A high resolution bremsstrahlung monochromator for photo-nuclear experiments

Abstract An improved bremsstrahlung monochromator, based on the original University of Illinois design, is being used with the Illinois C.W. microtron for (γ, γ) and (γ, f) measurements. The monochromator includes an electron energy disperser followed by an electron energy analyser operating in the energy loss mode. The disperser, which includes two quadrupoles and one 90° n = 0.5 dipole, focuses a demagnified image of the electrons transmitted by a 2 mm defining slit onto a thin Ni foil, termed the radiator. Electrons scattered by the radiator are analysed by a 180° inclined-plane flat pole-piece magnetic spectrometer and are detected in the focal plane by a hodoscope consisting of a 72-wire proportional gas-counter backed by a 12 segment plane of Ne-102 plastic scintillators. Bremsstrahlung emitted by the Ni foil is incident on a target and photons or fission particles emitted by the target are detected in coincidence with pulses from the scintillators and wire counter. The limit of energy resolution, determined primarily by the 3 mm wire spacing, is 14 keV for 3.5 MeV electrons. The limit of resolving time is set by the 3.5 ns jitter of the plastic scintillator-photomultiplier system. The factors which have contributed to improved resolution are: the use of a proportional gas-counter with closely spaced wires, the use of a reduced image of the defining slits on the radiator, the use of thin radiator foils, 5.4 mg/cm 2 , and the location of the radiator close to the minimum gap between the pole-pieces. The performance of the monochromator is illustrated by data from the resonance fluorescence of 208 Pb and the photofission of 232 Th.