CA1038441A - Process for forming developable electrostatic charge patterns and devices therefor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method of recording in terms of an electrostatic charge pattern, a pattern of emitted charged particles repre-senting information to be recorded and generated in the interior of an airtight envelope comprising an ionizable gas, is characterized in that an electron or positive ion emission pattern formed by an imagewise electrical discharge in said gas is projected onto an area of the inner side of said envelope which area contains closely spaced solid conductors, which are separated by a solid electrically insulating material and wherein said conductors receiving imagewise electrons or positive ions formed by said discharge obtain a charged condition or change in charged condition at their exterior ends at the outer side of the envelope, and during or after the electrical discharge in said gas the outer 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 the rear side of which is backed with an electrode whereto a voltage is applied being such that an electrostatic charge pattern is formed on said insulating surface.

Description

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~ his application is related to co-pending Canadian appli-cations Serial Numbers 200,055 filed May 16, 1974 and 200,403 filed May 21, 1974 by Agfa-Gevaert N.V.
This invention relates to a process for forming deve-lopable electrostatic charge patterns and devices for pro-ducing such patterns.
From the German Patent 1,497,093 filed November 8, 1962 by Siemens ~.G. an imaging technique is kn4wn 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 monobromotrifluoro-methane (1:5) and is provided with a photocathode and an anode, the latter being covered by an insulating recording material e.g. insulating resin sheet. Simultaneously with an object-wise modulated X-ray exposure a direct-current potential is applied across the electrodes so that photo-electrons, which are ejected image-wise from the photocathode, are strongly intensified by an avalanching process in the ionizable gas. ~he electrons are collected on the insulating material in an image pattern corresponding to the pattern Or distribution of the imaging radiation absorbed in the photocathode.
In the recording appliances described in said German Patent Specification a polyester foil is used as 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 said sheet. Such procedure requires for each print the inlet of air in the interspace filled with ionizable gas GV.758 PC~
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and the removal of a sufficient amount of air and replacement of the air by the desired ionizable gas before the production of a new print can start~
From the Belgian Patent 792,334 filed December 6, 1972 by Xonics Inc. corresponding with the United States Patent 3,774,029 of ~ric P.Muntz, Andrew P.Proudian and Paul B.Scott issued November 20, 1973 a process for the production of an electrostatic image is known which process is characterized by the steps of (1) placing a dielectric sheet between an anode and a cathode,-(2) causing absorption of X-rays in the interspace between the anode and cathode in which interspace a gas ~th an atomic number of at least equal to 36, preferably xenon, is kept at a pressure above atmospheric pressure so that by the X-ray absorption electrons and positive ions are formed, (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 said process as exemplified by the Fig. 1, 5 and 6 of said Belgian Patent a cassette is used which has to be ; opened and filled with the ionizable gas before the production of a new print ~an start.
~his makes the production of several subsequent prints rather 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 particles emission - GV.758PCT - 2 -1~38'~41 pattern is transferred outside said miedium without opening the imaging chamber containing saidmedium.
A further object of the present invention is to provide 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 a method of recording in terms of an electrostatic charge pattern a pattern of emitted charged particles representing information to be recorded and generated in the interior of an airtight envelope comprising an ionizable gas, is characterized in that an electron or positive ion emission pattern formed by an image-wise electrical discharge in said gas is projected onto an area of the inner side of said envelope which area con-tains closely spaced solid conductors, which are separated by a solid electrically insulating material and wherein said conductors receiving image-wise electrons or positive ions formed by said discharge obtain a charged condition or change in charged condition at their exterior ends on the outer side of the envelope, and during or after the electrical discharge in said gas the outer 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 that at its rearside is provided with an electrode whereto a voltage is applied being such GV.758PCT ~ 3 ~

~ 1~38~1 that an electrostatic charge pattern is formed on said in-sulating surface. The charged particles produced in the electrical discharge in the ionizable gas are electrons (photo-electrons and optionally secondary emission electrons) and positive ions.
~ he part o~ the emission pattern consisting of said electrons is called "the electron image". ~he 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 is improved by an electron accelerating DC potential difference that is applied across or between the means generating the charged particles image and the rear side of said charge receiving material.
~ he basic elements of recording apparatus of the present invention and the steps of embodiments of recording and charge transfer are illustrated by the acco~panying sche-matic drawings wherein ~ig.1 is a cross-sectional representa-tion of a recording system structure of the present invention, ~ig.2 to 7 are cross-sectional representations of alternative imaging structures.
It should be understood that in these figures some dimen-sions 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 dimen-sions of the layers or spacings separating the various elemen-G~J.758PC~ - 4 -.
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1~3~g41 tal parts of the imaging apparatus.
The reproducing system according to the invention in the form illustrated schematically in ~ig. 1 is suitable for X-ra~ recording and opera'es with an ionizable gas under atmos-pheric pressure or slight over- or underpressure e.g. of 1 to 10 ~orr.
~ he envelope 1 is filled with an ionizable gas or gas mix-ture in ad~ixture with a discharge quenching substance e.g.
ethanol as described e.g. in the German Patent 1,497,093 mentioned before. ~he filling gas is advantageously kept under an over-pressure above atmospheric pressure of a few ~orr, e.g. 10 ~orr.
A useful gas mixture consists e.g. of argon and monobromotxi-fluoromethane (CF3Br) in the volume ratio 1:5. When using the -above fluoromethane a separate quenching additive is not required.
~he applied D.C. voltage is preferably not more than 5 % above the brea~down voltage of the gas.
~ he photocathode 2 is of the type described in the German Patent 1,497,093 mentioned before e.g. is a 1.5 micron layer of lead or a ~.0 micron layer of uranium applied on an aluminium sheet 8. During the information-wise X-ray exposure of the photocathode 2 a DC-potential difference is applied by means of the potential source 10 between the photocathode and the electrode 6.
~ he distance between the photocathode 2 and the input-ends of the conductive pins 3 is preferably in the range of 0.3 to 5 mm. ~he potential difference between the photocathode

2 and the electrode 6 contacting the conductive rear side 4 GV.758PCT _ 5 _ of the insulatin~ web material 5 defines an accelerating field acting upon the electrons and determines together with ; ~aid distance the kind of ionizable gas and its pressure the degree of the electron multiplication (if any) by secondary emission (avalanching effect).
According to the embodiment illustrated in Fig~ 1 the photocathode 2 is provided with a screen 7 having minute holes ; for preventing the divergence of the electrons and improving the image sharpness.
~he 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 microohannels is preferably at least 1:4.
~he distance 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.
~ he 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. ~he 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 the ones diverging by more than 15 from the perpendicular to the plane of the electrode become absorbed.

According to a special embodiment the screen plate 7 i8 an assembly of electrically insulating resin or glass GV.75~PCT - 6 - -.
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.;, , ,, , ",,",,"", ~ ,.. ,",",,,",, ,;; ,,,,",, ~ " , " ~ ,, ~38441 sheets or semi-conductive or conductive sheets e.g. metal sheets preferably of a metal with high atomic number e.g. higher than 5O. Said metal sheets are whether or not coated with an in-sulating material e.g. insulating resin. ~aid sheets are cor-rugated in a direction parallel to the desired path of electron i flow, so that said corru~ations cooperate to provide parallel channels or conduits for the electrons. The manufacture of such channel plates but having secondary-emissive charac-teristics has been described in the United Eingdom Patent 954,248 filed May 31,1962 by Bendix Corporation. Other techniques for producing microchannel plates are described in the United Eingdom Patents 1,064,072 filed January 3, 1964 and 1,064,075 filed April 7, 1964 bOth by Mullard ~td. For the pur-pose of the embodiment discussed in connection with ~ig. 1 the materials of the plate are chosen in such a way that secondary emission onthe screen walls does not or not substantially occur under the conditions of voltage, gas composition and gas -~
pressure applied in the imaging apparatus of ~ig. 1. Indeed, secondary emission resulting from so-called "ionic feed-back"
(see IEEE ~ransactions on ~uclear Science Vol. NS-13 June 1966, pages 88-99) has to be avoided since at a particular gas ; pressure normally above 10 4 Torr (see Advances in Electronics and ~lectron Physics Vol. 28 (1969) pages 499-506) a self-sustaining discharge is obtained in~he gas-medium which dis-charge degrades the electrostatic charge image on the insulating charge receiving medium. ~he ionic feedback is a phenomenon which arises when ions produced at the output end of the channels are accelerated back down the charnels and set free secondary GV.758PCT - 7 -:-. . ,, . .. - .
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38~41 electrons by striking the secondary emissive mner walls at the input ends of the channels. ~he electron density may be increased thereby in such a degree that a self-sustaining discharge occurs, which has to be avoided.
Using a conductive internal collimator (see element 7 in fig.1) no direct electrically conductive contact with the wires of the pin matrix may exist. ~herefore the bottom end of the collimator is either supported by insulating mate-rial or is spaced from the pin mabrix at a distance sufficient to avoid electrical breakdown in the direction of the wires of the pin matrix when operating the apparatus under the imaging voltage co~ditions.
~ he electrons being multiplied by secondary emission in the gas medium are projected onto the inner ends of the con-dùctive wires 3 of the pin matrix plate 11 forming part of the vacuum envelope wall. ~he conductive wires 3 are substan-tially parallel arranged in and separated by a solid electrical-ly insulating material 12 e.g.glass, and are directed with their outer ends at the outer side of the envelope 1 towards the electrically insulating surface of the charge recèiving web 5 e.g. polyester resin film web or dielectric paper web having an electrically conductive coating 4 or support in electrically conductive contact with the conductive guiding plate 6. ~he 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 electro-photo OE aphy. From that station the web moves into a cutting GV.758PC~ - 8 - -:' :

., , .~ , . ,.. . . .~, 1~38441 -~ device comprising e.g. a movable knife and a station~ry knife~
After leaving the cutting device the obtained sheets are ~- collected in a collector tray.
The envelope material 1 is at the inside of the envelope , coated with electrically insulating material 9. ~he envelopc1 can be evacuated and provided with ionizable gas and quenching gas at the desired pressure through the 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 avalan-che dischargings so that it will be necessary to replace the ionizable gas and quenching gas by a fresh gas mixture aftcr a number of exposures.
~ he exposure of the photocathode e.g. with information-wise modulated ~-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.
proceeds as described in the United States Patents 2,221,776 of Chester F.Carlson issued September 8, 1938 and 3,526,767 of Walter Roth and Alex F.Jvirblis issued September 1, 1970 or as in the published German Patent Application (Dt-OS) 2,2~1,954 filed June 29, 1972 by Diagnostic Instruments. ~he photocathode may itself be in the form of a screen or an assembly of lamellac as described in our co-pending Canadian Patent Application ~ -200,119 filed May 26, 1974 by Agfa-Gevaert N.V., which has to be read in conjunction herewith.

According to a special method (embodiments of which are GV.758 PC~ - 9 -.
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~ ~38441 llustrated in Fig. 2 to 7) the X-ray recording in the form of an electrostatic charge pattern is carried out with an imaging device comprising i~ an air-tight envelope an electrode that is separated from a pin matrix wall by an i~terspace filled up with an ionizable gas having an atomic number of at least 36. ~he ionizable gas, being preferably xenon, is kept u~der a pressure above atmospheric pressure.
In said embodiment for improving the image sharpness the voltage applied over the electrodes is preferably such that no substantial electron multiplication, so no electron avalanching, by secondar~ emission takes place, i~ other words the i~aging chamber is operated in the horizontal part of the ~ownsend curve.
For the explanation of that curve see Molecular Science and Molecu~ar Engineering by Arthur R.von Hippel et al (1959) ~he ~echnology Press of M.I.~. and John Wiley and Sons, Inc., New York~ page 78.
The pressure under which the ionizable gas is kept may be rather high as described in the Belgian Patent 792,334 mentioned before. When operating in the present invention wich said high ~ -at~mic number gas as above referred to the product of pressure and thickness of the interspace between the electrode and input ends of the pin matrix may bet e.g. in the range of 10 mm.
atmospheres and 200 mm.atmospheres. A pressure of 6 atmospheres combined with an interspace of 15 mm gives good results.
The superatmospheric pressure, however, poses a problem with regard to the thickness and mechanical strength of the walls of the envelope taking into account that the wall of the envelope directed to the X-ray GV.758PC~ - 10 -.

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exposure source preferably i~ not macle of materials that con-tain or consist of elements having a high atomic number. So -there will be only a limited choice of X-ray penetrative wall ~aterial when the X-ray exposure dose has to be kept low which is the case e.g. for medical 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 said Belgian Patent 792,334 as mentioned before. 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 keeping of the X-ray dose as low as possible -is not a main demand 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 for obtaining a substantial mechanical strength. In order to keep the X-ray absorption in the pin matrix wall within certain limits the wires of the matrix are made of a metal or metal alloy of low atomic number(s) e.g. aluminium, beryllium or an alloy con-taining magnesium. ;
Optionally the ends of the wires of the pin matrix insidethe envelope containing ionizable gas are coated with a photo-GV.758PCT

. , . : . , 03~4~~athode material e.g. as described in the United States Patent

3,508,477 of H.R. Groo III issued April 28, 1967.
In X-ray recording the ends of the wires are pre-ferably covered with a suitable ~-ra~ photocathode material e.g. lead or the wires are made of a high atomic number metal.
In the latter cases, how~ver, 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 with negative charges and is in-formation-wise discharged through the wires of the pin matrix.
The apparatus illustrated in ~ig. 2 relates to an embodi-ment in which the ~-ra~ exposure is e~ected from the side of the receiving material directly through the pin matrix and in which the pin matrix has a sufficient mechanical strength to withstand -A superatmospheric gas pressure, e.g. up to 10 atmospheres, of an ionizable X-ray-absorbing gas as described in the Belgian Patent 792,334 as mentioned hereinbefore.
The elements indicated with the numbers 1 and 3 to 6 and - 9 to 13 have the same significance as described in ~ig. 1.-~he wires 3 are preferably made of aluminium or beryllium and the material 12 incorporating the wires is preferably glass that does not contain high atomic number elements e.g. is made of borosilicate glass. ~he 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 a GV.758PCT - 12 -.. . . . .
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means that keeps or presses 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 ~e 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 there-for means are used to increase and decrease respectively the pressure inside the imaging chamber and on the pin matrix at the outerside of the imaging chamber.
T e application of a pressure to the recording material offers a more intimate contact with the wire ends of the pin matrix and improves the image sharpness and charge densit~.
When applying the counter pressure technique there is no need for a rather thick and mechanicall~ very strong pin matrix wall;
a relatively thin wall may be used so that the radiation dose may be reduced when effecting the irradiation directly through the pin matrix. ~he counter pressure technique allows the use of a pin matrix that has short pins. ~he pins are preferably short in-order to avoid capacity coupling with neighbouring pins and charge leakage to neighbouring pins causing image damage (see Xerography and Related Processes by John H.Dessauer, The ~ocal Press, London and New York (~965) page 445).
Therefore, according to a preferred embodiment of the present invention an ionographic imaging device for the pro-duction of an electrostatic charge pattern on an insulating charge receivlng material comprises :
- an air tight envelope having a first wall portion through which a pattern of ionizing radiation may be projected, GV.~58PC~ - 13 -.

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1(~38441 in said envelope a gas that is ionizable by said radiation under a pressure substantially above atmospheric pressure preferably at least about 5 kg per sg.cm, in said envelope an electrode the rear side of which is adjacent to said first wall portion, in said envelope a second wall portion that faces said first wall portion wherein said second wall portion contains a plurality of closely spaced conductors embedded in an in-sulating material through which they pe~etrate, the input end~
of said conductors being in contact with said gas, a means for introducing the gas under superatmospheric pressure in said envelope and to allow it to leave the envelope again ~ ;
up to a pressure being such that the wall portion containing the conductors can withstand it without support, a means for supporting the wall portion containing said conductors between the moment of introducing the gas ~der superatmospheric pressure in the envelope and the moment of restoring the pressure to the level of pressure which can be withstood by said wall portion without support, a means for moving the insulating charge receiving material to bring it firstly in and secondl~ out of the position wherein it is in contact under pressure with the exterior ends of said conductors, an electrode backing said insulating charge receiving material, and means for applying a potential between said electrode in the envelope and the electrode backing the charge receiving material.

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In a particular imaging device according to said embodlment the means, e.g. a piston, for supporting the wall portion con-taining said conductors can be moved to and fro for keeping the pressure at both sides of the wall portion containing the conductors substantially equal during the introduction and irradiation of the gas in ~e envelope and flow out of the gas out of the envelope (see Fig. 7).
In another particular imaging device the imaging envelope is mounted on a means, e.g. a piston that can be moved to and fro , and is provided with a means, e.g. a rigid plate, that can withstand the pressure acting from the inner side of the envelope on the wall containing said conductors (see Fig. 3).
In still another embodiment the imaging device at the side of the wall containing said conductors contains a cover with sealing ring which cover can be filled with gas or liquid building up a counterpressure with respect to the pressure inside -the envelope (see Fig. 4). ~
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In a further imaging device the imaging envelope is -mounted in a supporting frame and is provided at the side of the wall containing said conductors with a pressure cushion comprising (1) a flexible membrane contacting the rear side of the electrostatic charge receiving material which is directed to the outer ends of the conductors and (2) a plate opposite to that membrane, and means for introducing aliquid or gas under pressure in said cushion and another means for introducing simultaneously ionizable gas under pressure in said envelope (see ~ig. 5). ~ -According to a preferred method the recording of a pattern `
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8~41 of X-rays or ~-rays in terms of an electrostatic charge pattern on a charge receiving insulating material comprises the steps of :
- introducing under superatmospheric pressure an ionizable gas having an atomic number of at least 36 in an air tight envelope having a first wall portion provided with an :~
electrode through which a pattern of ionizing radiation may be projected, - supporting during said introducing a second wall portion facing said first wall portion to prevent that the second wall portion becomes damaged by the pressure exerted on it from the innerside of the envelope, said second wall portion containing a plurality of closely spaced conductors embedded in an insulating material through which they penetrate, the input ends of said conductors being in contact with said gas, said supporting proceeding through the intermediary of a means ;.
which contacts the insulating charge receiving material and allows or forces it to become pressed between said means and the exterior ends of said conductors, - before said gas introduction positioning said charge re-ceiving material in close proximity with said exterior ends of said conductors, - exposing said ionizable gas being at a superatmospheric pressure in the air tight envelope with an X-ray or ~-ray pattern penetrating directly said first wall portion or said second wall portion, - generating electrons and positive ions in the gas, GV.758PC~ _ 16 - ~.

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~' - - attracting electrons or positive ions towards said electrode and attracting respectively positive io~s or electrons towards said wall portion containing said closely spaced conductors the output ends of which stand under pressure in contact with the insulating charge receiving material which is backed with an electrode having during the exposure a DC-potential differ-ence with respect to the other electrode in the envelope, here-by transferring charge through said conductors whereby the in-sulating charge receiving material becomes image-wise electro-statically charged or discharged, - depressurizing the envelope up to a pressure being such that the wall portion with the conductors can withstand the ~
pressure without support and simultaneously and substantially ;
egually releasing the pressure at the outer side of the wall portion containing the conductors, and -~ --- moYing the image receiving material containing a charge ~attern away from the conductor ends and bringing another image receiving material or image frame thereo~ in close proximity with the exterior ends of said conductors.
According to the embodiment illustrated in fig. 3, the pin matrix wall is pressed in contact with the charge image receiving material that is arranged in contact with a pressure-withstanding means, in this case plate 2~ .
~he imaging chamber is of the same type as described in ~ig. 2. ~he 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 b~ the pressure applied in the cylinder 22 on a gaseous medium e.g.
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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 co~ed 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 electrode and as supporting wall for withstanding the pressure applied to the pin matrix 11 by the introducing of the ionizable gas under pressure in the imaging chamber.
~he 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 and controlling the action of the p~essure cylinder 27 with that of the cylinder 24 from which the X-ray-absorbing ionizable gas e.g. xenon gas is fed with 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 up the force applied to the electrode 23 when moving the piston 21 upwardly. AD insulating sheet or plate 28 covers the electrode 23 and avoids direct contact of the operating personal or patient with the voltage of the voltage source 10.
- According to the embodiment illustrated in ~ig. 7 the radiographic imaging chamber 70 contains a cover 71 in the form of a spherical cap.

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- 1~38441 ~ he cover is composed o~ a thin sheet 72 of a low atomic number metal or metal alloy comprising one of the metals of the group of aluminium, magnesium and berylliu~ having low X-ray absoIption. ~or example, the thin sheet 72 of the spherical cap is made of beryllium having a thickness of 4 mm. ~he space 73 be-tween the co~er sheet 72 and the insulating support 74 carrying a thin X-ray transparent electrode 75 is filled with a low X-ray absorbing liquid. The cover 71 is gas tightly united through the ring spacer 76 (determining the gas gap width) with the pin matrix wall 77. ~he chamber 70 is firmly attached to the frame studs 78. ~hese studs -78 are either fixed to the floor 79 (which is the case according to ~ig. 7) or are mounted on floor rails that allow the X-ray table to be tilted in the desired diagnostic position (see for such type of table used in silver halide photography the book "Medical X-ray Technique - -Principles and Applications by G.J. Van der Plaats (1959), Philips Technical ~ibrary, page 274).
According to the embodiment illustrated by ~ig. 7 the image-wise modulated X-ray exposure is effected from the side of the cover 71 while having a superatmospheric pressure in the chamber 70, which at the moment of the exposure contains an X-ray-absorbing ionizable gas preferably xenon at a pressure of at least 5 kg per sq.cm. -.
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- 1~38441 ~ he ionizable gas is introduced under pressure in the chamber 70 from the cylinder 80 with the piston 81. Simul-taneously with the feeding of the ionizable gas under pressure into the imaging chamber 70 a web-like electrostatic charge-receiving material containing a resin support 82 coated with a conductive layer 83 is pressed against the pin-matrix wall 77 with the piston 84, which has an electricall~
insulating surface (e.g. made of polytetrafluoroethylene) and contains an electrode 85. ~he piston 84 is guided in the cylinder 86. ~he pressure applied inside the chamber 70 by the gas introduction and the pressure applied by the piston 81 are substa~tially equal; hereby, bending and damaging of the pin matrix wall is prevented.
The piston 84 can be moved pneumatically or hydraulicall~
in upward and downward direction with the pressure applied in the cylinder 87 on a gaseous or liquid medium compressed or decompressed by the action of the piston 88 that builts up through said medium in the cylinder 89 the pressure desired.
During the ~-ray exposure a DC voltage is applied between the electrodes 75 and 85 with a voltage source 90. The voltage between the electrodes 75 and 85 lS adjustable with a variable resistor (not shown in ~ig. 7~. The variable resistor brings preferably the voltage at a value that allows to operate the charge generation inthe horizontal part of the Townsend curve (current versus voltage). In operation the applied voltage is preferably at least a thousand volts.
After the X-ra~ exposure the pressure inside the chamber 70 GV.758PCT - 20 _ - - - - - ;.- - .. - - . :
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: . ' ' is reduced again. The piston 84 is moved downwardly over a distance sufficient to allow the removal of the charge-re-cei~ing material from the pin matrix wall in a direction per-pendicular to the pins. Thereupon the positioning of a next image frame of the charge-receiving material takes place.
~ he cover 71 is kept out of direct contact wlth the object e.g. patient to be radiographed by a table-top 91 of a common X-ray table.
According to the embodiment illustrated in Fig. 4 a con- -trolled counterpressure is ap~ied to the outer side of the pin-matrix wall.
The imaging chamber is likewise of the type illustrated in Fig. 2 and 3. ~he en~elope wall of the imaging chamber 1 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 thec~linder 1, --~ith piston 30'. After positioning the receiving ma~rial (4,5) in front of the pin matrix 11, a rectangular or sguare cover 30 provided at its edges 31 with a pressure sealing ring 32 is pressed onto a thin metal electrode 33 e.g. made of beryllium and forms a closed chamber of which the top 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 against said edges 31 with the .
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3~441 piston 21 and by the cylinder 37 and ~ston ~8 through the pipe fitting 39 gas e.g. air is introduced with the same pressure in the space formed by the cover 30.
After the ~-ray exposure the pressure is reduced again and the web-like receiving material is moved ~ver a distance sufficient to allow the positi~ning of the next image ~rame.
~ he film web containing the electrostatic charge pat-tern 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 tothose skilled in the art of-sheet feeding i~ copying machines. According to a particular embodiment the separate sheets are carried by a supporting web that is moved with a dryving mechanism (not shown in the drawing).
According to the embodiment illustrated in Fig. 5 the imaging chamber is likewise of the type illustrated in fig.2.
~he X-ray exposure source is element 63.
~ he 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 chàrge : receiving material composed of a charge recelving 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 pressure cushion 41 having a flexible membrane 42 andt~p plate 4~ is joint by a holiow rectangular or square flexible sealing ring 44 which is provided with a safety expansion valve 45. ~he cushion 41 is present in front of the pin matrix 11 and re- -`
ceives a gaseous or liquid medium from the cylinder 46 in GV.758PC~ - 22 -... . . .

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1038441 -~ -: which the piston 47 is operated simultaneously with the piston48 of the cylinder 49 containing an X-ray-absorbiDg ionizable gas in order to obtain an even pressure on both sides o- the ; pin ma~rix 11. ~he ioni7able 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 of that membrane. ~he membrane is made e.g. of an elastomer e.g. a s~thetic rubber that is vacuum coated with aluminium in the area contacting the conductive backing 4 of the film web 5.
~ he reproducing system illustrated in Fig. 6 is a modified system of the one illustrated in Fig. 2. ~he material 50 is a steel tube of which the internal structure of the welding joint has to be inspected. A dielectric web 51 covered wlth a conductive rear coating 52 is wrapped around the welding joint. ~he recording head 54 containing ionizable gas contains a single row of substantially parallel conduc~ve pins 55 embedded in t~e insulating wall 56 of the recordlng 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 absorption power for X-rays and ~-rays. ~he input openings of the row of pins 55 are ~acing an electrode strip 57.
~ he 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 re-~`
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;38441 cording head 54 along the circumference of the tube 50 while in the tube 50 at a short distance of i~ weldiDg joint a

4 radio-active source 58 e.g. a caesium 1~7 or cobalt 60 source emits ~ -rays. By employing magnetic or electrostatic focussing 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 la~er a DC potential of 3everal kV is applied with a potential source99 of which the positive pole is connected to the grounded tube 50 and the negative pole to said electrode 57. The element 60 is a radiation shield containing a lead sheet 62 fixed to an electricall~ insulating resin layer 61.
Depending onthe radiation-sensitivit~ of the ionizable gas and optionally of the photocathode used in the imaging chamber radiation patterns of penetrating radiation e.g.
~-rays, ~-rays, neutron ra~s, ~3-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~ ~-rays and neutrons may be transformed in ultraviolet andior visible light by fluorescent layers that are positioned in olose pro~imity or contact with a photocathode member ~at is sensitive for the fluorescent light. .
According to a preferred embodiment of the present inven-tion applicable in medical X-ray radiography (see Fig. ~ to

5 and 7) the imaging chamber does not contain a solid state photocathode and the ionizable gas itself serves as photo-electron emitter, which is the case when at superatmospheric GV.758PC~ - 24 -., .~

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-- 1~38441 pressure a high atomic number gas9 e.g. xenon, is used in the exposure step with pene~ating radiation, e.g. X-ra~s and ~-rays.
It is clear, however, that depending on the type of the optionally present photocathode and the ionizable gas all kind of reproduction and copying work may be made according - -to the present invention, e.g. document copying, micro-film enlargement, fac-simile, X-ray photography and even cinemato-- graphy e.g. by operating at 6 to 16 image frames per second.
In connection with facsimile the attention is ~rawn to the embodiment represented in Fig. 6 in-which the production of the charge pattern proceeds line-wise ~ 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 a non-previously charged 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 insuIating 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 pin-matrix 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 GV~758PC~ - 25 _ : . .: -' ., 1~3844~
a pin-matrix the conductive wires of which protrude from the external side of the imaging chamber. ~he protruding wire ends are contacted with a charged electrode be~ore effecting the image-wise photoexposure o~ the emitter of charged particles in the imaging chamber. In the exposed areas the charged wires are discharged and the charge of the ~Jdis-- charged wires transferred to an insulating charge receiving material.
According to a second embodiment the external wire ends are non-differentially charged e.g. with corona without charging the insulating material of the pin-matrix through a screen pla~ the openings of which correspond with the external ends of the wires of the pin matrix. ~his embodi-~ ment asks for a great care in bringing thescreen holes in ; alignment with the wire ends.
Although for reason of decreasing the X-ray radiation dose the wires of the pin matrix - when the X-ray exposure is made directly through the pin-matrix - are advantageously made of a low atomic number metal or metal alloy, preference ; 20 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 tungsten or platinum.
~he heavy metal wires acting as an X-ray grid analogously to a Bucky grid are directed towards the X-ray source. The wires are interleaved with an electrically insulating material that is more transparent to X-rays e.g. glass or resin. ~he primary rays (the rays coming in straight line GV.758PC~ - 26 _ 1~38441 from the X-ray source) pass through the pin matrix while the rays scattered in the object, the oblique rays, are mostly absorbed in the wires so that an improved image contrast is obtained.
A pin matrix element appropriate for serving as multi-conductor wall of the imaging chamber is available under the trade mark "Multilead" from Corning Glass works, Industrial Bulb Sales Department, Corning, N.Y~ It is avai-lable with a number of different conductor materials and sizes and a number of different spacings between the con-ductors. The "Multilead" material comes in sheet or strip ~ -form and can be incorporated into the imaging env~lope 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 1,064,072 as mentioned before. According to ~id technique ~-metal-cored glass fibres are drawn down tiil a sufficient length of 200-300 ~m fibre is obtained. A bundle of fibres is made by sealing the fibres together and is then cut into lengths of say, 10 cm. Each of these lengths of bundle is then drawn down in the same way as the original tub-, -equipped with an external cladding of thin insulating glass and drawn down till it is about 50 ~m in diameter. This glass fiber containing a metal wire e.g. copper is quite easy to handle. According to that technique 10 ~ fibres can be made.
Such a technique has been discussed also in Philips ~echnisch Ti~dschrift (1g69) No. 8/9/10, page 259.

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The wires or pins in the matrix should be preferably short and the dielectric constant of the binder material low, this for obtaining high charge transfer speed and maximum image resolution.
The transfer of the electrostatic images may proceed by conduction of electrica~ 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.
Image sharpness is practically not affected by charge transfer by contact. ~his requires, however, a close and direct contact of the ends of the conductive wires with the ins~lating materials. Such in~mate 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 deYel~pment or transfer of the electrostatic charge pattern onto the insulating target, (charge ~eceiving 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 non-transparent for visible light, but may proceed by surface deformation which is a technique known as "~hermoplastic Recording" (see e.g. J.Soc.Motion Picture ~elevision ~ngrs., Vol. 74,p.666-668).

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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 electrostaticall~ 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 electro-static latent image include cascade, powder-cloud (aerosol), magnetic brush, and fur-brush development. ~hese are all based on the presentation of dry toner to the surface bearing the electrostatic image wherc 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, lS 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 GV.758PCT - 29 -.' . ' - . , ~. . ................... . . . .- . - .

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due to the zeta potential built up with respect to the liquid phase. The outsta~ing 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 2,907,674 of Eenneth Archibald Metcalfe and Robert John Wright issued October 6,1959 and the United Eingdom Patent 1,151,141 filed February 4, 1966 by Gevaert-Agfa N.V.
~ he electrostatic image can likewise be developed according to the principles of "wetting development" e.g. as described in the United Eingdom Patents 987,766 filed April 18, 1962 by Agfa AG, 1,020,505 and 1,020,503 both filed ~ovember 8, 1961 by Gevaert Photo-Producten ~.V.
According to a particular embodlment the charge pattern is developed in direct relation to the quantity of charge, instead of to the gradient of charge (fringe effect develop-ment). Theref3re 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. Phot.Sci.~ng., Vol. 5, 1961, page 1~9).
According to another embodiment a transferable tonèr 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 . ~. , GV.758PC~ - 30 -. ~ , , .. , : .

~C~3t3441 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 powder is used, the powder image can be trans-ferred by electrostatic attraction, e.g. according to the method disclosed in the United Eingdom Patent 658,699 ~i:led April 14, 1949 by BatelleMemorial-Institute. Further det~ils are contained in the United States Patents 3,384,488 of Vsevolod Tulagin and ~eonard M.Carreira issued May 21, 1968 aDd 3,565,614 of ~eonard M.Carreira, Ira S.Stein and Vsevolod Tulagin issued February 23, 1971.
If a powder with ferro-magnetic properties is used for developing the electrostatic latent image, the powder can be transferred by magnetic attraction. The transfer can like-wise be carried out by a &esive pick off with an adhesive -A tape or sheet e.g. SC0~ ~ 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 electro-graphlc recording material. For example a recording web co~-sisting 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 inter-face is used. For a survey of dielectric coated papers re-ference is made to AGC-Monthly (1972) ~o.9, pages 4-15. A
special electrographic paper is described in the United States Patent 3,620,831 of Floyd T.Could issued November 16, 1971, and special thermoplastic recording tapes are describe~
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in the J.SocOMotion Picture Television ~ngrs., Volume 74, p.666-668.
As substances suited for enhancing the conductivit~ of the rear side of a dielectric recording material e.g. a transparent resin sheet are particularly mentioned anti-static 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;
~Z~
H2 ,~
' and vapour deposited films of chromium or nickel-chromium 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., 1~0-119, Feb. 1963). Such films are over 90 % transparent and have surface resistivities as low as 1500 ohms per square. ~he conducting film is preferably over-coated with a relatively thin insulating layer as described e.g. iD the J.Soc.Motion Picture Television ~ngrs., Vol.74, p.667.
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Claims (31)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of recording information as a pattern of electro-static 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 conductors 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 co-extensive 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 ex-posure 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 mechanical 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 zenon gas and the arithmetic product of pressure and thickness of the inter-space 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 insulting 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.

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 electro-static 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 there-between 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 changer 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.

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 interspaced between 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 b-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 (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.

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, wherein the conductors are made of alluminium 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.