US3920992A

US3920992A - Process for forming developable electrostatic charge patterns and devices therefor

Info

Publication number: US3920992A
Application number: US420560A
Authority: US
Inventors: Den Bogaert Jan Van; Willy Joseph Palmans; Den Eynde Karel Alfons Van
Original assignee: Agfa Gevaert AG
Current assignee: Agfa Gevaert NV; Agfa Gevaert AG
Priority date: 1973-07-16
Filing date: 1973-11-30
Publication date: 1975-11-18
Anticipated expiration: 1992-11-18
Also published as: FR2238174B1; JPS5072635A; DE2433766A1; GB1471858A; FR2238174A1; CA1038441A; BE817408A

Abstract

A method of recording information as a pattern of electrostatic charges carried by an insulating charge receiving medium which comprises: EXPOSING TO A PATTERN OF X- or gamma -rays an imaging chamber containing an ionizable gas having an atomic number of at least 36 and adapted to produce therein upon such exposure electrostatic charges in a corresponding pattern, while maintaining said gas during said exposure under superatmospheric pressure; ELECTRICALLY BIASING THE CHARGES IN SAID PATTERN TOWARD AN ARRAY OF DISCRETE CLOSELY SPACED PHOTOCONDUCTORS DISPOSED ALONG ONE SIDE OF SAID CHAMBER GENERALLY NORMAL TO THE EXPOSURE DIRECTION WITHIN A FRANGIBLE INSULATING MATRIX; SAID CONDUCTORS EXTENDING FROM THE INTERIOR TO ONE EXTERIOR OF SAID CHAMBER WALL AND THUS AT THEIR EXTERNAL ENDS DURING SUCH EXPOSURE ACQUIRING A DIFFERENTIALLY CHARGED CONDITION ACCORDING TO SAID PATTERN; ARRANGING IN AT LEAST CLOSE PROXIMITY TO THE EXTERIOR ENDS OF SAID ARRAY AN INSULATING CHARGE RECEIVING MATERIAL, SAID MATERIAL DEVELOPING A DIFFERENTIALLY CHARGED CONDITION ACCORDING TO SAID PATTERN UNDER THE INFLUENCE OF THE DIFFERENTIALLY CHARGED CONDITION OF THE EXTERIOR CONDUCTOR ENDS; AND WHILE SAID CHAMBER IS MAINTAINED UNDER SAID SUPER-ATMOSPHERIC CONDITION, MECHANICALLY BIASING SAID CHARGE-RECEIVING MATERIAL OVER AN AREA AT LEAST COEXTENSIVE WITH SAID MATRIX AGAINST THE EXTERIOR SIDE OF SAID MATRIX WITH A FORCE OPPOSING THE GAS PRESSURE EXERTED AGAINST THE INNER SIDE OF SAID MATRIX.

Description

United States Patent [191 [451 Nov. 18,1975

Van den Bogaert et al.

[ PROCESS FOR FORMING DEVELOPABLE ELECTROSTATIC CHARGE PATTERNS I AND DEVICES THEREFOR v [75] Inventors: Jan Van den Bogaert, Schilde; Willy Joseph Palmans, Kessel; Karel Alfons Van den Eynde, Deurne-Zuid, all of Belgium [73] Assignee: Agfa-Gevaert, Mortsel, Belgium [22] Filed: Nov. 30, 1973 [21] Appl. No.: 420,560

[30] Foreign Application Priority Data July 16, 1973 United Kingdom 33744/73 [52] US. Cl. 250/315; 250/326; 250/323 [51] Int. Cl. G03b 41/16 [58]. Field of Search 250/213 VT, 315, 315 A,

250/324, 325, 326, 320, 323; 346/74 CR, 74 ES, 74 EB; 355/3 R, 17

[56] References Cited UNITED STATES PATENTS 3,508,477 4/1970 Groo 250/315 3,710,125 l/l973 Jacobs et al. 250/213 3,757,351 9/1973 Simms 346/74 3,774,029 11/1973 Muntz et al 250/315 Primary Examiner-James W. Lawrence Assistant ExaminerB. C. Anderson Attorney, Agent, or Firm.William J. Daniel [57] y ABSTRACT A method of recording information as a pattern of electrostatic charges carried by an insulating charge receiving medium which comprises:

exposing to a pattern of X- or y-rays an imaging chamber containing an ionizable gas having an atomic number of at least 36 and adapted to produce therein upon such exposure electrostatic charges in a corresponding pattern, while maintaining said gas during said exposure under superatmospheric arranging in at least close proximity to the exterior ends of said array an insulating charge receiving material, said material developing a differentially charged condition according to said pattern under the influence of the differentially charged condition of the exterior conductor ends; and

while said chamber is maintained under said super-atmospheric condition.- mechanically biasing said charge-receiving material over an area at least coextensive with said matrix against the exterior side of said matrix with a force opposing the gas pressure exerted against the inner side of said matrix.

31 Claims, 6 Drawing Figures Sheet 3 of5 3,920,992

U.S; Patent Nov. 18, 1975 VIIIIIIIIII US. Patent Nov. 18, 1975 Sheet40f5 3,920,992

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US. Patent Nov. 18, 1975 Sheet 5 of5 3,920,992

Fig.6

PROCESS FOR FORMING DEVELOPABLE ELECTROSTATIC CHARGE PATTERNS AND DEVICES THEREFOR This invention relates to a process for forming developable electrostatic charge patterns and devices for producing such patterns. V t

From the German Patent Specification 1,497,093 an imaging technique is known in which a photocathode is used to produce an electrostatic charge pattern on a non-photosensitive insulating material. In this technique an air-tight chamber is filled with an ionizable gas e.g. a mixture of argon and monobromotrifluoromethane (1:5) and is provided with a photocathode and an cathode.

In the recording appliances described in this German Patent Specification, a polyester foil is used as a charge receiving insulating sheet. At the sides the polyester sheet is pressed on sealing strips in order to keep the interspace filled with the ionizable gas. Each electrostatic image is obtained on a separate insulating sheet and is toner-developed on that sheet. Such procedure requires for each print the refilling of the interspace with ionizable gas before the production of a new print canstart.

From the Belgian Patent 792,334 corresponding to US. Pat. No. 3,774,029 a process for the production of an electrostatic image is known which is characterized by the steps of(1 placing a dielectric sheet between an anode and a cathode, (2) filling at a pressrue above atmospheric pressure the interspace between the anode and cathode with a gas with an atomic number of at leastequal to 36, preferably xenon, which upon exposure to X-ray forms electrons and positive ions, (3) attracting the electrons towards the anode and the positive ions to the cathode by applying a potential difference between the electrodes in order to cause deposition of one of the types of the charged particles onto the dielectric sheet.

In the process exemplified by the FIGS. 1, 5 and 6 of the Belgian Patent 'a cassette is used which has to be opened and filled with the ionizable gas before the production of a new print can start.

This makes the production'of a series of prints rather 7 time consuming. Further it will be difficult to avoid the loss of ionizable gas.

It is an object of the present invention to provide a process wherein charged particles are emitted information-wise in an ionizable gas medium and a charged particle emission pattern is transferred outside said medium without, disturbing the pressure condition of said medium.

A further object of the present invention is toprovide devices for achieving the above object.

. Still further objects, features and advantages of the present invention will become apparent'upon consideration of the following disclosure.

According to the present invention information is recorded in terms of an electrostatic charge pattern formed of emitted charged particles generated in the interior of an airtight envelope, containing an ionizable gas, by a method characterized in that an electron or positive ion emission pattern created by an image-wise electrical discharge in thegas is projected in the same pattern onto an area of the inner side of the envelope which contains an array of closely spaced solid conductors separated by a solid electrically insulating material and extending from the interior to the exterior of the envelope such conductors receiving the pattern of electrons or positive ions thus formed and consequently undergoing a change in the charge condition at their exterior ends, and during or after the creation of the electrical discharge in said gas the exterior side of the envelope which contains the exterior ends of these conductors is held in contact or in close proximity with an insulating surface of a charge receiving material backed in its rearside with an electrode to which is applied a voltage such that an electrostatic charge pattern is formed on the insulating surface. The charged particles produced by the electrical discharge in the ionizable gas are electrons (photoelectrons and optionally secondary emission electrons) and positive ions.

The part of the emission pattern consisting of electrons is called the electron image. The part of the emission pattern consisting of positive ions is called the ion image.

The transfer of electrons from the conductors to the receiving material or vice versa can be improved by applying an electron accelerating DC potential difference across or between the means generating the charged particles image and the rear side of the charge receiving material.

The basis elements of recording apparatus of the present invention and various embodiments for associating these elements into operative units are illustrated by the accompanying schematic drawings wherein FIG. 1 is a cross-sectional representation of a recording system structure of the present invention, FIGS. 2 to 6 are cross-sectional representations of alternative imaging systems.

It should be understood that in these figures some dimensions of the layers of the possibly present photocathode, of the insulating target, etc., have been greatly exaggerated to show the details of construction.

No inferences should be drawn as to the relative dimensions of the layers or spacings separating the various elemental parts of the imaging appparatus.

The reproducing system according to the invention in the form illustrated schematically in FIG. 1 is suitable for X-ray recording and operates with an ionizable gas under atmospheric pressure or slight overor underpressure e.g. of 1 to 10 Torr.

The envelope 1 is filled with an ionizable gas or gas mixture in admixture with a discharge quenching substance e.g. ethanol as described e.g. in the German Patent Specification l,497,093. The filling gas is advantageously kept under an overpressure above atmospheric pressure of a few Torr, e.g. 10 Torr. A useful gas mixture consists e.g. of argon and monobromotrifluoromethane (CF Br) in the volume ratio 1:5. When using the above fluoromethane a separate quenching additive is not required. The applied D.C. voltage is preferably not more than 5% above the breakdown voltage of the gas.

The photocathode 2 is of the type described in the German Patent Specification 1,497,093 e.g. is a 1.5 micron layer of lead or a 1.0 micron layer of uranium applied on an aluminium sheet 8. During the informationwise X-ray exposure of the photocathode 2 a DC- potential difference is applied by means of the potential source between the photocathode and backing electrode 6.

The distance between the photocathode 2 and the input-ends of an array of conductive pins 3 embedded in an insulating matrix is preferably in the range of 0.3 to 5 mm. The potential difference between the photocathode 2 and the electrode 6 contacting the conductive rear side 4 of the insulating web material 5 defines an accelerating field acting upon the electrons and determines together with the separation the kind of ionizable gas and its pressure the degree of the electron multiplication (if any) that is possible by secondary emission (i.e. an avalanching effect).

According to the embodiment illustrated in FIGv 1, the photocathode 2 is provided with a screen 7 having minute holes for preventing the divergence of the electrons and improving image sharpness.

The height of the screen 7 and the cross-section of each individual microchannel of the screen determine the image sharpness. The ratio of cross-section diameter to the height of the individual microchannels is preferably at least 1:4. The clearance between the output openings of the screen and of the input ends of the conductive wires 3 is preferably not larger than 1.5 mm.

The minute holes of the screen 7 may have a diameter of e.g. 0.2 mm and a depth of e.g. 0.8 mm. The screen can be made of a plastic material or metal. By means of a screen having the above hole dimensions, the photo-electrons which, when liberated by X-rays, are emitted in all directions from the photocathode layer 2 are directed in such a way that those diverging by more than from the perpendicular to the plane of the electrode become absorbed.

According to a special embodiment the screen plate 7 is an assembly of electrically insulating resin or glass sheets or semi-conductive or conductive sheets e.g. metal sheets preferably of a metal with high atomic number e.g. higher than 50. Such metal sheets may be coated with an insulating material e.g. an insulating resin. These sheets are corrugated in a direction parallel to the desired path of electron flow, so that the corrugations cooperate to provide parallel channels or conduits for the electrons. The manufacture of such channel plates but having secondary-emissive characteristics has been described in the United Kingdom Patent 954,248. Other techniques for producing microchannel plates are described in the United Kingdom Patent Specifications 1,064,072 and 1,064,075. For the purpose of the embodiment discussed in connection with FIG. I the materials of the plate are chosen in such a way that secondary emission on the screen walls does not occur at least to any substantial extent under the conditions of voltage, gas composition and gas pressure applied in the imaging apparatus of FIG. 1. Indeed, secondary emission resulting from so-called ionic feedback (see IEEE Transactions on Nuclear Science Vol. NS-l3 June 1966, pages 8899) has to be avoided since at a particular gas pressure, normally above 10 Torr (see Advances in Electronics and Electron Physics Vol. 28 (1969) pages 499-506), a self-sustaining discharge is obtained in the gas-medium which discharge degrades the electrostatic charge image on the insulating charge receiving medium. Ionic feedback is a phenomenon which arises when ions produced at the output 4 end of the channels are accelera ed back down the channels and set free secondary electrons by striking the secondary emissive inner walls at the input ends of the channels. The electron density may be increased in this way to such a degree that a self-sustaining discharge occurs, which has to be avoided.

Using a conductive internal collimator (e.g. element 7 in P16. 1) no direct electrically conductive contact with the wires of the pin matrix 11 may exist. Therefore the bottom end of the collimator is either supported by insulating material or is spaced from the pin matrix at a distance sufficient to avoid electrical breakdown toward the pin matrix when operating the apparatus under the imaging voltage conditions.

The electrons. multiplied by secondary emission in the gas medium. are projected onto the inner ends of the conductive wires 3 of the pin matrix plate 11 forming part of the vacuum envelope wall. The conductive wires 3 are held in substantially parallel separated relation by a matrix of solid electrically insulating material 12, e.g. glass, and extend at their outer ends at the outer side of the envelope 1 towards the electrically insulating surface of the charge receiving web 5, e.g. a polyester resin film web or dielectric paper web having an electrically conductive coating or support 4 in electrically conductive contact with the conductive backing plate 6. The charge receiving web 5 is supplied by a supply reel (not shown) and moved over a guiding roller (not shown) into a toner applicator and fuser station known from electrophotography. From that station, the web moves into a cutting device comprising e.g. a movable knife and a stationary knife, and after leaving the cutting device the obtained sheets are collected in a collector tray. all as known in the art and thus not illustrated here.

The envelope material 1 is coated on its inside surface with electrically insulating material as at 9. The envelope 1 can be evacuated and filled with ionizable gas and quenching gas at the desired pressure through a pipe fitting 13.

As described by Lansiart et al. in Nuclear Instruments and Methods 44 (1966) page 46, the organic quenching vapour will be decomposed after a certain number of electron avalanche dischargings so that it will be necessary to replace the ionizable gas and quenching gas by a fresh gas mixture after a given number of exposures.

The exposure of the photocathode e.g. with information-wise modulated X-rays may proceed from the rear side (i.e. the side of the conductive backing 8 of the photocathode or front side (i.e. the side of the charge receiving material (5,4) e.g. as described in the U.S. Pat. Nos. 2,221,776 and 3,526,767'or as in the published German Patent Application 2,231,954. The photocathode may itself be in the form of a screen or an assembly of lamellae as described in our co-pending United Kingdom Patent Application filed 21st May 1973 and titled: Electrostatic Imaging Device and Process Using Same which has to be read in conjunction herewith.

According to a special embodiment the X-ray recording in the form of an electrostatic charge pattern is carried out with an imaging device comprising an airtight envelope with an electrode that is separated from a pin matrix wall by an interspace filled with an ionizable gas having an atomic number of at least 36. The ionizable gas, being preferably xenon, is kept under a pressure above atmospheric pressure.

In this embodiment, in order to improve the image sharpness the voltage applied over the electrodes is preferably such that no substantial electron multiplication, i.e. no electron avalanching, by secondary emission takes place; in other words the imaging chamber is operated in the horizontal part of the Townsend curve. For the explanation of that curve see Molecular Science and Molecular Engineering by Arthur R. von Hippel et al. (1959) The Technology Press of M.I.T. and John Wiley and Sons, Inc., New York, page 78. I

The pressure under which the ionizable gas is kept may be rather high as described in the Belgian Patent 792,334. When operating in the present invention with a high atomic number gas as referred to above the product of the pressure and the thickness of the interspace between the electrode and the input ends of the ,pin matrix may be, e.g. in the range of mm.atmospheres and 200 mm.atmospheres. A pressure of 6 atmospheres combined with an interspace of mm gives very good results.

This, however, poses a problem with regard to the thickness and mechanical strength of the walls of the envelope taking into account the fact that the wall of the envelope directed to the X-ray exposure source preferably is not made of materials that contain or con sist of elements having a high atomic-number. So there will'be only a limited choice of X-ray penetrative wall material when the X-rary exposure dose has to be kept low which is the case e.g. formedical X-ray purposes.

' According to an embodiment suited for rather low dose X-ray recording the X-ray exposure is effected from the side opposite to the pin matrix. For example, opposite the side of the pin matrix the envelope contains a wall of curved beryllium plate of a thickness of 0.4 cm and of which the radius of curvature is'300 cm as described in Belgian Patent 792,334. Such curved plate wall offers a sufficiently high resistance to the mentioned gas pressure and presents a not too high absorption for the X-rays.

For industrial (e.g. non-destructive testing purposes) X-ray radiography, the X-ray dose need not be kept as low as possible and the wall of the envelope directed to the X-ray source may be rather highly X-ray absorptive. In that case the X-ray exposure may be made directly through the pin matrix wall that is rather thick to sium.

Optionally the ends of the wires of the pin matrix inside the envelope containing ionizable gas are coated with a photocathode material e.g. as described in the U.S. Pat. No. 3,508,477. In X-ray recording the ends of the wires are preferably covered with a suitable X-ray photocathode material e.g. lead or the wires are made ofa high atomic number metal. In the'latter cases, however, the X-ray exposure is preferably effected from the anode side and the charge image formation proceeds on an insulating surface that prior to the X-ray exposure has been charged uniformly with negative charges and is information-wise discharged through the wires of the pin matrix.

The apparatus illustrated in FIG. 2 relates to an embodiment in which the X-ray exposure is effected from the side of the receiving material directly through the pin matrix and in which the pin matrix has a sufficient 6 mechanical strength to withstand a gas pressure of 10 atmospheres.

The elements indicated in FIG. 2 by the numbers 1 and 3 to 6 and 9 to 13 have the same significance as described in FIG. 1. The wires 3 are preferably made of aluminum or beryllium and the matrix material 12 incorporating the wires 3 is preferably glass that does not contain high atomic number elements e.g. is made of borosilicate glass. The element 20 is an electrode (cathode) that optionally has photoelectron emitting properties when struck by X-rays e.g. is made of lead.

According to a special embodiment of the present invention the pressure inside the imaging chamber acting upon the pin matrix wall is counteracted outside the imaging chamber by means operative to press the recording material against the outer wall of the pin matrix at a pressure that is substantially equal to the pressure inside the imaging chamber. In order to allow for transport of the recording material (its positioning in contact with the pin matrix and its removal therefrom) the pressure is not maintained during the transport step and therefor means are used to increase and decrease respectively the pressure inside the imaging chamber and on thepin matrix at the exterior side of the imaging chamber.

The application of pressure to the recording material give more intimate contact with the wire ends of the pin matrix and improves the image sharpness and chargedensity. When applying the counter pressure technique there is no needfor a rather thick pin matrix wall; a relatively thin wall may be used so that the radiation dose may be reduced.

According to a first embodiment illustrated in FIG. 3 the imaging chamber is of the same type as described in FIG. 2. The envelope wall 1 of the imaging chamber carrying the pin matrix 11 is made of steel and mounted onto a piston 21 that can be moved in upward and downward direction by the pressure applied in the cylinder 22 on a gaseous medium e.g. air or liquid medium e.g. oil supplied by the cylinder 27. Preceding the X-ray exposure a web-like charge receiving material composed of a charge receiving resin film web 5 coated with a conductive backing 4 is positioned in front of the outer side of the pin matrix wall 11. A conductive rigid backing plate 23 e.g. made of beryllium serves as an electrode and as a supporting wall for withstanding the pressure applied on the pin matrix 11 when the ionizable gas is introduced under pressure in the imaging chamber.

The pressure applied by the piston 21 and the pressure built up in the imaging chamber are kept substantially equal so that at any stage of the imaging procedure the pressure at both sides of the pin matrix wall is substantially equal.

This is realized e.g. by coupling the action of the pressure cylinder 27 with that of the cylinder 24 from which the ionizable gas, e.g. xenon gas, is fed by the piston 25 through the fitting 13 into the imaging chamber 1. A frame 26 fixed to the floor 29 keeps the rigid electrode 23 in front of the imaging chamber and takes the force applied to the electrode 23 when piston 21 moves upwardly. An insulating sheet or plate 28 covers electrode 23 and avoids direct contact of the operating personal or patient with the voltage of the voltage source 10.

According to a second embodiment illustrated in FIG. 4 the imaging chamber is likewise of the type illustrated in FIGS. 2 and 3. The envelope wall of the imag- 7 ing chamber 1 opposite the pin matrix 11 is made of steel and mounted onto a piston 21 that can be moved in upward and downward direction by the pressure applied in the cylinder 22 by a gaseous medium e.g. air or liquid medium e.g. oil supplied by the cylinder 27. After positioning the receiving material (4,5) in front of the pin matrix 11, a rectangular or square cover 30 provided at its edges 31 with a pressure sealing ring 32 is pressed against a thin metal electrode 33 e.g. made of beryllium, the form a closed chamber of which thetop wall 34 is preferably made of a metal having a low atomic number and high mechanical strength e.g. beryllium. Simultaneously with the feeding of the ionizable gas under pressure into the imaging chamber with the cylinder 35 and piston 36 the imaging chamber is pressed by piston 21 against the edges 31 while the cylinder 37 and piston 38 through the pipe fitting 39 introduce gas e.g. air, at the same pressure in the space formed by the cover 30.

After the X-ray exposure the pressure is reduced again and the web-like receiving material is moved over a distance sufficient to allow the positioning of the next image frame.

The film web containing the electrostatic charge pattern is before or after development cut into sheet form. According to a modified embodiment instead of a film web separate charge receiving sheets are used that are supplied by a dispenser known to those skilled in the art of sheet feeding in copying machines.

According to a third embodiment illustrated in FIG. the imaging chamber is likewise of the type illustrated in FIG. 2. The X-ray exposure source is element 63.

The envelope wall 1 of the imaging chamber carrying the pin matrix 11 is made of steel and mounted in a supporting frame 40. Before the X-ray exposure a web-like charge receiving material composed of a charge recieiving resin film web 5 coated with a conductive backing 4 is positioned in front of the outer side of the pin matrix wall 11. Above the pin matrix 11 is a pressure cushion 41 having a flexible membrane 42 and rigid top plate 43 joined by a hollow rectangular or square flexible sealing ring 44 which is provided with a safety expansion valve 45. The cushion 41 receives a gaseous or liquid medium from a cylinder 46 in which the piston 47 is operated simultaneously with the piston 48 of a cylinder 49 containing an ionizable gas in order to obtain an even pressure on both sides of the pin matrix 11. The ionizable gas is introduced through the pipe fitting 13 into the imaging chamber 1. A DC-voltage is applied between the conductive flexible membrane 42 or conductive coating on that membrane. The membrane is made e.g. of an elastomer e.g. a synthetic rubbere that is vacuum coated with aluminium in the area contacting the conductive backing 4 of the film web 5.

The reproducing system illustrated in FIG. 6 is a modified version of the one illustrated in FIG. 2. Here, the numeral 50 designates a steel tube of which the internal structure of the welding joint has to be inspected. A dielectric web 51 covered with a conductive rear coating 52 is wrapped around the welding joint. The recording head 54 containing ionizable gas contains a single row of substantially parallel conductive pins 55 embedded in the insulating wall 56 of the recording head 54. The pins 55 penetrate the envelope from the outer face to the inner face and stand at the inner face of the envelope in contact with an ionizable gas that is kept under high pressure, preferably xenon, that has a high 8 absorption power for X-rays and 'y-rays. The input openings of the row of pins 55 are facing an electrode strip 57.,

The charge pattern is produced line-wise by progressively moving the tube 50 with the transport rollers 64 with respect to the recording head 54 or by progressively moving the recording head 54 along the circumference of the tube 50 while in the tube 50 at a short distance from its welding joint a radioactive source 58 e.g. a caesium 137 or cobalt 60 source emits 'y-rays. By employing magnetic or electrostatic focusing coils around the recording head image sharpness and charge density may be improved. Between the conductive electrode strip 57 and the conductive rear coating 52 or electrode layer contacting that layer. a DC potential of several kV is applied by a potential source 59 having the positive pole' connected to the grounded tube 50 and the negative pole to electrode 57. The element 60 is a radiation shield containing a lead sheet 62 fixed to an electrically insulating resin layer 61.

Depending on the kind of emitter of charged particles e.g. the photocathode, used in the imaging chamber and its radiation sensitivity, electromagnetic radiation patternsof penetrating radiation e.g. X-rays, 'yrays, neutron rays, fi-rays and of ultraviolet, visible light and/or infrared may be used in the exposure step. The penetrating rays e.g. X-rays, 'y-rays, B-rays and neutrons may be transformed into ultraviolet and/or visible light by fluorescent layers that are positioned in close proximity or contact with a photocathode member that is sensitive for the'fluorescent light.

According to a preferred embodiment the imaging chamber does not contain a solid state photocathode and the ionizable gas itself serves as a photo-electron emitter which is the case e.g. when using a high atomic number gas e.g. xenon in an exposure step with penetrating radiation e.g. X-rays and y-rays.

Depending on the type of the optionally present photocathode and the ionizable gas all kind of reproduction and copying work e.g. document copying, microfilm enlargement, facsimile, X-ray photography and even cinematography e.g. by operating at 6 to 16 image frames per second is possible. In this connection the attention is drawn to the embodiment represented in FIG. 6 in which the production of the charge pattern proceeds line-wise and the transfer of the charge pattern proceeds with a wire-matrix containing a single row of wires.

The process of the invention includes apart from the direct charging of previously uncharged insulating charge receiving surface also those embodiments in which the charge receiving material has been charged overall prior to the image-wise charge transfer from the pin matrix. So, according to one of the embodiments of the invention an insulating surface that has been overall charged with charges opposite in sign with respect to the charges conducted by the wires of the pin matrix is image-wise discharged or its charge is image-wise substantially lowered.

According to another method the wires of the pinmatrix are non-differentially pre-charged from outside the envelope without charging the insulating material in which the wires are embedded. This may proceed in a first embodiment by using a pin-matrix having the conductive wires protruding from the external side of the imaging chamber. The protruding wire ends are contacted with a charged electrode before effecting the image-wise photoexposure of the emitter of charged I 9 particles in the imaging chamber. In the exposed areas the charged wires are discharged and the charge of the According to a second embodiment the external wire ends are non-differentially charged e.g. with a corona, without charging the insulating material of the pinmatrix through a screen plate the openings of which correspond with the external ends of the wires of the pin matrix. This embodiment requires a great care in bringing the screen holes in alignment with the wire ends.

Although in order to decrease the X-ray radiation dose the wires of the pin matrix are advantageously made of a low atomic number metal or metal alloy, preference may be given in some cases to a pin matrix with metal wires having a relatively high X-ray absorption power e.g. steel, copper, nickel and in particular cases Wolfram or platinum. The heavy metal wires are directed towards the focus of the X-ray source and are interleaved with a material that is more transparent to X-rays e.g. glass or resin. The primary rays (the rays coming in straight line from the X-ray source) pass through the pin matrix while the rays scattered in the object, i.e. the oblique rays, are mostly absorbed in the wires so that improved image sharpness is obtained.

A pin matrix element appropriate for serving as multiconductor wall of the imaging chamber is available under the trade mark Multilead from Corning Glass works, Industrial Bulb Sales Department, Corning, NY. It is available with a number of different conductor materials and sizes and a number of different spacings between the conductors. The Multilead material comes in sheet or strip form and can be incorporated into the vacuum envelope wall by a suitable glass fusion technique.

A pin matrix can be produced with glass fibres containing a metal core as is described in the United Kingdom Patent Specification 1 ,064,072. According to this technique metal-cored glass fibres are drawn down till a sufficient length of 200-300 micron fibre is obtained. A bundle of fibres is made by sealing the fibres together 7 and is then cut into lengths of say, cm. Each of these lengths of bundle is then drawn down in the same way as the original tube, equipped with an external cladding of thin insulating glass and drawn down till it is about 50 micron in diameter. This glass fiber containing a metal wire e.g. copper is quite easy to handle. According to that technique 10 p. fibres can be made. Such a technique has been discussed also in Philips Technisch' Tijdschrift (1969) No. 8/9/10, page 259.

The wires or pins in the matrix should be preferably short and, the dielectric constant of the binder material low, to promote high charge transfer speed and maximum image resolution.

' The transfer of the electrostatic images may proceed by conduction of electrical charges across a gas or air gap or by direct charge transfer when a gas or air gap is not present, e.g. by applying pressure or by effecting the transfer in a vacuum frame. 7

Image sharpness is practically not affected by charge transfer by contact. This requires, however, a close and direct contact of the ends of the conductivewires with the insulating materials. Such intimate contact is obtained in practice by operating with very smooth surfaces that are placed together under pressure.

The invention is not restricted to any particular type of development or transfer of the electrostatic charge undischarged wires transferred to an insulating charge receiving material.

pattern onto the insulating target, (charge receiving material) facing the conductive pins of the pin matrix. The development of the electrostatic charge image proceeds preferably, with finely divided electrostatically attractable material that is preferably sufficiently nontransparent for visible light, but may proceed by surface deformation which is a technique known as Thermoplastic Recording (see e.g. Journal of the SMPTE, Vol. 74, p. 666-668.

According to a common technique the development proceeds by dusting the insulating film or film layer bearing the electrostatic image with finely divided solid particles that are image-wise electrostatically attracted or repulsed so that a powder image in conformity with the charge density is obtained:

The expression powder" denotes here. any solid material e.g. finely divided in liquid or gaseous medium, which can form a visible image in conformity with an electrostatic charge image.

Well-established methods of dry development of the electrostatic latent image include cascade, powdercloud (aerosol), magnetic brush, and fur-brush development. These are all based on the presentation of dry toner to the surface bearing the electrostatic image where coulomb-forces attract or repulse the toner so that, depending upon electric field configuration, it settles down in the electrostatically charged or uncharged areas. The toner itself preferably has a charge applied by triboelectricity.

The present invention, however, is not restricted to the use of dry toner. Indeed, it is likewise possible to apply a liquid development process (electrophoretic development) according to which dispersed particles are deposited by electrophoresis from a liquid medium.

The dispersed toner particles may be any powder forming a suspension in an insulating liquid. The particles acquire a negative or positive charge when in contact with the liquid due to the zeta potential built up with respect to the liquid phase. The outstanding advantages of these liquid developers are almost unipolarity of the dispersed particles and their appropriateness to very high resolution work when colloidal suspensions are applied.

Suitable electrophoretic developers are described e.g. in the United States Patent Specification 2,907,674 and the United Kingdom Patent Specification l,l5l,l4l.

The electrostatic image can likewise be developed according to the principles of wetting development" e.g. as described in the United Kingdom Patent Specifications 987,766, 1,020,505 and 1,020,503.

According to a particular embodiment the charge pattern is developed in direct relation to the quantity of charge, instead of to the gradient of charge (fringe effect development). Therefor the developer material is applied while a closely spaced conductor is situated parallel to the insulating charge receiving member. (See for such type of development e.g. PS&E, Vol. 5, 1961, page 139).

According to another embodiment a transferable toner is used and the powder deposit forming the developed image is transferred from the support containing the electrostatic charge image to e.g. a flexible support e.g. transparent film or paper support. In the latter case, any known process for transferring powder image-wise from one support to another can be used; such powder transfer processes are well known in the art of electrophotography. If an electrostatically attractable 1 1 powder is used, the powder image can be transferred by electrostatic attraction, e.g. according to the method disclosed in the United Kingdom Patent Specification 658,699. Further details are contained in the U.S. Pat. 3,384,488 and 3,565,614.

If a powder with ferr -magnetic properties is used for developing the electrostatic latent image, the powder can be transferred by magnetic attraction. The transfer can likewise be carried out by adhesive pick off with an adhesive tape or sheet e.g. SCOTCH brand cellophane tape.

The final powder image is e.g. fixed by heat or solvent treatment.

The charge pattern may be formed on any type of electrographic recording material. For example a recording web consisting of an insulating coating of plastic on a paper base having sufficient conductivity to allow electric charge to flow from the backing electrode to the paper-plastic interface is used. For a survey of dielectric coated papers reference is made to AGC- Monthly (1972) Nr. 9, pages 4-15. A special electrographic paper is described in the U.S. Pat. No. 3,620,831, and special thermoplastic recording tapes are described in the Journal of SMPTE Volume 74, p. 666-668.

As substances suited for enhancing the conductivity of the rear side of a dielectric recording material e.g. a transparent resin sheet are particularly mentioned antistatic agents preferably antistatic agents of the polyionic type, e.g. CALGON CONDUCTIVE POLYMER 261 (trade mark of Calgon Corporation, Inc. Pittsburgh. Pa., U.S.A.) for a solution containing 39.1% by weight of active conductive solids, which contain a conductive polymer having recurring units of the following type:

and vapour deposited films of chromium or nickelchromium about 3.5 micrometer thick and that are about 65 to 70% transparent in the visible range.

Cuprous iodide conducting films can be made by vacuum depositing copper on a relatively thick resin base and then treated with iodine vapour under controlled conditions (see J.Electrochem.Soc., 110-119, Feb. 1963). Such films are over 90% transparent and have surface resistivities as low as 1500 ohms per square. The conducting film is preferably overcoated with a relatively thin insulating layer as described e.g. in the Journal of the SMPTE, Vol. 74, p. 667.

What we claim is:

l. A method of recording information as a pattern of electrostatic charges carried by an insulating charge receiving medium which comprises:

a. exposing to a pattern of X- or 'y-rays an imaging chamb'er containing an ionizable gas having an atomic number of at least 36 and adapted to produce therein upon such exposure electrostatic charges in a corresponding pattern, while maintaining said gas during said exposure under superatmospheric pressure;

b. electrically biasing the charges in said pattern toward an array of discrete closely spaced photoconductors disposed along one side of said chamber generally normal to the exposure direction within a frangible insulating matrix, said conductors extending from the interior to the exterior of said chamber wall and thus acquiring at their external ends during such exposure a differentially charged condition according to said pattern;

c. arranging in at least close proximity to the exterior ends of said array an insulating charge receiving material, said material developing a differentially charged condition according to said pattern under the influence of the differentially charged condition of the exterior conductor ends; and

d. while said chamber is maintained under said superatmospheric condition, mechanically biasing said charge-receiving material over an area at least coextensive with said matrix against the exterior side of said matrix with a force opposing the gas pressure exerted against the inner side of said matrix.

2. The method of claim 1 including the steps of removing said mechanical bias from said receiving material after completion of said exposure to permit removal of said material and concurrently reducing the pressure in said chamber.

3. The method of claim 2 wherein the pressure in said chamber is increased and decreased and said mechanical biasing force applied and removed in substantial synchronism.

4. The method of claim 2 including the step of advancing said receiving material to bring a fresh area thereof into proximity to said conductor array while said mechanican bias is removed therefrom.

5. The method of claim 2 wherein said chamber is sealed generally pressure-tight and said pressure therein is reduced without disturbing said seal wherein loss of said ionizing gas is minimized.

6. A method according to claim 1, wherein the chamber contains said gas in contact with an electrode and said electrode through a DC-potential source is connected to another electrode that makes contact during exposure with the charge receiving material at the side opposite to said conductors.

7. A method according to claim 6, wherein the chamber contains xenon gas and the arithmetic product of pressure and thickness of the interspace between the electrode in the chamber and the interior ends of the conductors is in the range of 10 mm atmospheres and 200 mm atmospheres.

8. A method according to claim 1, wherein the pattern of charges is generated by imagewise irradiating the ionizable gas in the chamber from the side opposite to the exterior ends of said conductors.

9. A method according to claim 1, wherein the pattern of charges is generated by imagewise irradiating the ionizable gas in the chamber with radiation penetrating through said conductors.

10. A method according to claim 6, wherein the interior ends of said conductors are coated with a photocathode material and prior to the generation of the charges the insulating surface of the receiving material 12. A method according to claim 1, wherein the receiving material is a transparent film material.

13. A method according to claim 1, wherein the electrostatic charge pattern obtained on the receiving material is developed with electrostatically attractable material.

14. An ionographic imaging system for producing an electrostatic charge pattern on an insulating charge receiving material which comprises:

1. an imaging chamber including (a) one wall incorporating a frangible array of discrete closely spaced conductors arranged in an insulating matrix, said conductors extending through said wall from the interior to the exterior of said chamber and (b) an opposite wall spaced from said one wall and defining therewith an enclosed space,

2. means for intermittently pressurizing said enclosed space with a gaseous medium at superatmospheric pressure and releasing said pressure,

3. electrode means in said enclosed space spaced from said conductor array,

4. means in said enclosed space for emitting electrical charges upon exposure thereof to ionizing radiation,

5. means for exposing said imaging chamber to a pattern of ionizing radiation whereby a pattern of electrical charges is produced therein,

6. electrode means arranged exteriorly of said one wall in closely spaced parallel relation to said conductor array and generally coextensive with said array,

7. an insulating charge receiving material disposed between the exterior side of said conductor array and said exterior electrode means in intimate contact with the exterior ends of the conductors in said array,

8. means for applying an electrical potential between said electrode means to attract the charges of said pattern to the interior ends of said conductors thereby producing a differentially charged condition in accordance with said exposure pattern at the exterior ends of said conductors and upon the insulating charge receiving material in contact therewith, and

9. means for mechanically supporting said exterior electrode means and for applying between said exterior electrode means and said conductor array with said insulating receiving means pressed therebetween a mechanical force opposite to the action of the pressure in said enclosed space on said array.

15. The system of claim 14, wherein said mechanical force is applied intermittently in substantial synchronism with the intermittent pressurization of said enclosed space.

16. The system of claim 15 including means for increasing the pressure in said space and the mechanical opposing force at substantially the same rate.

17. The system of claim 14 wherein said exterior electrode means and said imaging chamber are mounted for relative bodily movement towards and away from one another and including means for biasing the same together to apply said mechanical force be tween said exterior electrode means and said conductor array.

18. An imaging 'system according to claim 14, wherein the internal electrode means is a photocathode.

19. An imaging system according to claim 18, wherein the photocathode is directly sensitive to X- rays.

20. An imaging system according to claim 14, wherein the enclosed space contains a gas having an atomic number of at least 36.

21. An imaging system according to claim 20, wherein the enclosed space contains xenon gas and the product of pressure and thickness of the interspace be tween the internal electrode means and the input ends of the conductors is in the range of 10 mm atmospheres and 200 mm atmospheres.

22. An imaging system according to claim 21, wherein the pressure inside the enclosed space is in the range of about 5 to about 10 kg per sq.cm.

23. An imaging system according to claim 14, wherein the input ends of the conductors are covered with photocathode material that is directly sensitive to X-rays.

24. An imaging system according to claim 14, wherein the imaging chamber is mounted on a piston that can be moved to and fro and is provided with means that can keep the pressure at both sides of the wall containing said conductors substantially equal.

25. An imaging system according to claim 24, including at the side opposite to the piston a cover with sealing ring which cover can be filled with gas or liquid building up a counterpressure against said electrode means with respect to the pressure inside the chamber.

26. An imaging system according to claim 14, wherein the chamber is mounted in a supporting frame and said electrode is backed with a pressure cushion comprising (l) a flexible membrane and (2) a rigid plate opposite to that membrane, and means for introducing a liquid or gas under pressure in said cushion.

27. An imaging system according to claim 14, wherein the conductors are made of a low atomic number metal or metal alloy.

28. An imaging system according to claim 27, I

wherein the conductors are made of aluminium or beryllium.

29. An imaging system according to claim 14, wherein the conductors are made of a high atomic number metal.

30. An imaging system according to claim 14, wherein the conductors form a single row.

31. An imaging system according to claim 14, wherein the solid conductors are disposed in glass as insulating material and form a pin matrix element that forms part of the chamber wall.

Claims

1. A method of recording information as a pattern of electrostatic charges carried by an insulating charge receiving medium which comprises: a. exposing to a pattern of X- or gamma -rays an imaging chamber containing an ionizable gas having an atomic number of at least 36 and adapted to produce therein upon such exposure electrostatic charges in a corresponding pattern, while maintaining said gas during said exposure under superatmospheric pressure; b. electrically biasing the charges in said pattern toward an array of discrete closely spaced photoconductors disposed along one side of said chamber generally normal to the exposure direction within a frangible insulating matrix, said conductors extending from the interior to the exterior of said chamber wall and thus acquiring at their external ends during such exposure a differentially charged condition according to said pattern; c. arranging in at least close proximity to the exterior ends of said array an insulating charge receiving material, said material developing a differentially charged condition according to said pattern under the influence of the differentially charged condition of the exterior conductor ends; and d. while said chamber is maintained under said superatmospheric condition, mechanically biasing said charge-receiving material over an area at least coextensive with said matrix against the exterior side of said matrix with a force opposing the gas pressure exerted against the inner side of said matrix.

3. electrode means in said enclosed space spaced from said conductor array,

10. A method according to claim 6, wherein the interior ends of said conductors are coated with a photocathode material and prior to the generation of the charges the insulating surface of the receiving material is uniformly negatively charged.

11. A method according to claim 1, wherein the charge pattern on the receiving material is produced by means of a line-wise arranged single row of conductors and the image-wise exposure of the ionizable gas in the chamber proceeds line-wise.

12. A method according to claim 1, wherein the receiving material is a transparent film material.

17. The system of claim 14 wherein said exterior electrode means and said imaging chamber are mounted for relative bodily movement towards and away from one another and including means for biasing the same together to apply said mechanical force between said exterior electrode means and said conductor array.

18. An imaging system according to claim 14, wherein the internal electrode means is a photocathode.

19. An imaging system according to claim 18, wherein the photocathode is directly sensitive to X-rays.

21. An imaging system according to claim 20, wherein the enclosed space contains xenon gas and the product of pressure and thickness of the interspace between the internal electrode means and the input ends of the conductors is in the range of 10 mm atmospheres and 200 mm atmospheres.

26. An imaging system according to claim 14, wherein the chamber is mounted in a supporting frame and said electrode is backed with a pressure cushion comprising (1) a flexible membrane and (2) a rigid plate opposite to that membrane, and means for introducing a liquid or gas under pressure in said cushion.

28. An imaging system according to claim 27, wherein the conductors are made of aluminium or beryllium.