Monolithic Silicon Telescope Research Articles

Development of the ACSpect neutron spectrometer: Technological advance and response against an accelerator-based neutron beam

Advances in neutron-based radiation therapies such as Boron Neutron Capture Therapy (BNCT) pushes towards the development of new neutron spectrometers, whose key features are to be their practicability, reliability, energy resolution and detection range. The ACSpect is a novel neutron spectrometer based on a two-stages monolithic silicon telescope detector coupled to an organic scintillator working as an active neutron converter.This paper reports the latest developments of the ACSpect and the results of the measurements of an accelerator-based neutron beam moderated by AlF3. The AlF3 is a moderator material optimised to obtain an epithermal neutron beam for accelerator-based BNCT of deep seated tumours. The experiments carried out are the first neutron spectrometry of a neutron beam moderated by AlF3.The performances of the ACSpect have been compared against Monte Carlo simulations, literature data and the gold-standard neutron spectrometer DIAMON. While the agreement between experiments and simulations allowed to validate the Monte Carlo model used to simulate the new moderator material, the agreement between literature data, ACSpect and DIAMON results confirmed the ACSpect as a compact and relatively easy-to-use high-resolution neutron spectrometer, capable of reliably operating in the energy range 250 keV - 4 MeV.

Characterization of a pixelated silicon microdosimeter in micro-beams of light ions

The monolithic silicon telescope technology allows to produce solid state microdosimeters. A detector constituted by a matrix of pixels (2 μm in thickness) coupled with a deeper stage (about 500 μm in thickness) was designed, developed and characterized by comparing its response against Tissue Equivalent Proportional Counters (TEPCs) in different irradiation fields, showing promising results. Each pixel of the ΔE stage is a thin diode of 9 μm in diameter delimited by a guard ring of 14 μm in diameter.The aim of this work is the study of the charge collection efficiency of the single pixel of the silicon microdosimeter by irradiating a prototype at the micro-beam facility of the Ion Beam Center - IBC of the University of Surrey. Different light ions were exploited (protons, lithium ions and carbon ions) and scanned on different positions on and around a single pixel of the detector.The results show that the charge generated by events on the sensitive region of the ΔE stage is completely collected, while a charge collection efficiency lower than one characterizes the region between the central pixel and its guard ring. The guard ring delimits properly the sensitive region: no signals arise from events outside the 14 μm in diameter ring.Nevertheless, the microdosimetric distributions of the different charged particles highlight a minor distortion in terms of the absorbed dose. This latter result justifies the good agreement with TEPCs in terms of microdosimetric distributions in different hadrons beams described in previous works.

A telescope detection system for direct and high resolution spectrometry of intense neutron fields

A high energy- and spatial-resolution telescope detector was designed and constructed for neutron spectrometry of intense neutron fields. The detector is constituted by a plastic scintillator coupled to a monolithic silicon telescope (MST), in turn consisting of a ΔE and an E stage. The scintillator behaves as an “active” recoil-proton converter, since it measures the deposited energy of the recoil-protons generated across. The MST measures the residual energy of recoil-protons downstream of the converter and also discriminates recoil-protons from photons associated to the neutron field. The lay-out of the scintillator/MST system was optimized through an analytical model for selecting the angular range of the scattered protons. The use of unfolding techniques for reconstructing the neutron energy distribution was thus avoided with reasonable uncertainty (about 1.6% in neutron energy) and efficiency (of the order of 10−6 counts per unit neutron fluence). A semi-empirical procedure was also developed for correcting the non-linearity in light emission from the organic scintillator. The spectrometer was characterized with quasi-monoenergetic and continuous fields of neutrons generated at the CN Van De Graaff accelerator of the INFN-Legnaro National Laboratory, Italy, showing satisfactory agreement with literature data.

Response of a silicon telescope microdosimeter to 400 AMeV carbon ions

A monolithic silicon telescope consisting of a surface ΔE detector 2 μm in thickness coupled to an E detector about 500 μm in thickness made out of a single silicon wafer was recently proposed for the microdosimetric characterization of hadron beams. This device and its pixelated version are intended to be irradiated at the Centro Nazionale di Adroterapia Oncologica (CNAO, National Centre for Oncological Hadrontherapy, Pavia, Italy) with a 400 AMeV carbon ion beam. The response of the silicon microdosimeter under these conditions was calculated with Monte Carlo simulations by using the FLUKA code, in order to have reference available for comparison with the experimental results.

A monolithic silicon telescope for hadron beams: numerical and experimental study of the effect of ΔE detector geometry on microdosimetric distributions

A monolithic silicon telescope consisting of a surface ΔE detector 2 μm in thickness coupled to an E detector about 500 μm in thickness made out of a single silicon wafer was recently proposed for the microdosimetric characterization of hadron beams. This work discusses the study of the effect of the geometrical structure of the ΔE detector on microdosimetric distributions measured by the silicon device in hadron beams. Two different devices, i.e. a single-diode ΔE detector 1 mm in sensitive area and a ΔE detector geometrically segmented in micrometric cylinders, were irradiated at different phantom depths with 62 AMeV carbon ions in the same experimental conditions. In order to reproduce and deeply analyze the experimental results, a detailed numerical study based on Monte Carlo simulations was carried out through the FLUKA code version 2012, a recent release able to transport heavy ions at energies lower than 100 AMeV (the older lower transport limit) by exploiting the new Boltzman Master Equation model. The comparison between microdosimetric distributions measured by the two different detectors highlighted discrepancies at low lineal energies. Those differences, not reproduced by simulations, are probably due to charge sharing between the ΔE electrodes and the surrounding guards of the segmented device.

Study of the direct response of a monolithic silicon telescope to charged particles at different energies

Stefano Agosteo, Enrico Borsato, Flavio Dal Corso, Alberto Fazzi, Franco Gonella, Maria Vittoria Introini, Michele Lorenzoli, Matteo Pegoraro, Andrea Pola, Vincenzo Varoli and Pierluigi Zotto Politecnico di Milano, Dipartimento di Energia, Sezione di Ingegneria Nucleare, via Ponzio 34/3 20133 Milano, Italy; b INFN, Sezione di Milano, via Celoria 16, 20133 Milano, Italy; INFN, Sezione di Padova, via Marzolo 8, 35131 Padova, Italy; Universita di Padova, Dipartimento di Fisica, via Marzolo 8, 35131 Padova, Italy

Study of a monolithic silicon telescope for solid state microdosimetry: Response to a 100 MeV proton beam

Abstract A monolithic silicon telescope was recently proposed and studied for solid state microdosimetry. It consists of a thin surface ∆E stage (about 2 μm in thickness), at study for silicon microdosimetry, coupled to an E stage about 500 μm in thickness, which provide information about the energy of the impinging particle. In order to study the response of the detection system to high energy charged hadrons, the silicon telescope was placed in a polystyrene phantom and irradiated with a 100 MeV un-modulated proton beam at the Loma Linda University Medical Centre. The experimental results were compared with those obtained with a numerical study based on Monte Carlo simulations carried out with the FLUKA code. The agreement between experimental and simulation results was satisfactory.

Feasibility study of radiation quality assessment with a monolithic silicon telescope: Irradiations with 62 AMeV carbon ions at LNS-INFN

Abstract The Segmented Monolithic Silicon Telescope, a two-stage detector recently proposed as a solid state microdosimeter, demonstrated to be capable of measuring the microdosimetric spectra of clinical proton beams similar to those measured by reference tissue-equivalent proportional counters. This work concerns the study of the possibility of exploiting this feature for carbon beam quality assessment. To this aim, the silicon device was placed in a polymethylmetacrylate phantom and irradiated with a 62 AMeV un-modulated carbon beam at the cyclotron facility of the Laboratori Nazionali del Sud of the Italian Institute of Nuclear Physics. The results of these preliminary measurements will be discussed in details.

Characterization of the energy distribution of neutrons generated by 5MeV protons on a thick beryllium target at different emission angles

Neutron energy spectra at different emission angles, between 0° and 120° from the Be(p,xn) reaction generated by a beryllium thick-target bombarded with 5 MeV protons, have been measured at the Legnaro Laboratories (LNL) of the Italian National Institute for Nuclear Physics research (INFN). A new and quite compact recoil-proton spectrometer, based on a monolithic silicon telescope, coupled to a polyethylene converter, was efficiently used with respect to the traditional Time-of-Flight (TOF) technique. The measured distributions of recoil-protons were processed through an iterative unfolding algorithm in order to determine the neutron energy spectra at all the angles accounted for. The neutron energy spectrum measured at 0° resulted to be in good agreement with the only one so far available at the requested energy and measured years ago with TOF technique. Moreover, the results obtained at different emission angles resulted to be consistent with detailed past measurements performed at 4 MeV protons at the same angles by TOF techniques.

Study of a solid state microdosemeter based on a monolithic silicon telescope: irradiations with low-energy neutrons and direct comparison with a cylindrical TEPC

A silicon device based on the monolithic silicon telescope technology coupled to a tissue-equivalent converter was proposed and investigated for solid state microdosimetry. The detector is constituted by a ΔE stage about 2 µm in thickness geometrically segmented in a matrix of micrometric diodes and a residual-energy measurement stage E about 500 µm in thickness. Each thin diode has a cylindrical sensitive volume 9 µm in nominal diameter, similar to that of a cylindrical tissue-equivalent proportional counter (TEPC). The silicon device and a cylindrical TEPC were irradiated in the same experimental conditions with quasi-monoenergetic neutrons of energy between 0.64 and 2.3 MeV at the INFN-Legnaro National Laboratories (LNL-INFN, Legnaro, Italy). The aim was to study the capability of the silicon-based system of reproducing microdosimetric spectra similar to those measured by a reference microdosemeter. The TEPC was set in order to simulate a tissue site about 2 μm in diameter. The spectra of the energy imparted to the ▵E stage of the silicon telescope were corrected for tissue-equivalence through an optimized procedure that exploits the information from the residual energy measurement stage E. A geometrical correction based on parametric criteria for shape-equivalence was also applied. The agreement between the dose distributions of lineal energy and the corresponding mean values is satisfactory at each neutron energy considered.

Improvement of the minimum detectable energy of a recoil-proton spectrometer based on a silicon telescope

Abstract A recoil-proton spectrometer based on a monolithic silicon telescope coupled to a polyethylene converter was recently proposed and discussed in the literature. The device consists of a ΔE and an E-stage detector (about 2 μm and 500 μm in thickness, respectively) made out of a single silicon wafer and separated by a highly-doped layer acting as a common electrode. The detection system allowed continuous neutron spectra to be measured down to about 400 keV by discriminating against the contribution of low-LET radiation generated by photons from the distribution of the energy deposited in the E stage. This discrimination was carried out by selecting detected particles, event-by-event, with a ΔE − E correlation. At neutron energies lower than 400 keV recoil-protons cannot reach the E stage owing to the thickness of the ΔE stage and therefore the discrimination failed. In order to further reduce the minimum detectable energy, an improved detection system, which also accounts for the energy deposited by recoil-protons in the ΔE stage, was studied and tested. The new set-up permits the total energy deposited in the telescope to be measured directly by collecting the charge carriers, generated in both stages, at the deep common electrode. The capability of reproducing continuous neutron spectra was also verified by irradiating the improved set-up with neutrons generated by protons striking a thick beryllium target at INFN – Laboratori Nazionali di Legnaro (Legnaro, Italy). The agreement of the unfolded spectra with literature data was satisfactory at energies higher than about 200 keV.

Monolithic silicon telescope as position sensitive detector

We show the possibility to use a new kind of monolithic silicon telescope as position sensitive detector.

Feasibility Study of a Monolithic Silicon Telescope for BNCT Applications

A monolithic silicon telescope, consisting of a thin ΔE stage (of ~2 μm in thickness) coupled to a residual-energy stage E (thickness 500 μm), was studied and tested for measuring the boron concentration in biological samples for boron neutron capture therapy (BNCT). At the Laboratorio di Energia Nucleare Applicata BNCT facility, Pavia, Italy, this information is derived by placing the tissue sample in front of a surface barrier diode and by irradiating the system with neutrons generated in the thermal column of the TRIGA Mark II reactor. The boron concentration is estimated through the measurement of the energy deposited in the detector by the products of the 10B(n,α)7Li reaction. However, the low-energy part of the measured spectra is typically distorted by secondary electrons produced by photon background and by protons generated via the 14N(n,p)14C reaction in tissue. This work discusses the possibility of using a different silicon device, namely, the monolithic silicon telescope, for improving the accuracy of the method by exploiting its particle discrimination capabilities. A device with a sensitive area of 1 mm2 was irradiated (in vacuo) bare, faced both with a certified boron implanted silicon wafer and with a thin sample of rat lung loaded with boron. The preliminary results showed that (a) alpha particles and lithium ions produced by neutron capture on boron are well identified, (b) the events due to protons generated in tissue by neutron capture on nitrogen can be well discriminated, and (c) the presence of nitrogen inside the detector dead layer gives rise to an additional contribution of protons from neutron absorption. These preliminary studies gave confidence about the possibility of improving the accuracy for the assessment of the 10B concentration in biological samples for BNCT.

Analytical model for a monolithic silicon telescope – Response function of the E stage

A monolithic silicon telescope coupled to a polyethylene radiator was studied and tested as a neutron spectrometer. The detector consists of a Δ E and an E stage detector, about 1.9 and 500 μm in thickness, respectively. The neutron spectra were derived from the measurement of the energy deposited in the E stage by recoil-protons generated in the radiator. The detectable energy range of the present detection system is about 0.350–8 MeV. In order to optimize the reconstruction of the impinging neutron spectra, an analytical model of the response functions of the detection system to mono-energetic neutrons was developed and implemented in an unfolding procedure. The model is based on the kinematics of neutron elastic scattering on hydrogen and takes into account the actual geometrical structure of the silicon telescope. The calculated response functions were compared with the results of Monte Carlo simulations (by using the FLUKA code) and those obtained by an experimental characterization.

Neutron spectrometry with a monolithic silicon telescope

A neutron spectrometer was set-up by coupling a polyethylene converter with a monolithic silicon telescope, consisting of a DeltaE and an E stage-detector (about 2 and 500 microm thick, respectively). The detection system was irradiated with monoenergetic neutrons at INFN-Laboratori Nazionali di Legnaro (Legnaro, Italy). The maximum detectable energy, imposed by the thickness of the E stage, is about 8 MeV for the present detector. The scatter plots of the energy deposited in the two stages were acquired using two independent electronic chains. The distributions of the recoil-protons are well-discriminated from those due to secondary electrons for energies above 0.350 MeV. The experimental spectra of the recoil-protons were compared with the results of Monte Carlo simulations using the FLUKA code. An analytical model that takes into account the geometrical structure of the silicon telescope was developed, validated and implemented in an unfolding code. The capability of reproducing continuous neutron spectra was investigated by irradiating the detector with neutrons from a thick beryllium target bombarded with protons. The measured spectra were compared with data taken from the literature. Satisfactory agreement was found.

A solid state microdosimeter based on a monolithic silicon telescope

A monolithic silicon telescope, consisting of a DeltaE and an E stage-detector ( approximately 1.9 microm and 500 microm thick, respectively), was coupled to a polyethylene converter in order to investigate the feasibility of a solid state microdosimeter with respect to the field-funnelling effect. This work discusses the preliminary results of an analytical approach for the correction of a spectrum measured with this silicon-based microdosimeter for tissue-equivalence and geometrical effects. The device was irradiated with 2.7 MeV monoenergetic neutrons at the INFN-Laboratori Nazionali di Legnaro (Legnaro, Italy). The non tissue-equivalence of silicon was corrected by exploiting the signals generated in the E-stage. The correction for the sensitive volume geometry was optimised by taking into account the track length distribution of the recoil-protons generated in the converter. The derived dose distribution of the energy imparted per event was compared to the one measured with a cylindrical tissue-equivalent proportional counter (TEPC). The agreement is satisfactory.

Performance and perspectives of silicon detector telescopes

A new concept of monolithic silicon telescope sensitive to the position is studied along with an integrated front-end electronics.

A feasibility study of a solid-state microdosimeter

A solid-state silicon detector is a challenging device for microdosimetry, mainly because it can provide sensitive zones of the order of a micrometer. Moreover, these detectors are characterized by a high spatial and a good energy resolution. However, they may present some limitations, such as: (i) the minimum detectable energy which is limited by the electronic noise; (ii) radiation hardness; (iii) the geometry of the sensitive volume; (iv) the field-funnelling effect; (v) the non-tissue-equivalence of silicon. This work discusses a feasibility study of a microdosimeter based on a monolithic silicon telescope, consisting of a Δ E and an E stage-detector, about 1 and 500 μm thick, respectively. Charges are collected separately in the two stage-detectors. The use of the Δ E stage coupled with a tissue-equivalent converter was investigated as a solid-state microdosimeter. Irradiations with monoenergetic neutrons were performed at the INFN-Laboratori Nazionali di Legnaro (Italy). The field-funnelling effect appears to be negligible from the comparison of the experimental data with the results of Monte Carlo simulations, performed with the FLUKA code. The preliminary results of an analytical approach for the correction for geometrical effects and tissue-equivalence are also presented.

A New approach for cross section measurements at low energies using a monolithic silicon telescope

We propose a new experimental technique for the study of low-energy nuclear reactions using a monolithic silicon telescope, which has a thin silicon layer with a thickness of 1 μ m. A feasibility study of this technique for the investigation of the 8 Li( α ,n) 11 B reaction at low energies has been performed using the GEANT4 simulation code and a calibrated alpha source. Our results indicate the possibility of a novel technique for low-energy nuclear physics experiments relevant to astrophysics.

A new large area monolithic silicon telescope

A new prototype of large area (20×20 mm 2) monolithic silicon telescope with an ultrathin Δ E stage (1 μm) has been built and tested. A particular mask for the ground electrode has been developed to improve the charge collection reducing the induction between the E and Δ E stages. A special designed preamplifier has been used for the readout of the signal from the Δ E stage to overcome the problem of the large input capacitance (40 nF). A rather low energy threshold charge discrimination has been obtained. Small side effects due to the electric field deformation near the ground electrode were observed and quantified.