We will deal with the production and detection of subatomic particles, such as electrons and nuclei. As these interact with matter, including air, many of the experiments need to be carried out inside glass tubes out of which the air has been pumped.
The normal atmospheric pressure at sea level is right around 1.0 atmosphere, or 760 Torr. The composition of air is approximately 78% nitrogen and 21% oxygen, with the remainder consisting of a mix of carbon dioxide, water vapor, and other trace gases. At atmospheric pressure, there are roughly 1019 of these molecules per cubic centimeter, which makes it very difficult for a subatomic particle to travel even a short distance without hitting an atom. The average distance that a particle can travel before colliding with other particles is known as the mean free path. However, this number depends very much on the particles involved. For example, at 1 Torr, an air molecule can go about 0.05 mm before colliding with another molecule, but an electron can travel about 1 cm before hitting an air molecule. At a pressure of 1 mTorr, these distances increase to 5 cm and 9 m, respectively.
The objective of running subatomic particle experiments in a vacuum is exactly to prevent collisions between the particles and air. The depth of the vacuum needed depends on many factors, including the type of particles being studied, the number of collisions that will not affect measurements too much, and the physical size of the apparatus.
Table 3 shows the conventional ranges into which vacuum levels have been divided. These ranges are not standard, and each technical organization or country defines them differently. However, this table should give you an idea of the approximate levels used for different applications.
TABLE 3 Pressure within Various Vacuum Rangesa
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