US3275827A

US3275827A - Detector utilizing a scintillator and photoconductive material

Info

Publication number: US3275827A
Application number: US330888A
Authority: US
Inventors: Edward L Lind
Original assignee: Harshaw Chemical Co
Current assignee: Harshaw Chemical Co
Priority date: 1963-12-16
Filing date: 1963-12-16
Publication date: 1966-09-27
Anticipated expiration: 1983-09-27
Also published as: NL6414579A

Description

Sept. 27, 1966 I E, L. U'ND 3,275,827

DETECTOR UTILIZING A SCINTILLATOR AND PHOTOCONDUCTIVE MATERIAL Filed Dec. 16, 1963 2 Sheets-Sheet 1 I" Guam-TIGHT }L|eHT-TleHT 10% :CONTMNER 4 CONTAINER V I \\:SCINTILLATOR "1 rSCINTILLATOR :jpHoTocoN0umoR j PHOTQQONDUCTOR ISiL I7 I Bi- !7 2O |4-i 3 8 E5 I5 v FIGJA FIG.1B

22 v F1G.3

FIG.2

sans

ACdSe 1 2 QOVOLTS 2. 4O BIAS 2 U=RESPONSE 2 TIME 1N mo 200 300 400 500 SECONDS RADlATION LEVEL m ROENTGENS/HR W FIGA FIG-5 EDWARD LLIND, INVENTOR I Sept. 27, 1966 I E. L. LlND 3,275,827

DETECTOR UTILIZING A SCIN'I'ILLATOR AND PHOTOCONDUCTIVE MATERIAL Filed Deck 16, 1963 LIGHT EMISSION (ARe/TRA/ey uN/rs,/VoRMAL/zE0 m /00) DHOTO-CURR E NT (ARe/rRARY u/v/rs; NORMAL/ZED To /00) a 8 2 Sheets-Sheet 2 WAVELENGTH IN MlLLlMlCRONS Q) PHOETOCONDUCTWITY PURE CdS C2) TYPlCAL PHOTOCONDUCTIVE RESPONSE OF Cu ACTWATED CdS SlNTERED LAYER EMlSSlON csI=T1 C4) PHOTOCONDUCTIVITY PURE CdSe )1 EMiSSlON NGI=T1 EDWARD Z. LIND. INVENTOR.

United States Patent DETECTOR UTILIZING A SCINTILLATOR AND PHOTOCONDUCTIVE MATERIAL Edward L. Lind, Chagrin Falls, Ohio, assignor to The Harshaw Chemical Company, Cleveland, Ohio, a corporation of Ohio Filed Dec. 16, 1963, Ser. No. 330,888

5 Claims. (Cl. 250-715) This invention relates to radiation detectors and especially to detectors making use of the combination of a scintillator and a photoconductive material of the class consisting of cadmium sulfide and cadmium selenide.

By scintillator is meant a body of material capable of converting radiation such as for example gamma rays into scintillations or flashes of visible light. By photoconductive material is meant material which is more conductive in the presence of light than in darkness. Cadmium sulfide, cadmium selenide and single solid solutions Cd(S, Se) are known to be photoconductive or photosensitive, for example, when produced by sublimation and recrystallization of substantially pure material, however, no one has to my knowledge heretofore made use of whiskers of cadmium sulfide or cadmium selenide as -a part of a detector of the type which is based upon photoconductive material and illumination of the photoconductive material by a scintillator.

Examples of methods of making whiskers or needles which yield photosensitive crystals may be found in the paper by Bube and Barton, RCA Review, December 1959, vol. XX, No. 4, pages 591 and 593 (where the nitrogen or argon or other inert gas is preferred but not essential) and in a paper by Sears in Acta Metallurgica, volume 3, July 1955, page 367. It can thus be seen that Whiskers or needles of a photosensitive character are known to the art and that they may be produced by following the teachings of the art. Whiskers which includes needles, platelets, and production thereof, are described in the above cited art. It is preferred to use whiskers of an approximately cylindrical shape but other shapes can be used as, for example, crystals in the shape of a ribbon or platelet (two dimensional growth) described in Bube et al. While pure material is preferable for making the whiskers, I may utilize a slightly impure material if it is substantially free of oxygen. Small amounts of chlorine and/or iodine can be present, for example, as much as 1.0 part per million of chlorine or iodine. Oxygen should be below 1.0 part per million, preferably below 0.1 part per million.

Accordingly, it is an object of the invention to provide a detector comprising a scintillator transparent to its own em'anations, and a body of photoconductive material of the class consisting of cadmium sulfide and cadmium selenide, the scintillator being positioned to illuminate the photoconductive body and said photoconductive body being connected in series with a meter and an applied voltage and said photoconductive body being in the form of whiskers. The term whiskers includes needles, ribbons and platelets. A further object is to provide a module comprising a pair of conductors, a plurality of circuits connecting said conductors at a series of points and each said circuit made up of a photoconductor of the class consisting of CdS and CdSe and sub-conductors connecting said circuits through said photoconductors to said first mentioned conductors, said module also containing a scintillator adjacent said photoconductor adapted to illuminate such photoconductor responsive to impingement of radiation on said scintillator. The Whiskers are preferably about one-fourth to about one-half inch length.

With these and other objects in view which will become apparent as the description proceeds, the invention consists in the novel features herein described and illustrated in the accompanying drawings, wherein the same reference characters are used to indicate the same parts in the various figures.

FIG. 1A is a diagram showing the juxtaposition of the scintillator and the photoconductor together with a circuit of which the photoconductor is a part.

FIG. 1B shows a similar diagram wherein the circuit is somewhat different, making use of a resistor with a meter in parallel.

FIG. 2 shows a more detailed diagram wherein a plurality of whiskers are connected in parallel to form what may be called a grid.

FIG. 3 shows another type of grid wherein a whisker of larger size is used and a plurality of parallel circuits are provided through the same Whisker.

FIG. 4 is a graphic representation showing properties of CdS and CdSe whiskers plotting the current in microamperes as ordinates against the radiation level in roentgens per hour. These values may be obtained most easily from a device according to FIG. 1A or 1B.

FIG. 5 is a portion of a device functioning essentially like FIG. 1A or FIG. 1B but adapted for use where a small diameter probe or similar device is desired.

FIG. 6 is a diagram showing relative photocurrent in the photoconductor in arbitrary units normalized to plotted as ordinates against the wave length of the light in millimicrons. A second scale of ordinates shows the emission of light in the scintillator in arbitrary units plotted against wave length, normalized to 100.

The basic concept of a photoconductor type of detector can be seen in FIGS. 1A and 1B where the photoconductor material is not specified. This concept by itself does not come Within the scope of the present invention, but the present invention is realized in connection with such a device. By reference to FIGS. 1A and 1B it will be seen that a scintillator 10 has been juxtaposed with a photoconductor 12. The resulting unit may be housed in a light-tight housing 11.

Leads

13 and 17 may be taken ofi from the photoconductor and connected in series with the source of current 14 and meter 15, through conductor 16. The source can be at a voltage to afford a suitable bias. See FIG. 4. In FIG. 1B there is shown a slight variation from FIG. 1A. A resistor 18 is connected in series through the

conductors

19 and 20, and the meter 15 is connected across the resistance 18. Again, a suitable bias is used. See FIG. 4.

From FIG. 6 it can be seen that comparison can be made as between 1 plus 3 which is preferred and 1 plus 5 to ascertain which combination is best. These curves show in what part of the spectrum the emission from the scintillator matches the wave length response. If photoconductivity and emission occupy nearly the same area under the curves in FIG. 6, then that scintillator .and photoconductor are best matched. If the curves for 1 and 3 do not overlap, no light is available to change the resistance of the photoconductor. Consider curves 1 and 3. Here the overlap is less than in 2 and 3 but 2 is too slow in response and cannot, therefore, compare favorably with 3. Other things being equal, one should choose the pair having most overlap. Other combinations can be compared, for determination of preferable uses, 1, 2, or 4 with 3 or 5.

An important feature of the inventionis the combination of the scintillator body, such as for example NaIzTl with the photoconductive body in such manner that light generated in the scintillator falls on the photoconductor, th e photoconductor being connected in circuit with a source of current and a meter and at least the portion of the photoconductive body through which the current passes being monocrystalline, the photoconductive body being chosen from the class consisting of CdS and CdSe though not necessarily,

needles or monocrystalline plates of photosensitive material.

In place as CsI:Tl,CaI, CaIzTl, CaIzEu and LiIzEu. Other scintillators may be used. Naturally the most eflicient in light production will ordinarily be chosen. The scintilator and the photoconductor will normally be housed in a single, light-tight housing 11. In FIG. 2 will be seen diagrammatically represented an embodiment of the present invention. indicates generally the whisker type of photoconductor. It also represents specifically a cylindrical glass support or substrate upon which have been applied two spaced, preferably parallel, strips 21 and 22. These may be spaced, for example, about 1 millimeter to about 5 millimeters apart and preferably are composed of indium or silver. Lying across said spaced indium or silver strips are whiskers 23. A conductor 13 leads-off from one of said strips While the conductor 17 leads oif from the other. The indium or silver or other suitable metal strips may be applied to the glass substrate 12 in various ways such .as by evaporatiommoving a molten body of indium or silver, for example, by means of a ruling pen, in spaced,,preferably parallel or near parallel, lines across the substrate or by melting lines of metal paste. The whiskers (needle shaped or flat platelets) are then laid across the indium or silver lines or strips preferably, alparallel to each other. The assembly is thenheated whereby to melt the indium or silver or other metal or alloy strips or lines and make electrical connection between the whiskers and said lines. The substrate need not be cylindrical but may be any shape, for example, a square or oblong plate.

Not only indium strips can be used to connect the whiskers but other suitably conductive metals may be The following thallium,

used if they have a suitable melting point. may be noted: Silver (paste), gallium, Hg-Ga-Tl alloy.

Fabrication of the photoconductive array must be done carefully in order to achieve optimum results. One method which gives good results is to mount a large number of the whiskers according .to the invention between two indium strips on a glass plate such as shown in FIG. 2. When the indium is melted withthe whiskers resting on top, ohmic contact is made to the whiskers and electrical. connection is made by soldering wires. to the indium strips. a

FIG. -3 shows a variant form of the photoconductor which is well adapted to the use of larger whiskers, where- .as the device of FIG. 2. is,more adapted to the use of smaller whiskers. In FIG. 3 at 24, there is shown diagrammatically one of the metallic strips generally similar to the 'strip shown at 21 of FIG. 2. The needles or whiskers, of course, are connected in parallel, but in this instance at several points, as shown at 25,-the leads 25,'in turn, being connected to the conductors 26 and 27., which are equivalent to

conductors

13 and 17 of FIG. 2. However, the devices of FIGS.'2 and 3 can be used in connection with whiskers in the range of 1 to 200 microns diameter. Preferably, connections such as shown in FIG.

2 are used and preferably the diameter of the whisker is.

from 1 to 30 microns. Preferably, the length of the whisker is from one-fourth of an inch up.

Previous investigators have used large crystals of sintered material which shows photosensitivity. While these materials are of some use, the response and decay times at low light levels produced at low radiation levels, are large and severely limit the usefulness of the device. 0n the other hand, whiskers of cadmium'sulfide and cadmium selenide exhibit much lower response and decay time at low light levels. I

Whiskers according to the invention can be produced by crystal growth about a single crystal nucleation center in an atmosphere of CdS or CdSe at an elevated temof NaI:Tl I may use other scintillators such The numeral 12 perature or by reaction of Cd with S or Se or a mixture of both. The reaction will be carried out at an elevated temperature at which the elements are in vapor phase and recrystallized. The temperature in either case may be for example from 850 to 1100, prefer-ably 900 to 950,

degree Centigrade.

Needle shaped vention may have the following characteristics:

(1) Composed of CdS single crystals inroughly cylin drical shape (2) Diameter from 1 to 200 microns (3) Length not less than one-fourth of an inch.

(4) Resistance in light, 6.0 times 10 to 7.7 times 10 ohms (at 30 ft. candles) a (5) Resistance in dark, 1.6 times 10" to 5.0 times .10

ohms.-

(6) Light resistance darkresistance 7.3 times 10 to 1.7 times 10 I The corresponding figures for CdSe are:

(1) Composed of CdSe single crystals in roughly cylindrical shape (2) Diameter of 1 to 200 microns (3) Length not less than one-fourth of an inch (4) Resistance in light 9.5 times 10 to 1.7 times 10 ohms (at 30 ft. candles) (5) Resistance in dark 1.1 times ohms.

10 to 4.34 times 10 (6) Light resistance -e dark resistance.6.5 times10 to 3.9 times 10 The response time of these photoconducting whiskers. can be made even lower by incorporating :a permanent low level light source or radiation source inside the de-.

vice. This can be done conveniently by using a scintillator crystal grown with a small amount of long-lived radioactive isotope or by placing a. dot of luminescent a long-lived radioactive isotope material prepared with incorporated in it. The small amount of light produced by such means falls on the photoconductor and enables it to respond faster to an increase in light level, caused by radiation incident upon the scintillator crystal;

intensity than is needed to produce the desired response time since incorporation of a permanent lightsource decreases the sensitivity of the device. This feature is useful but not indispensable .and the invention can ;be realized to a large extent without it.

A mechanically stable array canbe made :by placing high resistivity epoxy cement over the whiskers witha 1 white reflecting powder mixed therein.- This plate is then placed next to the scintillator crystal and the wholelsur-- rounded by a reflector such as MgO. powder. The unit'is then sealed into a light-tight container with projecting electrical leads for connection to the associated components.

proportions to yield a paste. The medium may be a pine oil paste medium. The paste includes. a ground glass frit, saidoil and silver metal. I may, for example, use parts by weight powdered silver metal, 1 5 parts by weight'frit and enough pine oil to make a paste. Alternatively, I may use DuPont Silver Mixture No. 4817-. The other metals need not be in paste form. The photoconductive element and the scintillator may beoptically coupled and both may be coated with a highly difiuse.

oxide powder as MgO or A1 0 except a suitable area for permitting the light from the scintillator crystal to fall on the photoconductive-material. The optical coupling may be effected by a silicone resin or other well known optical coupling resin but no such coupling is essential.- Any metal substituted for indium should have a melting point belowl5 0 C.

whiskers of CdS accordingto the in-- The low level permanent light source should be no greaterin The response time of the photoconductive whiskers can be reduced as stated above by a light source or radiation source inside the housing 11. It can be reduced equally well by a light source or radiation source outside the housing 11 if it be kept in close proximity thereto.

Having thus described the invention, what is claimed is:

=1. A radiation detector comprising in combination a scintillation crystal, a photoconductive body positioned adjacent said crystal and illuminable thereby, and means, including a source of connected in circuit with said photoconductive body for .indicating conductance changes therein responsive to changes in illumination thereof, said photoconductive body comprising a plurality of needles of cadmium sulfide connected in parallel with each other and in series with said source of and said body for indicating conductance changes.

2. A radiation detector comprising in combination a scintillator crystal, a photoconductive grid positioned adjacent said crystal and illuminable thereby, and means, including a source of connected in circuit with said photoconductive grid for indicating conductance changes in said grid responsive to changes in illumination thereof, said grid comprising mono-crystalline needles of a material of the class consisting of cadmium sulfide and cadmium selenide, having their minimum dimensions in the range from 1 to 200 microns, said needles being positioned across and in electrical and mechanical contact with spaced approximately parallel strips of metal of the class consisting of indium, silver, gallium, thallium, and alloys of said elements other than silver having melting points below about C.

3. The device according to claim 2 wherein said scintillation crystal and said grid are enclosed in a light-tight housing.

4. The device of claim 2 wherein said photoconductive grid is a plurality of whiskers connected in parallel.

5. The device of claim 1 wherein said needles are less than 4 inch long, and from 1 to 200 microns in minimum dimension.

References Cited by the Examiner UNITED STATES PATENTS 2,899,560 8/1959 Nemet 250-71.5 3,119,016 l/1964 Attix 25071.5 3,143,653 8/1964 Adams l- 250208 OTHER REFERENCES Semiconductors, by Harmay, Reinhold Publishing Corporation, New York, 1959, pages 138-10 140.

RALPH G. NHJSON, Primary Examiner. JAMES W. LAWRENCE, Examiner. A. R. BORCHELT, Assistant Examiner.

Claims

1. A RADIATION DETECTOR COMPRISING IN COMBINATION A SCINTILLATION CRYSTAL, A PHOTOCONDUCTIVE BODY POSITIONED ADJACENT SAID CRYSTAL AND ILLUMINABLE THEREBY, AND MEANS, INCLUDING A SOURCE OF E.M.F. CONNECTED IN CIRCUIT WITH SAID PHOTOCONDUCTIVE BODY FOR INDICATING CONDUCTANCE CHANGES THEREIN RESPONSIVE TO CHANGES IN ILLUMINATION THEREOF, SAID PHOTOCONDUCTIVE BODY COMPRISING A PLURALITY OF NEEDLES OF CADMIUM SULFIDE CONNECTED IN PARALLEL WITH EACH OTHER AND IN SERIES WITH SAID SOURCE E.M.F. AND SAID BODY FOR INDICATING CONDUCTANCE CHANGES.