The waterfall foil(s) is (are) produced in a cell mounted in the standard scattering chamber of the Hall A.
The water, continuously pumped from a reservoir, goes, through an heat exchanger, into the target zone, and then back into the reservoir. All parts in contact with the water are made of stainless steel. The water pressed through a system of slits and holes forms one or more (in this case three) flat rectangular films which are stable due to the tension surface and to the adherence to stainless steel bars.
The thickness of the foil(s) is (to some extent) function of the pump speed that is of the flow rate. Once the target is formed (there is a minimum value of the pump speed/flow rate for this, depending on the particular target) the thickness increases with the pump speed up to a maximum value which depends essentially on the dimensions of the slits and holes the water passes through.
An absolute calibration has to be done for a particular target to know the thickness for a given pump speed. The raw counting rate of the detector as function of the pump speed will give the thickness of the target for any pump speed value.
The target is supposed to stay at a fixed angular position during the experiments; the waterfall target container is hence a box mm3.
The entrance and exit windows are circular (40 mm in diameter) and made of Be (75 m thick) or Ti (75 m) or stainless steel (25 m) (fig. 2.3.3.3, 2.3.3.4, 2.3.3.5). Scattered particles go through the `lateral' windows, made of kapton (75 m thick) or stainless steel (25 m thick) ???? ( 320 X 8 mm2). Some schematic views of the target container are shown in figures 2.3.3.3, 2.3.3.4, 2.3.3.5,2.3.3.6, and 2.3.3.7.
The three foils are identical, 12 mm wide, supported by poles of 2 mm by 2 mm. In the direction normal to the target, the foils are 22 mm apart. In the direction along the target, with respect to the central foil, the first one is shifted donwards by 13 mm, the third one is shifted upward by 13 mm.
The rotational axis is the intersection of the beam direction at the central foil, the distance from the rotational axis to the nearest pole side is 5 mm. The foils are parallel, the angle between beam direction and direction normal to the target is exactely 30 degrees. The tolerance of the machining is less than 0.2 mm.
Under the cell a target holder allows one to put up to 5 solid targets. The water coming back to the container goes through two cylinders in contact with the solid target holders, allowing the cooling of such targets.