Vacuum technology is extensively used in space science, nuclear energy, surface science, materials science, electric vacuum industry, microelectronics, semiconductor, metallurgy, and so on. More and more fields need ultra high vacuum (UHV) or extreme high vacuum (XHV) for scientific research and production. Both the rigorous vacuum processing technics and highly developed sophisticated vacuum technology are the necessities for the acquisition and conservation of UHV or XHV ambience. In this paper, a vacuum decline was analyzed and the UHV system was recovered. Using the residual gas analyzer to measure the partial pressure of residual gas and identify the type of gas, the outgassing source was found out by comparing the measure results. The UHV system was recovered back to 10<sup>-9</sup>Pa through treatment. The residual gas analyzing method can effectively identify the type of gas about outgassing, narrow the scope of outgassing subassembly, shorten the vacuum system recovery time, and improve work efficiency. The method of determining the outgassing source by residual gas analyzer can be applied in the fields of UHV and XHV, and it has a certain reference significance for further improving the system vacuum degree.
The gallium arsenide (GaAs) photocathode was generally cleaned by radiant heating, direct heating, ion bombardment annealing, and so on. In this paper, the radiant heating method, namely thermal cleaning method, was adopted for GaAs photocathode surface purification. Using this method could obtain an atomic clean surface, ensure the integrity of the GaAs surface lattice, and guarantee the uniformity of surface cleaning effect at the same time. But because the accurate measurement of the GaAs photocathode surface temperature in the vacuum system was very difficult, the residual gas analyzer (RGA) was used in this experiment to monitor the residual gas composition in ultrahigh vacuum during the thermal cleaning process and determine the thermal cleaning temperature by the partial pressure curves of As and Ga. It was found that the first peaks of As and Ga elements both appeared after heating about one hour, accompanied with H<sub>2</sub>O, N<sub>2</sub>/CO, CO<sub>2</sub> and other common gas. According to partial pressure curves of H<sub>2</sub>O, N<sub>2</sub>/CO, CO<sub>2</sub> and the heating time, it could be judged that the temperature at that time was not high, which should be under 150°C.After thermal cleaning experiment of three GaAs photocathodes, it was found that the peak value of As partial pressure at low temperature was generally within 10<sup>-11</sup>mbar~10<sup>-10</sup>mbar, and the peak value was at 10<sup>-10</sup>mbar at high temperature. Sometimes it was appeared that the peak value of As partial pressure at low temperature was even higher than the peak value at high temperature. The As volatilization phenomenon occurred at low temperature indicated that the elemental As exist on the GaAs photocathode surface or near surface after the chemical etching process, and the As could volatilize from GaAs photocathode at low temperature in the beginning of thermal cleaning. This research has guiding significance for further understanding the thermal cleaning mechanism of GaAs photocathode and improving the thermal cleaning technology.
Single-MCP Cs2Te solar blind ultraviolet image intensifier couldn’t detect weaker ultraviolet radiation, such as the
ultraviolet radiation near high-voltage wire insulating column. To increase the ultraviolet radiation gain, the double-MCP,
tri-MCP or multi-MCP units are introduced into ultraviolet image intensifiers. In this paper, two pieces of MCP are
cascaded in "V" shape as the electron multiplier of ultraviolet image intensifiers. Processed and scrubbed by the
single-MCP electron scrubbing traditional technology, the desired effect after electron scrubbing still could not be
achieved and flicker phenomenon appeared in field of view. The flicker noise appeared when the image intensifier was
working because the second MCP was not efficiently scrubbed. In order to completely scrub the two pieces of MCP
simultaneously, eliminate the flicker noise, reduce the dark current and achieve a stable MCP gain, the double-MCP
electron scrubbing method should be optimized without changing the assembly process. Combined with MCP
pre-treatment and pre-electron-scrubbing before assembled and scrubbed in “V” shape, flicker noise could be eliminated
effectively, and dark current could be lowered, which could increase the gain and get a clear field of view. Comparing
with two different methods of double-MCP electron scrubbing, either method has its own advantages and disadvantages.
Ultraviolet radiation gain can be increased from 10<sup>3</sup>~10<sup>4</sup> cd·m<sup>-2</sup>/W·m<sup>-2</sup> to 1.0×10<sup>5 </sup>cd·m<sup>-2</sup>/W·m<sup>-2 </sup>by using method of pre-treatment and pre-electron-scrubbing. With prospective ultraviolet radiation gain achieved, double-MCP Cs<sub>2</sub>Te solar blind ultraviolet image intensifier is manufactured.
In order to know more about the surface state of GaAs(100) epitaxial wafer during a storage period of two years, the
XPS analysis was carried out four times on the surface, respectively polished by chemical etching, stored in desiccator
for half a year, one year and two years. The results indicated that even after cleaned by proper etchant solutions, the fresh
surface was slightly oxidized with Ga<sub>2</sub>O<sub>3</sub>, As<sub>2</sub>O<sub>3</sub> and organic contaminant. The epi-wafer was always exposed to air during the storage period, so more and more oxides turned out. The mixed oxide layer comprised of C-OR, COOR, Ga<sub>2</sub>O<sub>3</sub>, As<sub>2</sub>O<sub>3</sub> and As<sub>2</sub>O<sub>5</sub> appeared after only half a year. In the ageing process of two years, the oxide types of gallium or arsenic did not change with stable content of Ga<sub>2</sub>O<sub>3</sub> and remarkably fluctuating relative contents of As<sub>2</sub>O<sub>3</sub> and As<sub>2</sub>O<sub>5</sub>. Based on the intensity ratio of Ga 3d-Ga<sub>2</sub>O<sub>3</sub> to Ga 3d-GaAs, the thickness of oxide layer was estimated. The oxide layer generated after chemical polishing was very thin, just only 0.435nm thick, and then it grew rapidly, approximately 1.822nm after one year while almost no change any more subsequently. It was indicated that after the epi-wafer was
stored for one year, because of volatile As<sub>2</sub>O<sub>3</sub> or As<sub>2</sub>O<sub>5</sub>, there remained a large amount of Ga<sub>2</sub>O<sub>3</sub> in oxide layer, which prevented the reactions between bulk material and oxide layer with oxygen. So native oxide layer plays a role as passive film to protect epi-wafer against the environment during a long storage period.
In order to research the influence of the quantity of the Micro-Channel Plates (MCP) on the detectable threshold
of the ultraviolet image intensifier tube, the wide spectrum image intensifier gain tester produced by Nanjing University
of Science and Technology is employed to test the relation curves between self-made one single MCP ultraviolet image
intensifier tube, two double MCP ultraviolet image intensifier tubes, and photocathode incidence radiation illumination
respectively. With reference to the 3<sup>rd</sup>-generation low-light image intensifier failure theory, if the radiation gain of the
ultraviolet image intensifier tube is defined as 1,000cd/m<sup>2</sup>, the tube will lose the effect of image intensification, when
the corresponding photocathode incidence radiation illumination will be the minimum detectable threshold. Viewed
from the test results, the minimum detectable threshold of the single MCP ultraviolet image intensifier tube is 3.0×10<sup>-6</sup>
W/m<sup>2</sup>, with the radiance gain linear interval between 3.0×10<sup>-6</sup> W/m<sup>2</sup> ～4.6×10<sup>-5</sup> W/m<sup>2</sup>; and that of the double MCP ultraviolet image intensifier tubes is 4×10<sup>-7 </sup>W/m<sup>2</sup>, with the radiance gain linear interval between 4.0×10<sup>-7 </sup>W/m2 ～2.0×10<sup>-5</sup> W/m<sup>2</sup>. The test results were analyzed on the basis of the MCP self-saturation effect, concluding that the saturation current density of the single-unit MCP is a fixed , but there may be certain difference among the saturation current density of different MCPs due to different materials and manufacturing processes. The test results show that the maximum of the radiation gain linear interval of the three ultraviolet image intensifier tubes are at the magnitude of 10<sup>-5</sup> W/m2, and the non-significant differences also verified the theory. In the double MCP ultraviolet image intensifier tubes, the photocathode-produced photocurrent is multiplied in passing the first MCP and then reaches the second MCP, so the second MCP will reach the state of current saturation earlier than the first MCP, making the minimum detectable threshold of the double MCP ultraviolet image intensifier tubes is lower than that of the single ultraviolet image intensifier tube by one order of magnitude, with the linear gain interval increasing by one magnitude, and the absolute of the corresponding radiation gain of the same radiation illumination within the linear gain interval increasing by 10
times, verifying that the double MCPs can detect much lower and weaker ultraviolet radiation and realize the high gain
theory. The research results has certain guiding effect towards the promotion and application of the double ultraviolet
image intensifier tubes, and has great significance on enhancing the high ultraviolet radiation detection and imaging