High-temperature photomultiplier tubes (PMTs) play an important role in oil logging. Till now, there has been a lack of their research and manufacturing capabilities domestically, and a significant gap in their performance compared to the world's advanced level. Therefore, it is urgent to develop independent and controllable technology for achieving domestic alternative high-temperature PMTs. The photocathode quality exerts a decisive effect on the performance of PMTs. It is necessary to optimize the photocathodes preparation techniques for high-quality photocathodes. To develop a Na-K-Sb high-temperature cathode that can be applied to high-temperature PMTs and further improve its photoelectric performance, we compare the performance of high-temperature PMTs fabricated by mono-evaporation and co-evaporation techniques and reveal the inherent mechanism affecting the performance. Additionally, we also test and evaluate the performance of the prepared high-temperature PMTs in practical applications.

Quantum efficiency and photocathode sensitivity are tested to explore the effects of preparation techniques on the properties of high-temperature PMTs. The ability to distinguish energy peaks is studied by measuring the ¹³⁷Cs energy spectrum after coupling PMTs with NaI (Tl) scintillation crystals using a multi-channel analyzer. The plateau characteristic curves of high-temperature PMTs prepared by different techniques are measured. To explore the underlying reasons for different performances, we test spectral response curves and high- and low-temperature curves on the two high-temperature PMTs. Meanwhile, the film thickness and surface morphology information of the Na-K-Sb high-temperature photocathode prepared by the two techniques are analyzed by scanning electron microscope, which can reveal the microscopic mechanism. The high-temperature PMTs coupled with high-temperature scintillators are placed in a chamber at 175 °C to study their performance in practical applications.

The test results show that the high-temperature PMTs prepared by the co-evaporation technique exhibit better performance. Compared with the PMTs prepared by the mono-evaporation technique, the quantum efficiency is increased by 55.4% and the photocathode sensitivity is enhanced by 88.3%, with the energy resolution increasing by 15.7% and counting stability improving by 56.9%. This indicates that the proposed co-evaporation preparation technique is more suitable for preparing high-temperature PMTs with high performance. The analysis of the spectral response curve and the high- and low-temperature curves shows that the essential reason for significant performance improvement of PMTs prepared by the co-evaporation technique is that the Na-K-Sb high-temperature photocathode prepared by the co-evaporation technique has better photoelectric emission ability and weaker thermionic emission ability. The microscopic mechanism of different photoelectric emission ability and thermionic emission ability for the high-temperature photocathode is further revealed by microscopic characterization. It reveals that the Na-K-Sb high-temperature photocathode prepared by the co-evaporation technique has a more uniform thickness, a denser and smoother film, and better morphology uniformity. In practical applications, the prepared high-temperature PMTs show relatively poor energy resolution, plateau characteristics, and gain at 175 °C compared with like products internationally.

We study the effects of photocathode preparation techniques on the performance of high-temperature PMTs. The two preparation techniques for preparing high-temperature Na-K-Sb photocathodes are introduced, and their effects on the performance of PMTs are compared. The test results indicate that PMTs prepared by the co-evaporation technique have better quantum efficiency, cathode radiant sensitivity, energy resolution, and plateau characteristics. By analyzing the spectral response curve and high- and low temperature curves, we can conclude that the photocathode prepared by the co-evaporation technique has stronger photoelectric emission capacity and weaker thermionic emission ability, which is the fundamental reason for the performance improvement. Combined with the microscopic morphology characterization analysis, it is found that the photocathode film layer prepared by the co-evaporation technique is denser, smoother, and more uniform, which reveals the underlying mechanism for the different photoelectric emission and thermionic emission abilities between the two techniques. This mechanism can be applied to prepare other photocathode types, which helps promote the development of high-performance PMTs. In practical applications, the prepared high-temperature PMTs still show relatively poorer performance than like products internationally. Thus, advanced research on noise reduction of photocathode and CuBe sensitization technique is needed to improve the performance of high-temperature PMTs to the international advanced level.