[Photoemission characteristic of transmission-mode extended blue GaAs photocathodes].

Based on the theoretical models of computing the photocathode optical performance, quantum yield and integral sensitivity, the photoemission characteristics of the domestic and ITT's transmission-mode extended blue GaAs photocathodes, namely the cathode optical properties and performance parameters, were respectively investigated. The compared results show that the integral sensitivity of the domestic transmission-mode extended blue photocathode has achieved 2,100 microA x lm(-1), still falling behind the ITT's integral sensitivity of 2,750 µA x m(-1). The reasons for the difference in quantum yield curves are that, on one hand, the thickness of GaAlAs window-layer and the Al mole fraction play a critical role in the short-wavelength response, especially in the extended blue region. On the other hand, the cathode performance parameters such as electron diffusion length and back interface recombination velocity work on the long-wavelength and short-wavelength response. All these factors are subject to the backwardness of basic industrial manufacturing level.

[The spectral response analysis of activated GaN photocathode].

GaN photocathode has a wide applicaion in ultraviolet detection because of the outstanding performance. GaN photocathode was activated in ultrahigh vacuum (UHV) system by Cs/O, and the reflection-mode quantum efficiency (QE) was analyzed. The QE is 30%-10% corresponding to the wavelength 240-350 nm, and the QE curve is flat. The QE reaches the maximum of 30% at 240 nm. Compared with the abroad result, the QE obtained by us is still inadequate at the short wavelength The atom arrangement of GaN (0001) was studied. The atom arrangement on the surface was simulated by 3D, and in this way the adsorption of Cs on the GaN(0001) was speculated.

[Comparative study of uniform-doping and gradient-doping negative electron affinity GaN photocathodes].

High temperature annealing and Cs/O activation are external incentives, while the property of GaN material is internal factor in the preparation of negative electron affinity GaN photocathode. The similarities and differences of the performance of the two structure photocathodes are analysed based on the difference of the structure between uniform-doping and gradient-doping negative electron affinity GaN photocathodes and the changes in photocurrents in activation and the quantum yield after successfully activated of GaN photocathodes. Experiments show that: the photocurrent growth rate is slower in activation, activation time is longer and quantum efficiency is higher after successfully activated of gradient-doping GaN photocathode than those of uniform-doping photocathode respectively. The field-assisted photocathode emission model can explain the differences between the two, built-in electric field of gradient-doping structure creates additional electronic drift to the photocathode surface, and the probability of electrons to reach the photocathode surface is improved correspondingly.

[Research on high temperature Cs activated GaAs photocathode].

Mechanism about the change of GaAs phtotocathode's surface barrier during Cs activated process was studied. Ionized Cs and p type doping impurity(Be) form a dipole that decreases the vacuum level of GaAs. Generally, Cs activated GaAs photocathode could achieve zero electron affinity state. The quantum yield formula of reflection-mode photocathode has been solved from the 1-dimension continuity equations and the escape probability formula has been solved from the Schrodinger equation. It was found from the formula that the quantum efficiency of Cs activated GaAs photocathode is directly proportional to the electric field intensity of Be- -Cs+ dipole. A Cs activation experiment was carried out, the experiment process tallies with the theory mentioned above and the integral sensitivity of Cs activated GaAs photocathode is 453 microA x lm(-1), which could be inferred as a zero electron affinity state.

[Analysis of spectral performance of varied-doping GaAs photocathodes].

For a quantitative analysis of the effect of varied doping structure on the quantum efficiency of cathode, two kinds of reflective gradient doping GaAs photocathode samples were designed and grown respectively. After activation, the dynamic spectral response curves of the two samples were obtained. The spectral response curves were transformed into quantum efficiency curves. The quantum efficiency curves in different wavebands were fit, and the varied doping coefficient K that reflects the contribution of the varied doping structure to the cathode quantum efficiency was obtained. Results show that, the functional effects of the same material varied doping structure responding to the incident photon of different wavebands are different from each other. The same is true for the different material varied doping structures responding to the same waveband incident photon. The fundamental reasons for these differences are that the positions and the intensities of the inside electric fields which build within the materials under different varied doping structures are different from each other. It offers an effective analysis means to evaluate the structure performance of cathodes grown under different doping ways, and has great value for the study on optimization design of cathode varied doping structure.

[The novel combined x-ray image intensifier].

By combining X-ray imaging technology and low light level imaging technology, the authors' research group designed the novel combined X-ray image intensifier. Different from the imported conventional one, it is an X-ray screen lens coupled with a brightness intensifier, and is a non-vacuum device. In this paper, the novel combined X-ray image intensifier is described in terms of its structure, imaging principle and imaging performance, and the comparisons of the structure, imaging principle and imaging performance between the novel combined one and the conventional one are given in detail. It is clear that the conventional medical tube is a little better than the novel combined one in imaging performance, whereas the novel combined one has a very high cost effectiveness and the satisfied imaging performance, and moreover, the quality disparity between the novel combined one and the imported medical one is being shortened. The novel combined one meets the X-ray imaging standard in many science and research fields and is convenient to be accepted by the usual users and has wide application fields such as industrial detection, nondestructive test and airport security check.

[The compensatory light for car plate recognition].

In the intelligent traffic system, the automatic recognition system of the car plate is an important development field. This system must satisfy the requirement of all-day long inspection. In the course of inspection, the lack of natural light(such as at night, in rainy day...) could be a problem, that is to say there could be backlighting problem, and this problem can be solved by using compensatory light. By now, the high-power floodlight has been adopted. Because of its own characteristics of the floodlight, the satisfied result cannot be obtained. In the present thesis, how to select the right light is analyzed, including car lights' spectra and visual response, and so on. The results of the practices are very satisfied.

[Design and application of a circuit for measuring frequency & duty cycle of stimulated bioelectrical signal].

Objective. To design a new type of circuit for measuring frequency & duty cycle of stimulated bioelectrical signal for the project of "the map of neuron-threshold in human brain and its clinical application". Method. This circuit was designed according to the character of stimulated bioelectrical signals. It was tested and improved and then used in the neuron-threshold stimulator. Result. The circuit was found to be very accurate for measuring frequency and the error for measuring duty cycle was below 0.2%. Conclusion. This circuit is well-designed, simple, easy to use, and can be applied in many systems.

[The method and implementation of measuring the peak of bioelectrical signals].

It is always very difficult to process the bioelectrical signals, sampling, amplifying and quantifying because of the different sources it comes from and the mechanism it depends on. This paper mainly introduces a method of measuring the peak value of bioelectrical signals, expatiates on the structure, testing result, analysis for error and measure taken for improvement of the circuit designed by this method. At last, the paper also points out that the method is wonderful, simple, easy to carry out, and can be used in many medical systems.

[The design of precision measure circuit for frequency & duty cycle of bioelectrical signal and its application in neuron-threshold stimulator].

This paper mainly introduces the principles of a type of circuit for measuring frequency & duty cycle of low-frequency signals, and its applications in neuron-threshold stimulator. The circuit has the advantages of simple structure and accurate measurement.