Bipolar Junction Transistor As a Detector for Measuring in Diagnostic X-Ray Beams
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2013 International Nuclear Atlantic Conference - INAC 2013 Recife, PE, Brazil, November 24-29, 2013 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-05-2 BIPOLAR JUNCTION TRANSISTOR AS A DETECTOR FOR MEASURING IN DIAGNOSTIC X-RAY BEAMS Francisco A. Cavalcanti1,2, David S. Monte1,2, Aline N. Alves1,2, Fábio R. Barros2, Marcus A. P. Santos2, and Luiz A. P. Santos1,2 1 Departamento de Energia Nuclear Universidade Federal de Pernambuco Av. Prof. Luiz Freire, 1000 50740-540 Recife, PE [email protected] 2 Centro Regional de Ciências Nucleares do Nordeste (CRCN-NE / CNEN) Av. Prof. Luiz Freire, 1 50740-540 Recife, PE [email protected] ABSTRACT Photodiode and phototransistor are the most frequently used devices for measuring ionizing radiation in medical applications. The cited devices have the operating principle well known, however the bipolar junction transistor (BJT) is not a typical device used as a detector for measuring some physical quantities for diagnostic radiation. In fact, a photodiode, for example, has an area about 10 mm square and a BJT has an area which can be more than 10 thousands times smaller. The purpose of this paper is to bring a new technique to estimate some physical quantities or parameters in diagnostic radiation; for example, peak kilovoltage (kVp), deep dose measurements. The methodology for each type of evaluation depends on the energy range of the radiation and the physical quantity or parameter to be measured. Actually, some characteristics of the incident radiation under the device can be correlated with the readout signal, which is a function of the electrical currents in the electrodes of the BJT: Collector, Base and Emitter. Samples of BJT are classified and submitted to diagnostic X- ray beams. The results show that the BJT could be a new semiconductor sensor type for measuring either the ionizing radiation dose or some characteristics of diagnostic X-ray beams. 1. INTRODUCTION Bipolar Junction Transistor (BJT) has more than 60 years and it is one of the most known semiconductor electronic devices. In fact, the BJT gave a physics Nobel Prize in the middle of the last century [1] and leveraged a true technological revolution bringing real change in our lives with the solid-state electronics equipment. Nowadays, electronics are increasingly being used to innovate both sensors as measurement technique whatever physical quantity. In the case of the ionizing radiation, photodiode and phototransistor are the most frequently used devices for measuring some characteristic of the X-ray beam in medical applications [2-3]. These electronic components provide an output signal, which consists of an electrical current, when the X-ray photons cause interactions with the whole device and they transfer part of the ionizing radiation energy. Although photodiodes have the operating principle well known for health physics instrumentation, the bipolar junction transistor (BJT) is not a typical device used as a detector for measuring some physical quantities for diagnostic radiation. The basic difference between a BJT and the above electronic components resides in their geometry and consequently the electric field configuration in the internal structure of the devices. In fact, a photodiode, for while, has an area about 10 mm square and a BJT has an area which can be more than 10 thousands times smaller. This feature of the BJT elects such a device as a strong candidate to be used for measuring some characteristic of the ionizing radiation at the point or micro-dosimetry. The aim of this paper is to present how a BJT can operate as an ionizing radiation detector for diagnostic X-ray beams besides the apparent trouble from its loss of sensitivity to ionizing radiation. 2. BIPOLAR JUNCTION TRANSISTOR 2.1. Operating principle Basically, a BJT consists of 3 types of semiconductor materials, NPN for example. In this case, the base (B) is a P type semiconductor sandwiched between two N type semiconductors [4] called collector (C) and emitter (E) as shown in Fig. 1. In general, a BJT can be biased with a current source in the base-emitter junction and a voltage source is applied to the collector and emitter electrodes, which it is so called the operating point, VCE [5]. Frequently, the base current IB is the input signal and the collector current IC is the output signal. The ratio IC/IB is the amplification factor of the BJT also called the forward current gain, β. In short, the BJT can operate in three configuration regions: 1) forward-active, for IB>0 and β=IC/IB; 2) cut, practically for IB=0 and IC=0; 3) saturation, for IB>0 and IC<β·IB. The last two operation modes are normally used in logical circuits or computers and they correspond to the binary numbers 0 and 1, respectively. Actually, for the radiation detector purpose the BJT operates in the forward-active region, which corresponds to an analog signal amplifier with the gain β. Figure 1: A simple illustration of a NPN BJT and its electronic symbol. 2.2. BJT as radiation sensor BJT has already been submitted to some type of radiation [6-9], as protons and neutron- gamma fields, to verify how is the behavior of the characteristic curve IC×VCE in function of the radiation dose [9]. Recently, it has been used as a radiation detector [10] to correlate some characteristic of ionizing radiation beam for health physics instrumentation. In this case, Santos et al. [10] have evaluated the loss of sensitivity to ionizing radiation and they correlate it with the dose in the therapy radiation energy range. For diagnostic radiation beams the loss INAC 2013, Recife, PE, Brazil. of sensitivity to X-ray is not significant and it is the focus of this paper. To take real time measurements with a BJT under an X-ray beam, a low intensity current IB must be injected in the base as suggested by Santos et al. Then, the output signal, IC, can be correlated with the kilovoltage or the deep dose in diagnostic X-ray analysis as it will be presented in this paper. 3. MATERIALS AND METHOD 3.1. Bipolar junction transistors Two commercial BJTs were chosen to submit them to the X-Ray beams: 2N3904 and BC846 (Fig. 2). The former has a TO-92 package and it was directly positioned into the radiation field. The second one is a surface mount device (SMD) with SOT23 package, and it was soldered to a printed circuit board aiming to make easy get readout. Figure 2: 2N3904 and BC846 BJTs; Printed circuit board with BC846. 3.2. Instruments During the irradiation of 2N3904, the input signal IB was generated from a 6430 Keithley® system, named as K6430, whereas the output signal IC was measured with 2400 Keithley® system, named K2400 (Fig. 3). The operating point of the 2N3904 was set to be VCE=5V from the K2400, which works simultaneously as voltage source (VCE) and current meter (IC). An EFF1201 Scients® electrometer (Fig. 4) was used for measuring the signal from the BC846 transistor. Such an instrument can provide the input signal IB and take measurements of the collector current IC simultaneously. Another type of instrument, the Thin-X system, Unfors®, was used to measure the dose rate at the point where the BC846 was positioned for the irradiation procedure. It allows us to find a conversion factor and make correlation between the BJT readout and the deep dose. IC C B VCE IB E + K6430 - K2400 Figure 3: Connection diagram for measuring X-rays with a NPN BJT. INAC 2013, Recife, PE, Brazil. Figure 4: EFF1201 electrometer, SCIENTS®. 3.3. Radiation beams and procedures In the first experimental set up an HF-320 Pantak X-ray equipment was used to generate a 2.5mm Al filtered radiation field. To test if the 2N3904 BJT can give information about the peak kilovoltage (kVp), this parameter was varied from 60 kV up to 160 kV to cover almost all energy range used in diagnostic X-rays. The tube current was set to be 10 mA and the exposure time was about 10 s to avoid significant damage [9]. The distance from the radiation source focus to each device was set to be 0.43 m. At least three samples of each BJT were used to make statistics and then build the graph. Another experimental set up was based on the clinical X-ray equipment Polymat Plus 30/50, Siemens, at the Centro de Regional de Ciências Nucleares do Nordeste (CRCN-NE/CNEN). The BC846 BJT was used to take measurements of deep dose varying the thick of poly(methyl methacrylate) (PMMA - to simulate a human tissue - Fig. 5) each 0.5 mm step from 0 mm up to 10.5 mm. The parameters of the X-ray beam were: 70kV; 100 mAs; and 1000 ms for the exposure time. The source-detector distance was set to be 0.50 m. Figure 5: Human tissue simulator for measuring in clinical X-ray equipment using the BC846 BJT. INAC 2013, Recife, PE, Brazil. 4. RESULTS The analysis of the peak kilovoltage from the 2N3904 BJT readout, which was submitted to the X-rays, has demonstrated that the kVp and IC are correlated (Fig.6). Actually, as suggested by Santos et al. [9], we can conclude that if the magnitude of IB is comparable with the radiation-induced current, which is called in this paper by the photocurrent IR. We have found for IB=100pA the 2N3904 BJT has the best sensitivity and it was a parameter to be chosen due to the fact that IR≈100pA. In fact, increasing IB the photocurrent IR is totally neglected and for that reason the effect of the X-ray photons begins to disappear, i.e., the BJT could not see the effect of the X-ray beam.