1. L. Hand andC. Schaerf:Phys. Rev. Lett.,6, 229 (1961).
2. In the previous experiment the gamma-ray energy resolution was under-estimated. Its real value was not 15 MeV as quoted but 20 MeV.
3. K. L. Brown:Rev. Sci. Instr.,27, 615 (1956).
4. H. Fisher andC. Schaerf:Rev. Sci. Instr.,35, 615 (1964).
5. Dr.E. A. Allton kindly provided an improved version of the program described byR. A. Alvarez jr.:Thick Radiator Bremsstrahlung, HEPL-228.