1. This work was supported by the U.S. Atomic Energy Commission. We wish to acknowledge gratefully the constructive comments and sug- gestions of R. S. Benua M. Berman A. M. Brues E. M. Chapman V. P. Dole D. D. Dziewiatkowski G. Failla L. E. Farr R. J. Hasterlick I. London H. G. Parker E. H. Ouimby J. E. Rall and J. S. Robertson.
2. G. J. Hine and G. L. Brownell Radiation Dosimetry (Academic Press New York 1956).
3. L. B. Marinelli , E. H. Quimby , G. J. Hine , Am. J. Roentgeriol. Radium Therapy 59 , 260 ( 1948 ).