热循环后处理提升高温光电倍增管计数率稳定性分析

Analysis of Count Rate Stability Improvement of High-Temperature Photomultiplier Tubes Based on Thermal Cycling Post-Treatment

计数稳定的高温光电倍增管能保证石油测井的准确性,针对高温光电倍增管的计数率不稳定性,通过引入热循环后处理方法,从多方面大幅提升高温光电倍增管的计数率稳定性。与常温相比,高温下的原始光电倍增管的最佳坪区向右偏移50 V、坪斜增大约117%、高温与常温的计数比在84.5%~92.5%范围内,而经过热循环后处理的光电倍增管在高温下的最佳坪区不发生移动、坪斜仅增大35.7%、高温与常温的计数比不低于94.1%。此外,改进后的光电倍增管在175℃高温下工作400 h后,其计数率仍能保持在初始值的97.0%以上。通过分析高温下暗噪声随电压变化曲线以及高压下暗噪声随温度变化曲线,可以推测热循环后处理能降低残余在管壁或管脚的碱金属在高温条件下蒸发时所带来的离子反馈暗噪声,从而提升计数率稳定性。光谱响应曲线的结果表明,管内残余的碱金属在热循环后处理时与光电阴极发生反应,使得管内残余碱金属量变少。所提出的热循环后处理方法简单有效,有望加快高温光电倍增管迈向实际应用的进程。

Objective Hightemperature photomultiplier tubes(PMTs)play an important role in oil logging,geological exploration,aerospace,and other applications.By taking oil logging for example,count rate stability is crucial to oil exploration accuracy,especially when the gamma rays emitted by natural radionuclides are weak.In practical applications,it is required that the count rate of hightemperature PMTs changes slightly when the temperature rises after going down the well.Most of the reported references focus on backend circuit optimization,and there are few studies on the improvement and mechanism of count rate fluctuation of the hightemperature PMTs.To improve the count rate stability of hightemperature PMTs,we propose the thermal cycling posttreatment method.The plateau curves and stability curves of the original tube and the tube after thermal cycling posttreatment are compared,and the internal mechanism of stability improvement is analyzed and revealed.Methods The plateau characteristic curves of PMTs without and after thermal cycle posttreatment measured at 25 and 175℃are tested,which can obtain the stability difference between room temperature and high temperature,along with the change after thermal cycling posttreatment.To explore the effect of thermal cycling posttreatment on the thermal stability of hightemperature PMTs,we record the counting rate curves of two tubes without and after thermal cycling posttreatment at three working voltages at 175℃for more than 400 h.The curve of dark count rate versus voltage at a constant high temperature of 175℃and the curve of dark count rate versus temperature at a constant high voltage of 1900 V are measured respectively.Additionally,the spectral response curves of the two tubes are analyzed to reveal the internal mechanism for the reduction in hot electron emission and residual alkali metal after thermal cycling posttreatment.Results and Discussions Compared with the room temperature,the optimal plateau area of the pristine hightemperature PMTs at high temperature increases by 50 V,the plateau slope rises by about 117%,and the count rate with the normal temperature is between 84.5%and 92.5%.By contrast,the optimal plateau area of the hightemperature PMTs at high temperature after thermal cycling posttreatment does not move,the plateau slope only grows by 35.7%,and the count rate with the room temperature is not less than 94.1%.Additionally,the count rate of the improved hightemperature PMTs can still maintain more than 97.0%of the initial value after working at 175℃for 400 h.Analysis of the curve of dark noise with voltage at high temperature and the curve of dark noise with temperature at high voltage indicates that the thermal cycling posttreatment can reduce the ion feedback dark noise caused by the evaporation of the residual alkali metal on the tube wall or pin in hightemperature conditions,which thereby improves count rate stability.The results of analyzing the spectral response curve reveal that the residual alkali metal in the tube reacts with the photocathode after thermal cycling posttreatment,thus reducing the amount of residual alkali metal in the tube.The count rate stability after thermal cycling posttreatment is comparable with like products from abroad.Conclusions Given count rate instability caused by high temperature,a simple and effective strategy called thermal cycling posttreatment is proposed to improve the stability of hightemperature PMTs and thus ensure oil logging accuracy.The test results show that PMTs after thermal cycling posttreatment are improved in many aspects.By analyzing the curve of dark noise versus voltage at high temperature and the curve of dark noise versus temperature at high voltage,we can infer that the strategy can improve the count rate stability by reducing the ion feedback dark noise caused by unstable residual alkali metal.The internal mechanism reveals that residual alkali metal reacts with the photocathode and stabilizes after many high and low temperature cycles.Therefore,the proposed thermal cycling posttreatment is expected to accelerate hightemperature PMTs toward practical applications.