A NEW EXPRESSION FOR DETERMINATION OF FLUENCES FROM A SPHERICAL MODERATOR NEUTRON SOURCE FOR THE CALIBRATION OF SPHERICAL NEUTRON MEASURINGDEVICES

A new expression modifying the inverse square law for determination of neutron fluences from spherical moderator neutron sources is reported . The formalism is based on the neutron fluence at a point outside the moderator as the summation of fluxes of two groups of neutrons: direc t neutrons from the central region of the moderator, and moderated neu trons which, to a first approximation, are scattered from the outermos t layers of the spherical moderator. The expression has been further d eveloped for spherical neutron measuring devices with an appropriate g eometry factor which corrects the reading of the device for non-unifor m irradiation of the detector. The combination of the new fluence func tion and those of the air and room scattered components introduce a ca libration model. The fluence relationship obtained for moderated sourc es may conveniently be used for calculating the more rapid change of n eutron dose at close distances than that which is based on the inverse square dependence.