Several Environmental Factors that Affect PMT Performance (part3) - Air Pressure and Helium Gas

The previous two parts introduced the effects of temperature, humidity and magnetic field on the performance of photomultiplier tubes (PMTs). Let's talk about how air pressure and helium gas affect the use of photomultiplier tubes.

1. Air pressure

The photomultiplier tubes are not only used at 1 atmosphere (1 atm.≈0.1Mpa) but also likely to be used at negative pressure environment such as aircraft and artificial satellites, and also underwater high pressure environment such as proton decay experiments and neutrino experiments.

Due to the inside vacuum of 10^-5pa and the sufficient distance between the electrode and the lead wire, no discharge occurs inside the PMT under normal conditions. The design of base and socket can also ensure no discharge occurring in environment at 1 atm. or in vacuum. However, the PMTs tend to discharge most likely at pressure between 100 to 1, 000pa. Therefore, when PMTs are used in negative pressure, the following precautions should be taken:

Ensure that a sufficient vacuum level is obtained (lower than 0.1pa) before applying high voltage gradually (voltage gradually increased from low to high).

When using PMTs with base, drilling holes in the base to ensure no pressure difference between the inside and outside of the base.

2.Helium gas

As we all know, helium gas can penetrate glass. If a PMT is placed in environment with helium gas, the helium gas will penetrate the glass and enter into the PMT. The vacuum level will decrease and the dark current will increase accordingly, leading to discharge and the end of PMT life. The permeability of helium gas depends on glass materials and storage temperatures. Quartz has a relatively high permeability for helium gas among glass materials. For example, if a PMT with quartz shell is placed in the environment of helium gas at 1 atm., its after-pulse will increase sharply after about 30 minutes so that the PMT can’t work normally anymore. Thus, when a PMT is used for detecting helium gas, the design of helium gas isolation should be considered to ensure the PMT life. In order to reduce the effects of helium gas, it is better to store the PMT in environment without helium gas, especially the PMTs with quartz shells.

Above environmental factors about PMT are shared. If you encounter any problems during the use of PMTs, please consult BHP.