Energy sensitive Timepix silicon detector for electron imaging

doi:10.1016/j.nima.2011.01.148

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 652, Issue 1, 1 October 2011, Pages 458-461

https://doi.org/10.1016/j.nima.2011.01.148 Get rights and content

Abstract

We present the first measurements with the energy sensitive Timepix pixel detector for electron imaging. The hybrid pixel detector consists of a silicon detector, 300 μm thick, bump-bonded to the Timepix readout chip developed by the Medipix2 collaboration (256×256 pixels, 55 μm pitch, 14.08×14.08 mm² sensitive area). Each Timepix pixel can be independently operated in one of three modes: 1. counting of the detected particles; 2. measurement of the particle energy; 3. measurement of the time of particle interaction. The energy measurement in the second mode is performed via the determination of the “Time-Over-Threshold” (TOT). The ionization charge generated by the particle along its track is often registered by several adjacent pixels forming a cluster. The shape of the cluster is affected also by lateral charge spread. It is often possible to determine a particle type, its energy, entrance point and direction by online or offline analysis of shapes of recorded clusters. This way an influence of background or noise can be significantly reduced in measured data.

The energy spectrum for ⁹⁰Sr/⁹⁰Y electrons was measured by Timepix detector and compared with the β⁻ decay spectrum and the Monte Carlo simulated spectrum. In order to improve spatial resolution, we analyzed the tracks of all electrons and substituted each cluster with the position of its centroid. We measured the spatial resolution with a ⁹⁰Sr/⁹⁰Y source irradiating at 10 cm distance, a 100 μm thick steel edge slightly tilted with respect to the detector lines. The oversampled Line Spread Function shows an FWHM of 27.5±1.1 μm. The Timepix Si detector will be used for digital autoradiography with β⁻ and β⁺ tracers, and it could be used for electron microscopy. First tests were performed with a ¹⁴C autoradiography sample.

Introduction

We present the first measurements with the energy sensitive Timepix silicon detector for electron imaging. Timepix [1] is a hybrid pixel detector (also in matrix size, 256×256, and pixel pitch, 55 μm) similar to the well known Medipix2 single photon counting pixel detector, but with the added capability of energy sensitivity, since Timepix can record the energy deposited in each pixel by interacting radiation. This gives spectroscopic capabilities at pixel level. In this paper we present the spectroscopic and imaging characterization of a Timepix silicon detector with low energy and high energy electrons from laboratory radioactive sources, having in mind application to beta autoradiography and electron microscopy.

For fine pitch (<100 μm) pixel detectors the charge sharing phenomenon has been shown to produce a severe degradation of the detector performance in terms of energy resolution and spatial resolution [2]. Charge sharing, here defined as splitting of the charge generated by an interacting particle in more than one detector pixels, arises from diverse causes, including primary or secondary particle range, charge lateral diffusion, X-ray fluorescence and Bremsstrahlung production. In photon counting pixel detectors, charge sharing has been shown to produce a degradation of the photon energy resolution [3] or even its suppression [4], for ∼50 μm pitch detectors, and it has been considered as a limiting factor in developing energy sensitive X-ray imaging detectors with high spatial resolution [2]. In pixel threshold discrimination mode—the usual signal processing scheme for single particle counting pixel detectors—a valid hit in a pixel is produced if its energy deposit is above the pixel (energy) threshold. Then, a possible scheme for charge sharing reduction or suppression is to raise the pixel threshold, but this produces a limitation of the useful energy range and of the counting efficiency. Software and hardware schemes have been proposed and used to overcome this problem, including the summation of the energy deposited in adjacent pixels [1], [5]. On the other hand, fine pitch detectors operating at a very low energy threshold (a few keV) allow to record with good accuracy the projected track of the primary and the secondary charged particles on the pixelated readout electrode side. Assuming a charge-collecting applied electric field in the detector with field lines perpendicular to the front/back detector surfaces, the energy recorded in each pixel can be regarded as the total energy deposited in the detector active volume under the given pixel. This permits to reconstruct the shape of the (2D) particle track in the detector and also its total deposited energy, by summing up the energy deposited in each pixel of the track. In this detection scheme, for which a very low energy threshold and a large energy range per pixel are necessary, the occurrence of charge sharing could not be considered as a problem but a phenomenon to be exploited with advantage, since it could recover energy resolution even for very fine pitch (e.g., 50 μm pitch or lower) imaging. This scheme is adopted in this work.

Section snippets

Detector

Timepix is a hybrid pixel detector developed by the CERN-based Medipix2 European collaboration [1]. It consists of a CMOS readout circuit (Timepix) bump-bonded to a 300 μm thick silicon pixel detector with a matrix of 256×256 square pixels of 55 μm pitch. Each cell in the matrix of the Timepix chip contains a charge sensitive preamplifier, a single threshold discriminator, a time based logic and a 14 bit pseudorandom counter provided with overflow control logic. It can work in three different

Results

The energy spectrum for ⁹⁰Sr/⁹⁰Y electrons (end-point energy of 2.28 MeV; source placed at about 10 cm above the detector surface) was measured via track analysis and compared with the β⁻ decay spectrum and the Monte Carlo simulation (Fig. 1, Fig. 2a). One million 0.02 s frames were acquired at a rate of 4 frames per second, for a total of 1.35×10⁸ clusters recognized. In Fig. 2b is shown the cluster size distribution for ⁹⁰Sr/⁹⁰Y electrons. With the source placed at about 10 cm above the detector

Discussion

The measured and simulated energy spectra for ⁹⁰Sr/⁹⁰Y electrons are in good agreement (Fig. 2a) except at low energy (E≤50 keV), where the measured spectrum is higher. This was attributed to electronic noise and energy losses arising from lateral diffused charge below threshold. This behavior was not taken into account in Monte Carlo simulations and, therefore, it is not present in the simulated results. A pile-up shoulder, due to overlapped tracks, is present in the measured spectrum at high

Conclusions

The Timepix silicon hybrid pixel detector has been characterized as an energy sensitive system for electron imaging. The new time based logic of this detector allows to implement the “single particle tracking” and the “first pixel hit” approximation via recognition of the pixel-by-pixel energy deposit of each electron, thus turning to an advantage the phenomenon of charge sharing, which can limit the performance of particle counting pixel detector as the companion Medipix2 detectors [21]. The

Acknowledgement

This work has been done in the framework of the Medipix2 European collaboration (www.cern.ch/medipix).

References (22)

X. Llopart
Nucl. Instr. and Meth. A
(2007)
H.E. Nilsson
Nucl. Instr. and Meth. A
(2007)
H.E. Nilsson
Nucl. Instr. and Meth. A
(2002)
V. Tichy
Nucl. Instr. and Meth. A
(2008)
J. Jakubek.
Nucl. Instr. and Meth. A
(2009)
T. Michel
Nucl. Instr. and Meth. A
(2009)
Zemlicka
Nucl. Instr. and Meth. A
(2009)
J. Jakubek
Nucl. Instr. and Meth. A
(2008)
J. Jakubek
Nucl. Instr. and Meth. A
(2009)
J. Vallerga
Nucl. Instr. and Meth. A
(2008)

M. Battaglia

Nucl. Instr. and Meth. A

(2009)

Cited by (31)

Monte Carlo and experimental evaluation of a Timepix4 compact gamma camera for coded aperture nuclear medicine imaging with depth resolution
2023, Physica Medica
We designed a prototype compact gamma camera (MediPROBE4) for nuclear medicine tasks, including radio-guided surgery and sentinel lymph node imaging with a ^99mTc radiotracer. We performed Monte Carlo (MC) simulations for image performance assessment, and first spectroscopic imaging tests with a 300 μm thick silicon detector.
The hand-held camera (1 kg weight) is based on a Timepix4 readout circuit for photon-counting, energy-sensitive, hybrid pixel detectors (24.6 × 28.2 mm² sensitive area, 55 μm pixel pitch), developed by the Medipix4 Collaboration. The camera design adopts a CdTe detector (1 or 2 mm thick) bump-bonded to a Timepix4 readout chip and a coded aperture collimator with 0.25 mm diameter round holes made of 3D printed 1-mm thick tungsten. Image reconstruction is performed via autocorrelation deconvolution.
Geant4 MC simulations showed that, for a ^99mTc source in air, at 50 mm source-collimator distance, the estimated collimator sensitivity (4 × 10⁻⁴) is 292 times larger than that of a single hole in the mask; the system sensitivity is 0.22 cps/kBq (2 mm CdTe); the lateral spatial resolution is 1.7 mm FWHM. The estimated axial longitudinal resolution is 8.2 mm FWHM at 40 mm distance. First experimental tests with a 300 μm thick Silicon pixel detector bump-bonded to a Timepix4 chip and a high-resolution coded aperture collimator showed time-over-threshold and time-of-arrival capabilities with ²⁴¹Am and ¹³³Ba gamma-ray sources.
MC simulations and validation lab tests showed the expected performance of the MediPROBE4 compact gamma camera for gamma-ray 3D imaging.
Double-crystal measurements at the CERN SPS
2021, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
The UA9 setup, installed in the Super Proton Synchrotron (SPS) at CERN, was exploited for a proof of principle of the double-crystal scenario, proposed to measure the electric and the magnetic moments of short-lived baryons in a high-energy hadron collider, such as the Large Hadron Collider (LHC). Linear and angular actuators were used to position the crystals and establish the required beam configuration. Timepix detectors and high-sensitivity Beam Loss Monitors were exploited to observe the deflected beams. Linear and angular scans allowed exploring the particle interactions with the two crystals and recording their efficiency. The measured values of the beam trajectories, profiles and of the channeling efficiency agree with the results of a Monte-Carlo simulation.
Feasibility study on the use of CMOS sensors as detectors in radioguided surgery with β<sup>−</sup>- emitters
2020, Applied Radiation and Isotopes
Radioguided surgery (RGS) is a medical practice which thanks to a radiopharmaceutical tracer and a probe allows the surgeon to identify tumor residuals up to a millimetric resolution in real-time. The employment of $β^{-}$ emitters, instead of $γ$ or $β^{+}$ , reduces background from healthy tissues, administered activity to the patient, and medical exposure. In a previous work the possibility of using a CMOS Imager (Aptina MT9V011), initially designed for visible light imaging, to detect $β^{-}$ from ⁹⁰Y or ⁹⁰Sr sources has been established. Because of its possible application as counting probe in RGS, the performances of MT9V011 in clinical-like conditions were studied.¹
Through horizontal scans on a collimated ⁹⁰Sr source of different sizes (1, 3, 5, 7 mm), we have determined relationships between scan fit parameters and the source dimension, namely A quadratic correlation and a linear dependency of, respectively, signal integrated over scan interval, and maximum signal against source diameter, are determined. Horizontal scan measurements on a source, interposing collimators of different size, aim to determine relationships or correlations between scan fit parameters and source dimension. A quadratic correlation and a linear dependency of, respectively, signal integrated over scan interval, and maximum signal against source diameter are determined.
In order to get closer to clinical conditions, agar–agar phantoms containing $^{90} Y$ with different dimensions and activities were prepared. A $^{90} Y$ phantom is characterized by a central spot and a ring all around, for simulating both signal (tumor) and background (surrounding healthy tissue). The relationship found between scan maximum and $^{90} S r$ source diameter is then exploited to extract the concentration ratio between spot and external ring of the $^{90} Y$ phantom. This observable, defined as the ratio between the tumor and the nearby healthy tissues uptake simulates the Tumor-to-Non-tumor Ratio (TNR). With the aim of evaluating the sensor’s ability to discriminate signal from background relying on the significance parameter, a further $^{90} Y$ phantom, featuring a well-known and clinical-like activity will mimic the signal only condition. This result is used to extrapolate to different source sizes, after having estimated the background for various TNR. The obtained significance values suggest that the MT9V011 sensor is capable of distinguishing a signal from an estimated background, depending on the interplay among TNR, acquisition time and tumor diameter.
CdTe compact gamma camera for coded aperture imaging in radioguided surgery
2020, Physica Medica
Citation Excerpt :
This imaging probe – also adopted in an experimental CT/SPECT system dedicated to the breast [34], and for imaging at nuclear facilities [27,28] – could be adopted for various tasks as preoperative or intraoperative SLN localization with 99mTc radiotracer in breast cancer or melanoma patients [18,35–38], breast imaging with 99mTc-Sestamibi [39], thyroid imaging [40], breast RGS with 125I and/or 99mTc radiotracers [41], and in hybrid imaging systems [42]. The same detector unit – developed within the Medipix2 Collaboration at CERN – provided the γ-ray head for planar and SPECT small animal imaging system [43–47] and also for a thermal neutron detector system [48] and for electron imaging [49,50]. Preliminary results of this work have been reported in Ref. [51].
The aim of this work was to assess the performance of a prototype compact gamma camera (MediPROBE) based on a CdTe semiconductor hybrid pixel detector, for coded aperture imaging. This probe can be adopted for various tasks in nuclear medicine such as preoperative sentinel lymph node localization, breast imaging with ^99mTc radiotracers and thyroid imaging, and in general in radioguided surgery tasks. The hybrid detector is an assembly of a 1-mm thick CdTe semiconductor detector bump-bonded to a photon-counting CMOS readout circuit of the Medipix2 series or energy-sensitive Timepix detector. MediPROBE was equipped with a set of two coded aperture masks with 0.07-mm or 0.08-mm diameter holes. We performed laboratory measurements of field of view, system spatial resolution, and signal-difference-to-noise ratio, by using gamma-emitting radioactive sources (¹⁰⁹Cd, ¹²⁵I, ²⁴¹Am, ^99mTc). The system spatial resolution in the lateral direction was 0.56 mm FWHM (coded aperture mask with holes of 0.08 mm and a 60 keV source) at a source-collimator distance of 50 mm and a field of view of 40 mm by side. Correspondingly, the longitudinal resolution in 3D source localization tasks was about 3 mm. MediPROBE showed a significant improvement in terms of spatial resolution when equipped with the high-resolution coded apertures, with respect to the performance previously reported with 1–2 mm pinhole apertures as well as with respect to adopting a 0.35 mm pinhole aperture.
Directional detection of charged particles and cosmic rays with the miniaturized radiation camera MiniPIX Timepix
2018, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Citation Excerpt :
The resulting miniaturized radiation camera stands as an online active nuclear emulsion [10,11] with sensitivity to X-rays and primarily to charged particles in wide range of fluxes, energies and direction [12,13]. The spectrometric and tracking capabilities of the detector find application in a variety of techniques in radiation imaging [2,14,15], electron microscopy [16,17], ion detection [18,19], charged particle tracking [13,20–23], ion beam therapy [21,24,25], radiation dosimetry [26–28], particle accelerator environments [29], nuclear reaction physics [30], environmental, atmospheric and cosmic ray detection [31,11,12] and space radiation detection in Low Earth Orbit (LEO) orbit on board the International Space Station (ISS) [5,32,33], spacecraft micro-satellites [34,35] and cubesats [36]. In tasks involving energetic charged particles, such as mixed radiation environments in ion beam therapy and outer space, the Timepix detector can provide particle-type data products [37] in wide-dynamic range.
We evaluate and characterize the directional response of the semiconductor pixel detector Timepix to charged particles and secondary cosmic rays. The detector granularity and per-pixel spectrometric response enable to perform directional sensitive tracking of energetic charged particles in wide field-of-view. In the highly integrated MiniPIX readout the detector serves as a miniaturized, low-power and easily deployable particle micro-tracker. Angular measurements require normalization for the acceptance angle and geometric factor which are described and taken into account. The methodology and evaluation of directional response are developed for light charged particles and atmospheric secondary cosmic rays. Tests and calibration of angular resolution were performed with electron and proton beams. Resulting angular distributions are expressed in terms of elevation and azimuth angles in 14 bins and 36 bins, respectively, over the full 2 $π$ acceptance range. For zenith angle $β$ > 28° the angular distribution is fitted by $\sim$ cos $^{n}$ ( $β$ ) with n = 2.0 ± 0.2 expected from secondary cosmic ray muon distribution.
Resolving power of pixel detector Timepix for wide-range electron, proton and ion detection
2018, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Citation Excerpt :
Using thick CdTe sensors this photon range can be extended to few-hundred keV (limited efficiency, few %) up to 1 MeV (low efficiency, below 1%). The Timepix instrumentation and developed methodology prove attractive and valuable for a variety of applications [30,42,43] ranging from radiation imaging [42–44] to electron microscopy [45,46], ion detection [8,47], charged particle tracking [10,31,32,48,49], detection and characterization of mixed radiation fields at the ATLAS-LHC experiment [2,4], particle accelerators [8,31] and ion beam radiotherapy environments [11,12,14,50], ion beam delivery imaging [25,51], radiation dosimetry [28,52–54], environmental, atmospheric and cosmic ray detection [30,53,55] and space radiation detection in LEO orbit [35,56–60]. Also, neutron detection with enhanced signal-to-noise sensitivity and large dynamic range [61,62] the field of which is accompanied by intense background and unwanted radiation components.
The resolving power of the Timepix detector for wide-range charged particle detection has been examined and evaluated in defined radiation fields. The goal is to broadly characterize mixed-radiation fields consisting namely of X-rays and charged particles in terms of particle-types (species), spectral response (energy loss) and direction in wide field-of-view (essentially 2 $π$ ) with a single compact tracking detector. Tests and calibration measurements were performed with the same device at electron, proton and ion fields at various energies and incident directions. Event-by-event detection, together with pattern recognition analysis of the single particle tracks, are exploited to analyze events according to three degrees of freedom—the particle type (X-rays, light and heavy charged particles), energy range (low or high energy—depending on their range being smaller or larger than the pixel size of the detector semiconductor sensor) and direction (incident angle to the sensor plane). Characteristic values are determined for the cluster analysis morphology parameters, the particles stopping power or Linear Energy Transfer and derived correlated quantities. Ratios and correlations between selected parameters are analyzed including 2D-scatter plots. A physics-based wide-range classification is proposed for a total of 8 broad event groups—in terms of light charged particles (electrons, muons) of both low and high energy incident perpendicular (type 1, including X-rays) or high energy non-perpendicular (type 5), protons of low energy omnidirectional (type 2) and high energy non-perpendicular (type 6), alpha particles and light ions of low energy omnidirectional (type 3) and high energy non-perpendicular (type 7), and heavy ions of low energy omnidirectional (type 4) and high energy non-perpendicular (type 8).

View all citing articles on Scopus

View full text