Method for preparing transmission-type polyalkali photocathode by automatically baking residual alkali in combination with photocurrent monitoring

Abstract

The invention discloses a method for preparing a transmission-type polyalkali photocathode by automatically drying residual alkali in combination with photocurrent monitoring. According to the method, through photocurrent feedback closed-loop control, photocurrent real-time monitoring and a residual alkali drying process are linked for the first time, and dynamic optimization of an activation state is realized; staged self-adaptive baking is adopted, baking stages are automatically switched based on photocurrent change characteristics, and over-baking or under-baking is avoided; an adsorption inhibition design is adopted, and the influence of residual alkali metal on the cathode is reduced through double means of temperature gradient and electric field regulation and control. The transmission-type polyalkali photocathode prepared by the invention has the advantages that the performance is improved, the sensitivity is improved by 40%, the sensitivity attenuation rate is less than or equal to 5% after the transmission-type polyalkali photocathode is stored for 3 months, and the residual alkali time is shortened by 20-40%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photomultiplier tube (PMT) manufacturing, and particularly relates to a preparation process optimization of a transmission type multi-alkali photocathode (Na-K-Cs-Sb), and especially aims to the problems of insufficient activation of cathode sensitivity and poor long-term stability, and the method for preparing the transmission type multi-alkali photocathode through the synergistic effect of an automatic residual alkali baking process and real-time monitoring of photoelectric current can realize the improvement and stability of the cathode performance. BACKGROUND

[0002] The transmission type multi-alkali photocathode photomultiplier tube is a kind of high-sensitivity photoelectric detection device, the core of which adopts a transmission type photocathode composed of multi-alkali metal antimonide (such as Na-K-Cs-Sb), has wide spectral response and high quantum efficiency, and is widely used in fields such as particle physics detection, fluorescence analysis, laser radar and astronomical observation, and is especially suitable for weak light signal detection scenes. Among them, the cathode sensitivity is a key performance index of the transmission type multi-alkali photocathode photomultiplier tube. The cathode sensitivity refers to the ability of the photocathode itself to convert the incident light radiation into a photoelectric current, which is specifically manifested as the integral sensitivity of the photocathode under specific working conditions, reflects the photoelectric emission efficiency of the photocathode material, and the physical meaning is the photoelectric current excited by unit light flux (unit: lumen, lm). It is believed in the past that the improvement of the cathode sensitivity mainly depends on the multi-alkali cathode manufacturing process and related process procedures, including antimony evaporation, alkali metal evaporation, and multiple antimony-alkali alternation. After the process is completed, the photomultiplier tube is sealed. However, at this time, the cathode sensitivity of the photomultiplier tube may still not reach the best state of exhaust activation, and even the sensitivity may decrease after long-term storage due to the fatigue effect, mainly due to the adsorption of residual alkali metal to the cathode surface. At present, the adsorption of residual alkali metal in the production process of the multi-alkali photocathode photomultiplier tube is obviously lacking in attention, which not only leads to the multi-alkali photocathode photomultiplier tube not being effectively activated to realize high-sensitivity performance, but also causes the cathode sensitivity to decrease or even fail in the use process. SUMMARY

[0003] Purpose of the Invention: To address the issues of suboptimal cathode sensitivity in transmission-type multi-alkali cathode photomultiplier tubes after sealing and sensitivity degradation after long-term storage, this patent proposes a method for improving the sensitivity and stability of transmission-type multi-alkali cathodes by combining automatic residual alkali drying with photocurrent monitoring. In the actual fabrication process of transmission-type multi-alkali cathode photomultiplier tubes, after the alternation of antimony and alkali, residual alkali metals remain. These residues adsorb onto the cathode surface after the tube cools to room temperature, reducing cathode sensitivity, and diffuse into the photocathode, continuing to affect it. This patent addresses these issues by setting an automatic baking program with an exhaust platform, combined with a photocurrent monitoring system, and setting a photocurrent threshold to dynamically regulate the chemical equilibrium state of the photocathode. Through the combination of automated processes and real-time monitoring, the following objectives are achieved: 1. Improving initial cathode sensitivity; 2. Suppressing sensitivity decay after long-term storage; 3. Eliminating the negative impact of residual alkali metals on the uniformity of photocathode sensitivity.

[0004] Core technology: To achieve the above objectives, the technical solution adopted by this invention is as follows: A method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring includes the following steps: Step 1: Pre-treatment and vacuum packaging (1) After completing the cesium-antimony alternating activation process of the cathode, the photomultiplier tube is cooled down, but it is not sealed off and is placed in the baking chamber of the exhaust platform. (2) Evacuate the gas to reduce residual gas, and start the residual gas analyzer to monitor the composition of the released gas; Step 2: Start the photocurrent monitoring system (1) Irradiate the cathode with a light source of a certain brightness; (2) A high-precision galvanometer is used to collect photocurrent signals in real time; (3) The data acquisition system plots the photocurrent-time curve and calculates the rate of change dI / dt to determine the cathode activation state; Step 3: Execute the adaptive staged residual alkali drying process based on the photocurrent monitoring system in Step 2. Phase 1: Low-temperature baking. When the RGA detects that the H2O or CO concentration in the pipeline has dropped to the threshold, the temperature is increased to Phase 2. Phase Two: Medium-temperature baking. When the photocurrent rises to 70-80% of its peak value, Phase Three begins. Phase 3: High-temperature baking. When dI / dt approaches zero, heating is immediately stopped and the cold trap is activated. Step 4: Adsorption Inhibition Measures (1) After baking, apply a reverse bias voltage between the cathode and the first multiplier electrode for a period of time to drive away the residual alkali metal; (2) By setting a semiconductor refrigeration low-temperature cold trap at the lower end of the exhaust pipe, the probability of alkali metal adsorption is further reduced.

[0005] As a preferred solution, the method for preparing a transmission-type multi-alkali photocathode by combining automatic baking of residual alkali with photocurrent monitoring described above comprises the following steps: Step 1: Pretreatment and vacuum packaging (1) After the cesium-antimony alternate activation process of the photocathode is completed, the photomultiplier tube is quickly reduced to below 100°C, but is not sealed from the photomultiplier tube, and is continuously placed in the baking cavity of the exhaust station; (2) Vacuum extraction to ≤1×10 -6 Pa, to reduce residual gas, and start the residual gas analyzer to monitor the released gas composition.

[0006] Step 2: Start of the photocurrent monitoring system (1) Irradiate the photocathode with a light source of a certain brightness; (2) Real-time acquisition of the photocurrent signal by a high-precision ammeter; (3) The data acquisition system draws a photocurrent-time curve and calculates the change rate dI / dt to determine the activation state of the photocathode; Step 3: Perform adaptive staged baking of residual alkali according to the photocurrent monitoring system of Step 2 Stage One: Low-temperature baking. When the RGA detects that the concentration of H2O or CO in the pipeline has dropped to <5×10 -8 Pa, increase the temperature to Stage Two; Stage Two: Medium-temperature baking. When the photocurrent rises to 75%-80% of the peak value, proceed to Stage Three; Stage Three: High-temperature baking. When dI / dt approaches zero, it is determined to be the optimal activation point, immediately stop heating and start the cold trap; Step 4: Adsorption inhibition measures (1) Apply a reverse bias of -300 V to -350 V for 10-30 minutes after the baking is completed to drive away the residual alkali metal; (2) Further reduce the adsorption probability of alkali metal by setting a semiconductor refrigeration low-temperature cold trap of 0-10°C at the lower end of the exhaust pipe.

[0007] The core technology of the present application is: 1. Dynamic photocurrent monitoring system: During the baking of residual alkali, irradiate the photocathode with a light source of a certain brightness, and monitor the photocurrent response in real time to establish a mapping relationship between the photocurrent and the baking parameters.

[0008] Determine the activation state of the photocathode by the change rate of the photocurrent, for example: Photocurrent rising stage: alkali metal migrates to the surface of the photocathode, and the quantum efficiency improves; Photocurrent peak stage: optimal activation state; Photo-current drop phase: Over-baking leads to cathode structure damage.

[0009] 2. Adaptive baking process: Temperature control in stages: First stage (low-temperature baking, 100-150℃): Remove volatile residual gases (such as H2O, CO); Second stage (medium-temperature baking, 150-200℃): Promote alkali metal atoms to diffuse to the cathode surface; Third stage (high-temperature baking, 200-250℃): Combine photo-current feedback to dynamically adjust temperature and time, lock the best activation point, and then quickly cool down.

[0010] Vacuum environment optimization: The vacuum degree of the baking chamber is maintained at ≤1×10⁻ 6 Pa, inhibit gas re-adsorption.

[0011] 3. Residual alkali metal adsorption inhibition technology: A low-temperature cold trap is installed at the lower end of the exhaust pipe to guide the residual alkali metal to migrate in the direction of the exhaust pipe through the temperature gradient difference; Electric field assisted adsorption control is used to guide the residual alkali metal to migrate to the dynode and anode direction by applying a reverse bias.

[0012] Beneficial effects: The innovation of the present invention is: 1. Photo-current feedback closed-loop control: Real-time monitoring of photo-current is first linked with baking process to realize dynamic optimization of activation state; 2. Adaptive baking in stages: Automatically switch baking stages based on photo-current change characteristics to avoid over-baking or under-baking; 3. Adsorption inhibition design: Reduce the impact of residual alkali metal on the cathode through temperature gradient and electric field regulation.

[0013] 4. The performance of the transmission type multi-alkali photocathode prepared by the present invention is improved, the sensitivity is increased by 40%; the sensitivity attenuation rate is ≤5% after storage for 3 months. The baking time is shortened by 20%-40%. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The exhaust station is combined with photo-current monitoring function and automatic baking function.

[0015] Figure 2 The photo-current-time curve is shown in the schematic diagram.

[0016] Figure 3 The automatic program temperature rise curve is shown in the schematic diagram. DETAILED DESCRIPTION

[0017] Hereinafter, the present application will be described in detail with reference to the accompanying drawings and embodiments. It is to be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0018] Embodiment 1

[0019] 1. A method for preparing a transmission-mode multi-alkali photocathode by automatic baking residual alkali combined with photocurrent monitoring, comprising the following steps: Step 1: Pretreatment and vacuum packaging (1) After completing the cesium-antimony alternate activation process of the photocathode, the photomultiplier tube is quickly reduced to below 100°C, but is not sealed from the photomultiplier tube, and is continuously placed in the baking cavity of the exhaust platform ( Figure 1 ); (2) Vacuum extraction to ≤1×10 -6 Pa, reducing residual gas, and starting a residual gas analyzer (RGA) to monitor the released gas composition.

[0020] Step 2: Photocurrent monitoring system starts (1) Irradiate the photocathode with a light source of a certain brightness; (2) Real-time acquisition of photocurrent signals by a high-precision ammeter (resolution 0.1 μA); (3) The data acquisition system draws a photocurrent-time curve ( Figure 2 ), and calculates the change rate (dI / dt).

[0021] Step 3: Self-adaptive baking residual alkali is executed As shown in Figure 3 , it is mainly divided into three stages: Stage one (low-temperature baking): when the RGA detects that the concentration of H2O or CO in the pipeline decreases to a threshold value (such as <5×10 -8 Pa), it is warmed up to stage two; Stage two (medium-temperature baking): when the photocurrent rises to 80% of the peak value, it enters stage three; Stage three (high-temperature baking): when dI / dt approaches zero, it is determined as the best activation point, and immediately stops heating and starts the cold trap.

[0022] The specific process parameters are shown in Table 1: Table 1 Specific parameters of self-adaptive baking residual alkali process Stage Temperature (°C) Time (min) Termination condition Stage one 120±10 20-30 H2O partial pressure < 5 x 10 -8 Pa Stage two 180±10 20-30 80% of the peak photocurrent Stage three 230±10 Dynamic adjustment dI / dt < 0.1 μA / min Step 4: Adsorption inhibition measures (1) After the baking is completed, a reverse bias voltage (-300 V to -350 V) is applied between the photocathode and the first dynode for 10 minutes to drive away the residual alkali metal; (2) By setting a semiconductor refrigeration low-temperature cold trap (0-10℃) at the lower end of the exhaust pipe, the probability of alkali metal adsorption is further reduced.

[0023] Performance detection of example 2 The performance of the multi-alkali photomultiplier prepared by the method of the present application is compared with that of the multi-alkali photomultiplier prepared by the prior art (post-sealing residual alkali baking) as shown in Table 2.

[0024] Table 2 Tube number Treatment Sensitivity Sensitivity after 3 months Bake-out time J0085 Post-seal bake-out 122 μA / lm 112 μA / lm 1h J0086 Automatic bake-out + photocurrent monitoring 171 μA / lm 166 μA / lm 0.6 h - 0.8 h The performance comparison results in Table 2 show that the performance of the photocathode is improved: the sensitivity is improved by 40%; the sensitivity attenuation rate after storage for 3 months is ≤5%. The process efficiency: the residual alkali baking time is shortened by 20%-40%.

[0025] The method of the present application can also be extended to the process optimization of other alkali metal antimonide cathodes (such as K2SbCs cathode).

Claims

1. A method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring, characterized in that, Includes the following steps: Step 1: Pre-treatment and vacuum packaging (1) After completing the cesium-antimony alternating activation process of the cathode, the photomultiplier tube is cooled down, but it is not sealed off and is placed in the baking chamber of the exhaust platform. (2) Evacuate the gas to reduce residual gas, and start the residual gas analyzer to monitor the composition of the released gas; Step 2: Start the photocurrent monitoring system (1) Irradiate the cathode with a light source of a certain brightness; (2) A high-precision galvanometer is used to collect photocurrent signals in real time; (3) The data acquisition system plots the photocurrent-time curve and calculates the rate of change dI / dt to determine the cathode activation state; Step 3: Execute the adaptive staged residual alkali drying process based on the photocurrent monitoring system in Step 2. Phase 1: Low-temperature baking. When the RGA detects that the H2O or CO concentration in the pipeline has dropped to the threshold, the temperature is increased to Phase 2. Phase Two: Medium-temperature baking. When the photocurrent rises to 70-80% of its peak value, Phase Three begins. Phase 3: High-temperature baking. When dI / dt approaches zero, heating is immediately stopped and the cold trap is activated. Step 4: Adsorption Inhibition Measures (1) After baking, apply a reverse bias voltage between the cathode and the first multiplier electrode for a period of time to drive away the residual alkali metal; (2) By setting a semiconductor refrigeration low-temperature cold trap at the lower end of the exhaust pipe, the probability of alkali metal adsorption is further reduced.

2. The method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring according to claim 1, characterized in that, Includes the following steps: Step 1: Pretreatment and Vacuum Packaging (1) After completing the cesium-antimony alternating activation process of the cathode, the photomultiplier tube is rapidly cooled to below 100°C, but the photomultiplier tube is not sealed off and is placed in the baking chamber of the exhaust platform. (2) Evacuate to ≤1×10 -6 Pa, reduce residual gas, and start the residual gas analyzer to monitor the composition of the released gas; Step 2: Start the photocurrent monitoring system (1) Irradiate the cathode with a light source of a certain brightness; (2) A high-precision galvanometer is used to collect photocurrent signals in real time; (3) The data acquisition system plots the photocurrent-time curve and calculates the rate of change dI / dt to determine the cathode activation state; Step 3: Execute the adaptive staged residual alkali drying process based on the photocurrent monitoring system in Step 2. Phase 1: Low-temperature baking, when RGA detects that the H2O or CO concentration in the pipeline has decreased to <5×10⁻⁶. -8 At Pa, the temperature is increased to stage two; Phase Two: Medium-temperature baking. When the photocurrent rises to 75%-80% of its peak value, Phase Three begins. Phase 3: High-temperature baking. When dI / dt approaches zero, it is determined to be the optimal activation point. Heating is stopped immediately and the cold trap is activated. Step 4: Adsorption inhibition measures (1) After baking, apply a reverse bias voltage of -300 V to -350 V for 10 to 30 minutes to remove residual alkali metals; (2) By setting a semiconductor refrigeration low-temperature cold trap at the lower end of the exhaust pipe to maintain 0-10℃, the probability of alkali metal adsorption is further reduced.

3. The method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring according to claim 2, characterized in that, Step 3, the adaptive phased drying of residual alkali execution parameters are as follows:

4. The method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring according to claim 3, characterized in that, Step 3, the adaptive phased drying of residual alkali execution parameters are as follows:

5. The method for preparing a transmission-type multi-alkali photocathode by automatically drying residual alkali combined with photocurrent monitoring according to any one of claims 1 to 4, characterized in that, Step 2 determines the cathode activation state by the photocurrent change rate. When the photocurrent is in the rising stage, the alkali metal migrates to the cathode surface, reduces the surface work function, enhances the photoelectric emission probability, and improves the quantum efficiency. Photocurrent at its peak: Activation of optimal state; The photocurrent is in a decreasing phase: over-baking has caused damage to the cathode structure.

6. The special tooling for the method of claim 5, characterized in that, include: Oven cavity, photocurrent monitoring and voltage system, cold trap, gas analyzer, vacuum system; The oven cavity is equipped with an alkali source and a pipe connected to the photomultiplier tube. The pipe is connected in sequence to a cold trap, a gas analyzer, and a vacuum system. The photocurrent monitoring and voltage system is connected to the photomultiplier tube. An illumination light source is provided on one side of the photomultiplier tube.

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