Measurement of the efficiency of the pattern recognition of tracks generated by ionizing radiation in a TIMEPIX detector

A hybrid silicon pixelated TIMEPIX detector (256 × 256 square pixels with a pitch of 55 μm) operated in Time Over Threshold (TOT) mode was exposed to radioactive sources and protons after Rutherford Backscattering on a thin gold foil of protons beams delivered by the Tandem Accelerator of the Montreal University. Simultaneous exposure of TIMEPIX to radioactive sources and to protons beams on top of the radioactive sources allowed measurements with different mixed radiation fields of protons, alpha-particles, photons and electrons. All measurements were performed in vacuum. The comparison of the experimental activities (determined from the measurement of the number of tracks left in the device by incoming particles) of the radioactive sources with their expected activities allowed the test of the device efficiency for track recognition. The efficiency of track recognition of incident protons of different energies as a function of the incidence angle was measured. The cluster size left by protons in the device was measured as a function of their incident energy at normal and large (75°) incident angles. The operation of TIMEPIX in TOT mode has allowed a 3D mapping of the charge spreading effect in the whole volume of the silicon sensor. The results of the present measurements demonstrate the TIMEPIX capability of differentiating between different types of particles species from mixed radiation fields and measuring their energy deposition. Single track analysis gives a good precision (significantly better than the 55 μm size of one detector pixel) on the coordinates of the impact point of protons with normal incidence interacting in the TIMEPIX silicon layer.

ABSTRACT: A hybrid silicon pixelated TIMEPIX detector (256 × 256 square pixels with a pitch of 55 µm) operated in Time Over Threshold (TOT) mode was exposed to radioactive sources and protons after Rutherford Backscattering on a thin gold foil of protons beams delivered by the Tandem Accelerator of the Montreal University. Simultaneous exposure of TIMEPIX to radioactive sources and to protons beams on top of the radioactive sources allowed measurements with different mixed radiation fields of protons, alpha-particles, photons and electrons. All measurements were performed in vacuum. The comparison of the experimental activities (determined from the measurement of the number of tracks left in the device by incoming particles) of the radioactive sources with their expected activities allowed the test of the device efficiency for track recognition. The efficiency of track recognition of incident protons of different energies as a function of the incidence angle was measured. The cluster size left by protons in the device was measured as a function of their incident energy at normal and large (75 • ) incident angles. The operation of TIMEPIX in TOT mode has allowed a 3D mapping of the charge spreading effect in the whole volume of the silicon sensor. The results of the present measurements demonstrate the TIMEPIX capability of differentiating between different types of particles species from mixed radiation fields and measuring their energy deposition. Single track analysis gives a good precision (significantly better than the 55 µm size of one detector pixel) on the coordinates of the impact point of protons with normal incidence interacting in the TIMEPIX silicon layer.
KEYWORDS: Particle identification methods; Particle tracking detectors

Introduction
The TIMEPIX device [1] used in the present experiments (figure 1) consists of a silicon detector chip (300 µm thick) bump-bonded to a readout chip. The detector chip is equipped with a single common backside electrode and a front side matrix of electrodes (256 × 256 square pixels, each of 55 × 55 µm 2 area). Each pixel is connected to its respective preamplifier, discriminator with an adjustable threshold and digital counter integrated on the readout chip. A globally applied shutter signal determines when all pixels are active. The device was operated in Time Over Threshold (TOT) mode allowing the direct measurement of the energy deposited in each pixel. A particle striking the detector deposits its energy in silicon generating free charge carriers. If the deposited energy exceeds the pre-set threshold, one or several pixels will be activated forming a cluster of adjacent pixels. The data are recorded as images called frames that contain the status of all the pixels (65536) after a given exposure time. Different shapes of clusters of illuminated pixels are visible as tracks in the recorded frames. The lateral spread of the charge carriers from the interaction of an ionizing particle in the silicon layer of TIMEPIX causes a spreading of the charge among adjacent pixels, resulting in different track patterns for different interacting particles [2]. Several experiments were performed in Montreal for measuring the efficiency for pattern recognition of tracks generated in a TIMEPIX detector operated in TOT mode by ionizing particles from radioactive sources and protons of different energies after Rutherford Backscattering on a thin gold foil of protons beams delivered by the van der Graaff Tandem Accelerator of the Montreal University (UMTA). The analysis framework (MAFalda) [3] was used to perform the pattern recognition of tracks in TIMEPIX and associate their shapes to specific types of particles species interacting in silicon. The results of the measurements, regarding the TIMEPIX capability of differentiating between different types of particles species from mixed radiation fields and measuring their energy deposition, are reported in this article together with results on the measurement of the impact coordinates of protons of normal incidence interacting in the TIMEPIX silicon layer as obtained from single cluster analysis. Conclusions and a brief outline of future work follow.  [4] used for the measurements presented in this work. The chip was surrounded by a protective sheet (marbled grey on the picture) to avoid direct contact of the chip surface with materials in the chamber. This sheet has limited the incoming particle incidence angle to values ≤ 75 • . The TIMEPIX detector was exposed to three radioactive sources (on a holder) and proton beams delivered by UMTA (see text).

Experimental setup
The measurements were performed at the UMTA. The TIMEPIX device, fully controlled by a Fitpix interface [4] and the Pixelman software [5], was located in a vacuum chamber (pressure of ∼10 −7 Torr) of the accelerator. It was struck by protons of different energies from 1.98 MeV up to 9.89 MeV after Rutherford Backscattering (RBS) of 2 MeV up to 10 MeV proton beams delivered by the UMTA on a 0.12 µm thick gold foil. The TIMEPIX detector was set at a scattering angle of 90 • after the foil. Rotation of the TIMEPIX detector, supported by a goniometer with a precision of 0.5 • , allowed the test of tracks recognition at different incidence angles (from 0 • up to 75 • ). The gold foil was located in front of the TIMEPIX detector in a contiguous accelerator chamber, at an angle of 45 • with respect to the beam direction (figure 1a). The TIMEPIX detector was also exposed to three radioactive sources (a source of alpha particles-241 Am, a source of electrons-106 Ru, a sources of photons-137 Cs), separately and simultaneously (two by two and then altogether, giving mixed radiation fields of photons, electrons and alpha-particles). The sources were mounted on a holder placed inside the chamber at different heights and located at 6.3 cm from the TIMEPIX detector (figure 1b). The average incidence angle of the particles emitted by the 241 Am, 106 Ru and 137 Cs sources was 29 • , 13 • and 22 • , respectively. For several experiments the TIMEPIX detector was exposed to protons from RBS beams on top of the radioactive sources giving for each proton energy a mixed radiation field of heavy charged particles (protons and alpha-particles), photons and electrons. The operation of the TIMEPIX detector in TOT mode allowed direct energy measurement in each pixel, with a low threshold of 10 keV and usually at a bias voltage of 100 V (the full depletion voltage is 25 V corresponding to a resistivity of 13 kΩcm). The exposure time was set to values short enough to avoid large tracks overlaps.

Reliability
The analysis framework (MAFalda) [2] was used to perform the pattern recognition of tracks at low threshold in TIMEPIX. MAFalda is a set of algorithms written in C++ language and based on the ROOT framework and dedicated to data analysis of any device of the Medipix family. MAFalda uses the cluster shape to recognize the different types of clusters which can be associated to a specific type of particle. Examples of pixel shapes generated by different types of particles in TIMEPIX are shown in figure 2.
Frames showing the tracks left in TIMEPIX by different types of ionizing particles are displayed in figure 3.
The reliability of track recognition was tested by comparing the activities (A reconst ) of the 241 Am, 106 Ru and 137 Cs radioactive sources (separate exposure) extracted from the experimental data (eq. (3.1)) with the expected activities (A expected ) (table 1). The count of the number of tracks N left in the detector by particles emitted by a radioactive source, allowed the extraction of its activity from the experimental data. The activity can be calculated using eq. (3.1): where A reconst is the activity of the radioactive source from the experimental data, t the exposure time, ε geom the geometric efficiency and ε det the particle detection efficiency. The detection efficiency is practically 100% for charged particles with energies above the pre-set threshold of 10 keV. For the 661 keV photons emitted by 137 Cs, the detection efficiency is 0.54% [6]. Table 1 shows the deviation -defined as (A reconst − A expected )/A expected [%] -of reconstructed activity with the data of tracks measurements from expected activity. This reconstruction error is 2.0% and 2.3% for 241 Am and 106 Ru, respectively. The larger error (4.1%) observed for 137 Cs is possibly due to photons originated in the chamber wall. The results reported in table 1 show that the pattern recognition software used can recognize tracks efficiently. ranges in matter are small and the sensitivity to distinguish incidence angle is small as it is difficult to distinguish heavy blobs from heavy tracks. Figure 7 shows the cluster size as a function of the incident energy (from 1.98 MeV up to 9.89 MeV) for protons striking TIMEPIX at 0 • and 75 • incident angles. Protons with perpendicular incidence (0 • ) on the sensor with kinetic energy above 6.93 MeV have a range in silicon larger than the detector thickness of 300 µm and can traverse the detector thickness. They cannot deposit all of their energy in the sensor, then, the cluster size has to decrease. Conversely, protons above 4.95 MeV have a range in silicon exceeding 300 µm and with an incidence of 75 • remain inside the detector. They can deposit their kinetic energy completely in the sensor. Then, the cluster size largely increases.

Single track analysis
Due to charge spreading effects, a 2D Gaussian can be fitted to each energy deposition point left by a track in the sensor plane. Thus, for 0 • incidence angle, a Gaussian can be fitted to a track left by a highly ionizing particle to determine the exact position of impact of the interacting particle in silicon. An example is shown in figure 8

Conclusion and future work
The results of the present measurements demonstrate the TIMEPIX capability, based on efficient track pattern recognition, of differentiating between different types of particles species from mixed radiation fields and measuring their energy deposition. The deviation (A reconst − A expected )/A expected [%] of the activities (A reconstr ) reconstructed from the count of the number of tracks left in the detector by particles emitted by individual radioactive sources ( 241 Am, 137 Cs, and 106 Ru) from their expected activities (A expected ) is below 5%. Distinction is achieved at ∼90% between heavy tracks and heavy blobs for protons with a large incidence angle (≥35 • ) and with energy larger -7 -than 6.93 MeV. The use of TOT has allowed a 3D mapping (position X, position Y in the pixels plane and the number of clock periods over threshold TOT [count] for each pixel along the normal axis to the pixels plane) of the charge spreading effect in the whole volume of the silicon sensor. Single cluster analysis currently under development already gives a good precision (significantly better than the 55 µm size of one pixel) on the position of impact point of a proton with normal incidence interacting in silicon. The single cluster analysis is currently being developed to account for incoming particles with any incidence angle. Further studies (higher energy protons and neutrons) with TIMEPIX detectors will be done towards their installation in the ATLAS detector at the CERN/LHC. For neutron detection, the TIMEPIX silicon layer will be covered with a mosaic of neutron converters ( 6 LiF and polyethylene for a position sensitive detection of slow and fast neutrons, respectively).