The circuit of Figure 1 includes a PIN photodiode that detects individual photons of gamma radiation. The reverse bias on the photodiode sometimes creates a depletion region. When such a photon strikes this depletion region, a small amount of charge develops. This charge is proportional to the photon's energy. Four amplifiers following the PIN photodiode amplify and filter the resulting signal. A final comparator distinguishes between the signal and the noise. Thus, the comparator's output pulses high each time a gamma photon with sufficient energy strikes the photodiode. Small signal levels make this design an interesting challenge. The design requires very-low-noise circuitry because the individual gamma photons generate a small amount of charge and because lowering the overall noise level allows the circuit to detect lower energy gamma photons. You must pay special attention to the first stage, which is the most noise-critical.
The most critical component is the PIN photodiode, whose selection often involves conflicting considerations. Detector sensitivity (the number of photons detected for a given radiation field), for example, depends on the size of the depletion region, which in turn depends on the area of the diode and the reverse bias applied to the diode. To maximize sensitivity, therefore, you should choose a large-area detector with high reverse bias. Large-area detectors tend to have high capacitance, which increases the noise gain of the circuit. Similarly, a high bias voltage means high leakage current. Leakage current also generates noise. The circuit in Figure 1 includes the QSE773 PIN photodiode. Though readily available and inexpensive, it is probably not the optimal choice. Certain PIN diodes from Hamamatsu can work nicely in this application. Choosing a detector with 25- to 50-pF capacitance with reverse bias applied provides a fair compromise between sensitivity and noise.
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| Figure 1. | When a single gamma photon with sufficient energy strikes the PIN photodiode in this circuit, the output of the comparator pulses high. |
Important considerations for the first-stage op amp include input-voltage noise, input-current noise, and input capacitance. Input-current noise is directly in the signal path, so the op amp should keep that parameter to a minimum. JFET- or CMOS-input op amps are a must. Also, if possible, the op amp's input capacitance should be smaller than that of the PIN photodiode. If you use a high-quality PIN photodiode and an op amp with low current noise and pay careful attention to design, the limiting factor for noise should be the first-stage op amp's input-voltage noise multiplied by the total capacitance at the op amp's inverting node. That capacitance includes the PIN-photodiode capacitance; the op-amp input capacitance; and the feedback capacitance, C1. Thus, to minimize circuit noise, minimize the op amp's input-voltage noise. The op amp in this circuit, IC1A, a MAX4477, well suits this design. It has negligible input-current noise and low input-voltage noise of 3.5 to 4.5 nV/√Hz at the critical frequencies of 10 to 200 kHz. Its input capacitance is 10 pF.
R1 and R2 contribute equally to noise because they are directly in the signal path. Resistor-current noise is inversely proportional to the square root of the resistance, so use as large a resistance value as the circuit can tolerate. Keep in mind, though, that leakage current from the PIN diode and first-stage op amp place a practical limit on how large the resistance can be. The MAX4477's maximum leakage current is only 150 pA, so R2 could be much larger than the 10 MΩ shown. R1 can also be substantially larger when the circuit operates with a high-quality PIN photodiode. C1 affects the circuit gain, and smaller values benefit both noise and gain. Use a capacitor with low temperature coefficient to avoid gain changes with temperature. This capacitance value also affects the requirement for gain-bandwidth product in the op amp. Smaller capacitance values require a higher gain-bandwidth product.
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| Figure 2. | These waveforms from the Figure 1 circuit show the signal at Test Point 1 (top trace) when a gamma photon strikes the PIN photodiode, and the resulting comparator output (bottom trace). |
To ensure that the circuit measures gamma radiation and not light, cover the PIN photodiode with an opaque material. To block radiated emission from power lines, computer monitors, and other extraneous sources, be sure to shield the circuit with a grounded enclosure. You can test the circuit by using an inexpensive smoke detector. The ionizing types of smoke detectors use americium 241, which emits a 60-keV gamma photon. (The more expensive photoelectric smoke detectors do not contain americium.) A 60-keV gamma is close to the circuit's noise floor but should be detectable. A graph shows the result of a typical gamma strike (Figure 2). The top waveform is from Test Point 1, and the bottom waveform represents the comparator's output. A possible improvement would be to replace C1 with a digitally trimmable capacitor, such as the MAX1474, which provides the circuit with digitally programmable gain. Similarly, replacing the mechanical potentiometer with a digital potentiometer, such as the MAX5403 allows digital adjustment of the comparator threshold. Finally, driving the comparator's noninverting input with a reference instead of the 5 V supply improves the comparator's threshold stability.
