Measurements have been made of the density of charged‐particle tracks in the Brookhaven 20‐in. hydrogen bubble chamber as a function of particle velocity and chamber operating conditions. The track density m is given by the function A (Ps)nβ-2 where, at 27°K, A=7.5×10-8 bubbles/cm of track, n=5.4, and Ps is the superheat pressure in psia in the chamber. The pressure superheat dependence Δm/(mΔPs)≅10% per psia; for tracks at different temperatures but with the same pressure superheat, the temperature dependence Δm/(mΔT) is in the range 5–15% per 0.1°K, over the region of normal chamber operation. Macroscopic bubbles grow at the rate 0.21 mm msec-½, until they are quenched by chamber recompression in proportion to their degree of internal energy. The measured track density dependencies are consistent with the view that bubbles originate on stopped secondary electrons produced in close collision processes, where distant collision influence is suppressed by the density effect much more for hydrogen than for the heavier bubble chamber liquids. It is shown that formation and growth of bubbles to critical size may be understood in terms of conventional fluid dynamics provided the expansion is extremely rapid. The secondary electron energies required for bubble formation at 27°K range from 80 to 292 eV depending on the superheat of the chamber.
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