- Email: [email protected]
Fast neutron detection efficiency of ATLAS-MPX detectors for the evaluation of average neutron energy in mixed radiation fields
Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Fast neutron detection efficiency of ATLAS-MPX detectors for the evaluation of average neutron energy in mixed radiation fields$ J. Bouchami a, A. Gutie´rrez a,n, T. Holy´ b, V. Kra´l b, C. Lebel a, C. Leroy a, J. Macana a, S. Pospı´sˇ il b, O. Scallon a, ˇ ˇ ka b M. Suk b, M. Tartare a, C. Teyssier a, Z. Vykydal b, J. Zemlic a b
Universite´ de Montre´al, Montre´al, Que´bec, Canada H3C 3J7 Institute of Experimental and Applied Physics of the CTU in Prague, Horska 3a/22, CZ-12800 Praha 2, Albertov, Czech Republic
a r t i c l e in f o
a b s t r a c t
Available online 23 June 2010
Within the framework of the ATLAS-MPX project, the ATLAS-MPX detectors (based on Medipix2 silicon devices) are covered with converting layers of 6LiF and polyethylene (PE) to make them sensitive to thermal and fast neutrons, respectively. Two ATLAS-MPX reference detectors were exposed to two calibrated neutron sources, 252Cf (2.2 MeV mean neutron energy) and 241AmBe (4.08 MeV mean neutron energy), in order to determine their fast neutron detection efficiency. Measurements were performed at low energy threshold ( 8 keV) and high energy threshold ( 230 keV). Fast neutron detection efficiency is primarily achieved via the use of a 1.3 mm thick polyethylene (PE) converter. Recoil protons from the elastic collision between neutron and hydrogen are detected from their tracks in the 300 mm thick silicon pixel detector. Calibrated neutron sources were placed at different distances from the detectors, both separately and simultaneously in order to obtain single and superposed neutron energy spectra. As expected, the neutron detection efficiency in the PE layer increases when the neutron mean energy increases due to the decrease of proton self-absorption in the PE converter itself. The variation of the cluster size as a function of the proton and alpha energy (at low energy threshold) was also studied for better understanding of the neutron response using the PE converter. The determination of the ratio of the fast neutron responses in each detector region at high energy threshold made it possible to establish a relation between the ratios and the mean neutron energy. At low energy threshold, a relation between the neutron energy spectrum and the cluster size distribution of heavy charged particles has been established. & 2010 Elsevier B.V. All rights reserved.
Keywords: Atlas Fast neutrons Heavy ionizing particle Medipix Pixel detectors Pattern recognition Radiation fields
1. Introduction Two reference ATLAS-MPX detectors (based on Medipix2 silicon devices) [1] covered with different converter masks were exposed to two fast neutron sources: 252Cf (2.2 MeV mean neutron energy) and 241AmBe (4.08 MeV mean neutron energy). Measurements were taken for each source separately and for both simultaneously, at different distances from the detectors, for the evaluation of the fast neutron detection efficiency. The measurements were performed at two energy thresholds ( 8 and 230 keV) for each experimental setup. The two responses are analyzed using different methods depending on the threshold value. The low energy threshold ( 8 keV) allows the detection of all events from the sources (neutrons, photons and electrons), leading to the use of a pattern recognition algorithm (Pixelman) [2] to identify the response to neutrons exclusively. $
This work was carried out within the CERN Medipix Collaboration n Corresponding author. Tel.: +1 514 343 6111; fax: +1 514 343 6215. E-mail address: [email protected] (A. Gutie´rrez). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.174
The high energy threshold ( 230 keV) is high enough to filter out the background from the light particles (photons and electrons). The bias voltage used for all measurements was 100 V.
Fig. 1. ATLAS-MPX detector X-ray image of six different regions: (1) 6LiF (5 mg/ cm2) above the silicon sensor and below a 50 mm thick aluminum (Al) foil, (2) polyethylene (PE) 1.3 mm thick, (3) PE (1.3 mm) layer above Al (100 mm), (4) Al (100 mm) layer, (5) uncovered region (Si) and (6) Al (150 mm) layer.
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
2. ATLAS-MPX detectors The ATLAS-MPX detectors are covered with a mask of different materials [3], as shown in Fig. 1:
S227
The 6LiF region in the ATLAS-MPX has a high sensitivity to thermal neutrons. The nuclear reaction 6Li(n,a)3H is dominant (cross-section of 950 barns for 25 meV neutrons). The PE layer is relevant for the fast neutron measurements.
Fig. 2. ATLAS-MPX detector located at 12 cm from: 252Cf source (a), 241AmBe source (b) and both sources (c). Neutron energy spectra of the sources: 241 AmBe source (D), and the solid line is the superposition of 252Cf and 241AmBe sources (d). The acquisition time is 1000 s.
Fig. 3. Responses of an ATLAS-MPX detector at low energy threshold. The detector is located at 12 cm from the time is 0.1 s.
252
Cf source (a) and
241
252
Cf source (K),
AmBe source (b). The acquisition
S228
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
1
H(n,n0 )p elastic collisions are produced in the PE layer. Aluminum (Al) layers are used to create a kinematic threshold for recoiling protons and for the attenuation of electrons and photons.
response to 241AmBe photons (14 and 60 keV) as well as electrons scattered in the air from associated photons. Since the response of neutrons at low energy threshold has to be isolated, the pattern recognition analysis from Pixelman was
3. Response at high energy threshold ( 230 keV) Since all photons and electrons are assumed to give no response [4] for this high energy threshold value, the number of detected neutrons is approximated as the total number of clusters detected (without shape discrimination). A cluster is defined as a track of activated pixels produced by the radiation of a charged particle in the silicon layer of the ATLAS-MPX detector [5]. Figure 2 shows the response of the ATLAS-MPX detector to the neutron sources at high energy threshold.
4. Response at low energy threshold ( 8 keV) At low energy threshold, the ATLAS-MPX detector records not only the response from fast neutrons but also from photons and electrons. Fig. 3 shows that the response from both sources is mainly composed of tracks from light particles. In Fig. 3(a) (curved) tracks can be distinguished, coming from the 252Cf photons with 0.9 MeV average energy. Fig. 3(b) shows the
Fig. 6. Fast neutron detection efficiency of the PE region. e(PE) as a function of the average neutron energy at high energy threshold (D) and low energy threshold (K).
Fig. 4. Proton (a) and alpha particle (b) cluster size at low energy threshold as a function of kinetic energy. The incidence angles of heavy particles to the surface of the detector are: 01 (K), 451 (&) and 851 (D).
Fig. 5. Integration (over 200 s) of the signal of heavy particles at low energy threshold for
252
Cf source (a) and
241
AmBe source (b).
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
configured to distinguish the heavy particle response from the light particle response. In order to successfully recognize the clusters of heavy particles (tracks from protons and alpha particles), a study of the cluster size dependence on the angle of incidence was performed using a Medipix2 detector without converter. These particles were accelerated at the linear accelerator tandem (6 MV)
S229
of the University of Montreal [6]. The cluster size is related to the type of particle, its energy and its incidence angle (Fig. 4). Protons with a kinetic energy above 6 MeV and with a perpendicular incidence (0o with respect to normal) on the detector are only partially absorbed in the silicon. For this reason, the cluster size decreases beyond 6 MeV (Fig. 4). In contrast, the cluster size from an alpha particle always increases with kinetic energy because the alpha particle is completely absorbed in the silicon detector. The energy of the recoil proton resulting from the elastic collision of a fast neutron (252Cf or 241AmBe source) in the PE layer is expected to be below 10 MeV. The information about the shape of the clusters allows the separation of the signal of heavy particles from light particles. After categorization, the fast neutron detection efficiency is evaluated. Fig. 5 shows the response to heavy particles only integrated over time.
5. Fast neutron detection efficiency The fast neutron detection efficiency of the PE layer of an ATLAS-MPX detector is evaluated for both energy thresholds: n =A nSi =ASi ð1Þ eðPEÞ ¼ PE þ Si PE þ Si jt Fig. 7. Ratio of events per cm2 of the PE + Si region over the PE + Al+ Si region (D) and the uncovered (Si) region (K) at high energy threshold.
e(PE) is the fast neutron detection efficiency of the 1.3 mm thick PE layer. nPE + Si and nSi are the number of clusters produced
Fig. 8. Neutron energy spectrum (first row) and cluster size distribution of heavy charged particles (second row) for 241 AmBe source together (c) at low energy threshold.
252
Cf source (a),
241
AmBe source (b) and
252
Cf and
S230
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
by interaction of fast neutrons in the PE+ Si region and the Si region, respectively. f is the neutron flux and t is the total acquisition time. APE + Si and ASi are the areas of the PE+ Si and the Si regions, respectively. The fast neutron detection efficiency in the 1.3 mm PE layer at different thresholds is shown in Fig. 6. e(PE) increases as the average neutron energy increases. The efficiency at low energy threshold is lower than the efficiency at high energy threshold. In fact, protons with energy below 1 MeV are not counted because their tracks cannot be distinguished from photons and electrons tracks [5]. The average neutron energy is evaluated from the superposition of the neutron spectra at different distances between the sources and the detector.
6. Estimate of the average energy of fast neutrons At the high energy threshold, the response of the detector to different neutron mixed fields can be differentiated. Fig. 2 shows, for example, that the region PE+ Al+Si is more sensitive to the 241 AmBe source (4.08 MeV mean energy) than to the 252Cf source (2.2 MeV mean energy). When the average energy of neutrons increases, recoil protons from the elastic collision in the PE layer are less absorbed in the Al layer. The ratios of the number of events of the PE +Si region over the PE+ Al+ Si and Si regions (normalized by the area) are evaluated (Fig. 7) according: Ri ¼ j
ni =Ai nj =Aj
ð2Þ
which corresponds to a proton energy between 2 and 3 MeV and above 7 MeV (Fig. 4a). The response to the 241AmBe source has a peak at 14 pixels corresponding to protons above 3 MeV with an incidence angle between 01 and 451. When both sources are used simultaneously, the resulting cluster size distribution is the superposition of the individual cluster size distributions (Fig. 8).
7. Conclusions A study of the capability of ATLAS-MPX detectors to provide information about the spectral characteristics of neutron fields has been completed. The ATLAS-MPX were exposed to fast neutrons provided by calibrated neutron sources located at different distances from the detectors, both separately and simultaneously, in order to obtain single and superposed neutron energy spectra. Measurements at low and high energy threshold settings allowed the evaluation of the fast neutron detection efficiency of the ATLAS-MPX detectors for these sources in various configurations. The results of the study will be helpful in establishing the reliability of track recognition with these devices, proper interpretation of the calibration data and future real time measurements of the unknown neutron field at the LHC.
Acknowledgment The technical assistance given by M. Kralik from the Czech Metrology Institute in Prague in setting up and operating the neutron sources is gratefully appreciated. References
Eq. (2) gives the ratio (R) between events (n) per cm2 (A) of different regions, where i ¼PE+ Si region and j¼PE +Al+ Si and Si regions. At low energy threshold, the size of the clusters in the PE+ Si region depends on the energy of the heavy particle (proton and alpha particle) detected and on its incidence angle. A link between the cluster size distribution of heavy charged particles and the neutron energy spectrum can be seen in Fig. 8. The cluster size distribution in the PE+ Si region from 252Cf has a peak at 12 pixels,
MedipixCollaboration: /http://medipix.cern.ch/MEDIPIXS. T. Holy´, et al., Nucl. Instr. and Meth. A 591 (1) (2008) 287–290. Z. Vykydal, et al., IEEE NSS/MIC Conference Proceedings N30-260, 2008. Z. Vykydal, et al., Nucl. Instr. and Meth. A 607 (1) (2009) 35–37. J. Bouchami, et al., Measurement of pattern recognition efficiency of tracks generated by ionizing radiation in a Medipix2 device, Nucl. Instr. and Meth. A, these proceedings. [6] J. Bouchami, et al., Study of the charge sharing in silicon pixel detector by means of heavy ionizing particles interacting with a Medipix2 device, Nucl. Instr. and Meth. A, these proceedings. [1] [2] [3] [4] [5]
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Fast neutron detection efficiency of ATLAS-MPX detectors for the evaluation of average neutron energy in mixed radiation fields$ J. Bouchami a, A. Gutie´rrez a,n, T. Holy´ b, V. Kra´l b, C. Lebel a, C. Leroy a, J. Macana a, S. Pospı´sˇ il b, O. Scallon a, ˇ ˇ ka b M. Suk b, M. Tartare a, C. Teyssier a, Z. Vykydal b, J. Zemlic a b
Universite´ de Montre´al, Montre´al, Que´bec, Canada H3C 3J7 Institute of Experimental and Applied Physics of the CTU in Prague, Horska 3a/22, CZ-12800 Praha 2, Albertov, Czech Republic
a r t i c l e in f o
a b s t r a c t
Available online 23 June 2010
Within the framework of the ATLAS-MPX project, the ATLAS-MPX detectors (based on Medipix2 silicon devices) are covered with converting layers of 6LiF and polyethylene (PE) to make them sensitive to thermal and fast neutrons, respectively. Two ATLAS-MPX reference detectors were exposed to two calibrated neutron sources, 252Cf (2.2 MeV mean neutron energy) and 241AmBe (4.08 MeV mean neutron energy), in order to determine their fast neutron detection efficiency. Measurements were performed at low energy threshold ( 8 keV) and high energy threshold ( 230 keV). Fast neutron detection efficiency is primarily achieved via the use of a 1.3 mm thick polyethylene (PE) converter. Recoil protons from the elastic collision between neutron and hydrogen are detected from their tracks in the 300 mm thick silicon pixel detector. Calibrated neutron sources were placed at different distances from the detectors, both separately and simultaneously in order to obtain single and superposed neutron energy spectra. As expected, the neutron detection efficiency in the PE layer increases when the neutron mean energy increases due to the decrease of proton self-absorption in the PE converter itself. The variation of the cluster size as a function of the proton and alpha energy (at low energy threshold) was also studied for better understanding of the neutron response using the PE converter. The determination of the ratio of the fast neutron responses in each detector region at high energy threshold made it possible to establish a relation between the ratios and the mean neutron energy. At low energy threshold, a relation between the neutron energy spectrum and the cluster size distribution of heavy charged particles has been established. & 2010 Elsevier B.V. All rights reserved.
Keywords: Atlas Fast neutrons Heavy ionizing particle Medipix Pixel detectors Pattern recognition Radiation fields
1. Introduction Two reference ATLAS-MPX detectors (based on Medipix2 silicon devices) [1] covered with different converter masks were exposed to two fast neutron sources: 252Cf (2.2 MeV mean neutron energy) and 241AmBe (4.08 MeV mean neutron energy). Measurements were taken for each source separately and for both simultaneously, at different distances from the detectors, for the evaluation of the fast neutron detection efficiency. The measurements were performed at two energy thresholds ( 8 and 230 keV) for each experimental setup. The two responses are analyzed using different methods depending on the threshold value. The low energy threshold ( 8 keV) allows the detection of all events from the sources (neutrons, photons and electrons), leading to the use of a pattern recognition algorithm (Pixelman) [2] to identify the response to neutrons exclusively. $
This work was carried out within the CERN Medipix Collaboration n Corresponding author. Tel.: +1 514 343 6111; fax: +1 514 343 6215. E-mail address: [email protected] (A. Gutie´rrez). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.174
The high energy threshold ( 230 keV) is high enough to filter out the background from the light particles (photons and electrons). The bias voltage used for all measurements was 100 V.
Fig. 1. ATLAS-MPX detector X-ray image of six different regions: (1) 6LiF (5 mg/ cm2) above the silicon sensor and below a 50 mm thick aluminum (Al) foil, (2) polyethylene (PE) 1.3 mm thick, (3) PE (1.3 mm) layer above Al (100 mm), (4) Al (100 mm) layer, (5) uncovered region (Si) and (6) Al (150 mm) layer.
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
2. ATLAS-MPX detectors The ATLAS-MPX detectors are covered with a mask of different materials [3], as shown in Fig. 1:
S227
The 6LiF region in the ATLAS-MPX has a high sensitivity to thermal neutrons. The nuclear reaction 6Li(n,a)3H is dominant (cross-section of 950 barns for 25 meV neutrons). The PE layer is relevant for the fast neutron measurements.
Fig. 2. ATLAS-MPX detector located at 12 cm from: 252Cf source (a), 241AmBe source (b) and both sources (c). Neutron energy spectra of the sources: 241 AmBe source (D), and the solid line is the superposition of 252Cf and 241AmBe sources (d). The acquisition time is 1000 s.
Fig. 3. Responses of an ATLAS-MPX detector at low energy threshold. The detector is located at 12 cm from the time is 0.1 s.
252
Cf source (a) and
241
252
Cf source (K),
AmBe source (b). The acquisition
S228
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
1
H(n,n0 )p elastic collisions are produced in the PE layer. Aluminum (Al) layers are used to create a kinematic threshold for recoiling protons and for the attenuation of electrons and photons.
response to 241AmBe photons (14 and 60 keV) as well as electrons scattered in the air from associated photons. Since the response of neutrons at low energy threshold has to be isolated, the pattern recognition analysis from Pixelman was
3. Response at high energy threshold ( 230 keV) Since all photons and electrons are assumed to give no response [4] for this high energy threshold value, the number of detected neutrons is approximated as the total number of clusters detected (without shape discrimination). A cluster is defined as a track of activated pixels produced by the radiation of a charged particle in the silicon layer of the ATLAS-MPX detector [5]. Figure 2 shows the response of the ATLAS-MPX detector to the neutron sources at high energy threshold.
4. Response at low energy threshold ( 8 keV) At low energy threshold, the ATLAS-MPX detector records not only the response from fast neutrons but also from photons and electrons. Fig. 3 shows that the response from both sources is mainly composed of tracks from light particles. In Fig. 3(a) (curved) tracks can be distinguished, coming from the 252Cf photons with 0.9 MeV average energy. Fig. 3(b) shows the
Fig. 6. Fast neutron detection efficiency of the PE region. e(PE) as a function of the average neutron energy at high energy threshold (D) and low energy threshold (K).
Fig. 4. Proton (a) and alpha particle (b) cluster size at low energy threshold as a function of kinetic energy. The incidence angles of heavy particles to the surface of the detector are: 01 (K), 451 (&) and 851 (D).
Fig. 5. Integration (over 200 s) of the signal of heavy particles at low energy threshold for
252
Cf source (a) and
241
AmBe source (b).
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
configured to distinguish the heavy particle response from the light particle response. In order to successfully recognize the clusters of heavy particles (tracks from protons and alpha particles), a study of the cluster size dependence on the angle of incidence was performed using a Medipix2 detector without converter. These particles were accelerated at the linear accelerator tandem (6 MV)
S229
of the University of Montreal [6]. The cluster size is related to the type of particle, its energy and its incidence angle (Fig. 4). Protons with a kinetic energy above 6 MeV and with a perpendicular incidence (0o with respect to normal) on the detector are only partially absorbed in the silicon. For this reason, the cluster size decreases beyond 6 MeV (Fig. 4). In contrast, the cluster size from an alpha particle always increases with kinetic energy because the alpha particle is completely absorbed in the silicon detector. The energy of the recoil proton resulting from the elastic collision of a fast neutron (252Cf or 241AmBe source) in the PE layer is expected to be below 10 MeV. The information about the shape of the clusters allows the separation of the signal of heavy particles from light particles. After categorization, the fast neutron detection efficiency is evaluated. Fig. 5 shows the response to heavy particles only integrated over time.
5. Fast neutron detection efficiency The fast neutron detection efficiency of the PE layer of an ATLAS-MPX detector is evaluated for both energy thresholds: n =A nSi =ASi ð1Þ eðPEÞ ¼ PE þ Si PE þ Si jt Fig. 7. Ratio of events per cm2 of the PE + Si region over the PE + Al+ Si region (D) and the uncovered (Si) region (K) at high energy threshold.
e(PE) is the fast neutron detection efficiency of the 1.3 mm thick PE layer. nPE + Si and nSi are the number of clusters produced
Fig. 8. Neutron energy spectrum (first row) and cluster size distribution of heavy charged particles (second row) for 241 AmBe source together (c) at low energy threshold.
252
Cf source (a),
241
AmBe source (b) and
252
Cf and
S230
J. Bouchami et al. / Nuclear Instruments and Methods in Physics Research A 633 (2011) S226–S230
by interaction of fast neutrons in the PE+ Si region and the Si region, respectively. f is the neutron flux and t is the total acquisition time. APE + Si and ASi are the areas of the PE+ Si and the Si regions, respectively. The fast neutron detection efficiency in the 1.3 mm PE layer at different thresholds is shown in Fig. 6. e(PE) increases as the average neutron energy increases. The efficiency at low energy threshold is lower than the efficiency at high energy threshold. In fact, protons with energy below 1 MeV are not counted because their tracks cannot be distinguished from photons and electrons tracks [5]. The average neutron energy is evaluated from the superposition of the neutron spectra at different distances between the sources and the detector.
6. Estimate of the average energy of fast neutrons At the high energy threshold, the response of the detector to different neutron mixed fields can be differentiated. Fig. 2 shows, for example, that the region PE+ Al+Si is more sensitive to the 241 AmBe source (4.08 MeV mean energy) than to the 252Cf source (2.2 MeV mean energy). When the average energy of neutrons increases, recoil protons from the elastic collision in the PE layer are less absorbed in the Al layer. The ratios of the number of events of the PE +Si region over the PE+ Al+ Si and Si regions (normalized by the area) are evaluated (Fig. 7) according: Ri ¼ j
ni =Ai nj =Aj
ð2Þ
which corresponds to a proton energy between 2 and 3 MeV and above 7 MeV (Fig. 4a). The response to the 241AmBe source has a peak at 14 pixels corresponding to protons above 3 MeV with an incidence angle between 01 and 451. When both sources are used simultaneously, the resulting cluster size distribution is the superposition of the individual cluster size distributions (Fig. 8).
7. Conclusions A study of the capability of ATLAS-MPX detectors to provide information about the spectral characteristics of neutron fields has been completed. The ATLAS-MPX were exposed to fast neutrons provided by calibrated neutron sources located at different distances from the detectors, both separately and simultaneously, in order to obtain single and superposed neutron energy spectra. Measurements at low and high energy threshold settings allowed the evaluation of the fast neutron detection efficiency of the ATLAS-MPX detectors for these sources in various configurations. The results of the study will be helpful in establishing the reliability of track recognition with these devices, proper interpretation of the calibration data and future real time measurements of the unknown neutron field at the LHC.
Acknowledgment The technical assistance given by M. Kralik from the Czech Metrology Institute in Prague in setting up and operating the neutron sources is gratefully appreciated. References
Eq. (2) gives the ratio (R) between events (n) per cm2 (A) of different regions, where i ¼PE+ Si region and j¼PE +Al+ Si and Si regions. At low energy threshold, the size of the clusters in the PE+ Si region depends on the energy of the heavy particle (proton and alpha particle) detected and on its incidence angle. A link between the cluster size distribution of heavy charged particles and the neutron energy spectrum can be seen in Fig. 8. The cluster size distribution in the PE+ Si region from 252Cf has a peak at 12 pixels,
MedipixCollaboration: /http://medipix.cern.ch/MEDIPIXS. T. Holy´, et al., Nucl. Instr. and Meth. A 591 (1) (2008) 287–290. Z. Vykydal, et al., IEEE NSS/MIC Conference Proceedings N30-260, 2008. Z. Vykydal, et al., Nucl. Instr. and Meth. A 607 (1) (2009) 35–37. J. Bouchami, et al., Measurement of pattern recognition efficiency of tracks generated by ionizing radiation in a Medipix2 device, Nucl. Instr. and Meth. A, these proceedings. [6] J. Bouchami, et al., Study of the charge sharing in silicon pixel detector by means of heavy ionizing particles interacting with a Medipix2 device, Nucl. Instr. and Meth. A, these proceedings. [1] [2] [3] [4] [5]