US2895068A - Photo-electric cells - Google Patents

Description

i July 14, 17959 v s. RoDDA 2,895,068

PHOTO-ELECTRIC CELLS @NVENTOR ATTOR NEYS United States Pat 2,895,068 Y moro-ELECTRIC CELLSA Sidney* Rodda, New Barnet, England, assignor, by mesne assignments, to Siemens Edison Swan Limited, VWestminster, London, England, a British company ApplicationlDecember 12, 1955, Serial No. '552,581

Claims priority, application Great Britain December 14, 1954 4 Claims. (cl. 313-102) Thisv invention relates to photo-electric cells and has an important application in photo-electric multipliers.

It has been found that in photo-electric cells there is a minimum residual current which is thought to be due -to thermionic emission of the electrons from the photoelectric surface of the cathode, and it has been found `that this occurs at ordinary atmospheric temperatures. It will be appreciated that this residual current sets a limit to the minimum level of radiation which can be detected. This eifect is particularly noticeable in the case of photo multiplier tubes in which the primary electron beam is magnified by successive secondary emission with the result that the initial residual current is appreciably magnied in the output current.

Whilst in general there is no material diiference between the thermionically emitted electrons and the photo electrons which are emitted as a result of radiation, there is the important distinction that the thermionic elecrons at emission have a much 'lower kinetic energy and hence a lower velocity than the photo electrons.

The main object of the present invention is to provide an improved photo-electric cell in which the residual current is appreciably reduced.

The present invention comprises a photo-electric cell provided with an intercepting electrode or electrodes which occupies a small proportion of the electron beam cross section together with focusing means adapted to focus slow moving electrons on to the intercepting electrode or electrodes Whilst allowing electrons with higher transverse velocities to pass by.

It will be appreciated that with such an arrangement it is possible to focus on to the intercepting electrode a Vlarge proportion of the electrons due to thermionic emission since these low velocity electrons can be readily focused on to a smaller area than the electrons due to photo-electric emission with the result that the faster electrons do not impinge on the intercepting electrode but by-pass it and subsequently impinge on either a secondary emissive electrode or collector electrode.

The focusing may be magnetic or electrostatic and in the case of electrostatic focusing may comprise an annular electrode through which the beam passes or a multi-apertured plate having an intercepting electrode associated with each aperture on to twhich the thermionic electrons are directed.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings, in which: K Y

Figs. l and 2 explain the principle underlying the invention.

Figs. 3 and 4 respectively 'show alternative 'arrangements of intercepting electrodes, whilst Fig. 5 is a plan view of the intercep'ting electrode in the arrangement shown in Fig. 4. A

Fig. y6 shows diagrammatically a photo multiplier ernbodying the intercepting electrode arrangement shown-in Figs. l and 2, and Fig. 7 similarly shows a photo multiplier embodying the electrode arrangement of Figs. 4 and 5.

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In the arrangement of Fig. 1 the reference 1 indicates the envelope of the tube which is formed with a photoemissive coating 2 indicated by VAchain lines on the inner end face. 3 is a small circular disc, maintained at a positive potential with respect to the photo-emissive coat'- ingr2, constituting anintercepting electrode and extending across the centre of the beam path. Itis assumed` that the beam path extends vertically downwards and has an axis indicated by the chain dotted line. 4 is a cylindrical electrostatic focusing electrode, maintained at Va negative potential.

In operation the tube is so designed that the slow mov ing thermionic electrons -will be focussed to impinge on electrode 3, and hence will be intercepted. The photoelectrons, however, which have high initial velocities tangential to the cathode surface, will not be focussed to anything like the same extent in the plane of electrode 3, with the result that few of them will strike on this electrode and the majority will pass uninterrupted down the tube and impinge on collector electrodes 16, 17 and 18. In Fig. l, the paths of the slow moving thermionic ele@ trons are indicated by dotted lines but in Fig. 2 the paths of the photo-electrons emitted with high initial velocities tangential to the cathode surface are findicaed and it will be observed that these mainly pass by the electrode 3.

Fig. 3 shows a modified arrangement in which the photo-electric cathode is formed as a semi-transparent layer 2" on the spherical end face of the envelope 1. Electrons are accelerated from the photo-emissive cathode by a radial electro-static eld produced by 4an annular focusing electrode 5 to inlpinlge on the intercepting electrode 3 at the centre of curvature of the end face. The annular electrode 5 has a part spherical contour. It will be appreciated that here again the low velocity elecy trons will impinge on the electrode 3 but the electronsof high initial transverse velocities will pass through the annular space between the electrode 3 and the accelerating electrode 5 and will impinge on the first secondary emissive electrode y6 and on collector electrodes 7 and 8'.

Figs. 4 and 5 show a further arrangement comprising a pair of spaced electrode plates 7 and 8. The plate 17 is the focusing electrode and is formed with a series of apertures 9 which are aligned with apertures 10 in the plate 8, each of which latter apertures has a centraldisc shaped electrode l1 lwhich is supported by conducting wires and is therefore at the same potential as the plate 3. Electrons passing through the apertures 9 are focussed by the potential on the electrode 7 to pass through the apertures 10 and Whilst the slow moving electrons will be largely focussed on to intercepting electrodes 1'1, the electrons with higher Vini-tial transverse velocities will pass through the apertures between the intercepting electrodes i1 and the edges of the apertures and impinge on col,- lector electrodes 162.17 and 18. It will be appreciated that Iwhilst for simplicity only seven apertures have been shown, in lgeneral a larger number of apertures may be used and the spacing between the plates will be adjusted accordingly.

When the apertures are circular, the relationship vbctween the spacing between the electrode plates 7 and 8 and the voltages required to be applied thereto is lgiven by the approximate formula:

where V1 is the potential between the first electrode 7 andthe emissive layer 2, V2 that between the second electrode 8 and the emissive layer 2; x1 is the spacing of the 'rst electrode from the emissive layer, andV x2 the spacing between the first and second electrodes.

At the plane of the second electrode an electron l lll lll photo electrons mayr be rnearly all transmitted.v

,y One can imagine an arrangement such as has just been. y f n described in kwhich x1=1-0 y mm., x2=l0-1 mm., 171:10

f volts, -V2=10O volts, with yapertureradii 1.6 with f a central blocking disc, at the centre of each aperture, lof

emitted from the kcathode with a tangential `energyl f the intercepting electrode which tepels secondary elecl Via, will cross lat adistance .ef froman axis .of an aligned system, where fthe focal length may be yshown rto be f Y l Photo-electric; electrons Vof energyrlQO e.v. can'pass f l 'through-ani aperture of radius f/Vlwhich is 40'times the Lblocked radius. It .is evident that the thermionic electrons may*y be substantially intercepted,- although the radius 0.25 mm.

intercepting electrode so asto'create a reta-rdin-g eld at.

, trons back .to it, rWhile allowing the photo electrons, which move forward with high velocities, to be transmitted'. `Iu-thetubeshown-n Fig.` 6 which embodies the intercepting elecn'ode arrangement fFigs. 1 and 2, the en` rvelope or' the tube ist xed'on a ,capr 12 .having contact pins 13 .for insertion in a socket Vin accordance with well cathodeZhas aconnectorIS on one sideof the envelope.`r

. The multiplier .part of the tube` is shown asconsisting 'of a ycascaded arrangement of secondary .electron-emissive electrodes ordynodes 16, 17 and 18 supported yon'axially extending leads '19 and surrounded by a mica screen 20.

Fig. 7A shows' a `dyno'de embodyingthe 'electrode arrangementshown in Figs. 4and 5.' In' this case, how-1 f.

ever,r the plates 7 and 8.r arer located` nearer the electron emissive electrode' 2y ,Whilst the. dynodes have different u contours as shown clearly; in the drawing.y

' kWhat. Iv claim as fnew and desireto secure by Letters i Patent of the United States is:

f l. The central vdisccan be ',Supll'tedbyy wires or strips'of width preferably not greater. than 0.50l mm.,r to avoidf yundue interception of photo electrons.` l l n y Clearly rthe .smaller the proportion of` the solid'xarea. `ofthe electrode plates to the total area, the .smaller Willi v be theinterception `,of photo electrons. :Normally it does; f .not appear practicable to reduce the relativefareazof the solid part. of the foraminatcd electrode beloivv.'.10r%}.fr y f Instead of the electrodes being plane, in an alternative arrangement the cathode and theflrst andsecond foram- .nated accelerating yelectrodes 'mayl form. concentric L'spherical or part Spherical surfaces, so that the axes which f y -passthrough the centres of the holes are radialclines di n rected rfrom the cathode surface toy the lcentre ofthe sphere which it formseither wholly or in part. In this instance f y v ralso the central portions ofy the holes inthetsecond elec? y .trode'are to beblocked to intercept .thermionic electronsy -f emittedbythe cathode, the.' electrode spacings and applied potentials being such that the ysecond'electrode lies in or near to the focal surface of the electron lens system so formed.

n The electrons which pass through the apertures will then proceed in a forward direction, and in the case of the spherical arrangement Will be concentrated to fall on a first multiplying target or dynode of relatively small area,

or in the case of the separate electron streams of the c plane parallel construction, they may be concentrated by a further electron lens system so that the separate electron streams fall on the same multiplying target.

In a further arrangement, the photoemissive cathode may be the inner surface of a cylinder or part cylinder, the electrons then being accelerated by an inner accelerating electrode maintained at a positive potential, and the `thermionic electrons then being intercepted by a strip or rod near the axis of the cylinder. The diagram for this arrangement is the same as for Fig. 3, if it is understood that Fig. 3 represents a section of the device taken normal to the axis, with suitable positive potentials applied to electrodes 3 and 5, and the secondary electron multiplying electrode 6. This arrangement will not'fully have `the advantage of a rotationally symmetrical system since t -a proportion of photoelectrons, viz, those emitted parallel n 1. A vphoto-electric cell comprising'a photo sensitivey 'cathode' and a collector electrode, amultiaperturedfocusing electrode extending across the electron path, means t for applying a potential'to said focusing electrode, .and a f p pluralityi of ielectrony absorbent intercepting electrodes.r l

with .a total. surface .area vwhich is smally comparedwith the cross-sectional' areay of the'beam' and -alignedwth they t beams passing through the apertures in said focusing elec'- l f trode .so' yas 1 to intercept and .absorb thev slow fmoving electrons. y y 22. `A rphoto-electric cell comprising aphoto-sensit'ivev f cathode, a ydiscv forming'an intercepting electrodellocated inI the path of a beam of lelectrons leaving said cathode,

plate extending across said path, .theedges of rsaid disc and of saidcooperating plate being spaced relatively to one .another vso as to f define an annularv space, and means.

for focusing `slowmoving electrons yon to the surface of said disc Without appreciably deecting the high velocity electrons from passing through said annular space.

3. A photo-electric cell comprising a photo-sensitive cathode, a disc forming an intercepting electrode located in the path of the beam of electrons leaving said cathode, the area of the surface of said disc being small relative to the cross-sectiona1 area of said beam, a cooperating plate extending across said path, the edges of said disc and of said cooperating plate being spaced relatively to one another so `as to define an annular space, and electrostatic focusing means for focusing slow moving electrons on to the surface of said disc Without appreciably deflecting the high velocity electrons from passing through said annular space.

4. A photo-electric cell comprising a photo-sensitive cathode, a disc forming an intercepting electrode in the path of the beam of electrons leaving said cathode, the area of the surface of said disc being small relative to 'the cross-sectional area of saidbeam, an annular focusing References Cited in the tile of this patent UNITED STATES PATENTS 2,135,615 Farnsworth Nov, 8, 1938 2,702,865 Herzog Feb. 22, `1955 2,728,014

Stoudenheimer et al Dec. 20, 1955

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