A double-target system for precision measurements of nuclear medium effects

A double-target system has been developed for precision measurements of nuclear medium effects in unpolarized electron scattering with 4–5 GeV electron beams. This system allows for a precise comparison of elementary targets such as deuterium and hydrogen to heavy solid targets to study subtle medium effects such as color transparency, transverse momentum broadening, and hadron attenuation. One cryo-target and one solid target were located in the beam simultaneously, separated by 4 cm to minimize acceptance correction differences in the large CLAS spectrometer while maintaining the ability to identify the target event-by-event. Because both targets were positioned in the beam simultaneously, time-dependent systematic effects such as drifting gains or inefficient detector channels cancel in ratios of observables, increasing the precision of the final results. Measurements were performed with hydrogen and deuterium in combination with 3 mm diameter targets of carbon, aluminum, iron, tin, and lead. The solid targets and deuterium target were comparable in thicknesses except for specialized diagnostic runs with ultra-thin aluminum. Switching of the solid targets was performed remotely and required only a few seconds to complete. An ultra-low mass vacuum chamber made from Rohacell® foam provided vacuum isolation of the cryotarget without adding significantly to multiple scattering of final-state particles.