Isotope calibrator ionization chamber for low energy gamma emitters

An ionization chamber with an aluminium walled re-entrant cavity and filled with argon at four atmospheres has been developed for the calibration of low energy gamma emitting radiopharmaceutical substances. It has an all-welded construction with an SS outer housing and base-plate. Explosion welded SS-aluminium clad material has been used as a buffer to weld an aluminium walled cavity to the chamber housing. The chamber can detect the 35 keV photon energy emission from 125I with a current sensitivity of 0.062 pA/MBq (2.29 pA/mCi) while a standard air-filled Type 1383A ion chamber has a corresponding figure of 0.006 pA/MBq (0.22 pA/mCi). The variation in signal due to minor displacement of the sample in the cavity is ± 2% while it is ± 10.5% in the case of the type 1383A chamber. The present chamber is compatible with the standard isotope calibrator providing direct readout of the activity under assay.