The Waterfall Target

The waterfall target system provides a target for experiments on $^{16}$O (see figure A.1). The conceptual design of the waterfall target system for Hall A is very similar to the one used at Saclay [13]. The thickness of the waterfall target can be modulated by changing the pump speed; this adds flexibility to the system and allows the user to choose the best value according to the wanted resolution and luminosity.

The hydrogen in the water can be used for calibration purposes. Elastic scattering from the hydrogen in the target can be used to measure and monitor the target thickness. The counting rate in the elastic peak is directly proportional to both the beam current and the target thickness.

Figure A.1: The target system.

The waterfall target can be single foil or multi-foils according to the need of the particular experiment. In fact, a modified version of this target has been built and used for experiments at NIKHEF (a $\sim 150$ mg/cm$^2$ single-foil and a three-foil one $\sim 60 \times 3$ mg/cm$^2$). The target built for Jefferson Lab Hall A for the two commissioning experiments on $^{16}$O (E89003 and E89033) is a three-foils one with thickness ranging from $\sim 130-200$ mg/cm$^2$ for each foil, depending on the pump speed.

The main components of the target system are:

1. The waterfall target container, also referred to as ``target cell'' in this document;
2. The solid target ladder and the solid targets;
3. The hydraulic system;
4. The gas system;
5. The movement system;
6. The slow-control system.

The waterfall foils are produced inside the waterfall target container, which is mounted in the standard Hall A scattering chamber.

The water, continuously pumped from a reservoir, goes through a heat exchanger into the target zone, and then back into the reservoir. All parts in contact with the water are made of stainless steel. In the target zone, the water pressed through a system of slits and holes and guided by the stainless steel bars forms one or more flat rectangular films, which are stable due to the surface tension and to the adherence to the guiding bars.

The thickness of the foil(s) is (to some extent) a function of the pump speed which determines the flow rate. Once the foil 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. The maximum pump speed depends essentially on the dimensions of the slits and holes the water passes through.

An absolute calibration of the target thickness as a function of the pump speed needs to be done before the experiment. One way of measuring the absolute target thickness is to measure the raw counting rate in the spectrometer.

The target used for E89003 and E89033 was at a fixed angle during the experiments. Therefore the waterfall target container is designed as a box with dimension of $630\times68\times8$ mm$^3$.

The entrance and exit windows of the target cell are circular (40 mm in diameter) and are made of gold-plated Be (75 $\mu$m thick) (Fig. A.2, A.3). Scattered particles go through the `lateral' windows, made of stainless steel (25 $\mu$m thick, dimension of  320 X 8 mm$^2$). Some schematic views of the target container are shown in Figures A.2, A.3, A.4 and A.5.

Figure A.2: A cutaway of the waterfall target cell.

Figure A.3: Detailed view of the 3-waterfoils geometry.

Figure A.4: Cutaway view of the waterfall target container.

Figure A.5: Side view of the waterfall target container.

The three foils are parallel and identical. Each foil is 12 mm wide, guided by two poles each of 2 mm by 2 mm cross-section. In the direction normal to the target, the foils are 22 mm apart. The normal direction of each foil is 30$^o$ with respect to beam line, and the normal direction points towards H-arm. The center of each foil is shifted 1 mm along the foil direction and towards E-arm, see Figure A.3. The tolerance of the machining is less than 0.2 mm.

A solid target ladder is attached to the bottom of the waterfall target container, which holds up to 5 solid targets. The water coming back to the container goes through two cylinders in contact with the side of solid target holders, providing some cooling for solid targets.