US2875344A - Protection system - Google Patents

Description

INVENTORI- JOSEPH J. GRINERICH BY.'-

ATToRNeY'; I

United States Patent O PROTECTION SYSTEM Joseph J. Griuerich, Milwaukee, Wis., assignor to General Electric Company, a corporation of New York Application March 9, 1955, Serial N0. 493,101

6 Claims. (Cl. Z50-95) The present invention relates in general to the control of penetrating rays, such as X-rays, and has more particular reference to a protective system for terminating the emission of rays from a ray source, at the conclusion of a safe ray emission interval, in the event that associated interval timing means should fail to function.

While the invention is especially well adapted for application to the control of penetrating ray sources comprising electrical power operated X-ray generating tubes, it will be obvious that the invention is not necessarily limited to the control of ray emission from power operated tubes, but that its principles may be usefully applied to the control of rays emitted from other sources, including bodies of ray emitting material, under the control of selectively operable emission blocking means, such as electrical power operated shutter means.

An X-ray generating tube commonly comprises an anode and a cooperating cathode enclosed in a sealed, evacuated envelope, such a tube operating for the production of X-rays as a result of impingement on the anode of electrons emitted by and at the cathode. The cathode may and commonly does comprise a filament adapted to emit electrons when electrically energized, as by the passage of electrical current therethrough. Electrons thus emitted by and at the filament may be impelled toward and caused to impinge upon the anode at high velocity under the influence of electrical potential maintained between the anode and cathode.

During operation, the anode of an X-ray generating tube becomes heated, as the result of electron impingement thereon. As a consequence, the anode may operate at a temperature of the order of the melting point of its constituent metal. Heat thus generated at the anode may be dissipated through the walls of the tube envelope, or by the circulation of a cooling iiuid in heat exchange relation with the anode, or by other heat disspating means. Each individual ray generating tube, of course, displays its own characteristic heat dissipation rate, and Xray generating tubes are usually operated at selected energy levels such that the tube will not be dangerously overheated during the interval of operation at the selected input energy levels, the permissible operating energy level varying inversely with the duration ofthe required operating time interval.

It is customary, therefore, n operating an X-ray tube, to adjust the level of tube energizing power in accordance with the required operating time interval, which, in turn, depends upon the particular use to which the generated rays are to be put. For example, X-ray equipment may be required to produce rays during time intervals which may vary between a short interval, say of the order of $430 of a second or less, and longer intervals of the order of several seconds, or even minutes, such equipment being adjustable to appropriately vary the tube operating energy level in accordance with the selected operating time interval so that the tube will not be injured or damaged when operated during the selected interval at the corresponding safe operating energy level. Tube operating energy levels 2,875,344 Patented Feb. 24, 1959 ICS employed for short interval operation, however, if con tinuously applied to a tube, during extended intervals of operation, will cause the temperature of the tube anode to increase above the melting temperature of its constituent materials, thereby damaging or destroying the device, or otherwise rendering it unt for further service.

X-ray generating tubes may be employed as ray sources for various purposes, including diagnostic, therapeutic and graphic or picturing purposes, requiring operation of the tube at various operating energy levels during intervals of unlike duration. To this end the tube may be employed as a ray source, in conjunction with ray confining means, for applying a ray beam as in an irradiation zone, forming a picturing or therapy station. Among the several picturing procedures requiring the employment of X-ray generating tubes, are the picture making procedures accomplished by means of spot film, Bucky diaphragm, cassette changer, and photoroentgeu equipment, the same forming picturing stations adapted to be irradiated by rays emitted from a controlled emission source.

Spot lilm picturing equipment, as shown more particularly in U. S. Letters Patent No. 2,552,858 of May 15, 1951, on the invention of R. J. Mueller et al., in Serialographic Apparatus, comprises means for supporting X-ray sensitive film in a suitable cassette, in normally retracted position outwardly of a picturing zone, and for projecting the cassette successively to relatively offset exposure positions in the zone so that several adjacent portions of the film may be successively exposed to the action of X-rays in the picturing zone.

Bucky diaphragm picturing equipment, as shown more particularly in U. S. Letters Patent No. 2,115,755 of May 3, 1938, on the invention of J. B. Wantz et al., in Radiographic Diaphragm, comprises means for applying, on suitable X-ray sensitive film, rays emitted from a suitable source and delivered thence in succession through an object to be pictured and a Bucky diaphragm prior to impingement upon the film.

Cassette changer apparatus, as shown more particularly in U. S. Letters Patent No. 2,277,330 of March 24, 1942, on the invention of A. J. Kizaur in Plate Changer, comprises means for presenting a plurality of cassette enclosed sheets or plates of X-ray sensitive lm in rapid succession, in position for exposure to X-rays emitted from a suitable source.

Photoroentgen apparatus, as shown more particularly in U. S. Letters Patent No. 2,248,976 of July 15, 1941, on the invention of G. W. Files in Radiographic Unit, comprises a screen of material adapted to become luminescent in response to the impingement of X-rays thereon, whereby a shadow picture of an examination object may be induced in the screen by applying X-rays from a suitable source through the object and upon the screen, the apparatus including camera means operable to produce an optical record of a ray induced shadow picture on the screen.

As shown more particularly in U. S. Letters Patent No. 2,588,124 of March 4, 1952, on the invention of A. J. Kizaur in X-ray Apparatus, Bucky diaphragm and spot film apparatus may both be mounted in a common table structure, including a support panel upon which an object to be examined, treated, or pictured may be disposed. As a consequence, either the spot film or the Bucky diaphragm apparatus may be used at will. As shown in said Letters Patent, the Bucky diaphragm apparatus is disposed beneath or behind the support panel. It may be used in conjunction with an X-ray generator tube, forming an X-ray source, disposed above or in front of the support panel in position to project X-rays through the support panel, an examination object thereon, the Bucky diaphragm and thence upon ray sensitive film carried in the Bucky diaphragm apparatus.

The spot film apparatus in said Letters Patent No 2,588,124 is disposed above or in front of the support panel. It may be used in conjunction with a ray source, such as an X-ray tube, disposed beneath or behind the support panel and mounted on suitable frame means which, in turn, is supported on the table structure. Such ray source may be used either in making pictures upon sensitive film supported in the spot iilm apparatus, or to induce shadow pictures in a lluorescent screen forming a part of said apparatus.

Obviously, separate ray sources may be employed for use in conjunction with the spot tilm and Bucky diaphragm apparatus of Letters Patent No. 2,588,124, although, as disclosed in U. S. Letters Patent No. 2,208,258 of July 16, 1940, on the invention of J. I. Grobe in X-ray Equipment, a single X-ray source may be provided and positioned either above or in front of the support panel of the table for use in conjunction with Bucky diaphragm apparatus, and alternately beneath or behind the support panel for use in conjunction with picturing equipment disposed above or in front of the support panel of the table.

It is thus common practice to utilize a single ray emitting source, not only in conjunction with spot film and Bucky diaphragm apparatus of the sort disclosed in U. S. Letters Patent No. 2,588,124, but also to use the same source in conjunction with photoroentgen and cassette changer apparatus of the sort shown, respectively, in U. S. Letters Patent No. 2,248,976 and No. 2,277,330. Indeed, it is usual to employ a single ray emitting source at will, not only for the accomplishment of ray therapy, but also for tluoroscopic screen examinations, as well as for picturing purposes in conjunction with spot lm, Bucky diaphragm, cassette changer, and photoroentgen equipment.

Where a single X-ray tube is so employed as a ray source, it is necessary to accurately adjust the energy level at which the ray generating tube is to be operated, as well as the duration of the operating interval for each kind of service to be performed. The energy level at which the ray generating tube is operated may, of course, be determined by adjustment of the electrical power supply system with which the generating tube is connected. The duration of the tube operating interval, however, is commonly determined by suitable timing apparatus, which may be controlled by sensitive ray detecting means, as disclosed, for example, in U. S. Letters Patent No. 2,401,289 of May 28, 1946, on the invention of R. H. Morgan et al., in X-ray Exposure Timing Apparatus. Such ray detecting means may be disposed in the path of rays emitted by the source during the exposure interval, each separate picturing apparatus having an associated detector. These detectors, in conjunction with suitable integrating equipment, determine the total quanta of rays delivered during an exposure interval and serve to terminate the exposure interval when a predetermined ray quanta has been applied.

Where several different devices, such as spot film, Bucky diaphragm, cassette changer, and photoroentgen apparatus, are used at will, it is usual to employ a timing unit for terminating irradiation intervals as by disabling the associated X-ray tube power supply system, selectively operable means being provided for connecting the timer with the ray sensitive detector which is associated with the picturing apparatus selected for operation. In such an arrangement, however, it is possible, through operational error, to connect with the timing unit a detector associated with a picturing device other than that selected for operation, in which case the timing unit will fail to function in response to ray emission from the ray source, thereby possibly subjecting the ray generating tube to dangerous, if not destructive, overloading, while, at the same time, exposing the object being pictured to possibly dangerously excessive X-ray exposure.

An important object of the present invention is to provide means for terminating the application of penetrating rays in an irradiation station, such as an X-ray therapy or picturing zone, after the expiration of an irradiation interval beyond which continued energy delivery at the selected level may deleteriously affect the device.

Another important object is to provide means operable to prevent the overloading of an electrically operated device adapted for operation, during intervals of limited duration, at adjusted safe operating energy levels, in the event of failure to terminate operation of the device at the conclusion of a safe operating interval.

Another important object is the provision, in conjunction with an X-ray generator tube and associated equipment, including timing means for terminating tube operation at the conclusion of a safe operating interval, at correspondingly safe operating energy levels, of protection means, operable in the event of failure of the timing means, to discontinue the supply of operating energy to the tube, after the same has been in operation, at a selected operating energy level, long enough to reach the safe limit of energy input to the tube.

Briey stated, the present invention may be applied in connection with apparatus, such as spot film, Bucky diaphragm, photoroentgen and cassette changer equipment, forming a plurality of irradiation stations, a penetrating ray source, such as an X-ray tube, adapted to apply rays at any selected one of said stations, and adjustable timing means operable to discontinue the application of rays at a so selected station, including ray sensitive detectors disposed each at a corresponding irradiation station and adapted to be controllingly connected selectively with the timer to actuate the same in accordance with ray quanta applied at the selected station, as measured by the associated detector at such selected station; and the objects of the invention may be attained by providing auxiliary ray sensitive detector elements, as of cadmium sulphide, each disposed at a corresponding station in position to be exposed to rays applied to the timing detector at said station, means being provided for controllingly connecting each of said auxiliary detector elements with suitable disabling means operable to discontinue the application of rays at the selected station when the timing detector at the selected station is controllingly connected with the timing means, the so connected auxiliary detector, when exposed to the action of X-rays, serving to hold said disabling means inactive.

The foregoing and numerous other important objects, advantages and inherent functions of the invention will become apparent when the same is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

To illustrate the invention, the single ligure comprising the drawing diagrammatically discloses a plurality of picturing devices including a Bucky diaphragm device 11, a spot lm device 12, a cassette changer device 13, and a photoroentgen device 14, said devices forming irradiation stations to which rays may be applied for picture making purposes. The illustrated equipment also includes suitable ray source means X adapted to emit rays and apply the same selectively in said stations. While the invention is not necessarily limited to any particular kind of ray source, the same may comprise a single X-ray tube X-l adapted selectively to be disposed in appropriate position to irradiate each of the several picturing devices. Alternately, the ray source means may comprise the generating tube X-l adapted for use in conjunction with certain of said picturing devices, such as the spot film device 12, the cassette changer device 13, and the photoroentgen device 14, and an additional ray generating tube X-2 adapted for use only in conjunction with the Bucky diaphragm apparatus 11. The generating tubes X-l and X-2 may be of any suitable or preferred character and may comprise each an anode 15, forming an electron target, a cathode 16 embodying a tilament adapted to be electrically energized for electron emission, and a sealed, evacuated envelope 17, hermetically enclosing the anode and cathode, means being provided for electrically connecting the anode and cathode with suitable sources of energizing power disposed outwardly of the envelope. An X-ray generating tube may be operated either as a self-rectifying device, by connecting the anode and cathode thereof directly with a suitable source of alternating current power, such as the secondary winding of a transformer, or the X-ray tube may be operated by applying unidirectional power between its anode and cathode, as by connecting a rectilier between the alternating current power source and the anode and cathode of the generating tube.

As shown, the generators X-l and X-2 may be energized from a primary source of alternating current power S respectively through transformers T and T', said transformers having secondary windings 18 and 18', respectively connected between the anode and cathode elements of the generator tubes X-l and X-2. The primary windings 19 and 19' of the transformers may be connected in parallel relation, one end of each winding being electrically connected with one side of the power source S. The opposite ends of the windings may be connected with the other side of said power source through a selector switch 20 and the normally open switch contactors 2l of a relay switch unit 22. Accordingly, the supply of operating power to the X-ray tube source means, and hence the application of rays at a selected irradiation station, is controlled by the switch 22. It is, however, within the contemplation of the present invention to control the application of rays at an irradiation station otherwise than by controlling the supply of actuating power to a ray generating source, as, for example, by providing ray blocking means, such as a shutter, disposed between the source and the irradition station and normally biased to prevent delivery of rays from the source to the station, and means, such as a solenoid, for retracting the shutter from ray blocking position when the solenoid is energized, a control switch, such as the switch'21, being connected in circuit with such shutter retracting solenoid and a source of solenoid energizing power, whereby to cause retraction of the shutter and consequent application of rays at an irradiation station when and so long as the control switch is energized.

The relay unit 22 may include an actuating solenoid 23 adapted, when electrically energized, to cause closure of the contactors 21, thereby supplying energizing power from the source S to cause rays to be delivered at the selected irradiation station. The relay actuating solenoid 23 may be connected in a control circuit C comprising a suitable source of energizing power S', a manually operable switch 24 and a normally closed switch 25 of a relay switch unit 26. The switch unit 26 may include an actuating solenoid 27 adapted, when electrically energized, to open said normally closed switch 25. Since the switch 25 is normally closed, closure of the switch 24 will cause application of rays at an irradiation station as the result of closure of the switch 21; and the application of rays at such station will continue until the switch 25 is opened.

To control the opening of the switch 25, each of the picturing devices 11, 12, 13, and 14 may include a timing detector D, which may conveniently comprise a iluorescent layer 28 and an associated photocell 29. The layer 28 may be disposed in the path of X-rays applied, for picture making purposes, at the irradiation station in which the layer is mounted, said layer being disposed in position to receive rays after the same have passed through an object being exposed to picturing rays at the irradiation station, the layer 28 being preferably mounted immediately behind the ray sensitive picturing film disposed at the station. The layer of material 28 is adapted, when excited by ray impingement thereon, to glow and emit visible light, the intensity of which varies as a function of the intensity of incident X-rays, light emitted by a layer 28 thus comprising a measure of ray quanta applied thereto. In conjunction with the layer 28, each of the detectors D associated with the devices 11, 12, 13, and 14 comprises a photoelectric cell 29 disposed in position to receive visible light rays emitted by the associated layer of uorescent material 28, the photoelectric cell 29 being thus excited in proportion to X-ray quanta applied on the associated layer 28. Accordingly, by connecting a cell 29 in circuit with the solenoid 27, a suitable power source B, and timing apparatus 30 of the sort disclosed, for example, in U. S. Letters Patent No. 2,401,289, the solenoid 27 may be energized to open the switch 25 after source emitted rays have been applied at the selected irradiation station during an interval measured by the timing apparatus 30 in terms of ray quanta impinging upon the layer 28 of the detector means D at said selected station.

Adjustable switch means 31 may be provided for selectively connecting the several photocells 29 individually with the timing apparatus 30, the photocells being also commonly connected with one side of the power source B, the timer being connected with the other side of said power source through the solenoid 27. By selective adjustment of the switch 3l, any one of the ray detecting photocells 29 may be controllingly connected with the timing equipment 30. Accordingly, if rays from a ray source be directed upon the picturing device or irradiation station containing the so connected cell 29, under the control of the relay 22 and the switches 24 and 25, application of rays at said station may be terminated upon the opening of the switch 2S, at the conclusion of an operating interval of duration determined by the timing apparatus 30, said apparatus being adjustable to measure any desired one of a number of different operating intervals in terms of ray quanta applied at the irradiation station during the measured interval. If, however, rays from the ray source be directed, in error, elsewhere than upon a picturing device or zone containing the timer connected ray detecting photocell, the timing apparatus will, of course, fail to operate. As a consequence, the application of rays at the irradiation station may be continued beyond safe limits.

To obviate the dangerous conditions thus made possible by faulty adjustment of the switch 31, an auxiliary or safety detector 32 may be mounted at each of the several irradiation stations at which rays from the source means may be applied, said auxiliary detectors 32 being mounted each adjacent the ray detecting photocell 29 at the station in which such auxiliary detector is mounted. As a consequence, each auxiliary detector 32 will be exposed to rays from the source means only when the photocell disposed at the same ray exposure station is also subjected to rays emitted from the same source.

While any suitable ray detecting means may be employed, the auxiliary detectors 32 preferably comprise crystalline elements of cadmium or mercury sulphide, or cadmium selenide, the same having desirable X-ray detecting characteristics, as set forth in an application for U. S. Letters Patent, Serial No. 228,333, filed May 25, 1951, now Patent No. 2,706,792 on the invention of John E. Jacobs in Semi-Conductors. In accordance with the present invention, the auxiliary detectors 32 are employed to control disabling means for discontinuing the application of rays at the selected station, in the event that rays emitted from the source should fail to be applied upon the auxiliary detector associated with the timer connected photocell. To this end the circuit C, which controls the application of source emitted rays at a selected radiation station, in addition to the switches 24 and 25, and the relay coil 23, may include a normally closed, time delay opening switch 33 forming a part of the relay 22 and adapted to open and thus break the circuit C at the conclusion of a preselected time interval following operation of the relay 22 in response to closure of the switch 24, such preselected time interval comprising a safe operating interval under all operatingconditions.

Apparatus embodying the present invention also includes means operable to maintain the circuit C at the switch 33, after closure of the switch 24, in the event that rays from the source be applied to the photocell component of the selected detector D. As shown, such means may conveniently comprise a relay 34 embodying a normally open switch 35, electrically connected in parallel with the switch 33, and a solenoid 36 adapted to close the switch 35 when electrically energized. The solenoid 36 may be connected in an electrical circuit including the power source B and a selector switch 31 connected with the several auxiliary detectors 32 and operable in unison with the switch 31. As a consequence, the switch 31' may be adjusted with the switch 31 to selectively connect the detectors 32 each respectively in circuit with the solenoid 36 and the power source B when the photocell 29, which is associated with the selected detector 32 at the same irradiation station, is also connected with the timing apparatus 30 by means of the switch 31.

In the event that the ray emission source means be disposed in position directing its useful ray beam within an irradiation zone or station other than the station containing the timer connected photocell 29, it will be seen that the ray beam will also fail to impinge upon the auxiliary detector 32 which is controllingly connected with the relay 34. The coil 36, accordingly, will remain de-energized and the switch 35 will remain open. As a consequence, upon closure of the switch 24 to initiate the application of rays at an irradiation station, the safety switch 33 will open after the expiration of a safe interval corresponding with the characteristic or set time delay opening interval of the switch, and application of rays at the selected irradiation station will be discontinued in response to the opening of the safety switch 33.

If, however, the ray source is oriented to apply rays at the irradiation station containing a timer connected photocell 29, the beam will also impinge upon the auxiliary detector 32 which is controllingly connected with the relay 34. The auxiliary detector, when so irradiated, will become conductive substantially instantaneously, thereby allowing the solenoid 36 to become energized and close the switch 35. Closure of the switch 35 will maintain the circuit C across the switch 33 when the same opens after its characteristic time delay interval following closurel of the switch 24. As a consequence, the application of rays at the selected irradiation station will continue until terminated in response to the opening of the switch 25 under the control of the timer equipment 30. lt will be seen, thus, that the provision of the auxiliary detectors 32 of normally nonconducting semi-conductor material, selectively connectible with the relay 34 in unison with the connection of the corresponding photocells 29 with the timing equipment 30, will allow properly timed application of rays at a selected irradiation station, in the event that the source means is correctly oriented to deliver rays at the irradiation station containing the timer connected photocell, but otherwise will operate to discontinue application of the ray beam within a safe interval.

lt is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction, and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

l. A protection system, for use in conjunction with penetrating ray emitting means, comprising the combination, with operable means for controlling the delivery of rays from said emitting means to an irradiation station, of timer means controllingly associated with said operable means to actuate the same to terminate the delivery of rays to the station, ray responsive detector means disposed at said irradiation station and controllingly connectible with said timer means to actuate the same, after the expiration of a safe exposure interval of minimum duration following actuation of said operable means to initiate ray delivery at said station, safety means operable to actuate said operable means for the discontinuation of ray delivery to the station, after the expiration of a safe irradiation interval of Iminimum duration, and means for disabling said safety means in response to ray delivery at said station.

2. A protection system, for use in conjunction with penetrating ray emitting means, comprising the combination, with relay switch means for controlling the delivery of rays from said emitting means to an irradiation station, of timer means controllingly associated with said relay switch means to actuate the same to terminate the delivery of rays to the station, ray responsive detector means disposed at said irradiation station and controllingly connectible with said timer means to actuate the same, after the expiration of a safe exposure interval of minimum duration following actuation of said relay switch means to initiate ray delivery at said station, safety switch means operable to actuate said relay switch means for the discontinuation of ray delivery to the station, after the expiration of a safe irradiation interval of minimum duration, normally inactive relay means operable to disable said safety switch means, a ray sensitive detector disposed at said station, and means to controllingly connect said ray sensitive detector with said normally inactive relay means.

3. A protection system for use in connection with penetrating ray emitting means, comprising the combination, with operable means for controlling the delivery of rays from said emitting means to an irradiation station, of a timer controllingly associated with said operable means to actuate the same to terminate the delivery of rays to the station, ray responsive detector means disposed at said station, means to connect said detector means with said timer to actuate the same, a time delay opening switch connected with said operable means for terminating the delivery of rays from said emitting means to said station, after the expiration of a safe emission interval of minimum duration following actuation of said operable means to initiate ray emission, an auxiliary ray sensitive detector unit at said station, and a relay and means to connect same with said auxiliary detector unit, said relay including a normally open switch connected in parallel relation with said time delay opening switch and closable in response to ray mpingement on said auxiliary detector unit to render the time delay opening switch ineffective to terminate ray delivery at the station.

4. A protection system, for use in conjunction with penetrating ray emitting means adapted to deliver rays selectively to any one of a plurality of irradiation stations, comprising the combination, with operable means for controlling the delivery of rays from said emitting means to a selected station, of a timer controllingly associated with said operable means to actuate the same to terminate the delivery of rays to the selected station, ray responsive detectors disposed each at a corresponding one of said stations in position exposed' to station delivered rays, after the expiration of a safe exposure interval of minimum duration following actuation of said operable means to initiate ray delivery at said station, selectively operable means for controllingly connecting any selected one of said detectors with said timer to actuate the same, safety means operable to actuate said operable means for discontinuation of ray delivery at a selected station, after the expiration of a safe exposure interval of minimum duration, and means for disabling said safety means only ig the event that rays be delivered to the irradiation station in which the timer connected detector is disposed.

5. A protection system, for use in conjunction with penetrating ray emitting means adapted to deliver rays selectively to any one of a plurality of irradiation stations, comprising the combination, with operable means for controlling the delivery of rays from said emitting means to a selected station, of a timer controllingly associated with said operable means to actuate the same to terminate the delivery of rays to the selected station, ray responsive detectors each mounted at a corresponding one of said stations in position exposed to station delivered rays, a selectively operable switch for controllingly connecting any selected one of said detectors with said timer, a time delay switch connected with said operable means for terminating the delivery of rays at a selected station, after the expiration of a safe exposure interval of minimum duration, a plurality of auxiliary ray sensitive detector units, each disposed at a corresponding one of said irradiation stations in position adjacent the station mounted ray responsive detector, whereby both the ray responsive detector and the auxiliary detector unit may simultaneously be exposed to rays delivered at the selected irradiation station, relay means including a normally open switch connected in parallel with the time delay switch and closable to render the same ineffective to terminate ray delivery at said station, and means operable in unison with said selectively operable means for selectively connecting said auxiliary detector units controllingly with said disabling means when the corresponding ray responsive detectors are controllingly connected with said timer.

6. A protection system, for use in conjunction with penetrating ray emitting means adapted to deliver rays selectively to any one of a plurality of irradiation stations, comprising the combination, with operable means for controlling the delivery of rays from said emitting means to a selected station, of a timer controllingly associated with said operable means to actuate the same to terminate the delivery of rays to the selected station, ray responsive detectors each mounted at a corresponding one of said stations in position exposed to station delivered rays, selectively operable means for controllingly connecting any selected one of said detectors with said timer, a time delay opening switch connected with said operable means for terminating the delivery of rays at a selected station, after the expiration of a safe exposure interval of minimum duration, a plurality of auxiliary ray sensitive detector units, each disposed at a corresponding one of said irradiation stations in position adjacent the station mounted ray responsive detector, whereby both the ray responsive detector and the auxiliary detector unit may simultaneously be exposed to rays delivered at the selected irradiation station, a relay having a normally open switch, connected in parallel with said time delay switch, and a coil operable to close said normally open switch, and means operable in unison with said selectively operable means for selectively connecting said auxiliary detector units controllingly with said coil when the corresponding ray responsive detectors are controllingly connected with said timer, to thereby energize the coil and render the time delay switch ineffective to terminate ray delivery at the selected station when the station mounted ray responsive detector is operatively connected with said timer.

References Cited in the tile of this patent UNITED STATES PATENTS 2,401,289 Morgan et al. May 28, 1946 2,650,258 Pantchechnikoif Aug. 25, 1953 2,706,792 Jacobs Apr. 19, 1955

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