear dose rate response was observed in both laboratory and clinical conditions, with calibration factors reasonably consistent with the reference system.Additional measurements were carried out in a clinical angiography environment to assess the sensor response in scattered fields representative of interventional practice. The sensor was able to reproduce the temporal evolution of both fluoroscopy and radiography techniques.\\Overall, these results support the feasibility of using the ARX3A0 sensor as a candidate sensitive element for active personnel dosimetry in Interventional Radiology. Further work is required to extend the calibration, quantify long term repeatability and assess system level performance under practical operating conditions.

Flexible hydrogenated amorphous silicon (a-Si:H) detectors are attractive candidates for clinical dosimetry, particularly for applications requiring lightweight devices and operation without external bias. In this work, three flexible a-Si:H detector architectures are experimentally compared under clinical photon and electron beams: a conventional n-i-p diode, a charge selective contact (CSC) detector, and a very thin p-i-n diode. All detectors show excellent dose linearity under both irradiation modalities. However, large differences in sensitivity are observed at zero bias, exceeding two orders of magnitude across architectures. The conventional n-i-p device shows a response that is limited by the internal electric field, while CSC and very thin p-i-n detectors achieve sensitivities in the pC/cGy range without applied bias. Electron beams yield higher charge per unit dose than photons, compatible with a larger local energy deposition in the active layer. The results demonstrate that sensitivity in flexible a-Si:H dosimeters is governed by contact design and intrinsic layer thickness rather than by active volume alone. Taken together, the present results suggest that combining charge selective contacts with thinner intrinsic layers may represent a promising strategy to further enhance zero-bias sensitivity. These findings provide clear design guidelines for the development of flexible dosimeters for surface and transmission measurements in advanced radiotherapy.

The INFN HASPIDE project investigates hydrogenated amorphous silicon (a-Si:H) as an innovative detector technology for dosimetry in FLASH radiotherapy. Conventional dosimeters, such as ionization chambers, show non-linear behavior when exposed to ultra-high dose-per-pulse beams used in FLASH radiotherapy, where several Grays are delivered in microsecond pulses. Thanks to its radiation hardness, mechanical flexibility and compatibility with large-area deposition, a-Si:H is a promising material for in-vivo and surface dosimetry. This work presents the response of a thin a-Si:H detector exposed to a high-dose-per-pulse 10 MeV electron beam, together with an analytical model used to describe the single-pulse dynamics and extract quantitative parameters. Measurements were performed at the University of Torino using a modified clinical LINAC operated in Ultra High Dose Rate regime, where the single-pulses satisfy the FLASH criteria. The a-Si:H detector, read out with a TetrAMM picoammeter (100 kHz sampling frequency), resolves individual radiation pulses with high repeatability. Both pulse charge and peak current scale linearly with the applied electric field up to 6 V (2.4 V/μm), beyond which the readout electronics saturates. The analytical model is validated through direct comparison with raw data, showing excellent agreement in extracted pulse parameters.

Radiation damage tests in hydrogenated amorphous silicon (a-Si:H) flexible flux and dose-measuring devices have been performed with a 3 MeV proton beam, to evaluate combined displacement and total ionizing dose damage. The tested devices had two different configurations and thicknesses. The first device was a 2 μm thick n-i-p diode having a 5 mm x 5 mm area. The second device was a 5 μm thick charge selective contact detector having the same area. Both the devices were deposited on a flexible polyimide substrate and were irradiated up to the fluence of 10¹⁶ n_eq/cm². The response to different proton fluxes has been measured before irradiation and after irradiation at 10¹⁶ n_eq/cm² for charge-selective contacts and n-i-p devices. The effect of annealing for partial performance recovery at 100°C for 12 hours was also studied and a final characterization on annealed devices was performed. This test is the first combined displacement and total ionizing dose test on flexible a-Si:H devices.

Hydrogenated amorphous silicon (a-Si:H) devices on flexible substrates are currently being studied for application in dosimetry and beam flux measurements. The necessity of in vivo dosimetry requires thin devices with maximal transparency and flexibility. For this reason, a thin (7 to 2.49 × 10¹⁰ p/(cm² s).

This paper presents a comprehensive study of hydrogenated amorphous silicon (a-Si)-based detectors, utilizing electrical characterization, Raman spectroscopy, photoemission, and inverse photoemission techniques. The unique properties of a-Si have sparked interest in its application for radiation detection in both physics and medicine. Although amorphous silicon (a-Si) is inherently a highly defective material, hydrogenation significantly reduces defect density, enabling its use in radiation detector devices. Spectroscopic measurements provide insights into the intricate relationship between the structure and electronic properties of a-Si, enhancing our understanding of how specific configurations, such as the choice of substrate, can markedly influence detector performance. In this study, we compare the performance of a-Si detectors deposited on two different substrates: crystalline silicon (c-Si) and flexible Kapton. Our findings suggest that detectors deposited on Kapton exhibit reduced sensitivity, despite having comparable noise and leakage current levels to those on crystalline silicon. We hypothesize that this discrepancy may be attributed to the substrate material, differences in film morphology, and/or the alignment of energy levels. Further measurements are planned to substantiate these hypotheses.

Hydrogenated amorphous silicon (a-Si:H) is a material having an intrinsically high radiation hardness that can be deposited on flexible substrates like Polyimide. For these properties a-Si:H can be used for the production of flexible sensors. a-Si:H sensors can be successfully utilized in dosimetry, beam monitoring for particle physics (x-ray, electron, gamma-ray and proton detection) and radiotherapy, radiation flux measurement for space applications (study of solar energetic particles and stellar events) and neutron flux measurements. In this paper we have studied the dosimetric x-ray response of n-i-p diodes deposited on Polyimide. We measured the linearity of the photocurrent response to x-rays versus dose-rate from which we have extracted the dosimetric x-ray sensitivity at various bias voltages. In particular low bias voltage operation has been studied to assess the high energy efficiency of these kind of sensor. A measurement of stability of x-ray response versus time has been shown. The effect of detectors annealing has been studied. Operation under bending at various bending radii is also shown.

The characteristics of a hydrogenated amorphous silicon (a-Si:H) detector are presented here for monitoring in space solar flares and the evolution of strong to extreme energetic proton events. The importance and the feasibility to extend the proton measurements up to hundreds of MeV is evaluated. The a-Si:H presents an excellent radiation hardness and finds application in harsh radiation environments for medical purposes, for particle beam characterization and, as we propose here, for space weather science applications. The critical flux detection limits for X rays, electrons and protons are discussed.