This devise was developed to find the shape of the E-835 hydrogen gas jet at the anti-proton intersection point. The devise makes use of a thin needle crossing the jet section. It is based on the following principle: when the hydrogen clusters hit the needle, they break, or evaporate, scattering atoms all around the chamber and causing an increase in local pressure. The higher the rate of hits (hydrogen flux), the higher the pressure rise.
To determine the jet shape a needle sweeps across the hydrogen jet (beam) in both the x and y (orthogonal) directions. This is accomplish with an L-shaped needle at the end of a long arm inserted, perpendicularly to the jet, in the interaction area. The head supporting the needle is equipped with two kinds of movements, each controlled by a stepping motor: Movements can be made back and forth along the z axis (thus allowing for horizontal sweep of the vertical needle) or it can rotate around the z axis (thus allowing for vertical sweep of the horizontal needle).