TimePix Pixel Research Articles

Development of Pattern Recognition Software for Tracks of Ionizing Radiation In Medipix2-Based (TimePix) Pixel Detector Devices

The principal aim of our project is to develop an efficient pattern recognition tool for the automated identification and classification of tracks of ionizing radiation as measured by a TimePix version of the hybrid semiconductor Medipix2 pixel detector system. Such a software tool would have a number of applications including dosimeters to assess the risk of human exposure to radiation, and area monitors to characterize the general background radiation environment harmful to humans and electronic equipment. We are particularly interested in the development of the real-time analysis software needed to support an operational dosimeter that can assess the radiation environment during space missions. Our software development project makes use of data taken in beams of heavy ions at HIMAC (Heavy Ion Medical Accelerator Facility) in Chiba, Japan, including data from several different heavy ions with similar Linear Energy Transfers (LETs) for calibration purposes. We describe two modules of our pattern recognition tool: feature generation and classification. Our first module builds on a segmentation algorithm that identifies tracks from the pixel image assuming an approximately elliptical form that varies in size and degree of elongation based on multiple factors, including the particle species and angle of incidence. Determining the charge and energy of the particles creating each track is a particularly challenging task because different energy and charge incident particles can produce very similar patterns. Our classification module invokes different algorithms such as decision trees, support vector machines, and Bayesian classifiers.

X-ray color imaging with 3D sensitive voxel detector

X-ray imaging is today widely used in a broad range of applications. Nevertheless some limitations are represented by the inability to distinguish between a thick layer of low Z material and a thin layer of high Z material, and by the beam hardening, where the incident X-ray spectrum is modified as the beam traverses the sample. Such effects cause problems in many applications (e.g. CT reconstruction) generating artifacts and worsening the spatial resolution. This work presents a new technique allowing spectral sensitivity using a new 3D voxel detector based on the Timepix pixel detector. The device is designed as a layered stack of several Timepix sensors. The readout chip is thinned down to reduce the amount of insensitive absorbing material. Every single layers in the stack act as a filter, i.e. each stack layer visualizes a different part of the spectrum attenuated by the object giving further information about the object composition. The comparison of attenuation levels observed in different detector layers can be used to estimate the extent of the beam hardening effect in the imaged object and thus point out differences in the material composition.

Energy sensitive Timepix silicon detector for electron imaging

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 mm 2 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 90 Sr/ 90 Y 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 90 Sr/ 90 Y 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 14 C autoradiography sample.

Precise energy calibration of pixel detector working in time-over-threshold mode

The semiconductor pixel detector Timepix (256×256 pixels with pitch of 55 μm) is a successor of the Medipix2 device. Each Timepix pixel can be independently operated in one of three possible modes: (1) counting of the detected particles; (2) measurement of the particle energy; and (3) measurement of the time of interaction. The energy measurement in the second mode is performed via the determination of the “time-over-threshold” (TOT). The energy measurement with the Timepix detector in TOT mode requires knowledge of the energy calibration of each pixel of the matrix. Such calibration is very nonlinear in the low energy range and can be described by a surrogate function depending on four parameters. The determination of all these parameters can be performed by measurement and evaluation of the response of each pixel in at least four calibration points. The procedure is extremely demanding: it requires the analysis of at least 250 thousand spectra and the performance of 330 thousand least-squares fits. In this article, it is demonstrated that even better result can be achieved with only two or three calibration points halving the number of least-squares fits needed. The method is based on precise analysis of the shape of spectral peaks. The article also discusses the performance of energy calibrated device for spectrometry of heavy charged particles.

Pixel detectors for imaging with heavy charged particles

State-of-the-art single quantum counting pixel detectors offer a large potential for different imaging applications. The TimePix pixel device can provide information about position and energy of the detected radiation allowing radiography with charged particles. Heavy charged particles of known initial energy lose their energy partially by going through a specimen material. If the resulting energies of particles passing the specimen are measured, then specimen structure can be revealed. This article shows experimental results of this technique acquired with alpha particles and the TimePix detector. The spatial resolution in detector plane depends on particle energy and can reach submicrometer level. The specimen thickness can be determined with precision up to 320 nm for organic materials if energy loss of individual alpha particle is measured.