Calibration of a vacuum gage by measuring collision cross sections of a molecular beam and a gas target

In the molecular beam technique, knowledge of the buffer gas pressure is necessary to determine the absolute scattering cross section, while a knowledge of the absolute scattering cross section enables the absolute pressure to be determined. A vacuum gage is calibrated by measuring the cross section for a beam of gas A in buffer gas B for which the gage is calibrated already. By reversing the roles of gases A and B, one can then calibrate the gage for gas A.