Properties of Photomultipliers

Andrew T. Young The University of Texas, Department of Astronomy, Austin, Texas 78712. Received 23 February 1967.

The recent article by Schleuter1 contains some errors and mis­conceptions about photomultipliers which I believe deserve com­ment:

(a) I t is not clear whether the author means the term "dark current" to include the dc component or just the fluctuating com­ponent, since he says first that it is a "noise current" but later says it has a dc component. In either case, Eq. (1) is nonsense, and is confused by using ID for both the dark current and the dy-node noise. The statement that "thermionic emission produces the largest noise current" is a common misconception; although a temperature-dependent component exists, it is orders of magni­tude larger than true thermionic emission, and in fact it is unlikely that the thermionic emission of an Sb-Cs cathode has ever been measured at 25°C or below. The dark current does not have "a dc component because the multiplier portion of the tube acts as a rectifier," but simply because the tube is a dc device.

(b) The statement tha t "IA = (GIc2 + ID

2)1/2; but when Ic

is large, little error is introduced by using algebraic addition", i.e., "IA = GIc + I D " , is mathematically incorrect, as brief in­spection will show.

(c) In view of the fact that fatigue and overload effects vary greatly from tube to tube2-3, the author's failure to mention the type of tube he used makes his overload data useless to anyone else.

(d) I t is not true that "as the signal current approaches the divider current, the current ga in . . . becomes unpredictable." The problem has been analyzed by Baker and Wyatt .4

In the interest of not seeing another OAO failure, I feel I should mention that the dark noise at balloon and satellite altitudes will be one or more orders of magnitude larger than at sea level, owing to cosmic rays.5 - 7 In the van Allen belts, particle fluxes large enough to saturate a photomultiplier may occur. Shielding is ex­tremely difficult, because the tube responds to the penetrating gamma-ray Bremsstrahlung produced by energetic particles strik­ing its housing.

There are several excellent practical discussions of photomul­tipliers in the literature; we recommend the user-oriented articles by Spicer and Wooten8, and Sharpe.9 Additional information is available from the manufacturers (see for example, E M I Docu­ment Ref. No. CP 5475 on dark current; and numerous Applica­tions Notes and Research Memos available from ITT) .

References 1. P . Schleuter, Appl. Opt. 6, 239 (1967). 2. J. P . Keene, Rev. Sci. Instr. 39, 1220 (1963).

3. W. Michaelis, H. Schmidt, and C. Weitkamp, Nucl. Instr. Methods 21 , 65 (1963).

4. D. J . Baker and C. L. Wyatt , Appl. Opt. 3 , 89 (1964). 5. G. Chodil, D. Hearn, R. C. Jopson, H. Mark, C. D. Swift,

and K. A. Anderson, Rev. Sci. Instr. 36, 394 (1965). 6. A. T. Young, Rev. Sci. Instr. 37, 1472 (1966). 7. R. L. Jerde, L. E. Peterson, and W. Stein, Rev. Sci. Instr.

(to be published). 8. W. E. Spicer and F. Wooten, Proc. I E E E 51 , 1119 (1963). 9. J. Sharpe, Electronic Technology, July 1961.

May 1967 / Vol. 6, No. 5 / APPLIED OPTICS 979