A new design of a re-entrant cavity ionisation chamber for use in brachytherapy

A theoretical model has been developed for evaluating the response of re-entrant cavity ion chambers to various gamma energies from simple geometry considerations. The method involves evaluating the charge generated in a differential volume of the chamber and integrating this over the entire volume of the detector. The theory has been applied to two chambers (one having SS cavity wall and the other having Al cavity wall) developed earlier. The calculated and experimental sensitivities of the SS cavity chamber are 12.78 and 12.41 +/- 0.76 pA/MBq for Co-60. For the Al cavity chamber the sensitivity values for 125 1 are, respectively, 0.047 and 0.049 +/- 0.003 pA/MBq. The technique described above has been used to design and develop re-entrant cavity ionisation chambers for the assay of brachytherapy sources. The design provides adequate sensitivity to assay 37MBq of Ir-192 and good saturation at 150 V for 37 GBq of Co-60. The design ensures that the change in the chamber current remains within +/- 2.5% over a 120mm long 'sweet spot'. Since the outer housing of the chambers carries high voltage a special alumina-titania coating by plasma arc spray has been given to prevent electrical shock hazard to users. Two detectors have been developed: one with SS construction is filled with argon while the other one with aluminium construction is filled open to atmosphere. The two detectors have calculated sensitivity of 3.22 and 1.46 pA/MBq for Co-60 The corresponding experimental sensitivities are 3.3 +/- 0.2 and 1.42 +/- 0.085 pA/MBq, respectively. (C) 2007 Elsevier B.V. All rights reserved.