  A prototype multiwire proportional chamber (MWPC) was developed to
demonstrate the feasibility of constructing a radiopure time projection chamber
with MWPC track readout to assay materials for alpha- and beta-emitting surface
contaminants for future rare-event-search experiments as well as other
scientific fields. The design features and assembly techniques described here
are motivated by the position and energy resolution required to reconstruct
alpha and beta tracks while efficiently rejecting backgrounds. Results from a
test setup using an $^{55}$Fe x-ray source indicate excellent operational
stability and a near-ideal energy resolution of 15.8% FWHM at 5.89 keV and a
gas gain of $\sim$10$^{4}$.
