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13	13	The neutral particles then enter our gas cell which has a length of 78.7 cm, beginning at the Quad.
14	14	The entrance aperture is a circular 1.6 cm diameter tube. He gas is let in via a leak valve. Typically
15		2x10^-4^ Torr of Helium pressure is used, while base pressure is ~3x10^-7^ Torr.
	15	2x10^-4^ Torr of Helium pressure is used, while base pressure is ~3x10^-7^ Torr. The pressure is meausred by a hot ion guage. In order to count the amount of H,,2,, molecules we need to know what portion of these are stripped when passing through the gas. This is done by using measured stripping cross sections and determining our He column density through the gas cell. The cross section is 1.04x10^-16^ cm^2^ (Browning et al, J Phys B, 1970).
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