FIBEROPTIC PROTON-BEAM INTENSITY MONITOR

The thermally induced change of the birefringence of a high-birefringe nce fused-silica optical fiber, when exposed to a proton beam (2 MeV, 0.5-44 nA, 5-mm spot diameter), was measured with a.fiber-optic polari metric sensor. A short length (5 cm) of 125-mu-diam, single-mode, pola rization-maintaining optical fiber was put orthogonally to the beam in vacuo and the linear polarization of a HeNe laser beam launched into the fiber was analyzed by a polarizer and recorded by a photodetector. The response of the sensor both to a continuous and to a modulated pr oton beam was studied. The photodetector output, which varies with the cosine of the induced relative phase retardance between the fiber eig enmodes, provides a dc or an ac (10 Hz test modulation frequency) meas urement of the beam current. This device acts as a proton beam intensi ty monitor, with minimum beam perturbation and a sensitivity of 0.1 nA . Its possible use also as a beam position monitor or as a wire-scanne r beam profiler is suggested.