Final Project: FEDX X-ray Radiation Detector

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1 Final Project: FEDX X-ray Radiation Detector Keita Todoroki Keita Fukushima December 12, 2011 Introduction The application of radiation detectors has played an important role in physical science, especially in high energy physics. The unwanted radiation dose that the instruments receive needs to be minimized in order to achieve accurate and precise measurements. The RadFET, a radiation-sensing field-effect transistor, is a useful device that allows us to measure the radiation dose by converting analog inputs to digital outputs. The Si-photodiode has also a practical use in detecting the dose rate. In this project we examined the practical application of the basic model of the radiation detectors by using both of the devices. Overview The goal of this project is to build a simple prototype readout circuit that is capable of measuring the radioactive doses emitted by radioactive sources. The basic components of the circuit are a RadFET, a Silicon photodiode (PIN diode), current sources, operational amplifiers, analog switches and an ADC. The project name FEDX stands for radiation-sensing Field Effect transistor and photodiode X-ray sensor. Functionality A block diagram of the project is shown in Fig.1. Both RadFETs and Si-photodiodes utilize the photoelectric effect in response to the radiation exposure. This causes ionization and a pair production of an electron and a hole. For RadFETs, the space charge is trapped in an inorganic insulator Si, where the holes accumulate on the surface as it s exposed to radiation, which causes the threshold voltage to shift [4]. This shift is then related to the radiation dose (Fig.2). A RadFET is essentially the same as an enhancement PMOS transistor, so the I-V characteristics resemble those of PMOS switching FETs. Some of the advantages are that it is suitable for long-term charge storage and it is compact and easily coupled with computer power. The main drawback would be that the ionization sensitive region is only 0.05, so the dose resolution is not as good as for gas tubes, for instance, which achieve milliard resolution [1]. In addition, the RadFET we used has two different channels, R and K, which differ in sensitivity to irradiation. The integrated dose for R range is 2.67 Rad/mV, while that of K range is 13.2 Rad/mV [4]. 1

2 Fig. 2 Chip layout for a RadFET. Both R and K channels are built on the same board. Fig. 3 shift of a RadFET versus accumulated radiation dose with source. Fast irradiations are at ~0.8 kgy/h and a slow irradiation is at ~16Gy/h [4]. Meanwhile, for the photodiode we used a PIN photodiode instead of p-n diode since a PIN diode produces a much faster and sensitive response. When a X-ray hits the PIN photodiode, electrons produced move toward the cathode (n-type semiconductor), while holes move to the anode (ptype semiconductor). This leads to accumulation of excess electrons and holes on the cathode and the anode, respectively. Therefore, a potential difference is created and thus we can measure the current output. 2

3 Fig. 4 Photoelectric effect and a electron-hole pair production mechanism. Note that this diagram uses p-n type diode instead of PIN diode, but the mechanism is essentially the same. The actual PIN diode has an intrinsic semiconducting layer, which is lightly doped, in between P- and N-type layers. Since the averaged energy that is produced by a electron-hole pair production is about 3.6eV, we were able to calculate the expected radiation dose rate for the photodiode. We used Eq. 1 to determine the produced current. (Eq. 1) where is the energy of the irradiation (X-ray), is the energy of the pair production and e is the charge of electron in Coulomb/electron. Suppose we use and the Si detector of 100Hz, we obtain = 4.44pA for each pair production. For the calculations of the dose rate, we used Eq. 2 with the unit to be rad/s. Dose rate = (Eq. 2) where m is the mass of the Si plate in kg, which was obtained to be By units conversion and using Ohm s law to relate the dose rate to voltage, we established the following relation. Dose rate =, where R is resistance. If we are to use R = 10MΩ we have 2.55µGy/sec*mV, where we used 1 Gy = 100 rad = 1 J/kg. It is also possible to vary the resistor values to work over a dynamic range of the radiation dose. 3

4 Fig. 5 Block diagram of FEDX. The whole project can be broken down into three basic steps: 1) measurements 2) sampling& timing 3) taking data and automatically logging it. Fig. 6 Schematic of FEDX. Labels in red show the specific product numbers. 4

5 Fig. 5 and 6 show the block diagram and the schematic. In our circuit, the two outputs of RadFET, R and K ranges, were connected to unity buffers, which convert a very high input impedance to a small one with gain = 1. Also, a current source made by MOSFET (ZVN3306) was installed to ensure a fixed current of 100µA. For the photodiode, we needed to have a zero bias current-to-voltage converter for each of the two outputs. The lead plate in between the two Si diodes is basically for estimating the energy spectrum of the X-ray. All of the four outputs were then connected to a analog switch, which is controlled by signals from an accelerator. This switch is connected to a capacitor to store the output voltages, and so this section of the circuit is for sampling and hold the data. Once the data are sampled and held, they are sent to an analog multiplexer to choose one of the four inputs by digital signals. We used a 555 oscillator and a counter for this part. Finally, an ADC is used for converting the analogue signal to digital one so that the radiation dose can be displayed on the computer screen. It will therefore be convenient to have a USB port on the circuit board to bypass the digital output to computer. Fig. 7 Actual circuit built. On the left we have a circuit with a RadFET, Si photodiode, and other discrete parts including current sources and analog switches installed. Note that the power sources are built on a separate board. The picture on the right just shows the scale of the circuit. Conclusions Based on the schematic and the actual built circuit, our radiation detector should have a promising future for practical applications. There is a great deal of work and progress that are required to reach that point, but the idea and concept of the functionality of FEDX are simple enough for immediate application. It is expected that the basic components of the circuit is built and completed for actual testing using radiation sources available in the high energy physics lab at University of Hawaii at Manoa. 5

6 References [1] RADFET Probes, Readers, Applications. REM Oxford. England, UK. [2] The RADFET. Tyndall National Institute. Cork, Ireland. [3] Silicon PIN Diode Radiation Detectors. Carroll & Ramsey Associates. Berkeley, CA. [4] Ruckman, L. et al. Development and Implementation of a Readout Module for Radiation- Sensing Field-Effect Transistors. IEEE Transactions on Nuclear Science, vol. 53, No.4. August [5] Silicon Photodiode Theory. Centronic. UK. 6%20photodiode%20theory.pdf 6