  We propose a new type of momentum spectrometer, which uses the RxB drift
effect to disperse the charged particles in a uniformly curved magnetic field.
This kind of RxB spectrometer is designed for the momentum analyses of the
decay electrons and protons in the PERC (Proton and Electron Radiation Channel)
beam station, which provides a strong magnetic field to guide the charged
particles in the instrument. Instead of eliminating the guiding field, the RxB
spectrometer evolves the field gradually to the analysing field, and the
charged particles can be adiabatically transported during the dispersion and
detection. The drifts of the particles have similar properties as their
dispersion in the normal magnetic spectrometer. Besides, the RxB spectrometer
is especially ideal for the measurements of particles with low momenta and
relative large incident angles. We present a design of the RxB spectrometer,
which can be used in PERC. The resolution of the momentum spectra can reach
14.4 keV/c, if the particle position measurements have a resolution of 1 mm.
