US2694162A - Radiation detection - Google Patents

Description

Nov. 9, 1954 J. F. ENGLISH, JR

RADIATION DETECTION Filed June 7, 1951 N mw %\NN om United States Patent RADIATION DETECTION James F. English, Jr., Lakewood, Ohio, assignor to Bailey Meter Company, a corporation of Delaware Application June 7, 1951, Serial No. 230,406

3 Claims. (Cl. 317-130) My invention is generally directed to the detection of radiation. Although the particular embodiment with which l express my invention is responsive to radiations in the infra-red region, the principle is applicable to any radiation which varies, pulsates, iiuctuates or is otherwise chopped."

lf direct the disclosed embodiment to the detection of radiation given oli' by what I recognize as commercial combustion processes. These combustion processes sustain fiame areas under process or power furnaces and may use oil, gas or coal as a fuel. Under any condition of combustion of this nature there is produced from the iiarne area continuous, band or line emission, part or all of which occurs within the infra-red wave length region. It is the general purpose of my invention to make use of variations in the intensity of the combustion radiations in the infrared region in order that a furnace operator will be informed when the flame of the combustion process is established, or having been established, fails.

It is sufficient for the purposes of my present invention to observe that flame areas do flicker in radiation as commercially propagated. in a general way we may surmise that the iiickering characteristic is possibly, in part, due to the turbulent mixing prior to and during combustion and possibly, in part, due to the convection currents created by the hot ame contacting the relatively cooler ambient surrounding it. Of course there may well be very short periods of steady combustion when flickering does not actually occur, but unsteadiness of this nature is predominant unless laboratory conditions of supply and environment are maintained.

Proportions of fuel and air to combustion processes may be varied in accordance with demand on the furnace, but whether the flame is momentarily yellowed by incandescent carbon, denoting incomplete combustion, or maintained clean and blue or colorless, with perfectly balanced proportions, measurable quantities of infra-red radiation are emitted with an unsteadiness which I now recognize as characteristic of commercial flame areas. Therefore, in connection with the device sensitive to the intensity of infra-red radiations, I have invented a circuit to detect the presence of the flame area in its production of an irregularly pulsating quantity of radiation.

It is well established that combustion processes of the commercial variety involving oil, gas or coal produce radiations emitting substantially through the visible range lying roughly between .4 to .7 micron and well into the infra-red range which lies roughly between .7 and 400 microns. It can generally be expected that these flames will produce radiations throughout the infra-red band but with the specific detector l employ, I am concerned primarily with the region between .7 and 20 microns which is within the so-called near infra-red region. lncidental identifications include what is known as the intermediate infra-red band from 20 to 40 microns and the far band which lies between 40 to 400 microns.

The specific detectors that I employ in my circuit are selectively responsive to a narrow band, or bands of infra-red radiation within the near infra-red region. I feel it advisable in specifying a specific detector to outline the differences between thermal detectors such as the bolometer, thermopile and radiometer, and selectiveradiation responsive devices such as the photoemissive, photovoltaic and photoconductive cells.

The thermal responsive device with which I am most familiar is the bolometer employing a tungsten filament 2,694,162 Patented Nov. 9, 1954 ICC after the manner shown in at least the Rutherford et al. patent 2,524,478. The tungsten lament of the bolometer will change resistance or is responsive, to all radiation to which it is exposed including the visible and infra-red ranges. There is substantially no discrimination in the bolometer response, the resistance changing in accordance with the intensity, or quantity, of the radiation. The tungsten element is enclosed in an evacuated housing whose one side is a transparent window which limits to some extent the length of the infra-red radiations passed to the filament. It can readily be understood that the thermal responsive device is essentially a filament upon which is concentrated all of the radiation coming to it for generating heat to change the temperature of the filament and raise its electrical resistance.

On the other hand the classes of selective light responsive devices are expanding at present. For example, there is the photoemissive cell which is essentially composed of two electrodes enclosed in a gas, or vacuum, and whose operation consists of having7 electrons ejected from the electrode-cathode by radiation impinging thereon and the released electrons fiowing to the other positively charged electrode-anode.

Secondly, there is the photovoltaic or barrier-plane i cell which depends for its function upon a current flow between a thin layer of semi-conductor and a metal when light is allowed to penetrate the former to the junction between the two.

A third type of the selective light responsive devices is the photoconductive cell operating on the principle of the inherent resistance change to radiation of a semiconductor material between two electrodes. The most common material in this usage has been selenium, although thallinm sulfide has been used and lead sulfide and germanium have been brought under recent investigation. The operation is on the principle of emission of electrons within the semi-conducting material due to light impact thereon. ln their motion due to applied potential, these electrons cause ionization, equilibrium being estabished when the rate of formation is equal to disassociation.

Lead sulfide is now in commercially available quantities and form for use in photoconductive cells in a variety of grid sizes and sensitivity resistance classifications. Specifically it is distributed as Cetron lead sulfide photoconductive cell which has a peak response near l micron and 21/2 microns. The speed with which the lead sulfide cell changes resistance in response to radiation makes it quite practical for use in mv invention. Germanium is also in commercial availability in complete cell form. Germanium photocells are referred to as point-contact rectiiiers having a useful photo response when radiant energy in the red and near infra-red region falls upon the crystal in the immediate vicinity of the point-contact. At least some of the germanium cell characteristics roughly parallel those of the lead sulfide cell and to such extent there is available another detectnr useful with mv invention.

Whatever specific form of detector is utilized with my invention, it is essential that it respond by establishing a voltage variable in accordance with a variable and that the speed with which it alters the established voltage follow the change in magnitude of the variable very closely. For detection of commercial flame area propagation, the detector may be one or another of those described supra. Their response is greatest in the near infra-red region of radiation and the high degree of selectivity is desirable for, as observed supra, the output in this region is generic to all types of these flames.

Therefore, a general object of my invention is the detection of the sustainment period of a source of some variable.

Another object of my invention is to sense the presence of a source of pulsating light radiations.

v Another object of my invention is to indicate when a flame is established which produces infra-red radiations at a variable rate.

It is another object of my invention to control the circuit. of a relay device that said relay will be actuated upon termination of a combustion process.

The single figure of the drawing represents a practical embodiment of my invent-ion employing agener-1c detector sensitive to the varyingradiation source of a flame bod Ig the drawing I show a crossl section of aafurnace por.- tion: into which'l propagate a, amebodyf 1, by'` any of a number of injectorsgof-4 various; typesl of fuels. I1 wish to operatemy system: to detect', at" station 2, radiations from this body 1 and establish an electrical' signal for amplification by the; electric` network I designate at The signal amplified at 3i is applied to the control. grid of thyratron tube trarranged in circuit with a relay device I showat 5. The circuit ofl relay'5 normally energized during.` periods: of fiame propagation so that the condition: of extinguishinent, or. disappearance, ofthe fiaine: will deenergize theN circuit and' release relay 5` for giving signal ofthez condition orI exerting a control action to. affect thezcondition.

The iectifying section dis; established to furnish the necessary D. C. supply to the detector at 2, amplifier and thyratron 4'. The components of section 6 are physically distributed: ink the` embodiment asreduced to practice and'niay take other specific formsv than I' disclose. I employ diagrammatic licenseto enable meto group the components of section 6 for clarity ofexplainiiig its functional relation to; thek remainder.' of thecircuit; Electronic lcomponents of section 6 as specifically disclosed include one half of a dual diode I designate (A)'.. A dual diode -24 mayk also be discerned' and` will.' be given. more specific reference and description hereinafter. Thedual diode (A) is of initial importance because it supplies the con.- starit' D.' C. for the detector circuit. Adiustable potentiometer 7 has a voltage drop across it supplied bythe (A) section. This adjustable voltage is arranged in series with the detector elementv I house atV 8 and a resistor 9.

The housing is specifically designated 10 and is ar` ranged to expose the detector element to the radiations of flame body 1 for alteration off the electrical resistance by the impinging radiations. The exposure of detector 8 to the radiations is regulated by structure designated at 11, 12 and 13 which may be alternate in a particular installation.

Device 11 represents a concentrating lens, 12 representsA aV filter unit and 13 represents the .aperture into the housing 10 which mayl be'sized for particular requirements. This structure is necessary when-the limitations of'variousV detectors 81arev individually considered. Some detectors whose resistance is changed with the varying degrees of radiation in a satisfactorily rapid manner can be saturated, that is, the change in resistance for a changev in radiation magnitude mayapproach .Azero Within a particular range of magnitude of radiation. If flame body 1', plus refractory background.` present, emits a saturating quantity of radiation, aperture 13 offers a means for limiting the total" radiation magnitude.

Should' the distance between the radiation source and detector 8', of itself, limit the totalradiation through the largest practical size of' aperture 13', lens 11 may be used to concentrate all radiation gathered into housing 10 on the relatively small' target area of detector 8. Beginning with the consideration ofthe distance between de` tector-target 8 and the source of'radiation, it Will'be necessary to size aperture 13 untilthe contribution by the 'ame body 1 will bean amount whose variation will measurably alter the resistance of' detector 8.'- If the total radiation then falls below the desiredy range of the detector, lens lll may be employed to concentrate' the radiation ori target 8 until its resistancev reaches the d'esired range of magnitude.

Filter 121 is also used. in connection withv the detector and' hasthe function of predominating they bands or lines of radiation to which the. detector 8V has the greatest response. band-pass filters. The devolpmentis largely empirical and IV have: selected a filter which narrowsy the frequency bandV allowed toz pass,v to include substantially only' that portion of the near infra-red radiation which causes the greatest resistance change; in4 theV cell.v The band-pass filter will cut down on the total' radiationrcomingthrou'gh the sighting tubefand lens. but it is effective to pass a. proportionately greater amount of the` infra-red band in which is found the greatest response in the:d'etector. rEhus; with. sighting tube: 13:,vv lens 11,. and; filter: 12:: I provide; astructurebyawhich. there isfpassed; to theJ detector 8, in housing 10, a:totali.amountoff radiatiomofiwhichga These filters are well-known in the art as y 4. large portion fluctuates in accordance with characteristic4 arnepropagation.

With detector element S properly exposed to the varying, or fiickering, radiations of flame body 1, it can be logically explained how the resistance variation can be utilized to establish a voltage drop across resistance 9 for amplification to energize the circuit of relay 5.

The two stage amplifying network I specifically employ, and designategenerally at 3,A is Well-known in the electronic art. The patents to at least Hornfeck 2,437,603y and Ryder 2,333,393. disclose the network which is characteristically dominated by a dual triode such as I show here.

The: grid of the triode in the first stage of amplification has the detector. circuit signal applied directly to it. Considering that the` total Voltage drop across load resistor 9 is composed of both A.C. components, a capacitor 14 is used to isolate, or block, the D.C. component in the drop across load resistor 9. The voltage drop across resistor 15 is therefore A.C. and represents, in its irregular and cyclicvariations, the flickering radiations ofy ame body 1. Electronic amplification ofthe first stage then has 16 as a load resistor across which the amplified signal is established, withl components of both- A.C.v and Dr-C. making up the total signal.

The. second stage of amplification is carried on in the samemanner. asiii the first stage. Capacitor 17 blocks D.C. components from the second stage triode grid and there is established across load resistor 1S the finally amplified signall made. up of components of both A.C. and` D.C. Capacitor 19 blocks the D.C. in the same manner as 14 and 17 and there is established across resistor 20- the signal finally representative of the radiation. sourcey at 1 which Varies with the resistance of detector 8..

Adjustable contact" 21 enables a portion of the amplified` flame-radiation signal to be selected for rectification by the (B) section of the dual diode, disclosed in the dividedmanner employed heretofore in rectifier section 6. The A.C. flame signal is` rectified by section (B) and. applied acrossa parallel resistance-capacitor circuit in the cathode circuit of the tube. Capacitance 22 and resistance 23` specifically form this parallel circuit, with 23f adjustableftogive means of varyingthe rate at which a voltage difference on 22'will bleed off, or reduce; toward zero.

The output' of. the rectifying and reducingV circuits 22, 23' is usedto activate a relay circuit which is thyratron controlled. Ihave reduced the principle ofoperationto practice by4 biasing` the control grid 4 of thyratron 4 with a potentialA of. negative polarity and opposing said potential' by asignal of positive polarity and normally greater magnitudek from the rectifying and reducing circuit 22, 23. The resistance-capacitor arrangement in thev cathode circuiti of the dual diode (B) gives the positive signal. output for opposition to the bias on the thyratrongridA. Therefore, with the ame body 1 near normal. thyratron 4- will fire continuously with the bucking signals on its` grid at predetermined magnitudes with the positive signal predominating.

Should. theV flame body' 1 disappear and the amplified-radiation signal across resistance 2f) drop to zero, the potential.l across capacitor 22 will drop toward zero as` it. discharges' through resistor 23. The negativev bias applied to` the? control grid of thyratron 4 now preL dominates` andthe tube ceases to pass current so the control circuitA is` deenergized and relay 5 falls to its alternate position under the force of gravity or a spring.

Rectifier section 6 has-been considered in its dualv diode (A) which supplies the energy for the detector circuit'. The. completev dualdiode I employ at 24 bears a signifi` cant relation to the complete function of the entire radiation detection circuit.' It is obvious to those familiar with the amplifierI circuit of Hornfeck and Ryder, mentioned-supra, how the right hand'halfT of the tube suppliessthe' D.C. energization necessary for the function of the amplifier; The components associated with the diode; such asthe'choke 25, are familiar and used in ways: well' known in the art to produce satisfactorily rectified: D.C.`V Specifically, choke 25, asa` reluctance; filters the pulsating ll-C.' produced by'rectifyinggA.-C. to. reduce-1 harmonics which appear in the process. The negative bias signa'lfortliyratronA 4' isi established', from this rectification, across. resistance 26'I and' is. made: adjustable foricalibrationfpnrposes;

The left side of dual diode 24 is placed in the thyratroncontrol relay circuit to insure that tube failure at this point will deenergize the relay circuit in a fail-safe manner. By far the larger percentage of tube failures occur in the filaments; should either filament of the dual diode 24 fail, both will fail, and as the energization of the circuit of relay 5 depends upon completion through the left hand side of dual diode 24, the relay will indicate a malfunction upon this circuit being opened by filament burn-out.

While I have chosen to illustrate and describe one preferred embodiment of my invention, it will be understood that this is by way of example only and is not to be considered as limiting.

What I claim as new, and desire to secure by Letters Patent of the United States, is:

l. In a system for the detection of the amplitude of the rapid fluctuations of a source of radiation, of the type having a resistance device whose conductivity varies rapidly in accordance with the intensity of radiation striking it included in a circuit producing a fluctuating voltage whose magnitude varies solely in accordance with the radiation and which is then amplified, the combination including, a reducing circuit for the amplified iiuctuating radiation voltage producing a potential of positive polarity which reduces in magnitude at a determinable rate, a thyratron, a rectifier connected to energize the anode-cathode circuit of said thyratron, a relay in said last mentioned circuit, a second rectifier having its negative output biasing the control grid of said thyratron, and means connecting said reducing circuit positive potential to said control grid, said bias voltage being adjusted to cause relay energization at predetermined radiation magnitude, said rectifiers being of the diode type with series connected filaments whereby failure of one terminates operation of the other and the relay will indicate lack of radiation.

2. In a system for detection of a rapidly and non-uniformly fluctuating variable having means responsive directly to the fluctuations of the variable establishing a fluctuating voltage directly representative thereof and means amplifying only said established voltage, the combination which includes; a rectier; a load for said rectifier comprising shunt connected capacitance and resistance units; means connecting the output of the voltage amplifying means, the rectifier and said load directly in series whereby the capacitance unit is charged solely in accordance with the amplitude, duration and frequency from said voltage amplifier; means providing a fixed potential of polarity opposite that from one pole of the capacitance unit; and means to compare said capacitance pole voltage with said potential to determine the amplitude of the variable including, a signal circuit including a thyratron whose grid is connected to both voltages, a rectifier for supply of the anode-cathode circuit of the thyratron, and a signal relay.

3. The system as deIined in claim 2 in which the values of the capacitance and resistance units are selected to provide a delay in voltage decline from the capacitance unit commensurate with known variations of inconsequential duration and range.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,194,559 Koch Mar. 26, 1940 2,295,366 Stout Sept. 8, 1942 2,435,896 McIlvaine Feb. 10, 1948 2,448,503 Wilson Aug. 31, 1948 2,517,554 Frommer Aug. 8, 1950 2,540,063 Victoreen Jan. 30, 1951 2,541,051 Hanert Feb. 13, 1951

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