Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas Timepix detector as a tool for X-ray and gamma dosimetry Peter Rubovič a,∗ , Benedikt Bergmann b , Daniela Ekendahl a , JiříHůlka a , Libor Judas a , Zdeněk Kohout c , Stanislav Pospíšil b , Ivan Štekl b a National Radiation Protection Institute, Bartoškova 1450/28, 140 00 Prague 4, Czech Republic b Czech Technical University in Prague, Institute of Experimental and Applied Physics, Horská 3a/22, 128 00 Prague 2, Czech Republic c Czech Technical University in Prague, Faculty of Mechanical Engineering, Technická 4, 166 07 Prague 6, Czech Republic ARTICLE INFO Keywords: Timepix Medipix Gamma X-ray Dosimetry ABSTRACT We present a concept of using Timepix detector for real-time and simultaneous measurements of various external dosimetry and operational quantities in X-ray/gamma fields with broad range of air kerma rates ranging from natural background levels to the order of Gy·h -1 , including intensive pulsed sources used in medicine. Timepix is a hybrid semiconductor detector of ionizing radiation which can visualize tracks of particles interacting in the semiconductor pixelated sensor bump bonded to the Timepix readout chip. Energy deposited in the individual pixels and properties of these tracks enable us to classify the incident radiation and to assign the correct con- version coefficient between a number of detected events and the desired quantity, e.g. an ambient dose equivalent rate. This paper shows calibration measurements of such dosemeter and discusses its properties. 1. Introduction The motivation for the present study was to develop a small and portable active gamma dosemeter based on the sophisticated tech- nology of semiconductor pixel detectors developed within the CERN collaboration (Medipix collaboration homepage, 2017). The detector should be able to measure quantities used in radiation protection and dosimetry such as air kerma K a and operational dose equivalents for area and personal monitoring, for instance ambient dose equivalent H (d) * , directional dose equivalent H’(d) or personal dose equivalent H (d) p , even in conditions of a broad range of dose rates. The hybrid semiconductor pixel detector Timepix is based on Medipix2 technology (Llopart et al., 2002). The sensor consists of a semiconductor diode with pixelated surface. It is possible to determine the energy deposited in each pixel and to identify the type of an in- teracting ionizing particle through a morphological analysis of the track of the particle registered by the detector (Holy et al., 2008). These tracks are sometimes called clusters in the literature and we will use both of the aforementioned terms throughout the paper. The fact that the active sensor is segmented also improves the linearity of the re- sponse of the detector at high flux rates. The detector is unique for its noiseless photon energy counting possibilities and a rather low threshold level set to 5 keV. There have already been several efforts to use a Medipix/Timepix detector for gamma dosimetry. A research group at the University of Erlangen used a Medipix detector for a detailed analysis and re- construction of X-ray spectra (Sievers et al., 2011; Talla et al., 2009) and also as a low energy X-ray dosemeter (Michel et al., 2009). Their work resulted in developing the Dosepix device (Wong et al., 2011; Zang et al., 2015). A different approach was chosen by Turecek et al. (2011) for dose estimation aboard the International Space Station, based on a calculation of LET and quality factors of heavy ions. Heijne et al. (2013) tried to estimate gamma dose in the ATLAS cavern by simply counting detected events and using a conversion coefficient for every track under the assumption that the measured radiation field is similar to the calibration one. However, none of these approaches are suitable for general gamma fields ranging from low energy X-rays up to high energy gamma rays and from natural radiation background levels to very high dose rates. 2. Materials and methods Irradiation All the measurements were performed in the gamma and X-ray calibration accredited laboratory of the National Radiation Protection Institute in Prague, which is equipped with a large variety of X-ray and gamma sources. It provides radiation qualities of S-Cs, S-Co and the narrow-spectrum series (N) specified in ISO 4037-1 (ISO4037- 1:1996, 1996). An overview of the X-ray spectra properties is given in Table 1. Apart from the ISO norm specified production, the X-ray spectra with energies up to 160 keV were experimentally verified by a http://dx.doi.org/10.1016/j.radmeas.2017.10.012 Received 3 July 2017; Received in revised form 2 October 2017; Accepted 27 October 2017 ∗ Corresponding author. E-mail address: peter.rubovic@suro.cz (P. Rubovič). Radiation Measurements 107 (2017) 39–42 Available online 31 October 2017 1350-4487/ © 2017 Elsevier Ltd. All rights reserved. MARK