X-Ray Isochromats of Copper Taken in Different Directions Relative to the Cathode Stream

Variation with potential of the intensity of monochromatic x-rays of wave-lengths 0.823 to 0.247A for directions making angles of 36°, 90° and 144° with the cathode stream.—X-ray isochromats of copper were taken in three different directions relative to the cathode stream, for a target face making an angle of 25° with the cathode stream. Corrections were made for stray radiation in the neighborhood of the measured beam, for radiation due to secondary hits of cathode electrons which had been reflected backward from the focal spot at large angles, and for absorption in the target. It is established that, within experimental error, an isochromat of frequency $\nu$ varies linearly with the potential from potentials about ($\frac{5}{4}$) $H\nu$ to $2H\nu$, where $H\nu$ is the quantum voltage for excitation of frequency $\nu$. The linear portions of the graphs were extrapolated to find the intercept on the intensity axis at $H\nu$. This intercept depended on $H\nu$, and also on $\theta$, the angle between the measured x-rays and the cathode stream. For $\theta =90\mathrm{}°$ the intercepts varied between $0.062I^{\prime }$ (for $H\nu =50\mathrm{}$ kv.) and $0.085I^{\prime }$ (for $H\nu =15\mathrm{}$ kv.) with an average of $0.072I^{\prime }$; for $\theta =36\mathrm{}°$ the respective values were $0.076I^{\prime }$, $0.096I^{\prime }$, $0.086I^{\prime }$; for $\theta =144\mathrm{}°$, $-0.035\mathrm{}{I}^{\prime }$, $0.010I^{\prime }$, $-0.017\mathrm{}{I}^{\prime }$. $I^{\prime }$ is the intensity of the isochromat at $2H\nu$. Values of $H\nu$ used were 15, 20, 30, 40, 50 kv. The intercept decreased, in general, for increasing $H\nu$, but for $\theta =36\mathrm{}°$ and $\theta =90\mathrm{}°$ the change was within the experimental error of $\pm 0.01I^{\prime }$.