on the photographic action of slow electrons

On the Photographic Action of Slow ElectronsAuthor(s): R. Whiddington and J. E. TaylorSource: Proceedings of the Royal Society of London. Series A, Containing Papers of aMathematical and Physical Character, Vol. 136, No. 830 (Jun. 1, 1932), pp. 651-662Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/95814 .

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Photographic Action of Slow Electrons. 651

velocity of 70 KV. are scattered successively through 900 by thin gold foils, and a change in the intensity is looked for as the azimuth of the second scattered beam is altered. As the change looked for is very small, precautions must be taken to eliminate all stray effects due to X-rays, secondary electrons, and 'faulty alignment of the apparatus. These precautions are discussed in detail.

The polarisation effect found is 1 7 ? 0 3 per cent. That predicted by Mott is 10 per cent. The polarisation when a light scatterer (aluminium) is sub- stituted for gold is shown to be negligible, confirming the view that the effect found is a true one. Reasons are given to show that the experimental value is likely to be too low rather than too high, but it is not anticipated that it can be so high as 10 per cent.

An unexplained asymmetry in the direction at right angles to the initial beam is also found, but its magnitude and direction are not constant, and it is doubtful whether it has real significance.

On the Photogracphic Action of Slow Electrons.

By R. WHIDDINGToN, F.R.S., and J. E. TAYLOR, B.Sc., University of Leeds.

(Received March 2, 1932.)

[PLATE 14.]

Introdwution.

The fact that fast moving electrons have a blackening efEect on photographic emulsion has been known for a quarter of a century and many investigators have utilised this important property in their researches.

In the important fields of f-ray and X-ray electron investigation, for example, electron spectra have been recorded photographically, Innes* leading the way in 1907, followed amongst others, by Robinson and Rawlinson,t de Broglie$ and Whiddington.? Within the last year or two, G. P. Thomson and Reidli have also used this method for the recording of electron diffraction patterns.

* ' ProC. Roy. Soc.,' A, vol. 79, p. 442 (1907). t 'Phil. Mag.,' vol. 28, p. 277 (1914). t 'J. P. hysique,' vol. 2, p. 265 (1921). ? 'Phil. Mag.,' vol. 43, p. 1116 (1922). I 'Nature,' vol. 119, p. 890 (1927).



652 R. Whiddington and J. E. Taylor.

In 1922 one of us (R.W.) in the course of some experiments on low speed electrons found it necessary to increase the sensitiveness of photographic films to electrons of about " 50 volts velocity." This led to a qualitative investigation by Brett,* in this laboratory, of the sensitising effect of a super- imposed film of oil, and the working out of a suitable technique for its applica- tion.

This method where applicable is preferable, in our experience, to that of Schumannt as used, for example, by Duclaux and Jeantetl in which, by the use of acid, the gelatin of the film is partially removed, as in this latter method the technique is so troublesome and the results inconsistent.

Brett's results showed that in the neighbourhood of 60 volts a great increase in sensitiveness of the plate resulted from the use of a thin coating of some fluorescent grease, such as vaseline.

Cole's? qualitative experiments confirmed these results and in addition brought out the not surprising point that, as with light, different plates have different sensitivenesses to a definite current of electrons of given speed.

Carrjl continued Cole's qualitative work using electrons generated at less than 35 volts and extended the work in a specially interesting direction. He found that it was possible to use a gold or silver plate instead of a photographic film, developing the image by means of an atmosphere of mercury vapour. One of the advantages of such a method is that the metal plate has high con- ductivity and there is therefore no possibility of the incident electrons diffusing over the plate as a result of electrostatic charges on its surface. This effect certainly occurs in a very disturbing manner with ordinary photographic films and plates when large electron current densities are used.

Little experimental work appears to have been carried out on the law of blackening of photographic plates by slow electrons along lines similar to the well investigated case of light blackening, although Seitz and Harig? investigated the photographic blackening produced by electrons generated at 1500 to 18,500 volts and found that for low current densities the photographic density was proportional to log It, where I was the electron current and t the time of exposure.

There seems to be little information available for low velocity electrons in the * ' Proc. Leeds Phil. Lit. Soc.,' vol. 1, p. 1 (1925). t 'Ann. Physik,' vol. 5, p. 349 (1901). j 'J. Physique,' vol. 2, p. 156 (1921). ? ' Phys. Rev.,' vol. 28, p. 781 (1926). 11 'Rev. Sci. Inst.,' vol. 1, p. 711 (1930). T ' Phys. Z.,' vol. 30, p. 758 (1929).




region of 100 volts and the present experiments were carried out in order to obtain data within a comparatively narrow range of electron velocity and for a particular type of photographic film which is being used in this laboratory for certain electron recording experiments.

This investigation, therefore, does not aim at completeness, but adequate information has been obtained for the purpose in view and it seems to be of sufficient general interest to place on record.

The Apparatus.

The apparatus is arranged to produce an electron beam of uniform and measurable current density which can be directed at will on selected parts of a photographic film. These two main parts of the apparatus termed the gun and the, camera respectively are shown diagrammatically in fig. 1.

I . b /~FIG 1.

S, K s2

cylinder 1 c.idimtrad1cmlogfms th ehnclbswe.Thi

....................... .................I. .. . .. .. . .. .. . .. .. . .. .. . .... .... . .

x 1

FIG. 1.

The electron beam itself is produced from a heated filament F placed very nLear a wire grid P, mainltained at a suitable potential. An insulating quartz cylinder I cm. in diameter and 1 * 5 cm. long forms the mechanlical base. This brass gun is mounted on a brass ground joint which permits orientation in the vacuum within the containing cylinder C.



654 R. Whliddington and J. E. Taylor.

The electron beam is directed along the axis of the iron cylinder I which carried a correctly aligned slit S at its far end. The camera, K, is just a rectangular box of substantially the same construction and size as that previously described by one of us (IR.W.),* but in this case used in conjunction with a long split solenoid S1 S2 which gives a substantially uniform magnetic field over the greater part of the camera.

The current in this solenoid could be adjusted (by means not shown in the diagram) so as to deflect the electron beam emerging from the slit S on to a photographic film placed in the guides B which must be understood to run at right angles to the plane of the diagram. Under suich circunstances the marks which appear on the film are due only to deflected electrons and are not caused by any light emerging from the slit, which will be collected by the cylimder A.

The cylinder A fulfilled another purpose, which is mentioned later, -as it was insulated and connected to an electrometer in order to act as a: Frada collecter.

A high vacuum was produced in the gun-camera system by means of a Pfeiffer oil pump and a mercury vapour diffusion pump. A pressuwe less than 0O0001 mm. mercury was readily anad rapidly attained after degassing of the metal parts of the apparatus had been carried out.c "Apiezon " low vapouir pressure grease was used for the joints.

The electrical circuits were of the simplest character and aie sufficiently indicated by fig. 1. They comprised (1) the filament heating circuit; (2) the anode circuit, in which an accelerating potential up to 300 volts produces an emission current of strength measured by the galvanometer G - and (3) the Faraday cylinder A which, connected to one of the two alternative sub- circuits X or Y, could measure " beam currents" over a sufficiently large range down to 10-3 amperes, using a frequently calibrated Lindemani electrometer.

Preliminary Investigations.

It was soon found that there was no easily discoverable relation between the emission current measured by G and the beam current entering Ant (zero field in St 82) and measured by E. This was not unexpected, although for obvious reasons there was still some advantage accruing from including G in the circuit.

* Jones and Whiddington, Phil. Mag.,' vol. 6, p. 889 (1928). t Previous close investigation by Dr. Seddon in this laboratory had shown that for this

purpose the right dimensions for this cylinder were 8 cm. long and Q 7 cm. diameter.




It was, therefore, necessary always to use the A circuit for measuring the beam current, thereafter switching it round by the magnetic field of S1 S2on to the film F for the required time of exposure.

The kind of photograph obtained is shown in fig. 2, Plate 14. This spectrum clearly shows that, while at the expected position (1) there is a sharply defined slit image due to electrons which have a speed corresponding to the accelerating voltage between F and P, yet there is in addition another, less well marked band (2) in a lower velocity position on the film. This is a well-known effect due to the emission of secondary electrons by the primary ones at various points along their pa.ths. It is clear, then, that these electrons will also be included in the reading of A when the magnetic field is zero, although they cannot contribute to the density of the line (1) in the photograph.

This matter was set right by applying a sufficiently large retarding potential on A to turn back these lower speed electrons, the high speed flight only being registered in this case by E.

The necessary imum value of this retarding potential was obtained from a curve of the type shown in fig. 3. Here, beam current in arbitrary units, is plotted against retarding potential in volts. It is clear that in the case of an accelerating voltage (F to P) of about 193 volts, about 100 volts retarding potential on A is necessary in order to ensure a correct reading in E.

4C-

85 301 rizi

w20(EEL--, ?! 1150 lOO lSO 20

J, (retarding potential, volts)

FIG. 3.

In fig. 3 the upper curve (A) was taken with the edges of S clean and bright- in the lower curve (B) the same slit was covered with a soot deposit. The, curve (B) is clearly representative of a better velocity distribution, in that the long horizontal part of the curve means an entire absence of unwanted electrons within the range 70 to 193 volts. Soot covered slits were therefore always used. This procedure never completely prevented the production of low velocity secondaries, but they were not troublesome, being recorded on the film at some distance from the important area (1) of fig. 2, Plate 14.




It was found convenient to remove the retarding potential on A when the photographic exposure was in operation.

Method of Experiment.

Broadly speaking, the method of experiment was to measure first the beam current I in the manner described, then to turn on a steady magnetic field for a time t, which produced on development a line of density D as measured by the photometer. It is the relation between D, t and I which is sought.

It is necessary now to give a brief summary of the precautions taken in regard to development and photometry.

The technique previously worked out by many investigators for ordinary light photography was closely followed.*

Development was carried out for a fixed time in a vigorously stirred vertical tube placed in a large thermostatically controlled bath kept at 200 0 1? C.t

The following metol quinol developer was employed:

Metol ... . . 0 7 gm. Pot. Brom. ....... 0-2 gm. Hydroquinone .... 1'4 gm. Sod. Carb 10 .0 gin. Sod. Sulph . ....... 10 0 gm. Water up to ...... 1200 c.c.

Fixing and lengthy washing were carried out in the usual way. The photometry presented some initial difficulties. But the final arrange-

ment comprised a half watt lamp run at about two-thirds its normal voltage, in train with a good optical condensing system, a narrow slit, a photoelectric cell and Lindemann electrometer.

The light absorption in the film was recorded by the electrometer and D was evaluated from the formula

D log,0 0O/ where

co = electrometer current when the clear part of the film was in the path of the light.

C -electrometer current when the exposed part of the film was in path of the light.

In the measurement of C, care was taken to ensure a linear relation between electrometer reading and current. Densities were also checked by the use of an optical wedge.

* Dobson, Griffith and Harrison, " Photographic Photometry." t We are indebted to Mr. Hume, of the Chemical Department, for valuable help in

setting up this thermostat.



Whiddington and Taylor. Proc. Roy. Soc., A, vol. 136, Pl. 14.

* ~~ -. ..l) -: (.2> . n .- : > . .

Hig velocy ed __________ _ Lowelcten

(4 hre separte eo .ZZl *at. d1E^t tig fie.4s,:Note-

e~~~~~~~~~~~~~~~~~~~~~Fcn p. 656.

. .~~~~~~~~~~~~~~~~~~~~~~~.e Hik .. . . ...

..mr O' . 1. ..t d AC ot..-

(H)~.; Sli -ooted,, *0-'fe.. lie. of dM rn:pto wt....:

: . . . _ : ,~~~~FI. 2..,

FIG.~~~~~~~~(Fcn p2.5.




In order to test the working of the whole apparatus, a set of lines was produced on the film, fig. 2, (i), Plate 14, the position of each exposure on the film being altered by varying the magnetic field slightly. The values of I and t were kept constant.

A typical case gave the following values:

D=034; 035; 035

for three exposures on the one film close together. Experiments of this sort showed that it was definitely possible to ensure

repetition on the same film of identical exposures so as to yield substantially the same values of D.

Similar experiments comparing densities on different strips cut from the same sheet showed somewhat larger variations of D up to about 8 per cent. but it was observed that when films cut from different film batches were tried, there were much larger variations.

It was very soon found that the well-known empirical light law connecting D, I and t held with some accuracy.

This law states that D =y. log Itt -

where ID is the density or blackening, measured photometrically and expressed

b g cident light transmitted light

y is a constant if the method of development and the nature of the light do not vary.

I is the intensity of the light incident on the film.

t is the time of exposure.

p is " Schwarzchild's constant."

i is the " inertia" of the plate or film.

In the case of electron blackening of the film, it is only necessary to replace I by electron beam current and D by log10 CQ/C and the formula holds but, of course, with different values for the constants p, i and y.

The evaluation of the constants was not difficult from the experimental resuIts and the following procedure was followed.

Observations were first taken keeping I constant, varying only t. From the

* Dobson, Griffiths and Harrison (loC. cit.).

VOL. CXXXVI-A. 2 u




values of D emerging, a curve was drawn showing D against log t. It was a straight line. The above formula therefore holds if I is constant.

But if on a film giving the result just described with intensity (IA) constant, another line be placed with a different intensity (1B) which, with an exposure tB gives a density DB within the range of observation, then it is possible to choose a point on the curve just described for which there is an equal density corresponding to time tA and intensity IA. In which case clearly

log IAt' = log IBtB , and therefore

log IA -log ]B log tB -log tA

The value of y was obtained by investigating the relation between D and log I, keeping t constant. This curve was also found to be linear, and y was obtained at once from the slope of the curve, since

D = y. logI + yplogt-i,

and the last two terms in this equation are constant. Moreover, if D 0_ ; i = y log I + yp log t, where log I is the intercept and t the constant value of the exposure in the experiments.

The following table gives the results obtained using " Imperial Duoplex" film, the probable mean error also being indicated. In fig. 4 is exhibited in

0* 2

0 AO 1S20 2280 3040

FIG. 4.-Continuous curve represents D = O*31 log It0-8 - 047.

graphical form some of the results incorporated in the table, the smooth curve being that calculated for the formnula D _ 0* 31 log It088 - 047.




p.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

O*88 ? 0.01 047 ? 003 031 A: 0*01 Electrons-Film untreated. 0 90 4 002 0 17 i 0 01 0-27 A 0 01 Electrons-Film oiled. 0-86 _ 0-41 Light.

It is to be noted that y and p are ratios and independent of the units of I and t. In the case of i, however, which here has the value of 0 * 47, the units of I and t are respectively amperes per square centimetre X 10-8 and seconds.

It is to be noted that the range of D was sevenfold, and that the relations hold over more than a hundredfold variation of I and of t.

Examination of the above table of results brings out that the value of the constants p and y do not change much for electrons striking an oiled or an untreated film. The increased sensitiveness for the oiled film being expressed in the formula by a large reduction in i-the so-called " inertia " of the film. To give a numerical idea of this increased sensitiveness; a certain I for exposure time 60 seconds gave a certain D on an untreated film; an oiled film would give the same D for the same I in only 5 seconds.

It is of interest finally, to record the important fact that whether oiled or untreated films be used, there is no observable change in D with the velocity of the electron beam at any rate between 60 and 300 volts. On the view that the increased sensitiveness of an oiled film is due to fluorescence of the oil, this result is readily understood. The investigations of Harrison and Leighton* show that when fluorescence is evoked by incident light of varying frequency, the number of quanta emitted is proportional only to the number incident and independent of the frequency. In the present case, assuming the electron energy to be far in excess of the excitation value, the number of quanta emitted by the oil (fluorescent light intensity) will be proportional only to the number of incident electrons and not on their energy-the result observed-thus the film is to be regarded as " panchromatic " within the above-mentioned range.

Comparison of Photographic Action of Electrons of Different Velocities. It may not be out of place to compare now the photographic effects of

electrons over the whole range of energy so far examined by different investigators. This can however only be done in a very general way, since the effects depend to some extent on the nature of the photographic emulsion.

* ' Phys. Rev.,' vol. 88, p. 899 (1931). 2 u 2



6(60 R. Whiddington and J. E. Taylor.

The earlier attempts at accurate measurement in the case of electrons were confined to the (-rays from radioactive preparations and Bothe* examining heterogeneous (-rays found the Reciprocity Law held, namnely that D =f(It) which furniished a linear characteristic curve up to values of 0 * 5 for D.

This result was not entirely confirmed by Salbacht or by Ellis and Wooster,t the latter, however, found that the Reciprocity Law held, suggesting the formula

D C log (It/xr + 1)

as expressing closely enough the characteristic curve which, under the conditions of their experiments was linear up to D c 0 3, the form of the curve being independent of the velocity.

This latter point was investigated in further detail by Ellis and G. H. Aston? who found a very rapid change of photographic action with electron velocity.

The results of the present experiments show that within the region of electron velocity, between 60 and 300 volts, the simple reciprocity law does not hold, but that a law of the Schwarzchild type D f (It ) must be employed, as in the case of light.

The value of p is sufficiently far from unity to make the diflerence between the Reciprocity and Schwarzchild relations very marked.

A further difference between the slow electrons and the faster (-rays lies in the fact that within the region considered there is no change in photographic action with velocity. This no doubt depends on the fact that in the case of the faster particles, owing to their high penetrating power, only part of the available energy is used within the film, whereas the slower particles used in our experiment are wholly absorbed and of the whole energy of the slower particles again, only a definite amount, in the case of each particle, is employed in photographic action.

If it be supposed further that this definite energy acts in a secondary manner on the emulsion via fluoreseence excitation, it can be understood that a relation of the Schwarzchild type might be required to explain the facts.

Acknowledgments.

It is a pleasure to record the fact that one of us (J.E.T.) is in receipt of a maintenance grant for a student in training from the Department of Scientific

* 'Z. Physik,' vol. 2, p. 243 (1922). t 'Z. Physik,' vol. 2, p. 107 (1922). $ ' Proc. Roy. Soc.,' A, vol. 114, p. 266 (1927). ? ' Proc. Roy. Soc.,' A, vol. 119, p. 645 (1928).



Photographic Actton of Slow Electrons, 661

and Industrial Research and that some of the apparatus was purchased out of a grant from the Royal Society.

Summary.

The photographic action of electrons (60-300 volts) has been experimentally investigated in the case of " Imperial Duoplex " films.

The considerable increase in sensitiveness previously discovered on oiling the film surfaces has been found to be associated mainly with a diminution of the value of i in the formula

D-y . log It" _-, where

D is the density or blackening, measured photometrically and expressed

,by log10 incident light transmitted light.

I is the intensity of the light incident on the film.

t is the time of exposure.

p is " Shwarzchild's constant."

i is the " inertia " of the plate or film.

The constants have the following values

0.88 i 0*01 0*47 0 O03 0 31 Zt 0 01 Electrons-Film untreated. 090 ? 0*02 0*17 ? 0*01 0-27 z? 0 01 Electrons-Film oiled. 0*86 - 0*41 Light.

The results in this table are based on the units, seconds and amperes per square centimetre X 10-8 for t and I respectively.

It has been found that these constants are independent of the velocity of the electrons within the range considered.

The evidence in favour of the sensitising effect of the oil being due to fluorescence is mentioned.

A preliminary account of this work was published in 1931 in the ' Proceedings of the Leeds Philosophical Society.'*

* Whiddington and Taylor, 'Proc. Leeds Phil. Lit. Soc.,' vol. 2, p. 264 (1931).



662 L. H. Gray and G. T. P. Tarrant.

[Note added.-Since writing the preceding we have seen a paper by Herr Valentin Weidner, of Heidelberg, entitled " die photographische Wirkung langsamer Kathodenstrahlen,"' 'Anin. Physik.,' vol. 12, p. 239 (1932).

This author has investigated in somne detail the photographic action of electrons of speeds between 30 and 1100 volts. The plates he used were not the same as ours and he finds a simple reciprocity law of blackening instead of the more complicated form which we find necessary. This difference may be due to the different emulsion, but it is rather difficult to see how the agree- ment that between 60 and 300 volts the blackening is independent of electron energy, fits in with this supposition.]

The Nature of the Interaction between Gamma-Radiation and the Atomic Nucleus.

By L. H. GRAY, Ph.D., Fellow of Trinity College, Cambridge, and G. T. P. TARRANT, M.A., Pembroke College, Cambridge.

(Communicated by Lord Rutherford, O.M., F.R.S.-Received April 14, 1932.)

? 1. Introduction. A numaber of independent investigations by Chao,* Meitner and Hupfeld,t

and Tarrant,J have shown that the absorption of strongly filtered thorium C" y-rays, both in magnitude and in the manner of its variation with the atomic number of the absorbing element, was in definite disaccord with what was independently known concerning the absorption of y-rays by extra-nuclear electronic systems. The additional absorption was attributed to interaction with the nuclei of the atoms concerned. In the case of quantum energies as high as 2y million electron-volts, interaction with a heavy nucleus is quite important. In lead, for example, it accounts for 20 per cent. of the total absorbing power of the atom. The amount of energy absorbed by a nucleus is roughly proportional to the square of its atomic number, and from the

* ' Proc. Nat. Acad. Sci. Wash.,' vol. 16, p. 431 (1930). t ' Z. Physik,' vol. 67, p. 147 (1930). j 'Proe. Roy. Soc.,' Av vol. 128, p. 345 (1930).



on the photographic action of slow electrons

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