X-Ray Isochromats of Molybdenum

Abstract

Variation of intensity of x-radiation from molybdenum with voltage, for wave-lengths 0.246 to 123 A.—The ordinary x-ray spectrum, intensity as function of wave-length, cannot be interpreted in terms of energy because of the unknown variation of several factors with frequency. In the case of isochromats, intensity per cathode ray for a given wave-length as function of voltage, the problem is simpler. Measurements up to twice the quantum voltage were made in each case except where that would exceed 70 kv. and both voltage and current were determined to 1/10 per cent. The angle between the x-rays and the beam of cathode rays was about 110°. To avoid correcting for the variation of size of focal spot, a single narrow slit was used near the crystal. The correction for crystal scattering was determined from measurements on each side of the reflected beam; it was less than one per cent. except near the quantum voltage. The correction for target absorption was taken to be $\mu \overline{x}$ where $\overline{x}$, the mean depth of penetration, was computed from the data reported in the preceding article. After making these corrections, the isochromats were found to be straight lines except for some downward concavity near the feet. The results for 0.41 to 1.23 Å agree within 1 or 2 per cent with an empirical equation for isochromats which is given. The values for the shorter wave-lengths are too unreliable at high voltages to serve as tests. From the above equation, an expression for the radiation from $a$ vanishingly thin target is derived which involves $b$ the coefficient of the Thomson-Whilddington law. If this law and the $V^2$ law for total intensity are correct, the probability of emission of a quantum with frequency between $\nu$ and $\nu +d\nu$ in distance $\mathrm{ds}$ is $\frac{\mathrm{idsd}\nu }{h\nu }$, where idsdv is the intensity per cathode ray, with the thin target.