- Email: [email protected]
Optimization of the Timepix chip to measurement of radon, thoron and their progenies
Applied Radiation and Isotopes 107 (2016) 220–224
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Optimization of the Timepix chip to measurement of radon, thoron and their progenies Miroslaw Janik a,n, Ondrej Ploc b, Michael Fiederle c, Simon Procz c, Norbert Kavasi a a
National Institute of Radiological Sciences, Chiba, Japan Nuclear Physics Institute of the CAS, Prague, Czech Republic c Albert-Ludwigs-Universität Freiburg, Freiburg, Germany b
H I G H L I G H T S
Energy calibration of the Timepix chip for alpha particle measurement. Linear dependency between cluster volume and energy. Radon, thoron and their progenies analysis.
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online 21 October 2015
Radon and thoron as well as their short-lived progenies are decay products of the radium and thorium series decays. They are the most important radionuclide elements with respect to public exposure. To utilize the semiconductor pixel radiation Timepix chip for the measurement of active and real-time alpha particles from radon, thoron and their progenies, it is necessary to check the registration and visualization of the chip. An energy check for radon, thoron and their progenies, as well as for 241Am and210Po sources, was performed using the radon and thoron chambers at NIRS (National Institute of Radiological Sciences). The check found an energy resolution of 200 keV with a 14% efficiency as well as a linear dependency between the channel number (cluster volume) and the energy. The coefficient of determination r2 of 0.99 for the range of 5 to 9 MeV was calculated. In addition, an offset for specific Timepix configurations between pre-calibration for low energy from 6 to 60 keV, and the actual calibration for alpha particles with energies from 4000 to 9000 keV, was detected. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Radon Thoron Timepix Calibration
1. Introduction Inhalation of radon (Rn) and its short-lived decay products, as well as inhalation of products of the thoron (Tn) series, account for about half of the world-average effective dose from natural radiation sources (UNSCEAR, 2008). Many types of passive monitors and devices for radon and thoron measurement have been developed. The most common monitors for large-scale and longterm radon measurements are passive type monitors (Janik et al., 2014) utilizing etched-track solid state nuclear track detectors (SSNTDs) commercially known as CR-39 or LR 115 (Dwivedi et al., 2001; Mishra and Mayya, 2008; Zhuo et al., 2002). Recently, new monitor types based on radon absorption in solid polymers n
Corresponding author. E-mail address: [email protected] (M. Janik).
http://dx.doi.org/10.1016/j.apradiso.2015.10.023 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
processes have been developed (Pressyanov et al., 2013; Tommasino, 2010; Tommasino et al., 2009). However, the results of measurements made using passive type monitors are obtained only after chemical or electro-chemical treatment of the detection materials and counting by manual or automatic systems. Some progress is being made using electronic passive monitors with online displayed results, e.g. the RadonScout and ThoronScout, the Corentium Digital Electronic Radon Gas Monitor, an active radon exposure meter developed by the Helmholtz Center Munich (Irlinger et al., 2014) as well as a new type of radon monitor based on the Timepix device (Caresana et al., 2014). In this study, the Timepix device (Llopart et al., 2007) developed at CERN was calibrated at the NIRS, checked for application suitability and utilized for visualization of alpha particles. Moreover, the performance of the Timepix was also confirmed through experiments in the NIRS radon and thoron chambers.
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
Fig. 1. Cluster of pixels caused by a single alpha particle from
241
Am.
Fig. 2. Log-normal fitting (solid line - 241Am, dashed line – 210Po) for the relationship between cluster height and bias voltage for alpha particles from 241Am (A) and 210Po (P).
2. Methodology Originally the Timepix device was designed for position-sensitive single X-ray photon detection. The hybrid silicon pixel device Timepix consists of a pixelated semiconductor detector chip (256 256 square pixels with pitch of 55 μm) bump-bonded to a readout chip. Each element of the matrix (pixel) is connected to its respective preamplifier, discriminator and digital counter integrated on the readout chip. Each pixel can independently work in one of three modes: Medipix mode (the counter counts incoming particles), Timepix mode (the counter works as a timer and measures the time when the particle is detected) and time over threshold (TOT) mode. The Timepix device running in the TOT mode measures the charge collected in each pixel. As the device contains 65,536 independent channels and as their responses can never be identical, it is necessary to perform an energy calibration for each of them. A single particle often generates a signal in a cluster of adjacent pixels. This is because the charge generated by the particle spreads out during the charge collection process and it can be collected finally by several adjacent pixels forming the cluster. Utilization of the Timepix chip based on measurement of heavy charged particle energy loss has been published in the literature (Jakubek et al., 2008). The Timepix device for the estimation of radon progenies using the filter
221
method with measurement of alpha decay products only is discussed elsewhere (Bulanek et al., 2015, 2014). The basic setup of the Timepix with the FitPix interface consists of setting the clock frequency, bias voltage, threshold equalization, and then energy per-pixel calibration. Threshold equalization is a procedure which makes the overall threshold as homogenous as possible. With the Pixelman software (Turecek et al., 2011), the procedure is automatic and uses noise pulses to find the distribution thresholds for each adjustment value, then selects for each pixel a suitable threshold adjustment that is as near as possible to the average of the means of threshold distributions (see http://aladdin.utef.cvut.cz/ofat/ others/Pixelman/Pixelman_manual.html#Thresholdequalization ). In some cases it’s not possible to equalize all pixels; some of them give a permanent signal and some of them are “dead”. In such cases the calibration procedure allows for such responses to be ignored. In the preliminary energy calibration, performed outside NIRS, the Timepix (the device used later in this study) was irradiated by a multi-energy calibration source constructed as a combination of an isotopic gamma source and a set of XRF (X-ray fluorescence) materials with the energy range from 6 to 60 keV. (Jakubek, 2011). However, isotopes in radon and thoron chains decay with emission of alpha particles with energy higher than 5 MeV. Therefore the calibration of the Timepix for alpha particles in the range from 5 to 9 MeV is much more complicated because the total charge can only be revealed by making a summation of all fractional charges, i.e. by determination of the cluster volume not for single pixel clusters as for gamma or X-ray radiation. An example of the cluster volume generated by a single alpha particle from 241Am (5.485 MeV) is shown in Fig. 1. The above-mentioned limitations were taken into account, and the dependence between alpha particle energy and response of Timepix device was evaluated at the NIRS. Alpha standard sources of 241Am (particle energy of 5.485 MeV with activity of 1 kBq) and 210 Po (5.305 MeV with activity of 0.4 kBq) were used for evaluating the energy dependence in the range below 5.5 MeV. As for the range above 5.5 MeV radon (Ichitsubo et al., 2004) and thoron (Sorimachi et al., 2010) chambers were utilized. In this experiment, the Timepix device collected alpha particles emitted from radon progenies, i.e. 214Po (7.7 MeV) and 218Po (6.00 MeV) as well as from thoron progenies, i.e. 212Bi (6.05 MeV) and 212Po (8.8 MeV). All stages of the experiment were carried out in the TOT mode. The measurement was performed with the threshold set to 330 and with a sensor bias voltage of 50 V. The clock frequency was set to 10 MHz and the constant current source parameter Ikrum was set to its minimal value of 1. The data acquisition and processing of clusters was done using Pixelman software described earlier. Before the energy calibration and efficiency estimation, the distortion of spectra regarding bias voltage in the range from 7.5 to 100 V was checked using the 241Am and 210Po sources. This check was done because the experiment described by (Jakubek et al., 2008) using alpha particles had shown a dramatically growing cluster height if bias voltage was higher than 40 V. They attributed this phenomenon to a problem with the constant current source parameter Ikrum, which does not work properly for large collected charge. In this study the Timepix chip was placed in a vacuum desiccator (volume, 6.9 dm3; vacuum conditions, 700 mm Hg) which was used as an exposure chamber. Results of the spectra distortion experiment utilizing the 241Am and 210Po sources are shown in Fig. 2. The log-normal relationship between bias voltage (7.5–100 V) and cluster height was observed for 241Am and also for 210 Po. It was concluded that the phenomenon of distorted pixels was not detected and Ikrum as well as other DAC (digital-to-analog converter) parameters of the Timepix chip were set up properly. In order to find the optimal DAC configuration, the influence of bias voltage variation on cluster size and height was analyzed.
222
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
Fig. 3. Visualization of tracks (cluster volume) from
241
Am source for different bias voltages: (a) 7.5 V, (b) 20 V, (c) 50 V and (d) 100 V.
Fig. 4. Relationship between bias voltage and sigma (energy resolution) for source.
241
Am
Fig. 3 show the registered clusters from alpha particles for different bias voltages. Increasing the bias voltage affected both size and height of the cluster area. On the other hand, Fig. 4 presents the relationship between energy resolution (sigma value from Gaussian fitting) and bias voltage in the range of 7.5 to 100 V. These results suggest that for better energy resolution bias voltage should not be lower than 40 V. For energy calibrations the same DAC parameters (as recommended by manufacturer) and bias voltage of 50 V, as before,
were used. Alpha particles emitted from 241Am and 210Po under the vacuum condition were collected on the Timepix and their respective spectra are presented in Fig. 5. Because of different activities of the sources and for better graphical presentation, counts were normalized to 1. The energy resolutions were calculated as 127 keV and 124 keV for 241Am and 210Po, respectively. In addition, as mentioned earlier, two exposures for identification of radon and thoron isotopes were performed in the NIRS radon and thoron chambers. The Timepix chip was exposed directly to radon or thoron gas without any filter. The measurements were conducted with a sensor bias voltage of 50 V. The other DAC parameters were fixed as for the spectra distortion measurement. The Timepix was placed in the Rn chamber for about 13 h. At this time about 1600 frames (each frame registers particles during 30 s measurement time) were logged. The radon concentration was measured with the AlphaGUARD ionization chamber and was about 8000 Bq m 3 on average with conditions of 40% relative humidity and 20 °C temperature. In the thoron atmosphere 6100 frames were collected in 50 h. The average thoron concentration measured by RAD7 device was about 40,000 Bq m 3. Fig. 5 illustrates the cluster volume spectra, registered by the Timepix, of alpha particles emitted from radon progenies (218Po, 214 Po) and thoron progenies (212Bi, 212Po). Because the concentrations of radon and thoron were different the counts were normalized to 1, the same as for 241Am and 210Po. At around 6 MeV the spectra from two isotopes, 212Bi and 218Po, from thoron and radon, respectively, were overlapped. The statistical analysis was done to find the relationship between energy and cluster volume registered by the Timepix using 241 Am, 210Po, and radon and thoron chambers. The linear regression with the correlation coefficient r2 ¼ 0.99 is presented in Fig. 6. The function for a specific Timepix configuration (e.g. bias voltage
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
223
Fig. 5. Energy (cluster volume) spectra of 241Am, 210Po (reference sources), 218Po and 214Po (radon exposure, red line) as well as 212Bi and 212Po (thoron exposure, blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Linear relationship between energy and cluster volumes with 95% CL.
of 50V, Ikrum parameter of 1) is given by the formula (Eq. (1)):
Energy[keV] = 1. 02 × ClusterVolume + 510
(1)
The shift of about 510 keV between the pre-calibration by the gamma source and expected energies was noted. This drift can be explained by differences in the DAC configuration, in particular, the bias voltage, as well as the Timepix’s response to the type of radiation. Pre-calibration was done pixel by pixel using low energy radiation in the range from 6 to 60keV. In the current experiment, carried out with a range from 4000 to 9000 keV, the Timepix was calibrated in relation to the cluster volume (generated by each alpha particle) of adjacent pixels. Parameters from the linear regression were used to recalculate the cluster volumes. Since the mean energies of 218Po, 214Po, 212Bi and 218Po were estimated to be 6.00 MeV, 7.69 MeV, 6.05 MeV and 8.78 MeV, respectively, all four alpha particles from radon and thoron could be detected with the present technique. In the final stage, the bias voltage was fixed to 50 V and efficiency calibration of the Timepix chip using the 241Am source was
carried out. Due to the relatively high number of alpha particles emitted from the source, the Timepix chip dead-time (the minimum amount of time that must separate two events in order that they are recorded as two separate pulses) was taken into account. The efficiency was calculated using the following equation (Eq. (2)):
ϵ=
N AAm − 241 × Ttotal
(2)
where N is the number of particles registered by the detector with the energy range; AAm 241 is the 241Am activity in Bq and Ttotal is measurement time including dead-time, is seconds. Finally, ϵ, in percent, was calculated for bias voltage ¼ 50 V as 14.1% which was similar to that reported by (Bulanek et al., 2014).
Conclusions The study findings showed that the calibration using only
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M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
gamma sources was not valid for alpha particles and energies higher than 4 MeV. It was also demonstrated that the Timepix chip, operating in the TOT mode, is a suitable device for measurement of radon and thoron progenies; however the efficiency (14%) and energy resolution (4200 keV) were substantially lower than e.g. a passivated ion-implanted planar silicon (PIPS) detector ( 30 keV) (Canberra). However, the Timepix chip offers an advantage for visualization of particle hits that is suitable for further research with etch-track detectors of radon, thoron and their progenies.
References Bulanek, B., Hulka, J., Jilek, K., Stekl, I., 2015. The portable device for continual measurement of radon progenies on filter using the detector. Timepix. Radiat. Prot. Dosim. 164, 493–496. Bulanek, B., Jilek, K., Cermak, P., 2014. Measurement of radon progenies using the Timepix detector. Radiat. Prot. Dosim. 160, 184–187. http://dx.doi.org/10.1093/ rpd/ncu077. Canberra, Passivated Implanted Planar Silicon (PIPSs) Detectors [WWW Document]. URL 〈http://www.canberra.com/products/detectors/pdf/passivated_ pips_C39313a.pdf〉 (accessed 6.26.15). Caresana, M., Garlati, L., Murtas, F., Romano, S., Severino, C.T., Silari, M., 2014. Realtime measurements of radon activity with the Timepix-based RADONLITE and RADONPIX detectors. J. Instrum. 9, P11023. Dwivedi, K., Mishra, R., Tripathy, S., Kulshreshtha, A., Sinha, D., Srivastava, A., Deka, P., Bhattacharjee, B., Ramachandran, T., Nambi, K.S., 2001. Simultaneous determination of radon, thoron and their progeny in dwellings. Radiat. Meas. 33, 7–11. http://dx.doi.org/10.1016/S1350-4487(00)00131-1. Ichitsubo, H., Yamada, Y., Shimo, M., Koizumi, A., 2004. Development of a radonaerosol chamber at NIRS—general design and aerosol performance. J. Aerosol Sci. 35, 217–232. http://dx.doi.org/10.1016/j.jaerosci.2003.08.002. Irlinger, J., Wielunski, M., Rühm, W., 2014. Thoron detection with an active Radon exposure meter – first results. Rev. Sci. Instrum. 85, 022106. http://dx.doi.org/ 10.1063/1.4865162.
Jakubek, J., 2011. Precise energy calibration of pixel detector working in time-overthreshold mode. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 633, 262–266. http://dx.doi.org/10.1016/j. nima.2010.06.183. Jakubek, J., Cejnarova, A., Holy, T., Pospisil, S., Uher, J., Vykydal, Z., 2008. Pixel detectors for imaging with heavy charged particles. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 591, 155–158. http://dx.doi. org/10.1016/j.nima.2008.03.091. Janik, M., Ishikawa, T., Omori, Y., Kavasi, N., 2014. Invited article: radon and thoron intercomparison experiments for integrated monitors at NIRS, Japan. Rev. Sci. Instrum. 85, 022001. http://dx.doi.org/10.1063/1.4865159. Llopart, X., Ballabriga, R., Campbell, M., Tlustos, L., Wong, W., 2007. Timepix, a 65 k programmable pixel readout chip for arrival time, energy and/or photon counting measurements. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 581, 485–494. http://dx.doi.org/10.1016/j. nima.2007.08.079. Mishra, R., Mayya, Y.S., 2008. Study of a deposition-based direct thoron progeny sensor (DTPS) technique for estimating equilibrium equivalent thoron concentration (EETC) in indoor environment. Radiat. Meas. 43, 1408–1416. http: //dx.doi.org/10.1016/j.radmeas.2008.03.002. Pressyanov, D., Foerster, E., Georgiev, S., Dimitrova, I., Mitev, K., 2013. Traceability of CDs/DVDs used as retrospective 222Rn detectors to reference STAR laboratory. Radiat. Meas. 59, 165–171. http://dx.doi.org/10.1016/j.radmeas.2013.06.002. Sorimachi, A., Ishikawa, T., Janik, M., Tokonami, S., 2010. Quality assurance and quality control for thoron measurement at NIRS. Radiat. Prot. Dosim. 141, 367–370. http://dx.doi.org/10.1093/rpd/ncq245. Tommasino, L., 2010. Radon film-badges versus existing passive monitors based on track etch. Nukleonika 55, 549–553. Tommasino, L., Tommasino, M.C., Viola, P., 2009. Radon-film-badges by solid radiators to complement track detector-based radon monitors. Radiat. Meas. 44, 719–723. http://dx.doi.org/10.1016/j.radmeas.2009.10.013. Turecek, D., Holy, T., Jakubek, J., Pospisil, S., Vykydal, Z., 2011. Pixelman: a multiplatform data acquisition and processing software package for Medipix2, Timepix and Medipix3 detectors. J. Instrum. 6, C01046. http://dx.doi.org/ 10.1088/1748-0221/6/01/C01046, C01046. UNSCEAR, 2008. UNSCEAR, Sources and Effects of Ionizing Radiation, United Nations Scientific Committee on the Effects of Atomic Radiation. Zhuo, W., Tokonami, S., Yonehara, H., Yamada, Y., 2002. A simple passive monitor for integrating measurements of indoor thoron concentrations. Rev. Sci. Instrum. 73, 2877. http://dx.doi.org/10.1063/1.1493233.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Optimization of the Timepix chip to measurement of radon, thoron and their progenies Miroslaw Janik a,n, Ondrej Ploc b, Michael Fiederle c, Simon Procz c, Norbert Kavasi a a
National Institute of Radiological Sciences, Chiba, Japan Nuclear Physics Institute of the CAS, Prague, Czech Republic c Albert-Ludwigs-Universität Freiburg, Freiburg, Germany b
H I G H L I G H T S
Energy calibration of the Timepix chip for alpha particle measurement. Linear dependency between cluster volume and energy. Radon, thoron and their progenies analysis.
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online 21 October 2015
Radon and thoron as well as their short-lived progenies are decay products of the radium and thorium series decays. They are the most important radionuclide elements with respect to public exposure. To utilize the semiconductor pixel radiation Timepix chip for the measurement of active and real-time alpha particles from radon, thoron and their progenies, it is necessary to check the registration and visualization of the chip. An energy check for radon, thoron and their progenies, as well as for 241Am and210Po sources, was performed using the radon and thoron chambers at NIRS (National Institute of Radiological Sciences). The check found an energy resolution of 200 keV with a 14% efficiency as well as a linear dependency between the channel number (cluster volume) and the energy. The coefficient of determination r2 of 0.99 for the range of 5 to 9 MeV was calculated. In addition, an offset for specific Timepix configurations between pre-calibration for low energy from 6 to 60 keV, and the actual calibration for alpha particles with energies from 4000 to 9000 keV, was detected. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Radon Thoron Timepix Calibration
1. Introduction Inhalation of radon (Rn) and its short-lived decay products, as well as inhalation of products of the thoron (Tn) series, account for about half of the world-average effective dose from natural radiation sources (UNSCEAR, 2008). Many types of passive monitors and devices for radon and thoron measurement have been developed. The most common monitors for large-scale and longterm radon measurements are passive type monitors (Janik et al., 2014) utilizing etched-track solid state nuclear track detectors (SSNTDs) commercially known as CR-39 or LR 115 (Dwivedi et al., 2001; Mishra and Mayya, 2008; Zhuo et al., 2002). Recently, new monitor types based on radon absorption in solid polymers n
Corresponding author. E-mail address: [email protected] (M. Janik).
http://dx.doi.org/10.1016/j.apradiso.2015.10.023 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
processes have been developed (Pressyanov et al., 2013; Tommasino, 2010; Tommasino et al., 2009). However, the results of measurements made using passive type monitors are obtained only after chemical or electro-chemical treatment of the detection materials and counting by manual or automatic systems. Some progress is being made using electronic passive monitors with online displayed results, e.g. the RadonScout and ThoronScout, the Corentium Digital Electronic Radon Gas Monitor, an active radon exposure meter developed by the Helmholtz Center Munich (Irlinger et al., 2014) as well as a new type of radon monitor based on the Timepix device (Caresana et al., 2014). In this study, the Timepix device (Llopart et al., 2007) developed at CERN was calibrated at the NIRS, checked for application suitability and utilized for visualization of alpha particles. Moreover, the performance of the Timepix was also confirmed through experiments in the NIRS radon and thoron chambers.
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
Fig. 1. Cluster of pixels caused by a single alpha particle from
241
Am.
Fig. 2. Log-normal fitting (solid line - 241Am, dashed line – 210Po) for the relationship between cluster height and bias voltage for alpha particles from 241Am (A) and 210Po (P).
2. Methodology Originally the Timepix device was designed for position-sensitive single X-ray photon detection. The hybrid silicon pixel device Timepix consists of a pixelated semiconductor detector chip (256 256 square pixels with pitch of 55 μm) bump-bonded to a readout chip. Each element of the matrix (pixel) is connected to its respective preamplifier, discriminator and digital counter integrated on the readout chip. Each pixel can independently work in one of three modes: Medipix mode (the counter counts incoming particles), Timepix mode (the counter works as a timer and measures the time when the particle is detected) and time over threshold (TOT) mode. The Timepix device running in the TOT mode measures the charge collected in each pixel. As the device contains 65,536 independent channels and as their responses can never be identical, it is necessary to perform an energy calibration for each of them. A single particle often generates a signal in a cluster of adjacent pixels. This is because the charge generated by the particle spreads out during the charge collection process and it can be collected finally by several adjacent pixels forming the cluster. Utilization of the Timepix chip based on measurement of heavy charged particle energy loss has been published in the literature (Jakubek et al., 2008). The Timepix device for the estimation of radon progenies using the filter
221
method with measurement of alpha decay products only is discussed elsewhere (Bulanek et al., 2015, 2014). The basic setup of the Timepix with the FitPix interface consists of setting the clock frequency, bias voltage, threshold equalization, and then energy per-pixel calibration. Threshold equalization is a procedure which makes the overall threshold as homogenous as possible. With the Pixelman software (Turecek et al., 2011), the procedure is automatic and uses noise pulses to find the distribution thresholds for each adjustment value, then selects for each pixel a suitable threshold adjustment that is as near as possible to the average of the means of threshold distributions (see http://aladdin.utef.cvut.cz/ofat/ others/Pixelman/Pixelman_manual.html#Thresholdequalization ). In some cases it’s not possible to equalize all pixels; some of them give a permanent signal and some of them are “dead”. In such cases the calibration procedure allows for such responses to be ignored. In the preliminary energy calibration, performed outside NIRS, the Timepix (the device used later in this study) was irradiated by a multi-energy calibration source constructed as a combination of an isotopic gamma source and a set of XRF (X-ray fluorescence) materials with the energy range from 6 to 60 keV. (Jakubek, 2011). However, isotopes in radon and thoron chains decay with emission of alpha particles with energy higher than 5 MeV. Therefore the calibration of the Timepix for alpha particles in the range from 5 to 9 MeV is much more complicated because the total charge can only be revealed by making a summation of all fractional charges, i.e. by determination of the cluster volume not for single pixel clusters as for gamma or X-ray radiation. An example of the cluster volume generated by a single alpha particle from 241Am (5.485 MeV) is shown in Fig. 1. The above-mentioned limitations were taken into account, and the dependence between alpha particle energy and response of Timepix device was evaluated at the NIRS. Alpha standard sources of 241Am (particle energy of 5.485 MeV with activity of 1 kBq) and 210 Po (5.305 MeV with activity of 0.4 kBq) were used for evaluating the energy dependence in the range below 5.5 MeV. As for the range above 5.5 MeV radon (Ichitsubo et al., 2004) and thoron (Sorimachi et al., 2010) chambers were utilized. In this experiment, the Timepix device collected alpha particles emitted from radon progenies, i.e. 214Po (7.7 MeV) and 218Po (6.00 MeV) as well as from thoron progenies, i.e. 212Bi (6.05 MeV) and 212Po (8.8 MeV). All stages of the experiment were carried out in the TOT mode. The measurement was performed with the threshold set to 330 and with a sensor bias voltage of 50 V. The clock frequency was set to 10 MHz and the constant current source parameter Ikrum was set to its minimal value of 1. The data acquisition and processing of clusters was done using Pixelman software described earlier. Before the energy calibration and efficiency estimation, the distortion of spectra regarding bias voltage in the range from 7.5 to 100 V was checked using the 241Am and 210Po sources. This check was done because the experiment described by (Jakubek et al., 2008) using alpha particles had shown a dramatically growing cluster height if bias voltage was higher than 40 V. They attributed this phenomenon to a problem with the constant current source parameter Ikrum, which does not work properly for large collected charge. In this study the Timepix chip was placed in a vacuum desiccator (volume, 6.9 dm3; vacuum conditions, 700 mm Hg) which was used as an exposure chamber. Results of the spectra distortion experiment utilizing the 241Am and 210Po sources are shown in Fig. 2. The log-normal relationship between bias voltage (7.5–100 V) and cluster height was observed for 241Am and also for 210 Po. It was concluded that the phenomenon of distorted pixels was not detected and Ikrum as well as other DAC (digital-to-analog converter) parameters of the Timepix chip were set up properly. In order to find the optimal DAC configuration, the influence of bias voltage variation on cluster size and height was analyzed.
222
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
Fig. 3. Visualization of tracks (cluster volume) from
241
Am source for different bias voltages: (a) 7.5 V, (b) 20 V, (c) 50 V and (d) 100 V.
Fig. 4. Relationship between bias voltage and sigma (energy resolution) for source.
241
Am
Fig. 3 show the registered clusters from alpha particles for different bias voltages. Increasing the bias voltage affected both size and height of the cluster area. On the other hand, Fig. 4 presents the relationship between energy resolution (sigma value from Gaussian fitting) and bias voltage in the range of 7.5 to 100 V. These results suggest that for better energy resolution bias voltage should not be lower than 40 V. For energy calibrations the same DAC parameters (as recommended by manufacturer) and bias voltage of 50 V, as before,
were used. Alpha particles emitted from 241Am and 210Po under the vacuum condition were collected on the Timepix and their respective spectra are presented in Fig. 5. Because of different activities of the sources and for better graphical presentation, counts were normalized to 1. The energy resolutions were calculated as 127 keV and 124 keV for 241Am and 210Po, respectively. In addition, as mentioned earlier, two exposures for identification of radon and thoron isotopes were performed in the NIRS radon and thoron chambers. The Timepix chip was exposed directly to radon or thoron gas without any filter. The measurements were conducted with a sensor bias voltage of 50 V. The other DAC parameters were fixed as for the spectra distortion measurement. The Timepix was placed in the Rn chamber for about 13 h. At this time about 1600 frames (each frame registers particles during 30 s measurement time) were logged. The radon concentration was measured with the AlphaGUARD ionization chamber and was about 8000 Bq m 3 on average with conditions of 40% relative humidity and 20 °C temperature. In the thoron atmosphere 6100 frames were collected in 50 h. The average thoron concentration measured by RAD7 device was about 40,000 Bq m 3. Fig. 5 illustrates the cluster volume spectra, registered by the Timepix, of alpha particles emitted from radon progenies (218Po, 214 Po) and thoron progenies (212Bi, 212Po). Because the concentrations of radon and thoron were different the counts were normalized to 1, the same as for 241Am and 210Po. At around 6 MeV the spectra from two isotopes, 212Bi and 218Po, from thoron and radon, respectively, were overlapped. The statistical analysis was done to find the relationship between energy and cluster volume registered by the Timepix using 241 Am, 210Po, and radon and thoron chambers. The linear regression with the correlation coefficient r2 ¼ 0.99 is presented in Fig. 6. The function for a specific Timepix configuration (e.g. bias voltage
M. Janik et al. / Applied Radiation and Isotopes 107 (2016) 220–224
223
Fig. 5. Energy (cluster volume) spectra of 241Am, 210Po (reference sources), 218Po and 214Po (radon exposure, red line) as well as 212Bi and 212Po (thoron exposure, blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Linear relationship between energy and cluster volumes with 95% CL.
of 50V, Ikrum parameter of 1) is given by the formula (Eq. (1)):
Energy[keV] = 1. 02 × ClusterVolume + 510
(1)
The shift of about 510 keV between the pre-calibration by the gamma source and expected energies was noted. This drift can be explained by differences in the DAC configuration, in particular, the bias voltage, as well as the Timepix’s response to the type of radiation. Pre-calibration was done pixel by pixel using low energy radiation in the range from 6 to 60keV. In the current experiment, carried out with a range from 4000 to 9000 keV, the Timepix was calibrated in relation to the cluster volume (generated by each alpha particle) of adjacent pixels. Parameters from the linear regression were used to recalculate the cluster volumes. Since the mean energies of 218Po, 214Po, 212Bi and 218Po were estimated to be 6.00 MeV, 7.69 MeV, 6.05 MeV and 8.78 MeV, respectively, all four alpha particles from radon and thoron could be detected with the present technique. In the final stage, the bias voltage was fixed to 50 V and efficiency calibration of the Timepix chip using the 241Am source was
carried out. Due to the relatively high number of alpha particles emitted from the source, the Timepix chip dead-time (the minimum amount of time that must separate two events in order that they are recorded as two separate pulses) was taken into account. The efficiency was calculated using the following equation (Eq. (2)):
ϵ=
N AAm − 241 × Ttotal
(2)
where N is the number of particles registered by the detector with the energy range; AAm 241 is the 241Am activity in Bq and Ttotal is measurement time including dead-time, is seconds. Finally, ϵ, in percent, was calculated for bias voltage ¼ 50 V as 14.1% which was similar to that reported by (Bulanek et al., 2014).
Conclusions The study findings showed that the calibration using only
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gamma sources was not valid for alpha particles and energies higher than 4 MeV. It was also demonstrated that the Timepix chip, operating in the TOT mode, is a suitable device for measurement of radon and thoron progenies; however the efficiency (14%) and energy resolution (4200 keV) were substantially lower than e.g. a passivated ion-implanted planar silicon (PIPS) detector ( 30 keV) (Canberra). However, the Timepix chip offers an advantage for visualization of particle hits that is suitable for further research with etch-track detectors of radon, thoron and their progenies.
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