Table 1.

Relationship of cavity-chamber and free-air-chamber source measurements, and ratios of collimated beam calibrations

Ratios of cavity-chamber exposure rate to free-air-chamber exposure rate. 60Co and 137Cs source measurements.			Ratio of weighted mean exposure rate to exposure rate based on cylindrical chamber measurements with radiation incident ⟘ to chamber axis. Calibration change in 1961.	Ratio of new weighted mean exposure rate, based on measurements with six spherical chambers, to 1961 weighted mean. Calibration change in
	Cylindrical chamber. Radiation incident ⟘ to chamber axis. (1959)	Weighted mean of cylindrical and small spherical chamber data. Cylindrical chamber axis at 45° to incident radiation. (1961)
Source				May 1972	July 1974*
60Co	1.003	1.019	1.016	0.993	0.993
137Cs	0.984	0.994	1.010	0.994	0.992
(1)	(2)	(3)	(4)	(5)	(6)

^*All new data provided in this publication reflect the July 1974 values.