Nuclear Electronics for Signal processing
| It is important to extract information by electronically processing the signal generated by the detector at its output. The electronics for each detection system and for some specific radiation has to be designed judiciously so as to obtain the desired output and with maximum information about the incident radiation in terms of its energy, count, position, timing etc. A typical pulse processing set ( to determine energy versus count) consists of the following stages of electronics. | ||
| Stage 1: Preamplifiers | ||
| Preamplifier does the initial stage of amplification of the weak signal coming from the detector and also couples the detector with rest of the electronic set up by matching impedance with the detector. Preamplifiers are normally mounted as close as possible to the detector and add least amount of noise. There are three basic types of preamplifiers, namely, voltage sensitive, current sensitive and charge sensitive. As detectors are charge-producing devices, charge sensitive preamplifiers are most widely used. A typical charge sensitive preamplifier is schematically represented in Fig. m2.23. In this preamplifier, the charge carried by the incoming pulse is integrated on the capacitor C2 . | ||
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FIGURE m2.23 Schematic diagram of a charge sensitive preamplifier. | ||
| It can be shown that the output voltage is always proportional to | ||
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| In this way the dependence of the detector capacitance (C 1) has been removed. To discharge the capacitor C 2, a resistor is also placed in parallel with it. This results in an exponential tail pulse output as shown in Fig. m2.24. This pulse is given to the input stage of amplifier. . | ||
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Figure m2.24 Exponential tail pulse from a preamplifier. | ||
| Stage 2: Amplifiers | ||
| In this stage, the signal from the preamplifier is amplified and also shaped in a manner convenient for further processing. In general, the output pulse height from the preamplifier which is proportional to the energy has a long tail lasting anywhere from τ ~few ms to 100 ms. If any signal arises within time τ, it will ride on the tail of the previous pulse and hence distorting the information of energy. This is known as pulse pile up. This problem can be eliminated without affecting the count rate by reshaping the pulse. In amplifiers, pulse-shaping process is done by RCdifferentiation-integration method. In this technique, signal is processed through a cascaded CR differentiator and RC integrator. For an input step pulse the typical output for CR–RC circuit is shown in Fig. m2.25. The pulse is also filtered at low (differentiator) and high (integration) frequencies, resulting in an improvement in signal to noise ratio. | ||
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FIGURE m2.25 CR-RC pulse shaping network | ||
| The pulse produced in a CR–RC circuit is unipolar, which can be made bipolar using CR-RC-CRshaping. In the amplifier stage, a semi-Gaussian shape is given to the signal with a network of CR-RC-RC-RC—cascade so as to obtain a good signal to noise ratio. This amplifier output is then given to a discriminator for creating a logic pulse. | ||
| Stage 3: Discriminator | ||
| In the discriminator, a threshold value is set in the input stage so that any input signal with a pulse height greater than this threshold generates an output signal as indicated in Fig. m2.26. Thus, discriminator gives a standard logic signal at its output. By this process, discriminator eliminates the low amplitude noise and the signal trigger the discriminator is then transferred into logic pulses for further processing. However, it is important to maintain a constant time relation between the arrivals of the input pulse and issuance of the output pulse. Therefore, discriminator works as a simple analog-to-digital converter. However, for spectroscopy, a complicated circuit is used to convert the information contained in an analog signal to an equivalent digital form. This is known as analog-to-digital converter (ADC). | ||
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FIGURE m2.26 Generation of logic pulse in discriminator | ||
| Stage 4: Analog-to-Digital Converters (ADC) | ||
| This device is the fundamental link between analog and digital electronics and gives some numbers corresponding to some analog voltage signals. If an ADC accepts an input pulse of 0 and 10 V and is capable of giving output digital numbers from 0 to a maximum of 1000, then an input signal with amplitude 2.5 V will thus be converted to a number 250 and similarly for other voltages. The resolution of the ADC depends on the range of digitization. The most widely used technique on which ADC works is known as successive approximation. Here, the incoming pulse is compared to a series of reference voltages to determine the height of the pulse. Any input signal from 0 to 10 volt can be digitized to some binary numbers following the method given below: | ||
| Suppose an 8V pulse comes in the input of ADC. ADC first compares this pulse to a reference of 5 V. If the signal is more than the reference, which is true for this case, the first bit of the digital number is set to 1. One half of previous reference is added to make a new reference of 7.5 V and the comparison is again made. Since the signal is greater, the second bit is again set to 1. One half is again added on to make 8.75 V. This time the signal is less than this value, so that the third bit is set to 0. Now half is subtracted from the reference and a comparison is made. This continues until the required number of bits is obtained. The digital numbers so generated in the ADC can be fed to the memory of a computer through a control unit or to a multichannel analyzer (MCA) for pulse height spectroscopy. A typical pulse height spectroscopy set up with MCA is shown in Fig. m2.27. | ||
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FIGURE m2.27 Block Diagram of Pulse height spectroscopy set up with MCA | ||
| After the pulse processing, a typical output of MCA is a spectrum with X-axis indicating channel no. which corresponds to ADC channels and is proportional to the energy of particles. Along Y-axis, the count is proportional to the number of particles falling on the detector. One can also determine timing, position etc. of an incident particle with appropriate electronics. Readers are suggested to go through the literature.1,2 | ||