Fig. (8) Magnetic spectrometer [93] with an electron orbit of 40 cm. The iron pole pieces produced the radially inhomogeneous magnetic field that yielded better focus at 180° than a homogenous field. The set of beam-defining slits made of aluminum minimized electron scattering. The instrument was adjusted for the relative momentum resolution⁸ of 0.5% and solid angle of about 0.1% of 4π. The size of radioactive sources was (0.4 x 2.5) cm². The energy scale was calibrated with photoelectrons ejected from the K-shell of a lead convertor by the 511 keV annihilation radiation.