On the Shape of the Sensitive Cathode Area of Electron Multiplier Photo-Tubes
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JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 40, NUMBER 4 APRIL, 1950 Letter to the Editor On the Shape of the Sensitive Cathode Area of Electron Multiplier Photo-Tubes E. W. PETERSON AND DAN H. HOLLAND* The Dow Chemical Company, Midland, Michigan January 30, 1950 I F one is measuring the intensity of a narrow beam of light with an electron multiplier photo-tube such as the RCA 931A, how precisely must he position his light beam to get accurate measurements? This problem has been treated in at least four publications, which show considerable disagreement. Hasler et al. 1 gave no data, but stated that the sensitive area appeared to be, "a broad plateau which usually extends one-eighth of an inch." Dieke 2 shows sensitivity curves which are very non-uniform and with a half-width of about 4 mm. Marshall et al. 3 mentioned half- widths of about 5 mm. In the publication from this laboratory, 4 a width of 1 or 2 mm was suggested and one curve was given which showed a half-width of less than 1 mm. This problem was recently reopened by J. H. Jurmain 5 of the Baird Associates, Inc., who made measurements on some tubes with a technique similar to that described later. Jurmain's results indicated a sensitive region appreciably wider than that reported in reference 4, and at his request we measured a few of his tubes and confirmed his results. On repeating our experiments, with some of the same tubes as reported in reference 4, we found that we had been incorrect in stating that the voltage between the cathode and the first dynode was set at 100 volts per stage—the same as other stages—for the curves which were published. We have since discovered that the size and shape of the sensitive cathode area is strongly influenced by this voltage, which we had been adjusting as a gain control. The experimental arrangement for obtaining all data presented here was essentially the same as was used before. 4 A small tung- sten lamp was placed 75 cm from a 0.2-mm slit about 1 cm long. The photo-tube was placed about 10 cm behind the slit, with the grid wires of the tube normal to the line joining the slit and the lamp. The vertical axis of the tube was set approximately parallel to the slit. The slit was moved by means of a screw to scan the cathode. Figure 1 is a set of curves obtained from one tube. The voltage between the cathode and the first dynode was set at the values shown on each curve, all other stages operating at 100 volts per stage. To prevent overlapping, each successive curve was dis- placed upward one unit from the previous curve. The left and right extremities of each curve correspond to zero sensitivity. It is suggested that the maximum occurring between 4 and 5 mm on the abscissa scale might be due to photo-electrons which by-pass dynode 1 and go directly to dynode 2. The small sensitivity maxima at each side are due to reflections from the grid wire supports. Although the width of the sensitive area increases rapidly with the voltage on the first stage at the lower voltages, the broadening increases more slowly at higher voltages so that little advantage results from operation of the tube with a first stage voltage higher than that on the other stages. Of five tubes measured with varying voltages on the first stage, all showed an essentially similar behavior. TABLE I. Result of measurements made of the width of the sensitive cathode area on 35 multiplier photo-tubes. No correction is made for angle of incidence or width of light probe. FIG. 1. Variation in the sensitivity of a multiplier photo-tube with position of illumination on the cathode, for various values of voltage be- tween the cathode and first dynode. There is considerable variation among tubes as to the size and shape of the sensitive cathode area. Table I is from data taken on 35 tubes with the experimental arrangement described. The widths reported are actual distances which the slit moved, with no allowance being made for the width of the light beam or angle of incidence on the cathode. All tubes were operated at 100 volts per stage on all stages. By "95 percent width" is meant the dis- tance the slit was moved to increase the tube output (dark current subtracted) from 95 percent of its maximum value to 100 percent and back to 95 percent. A similar meaning applies to "50 percent width." Table I gives the largest and smallest values of these widths found among all the tubes tested as well as the average value for all 35 tubes. The tubes measured included Types 931, 931A, 1P21, 1P22, and 1P28. No significant correlation was found between the shape of the sensitive area and the tube type. Measurements were also made with many of the same tubes operated at reduced voltage, down to 50 volts per stage on all stages. In most cases the 95 percent width became slightly smaller at lower voltages but in one case it became larger. The effect is small and in many cases could not be detected. As given in Table I, a beam of light centered on the sensitivity maximum can be moved on the average about ±0.55 mm for a five percent change in photo-current. It should be pointed out, however, that for a light beam centered elsewhere, the change in photo-current with position is greater than that given above by at least a factor of two and in most cases by a much larger factor. * Now at Cornell University. Ithaca, New York. 1 Hasler, Lindhurst, and Kemp, J. Opt. Soc. Am. 38, 789 (1948). 2 Dieke, War Production Board Report W-193. 3 Marshall, Coltman, and Bennett, Rev. Sci. Inst. 19, 744 (1948). 4 Saunderson, Caldecourt, and Peterson, J. Opt. Soc. Am. 35, 681 (1945). 5 Private communication. 253
Transcript of On the Shape of the Sensitive Cathode Area of Electron Multiplier Photo-Tubes
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 40, NUMBER 4 APRIL, 1950
Letter to the Editor
On the Shape of the Sensitive Cathode Area of Electron Multiplier Photo-Tubes
E. W. PETERSON AND DAN H. HOLLAND*
The Dow Chemical Company, Midland, Michigan January 30, 1950
IF one is measuring the intensity of a narrow beam of light with an electron multiplier photo-tube such as the RCA 931A,
how precisely must he position his light beam to get accurate measurements? This problem has been treated in at least four publications, which show considerable disagreement. Hasler et al.1
gave no data, but stated that the sensitive area appeared to be, "a broad plateau which usually extends one-eighth of an inch." Dieke2 shows sensitivity curves which are very non-uniform and with a half-width of about 4 mm. Marshall et al.3 mentioned half-widths of about 5 mm. In the publication from this laboratory,4
a width of 1 or 2 mm was suggested and one curve was given which showed a half-width of less than 1 mm.
This problem was recently reopened by J. H. Jurmain5 of the Baird Associates, Inc., who made measurements on some tubes with a technique similar to that described later. Jurmain's results indicated a sensitive region appreciably wider than that reported in reference 4, and at his request we measured a few of his tubes and confirmed his results.
On repeating our experiments, with some of the same tubes as reported in reference 4, we found that we had been incorrect in stating that the voltage between the cathode and the first dynode was set at 100 volts per stage—the same as other stages—for the curves which were published. We have since discovered that the size and shape of the sensitive cathode area is strongly influenced by this voltage, which we had been adjusting as a gain control.
The experimental arrangement for obtaining all data presented here was essentially the same as was used before.4 A small tungsten lamp was placed 75 cm from a 0.2-mm slit about 1 cm long. The photo-tube was placed about 10 cm behind the slit, with the grid wires of the tube normal to the line joining the slit and the lamp. The vertical axis of the tube was set approximately parallel to the slit. The slit was moved by means of a screw to scan the cathode.
Figure 1 is a set of curves obtained from one tube. The voltage between the cathode and the first dynode was set at the values shown on each curve, all other stages operating at 100 volts per stage. To prevent overlapping, each successive curve was displaced upward one unit from the previous curve. The left and right extremities of each curve correspond to zero sensitivity. I t is suggested that the maximum occurring between 4 and 5 mm on the abscissa scale might be due to photo-electrons which by-pass dynode 1 and go directly to dynode 2. The small sensitivity maxima at each side are due to reflections from the grid wire supports. Although the width of the sensitive area increases rapidly with the voltage on the first stage at the lower voltages, the broadening increases more slowly at higher voltages so that little advantage results from operation of the tube with a first stage voltage higher than that on the other stages. Of five tubes measured with varying voltages on the first stage, all showed an essentially similar behavior.
TABLE I. Result of measurements made of the width of the sensitive cathode area on 35 multiplier photo-tubes. No correction is made for angle of incidence or width of light probe.
FIG. 1. Variation in the sensitivity of a multiplier photo-tube with position of illumination on the cathode, for various values of voltage between the cathode and first dynode.
There is considerable variation among tubes as to the size and shape of the sensitive cathode area. Table I is from data taken on 35 tubes with the experimental arrangement described. The widths reported are actual distances which the slit moved, with no allowance being made for the width of the light beam or angle of incidence on the cathode. All tubes were operated at 100 volts per stage on all stages. By "95 percent width" is meant the distance the slit was moved to increase the tube output (dark current subtracted) from 95 percent of its maximum value to 100 percent and back to 95 percent. A similar meaning applies to "50 percent width." Table I gives the largest and smallest values of these widths found among all the tubes tested as well as the average value for all 35 tubes. The tubes measured included Types 931, 931A, 1P21, 1P22, and 1P28. No significant correlation was found between the shape of the sensitive area and the tube type.
Measurements were also made with many of the same tubes operated at reduced voltage, down to 50 volts per stage on all stages. In most cases the 95 percent width became slightly smaller at lower voltages but in one case it became larger. The effect is small and in many cases could not be detected.
As given in Table I, a beam of light centered on the sensitivity maximum can be moved on the average about ±0.55 mm for a five percent change in photo-current. I t should be pointed out, however, that for a light beam centered elsewhere, the change in photo-current with position is greater than that given above by at least a factor of two and in most cases by a much larger factor.
* Now at Cornell University. Ithaca, New York. 1 Hasler, Lindhurst, and Kemp, J. Opt. Soc. Am. 38, 789 (1948). 2 Dieke, War Production Board Report W-193. 3 Marshall, Coltman, and Bennett, Rev. Sci. Inst. 19, 744 (1948). 4 Saunderson, Caldecourt, and Peterson, J. Opt. Soc. Am. 35, 681 (1945). 5 Private communication.
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