High-temperature 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 high-temperature PMTs changes slightly when the temperature rises after going down the well. Most of the reported references focus on back-end circuit optimization, and there are few studies on the improvement and mechanism of count rate fluctuation of the high-temperature PMTs. To improve the count rate stability of high-temperature PMTs, we propose the thermal cycling post-treatment method. The plateau curves and stability curves of the original tube and the tube after thermal cycling post-treatment are compared, and the internal mechanism of stability improvement is analyzed and revealed.

The plateau characteristic curves of PMTs without and after thermal cycle post-treatment 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 post-treatment. To explore the effect of thermal cycling post-treatment on the thermal stability of high-temperature PMTs, we record the counting rate curves of two tubes without and after thermal cycling post-treatment 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 post-treatment.

Compared with the room temperature, the optimal plateau area of the pristine high-temperature 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 high-temperature PMTs at high temperature after thermal cycling post-treatment 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 high-temperature 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 post-treatment can reduce the ion feedback dark noise caused by the evaporation of the residual alkali metal on the tube wall or pin in high-temperature 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 post-treatment, thus reducing the amount of residual alkali metal in the tube. The count rate stability after thermal cycling post-treatment is comparable with like products from abroad.

Given count rate instability caused by high temperature, a simple and effective strategy called thermal cycling post-treatment is proposed to improve the stability of high-temperature PMTs and thus ensure oil logging accuracy. The test results show that PMTs after thermal cycling post-treatment 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 post-treatment is expected to accelerate high-temperature PMTs toward practical applications.