US3415990A - Invisible light sensor tube and faceplate material - Google Patents

Description

Dec. 10, 1968 Qigut R. T. WATSON INVISIBLE LIGHT sENsoR TUBE AND FACEPLATE MATERIAL Filed March 21, 1966 l Reefer 7.' wArSo/v Y mmm?.

ATTQRNEY n Patented Dec. 10, 1968 3,415,990 INVISIBLE LIGHT SENSOR TUBE AND FACEPLATE MATERIAL Robert T. Watson, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, a corporation of Delaware Filed Mar. 21, 1966, Ser. No. 535,842 11 Claims. (Cl. Z50-71.5)

ABSTRACT F THE DISCLOSURE Invisible light radiation sensitivity is increased by a tube faceplate having an internal layer of material responsive to two different light sources. An ultraviolet pump source raises and stores electrons at an intermediate energy level and an infrared source stimulates the electrons into a conduction level to emit light. A photo-cathode generates photoelectrons Iwhich pass through a control mesh and electron multiplier to the tube output electrode.

This invention relates to an improved light radiation sensing device and particularly to a novel image tube and screen arrangement which provides greater sensitivity to an extended range of invisible radiations.

The sensitivity of known photoemitters has generally been limited to the ultraviolet, visible and near infrared radiations because the energy of an absorbed infrared quantum is so small that an excited electron cannot overcome the surface barrier of the material. Several methods have been proposed to provide an excited electron with additional energy to allow the escape into the vacuum of the associated tube structure. One such device applies a high field in the phosphor containing solid state bul-k material perpendicular to the surface, which accelerates the excited electrons. However, these devices have not been successful to extend the threshold beyond the region of one micron in wavelength.

It is therefore the primary object of the present invention to provide an invisible light radiation sensing device havin-g an additional energy source and a novel structure which facilitates the release of electrons from a photoemitter and permits the extension of the range of radiation sensitivity.

This is accomplished by an external pump light source and a novel tube faceplate structure Iwherein an internal photocathode is in optical contact with an intermediate solid transducer layer having particular phosphor activator material therein. The electrons in the phosphor are energized by the external source of ultraviolet radiations and stored in a level between the conduction and valence band of the material. At the same time, infrared radiation is injected into the solid at an angle so that it is reflected back and forth and completely absorbed. This infrared energy is sufficient to lift the trapped electrons into the conduction band during the absorption process, from which they return to the |valence band, recombining and emitting visible light or luminescence. The photocathode absorbs this stimulated luminescence, generating excited photoelectrons |which escape through the surface barrier. To prevent the emission of undesired photoelectrons caused by the added light source and other thermally generated electrons, a control mesh at a slightly negative potential is placed adjacent the photocathode and is pulsed negatively in synchronism with the ultraviolet light source so that only electrons stimulated by the infrared energy are procesed. The details of the invention will be more fully understood and other objects and advantages will become apparent in the following description and accompanying drawings wherein:

FIGUREI 1 shows a schematic view of a tube embodyin-g the novel structure,

FIGURE 2 is a variation of the invention employing concentric elements, and

FIGURE 3 is a further variation using a plurality of like elements in a common tube envelope.

As shown in FIG. 1, a vacuum tube envelope 10 includes a planar transparent faceplate 12 at one end and an external obliquely positioned optical coupler 14 mounted thereon. The tube is positioned so that the coupler is at a particular angle 'with respect to an external source 16 of infrared or other invisible radiations. A solid transducer layer 18, containing a phosphor with at least two impurities or activators dispersed therein, is disposed on vthe inside of the faceplate and a photocathode layer 20 forms the interior surface on layer 18 at the faceplate end of the tube. Since the absorption coefficient for infrared radiation is small, for most efficient operation, the coupler 14 directs the infrared into layer 18 so that it will be reflected back and forth by total reflection, as shown by the dashed line 19 until it is completely absorbed ywithin the solid. The reflective indices of the preferable powdered phosphor particles and solid layer material should be about equal to avoid scattering of the infrared radiation and should be high for proper trapping of the luminescence. Most of the infrared stimulated luminescence is absorbed in the photocathode. The oblique angle increases the yield of photoelectrons since a very thin photocathode layer can be applied, resulting in absorption very near the surface where the probability of electron escape is highest.

The external ultraviolet or other suitable light source 22 provides energy during. the pumping cycle to cause storage of energized electrons in a level between the conduction and valence bands. A discussion of the theory of operation of this physical phenomena may be found in the text entitled Luminescence in Crystals, by D. Curie, 1963 edition and in an article by W. E. Spicer and F. Wooten, in the Proceedings of the IEEE, August 1963, pp. 1119-1126. The added infrared radiation is absorbed and lifts the trapped electrons into the conduction band, after which they return to the valence band to recombine 'while providing light emission or luminescence through the activator material in the solid. The light is reabsorbed in the photocathode generating further excited electrons which may escape into the vacuum.

The pumping light will also produce undesired photoelectrons either by direct absorption in the photocathode or by generation of luminescence 'with the same characteristics as the infrared stimulated emission. It is therefore necessary to avoid emission or processing of electrons generated during the pumping operation. This is accomplished by use of a control mesh 24 in front of the cathode 20 which is pulsed negatively during t-he time undesirable electrons are leaving the photocathode. The electrons are driven back into the photocathode during this time. Since the mesh can be pulsed with frequencies up to several hundred megacycles, a lwide variety of pumping times and cycles can be used. The mesh also i-mproves the signal to noise ratio if its potential during the infrared radiation is adjusted to such a small negative value with respect to the photocathode that photoelectrons can pass it while thermally emitted electrons are rejected. The pulses may be synchronized by a common pulse source 26 which applies pulses 28, 30 respectively, to the ultraviolet source 22 and control mesh 24.

The traps in the solid are filled periodically by irradiating the solid :with ultraviolet light. During this flashing, the mesh, which is normally at a small fractional negative potential with respect to the reference, is pulsed to about 3 volts negative. The solid transducer and photocathode can be held at a low temperature by a photoelectronic cooler (not shown) to prevent any thermal transitions of the trapped charges. The absorption of infrared leads to the recombination process which produces the visible luminescence. The majority of the luminescence quanta is absorbed by the photocathode because of the close proximity and the confinement of most of the luminescent light to the layer 18 which has a high refractive index. The photocathode is adjusted in thickness and material to the wavelength of the luminescence and can therefore convert a large part of the absorbed quanta into free photoelectrons. Typical thickness of the photocathode may be in the order of 200-500 A. with emission decreasing with increasing thickness.

The electrons pass the mesh 24 and enter the multiplier 32 Iwhich includes a plurality of dynode stages at progressively higher voltages as shown, and the output signal is taken from an anode 34. The multiplier signal can be further amplified externally by a conventional amplifier or converted back to visible light by a luminescent display screen 36, shown in FIG. 3. The useful electrons passing through the control grid 24 can be multiplied in a conventional electron multiplier to such a level that the noise of the following amplifier can be neglected. Typical voltages supplied by a direct voltage source 38 may be zero or ground potential on the photocathode, 200-800 volts on the stepped multiplier and 900 volts on the anode.

FIG. 2 shows the same device in a concentric arrange- -ment about a common tubular axis, and as shown in FIG. 3, several like transducer layers can be combined in multiple parallel structure to obtain imaging properties with information on a large number of different elements.

The choice of infrared sensitive phosphors is governed by the photocathode wavelength sensitivity. Known infrared phosphors are generally compounds of the sulfdes and selenides of Periodic Group II, including magnesium, calcium, strontium, zinc, and cadmium, containing two activators. The primary activator emission is decreased to an almost negligible amount, with the secondary activator present. When exposed to band gap excitation energy such as ultraviolet light, the secondary activator causes the ultraviolet energy to be stored in the phosphor and this energy is released when a specific infrared wavelength band is impinged on it, giving rise to the characteristic primary activator emission. The excitation process is relatively long, in the order of minutes, depending upon the intensity and wavelength of the ultraviolet source, hence a warm up time is needed. A pulse operation is therefore preferable in which the pulses can be several hundred milliseconds long. The response time to infrared radiation is usually very short, in the order of milliseconds, and one could have for example an excitation time of 100 milliseconds and a sensing time of 10 milliseconds or some other suitable time cycle.

The most sensitive phosphors to low intensity infrared are the alkaline earth sulfides activated with Europium or Cerium and Samarium. Europium and Cerium are the primary activators and Samarium is the secondary activator. Various known ymixtures of infrared senstive phosphors may be employed. One known phosphor includes zinc sulfide, lead and copper which exhibits infrared stimulation at room temperature in the 1.25-1.60 micron region with an emission peak at 5000 A. Another zinc sulfide activated with manganese and copper exhibits an emission 'band peaking at 6000 A. Both phosphors are suitable for use with standard photocathodes. One difficulty encountered with these materials is the long phosphorescence which is produced by the copper activator. A preferable choice of phosphors with respect to a short phosphorescence is the standard P11 phosphor, consisting of strontium sulfide, Cerium, samarium and lithium iluoride the latter of which is also excited by ultraviolet CTI energy. The phosphor is stimulated by 1.02 microns of infrared radiation and has an emission peak at 4850 A. This material is also subject to decomposition by moisture and does not have the longer infrared wavelength sensitivity. The material may be embedded in a clear polymer. The sensitivity to infrared is much faster in the SrS based phosphor, in the order of l millisecond, whereas in ZnStCutPb the response time and stimulated emission is the order of seconds. SrS also has a refractive index of approximately 2.1.

Some conventional infrared stimulable phosphors have relatively deep traps of varying energy, hence the use of a cooled phosphor exhibiting a single optical glow peak is preferred. A number of conventional phosphors may be employed which exhibit single strong thermal glow peaks at low temperature. One such phosphor is Zn2SiO4 which exhibits a green emission `at 260 K. By cooling to -40 C. and exciting or storing with 2537 A. ultraviolet energy, the traps may be filled which are subsequently emptied by infrared. The response time will be similar to the normal luminescence decay of Zn2SiO4Mn, which has a refractive index of 1.71 embedded in polystyrene of refractive index 1.59.

Organic compounds may also be useful as well as organometallic compounds which actually dissolve in the matrix material such as lucite. A recently devolped material is a rigid organic material of tetramethylparaphenylenediamine (TMPD) in 3-methylpentane which exhibits stimulated emission in the near ultraviolet and visible range of 1.0-2.5 micron radiation. The decay of phosphorescence is about 2 seconds. The device operates at a low temperature of 77 K. in a glass coated envelope with gold or aluminum to refiect the infrared. Other useful materials are a low temperature phosphor such as a rare earth chelate dissolved in lucite or a solid solution in dimethylsulfoxide or dimethylforrnamide in a glass envelope with an infrared refiecting coating. ZnMgS and Mg may also be employed as alternative phosphors and suitable liquids can be used in place of a solid layer. Alkaline earth tungstate and molybdate laser materials doped with rare earths have shown useful energy transfer properties. For example, calcium tungstate or molybdate containing two activators, europium and terbium, exhibits the phenomenon of energy transfer from Eu to Tb which may be stimulated by infrared radiation.

While several embodiments have been illustrated, it is apparent that the invention is not limited to the particular forms or uses shown and that many other variations .may be made in the particular design and configuration lwithout departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A device for sensing invisible light radiations comprising,

a vacuum tube envelope having a transparent faceplate at one end,

a light transmissive layer positioned within the tube on the inner surface of said faceplate, said layer having light emitting material dispersed therein responsive to light radiations of two different frequencies, said material being capable of storing electrons at an energy level between the conduction and valence energy levels,

a first light source projecting light radiations of a particular frequency range onto said faceplate and layer,

a second light source projecting light radiations of a higher frequency onto said faceplate and layer, means directing light radiation of one of said frequencies onto said faceplate and layer to cause refiection and absorption of said radiation of said one frequency within said layer, the electrons in said light emitting material being raised to and stored at said energy level between the conduction and yvalence energy levels by radiation of said higher frequency and being stimulated into the conduction level to cause light emission by radiation of said one frequency,

a photoemissive layer positioned on said light transmissive layer for emitting electrons in response to impingement of said light emission thereon,

a control :mesh spaced from said photoe-missive layer to control the ow of electrons therefrom,

electron multiplier means spaced from said control mesh,

output means disposed at the ot-her end of said tube,

and

direct voltage supply means providing progressively increasing voltage between said photocathode, electron multiplier and output means.

2. The device of claim 1 including means applying synchronized pulses between said second source of light radiations and said control mesh to periodically turn on said second light source and to prevent passage of electrons stimulated thereby during the occurrence of said pulses.

3. The device of claim 2 wherein said light directing means, transmissive layer, photocathode, control mesh and electron multiplier are disposed successively in a concentric arrangement about a common tubular axis.

-4. The device of claim 2 including a plurality of means directing light radiation in substantially parallel paths, a plurality of light transmissive layers and photocathodes disposed in respective paths, and a transversely positioned control mesh adjacent said photocathodes.

5. The device of claim 2 Iwherein said light emitting material and light transmissive layer have similar relatively high refractive indices.

6. The device of claim 2 wherein said light radiation of said one frequency is in the infrared frequency range and said light radiation of said higher frequency is in the ultraviolet frequency range.

7. The device of claim 6 wherein said light directing means directs light at a predetermined oblique angle into said light transmissive layer to cause said reflection and absorption.

8. The device of claim 7 wherein Said light emitting material includes phosphor particles having activator materials therein responsive to said infrared and ultraviolet radiations.

9. The device of claim 8 wherein said output means is an anode.

10. The device of claim 8 wherein said output means is a luminescent display screen.

11. A device for sensing invisible light radiation comprising,

a light transmissive layer having light emitting material dispersed therein responsive to radiations of two different frequencies, said material being capable of storing electrons at an energy level between the conduction and valence energy levels,

a first source projecting light radiations of a particular frequency range onto said layer,

a second source projecting light radiations of a higher frequency onto said layer,

means directing the light radiation of one of said frequencies onto said layer to cause reflection and absorption of said radiation of said one frequency lwithin said layer, the electrons in said light emitting material being raised to and stored at said energy level 'between the conduction and valence energy levels by radiation of the higher frequency and being stimulated into the conduction level to cause light emission by radiation of said one frequency,

means supporting and enclosing said layer with a vacuum,

means responsive to said light emission for emitting electrons lupon impingement of said light emission thereon,

means to control the flow of said electrons, and

output means providing an output in accordance with said electron How.

References Cited UNITED STATES PATENTS 3,062,962 11/1962 McBee 250--213 3,070,698 12/1962 Bloembereen.

RALPH G. NILSON, Primary Examiner. M. J. FROME, Assistant Examiner.

U.S. C1. X.R. 'Z50-83.31, 77, 213; 313-103

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