Determination of the exposure rate constant for 125 I using a scintillation detector

Abstract

The exposure rate constant for an 125I seed of the type used in therapeutic radiology was investigated with a scintillation spectrometer. Counting rate was converted to photon fluence incident upon the detector, and the exposure rate was calculated from the photon fluence. Corrections were required for absorption of the 125I photons in the air path, in the Al window of the NaI crystal and in the Al2O3 reflector. Corrections were required for the escape of I- K-characteristic radiation and the intrinsic peak efficiency of the crystal. The exposure rate constant in a direction perpendicular to the long axis of the seed was 1.309 R cm2/mCi h-1 .+-. 5.8%. A calculated value of 1.284 R cm2/mCi h was consistent with this value. The photon fluence as a function of angle of rotation about an axis perpendicular to the seed''s long axis was measured to estimate an average exposure rate constant for 125I seeds of 1.089 R cm2/mCi h-1.