The Penetration of Cathode Rays in Molybdenum

Penetration of cathode rays into a target.—(1) $X$-ray spectrum method. The farther a ray penetrates into the target the more the x-rays excited by it are absorbed by the target material before coming out. X-rays just harder than the $K$ absorption limit are absorbed more strongly than those just softer. If the intensity of the spectrum emitted, except for the characteristics, is a continuous function of the wave-length and if the discontinuity in absorption coefficient is ${\mu }_K$ and the observed ratio of intensities on each side of the discontinuity is $R$, then the logarithmic mean depth of penetration $\overline{x}={{\mu }_K}^{-1\mathrm{}}log R^{-1\mathrm{}}$, for voltages above the minimum necessary to excite radiation of the absorption limit frequency, 20 kv. in case of Mo. (2) Results for molybdenum. To reduce the effect of stray x-rays, a calcite crystal was used and narrow slits which limited the incident beam to 8′ 30″. The voltages needed to make the mean depth of excitation of this frequency equal to 1, 2 and 3 μ were found to be about 24, 44 and 67 kv. respectively. The increased difficulty of penetration beyond the first micron is probably due to the deflection of the cathode rays by the atoms.