CA2191471A1 - System and method for conditioning a radiation detector - Google Patents

Abstract

A system (10) and method for conditioning a photoconduc-tive radiation detector (12) achieve charge redistribution within a photoconductive layer (16) without the need for charge removal or charge injection techniques. A first conditioning voltage is ap-plied (46) across the detector to establish a first electric field. The first electric field is reversed relative to fields applied to the detec-tor during image exposure and image read-out operations. While the first conditioning voltage is maintained, the photoconductive layer is exposed for a period of time to first conditioning radiation (48) having one or more wavelenghts selected to penetrate at least a portion of the photoconductive layer (16). A second condition-ing voltage, less than the first conditioning voltage, then can be optionally applied (50) across the detector to establish a forward bias electric field. While the second conditioning voltage is main-tained, the photoconductive layer (16) is exposed for a period of time to second. Broad spectrum conditioning radiation (52). The detector (12) can then be placed in a dark environment for a period of time, in a shorted condition, to dark-adapt the photoconductive layer (54).

Description

~10 9S13SSII r~
~19~471 SYSTEM AND METIIOD FOR CONDITIONING
A RADIATION DETECTOR
Fidd of the Invention The present invention relates to imaging, and, more ~ " to 10 techniques for ~ , detectors useful in ~ O _, ' ,.
Di~r~ ion of Rdnted Art Cu..~. ' X-ray imaging systems employ an X-ray sensitive fluorescent screen and a j ' ~, film to form a visible analog IG~JU~ of a modulated X-ray pattern. The fluorescent screen absorbs X-ray radiation, and is thereby stimulated to emit visible light. The visible light exposes the j ' ~, film to form a latent image of the X-ray pattern. The film is then chemically processed to transform the latent image into a visible analog ~ t. L;u.. of the X-ray pattern.
2û More recently, efforts have been made to develop systems for acquiring digital X-ray images using various physical processes. Several approaches have focused on the use of radiation detectors made with I ' ' ;c materials such as amorphous selenium. One type of I ' ' - ~., radiation detector includes a multilayer structure having a first conductive layer, a ~ , layer disposed adjacent the first conductive layer, an insulative layer disposed adjacent the 1 .l .. .1. .~ ~. 1. .. 1: ~, layer opposite the first conductive layer, and a second conductive layer disposed adjacent the insulative layer opposite the Iayer. The adjacent surfaces of the I ~ c~ ;VG layer and the insulative layer define a r~ ~C ' -insulator interface. A junction layer is 30 formed between the adjacent surfaces of the ~ layer and the first conductive layer.
For a radiation detector as described above, an imaging operation is . ' ' ' by applying a first voltage across the first and second conductive layers, and exposing the r' ' '-~ Iayer to imaging radiation. With a -I -WO 95135~ 4 7 1 P~ J~6I ~
". I; ve layer of amorphous selenium, for example, the first voltage may have a positive polarity at the second conductive layer and a negative polarity at tile first conductive layer. The application of the first voltage creates a first electric fie~d across the detector. The absorption of imaging radiation by the 1,~ ,~v .. 1.. (iv~ detector creates electron-hole pairs in the 1~1 .tv~ .. ri ~; ve layer.
The first electric field separates the electron-hole pairs to form a set of charge carriers that create an Cl~ uc.Ldli~, latent image at the interface between the .-~1v~ T~ iv~ layer and the insulative layer.
The latent image can be read out and digitized to reaiize a digitai 10 ~ ,llL~Liul- of the radiation pattern. The read-out operation is ~, u .~ by applying a second voltage across the first and second conductive layers, therebycreating a second electric field. A pixel-sized beam of readout radiation is then raster scanned across the 1 ,l ~LI~ i ve detector. The scanned beam substantiaily completes the locai discharge of the 1 ,l .. ~t.~C. ~ li v~ layer at each pixel point, causing movement of a second set of charge carriers in the applied electric field. The imaged areas of the l.I ..~tv~ i v~ detector, which form thelatent image, respond to the beam by producing less charge movement than non-imaged areas. t'~ y, the level of detected current varies as an image-wise function of the position of the scanned beam on the ~ u- v --l~ ;ve layer. A
current detection circuit senses local current between the first and second conductive layers as each pixel point is scanned. The current detection circuit processes the current level detected at each pixel point to generate a .~ ,.,..L~,~io.
of the latent image, which may take the form of a digital l,"ul~;.,..llt~Lio~l.
A radiation detector as described above can be reused for subsequent

2~ imaging operations, but only after -,.,1;1;" :"~ to l~ Llii~u~. Iatent image charge and read-out charge collected at the rhotvc.~ ."-insulator interface. The oc ~ operation effectively erases the rh-~to~ v, 1~ ~ ~; v~ detector for the next imaging operation. To properly condition a phot~,~u~ (i v~ detector, charge collected at the l-I --~u~ -insulator interface should be either neutralized or removed. Unlike single-layer 1~ v~ L; v~ structures, such as those used in x~rography, the closed structure of the r~ - ,lu u l ~ ; v~ detector does not aliow the W095135511 . ._~v~. '1 2 ! 9 i 4 7 1 application of external charge to neutralize the collected chari~e. Specifically, the insulative layer of ehe detector covers the ~ i vc layer, making the u.,.. I ~ insulatorinterface i ' '- toexternal charge.
The inability to apply external charge to the interface has led to the study of ch`arge removal as an alternative ~.. l;l;.. -- .. ~ technique. Charge removal involves . transport of the collected charge from the r.l.. ~lu~ .. ,.l. .. l. ., -insulator interface, through the l,l ,~u~,,,,,l~.. I;ve layer, and to the junction layer. Transport can be . . ~".,l,i;~l,. .l to some extent by shorting the first and second conductive layers, and dark-adapting the shorted 1,1,... ~.. 1..... ~;v~ detector, i.e., placing the detector in a dark ~v;~u~ l for a period of time. A flood light exposure optionally can be performed prior to dark-adapting the detector. The shorting of the conductive layers creates an electric field that facilitates charge movement across the 1 ,l .. .1~,~ . . 1~ -- 1 i ve layer to the first conductive layer. The electric field would effectively remove the charge from the 1~ -, -insulator interface if the charge were free to transport. The charge typically is not all free, however, but held in interface trap sites. Because the trap sites prevent transport, charge removal generally has not been considered a completely effective technique for cnn~ a l~l . t.~c, ~- --l l- -. detector.
Other cnnA~ n~ techniques have focused on charge injection as a means to neutralize the collected charge, due to the ~l ~;.. ;.. : ~ ~,Il~,uu~ d with the charge removal technique. Charge injection involves the transport of nPIltr:~li7in charge from the first conductive layer to the l~ JtUC . -l ~ l ..-insulator interface.
C.... l;l ;.... .~ by charge injection has proved effective. u..ru. i 'y, the physical ,., U.~ of charge injection have placed u ~ - ~l .Ic constraints on the structure of the overall ph ~ iv~ detector. In particular, the charge injection tecbnique . , l fabrication of the junction layer separating the firstconductive layer amd the l-l " ~t~ ; v~ layer. The imaging and read-out operations involve the application of a high potential across the first and second conductive layers. To avoid premature discharge, the junction layer should be constructed to electrically block charge flow from the l.l .. ~ -~ - l ,- ~iv~ layer to the first conductive layer. ~or charge injection, however, the junction layer also

-3--wo 95135511 'I

should be constructed to allow charge flow from the first conductive layer to the ,l l;vc layer.
The charge injection ~"""11;....:..~ technique therefore generally requires the formation of a "~cc~iry;ll~;" junction layer between the first conductive layer and the ~ li ve layer. A "lc~,liryill~;" junction layer s~het ~nfi~lly blocks charge flow in one direction only, generally allowing charge flow in the opposite direction. The formation of a "rectifying" junction layer involves a . r process having an uncertain success rate. Specifically, the ''Ic.,liryil.~s'' junction layer generally is formed by creating a carefully controlled oxide layer on an aluminum surface of the first conductive layer, and then carefully controlling the initial phases of deposition of thel.l...~,~.~...l....~;ve layeroverthe oxide layer. The process for creating an oxide layer having suitable rectifying l.~ lir~ has not been fully understood by those skilled in the art, resulting in an; ~
success rate that produces ~ ci~;l~ly low ,.... r 1... ;..~ yields. In addition, the need for an aluminum surface constrains the choice of materials that can be usedfor the first conductive layer. Finally, the process for forming the oxide layer also places constraints on deposition process parameters for the l.l..,l..~ ....l -.~ive layer.
In surnmary, ~.. 1;1 ;.. ; . ~ a ~ ; ve detector by the application of external charge has not been considered feasible due to the disposition of the insulative layer adjacent the ~ i ve layer. ('I, l;~ g by charge removal has been found to be ineffective due to retention of charge carriers in interface trap sites. Finally, ~ , by charge injection provides effective charge IC'I ~ 1 ;- .", but has been found to require the use of a "lccliryill~"
junction layer formed by a ~ ~ l. .r~. 1... ;..Q process.

S of ' I
In view of the foregoing d;~cd~ l~gc~ associated with existing techniques for . ~,...li~i,~":.,e a radiation detector for reuse, the present invention is directed to a c - .-l:~;--- .e system and method that achieve effective (~on~1iti~ninQ without the 30 need for charge injection or charge removal techniques. rhe ' ,, system and method of the present invention are not only applicable to a detector structure

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~ W095135511 2 ~ 9 i 4 71 r~"~
haYing a "Ic.,~ir~ " junction layer, but also enable the use of a fully closed detector structure having both an insulative layer and a "blocking" )unction layer that sl~hrt~n~ ly blocks charge flow between the ~ ivc layer and the first conductive layer in both a forward bias direction and a reverse bias direction.

- 5 A fully closed detector structure would be a i~O.. L.~ ,ù~ because the "blocking"
junction layer could be formed by a thin insulative layer. In contrast to the ,."".~.1;. .t ~I " - ,..'`- I... ;.,~ processes generally necessary to form a "l C~liry;~
junction layer, a thin insulative layer can be formed by relatively simple processes such as vacuum deposition, growth by oxidation, or solution coating to form a thin 10 fiim.
Additionai features and advantages of the invention will be set forth in part in tbe description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention wili be reaiized and attained by the system and method ~ i.,ul~ly pointed out in the written description and claims hereof, as well as in the appended drawings.
To achieve the foregoing advantages, as broadly embodied and described herein, the present invention provides a method for .. lil;.. :.~t~ a radiation detector having a first conductive layer, a ~ ivc layer disposed adjacent the first conductive layer, an insulative layer disposed adjacent the rhnt~crn~ rtive layer opposite the first conductive layer, and a second conductive layer disposed adjacent the insulative layer opposite the phnto~ ..l Iivc layer. the radiation detector forming a latent image in response to both application of an image exposure voltage across the first conductive layer and the second conductive layer and exposure to imaging radiation, and the latent image being read out during application of an image readout voltage across the first conductive layer and the second conductive layer and exposure of the detector to readout radiation, the .: ol~ method comprising the steps of applying a ~.. 1;1 ;....;, .~ voltage across the first conductive layer and the second conductive layer, the ~ ' ~
voltage having a polarity opposite to a polarity of both the image exposure voltage and the image readout voltage, wherein the ~ .,.. lil;.. '.. ~ voltage establishes an electric field across the rhntocnn~l--rtive layer and exposing the rhntoc~,...l... ~ivc WO95/35511 2 ~ 91 4 7 ~ P~
layer to f~nin~ radiation having one or more ~dV~ ;LIl, selected to penetrate at least a portion of the ~ i ve layer, thereby releasing charge carriers trdpped within the ~ ;ve layer to transport within the electric field.
The present invention also provides a system for ç~ a radiation detector having a first conductive layer, a l.l.. .t .~ ive layer disposed adjacent the first conductive layer, an insulative layer disposed adjacent the .J. ~)...l ~iv~ layer opposite the first conductive layer, and a second conductive layer disposed adjacent the insulative layer opposite the l,l ., )t..~ ,.... l... l; v~ layer, the radiation detector forming a latent image in response to both application of an imdge exposure voltage across the first conductive layer and the second conductive layer and exposure to imaging radiation, and the latent image being read out during application of an image readout voltage across the first conductive layer and the second conductive layer and exposure of the detector to readout radiation, the ~f~,. l;l;- ,. .~ system comprising means for dpplying a f~. lili.. .æ voltdge across the first conductive layer and the second conductive layer, the .... ,.l;l;.. ,. -.p voltage having a polarity opposite to a polarity of both the image exposure voltage and the image reddout voltage, wherein the ~..., l;l;....: .æ voltage establishes an electric field across the l~ t .~f~ live layer, and means for exposing the 20 r.~ o( ~ l ~;vc layer to 1:1;.. ,: .~ radiation having one or more ~a~ ;lla selected to penetrate at least a portion of the rhf~to~ u~ l. (i ve layer, thereby releasing charge carriers trapped within the ~ ut~J~ i ve layer to transport within the electric field.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and c r I ' ,y only, and not restrictive of the invention, as claimed.
Bri~f ~ " of '' -The dCCulllually;ll~ drawings are included to provide a further .,.,.l. . ~1~ ,.1;"~ ofthe invention and are ;~,vluuld~ed in and constitute a part of this

-6 -wo 95/35511 r~
2 1 9 ~ 47 ~
1;..,. The drawings illustrate exemplary ~ .o.l;.... ~ of the invention and together with the description serve to explain the principles of the invention.
Fig. I is a schematic ~culc~ ion of a system for acquiring a digital image with a reusable radiation detector;
Fig. 2 is a schematic cross-sectional IC~ IIt.ll;UII of the radiation detector shown in Fig. l during an image exposure operation;
Fig. 3 is a schematic cross-sectional l~nc.~ dliu.. of the radiation detector shown in Fig. I illustrating an example of charge tlietrihl.tinn afler an image exposure operation;
Fig. 4 is a schematic cross-sectional IculC~c~llLaLiull of the radiation detector shown in Fig. I during an image read-out operation;
Fig. 5 is a schematic cross-sectional ICUlC~ClllatiUII of the radiation detectorshown in Fig. I illustrating an example of charge .I:~1, ;1,.,1;.". afler an image read-out operation;
Fig. 6 is a flow diagram illustrating an exemplary ~ o.l;~ of a method for I ' ~ a radiation detector in accordance with the present invention;
Fig. 7 is a schematic cross-sectional ICIJlC~CI~t~l~iUII of the radiation detector shown in Fig. I during a first stage of a system and method for rnm~itil ~ a radiation detector in accordance with the present invention;
Fig. 8 is a schematic cross-sectional, ~I)IU~ IIiUII of the radiation detector shown in Fig. I illustrating an example of charge d;ai '' " after the .. ,1;1 ;u.. : .
stage of Fig. 7;
Fig. 9 is a schematic cross-sectional IclJlc~.l.;dliull of the radiation detector shown in Fig. I during another stage in a system and method for ~~ 1; l ;:, .~; a radiation detector in accordance with the present invention; and Fig. 10 is a schematic cross-sectional l.,~lc,.llLlliO.I of the radiation detector shown in Fig. I illustrating an example of charge .I; . ;1, ~1;. ." afler the ~,..,,I;I;,,,.;,,p stage of Fig. 9.

-7-W09S~3SS11 2 ~ 9 1 4 7 1 of " - PrPt`Prred E ' Fig. I is a schematic ~ lL lliul~ of a system 10 for acquiring a digital image in response to imaging radiation. The system 10 ;IICVIIJ ' an exemplary reusableradiationdetectorl2suitableforapplicationofa~..,..~1;1;....:.,esystemandmetbod irl accordance with the present invention. The detector I Z comprises a multi-layered stack having a first conductive layer 14, a ~ v~ ;v~ layer 16 disposed adjacene the first conductive layer, an insulative layer 18 disposed adjacent the ~ v~ - I;Ye layer opposite the first conductive layer, and a second conductive layer 20 disposed adjacent the insulative layer opposite the ~ t. .~ . .. 1- - 1; ve layer. The first conductive layer 14 may be a planar sheet of conductive material or, as shown in Fig. I, a segmented array of conductive electrodes 14a-14p arranged in a striped pattern across detector 12. Adjacent surfaces of l.l --lv .. li-- liv~ layer 16 and insulative layer 18 define a rhnto~...... l .. I-.. -insulator interface 22. An example of a known radiation detector I S substantially - r...... : .~ to that described above is disclosed in United States Patent No. 4,176,275 to Korn and Nelson.
A jumction layer 24 is formed between first conductive layer 14 and l,l...lv. .. 1~ ive layer 16. A ~ e system and method, in accordance with tl-e present invention, enablcs junction layer 24 to be realized as either a "rectifying" junction layer or a "blocking" junction layer. The term ''l~liryi.. ~'' junction layer, as used herein, refers to a junction layer that is ' Ily electrically blocking to charge flow from rhl-toronf~ tive layer 16 to first conductive layer 14, but substantially electrically non-blocking to charge flow from the first conductive layer to the J ~ ' 1 ' V~ layer. In contrast, the term 2~ "blocking" junction layer, as used herein, refers to aJunction layer that, like a iry;~ " junction layer, is substantially electrically blocking to charge flow from 1,l ,.. 1.. ~.. 1, .. ~i ve layer 16 to first conductive layer 14, but which, unlike a "rectifying" junction layer, is substantially electrically blocking to charge flow from the first conductive layer to the 1,l.. 1-,~ - ....1.,. 1; v~ layer. Thus, in accordance with the ~o~ ;. ., .: ,.e system and method of the present invention, junction layer 24 should be substantially electrically blocking to the flow of charge from

-8-~ W095135511 ~ ~ q i 4 71 r~
I;ve layer 16 to first conductive layer 14. However, junction layer 24 may be either e1lhct~ntiol1y electrically non-blocking or sl-hcf~nfi~lly electrically blocking to the fiow of charge from first conductive layer 14 eO ~ u~ i ve ~ayer 16. C~ , detector 12 may, if desired, comprise a fully closed detector structure in which ~ (i ve layer 16 is enclosed by both insulative Iayer 18 and a "blocking" junction layer 24.
Nurnerous variations in the structure of detector 12 are ,O~ d~ . For exa~nple, the first and second conductive layers 14, 20 can be il~b~ such that first conductive layer 14 is disposed adjacent insulative layer 18 and second conductive layer 20 is disposed adjacent I ' ' ive layer 16. In addition, detector 12 can be mounted on a substrate and~or a mechanical frame for added support. The layers of detector 12 can be fommed from various materials providing suitable electrical and . ,- I;o~ yl :r .,1.~.,.,.~,. i~i.,~. If a substrate is employed, first conductive layer 14 can be fommed over the substrate by depositing a planar sheet of conductive material . The segmented array of electrodes 1 4a- 1 4p, as shown in Fig. 1, can be fommed by etching the deposited planar sheet to define a suitablestriped pattem. If a substrate is not employed, first conductive layer 14 can befommed by depositing a planar sheet on a surface of r~ u~ ive layer 16, and then etching the sheet to define electrodes 14a-14p, as desired.
The l l"-t,~ ;ve layer 16 preferably comprises amorphous selenium, which can be fommed over first conductive layer 14 by cu..~,...iu..~l techniques, or fommed as a separate laye} to which conductive layer 14 can be added by deposition, as described above. However, I.l .lù ~, 1 Iive layer 16 may compriseother materials suitdble for detecting radiation, such as lead oxide, cadmium sulfide, or mercurous iodide, as well as any of a variety of organic In general, l ~ u . " ..1 ~ ~; ve layer 16 should exhibit a low Cul..lu~,liviLy in the absence of imaging radiation, on the order of ~UIU~ y 1 09 ohm-~ or greater. The low COlldu~.li vi~y enables an electric field to be maintained across detector 12 for an extended period of time without ~.c .,;~l~ discharge. The l.l" .L.)~.. l.. ~; ve layer 16 also should be thick enough to absorb ~ 50 percent or more of the flux of the imaging radiation g Wo95/35sll 2 1 9 1 4 7 1 ~ 61 received by detector 12 during image exposure operations to form a afA~i~rf..luly latent image. With amorphous selenium, for example, IJ~ " fll~ ` (;V~: layer 16 preferably has a thickness of ~ / 250 to 550 microns.
The insulative layer 18 can be formed either from a material that is fluid at the operating t~ J.,lf~ ûf detectûr 12, which may be a gaseous material such as air, or from a layer of non-fluid material. The insulative layer 18 typically has a thickness of f~ 100 to 300 micrûns. The insulative layer 18 can be formed by vapor deposition of a po~ymeric material, such as parylene-C, lur~ d by Union Carbide, over ~ ; ve layer 16. The vapor depositiontechniquemaybeadv~ullf~f~)u~becauseitfacilitatestheformationofan insulative layer having uniform thickness. The insulative layer 18 ~1 v~.ly can be realized by a flexible polymeric film that is bonded to the surface of rh..l-.f~,l,.l... (;v~ layer 16 opposite first conductive layer 14 with an optical adhesive, typically I to 30 microns thick. The second conductive layer 20 can befommed by evaporation of a conductive material over the surface of insulative iayer 180pposite~ 1 -1iv~layerl6. If insulativelayerl8isformedseparately from the stack as a polymeric film, second conductive layer 20 can be evaporatedover the insulative layer either before or after bonding the film to rhntor~lnflllftive layer 16.
With reference to Fig. 1, system 10 further includes a voltage source 26, a first radiation source 28, a second radiation source 30, and a current detectioncircuit 32. For an image exposure operation, voltage source 26 applies an exposure voltage across detector 12. The first radiation source 28 then emits imaging radiation 34 that is received by detector 12, aRer passing through an object to be imaged. The object to be imaged alters the intensity of the imaging radiation 34received by detector 12 in an image-wise pattem, resulting in the formation of alatent image ~ n~ llillg the object, as described in United States Patent No.
4,176,275. For an image read-out operation, voltage source 26 applies a read-outvoltage across detector 12 while second radiation source 30 raster scans a pixel-sized beam 36 of readout radiation across detector 12 in a time-ordered pattern.The current detection circuit 32 detects and stores current flow values in W095135511 P~l/u., ' I

~y~ ull.... with the time-ordered pattem scanned by beam 36 to produce a aliull of the latent image, which may be in the form of a digital tdtiUII.
The voltage source 26 has a first temminal coupled to first conductive layer 5 14, as indicated by line 38, and a second terminal coupled to second conductive layer 20, as indicated by line 40. Fig. 2 is a schematic cross-sectional lCp~;i:lCillt~liUII of detector 12 during an image exposure operation, illustrating an example of initial charge ~ ;.,.. within the detector. As shown in Fig. 2, during the image exposure operation, voltage source 26 applies an exposure voltage VE that creates an electric field EE across detector 12. When amorphous selenium is used for ~ lu~ ive layer 16, the more mobile carriers are the holes. In this case, the exposure voltage VE applied by voltage source 26 preferably has a positive polarity at second conductive layer 20, as shown in Fig. 2.
The voltage VE W;ll be referred to as a "forward bias" voltage for the holes. The voltage source 26 dlt~ may apply an exposure voltage VE having an opposite polarity across detector 12 while achieving ~ali~ra~,luly imaging p. . r." . --- c In such a case, the opposite polarity voltage would be referred to as a "forward bias" voltage for the less mobile electrons.
The 1,~ liv~ layer 16 absorbs a portion of imaging radiation 34 emitted by first radiation source 28. Figs. I ~md 2 show imaging radiation 34 asbeing incident on detector 12 from the direction of second conductive layer 20.
However, imaging radiation 34 may be incident from the direction of first conductive layer 14. In either case, the conductive layer 14, 20 through which uc.~ iYe layer 16 receives imaging radiation 34 should be transparent to the ~lv~ SIII of the imaging radiation. As an exampte, for imaging radiation 34,having an X-ray wavelenL~th of atJIJ~u~ ly lo-8 to lO ll meters, either first conductive layer 14 or second conductive layer 20 can be formed from thin metallic, e.g., aluminum, layers that are sufficiently transparent to X-rays.
Altematively, a deposited film of r~ lr~in~ transparent indium tin oxide (ITO) may be suitable for fommation of either first conductive layer 14 or second wo 95/35511 P~ 61 2 1 q 1 47 1 conductiYe layer 20. If 1ll.. ,lo~ .. . 1 ~i ve layer 16 also receives imaging radiation 34 through a substrate, the substrate should be transparent to the radiation.
The imaging radiation 34 is absorbed by l~ t~ ;vc layer 16 in an image-~-vise pattern Icv.~ læ an object 41 to be imaged, which may be, for cxample, a portion of the human body, an industrial structure, or a contact film.
The object 41, as shown in Fig. 2, includes 5~hotD~lti~ly radiation-opaque areas41a and less radiation-opaque areas 41b that determine the pattern of incident rmaging radiation 34 received by detector 12. The absorbed radiation creates electron-hole pairs within ~ u~ , layer 16. The electric field EE separates the electron-hole pairs within ~ ~. ' vc layer 16 to mobilize a set of charge carriers. The electric field EE generated by the exposure voltage VE typically has a field strength of Cl,V,VlU~il.l~ly 5 to 15 volts/micron within ~ u~ ,. ..1. .. I ;ve layer 16. In particular, field strengths in the higher end of this range provide more effective carrier separation within ~ u~ ive layer 16.
The mobile carriers reduce the electric field EE across rh~l.. c.. l~ vc layer 16 in an i...~ pattern. The charge carriers having a first polarity move to first conductive layer 14, whereas the charge carriers having a second, opposite polarity move to interface 22, resulting in the formation of a latent cl~ u~ldtic image at the interface. For example, jfl~h ~u v ..l ~ I;ve layer 16 comprises amorphous selenium and exposure voltage VE has a positive polarity at second conductive layer 20, the latent elc.,l.u,L~ic image is formed by the collection of electrons at interface 22.
As shown in Fig. 2, prior to the image exposure operation, a negative charge QO exists at the first conductive layer 14, as determined by the exposurevoltage VE. Fig. 3 is a schematic cross-sectional Icl~ic~c~ liu~ of detector 12,illustrating an exatnple of charge ~ rihllti~n within the detector after an image exposure operation. As shown in Fig. 3, the electric fie~d EE causes a fractionfof the original charge QO on first conductive layer 14 to reside at interface 22 in an image-wise pattern. The voltage source 26 ~1 " v~ charge between first conductive layer 14 and second conductive layer 20 to maintain the .

WO95~355~1 2 1 9 1 4 7 1 ~ uw~ ~61 potential difference VE across detector 12. The chargefremains at interface 22, howeYer, thereby preserving the eL..,llu~ . latent image.
Fig. 4 is a schematic cross-sectional l~JlC~ ll of detector 12 illustrating charge ~ U~ within the detector during an image read-out operation. As shown in Fig. 4, voltage source 26 applies a read-out voltage VR that creates a second electric field ER across detector 12. When amorphous selenium is usedfor~ f '~ Livelayer 16,theread-outvoltage VR appliedbyvoltage source 26 preferably has a positive polarity at second conductive layer 20. Like the exposure voltage VE, the read-out voltage VR will be referred to as a "forward bias"
voltage for the more mobile holes. The read-out voltage VR ~ ,IY may have an opposite polarity across detector 12. In such a case, the opposite polarity voltage would be referred to as a "forward" bias voltage for the less mobile electrons. However, the polarity of the read-out voltage VR should be the same as the polarity of the exposure voltage VE.
The second radiation source 30 scans beam 36 across detector 12 in a time-ordered pattern. The time-ordered pattern preferably comprises a raster pattern that scarls a series of parallel lines, one at a time, until every pixel on detector 12 has been scarmed. In the example of Fig. 4, beam 36 is shown as being scanned from right to lefl across detector 12. The scarlning beam 36 may have a v~a~ s-lh~t~ntiS~lly similar to that of imaging radiation 34, or a 5-~h~t~ ti~lly different v.~ . For exarnple, beam 36 may comprise ultraviolet, visible, or irlfrared radiation, as appropriate to discharge the particular material selected for . "~ ; ve layer 16. If amorphous selenium if used for l,l .~ , l . " li vc layer 16, for example, bearn 36 may comprise visible radiation in the blue-greenspectral range. A typical beam 36 employed in read-out operations is realized by a laser having a ~ ll of a~ lUAil~ y 488 " ,,, ,. t, ,, ~
Figs. I arld 4 show beam 36 as being incident on detector 12 from the direction of second conductive layer 20. However, beam 36 may be incident from the direction of first conductive layer 14. In either case, the conductive layer 14, 2Q through which ~ c -ll-~ ivc layer 16 receives beam 36 should be transparent to the ~ .lcll~ of the beam. A deposited film of cnn~1llrtinp, transparent indium 11V0 95~ r~l~L~ 61 tin oxide (ITO), for example, may be suitable for for~nation of either first conductive layer 14 or second conductive layer 20. If ~ lu~ ; ve layer 16 receives beam 36 through a substrate, the substrate also should be transparent to the beam.
The ~ t.,~ li v~ layer 16 absorbs a portion of the radiation transmitted by scanned beam 36 to create additional electron-hole pairs. The electric field ER
applied across detector 12 separates the electron-hole pairs to mobilize a second set of charge carriers. The electric field ER typically has a field strength of a~ wd..~.~ ly I to 5 vrl / u... As in the image exposure operation, the charge carriers of a first polarity move to first conductive layer 14, whereas the charge carriers of a second polarity move to interface 22. The charge movement caused by electric field ER substantially completes the local discharge of l-l --~V~2.' 1 - ~ive layer 16 at each pixel point, as I~IVIG~ 2 by the dashed arrow 42 in Fig. 4. Thesecond radiation source 30 continues to scan beam 36 until every pixel has been æddressed. The resulting charge movement at each pixel leads to further - .., of charge between first conductive layer 14 and second conductive layer 20 by voltage source 26.
The current detection circuit 32 is coupled to electrodes 14a-14p, ly, via channels 44a-44p, as sho vn in Fig. 1. The current detection 20 circuit 32 detects the l~ l . ibl ~ .. of charge between first conductive layer l 4 and second conductive layer 20 as beam 36 scans eæh pixel. In this manner, current detection circuit 32 acquires a measure of local current flow at eæh pixel point.
The imaged areas of ~ .(uc ~.. l - liv~ layer 16, which coincide with the latentimage at interface 22, respond to bearn 36 by producing less charge movement than non-imaged areas. The imaged areas produce less charge movement because less charge is necessary to complete the discharge of ~ i v~ layer 16 in the imaged areas of interface 22 where latent image charge has been collected.
(`""'~'1 ~ly, the level of current detected by current detection circuit 32 varies as an image-wise function of the position of the scanned beam on l~ t~J~ ~ . I I. I i V~
layer 16. The current detection circuit 32 processes the current level detected at each pixel point to generate a l~l~ aliUII of the latent image.

.

~ W095135511 2 7 9 1 4 7 1 ~ u~
Fig. 5 is a schematic cross-sectional ..,~. ~ac~ iO~l of detector 12 illustrating an example of charge ~lictrihl-tinn within the detector after the image read-out operation. As sho~-vn in Fig. 5, the image exposure and image read-out operations result in the collection of both latent image charge and read-out charge at interface 22. To prepare the reusable detector 12 for a new imaging operation, ,. the detector should be, " ' to l~d;~ll;bLI~e this collected charge within J~l ~tu~ ;ve layer 16.
Fig. 6 is a flow diagram illustrating an exemplary ~ ~ " of a method for ...,...1:1;.... .~ detector 12, in accordance with the present invention. The ~ , method of the present invention achieves ~ ...................... of the charge collected at interface 22 without the need for charge removal or charge injection methods. As indicated by block 46 Of Fig. 6, a first, reverse bias ~ ,.g voltage is applied across first conductive layer 14 and second conductive layer 20 to establish a first, reverse bias electric field. While the first .. 1;~ voltage is maintained across detector 12, 1.II~JIU~. 1 - 1;ve layer 16 is exposed for a period of time to first rnn~ innin~ radiation having one or more ~ lla selected to penetrate at least a portion of the ~ ;ve layer, as indicated by block 48.
The first, ' ~ radiation is selected to release charge carriers trapped within photoc~/lldu.,~ive layer 16, enabling the released charge carriers to transport in the first electric field established by the first cnnflitinnine voltage. In some Uil' ~ it is collc-,;v~lc that acceptable ~.n~ :................. g can be achieved by application of the first C'"";;tinninF voltage and first ~ .. 1; 1 ;'J': æ radiation alone.
~or example, it may be suff cient that the application of the first ~
voltage and first, ~ ~ ,, radiation result in a 11~JIOd~IU;bIC charge density state at interface 22, which serves as a starting point for the next image exposure operation.
To achieve more complete, ' ., however, a method in accordance with the present invention preferably is realiæd by the following additional steps.
Specifically, a second, forward bias ~ntlitinnin~ voltage, less than the first .... ,~.1;1;.. ",.,~ voltage, can be applied to first and second conductive layers 14, 20 to establish a second, forward bias electric field, as indicated by block 50. The WOgS/35511 2 ~ 9 ~ 4 7 ~ r~l", , second ~ ", ,æ voltage preferably is achieved by shorting first conductive layer 14 and second conductive layer 20 to produce a voltage of ~ 'y zero. While the second ,:.",.1;1;.".~ voltage is maintained across first and second conductive layers 14, 20, r~ ; ve layer 16 is exposed for a period of time to second, broad spectrum ~ radiation, as indicated by block 52. As a further optional step, detector 12 then can be placed in a dark CIlV;l~ for a period of time, with first and second conductive layers 14, 20 shorted, to dark-adapt ~ ivc layer 16, as indicated by block 54.
Fig. 7 is a schematic cross-sectional l~ dliUII of detector 12 during a fir$ stage of the c ~ method of the present invention. As shown in Fig. 7, voltage source 26 applies the first ronA;~ionin~ voltage Vcl across first and second conductive layers 14, 20 to establish the first electric field ECI The application of first ~ onAitionin~ voltage Vc~ cul.c~ullla to the step indicated in block 46 of Fig.
6. The rnnAitionin~ voltage Vcl is a reverse bias voltage relative to both the forward bias exposure voltage VE and the forward bias read-out voltage VR applied during the image exposure and image read-out operations, .~ .,ly. Thus, the resulting first electric field Ecl similarly is a reverse bias field relative to fields EE
and ER established during the image exposure and image read-out operations, rc~ "ly. If the image exposure and image read-out voltages VE, VR are selected to bave positive polarities at second conductive layer 20, as generallywould be the case when ~ i ve layer 16 comprises amorphous selenium, the oppositely poled ~ voltage Vcl then is selected to have a positive polarity at first conductive layer 14. The polarities ofthe image exposure voltage VE, image read-out voltage VR, and first çl~nA~ voltage Vcl can, of course, be reversed according to the needs of the system user and/or the type of material used to form ~ ve layer 16.
~Vhile first ~..,...1;1;~,,,;.,~ voltage Vc~ is mo;nroin~fl, a first cnnAitinnin~
radiation source 56 exposes r~ ive layer 16 to first ~ ,g radiation 58, as also shown in Fig. 7. The application of first c~-nAitio; -~
3û radiation 58 CullcalJulld~ to the step indicated in block 48 of Fig. 6. The first ,.t rtt 5ahon 58 cont ins ~ .s select d lo ~nctt lG at letls~ t ~ W095BSSII 2 l 9 i 4 7 1 r~,,l,. -1 portion of rhnt~ live layer 16. The photons transmitted with first ~ radiation 58 release holes and electrons from trap sites at interface 22.
For enh~mced ~ , it is preferred that first ~;n~ , radiation 58 also contain ~r~v~,L .~ selected to penetrate deep within the bulk of ~ ive - 5 layer 16 to release holes and electrons from distributed trap sites. Once first radiation 58 releases the trapped charge carriers, they are free to transport within the reverse bias electric field ECI, along with charge carriers not held in trap sites, resulting in ~ . ;l .ul ;.... of charge supporting the latent image at interface 22.
10 The first 1~l .. 1; 1 ;. - ,; ,~ radiation 58 releases, in particular, a portion of the charge carriers held in trap sites at interface 22, enabling the released chargecarriers to leave the interface by light excitation and transport away from the interface in the reverse bias field ECI The collected charge carriers not held in interface trap sites are also free to transport away from interface 22 in field Ec,, thereby c~ e to I~J;~L-;I~ ofthe latent image at the interface. At the same time, first c~,. .. l;l ;. . , .~ radiation 58 releases charge carriers having a polarity opposite to that of the charge collected at interface 22. The oppositely poled charge carriers released by radiation 58 are thereby free to transport toward interface 22 in the reverse bias field Ec~, along wjth oppositely poled charge carriers not held in trap sites, to neutralize the remaining charge carriers collected at the interface. The opposite poled charge carriers also r ' ' at interface 22 in excess of the original latent image charge.
If l~l ~t~n~ l;ve layer 16 comprises amorphous selenium and image exposure voltage EE has a positive polarity at second conductive layer 20, for example, the charge collected at interface 22 is negative, as shown in Fig. 7. The first c.. 1;1;.. ,~ radiation 58 releases a portion ofthe electrons and holes held in trap sites at interface 22, as well as electrons and holes held in distributed trap sites deep within rhntocnn~ rtive layer 16. The electrons released from interface trapsites leave interface 22 by light excitation and transport away from the interface in the reverse bias field Ecl. The holes released from interface trap sites and distributed trap sites transport in the opposite direction toward interface 22 to WO95/35511 2!9~47~
neutralize remaining electrons. Thus, the IcL~Ll;lJulive effect is achieved both by transport amd . ~ , of electrons collected at interface 22.
The ~a ~ ;1]l of first ~ radiation 58 sufficient to release a ~aLi~r~.tul.y amount oftrapped charge carriers inevitably will vary with the absorption versus ~av~ ; of the particular ~ ; ve material chosen for ~ I i ve layer 16, as well as the intensity and exposure time of the frtst < .~.. 1;l ;....; ~ radiation. In addition, the optimum V~a~ ll of first " 'nnin~ radiation 58 will depend on the density of deep trap sites within1 .l ...~.,~.... ., .1 ; ve layer 16 and the spatial 1 ; ~I il ... l ; . of such trap sites throughout lû thel~h-.1u~.. l l;vclayer. Theabsorptionversuswa~.,llll~;lll.l-, ~.. ;~l;~ofthe r'=- ~ ive material determines the number of photons that will be absorbed at a particular wavelength before actually reaching trapped charge carriers at a umit depthwithin~ v~n~ 1;ve layer16. Theabsorptionversus va~ SLIl ..l,--_.l..;~l;~ofin5ulativelayerl8generallycanbed;~lcLald"difaproperchoice of materials is made. The total number of photons transmitted via first .æ radiation 58, l~u~willla~ld;ll~ absorption, is a function ofthe intensity of tbe ~ ..,..1;1;. ~. ~ .~ radiation and the exposure time.
The absorption edge of a ~ i ve material refers to a wavelength at which s-lh~nti~lly all of the incident radiation is absorbed within the first micron of depth. Effective ~ ", of trapped charge carriers can be achieved, in accordance with~ the present invention, by exposing l-l.-~to~ ive layer 16 to first ron~itinn np. radiation 58 that does not include v a~,h,ll~ greater than the pertinent absorption edge wavelength, provided intensity and/or exposure time are increased to Culll~ for excessive absorption. However, the need for higher energies and/or longer exposure times is inefficient, and therefore may be to the user. Thus, to achieve more efficient ~f l;~ .. for rhnto~-nntll-~tive materials having a significant degree of trapping within the bulk, first cc~...l;l;u~ æ radiation 58 should contain wavelengths that are not 511hct~nti ~lly less than the absorption edge wavelength of the chosen l.l.. .lu~ ~ I - live material, and preferably contains ~a~.,lcll~ greater than the absorption edge wavelength.

~ w095~3ss~ 9 ~ 4 7 ~ F~
~or example, when ~ ive layer 16 is formed with amorphous selenium, the pertinent absorption edge ~ is ~y~ '" 520 " . .. " ,.. t. . ~ Effective 1~.l;~l, ;l .. 1;- -~ can be achieved by exposing the arnorphous selenium ~ u~ ;ve layer 16 to first ~ ,fr radiation 58 that does not include ~ a greater than ,l~Jy~ / 520, - " .. . r. . ~ However, to realize effective .c,1 ~1 1 ;l l. .1 ;l l with a lower intensity level and shorter exposure time, ,u~. ~ ' ve layer 16 preferably is exposed to ~ e radiation 58 including ~r~ greater than or equal to 1,~".., '~r 500 ~ In particular, first Cr. nriitir ninfi radiation 58 having ~a~L.Il~;Llls greater than or equal to ~ wd~ ly 600 . ~ should be effective in releasing a sufficient amount of trapped charge carriers in a relatively short amount of time when amorphous selenium is used.
The reverse bias field Ec~ should have a strength sufficient to achieve effective l~ ;bu~ive transpûrt of charge carriers released from trap sites by first ,.-1~.l;l;.,.. ;,.~ radiation 58, as well as charge carriers not held in trap sites. The appropriate strength of the first c - " r. ninfr field Ec I will vary with the strength of the image readout field ER~ which norrnally is the major factor in the amount ofcharge collected at ~ r~l..l I ..-insulator interface 22. A r."~.l;l;~.";~.~, field strength suitable to Icdi~ll;bule the charge transported to interface 22 in the readout field ER can be generated by a first ~,~.. ,1;1;~",;,.~; voltage Vcl having an absolute value on the order of a~,ull ' 'y 0.2 to 1.0 times the image readout voltage VR
In particular, a r ,~. ..I;I ;l l. .; . ~g voltage Vcl of ~/,UI ~ 0.5 times the value of the image readout voltage VR has been found to establish a reverse bias field Ec, providing effective ~cd;~ll;blllive transport. A first r.... l;~ voltage Vcl less than the above range may generate a weak field producing incl-fflrir~nt transport, whereas a first ~ voltage Vcl substantially greater than the above range may create an excessive boost field at interface 22 that can complicate l~d;;~Uibuliull in a second stage of the c~ method, to be discussed.
Therefore, although a first . . " "1;1;~ voltage Vc~ having a value outside of the above range may support acceptable ~`O ~ , such çr.~n~ir,~r~tir. n~ may justify operation v~ithin the stated range in certain ~

WO95/35511 ~ 1 9 1 4 7 1 F~
As an example, a typical image readout voltage VR could have a value of a~ Iy 1000 to 3000 volts when detector 12 is formed with an amorphous selenium~' v~",-1-~liv~layer16havingathicknessof au,UI~ 425 microns and an insulative layer 18 having a thickness of a~,ul~ ' ' 'y 175 5 microns. For the above .. ,.. ri,,.. ~1;.. ,. with a readout voltage of 3000 volts, in particular, a suitable ~ voltage Vcl preferably would be ay~ y .
0.2 to 1.0 times the irnage readout voltage VR, falling in a range of a~ y ~
600 to -3000 volts. More specifically, for the above . r;~ , effective trarlsport has been observed with a c- - ~ lil inl~ voltage Vcl of a,u~
1500 volts, which l;UIII,.~IJVIIdS to a field strength of aU,UI~ ' ~ Iy 1.2 YUIL~ Ull.
For effective ... T jl i.... ~g, the ' application of first c- - ' Ig radiation 58 and reverse bias field Ecl should be maintained for a period of time sufficient to allow r~ ~Tictrih~ n of ~.batalllidlly all of the charge collected at interface 22. The period of time required for effective ,~
vdries with the strength of the reverse bias field ECI~ the amount of charge collected at interface Z, and the intensity of first .. I~ p radiation 58. As an example, however, when ~ ., layer l2 is formed with an amorphous selenium pl. ~l uc.... l ~ , layer 16 having a thickness of a,uulu~ ly 425 microns and an insulative layer 18 having a thickness of a,u,ul~ y 175 microns, a .,.. l i l ;.. ,: . ,~ voltage Vcl of alJ,ul~ Iy 1500 volts (relative to a readout voltage VR of a,uulw~;lll~.t~,ly 3000 volts) and a first ' radiation 58 having Y~a~ la greater than or equal to a~ .u~ .~ly 600 ~ amd am intensity of a~ y 0. 1 watt/cm2 should provide effective r~licfrih~tinn when applied for a period of time of at least one second, with longer periods oftime a~lua~,llillg ten seconds providing better results.
Fig. 8 is a schematic cross-sectional .~ ,..lalion of radiation detector 12 illustrating an example of charge ~i ~frihlltinn after the r - ~ stage of Fig. 7.
As shown in Fig. 8, the reverse bias field Ec~ transports the collected charge away0 from interface 22, but may result in an interface charge density that contains excess carriers ofthe opposite polarity. With a IJI.vlu.,u..duulive layer 16 ~ WO95135511 2 1 9 1 4 7 ~ r~
formed from amorphous selenium and an image exposure voltage VE having a positive polarity at second conductive layer 20, for example, the reverse bias field ECI transports both free and released electrons forming the latent image charge and read-out charge away from interface 22. However, the reverse bias field ECI alsomay transport more holes toward interface 22 than are necessary for 11. :.,~1;,-l;
resulting in a positive charge density. The arnount of excess 1- . .t~ .. 1;,;, .,~ charge collected at interface 22 may be acceptable for some ~ t ;.~ For more complete .. 1;1;.. : ~ of detector 12, however, the excess charge optionally can be , . . . .
Fig 9 is a schematic cross-sectional ~ m of detector 12 during arlother stage in the c~ L method of the present invention. As shown in Fig. 9, the excess ~ charge at interface 22 j5 ~ , ;bUt ~1 by applying a second c., --~ ,p voltage VQ across first conductive layer 14 and second conductive layer 20, and exposing ~ uc~ ;ve layer 16 to second ~-,.. 1,1;.. ~ - ~ radiation 60 emitted by a second radiation source 62. The application of the second condition ' ~ voltage VQ and second rrmrlitir,n;rl~
radiation 60 cullc~,uu~ to the steps indicated in blocks 50 and 52, respectively, of Fig. 6 The second .. ",1:1;.. ,.~ voltage should have an absolute value less than that of the first r.rnrl:tinnin~ voltage Vcl to establish a second, forward bias electric field EC2 across rl-- IU~ ive layer 16 The forward bias electric field EQ
preferably is reali_ed by simply shorting first and second conductive layers 14, 20 together, such that the second ' ~ voltage VQ ;S ,~Y,U., IY 7ero volts. A second cr~ voltage VQ of ~,U~JII ' ' ~y 7ero volts avoids the ;. .,. of latent image charge at interface 22.
The second, broad spectrum radiation 60 can be reali_ed, for example, by an ' light. The broad spectrum radiation excites 1~ .."1-- (;ve layer 16 to a limited degree suffcient to enable excess n~---t-ztli7in~ charge collected at interface 22 to transport back toward junction layer 24 in the forward bias field EQ. The strength of the forward bias field EQ should be sufficient to transport 5~h~t ~ti ~1ly all of the excess r, . .1, ,.l ;,, . .~ charge away from interface 22, but should be much less than the strength of the reverse bias electric field Ecl to avoid wo ssr35511 2 ~ ~ i 4 7 ~
the transport of oppositely poled charge back to interface 22. Thus, although small, non-zero voltages may be used for second r.nntliti~nin~ voltage Vc2 the use of large voltages, relative to first, " ~ voltage Vcl, generally is Im~ cirslhl~ In particular, the shorting of first and second conductive layers 14, 20 should establish a forward bias field EQ of sufficient strength to transport excess charge from interface 22 over a period of time without causing the collection ofoppositely poled charge at the interface.
The ~ u ~ application of second ... ,... l;l ;....; "p radiation 60 and forward bias field EC2 should be maintained for a period of time sufficient to allow r~tlictrihl~tionofcllh~t~nti~llyalloftheexcessn~-tr~li7inpchargecollectedat interface 22. This period of time varies with the amount of charge collected at interface 22, the strength of the reverse bias field Ec~, and the intensity of second c~ln~itinnin~ radiation 60. If forward bias field EC2 is produced by shorting first and second conductive layers 14, 20, the field strength is a direct function of the strengthofthereversebiasfieldEcl. Foral.l,ul~,~.. l l;vclayerl60f amorphous selenium subjected to a typical image exposure operation, for example,a second c. ~ I;I;u~ voltage Vc~ of al~y~ / zero volts and second, broad spectrum rnn~litioninp radiation 60 having an intensity of .1~l... ly 0.2 watt/cm2 should provide effective !~'~;`1 ' ;1 . ~l i. ..l of the excess l ,. .l . ,.l; ~; "~ charge when applied for a period of time of at least one second, with longer periods oftime alJ~ulu~ g ~ JIUAi~ .l,ly ten seconds providing better results.
Fig. 10 is a schematic cross-sectional I C~ ion of detector 12 afler the ....,.l;l;~...;,.~stageofFig.9. AsshowninFig.lO,theapplicationofsecond .:.. 1;1;.. :"~ voltage Vc2 in ~.omhin~ti~n with the exposure of rhnto~...... ,.l.. l;vc layer 16 to second, broad spectrum radiation 60 results in the ,f~ l.;l.. l;... of substantially all l~ charge collected at interface 22. However, a small number of free charge carriers may remain in rh-llu~ - li vc layer 16. The presence of a small number of free charge carriers may be acceptable for some ;....c but may contribute to u~ld~ ~lc dark current when the image exposure field EE ;S applied for the next image exposure operation. Therefore, the free charge carriers optionally can be eliminated by an additional step.

W095135511 ~ ~ 9 i 4 71 r~ _3~61 Specifically, the free eharge carriers can be readily eliminated by placing detector 12 in a dark C..V;IUIIIII~ for a period of time, with first conductive layer 14 and second conductive layer 20 shorted together, to dark-adapt ~ ; ve laycr 16. This dark-adaptation pcriod CUllC.~,UUlld:~ to the step indicated in block 54 of Fig. 6. The dark ellYilUlUll~,.lt allows some of the free charge carriers to transport to reduce residual fields within 1~ ive layer 16, but also changes the conductive state of the ~ t. ~ l - live material to capture some of the free charge carriers in distributed traps. The transported and trapped carriers reduce the number of carriers able to contribute to dark current at the start of the next image exposure operation. The ~ u~n~ ;ve layer 16 can be effectively dark-adapted by placing it in a dark ,ll~ilUIIIII~,III for an indefinite amount of time.
However, a minimurn dark adaptation time of at least two minutes should suffficiently reduce dark current, thereby preserving image contrast.
rhe following examples are provided to illustrate the rnntlitinninp method of the present invention, and, in particular, the cr.,~ ,l.. ,." of the rnntiitjnning method of the present invention for reusable radiation detectors having either "~c-,liryillt;" or "blocking" junction layers.
FXAMPLF. I
A detector 12 having a structure ~ y as described in Fig. I was used in this example. The first conductive layer 14 comprised an a'tuminum platesubstratehavinga width of a~,ul~ '~/ 12.7 ~ alengthof ~I,U,UI. '~12.7~ .1... ~..~,andathicknessof,~,u~ /0.2~- .t;.. ~
A rectifying junction layer 24 was formed over the aluminum plate substrate of first conductive layer 14 by plasma oxidation. The photo,,ù...lu,,live layer 16,formed over junction layer 24, comprised a vacuum deposited layer of amorphous selenium having a thickness of dhulo~d~ .t~,ly 425 microns. The insulative layer18 was a parylene film, having a thickness of l.UUlW~illl~.'~,ly 175 microns, bonded to the surface of 1~ ; ve layer 16 opposite first conductive layer 14 by an ûptical adhesive having a thickness of ~"u-~ '~/ 10 microns. A film of cnntl~mtir~, transparent indium tin oxide, having a thickness of ~,u~u~ ~ ~J 0.9 W095/35511 2 ~ ~ 1 47 1 P
microns, was deposited over the surface of insulative layer 18 opposite ,h- .~ l;VC layer 16 to form second conductive layer 20.
An image exposure operation was performed by applying an image exposure voltage VE of ~ / 7000 volts~ and then exposing . .".1 I;vc layer 16 to X-ray imaging radiation having a ~a~ "5~1 in the range of 10~ to 10 ~ ~ meters, via second conductive layer 20 and insulative layer 18. After the resulting first image was read-out with a readout voltage VR of ~Jplu~d~ ,ly 2000 volts and a scanning beam having a ~a~ of "~1"' Iy 488 , detector 12 was r.nnAitinn.-~ according to the knovvn charge injection method by shorting ffrst conductive layer 14 amd second conductive layer 20 together for a period of a~"Jl. 'y twenty minutes after an initial ten-second exposure to room light. A second image then was formed with detector 12, and was found to have nearly the same image contrast as the first image (104 pixel values versus 121 pixel values). The image contrast of the second image ,' ' thatjunction layer 24 exhibited reverse-bias injecting behavior.
The detector 12 was then ~ I in accorddnce with the system amd method of the present invention. A first, reverse bias c~ ., ,.1;1;, . - ~e voltage Vcl of ~pp., 'y -2000 volts was applied to the second conductive layer 20 relative to the first conductive layer 14. While the first cc ' ~ voltage was m~int~in.o~l, the l)~ ; ve layer 16 was exposed to first radiation for a period of a~ ten seconds. The first ' nn;n~
radiation was generated by filtering am; 1 - ..l light source using a Wrattan IA filter to pass ~wa~,h,~ in a r mge of ~ 'y 550 to 900 ~ -- .. t The resulting radiation was estimated to have an intensity of ~ 0.1 watt/cm2. The first ''''nnin~ radiation ~d.-,llld~ could be generated by operating an; - l - ,l light at low voltage to produce ~va~ falling in the red spectral range. The first and second conductive layers 14, 20 were then shorted together, and ~ n~ iv~ layer 16 was exposed to second, broad spectrum l~.on~iitinnin~ radiation provided by an; ~ light for a period of ., '~ ten seconds. The broad spectrum ~ ;,)";"~ radiation was WO 95/35511 P~ 'C~61 ;~1 91 471 estimated to have an intensity of a~u~ull ly 0.2 watVcm2. Finally, detector 12 was placed in a dark ~IIV;IUIUII.~It, with first and second conductive layers 14, 20 shorted together, for a period of au,ull '~/ twenty minutes.
A third image was then formed with detector 12, and was observed to have S an image contrast essentially the same as that of the first image (119 pixel values versus 121 pixel values). Thus, the, ' ~ performed in accordance with the system and method of the present invention provided effective ~ .. 1; 1 ;~ . , .~ for a detector 12 having a "l~..Liryill~S" junction layer 24 with reverse bias charge injection properties.
F.XAMPI F 2 A second detector 12 having a structure s~lhct~tiAlly identical to that employed in EXAMPLE I was used, except that junction layer 24 was formed by plasma ~ ;"-l;.J - of the aluminum first conductive layer 14 to a thickness of au~ 50 Angstrom to realize a "blocking" junction layer. The "blocking" junction layer 24 was designed to 5~het~ tiolly electrically block charge flow in both forward and reverse bias directions. Thus, blocking junction layer 24 was designed to ellhrts~ltiAlly electrically block charge flow from first conductive layer 14 to ~,1..,~"...1,,, ~;v~ layer 16, and to sllhet~lti~lly electrically block charge flow from the rhr~toc.~ ;v~ layer to the first conductive layer.
A first image was formed with detector 12 and read out via the image exposure and image read-out operations described in EXAMPLE 1. The detector 12 was then ~ .. d according to the known charge injection method by shorting first conductive layer 14 and second conductive layer 20 together for aperiod of a~uu~l ~y twenty minutes after an initial ten-second exposure to room light. A second image then was formed with detector 12, and was found to have s;~l.ir~.,au.lly lower image contrast than that of the first image (125 pixel values versus 241 pixel values). The image contrast of the second image 1 ' that junction layer 24 exhibited very limited reverse-bias injecting behavior, and therefore could be .1. ~ t ;~d as a "blocking" junction layer.
The detector 12 was then ro~ cl in accordance with the system and method of the present invention. As in the case of Example I, a first, reverse bias WO951355~ 91 4 71 r.~ c5~61 ~
. voltage Vc~ of ~ 2000 volts was applied to the second conductive layer 20 relative to the first conductive layer 14. While the first ~ voltage was m~ 7inP~ u ~ ive layer 16 was exposed to first ' nnin~ radiation for a period of a~", 'y ten seconds. The first s f.. ~;l;.. ~radiationagainincluded ~va~ lg~ inarangeof a,u~ y550 to 900 . . ~ , and had an estimated intensity of ~ VI ~ ' ' Iy 0. 1 watt/cm2.
The first and second conductive layers 14, 20 were then shorted together, and vtvC.~ ;ve layer 16 was exposed to second, broad spectrum ~
radiation, having an estimated intensity of ~.,u~., u~dlll..t~,ly 0.2 watt/cm2 for a period of ~ , ten seconds. Finally, detector 12 was placed in a dark V;IU~ with first and second conductive layers 14, 20 shorted together, for a pcriod of ~,U,UI~ y twenty minutes.
A third image was then formed with detector 12, and was observed to have an image contrast essentially the same as that of the first image (215 pixel values 15 versus 241 pixel values). Thus, the ~.~.. 1~ ;.. :.. ~ perforrned in accordance with the system and method of the present invention also provided effective ~ litifmin~
for a detector 12 having a "blocking" junction layer 24.
Having described the exemplary t ...ho.l;,.,...1~ of the invention, additional advantages and n~oAifir Iti/mc will readily occur to those skilled in the art from c~ - of the .cre~ifir~tinn and practice of the invention disclosed herein.
Therefore, the ~rPrifi~ tinn and examples should be considered exemplary only, with the true scope and spirit of the invention being indicated by the followingclaims.

Claims (11)

What is claimed is:

1. A method for conditioning a radiation detector having a first conductive layer, a photoconductive layer disposed adjacent the first conductivelayer, an insulative layer disposed adjacent the photoconductive layer opposite the first conductive layer, and a second conductive layer disposed adjacent the insulative layer opposite the photoconductive layer, the radiation detector forming a latent image in response to both application of an image exposure voltage across the first conductive layer and the second conductive layer and exposure to imaging radiation, and the latent image being read out during application of an image readout voltage across the first conductive layer and the second conductive layer and exposure of the detector to readout radiation, the conditioning method comprising the steps of:
applying a first conditioning voltage across the first conductive layer and the second conductive layer, the first conditioning voltage having a polarity opposite to a polarity of both the image exposure voltage and the image readout voltage, wherein the first conditioning voltage establishes an electric field across the photoconductive layer;
exposing the photoconductive layer to first conditioning radiation having one or more wavelengths selected to penetrate at least a portion of the photoconductive layer, thereby releasing charge carriers trapped within the photoconductive layer to transport within the electric field;
applying a second conditioning voltage across the first conductive layer and the second conductive layer, an absolute value of the second conditioning voltage being less than the first conditioning voltage, wherein the second conditioning voltage establishes a second electric field across the photoconductive layer in a direction opposite to the electric field; and exposing the photoconductive layer to second conditioning radiation, the second conditioning radiation being broad-spectrum radiation.

2. A method for conditioning a radiation detector having a first conductive layer, a photoconductive layer disposed adjacent the first conductivelayer, an insulative layer disposed adjacent the photoconductive layer opposite the first conductive layer, and a second conductive layer disposed adjacent the insulative layer opposite the photoconductive layer, the radiation detector forming a latent image in response to both application of an image exposure voltage across the first conductive layer and the second conductive layer and exposure to imaging radiation, and the latent image being read out during application of an image readout voltage across the first conductive layer and the second conductive layer and exposure of the detector to readout radiation, the conditioning method comprising the steps of:
applying a first conditioning voltage across the first conductive layer and the second conductive layer, the first conditioning voltage having a polarity opposite to a polarity of both the image exposure voltage and the image readout voltage, wherein the first conditioning voltage establishes a first electric field across the photoconductive layer; and exposing the photoconductive layer to first conditioning radiation having one or more wavelengths selected to penetrate at least a portion of the photoconductive layer, thereby releasing charge carriers trapped within the photoconductive layer to transport within the first electric field, wherein the photoconductive layer comprises a photoconductive material having an absorption edge, and the first conditioning radiation consists essentially of radiation having wavelengths greater than or equal to a wavelength corresponding to the absorption edge.

3. The conditioning method of claim 2, further comprising, after the step of exposing the photoconductive layer to the first conditioning radiation, the steps of:
applying a second conditioning voltage across the first conductive layer and the second conductive layer, an absolute value of the second conditioning voltage being less than the first conditioning voltage, wherein the second conditioning voltage establishes a second electric field across the photoconductive layer in a direction opposite to the first electric field; and exposing the photoconductive layer to second conditioning radiation, the second conditioning radiation including radiation having wavelengths less than awavelength corresponding to the absorption edge.

4. The conditioning method of either of claims 1 or 3, further comprising, after the step of exposing the photoconductive layer to the second conditioning radiation, the step of dark-adapting the photoconductive layer.

5. The conditioning method of any of claims 1 to 3, wherein an absolute value of the first conditioning voltage is a factor of approximately 0.2 to 1.0 times an absolute value of the image readout voltage.

6. The conditioning method of claim 1, wherein the photoconductive layer comprises a photoconductive material having an absorption edge, and the first conditioning radiation includes one or more wavelengths greater than or equal to a wavelength corresponding to the absorption edge.

7. The conditioning method of any of claims 1 to 3, wherein the photoconductive layer comprises amorphous selenium, and the first conditioning radiation includes one or more wavelengths greater than or equal to approximately 500 nanometers.

8. The conditioning method of any of claims 1 to 3, wherein the photoconductive layer comprises amorphous selenium, and the first conditioning radiation includes one or more wavelengths greater than or equal to approximately 600 nanometers.

9. The conditioning method of any of claims 1 to 3, wherein the radiation detector includes a junction layer formed between the first conductivelayer and the photoconductive layer, the junction layer being substantially electrically blocking to charge flow from the first conductive layer to the photoconductive layer, and the junction layer being substantially electrically blocking to charge flow from the photoconductive layer to the first conductive layer.

10. The conditioning method of either of claims 1 or 3, wherein the step of applying the second conditioning voltage includes shorting the first and second conductive layers together, the second conditioning voltage thereby being approximately 0 volts.

11. The conditioning method of any of claims 1 to 3, wherein the radiation detector is an X-ray detector, the imaging radiation being X-ray imaging radiation.