Characterization of a CT ionization chamber for radiation field mapping Ana P. Perini a , Lucio P. Neves a , Vitor Vivolo a , Marcos Xavier a , Helen J. Khoury b,1 , Linda V.E. Caldas a,n a Instituto de Pesquisas Energe´ticas e Nucleares (IPEN-CNEN), Comiss ~ ao Nacional de Energia Nuclear, Av. Prof. Lineu Prestes 2242, 05508-000, S ~ ao Paulo, SP, Brazil b Universidade Federal de Pernambuco, Departamento de Energia Nuclear, Av. Prof. Luiz Freire 1000, 50740-540, Recife, PE, Brazil article info Available online 30 November 2011 Keywords: Pencil ionization chamber Standard X-rays beams Radiation field mapping abstract A pencil-type ionization chamber, developed at Instituto de Pesquisas Energe ´ ticas e Nucleares (IPEN), was characterized with the objective to verify the possibility of its application in radiation field mapping procedures. The characterization tests were evaluated, and the results were satisfactory. The results obtained for the X radiation field mapping with the homemade chamber were compared with those of a PTW Farmer-type chamber (TN 30011-1). The maximum difference observed in this comparison was only 1.25%, showing good agreement. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ionization chambers are dosimeters widely used for measure- ments of air kerma and absorbed dose at hospitals and radiation metrology laboratories. There are several types of ionization chambers for different applications. For example, one type of chamber commonly used in computed tomography (CT) dosimetric measurements is the pencil-type ionization chamber. This chamber presents a special design and some particular properties. It has a cylindrical geometric shape with a sensitive length of 10 cm and a sensitive volume of approximately 3 cm 3 in most cases (Suzuki and Suzuki, 1978). In Brazil, there is an increasing interest in the development of ionization chambers and other radiation detectors, because there are difficulties in importing and servicing them. In order to construct low cost ionization chambers as an alternative for use at laboratories for calibration and dosimetric measurements, some ionization chambers were developed at the Calibration Laboratory of IPEN (LCI) for use in diagnostic radiology and radiotherapy energy ranges (Albuquerque and Caldas, 1989; Costa and Caldas, 2003; Oliveira and Caldas, 2007; Yoshizumi and Caldas, 2010). In this work a pencil ionization chamber was developed for a different application than the dosimetry in CT; the pencil ioniza- tion chamber was tested for diagnostic radiology calibration beam mapping. This ionization chamber differs from other com- mercial pencil ionization chambers mainly due to its volume and constituent materials. The sensitive volume of this chamber is only 1.0 cm length instead of the usual 10.0 cm of commercial pencil ionization chambers, and only Brazilian low cost materials were used for its construction. The beam mapping is an important step to assure a reliable calibration procedure. This test is required, because it allows the verification of the homogeneity level in the radiation field, and therefore, it will indicate if there is any irregularity in the radiation field caused, for example, by setup changes of the radiation system. Several services are offered at the LCI, such as calibration services of radiation detectors for X radiation, using an industrial X-ray unit. In order to offer reliable services to customer, measurements are periodically taken to evaluate the radiation beam homogeneity. Observing this need, a small ionization chamber of sensitive volume of 0.34 cm 3 was studied to evaluate the field homogeneity of standard diagnostic radiology beam qualities established in the industrial X-ray unit at LCI. The main quality control tests to characterize this ionization chamber were: saturation curve, polarity effects, ion collection efficiency, stabi- lization time, short- and medium-term stabilities, leakage cur- rent, linearity of response, angular dependence and energy dependence in standard diagnostic radiology calibration beams. 2. Materials and methods A new pencil ionization chamber, developed at IPEN, was manufactured using polymethyl methacrylate (PMMA), polyvinyl chloride (PVC) coated with graphite, aluminum electrode and co- axial cables. The sensitive volume is delimited by the PVC cylinder, coated with a thin layer of graphite, connected with two other cylinders of PMMA. Since this is a vented ionization chamber, a small hole was made in the PVC cylinder. To attach the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.11.051 n Corresponding author. Tel.: þ55 11 3133 9716; fax: þ55 11 3133 9671. E-mail addresses: aperini@ipen.br (A.P. Perini), lpneves@ipen.br (L.P. Neves), vivolo@ipen.br (V. Vivolo), mxavier@ipen.br (M. Xavier), hjkhoury@gmail.com (H.J. Khoury), lcaldas@ipen.br (L.V.E. Caldas). 1 Tel./fax: þ55 81 21 268708. Applied Radiation and Isotopes 70 (2012) 1300–1303