Journal of Medical Physics
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Year : 1986  |  Volume : 11  |  Issue : 3  |  Page : 146

Procedure For Absorbed Dose Calibration And Use Of BARC Make Secondary Standard Dosemeter As Per The Recent IAEA Code Of Practice



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A Kannan


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The new code of practice by IAEA for absorbed dose measurement for photon and electron beams recommends the usage of perturbation factor calculated from proximal wall effect factors for the chamber and appropriate (S/P) water/air and (Men/P) water air values instead of earlier recommended C & CE values by ICRU. This paper describes the calibration procedure adopted at Radiological Standards Laboratory for BARC Make SSD's (Tufnol Walled). Calculated factors are given to help the Physicists to adopt the new IAEA Code of Practice. To determine absorbed dose (Dw) in water at specified depth, the required quantities are: (i) ND : Absorbed dose to air calibration factor, (usually specified at Co-60); and (ii) Pu : The factor to allow for non-water equivalence of the ion chamber, when in the user's beam. Evaluation of Pu depends upon the fraction (α) of ionisation due to electrons from the chamber wall and stopping power ration (S/p) water air for the photon energy under consideration. The paper describes the air method of determining (α) and subsequently Pu. For tufnol walled (tuf) BARC chamber, Pu's will be determined at RSL for each chamber at Co-60 since no (S/P) tuf (men/p) tuf air are recommended by IAEA code of Practice. At higher energies (other than Co60) tufnol chamber factors have been evaluated empirically using available data. These calculated values of Pu for tufnol chamber have been used to evaluate Dw at Co-60 and at other photon energies. In order to establish the validity of these empirically calculated factors, the measurement of Dw were carried out by a calibrated graphite chamber for which IAEA recommended values are used and compared with measurement with tufnol walled chamber.


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