Critical Potentials in Secondary Electron Emission From Iron, Nickel, and Molybdenum

Secondary electron emission from iron, nickel and molybdenum targets for primary voltages 0 to 1500.—The tube used was designed so as to minimize disturbing factors, all metal parts were glowed before assembly, and the tube was baked before each set of runs. In addition the targets were thoroughly cleaned by heating to bright red by electronic bombardment from an auxiliary filament until a permanent condition was obtained as indicated by the curves. The pressure was kept below ${10}^{-6\mathrm{}}$ mm. Both the secondary emission $I_1$ and the primary current $I_2$ were measured by a null method of high precision (to 0.1 percent or less). The ratio $\frac{I_1}{I_2}$ was found (1) to vary only 2 percent as the retarding potential between the plate and surrounding box was increased from 3 to 10 volts, so 6 volts was considered sufficient; (2) to vary only slightly with the primary current; (3) to be unchanged when the earth's magnetic field was neutralized. The final curves, for all three metals after heat treatment, are nearly alike, the value of $\frac{I_1}{I_2}$ rising to a maximum of about 1.30 at about 348 volts for iron, 455 volts for nickel and 356 volts for molybdenum. From slight breaks (changes of slope) in the curves, some 25 critical potentials were located for iron, 16 for nickel and 20 for molybdenum. The values below 40 volts for iron (7.3, 11.6, 14.4, 18.3, 22.6, 25.0 and 29.0) and for nickel (6.2, 9.3, 9.9, 11.8, 15.1, 22.6, 24.4, 31.4, 35.0) agree fairly well with critical potentials for soft x-rays determined by Thomas. This suggests that there is a common phenomenon involved in the production of secondary emission and of soft x-rays which is evident at the lower voltages but which is masked at higher voltages by other processes not yet understood.