Target and Recoil System

The recoil system should measure the proton momentum and its output angle with sufficient accuracy to predict $\phi$ total energy and momentum direction to be used as a constraint in $K_S K_L$ reconstruction. This determination should be better than the overall detector resolution to be effectively used and should be an order of few percent.

Two obvious disadvantages of Be taget are: Fermi motion of nuclei in the Be atom what leads to about 200 MeV additional energy spread of secondary particles; and recoil proton should go through condense Be matiarial to be detected in the recoil system. These DE/DX losses can be minimized by making cylindrical target with small diameter.

As an alternative to Be target liquid hydrogen can be used. With density of 0.07 $g/cm^3$ the 70 cm length of $LH_2$ is needed for the same $\phi$ rate (8% rad.length). The transverse dimention can be made relatively smaller, than for Be taget and no effect from Fermi motion is expected.