Density Changes in Low Pressure Gas Targets for Electron Scattering Experiments.

A system of modular sealed gas target cells has been developed for use in electron scattering experiments at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). This system was initially developed to complete the $MARATHON$ experiment which required, among other species, tritium as a target material. The system has been used in several of the 12 GeV era experiments in Experimental Hall A using the Jefferson Lab Continuous Electron Beam Accelerator Facility (CEBAF). Thus far, the cells have been loaded with the gas species $^{3}$H, $^{3}$He, $^{2}$H, $^{1}$H and $^{40}$Ar and operated in nominal beam currents of up to 22.5~$\mu$A. Each cell is 25 cm long with a diameter of 1.3 cm. While gas density of the cells at the time of loading is known, the density of each gas varies uniquely when heated by the electron beam. To extract experimental cross sections using these cells the beam current dependent density of each target fluid must be determined. In this study, data from measurements with several beam currents within the range of 2.5 to 22.5 $\mu$A on each target fluid are presented. Additionally, expressions for the beam dependent fluid density of each target are developed.