The RMC Liquid Protium Target

The RMC liquid protium target used for experiment E838 is a 2.7 liter chemically pure isotopically enriched hydrogen (protium). The deuterium concentrations in the target are reduced to $\sim$ 10$^{-6}$, and $Z > 1$ contaminants in the liquid protium are less than 10$^{-9}$. The cylindrical target wall and dome-shaped front face, each 254 $\mu$m thick, are made of gold. The target flask dimensions are 15 cm in length and 16 cm in diameter to ensure most of the incident 81.5 MeV/$c$ pions stop before reaching the end of the target. Surrounding the flask are two silver heat shields each 508 $\mu$m thick, and a silver vacuum vessel 1.02 mm thick. A 254 $\mu$m thick film of aluminized myler forms the upstream window of the vacuum jacket (29).

For a beam of 81.5 MeV/$c$ pions incident on this protium target, the fraction of pions that actually stop in hydrogen was obtained from Monte Carlo analysis (29), and is given by $c_{stop} = 0.85 \pm 0.01$.