K X-ray yields from elements of low atomic number

Abstract

Absolute measurements have been made of the K X-ray quantum yields resulting from electron bombardment of solid targets, containing the elements beryllium, boron, carbon, oxygen, fluorine and aluminium. Beryllium oxide and lithium fluoride targets were used for the oxygen and fluorine measurements, respectively; targets containing at least 99% of the pure element were used for the remainder. The yield from each target was measured along a path inclined at 45° to the surface and at several electron accelerating voltages in the range 500 to 30 000 V ; the electron beam was then inclined at an angle of 45° to the surface of the target. The carbon yield was also measured at 10° to the surface with the electron beam inclined at 80° to the surface. The gas-flow proportional counter was operated at atmospheric pressure with a nitro-cellulose window; the window used for detecting beryllium and boron radiation was less than 1000 Å thick. The relative variation of the X-ray yield with the electron accelerating voltage is in excellent agreement with Archard’s (1960) predictions for the yield measured both at 45° and 10° to the target surface but with the electron beam normal to the surface. The absolute yield values, apart from those for beryllium, are 5 to 15% higher than those calculated from Archard’s theory and Burhop’s (1952) fluorescence yield formula; the beryllium yield is about half that predicted. For 5 ≤ Z ≤ 9, the greatest X-ray yield at 45° to the target surface is obtained when the accelerating kilovoltage is between 0.5 ( Z — 1) 2 and 0.6 ( Z — 1) 2 and amounts to 9 x 10 13 Z 5/2 photons per incident coulomb per steradian (1.45 x 10 -5 Z photons per incident electron per steradian).