Conversion of fast neutrons for neutron radiography with TPX2 detector

NUKLEONIKA 2024;69(2):135140 doi: 10.2478/nuka-2024-0020 ORIGINAL PAPER 0029-5922 © 2024 The Author(s). Published by the Institute of Nuclear Chemistry and Technology. This is an open access article under the CC BY-NC-ND 4.0 licence (http://creativecommons.org/licences/by-nc-nd/4.0/). Introduction The non-destructive analysis [1] is an important technique for the inspection of critical components in the industry, such as the new class of compact heat exchangers, the printed circuit heat exchanger (PCHE) applicable in Generation IV nuclear reactors or solar power plants. The PCHE behaves as a single metal block, with etched holes running through it, withstanding high-pressure differences between the primary and the secondary coolant sides accompanied by compact size and high thermal effi ciency, up to 98% [2, 3]. To reduce the time and cost associated with the maintenance tasks of future energy systems, engineers need to quickly identify the potential defects by non-destructive techniques. Due to the metal composition of the PCHE, fast- -neutron radiography is the most suitable technique. A new concept of neutron digital radiography with fast neutrons produced by a deuteron–deuteron (DD) neutron generator and a Timepix-based detector is planned to be used for PCHE examination. The neutrons will be produced by the D(d,n)3He nuclear reaction (Q = 3.269 MeV) [4], with neutron energies in the range of 2–5 MeV depending on the initial deuteron beam energy. For neutron registration, the MiniPIX TPX2 detector [5] with a 500 m thick silicon sensor was chosen. In order to improve the detection effi ciency of the TPX2 detector, conversion layers were applied on the sensor surface, to exploit the elastic scattering of neutrons on hydrogen. The Conversion of fast neutrons for neutron radiography with TPX2 detector Andrea Šagátová , Marko Fülöp, Andrej Novák , Branislav Vrban , Jakub Lüley , Štefan Čerba , Ivan Benkovský, Bohumír Zaťko A. Šagátová, A. Novák, B. Vrban, J. Lüley, Š. Čerba Slovak University of Technology in Bratislava Institute of Nuclear and Physical Engineering Faculty of Electrical Engineering and Information Technology Ilkovičova 3 St., 841 04 Bratislava, Slovak Republic E-mail: andr[email protected] M. Fülöp ABRS Ltd. Pomlejská 106 St., 931 01 Šamorín, Slovak Republic I. Benkovský Comenius University in Bratislava Faculty of Pharmacy Odbojárov 10 St., 832 32 Bratislava, Slovak Republic B. Zaťko Slovak Academy of Sciences Institute of Electrical Engineering Dúbravská cesta 9 St., 841 01 Bratislava, Slovak Republic Received: 21 November 2023 Accepted: 8 May 2024 Abstract. The Timepix2-based hybrid-pixel detector with a 500 m thick silicon sensor was employed for fast- -neutrons registration to be applied in neutron radiography of metallic printed circuit heat exchanger (PCHE). Two energies of neutrons were experimentally tested. The detection of 3.55 MeV neutrons from the deuteron– –deuteron (DD) reaction was compared to 15.7 MeV neutrons from the deuteron–tritium (DT) neutron generator. In order to distinguish the signal induced by the registered neutrons from the accelerator background, fi ltration of the recorded particle spectral tracks was applied. The benefi t of applying hydrogen-based converter layer for 3.55 MeV neutrons was observable. On the other hand, in the case of 15.7 MeV neutrons, the direct registration by interaction with the sensor Si signifi cantly dominates the conversion. Keywords: Hybrid pixel detector • Neutron conversion • Neutron radiography • Timepix2 136 A. Šagátová et al. effect of the conversion layers on the registration of two types of fast neutrons, the 3.55 MeV and 15.7 MeV, was evaluated. MiniPIX TPX2 detector The MiniPIX TPX2 detector is a hybrid semiconductor multi-pixel detector of the Timepix family designed by Advacam [5]. The Timepix type detectors utilize the readout electronics integrated on an ASIC chip [6–8] to evaluate the signal measured by a pixelated semiconductor sensor registering single particles of ionizing radiation. The sensor is typically made of Si (silicon), nevertheless, also other semiconductor materials are being used, e.g. CdTe, GaAs, or recently also SiC [9–13]. The MiniPIX TPX2 detector is prepared by bump-bonding a pixelated sensor to a Timepix2 readout ASIC chip [8], with a matrix of 256 × 256 pixels with 55 m pitch covering an area of 1.4 cm × 1.4 cm. The sensor of the used TPX2 detector was made of 500 m thick silicon with each of its 65 536 pixels representing an independent detecting unit (Fig. 1). Previous detectors of the Timepix type, the Timepix, and the Timepix3, have already been tested for the detection of fast neutrons [14–16], partially enhancing the detection effi ciency using a hydrogen-based converter layer. The problem of semiconductor detectors for neutron detection is that they are sensitive to all types of ionizing radiation and in mixed radiation fi elds like around neutron sources, the signal from neutrons is overlapped by other unwanted radiation types, mainly the Xand gamma rays. On the other hand, the pixelization of the sensor is suffi ciently small regarding the particle track (the charge cloud produced during particle interaction in the sensor) and the track usually overlaps a group of pixels creating the pixel cluster. Consequently, the hybrid pixel detector registers single particles with their tracks with attribution of energy loss in the sensor at each point of the track, with pixel spatial resolution, the so-called spectral track. One can utilize the track analysis to recognize the neutron product tracks from other registered particles as each type of radiation differs in track shape and energy released [17]. The idea of utilizing the pattern recognition algorithms on the spectral tracks of registered particles was introduced by Granja et al. [16] to decompose the neutron-induced tracks into particle event types, when measuring the 2.6 MeV and 14.1 MeV neutrons by the Timepix3 detector with 500 m Si sensor. The MiniPIX TPX2 detector utilizes the latest Timepix readout ASIC chip, the Timepix2. The chip provides fast frame-based readout, a wide spectral range for each pixel up to a few MeV, and a new function “the fi rst-hit of pixel” registering only the track of the particle that fi rst hits the pixel, preventing from summing multiple tracks into one. All these characteristics make the Timepix2 detector ideal for utilizing track fi ltration to recognize registered neutrons in mixed radiation fi elds. The TPX2 detector was enhanced for the registration of fast neutrons by converter layers with hydrogen to register neutrons through elastic scattering on hydrogen nuclei. To compare the effect of various conversion layers, the parts of the sensor surface were coated by different layers as shown in Fig. 2. The left part shows the photograph of the MiniPIX TPX2 detector with the applied layers. The middle part shows an X-ray image of the applied layers, precisely defi ning the position of the layer over certain pixels and the right image shows the areas of the sensor covered by converter layers in color: #0 without any layer, #1 Kapton polyimide fi lm (10 m thick), #2, #3, #4 polyethylene (PE) layers: 50 m, 100 m, and 150 m thick. Experiment The quality of the TPX2 detector in registering the neutrons from the neutron generator was examined at the Van de Graaff laboratory of the Institute of Experimental and Applied Physics CTU in Prague. Two different MeV energies of neutrons were compared, the neutrons from DD and DT nuclear reactions, respectively: (1) (2) During the fi rst experiment, the TPX2 detector was placed at an angle of 50° in front of the deuterium target impinged by the deuterons accelerated to energy of 1 MeV/A. The energy of neutrons hitting the detector was 3.55 MeV [4]. During the second experiment, the target was exchanged for a tritium one and was bombarded by 0.85 MeV/A deuterons. The energy of the released neutrons <50° was 15.7 MeV [4]. The photograph of the experimental setup is shown in Fig. 3. The TPX2 detector with Si sensor registers fast neutrons either directly through reactions on Si or indirectly utilizing converter layers. More than 92% of naturally occurring Si is created by the 28Si isotope. The neutrons can interact with the 28Si in the sensor by elastic scattering or by several nuclear reactions, which are energy limited by their Q value. Fig. 1. Schematic cross-section view of TPX2 chip bump- -bonded to Si sensor with examples of neutron registration: through conversion in polyethylene (left) or by nuclear reaction with Si nucleus (right). 3 D D He (0.82 MeV) n (2.45 MeV)  4 D T He (3.52 MeV) n (14.06 MeV)  137Conversion of fast neutrons for neutron radiography with TPX2 detector Some of the possible reactions are listed in Table 1 together with their calculated Q values. More reactions can be found elsewhere [18, 19]. In the case of our experiment with 3.55 MeV neutrons, only the elastic scattering (n,n), radiative capture (n,) and (n,) nuclear reactions were possible. With 15.7 MeV, more reaction channels are opened, including the (n,p) nuclear reaction. The electrically charged products of the interactions transfer their energy into the sensor by ionization of atoms, thus indirectly registering neutrons. The reaction products, the protons, alpha particles, or heavy ions, gradually ionize Si atoms forming a charge cloud on a short trajectory in the shape of a circle extending over several pixels in the sensor. Particles with higher energy will produce a larger cloud, the circle track will be spread over more pixels. The energetic per pixel calibration of the TPX2 detector enables to determine the released energy in the particle track, in other words in the cluster of active pixels. The total kinetic energy of the reaction products for both energies of the interacting neutrons is listed in Table 1. The trajectories of particles from the accelerator background, from gamma rays or beta particles, differ from the shape and the released energy of neutron recoils [20], enabling their fi ltration. The converter layers covering the TPX2 detector common electrode were used to convert the neutrons into ionizing particles directly registerable by the detector. The PE layers of 50 m, 100 m, and 150 m thickness were fi xed by the 10 m Kapton polyimide fi lm, where neutrons elastically scatter on hydrogen. Table 2 shows the projected ranges of scattered protons of maximum energy in the converter layers used. Fig. 2. The photograph of the MiniPIX TPX2 detector with applied layers (left). The X-ray image of the layers applied on the sensor substrate (middle). Areas of the sensor covered by different converter layers in color (right). Fig. 3. Arrangement of experimental testing of the TPX2 detector by fast neutrons. Table 1. Selected interactions of fast neutrons in Si sensor and conversion layer with their Q values and total energy of recoils for interacting neutrons with kinetic energy of 3.55 MeV and 15.7 MeV with parameters for elastic scattering on hydrogen in the converter layer Interaction Q value (MeV) Total kinetic energy of charged recoils (MeV) @ 3.55 MeV neutrons @ 15.7 MeV neutrons 28Si(n,n)28Si (elastic sc.) 0 up to 0.47 up to 2.09 28Si(n,n)28Si* –1.779 1.771 13.921 28Si(n,)25Mg –2.654 0.896 13.046 28Si(n,)25Mg* –3.239 0.311 12.461 28Si(n,)25Mg** –3.628 – 12.072 28Si(n,)25Mg*** –4.266 – 11.434 28Si(n,)25Mg**** –4.618 – 11.082 28Si(n,)25Mg***** –5.455 – 10.245 28Si(n,)25Mg****** –6.059 – 9.641 28Si(n,p)28Al –3.860 – 11.840 28Si(n,d)27Al –9.360 – 6.340 28Si(n,)29Si 8.474 12.024 24.174 H(n,p)n 0 up to 3.55 up to 15.7 *The fi rst excited state of the nucleus, up to ****** 6th excited state of the nucleus. 138 A. Šagátová et al. Results and discussion The tracks of the fi rst 2000 particles registered by the TPX2 detector with fi ltration applied to distinguish the signal of the neutrons from the gamma and the electron background are depicted in Fig. 4. Protons, alphas, and heavy ions from neutron interactions have expected energy >0.5 MeV (Table 1) and round tracks with size S >20 pixels. Various conditions for roundness from >0.5 (mixed fi eld), through >0.9, up to > 0.99 (well-fi ltered alphas, protons, and recoils with almost ideal circle shape (circle has roundness = 1)) in the fi rst up to the third row of Fig. 4 are depicted for 3.55 MeV and 15.7 MeV neutrons in two columns. The fi ltration of registered particle tracks was done utilizing the data processing engine (DPE) [22] developed by Advacam [5] and the Python programming language [23]. The DPE is a software which enables per-pixel calibration of the Timepix detector and calculation of various parameters of particle tracks, the clusters of activated pixels. The DPE gives a set of about 20 raw data for each particle track: e.g., cluster size, total energy, the highest per pixel energy of cluster, roundness, its position in the pixel fi eld, border pixel count, linear energy transfer (LET), which can be used to distinguish the track s based on the nature of particle interaction. One can observe that the 3.55 MeV neutron- -induced tracks are well-distinguished below the Table 2. Projected ranges of protons in converter layers applied on the sensor surface according to SRIM [21] Proton energy (MeV) Kapton polyimide fi lm C22H10N2O5 (1.42 g/cm3, 10 m) Polyethylene (C2H4)n (0.93 g/cm3, 50–150 m) Projected range (m) 3.55 152.24 190 15.70 2090 2700 Fig. 4. Tracks of the fi rst 2000 particles registered by the TPX2 detector with applied fi ltration are described in the left column for two energies of interacting neutrons: 3.55 MeV (middle column) and 15.7 MeV (right column) . 139Conversion of fast neutrons for neutron radiography with TPX2 detector converter layers, and in the case of 15.7 MeV neutrons, the tracks are distributed through the whole sensor. This can be explained by the probability of neutron interactions. The most probable are elastic scatterings on converter layers’ atoms and Si for MeV energies of interacting neutrons (Fig. 5). In the case of 3.55 MeV neutrons, the elastic scattering on atoms’ nuclei in converter layers dominates with about 2 barns cross-section over (n,) nuclear reaction in the sensor (3 × 10−4 barns). The elastic scattering on Si (2 barns), however, produces charged 28Si ions with energy <0.5 MeV, which tracks do not pass the fi lter. That is why the tracks occur in the area of the applied converter layers, where the recoils are lighter and gain a larger part of energy than Si during elastic scattering. In the case of 15.7 MeV neutrons all interactions, the elastic scattering in the converter layer or Si as well as the nuclear reactions on Si, have a similar order of cross-section. However, the volume of the Si (with 500 m thickness) exceeds the volume of the converter layers (10–150 m thick), which explains why the tracks of the neutron interaction products can be seen all over the area of the sensor, regardless the position of the converter layers . The analysis of raw data from the experiment with 3.55 MeV neutrons shows 571 842 separated tracks during 280 s, where from the fi rst 2000 tracks only those are depicted in Fig. 4, which passed the fi lter defi ned on the left (energy >500 keV, number of pixels in cluster >20 and roundness) supposed to be the heavily charged particle tracks, tracks of the products of neutron interaction (protons, alphas, and recoils). In the case of the fi lter with roundness >0.9, it is 30 tracks, dominantly under converter layers and Kapton fi lm. The neutron fl ux on the detector was 2700/s/cm2. The dominant effect of Kapton fi lm can be observed and it was calculated that it increased the detection effi ciency from the initial 0.04 ± 0.03% (bare detector) to 0.4 ± 0.2% (with converter layer) regardless of the PE layer presence. The Kapton fi lm density exceeds the density of PE. Conclusion s For neutron radiography of PCHE for nuclear reactors of Generation IV, the quasi-monoenergetic neutrons produced via the D(d,n)3He nuclear reaction (Q = 3.269 MeV) with energies in the range of 2–5 MeV depending on the initial beam energy are planned to be used. The neutrons will be registered by the Timepix2 detector with a 500 m thick Si sensor after penetration of the PCHE experimental model. To enhance the detection effi ciency of the Timepix2 silicon detector in this energy range, the Kapton fi lm would be benefi cial, when the recoils from elastic scattering of neutrons in the fi lm dominate the signal from direct registration of neutrons on Si atoms. The effect of the PE converter layer on the detection effi ciency of neutrons was not confi rmed by the experiment. Acknowledgment. This study has been fi nancially supported by FEEIT STU through the project NERAKO. This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under GA no. 101004730 and from the Slovak Research and Development Agency under grant no. APVV-22-0382. The neutron fi elds used for the measurements were provided in the open-access regime by the Van de Graaff accelerator facility of IEAP CTU in Prague, whose operation was supported by the Research Infrastructure Grant LM2018108 of the Ministry of Education, Youth and Sports of the Czech Republic. ORCID Š. Čerba http://orcid.org/0000-0002-4065-3976 J. Lüley http://orcid.org/0000-0001-9702-2895 A. Novák http://orcid.org/0000-0003-3308-7256 A. Šagátová http://orcid.org/0000-0002-8519-7206 B. Vrban http://orcid.org/0000-0002-8951-0052 B. Zaťko http://orcid.org/0000-0003-0612-6264 References 1. International Atomic Energy Agency. (2013). Training guidelines in non-destructive testing techniques. Vienna: IAEA. 2. Chang, C. Y., Che, W. H., Saw, L. H., & Arpia, A. A. (2021). Performance analysis of a printedcircuit heat exchanger with a novel mirror-symmetric channel design. Energies, 14(14), 4252. https://doi.org/10.3390/ en14144252. 3. Aakre, S. (2018). Nuclear code case development of printed-circuit heat exchangers with thermal and mechanical performance testing. In 6th International Supercritical CO2 Power Cycles Symposium, Pittsburgh. 4. Liskien, H., & Paulsen, A. (1973). Neutron production cross sections and energies for the reactions T(p,n)3He, D(d,n)3He, and T(d,n)4He. Atom. Data Nucl. Data Tabl., 11(7), 569–619. DOI: 10.1016/ S0092-640X(73)80081-6. Fig. 5. The cross-sections of selected reactions on 28Si and elastic scattering on 1H, 12C, 14N, and 16O as a function of neutron kinetic energy obtained from ENDF/B-VII.1 [24]. 140 A. Šagátová et al. 5. Advacam. (2023, November). MiniPIX TPX2. Retrieved November 13, 2023, from https://advacam. com/camera/minipix-tpx2/. 6. Llopart, X., Ballabriga, R., Campbell, M., Tlustos, L., & Wong, W. (2007). Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equ., 581(1/2), 485–494. 7. Poikela, T., Plosila, J., Westerlund, T., & Campbell, M. (2014). Timepix3: a 65K channel hybrid pixel readout chip with simultaneous ToA/ToT and sparse readout. JINST, 9(5), C05013. 8. Wong, W. S., Alozy, J., Ballabriga, R., Campbell, M., Kremastiotis, I., Llopart, X., Poikela, T., Sriskaran, V., Tlustos, L., & Turecek, D. (2020) Introducing Timepix2, a frame-based pixel detector readout ASIC measuring energy deposition and arrival time. Radiat. Meas., 131, 106230. 9. Bergmann, B., Burian, P., Manek, P., & Pospisiul, S. (2019). 3D reconstruction of particle tracks in a 2 mm thick CdTe hybrid pixel detector. Eur. Phys. J. C, 79, 165. https://doi.org/10.1140/epjc/s10052019-6673-z. 10. Zatko, B., Sagatova, A., Gal, N., Novak, A., Osvald, J., Bohacek, P., Polansky, S., Jakubek, J., & Kovacova, E. (2022) From a single silicon carbide detector to pixelated structure for radiation imaging camera. JINST, 17, C12005. 11. Sagatova, A., Zatko, B., & Necas, V. (2018). From single GaAs detector to sensor for radiation imaging camera. Appl. Surf. Sci., 461(2), 3–9. 12. Zaťko, B., Zaprazny, Z., Jakubek, J., Sagatova, A., Bohacek, P., Sekacova, M., Korytar, D., Necas, V., Zemlicka, J., Mora, Y., & Pichotka, M. (2018) Imaging performance of Timepix detector based on semiinsulating GaAs. JINST, 13, C01034. 13. Novak, A., Granja, C., Sagatova, A., Zach, V., Stursa, J., & Oancea, C. (2023) Spectral tracking of proton beams by the Timepix3 detector with GaAs, CdTe and Si sensors. JINST, 18, C01022. 14. Jakubek, J., & Uher, J. (2009) Fast neutron detector based on TimePix pixel device with micrometer spatial resolution. In Proceedings of the 2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), 24 October–1 November 2009, Orlando, FL, U.S.A. (pp. 1113–1116). DOI: 10.1109/NSSMIC.2009.5402420. 15. Bergmann, B., Nelson, R. O., O’Donnell, J. M., Pospisil, S., Solc, J., & Vykydal, Z. (2014). Time-of-fl ight measurement of fast neutrons with Timepix detectors. JINST, 9, C05048. 16. Granja, C., Uhlar, R., Chuprakov, I., Alexa, P., Sansarbayar, E., Gledenov, Y. M., Poklop, D., Olsansky, V., Marek, L., Vuolo, M., & Pacik, J. (2023). Detection of fast neutrons with the pixel detector Timepix3. JINST, 18, P01003. 17. Granja, C., Jakubek, J., Polansky, S., Zach, V., Krist, P., Chvatil, D., Stursa, J., Sommer, M., Plac, O., Kodaira, S., & Martisikova, M. (2018). Resolving power of pixel detector Timepix for wide-range electron, proton and ion detection. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equ., 908, 60–71. 18. Sedlackova, K., Sagatova, A., Zatko, B., Necas, V., Solar, M., & Granja, C. (2016) MCNPX simulation of the silicon carbide semiconductor detector response to fast neutrons from D-T nuclear reaction. Int. J. Modern Phys.-Conf. Ser., 44, 1660226. 19. Simakov, S., Majerle, M., & Kostal, M. (2023) Recoil and charged particle energy spectra from the natSi(n,x) reaction and the Si semiconductor detector response to the 14 MeV neutrons. Radiat. Phys. Chem., 203, 110624. 20. Jakubek, J., Granja, C., Hartmann, B., Jaekel, O., Martisikova, M., Opalka, L., & Pospisil, S. (2011). Selective detection of secondary particles and neutrons produced in ion beam therapy with 3D sensitive voxel detector. JINST, 6, C12010. 21. Ziegler, J. F., Biersack, J. P., & Ziegler, M. D. (2008). SRIM – The stopping and range of ions in matter. NC, USA: Lulu Press Co. 22. Marek, L., Granja, C., Jakubek, J., Ingerle, J., Turecek, D., Vuolo, M., & Oancea, C. (2023). Data Processing Engine (DPE): Data Analysis Tool for Particle Tracking and Mixed Radiation Field Characterization with Pixel Detectors Timepix. JINST, prepared for publishing. https://arxiv.org/pdf/2310.15723v1.pdf. 23. Python Software Foundation. (2023 November). pythonTM. [computer software]. Retrieved November 13, 2023, from https://www.python.org/. 24. Chadwick, M. B., Herman, M., Oblozinsky, P., Dunn, M. E., Danon, Y., Kahler, A. C., Smith, D. L., Pritychenko, B., Arbanas, G., Arcilla, R., Brewer, R., Brown, D. A., Capote, R., Carlson, A. D., Cho, Y. S., Derrien, H., Guber, K., Hale, G. M., Hoblit, S., Hollaway, S., Johnson, T. D., Kawano, T., Kiedrowski, B. C., Kim, H., Kunieda, S., Larson, N. M., Leal, L., Lestone, J. P., Little, R. C., McCutchan, E. A., MacFarlane, R. E., MacInnes, M., Mattoon, C. M., McKnight, R. D., Mughabghab, S. F., Nobre, G. P. A., Palmiotti, G., Palumbo, A., Pigni, M. T., Pronyaev, V. G., Sayer, R. O., Sonzogni, A. A., Summers, N. C., Talou, P., Thompson, I. J., Trkov, A., Vogt, R. L., van der Marck, S. C., Wallner, A., White, M. C., Wiarda, D., & Young, P. G. (2011). ENDF/B-VII.1 nuclear data for science and technology: Cross sections, covariances, fi ssion product yields and decay data. Nuclear Data Sheets, 112(12), 2887–2996. https://www.sciencedirect.com/ science/article/pii/S009037521100113X.