I[EEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-21, NO. 1, JANUARY 1974 11

vol. 42, pp. 800-810, May 1954. focusing with periodic permanent magnet fields,” Proc. IRE, permanent magnets for focusing of electron beams,” RCA Rev.,

[2] IC. K. N. Chang, ‘[Beam focuslng by periodic and complementary 141 J. E. Sterrett and H. Heffner, “The design of periodic magnetic

[a] I?. Sterzer and W. W. Siekanowicz, “The design of periodic pp. 35-42, Jan. 1958. fields,” Proc. IRE, vol. 43, pp. 62-71, Jan. 1955. focusing structures,” IRE Trans. Electron Devices, vol. ED-5,

vol. 18, p. 39, Mar. 1957.

Electron Position Distribution Measurements in the Dynamic

Crossed-Field Photomultiplier

Abstract-Direct measurements of the arrival position distribu- tions of electrons on multiplication steps in the dynamic crossed- field photomultiplier (DCFP) are reported. These measurements were performed using special devices with slotted dynodes behind which a fluorescent screen was placed. The electron position dis- tribu.tion could then be observed on the fluorescent screen and photographed. These experiments have shown that under all normal operating conditions of the DCFP good spatial resolution (no overlap) of electrons on all multiplication steps can be achieved.

I. INTRODUCTION

T IEIE dynamic crossed-field photomultiplier (DCFP) is a gated optical detector that utilizes a combina-

tion of dc and RF fields (of amplitude Eo and E, re- spectively) along with a crossed magnetic field ( B ) to provide for multiplication of photoelectrons along a single metallic dynode. The details of operation of this device have been described in previous publications [l], [2] and will not be repeated here except for a very brief discussion. The essential features of the DCFP operation consist of 1.) acceleration of photoelectrons away from the photocathode in a curved trajectory and their return to the dynode, with a net translation and gain in energy, in a time approximat.ely equal to the period of the RF electric field E, and 2) amplification of secondary elec- trons by a similar process on each subsequent cycle of the R F field until sufficient gain is achieved and the amplified electrons are collected through an aperture in the dynode.

Additional properties of interest in the DCFP opera- tion are I) not all photoelectrons and secondary electrons

Mmuscript received April 20, 1973; revised August 27, 1973. This research was supported by the National Science Foundation under Grant NSF GK-32869.

B. J. Williams and 0. L. Gaddy are with the Electrooptics Laboratory, Department of Electrical Engineering, University of Illinois at Urbana-Champaign, Urbana, Ill. 61801.

ment of Electrical Engineering University of Illinois at Urbana- D. L. Helfrich was with the Electrooptics Lsboratory, Depart-

Champaign, Urbana, Ill. He is now with the Emerson Electric Comlpany, St. Louis, Mo.

arrive back at the dynode on the first and subsequent multiplication steps with sufficient energy to produce a large number of secondary electrons, hence, the gating property of the device [4], [SI, and 2) those electrons that are multiplied are phase focused or bunched. The output of the DCFP therefore conlsists of a series of pulses short in duration compared t o the RF period and with an amplitude corresponding to the number of photo- electrons emitted in the gating interval during the nth previous RF cycle, where n is the number of multiplicrt- tion steps.

An application of the DCFP that has received con- siderable interest recently is its use as a gated receiver in high-data-rate optical communicaiion systems using mode-locked laser transmitters and pulse-code modula- tion [SI. The gating property of the DCFP is useful in minimizing the effect of background light on the performance of these systems. Leverens and Gaddy [4], [SI have reported the gating properties of the DCFP, and Ross et al. [SI have demonstrated high performance laboratory systems using the DCFP as a gated optical receiver.

A condition of operation that can degrade the per- formance of the DCFP, both as a gated receiver as previously described and as a wide bandwidth baseband detector, is the spatial overlap of electrons on different multiplication steps near the collector. This condition results in the collection of electronel that have under- gone a different number of multiplication steps and produces an effective transit time dispersion which limits the bandwidth in the baseband mod’e of operation [l], [a] and in pulse errors in the gated receiver application

The finite photocathode size and electron dynamics within the DCFP result in an electron arrival position distribution on the first and subsequent steps that has, in general, a larger area than the photocathode. Under certain operating conditions, the init:ial energy distribu- tion of secondary electrons can cause further spreading

c31.

12 IEEE TRANSACTIONS ON ELECTRON DEVICES, JANUARY 1974

of the arrival position distributions on subsequent steps, until one or more step distributions overlap at the col-, ,--FLUORESCENT SCREEN f - GLASS STRIP

lector, resulting in the aforementioned deleterious effects These arrival position distributions can be and havr

been calculated under certain limited conditions for the: DCFP [SI, [7]; however, many simplifying assumptiom must be made, and digital computers are required t c ’ perform the complicated computations in order to achievr: any reasonable results. An easier and more precise waJ’ of determining this information is by direct experimenta: measurement.

The purpose of this paper is to report on experimentzl in which we have directly measured the position distribu- tion of electrons on multiplication steps in the DCFP, We have determined threshold values of operating; parameters of the device which result in good resolutior. of electron position distribution of all multiplication steps. These results were obtained by constructing specia 1 200-MHz devices with a slot in the dynode behind which a fluorescent screen was placed. A small portion of thc: primary electrons incident upon the dynode on eacl.1 multiplication step pass through the slot and excite thc fluorescent screen, giving a visual indication of the spatia[ distribution of electrons on each step. These experimentig were performed with both a continuous-dynode DCFP ai3 well as the split-dynode device described by Leverenz and Gaddy [SI.

11. EXPERIMENTAL DEVICES The photomultipliers used in the experiments reported

here were almost identical to the 200-MHz devices da- scribed by Leverenz and Gaddy [4], [5] and by Schaeffer and Gaddy [SI. No collector was incorporat,ed in thew devices, however. Figs. 1 and 2 show schematic drawings of the cross section of the experimental photomultiplier. The essential components of the devices consist of 1) 11,

2.5-in-diameter 304 stainless-steel half-cylinder enclosing and shielding the resonant structure in which the RIP electric field is established, and 2) the resonator formed by the capacitance between the rail and the dynode and the two 23 turn coils of stainless-steel wire wrapped around the glass rail supports that serve as the inductom. The dynode is attached to the flat side of the half-cylinder with stainless-steel clips and insulating mica sheets. 11 cesium-antimonide (CssSb) photocathode 0.5 cm i : ~ diameter is formed near one end of the dynode. The flat side of the half-cylinder, the thin mica insulator sheetr;, and the dynode have a long narrow slot cut in t h e n through which a small sample of incident primary elec- trons pass on each.multiplication step. This slot is 25.5 crn in length and 2 mm in width. These electrons strike an’l energize the fluorescent screen located immediately above the slot allowing the electron spatial distribution to be observed. A 304 stainless-steel dynode was used t o provide low gain per step and, consequently, allolv observation of the spatial distribution on a large number of steps.

Two different dynode configurations were used io these experiments. One is a single-dynode and the othrr

DYNODE

RAIL ELECTRODE

I I LIGHT

Fig. 1. Schematic longitudinal cross section of the experimental 200-MHz DCFP.

/ GLASS

TOP PLATE -, // FLUORESCENT SCREEN

DYNODE

MICA

ELECTRODE

GLASS RAIL INDUCTANCE SUPPORT

Fig. 2. Schematic transverse cross section of the experimental 200-MHz DCFP.

a split-dynode device [SI. In the split-dynode DCFP, the shorter section of the dynode is the one on which the photocathode is fabricated and is called the sampling dynode. The length of this section of the dynode is ap- proximately one-fourth the total dynode length. The dc electric field in this region is designated Eos. The longer section is called the gain dynode and is the only slotted part of the dynode in this device. The dc electric field here is designated Eog. The purpose and advantages of the split-dynode DCFP have been described by Leverenz and Gaddy [5]. The experimental split-dynode device also incorporated a row of transverse holes and a fluores- cent screen to measure the transverse electron distribution.

In the devices reported here, the dynode-to-rail separa- tion d is 1 cm. The length of the sampling dynode is 10 ern and the gain dynode 30 cm. The length from the photo- cathode to the row of transverse holes is 35 cm. The total dynode and slot length in both cases was sufficient to observe the electron position distribution of the latter three-fourths of a t least 18 multiplication steps. A photo- graph of the single-dynode device used in these experi- ments is shown in Fig. 3.

111. EXPERIMENTAL RESULTS Resolution of the electron distributions may be

measured by observing and photographing the fluorescent screen as electrons strike it. It was observed in the case of the single-dynode device that good resolution could be obtained for values of Eo/E between 1 and 3 provided E was sufficiently large. The magnetic field parameter y (defined as the ratio of the electron cyclotron frequency t o R F electric field frequency) was set a t 0.85 since this is near the value that yields the highest gain and desirable gating properties in the DCFP. Fig. 4 shows a typical

WILLIAMS et al. : DYNANIC CROSSED-FIELD PHOTOMULTIPLIER 13

here were taken with the split-dynode device with both dynodes externally connected together. The results are identical t o those obtained with the single-dynode DCFP. This was determined by comparison of data photographs obtained with the two devices operating under identical conditions.

Operation of the device in the split-dynode configura- tion yields similar results. In these experiments, the two dynode sections were operated at different dc voltages. Fig. 6 shows photographs of the fluorescent screen showing the spatial resolution obtained for Eo,/E = 1, 2, and for Eos/E = 2 and 3, where V is greater than the voltage required for threshold of good spatial resolution. As was the case with the single-dynode experiments, lack of good resolution was observed in all cases for V below a threshold RF voltage.

'The double row of light on the screen in some of the photographs is believed to be due to the fact that the

14 IEEE TRANSACTIONS ON ELECTRON DEVICES, JANUARY 1974

mica and the acceleration of these secondary electrcans to an energy equal to Vo before they strike the screen. At higher gain and current density levels on the dynode, this effect wars not observed since the contribution of secondary electrons produced at the edges of the slot in the mica is small compared to that of the primaries passing directly t,hrough the slot.

The split-dynode device also incorporated a transverse row of small diameter holes and a fluorescent screen . a t the end of the longitudinal slot. The transverse distriba- tion is seen to be fairly uniform for all operating conditions. This indicates that the transverse electron spatial distribll- tion pattern has widened out to the full width of t,he rail electrode at the collect~or end of the dynode.

It should be pointed out that in this device the tran83- verse holes and screen are located a distance from tke photocathode which allows 18--20 multiplication step,$. A normal device would be much shorter (8-10 steps: , and much less transverse spreading would be expected. Further studies are necessary to determine the amount of spread of the transverse distribution in order that optimization of collector efficiency (collected fraction c f dynode current) can be achieved.

IV. TRAJECTORY INTERCEPTION RESULTS It was also observed during the course of the experi-,

ments that under certain conditions (large values of 1' and &/E) the electron distribution of a single-multiplica- tion step would not be symmetrical about the center O F ma,ximum intensity but rather asymmetrical where thc point of maximum intensity was located near the col- lector edge of the distribution rather than at its center This is illustrated in Figs. 7 and 8. When the field condi-, tions are such that the average elect'ron displacement along the dynode becomes large, the height of some electron trajectories are greater than d . These electrons, which strike and are intercepted by the rail, are lost t o the multiplication process. Therefore, that portion of the electron distribution that would normally have been observed in a device with larger d is intercepted by the rail electrode, and the pattern appears to be one-half to two-thirds the width of the normal pattern. The onset of this effect occurs when field conditions are such that the average displacement (x) on each step is approxi- mately equal to 2d ( 2 cm in this case).

The question of operational stability under these conditions has a t times been raised. We have found tha,t the device is stable under all normal operating conditions including those that result in electron interception by the rail, and it does not fall into an unstable mode of operation under any field conditions as large as Eo/E = 3 and V = 140 V.

Experiments were also performed with the split-dynode device to determine the dependence of the threshold of resolution on y (the magnetic field parameter). The value of E for several values of Eo,/E and E,,,/E was set at 20 percent above the resolution threshold value,

Fig. 7. Photograph of the fluorescent screen showing asymmetrical electron arrival position distributions ((x) 2 d) .

2 1 REGION OF ELECTRON TRAJECTORIES7 I

(a) REGION OF INTERCEPTED ELECTRON TRAJECTORIESr?

<x>

(b) Fig. 8. Schematic representation of electron trajectories and ar-

rival position distributions. (a) Symmetrical distributions, (x) < d. (b) Asymmetrical distributions, (x) 2 d.

and y was set at the value for maximum gain ymax. y was then varied about either side of ymax until either t.hreshold was reached or there was insufficient gain to give a visible distribution pattern on the screen. In- sufficient gain was the limiting factor in all cases indicating that spatial resolution a t sufficiently large operating gain is independent of the magnetic field strength. The range of y over which no deterioration of spatial resolu- tion was observed was approximately rt 20 percent of Ym ax.

WILLIAMS et al.: DYNAMIC CROSSED-FIELD PHOTOMULTIPLIER

V. SUMMARY OF RESULTS AND CONCLUSIONS ;Experimental results have been reported which deter-

mine the value of peak RF voltage V necessary to achieve the threshold of spatial resolution of electron arrival position distribution in the DCFP for different values of .Eo/E. The results of these experiments for the single- and split-dynode devices are summarized in Fig. 9. These results show that as Eo/E is increased, V (or E ) and consequently the amount of power to drive the device necessary to achieve threshold of spatial resolution decreases. This also applies to the split-dynode case in which Eog)E’ is held constant and Eo,/E is varied.

In summary, we have shown that the threshold of spatial resolution on all multiplication steps in the DCFP can be achieved and exceeded in either the single- or split-dynode devices under the range of field conditions Eo/E, Eo,/E, and Eo,/E between values of 1 and 3 provided that V‘ (or E ) is sufficiently large. Safe and stable operating Conditions of the device are not exceeded for these field conditions. These measurements included all field condi- tions yielding highest gain and desirable gating char- acteristics.

Although these experiments were performed with 200-;MHe devices, there is no reason to expect that these results cannot be extended to apply to higher frequency devices. As the device frequency increases, size decreases, the magnetic and electric field intensity required in- crease, but peak RF and dc voltage remain constant for equivalent operating conditions. Operating parameters required to achieve good spatial resolution of electrons on all multiplication steps in t,he DCFP under almost all conceivable operating conditions are now well estab- lished. These results should prove to be of great value in the design and operation of the DCFP as a gated optical receiver or as a wide bandwidth baseband photo- multiplier.

ACKNOWLEDGMENT

The authors wish t’o thank A. H. Wilson, K. Kuehl, E. Boose, and B. Marshall for technical assistance. Part

140 -

130 -

n 120 -

-I

I 0

K I- I y 110

t 100

-

-

> 90 -

80 -

15

-+---+--%- EOp / E = 1.5

Fig. 9. Variation of threshold voltage required for good spatial resolution with Eo/E. Bars indicate uncertainty of threshold of spatial resolution.

of the equipment used in the course of t,his research was provided by the Industrial Affiliates Program in Physical Electronics at the University of Illinois a t Urbana- Champaign, Urbana.

REFERENCES 0. 1,. Gaddy and D. F. Holshouser, “A microwave frequency dynamic crossed-field electron multiplier,” Proc. IEEE, vol. 51, pp. 153-162, Jan. 1963. 0. L. Gadd.y, “Microwave crossed-field secondary emission photomultrpllcation,” Ph.D. dissertation, Univ. Illinois at Urbana-Champaign, June 1962.

munications experiments,” Proc. IEEE’, vol. 58, pp. 1719-1726, M. Ross, S. I. Green, and J. Brand, “Short-pulse optical com-

Oct. 1970. D. J. Leverena and 0. L. Gaddy, “Measurement of the duration

multiplier,” Proc. IEEE (Lett.), vol. 57, pp. 2153-2154, Dec. 1969. of the sampling function in the dynamic crossed-field photo-

---, “Subnanosecond gating properties of the dynamlc crossed- field photomultiplier,” Proc. ZEEE, vol., 58, pp. 1487-1490, Oct. 1970. D. J. Leverenz, ‘‘Effects of initial electron energy in the DC biased dynamic crossed-field electron multiplier,” M.S. thesis, Univ. Illinois at Urbana-Champaign, Urba,na, 1966.

namic crossed-field electron multiplier,” Ph.D. dissertation, , “A study of the sampling process in the DC biased dy-

Univ. Illinois at Urbana-Champaign, Urbana, 1969. E. M. Schaeffer and 0. L. Gaddy, “Noise properties of the dynamic crossed-field photomultiplier,” IEEE Trans. Electron Devices (Corresp.), vol. ED-18, pp. 388-390, June 1971.

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