Design of Vernier anode detector and its electronic readout circuit

0 Introduction
Ultraviolet detection technology is another dual-use photoelectric detection technology for both military and civilian use that has been developed after infrared and laser detection technology. As early as the 1950s, people began to study ultraviolet detection technology. EUV detectors use extreme ultraviolet imaging technology with a wavelength of 30.4 nm to image the Earth's plasma layer, which can obtain the distribution of the entire magnetosphere around the Earth, and is used to detect the space environment and study changes during solar disturbances.
On October 24, 2007, China's "Chang'e" No. 1 satellite was successfully launched, marking that China has entered the ranks of countries with deep space exploration capabilities. At present, the second phase of the "Chang'e Lunar Exploration Program" is carrying out moon-based EUV imaging experiments on the Earth's plasma layer to study changes in the Earth's space environment and provide observation data for catastrophic environmental changes.
Our research group has conducted technical research on extreme ultraviolet imaging detection systems and has achieved good results in anode design and circuit signal processing.

1 Structure of Vernier anode detectors
Anode detectors can be divided into two types according to position sensitivity: one is a unit type, such as MAMA type; the other is continuous, such as resistor anode, WSA, Delay-line, Vernier, etc. Among them, the Vernier anode has a higher photon counting rate and position resolution than other anodes. Therefore, this article mainly introduces the Vernier anode.

The anode detector is mainly composed of a photocathode, MCP, position-sensitive anode and electronic readout circuit. The basic structure diagram of the anode detector is shown in Figure 1. The single-photon light source reaches the photocathode through the input window to generate electrons, which are then multiplied by the V-type cascade MCP to generate an electron cloud, which reaches the Vernier anode under the action of the accelerating electric field to form multiple electronic pulses. After the multiple signals are processed by the electronic readout circuit, they are decoded by software to form a grayscale image.

The structure of the anode panel used to collect electron clouds is shown in Figure 2. There are 6 electrodes to collect charges, and they are insulated from each other. In the horizontal direction, the area of ​​each electrode changes sinusoidally, and there is a 120° difference between them. The phase of the sine curve changes linearly with the horizontal direction. The amount of charge Q collected on each electrode also changes sinusoidally with the position, and the amount of charge Q is proportional to the area SQ of the electrode collecting the charge. Since the wavelength of the sine curve is much larger than the width of the electrode, the center of mass position of the electron cloud is proportional to the width of the electrode in each electrode area covered by the electron cloud. Therefore, the phase value θ of the center of mass position on the electrode can be obtained. The horizontal coordinate x value can be obtained through the θ value. When the x values ​​of the two groups of electrodes are the same, the coordinate position of the photon on the anode panel can be obtained.

2 Design of Vernier anode electronic readout circuit
The electronic readout circuit is mainly composed of a charge-sensitive preamplifier, a filter-shaping amplifier circuit and a peak-holding circuit. The charge-sensitive preamplifier is mainly used to convert the signal output by the anode into a voltage signal; the filter-shaping circuit is to make the shape of the signal meet the quasi-Gaussian waveform to meet the needs of subsequent processing and improve the signal-to-noise ratio; the peak-holding circuit is to widen the peak of the signal to improve the accuracy of the peak value obtained. The structural block diagram is shown in Figure 3.

In the preamplifier circuit, in order to increase the input impedance and reduce noise, low-noise junction field-effect transistors are used as the input stage of the charge-sensitive preamplifier, such as 2SKl52 and 2N4416. The feedback resistor and capacitor are set to Rf=500 MΩ and Cf=1 pF, so τ=500×106×10-12=500μs. The tail of the output waveform is long, which is prone to pulse accumulation. In order to increase the counting rate, CR differential processing is required, as shown in Figure 4. Its transfer function is , where , in the design, the purpose of pole-zero cancellation is achieved when τ1=τ. In Figure 4, C5=1 000 pF, R6=1 MΩ, W2=50 kΩ, and different photon rates incident on the system have different requirements for integration time. Different integration times can be obtained by selecting different R8 values. Under the condition of C = 1 000 pF, when τ2 = 1 μs, R8 = 1 kΩ; when τ2 = 2 μs, R8 = 2 kΩ; when r2 = 10 μs, R8 = 10 kΩ. In the actual circuit, a jumper is used to select R8.

The waveform output by the pole-zero phase cancellation circuit cannot be directly sampled by the circuit, and its waveform must be improved. In order to meet the needs of the acquisition card for the signal waveform, improve the signal-to-noise ratio, and accurately obtain the pulse peak data, it is necessary to further filter and shape the signal. Through the discussion of the optimal filter, it can be seen that the symmetrical infinite width peak pulse has the best signal-to-noise ratio, and the Gaussian waveform has the above characteristics. The top of the pulse is relatively flat, so the pulse is generally shaped in a Gaussian or quasi-Gaussian shape. The circuit structure shown in Figure 5 is used, and a two-stage active low-pass filter is used. Under C=1 000 pF, the values ​​of different integration times are taken, when τ2=1μs; when R=1 kΩ, when τ2=2μs, R=2 kΩ; when τ2=10μs, R=10 kΩ. Similarly, R is realized by using a jumper.
The basic principle of the peak hold circuit is shown in Figure 6. When the input signal is larger than the threshold, the comparator 1 outputs a high level, triggering the trigger 1 output Q to be high, and the trigger 2 outputs a high level to control the logic level of LF398, making LF398 in the adopted state.

When the signal reaches the peak, comparator 2 outputs a high level, causing the AND gate circuit to output a low level. At this time, LF398 is in a hold state, thereby maintaining the signal peak. The control circuit is mainly completed by two-stage monostables. When comparator 1 outputs a high level, the rising edge triggers the first-stage monostable, and the output temporary stable time can be adjusted by an external resistor. The falling edge of the temporary stable state triggers the second-stage monostable, and the high level of the output triggers the analog switch, causing the holding voltage capacitor of LF398 to discharge rapidly.

3 Conclusions
When testing the circuit, the circuit was connected to the ultraviolet single photon counting imaging system built by our research group. The output waveform of each circuit was observed with a Tek DPO 4104 oscilloscope, which met the quasi-Gaussian distribution, and the system was used to image the 4-hole mask. Figure 7(a) is a real picture of the 4-hole mask. The distances between the holes are 7 mm, 9 mm, and 15 mm, respectively, and the aperture is about 2 mm. Experimental conditions: The light source is a low-pressure mercury lamp, the voltage of the two MCPs is 2280 V, the voltage between the MCP and the anode is 300 V, the distance between the two MCPs is 50 μm, and the distance between the MCP and the anode is 15 mm. When the vacuum reaches 1.0×10-4Pa, the pulse signal output by the anode is connected to the electronic readout circuit. The circuit parameters used in this experiment are: the sensitivity of the preamplifier is A = 1 V/pc, the pulse shaping time is 2μs, the voltage amplification factor is 4 times, the pulse half-peak full width of the pulse output measured by the oscilloscope is 5μs, and the voltage amplitude meets the range of the acquisition card of 0~10 V. The acquisition card collects the voltage peak value, and the grayscale image obtained after software decoding is shown in Figure 7(b). It can be seen that the obtained image is consistent with the actual object.

4 Conclusion
A signal processing circuit for Vernier anode detector is designed. The voltage amplitude and signal-to-noise ratio of the signal processed by the circuit meet the design requirements of the anode detector. The designed circuit is tested experimentally and can meet the requirements of the single-photon detection imaging system, verifying the feasibility of the electronic readout circuit.