A defence of the use of the concept C-lambda for the determination of high energy photon dose.

In view of some recent publications in which the concept of Cλ seems to have been misunderstood and criticized, the basic equation for the calculation of Cλ is derived. The assumptions in this derivation are discussed and the errors which arise from them in practice are estimated. It is concluded that the use of Cλ remains the present method of choice for converting ion chamber readings into absorbed dose in water.

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