US 3476939 A
Description (OCR text may contain errors)
1969 R. H. GOODMAN ETAL 3,476,939
RADIOACTIVE-SOURCE ENCLQSURE Filed Feb. 9. 1966 3 Sheets-Sheet 1 Nov. 4, 1969 R. H. GOODMAN ETAL 3,476,939
RADIOACTIVE-SOURCE ENCLOSURE Filed Feb. 9, 1966 5 Sheets-Sheet 2 PATENT AGENT R. H. GOODMAN ETAL 3,476,939
RADIOACTIVE SOURCE ENCLOSURE 3 Sheets-Sheet 25 by 'BZTORB a.
FATE NT AGENT Nov. 4, 1969 Filed Feb. E), 1966 United States Patent M Int. CI. (52111 /00 US. Cl. 250-106 1 Claim ABSTRACT OF THE DISCLOSURE A radioactive-source enclosure having movable shielding gates which, when free to move under their own weight, assume a position sealing off the radioactive source preventing any escape of radiation from the enclosure. In normal operation these gates are held open by a locking mechanism which, on failure of the power supply to the sorting apparatus, allows the gates to close and locks them shut in the closed position.
This invention relates to a method and apparatus for sorting fragments of ore according to the wavelength of the X-r-ays emitted when the ore is irradiated by a source of beta radiation.
It has previously been known to sort ores containing naturally radioactive elements according to the intensity of the radioactivity detected from the various fragments of ore. A method of sorting ore is also known in which the fragments of ore are irradiated with neutrons and the energy spectrum of the resulting gamma rays is analysed to determine the presence of a particular element being sought in the ore. The disadvantages of this latter method are that neutron sources are expensive and that elements having a low capture cross-section for neutrons do not emit sufiicient gamma radiation for satisfactory analysis. A further disadvantage of neutron irradiation is the presence of induced radioactivity remaining in the ore. Metal obtained from such ore could not be used in connection with the packaging of photographic supplies, to give one example.
It is, therefore, an object of this invention to provide a novel method of sorting ore. In one form of this method the fragments of ore are moved along a suitable path, such as a moving belt, and exposed to a source of beta radiation. The beta particles interact with the surface atoms of the ore fragments to stimulate the emission of X-rays. The spectrum of X-rays emitted by a pure element under beta particle bombardment consists of two components. One component is a relatively uniform continuous wavelength spectrum known as bremsstrahlung; the other component consists of sharp peaks of intensity at certain wavelengths. The wavelengths at which these peaks of X-ray intensity occur differ for each element and hence afford a means of distinguishing between elements.
The X-rays emitted by the ore fragments are intercepted by a radiation detector which produces pulses of voltage whose amplitude is inversely proportional to the wavelength of the incident X-rays. The voltage pulses from the detector are fed to a conventional single channel pulse height analyser which selects and counts only those pulses having amplitudes in a predetermined range. It will be clear that the rate of occurrence of such pulses is a measure of the intensity of X-rays detected with wavelengths falling in a predetermined narrow range. The range of wavelengths to be measured can be altered by changing the setting of the pulse height analyser to select pulses having a different amplitude range.
3,476,939 Patented Nov. 4, 1969 In the method of this invention the setting of the pulse height analyser is adjusted to correspond to the pulse amplitudes produced by the detector when receiving X-rays having a wavelength corresponding to the sharp peak of X-ray intensity characteristic of a particular desired element. When the rate of occurrence of such pulses exceeds a predetermined value, this is indicative of the presence of such an element in significant quantities and conventional sorting methods are used to separate out the ore fragment producing this increased pulse rate.
The apparatus of this invention includes conventional feeding means for the ore fragments, a source of betaradiation in a novel fail-safe enclosure, an X-ray detector coupled to control circuitry and conventional ore sorting apparatus. It is, thus, a further object of this invention to provide a novel radioactive source enclosure having movable shielding gates which, when free to move under their own weight, assume a position sealing off the radioactive source preventing any escape of radiation from the enclosure. In normal operation these gates are held open by a locking mechanism which, on failure of the power supply to the sorting apparatus, allows the gates to close and locks them shut in the closed position.
Other objects and features of this invention will be more fully understood from the following description taken in conjunction with the accompanying drawings in which:
FIGURE 1 is a pictorial representation of the sorting apparatus according to this invention;
FIGURE 2 is a schematic representation of the detector and control apparatus for the sorter;
FIGURE 3 is a perspective view of the radioactive source enclosure of this invention;
FIGURE 4 is an end elevation, partially in section, of the enclosure of FIGURE 3,
FIGURE 5 is a side elevation of the enclosure of FIGURE 3,
FIGURE 6 is a section taken along the line 6-6 of FIGURE 4,
FIGURE 7 is an end elevation of the enclosure showing the shielding gates in the open position, and
FIGURE 8 is a circuit schematic showing the control circuit for the solenoids in the source enclosure.
An overall view of the sorting apparatus of this invention is shown in FIGURE 1. Ore fragments 10 are fed from a hopper 11 on to a conveyor belt 12. The conveyor belt carries the fragments of ore underneath a radioactive source enclosure 13, which is shown with shielding gates 58 and 59 open, where the ore is irradiated with beta radiation. Radioactive source enclosure 13 is suspended at a suitable height above conveyor 12 by supporting means (not shown) attached to handle 85. An X-ray detector 14 is arranged to detect X-rays emitted by ore fragments While under irradiation from the beta source.
The output signals from detector 14, which are in the form of voltage pulses, are fed to detection and control circuit means 15, which is explained in more detail in connection with FIGURE 2. When the voltage pulses are Within the amplitude range which indicates the presence of X-rays having wavelengths characteristic of a desired element and occur at a rate indicative of the presence of a sufficient amount of such element, control circuit 15 actuates the control valve of a conventional kicker-type sorter 16 which displaces the ore fragment containing this element from the conveyor 12 into a hopper feed 17 for concentrated ore. Those fragments of ore which do not contain sufiicient amounts of the desired element to actuate the control circuit pass off the end of conveyor 12 into a hopper feed 18 for a tailings bin.
The detector 14 along with the detection and control circuits is shown in greater detail in FIGURE 2. Since the individual elements shown in FIGURE 2 are conventional elements which are well known in the art they have been represented in schematic form. The detector is of the scintillation detector type and contains a sodium iodide scintillator crystal 21 and its associated photomultiplier tube 22. The scintillator crystal and photomultiplier tube are surrounded by lead shielding 23 to prevent background X-ray radiation or scattered beta particles reaching the crystal through the sides of the detector and causing spurious pulses. The X-rays incident on the crystal 21 pass through a collimator 24, directed at the ore fragment being irradiated, and through a steady magnetic field provided by magnet pole pieces 25 and 26. Supporting bars 27 hold the collimator and magnet in position against the detector housing.
The operation of a scintillation detector is well known. The incident X-rays are absorbed in the crystal and in losing their energy emit a flash of light. The intensity of. the flash is proportional to the energy of a quantum of the X-rays, that is, inversely proportional to the X-ray wavelength. The number of flashes emitted under irradiation by a beam of X-rays is proportional to the intensity of the X-ray beam. The photo-multiplier tube is responsive to the flash of light to produce a voltage pulse whose amplitude is proportional to the intensity of the light flash.
The collimator ensures that only X-rays emitted from the irradiated ore fragment are incident on the crystal 21 and background radiation is blocked. The steady magnetic field produced by pole pieces 25 and 26 deflects any beta particles scattered by the ore which would otherwise give spurious indications at the detector.
A preamplifier 28, shown in schematic form in FIG- URE 2, is mounted on the detector housing and receives the output signal from the photomultiplier tube. The signal from the preamplifier is fed to a pulse amplifier 29. The necessary high voltage for the operation of the photomultiplier tube is supplied from high voltage source 30.
The amplified pulse signal, whose amplitude represents the wavelength of the X-rays emitted by the ore fragment, is supplied to a conventional signal channel pulse height analyser 31. This instrument gives an output pulse whenever an input pulse has an amplitude falling within a preset range. The preset range and its lower limit are variable and can be separately adjusted in the instrument. It will be seen that the range and lower limit can be calibrated directly in terms of the wavelength of the X-rays incident on the detector or, more commonly, in terms of the quantum energy of the X-rays. The relationship between the wavelength 7\ of X-rays expressed in centimeters and their quantum energy W expressed in electron volts is The output pulses from the pulse height analyser, each of which represents the presence of a quantum of X-ray radiation in the desired wavelength range, are supplied to a rate meter 32 which produces an analog voltage proportional to the rate of occurrence of input pulses. When this analog voltage exceeds a predetermined value, trip circuit 33, connected to rate meter 32, produces an output signal on lead 38 which actuates sorter control 35 through delay circuit 34. The delay of circuit 34 is adjusted to compensate for the time taken by the ore fragment to travel from underneath the source enclosure 13 to the kicker-sorter 16. Trip circuit 33 is adapted to produce an output signal on lead 37 when the analog signal from rate meter 32 falls below the value normally present due to the detection of background radiation. The signal on lead 37 actuates low level alarm 36 to indicate that the radioactive source has ceased to function.
Alarm 36 may be used to interrupt the electrical power supply to the sorting apparatus. As is more fully described below this will automatically close the shielding gates of the radioactive source enclosure 13. Thus should the source enclosure fall or otherwise be displaced from its normal position there is no danger of unwanted radiation escaping.
The wavelength of X-rays emitted by an element varies inversely with the square of the atomic number of the element. Thus the elements of low atomic number emit X- rays of long Wavelength, i.e. of low quantum energy, which produce small voltage pulses in the scintillation detector, which are hard to detect. For this reason it is found that the apparatus of this invention is most useful in detecting and sorting elements with an atomic number greater than 36.
It has been found that the presence of an element in the amount of 5% of an ore fragment can be easily detected. For elements having high atomic numbers, their presence in the amount of 1% can be detected. The following table gives the settings used for the upper and lower limits of the pulse height analyser in detecting the presence of the noted elements in ore:
Element: Analyser setting, kev. Molybdenum 14-22 Tin 21-29 Cesium 26-34 Mercury 65-73 These tests were carried out using a Strontium 90 source for the beta radiation. This source has the advantage that no gamma rays are emitted, which would otherwise reduce the sensitivity of the apparatus.
The novel radioactive source enclosure will now be described with reference to FIGURES 3-8 of the drawings. FIGURE 3 shows the radioactive source enclosure, indicated generally at 13, consisting of a main body housing 51 with a supporting handle mounted thereon. As seen most clearly in FIGURE 6, body housing 51 is formed by a shell 52, made of brass or some other suitable material, filled with a dense shielding material 53 such as lead. A downwardly facing opening 54 is provided in the body housing to contain the radioactive source. Lugs 55 are provided in opening 54 to support plate 56 carrying the radioactive source.
Two gates 58 and 59 are pivoted on hinge pins 60 and 61 which extend longitudinally in body housing 51. The lower sections of gates 58 and 59 are formed with shells 62 and 63 respectively, made of brass or some other suitable material, filled with a dense shielding material 64 such as lead. A bar 65 is provided at the foot of each gate to form a support for the enclosure when placed on a horizontal surface.
The gates 58 and 59 normally assume one of two possible positions. The first position, shown in FIGURES 3 and 4, is with the gates closed so that the shielding material 64 prevents any radiation from a radioactive source in opening 54 escaping from the enclosure. The second position, shown in FIGURE 7, is with the gates drawn back allowing radiation from a radioactive source in opening 54 to irradiate an area below the container. If free to move under the influence of gravity the gates close to assume the first position and shield the radioactive source.
Locking bars 66 and 67 are provided to maintain either the first or second position of the gates. When the gates are in the first position, as shown in FIGURES 3 and 4, locking bars 66 and 67 extend down the outside of the gates and prevent any movement. When the gates are in the second position, as shown in FIGURE 7, locking bars 66 and 67 enter slots 68 and 69 on the inner face of gates 58 and 59 respectively. This prevents any movement of the gates from the second position.
Locking bars 66 and 67 are supported for vertical sliding movement by guides 70 attached to main body housing 51. A yoke 71 attached to locking bar 66 carries a roller 72 which cooperates with a cam surface 73 carried on a solenoid plunger 74 to move the locking bar vertically. The movement of plunger 74 is controlled by solenoid 75 mounted on body housing 51. Plunger 74 is normally urged in a direction outwardly of the solenoid by a compressive spring 76 mounted on a guide 77. The electro-mechanical force exerted on plunger 74 by solenoid 75 can overcome the force exerted by spring 76 and move the plunger inwards. Thus when the solenoid is energized roller 72 is moved vertically by cam surface 73 and the locking bar 66 is raised leaving gate 58 free to move.
Locking bar 67 is raised and lowered by an identical mechanism consisting of yoke 77, roller 78 and solenoid plunger 80 having a cam surface 79. Plunger 80 is controlled by solenoid 81 and is maintained in a normally outward position, when the solenoid is not energized, by means of compressive spring 82 mounted on guide 83.
The movement of the locking bars is controlled by the circuit of FIGURE 8 with the object that; in the event of a failure in the power supply to the ore sorting equipment, the radioactive source enclosure will be automatically sealed to prevent unnecessary radiation.
Referring to FIGURE 8, 91 and 92 denote the normal AC. power supply lines. An A.C. sensitive relay 93 is connected across the power lines and controls switches 94 and 95 which assume the positions shown in FIGURE 8 when the relay 93 is energized. A source of DC. power, indicated symbolically as battery 96, is connected to a capacitor 97 through switch 95. The coils of solenoids 75 and 81 are connected to capacitor 97 through switch 94.
Consider the enclosure to be in its second or open position with the gates held open by locking bars 66 and 67 engaging slots 68 and 69 in the gates. If the power supply on lines 91 and 92 then fails, relay 93 ceases to be energized, D.C. source 96 is disconnected from capacitor 97 and solenoid coils 75 and 81 are connected across capacitor 97. The discharge of capacitor 97 through solenoid coils 75 and 81 actuates the solenoids and raises the locking bars thereby permitting the gates to close under their own weight. The solenoids are actuated for only a brief interval as the capacitor discharges through coils 75 and 81 and when the capacitor has discharged the locking bars return to their downward position and lock the gates in the first or closed position.
If a temperature sensitive fuse is placed in series with one of the power supply lines 91, 92 it will be seen that the radioactive source enclosure 13 will automatically be sealed off in the event of fire.
Thus there has been described a novel method and apparatus for sorting ore fragments according to the wavelength of the X-rays emitted when the ore fragments are irradiated by beta radiation and a novel enclosure for a radioactive source particularly adapted for use with the apparatus.
1. An enclosure for radioactive materials comprising: a main body housing containing radiation shielding dense material having an opening therein; means on said housing for supporting a radioactive source in said opening; a pair of shielding gates hingeably mounted on said body housing; each said gate being pivotally supported by a hinge pin extending longitudinally of said body portion and said gate having a first position covering said opening with a layer of radiation shielding dense material and a second position displaced from said opening; each gate having an outer surface and an inner surface with the inner surface defining a slot therein; a locking bar associated with each gate mounted on said body housing for reciprocal movement, at one extremity of said movement abutting on the outer surface of said gate in said first position to lock said gate; said locking bar engaging with said slot at said extremity of movement to lock said gate in said second position; and a solenoid associated with each locking bar mounted on said body housing having a plunger with a cam surface co-operating with its associated locking bar to produce said reciprocal movement.
References Cited UNITED STATES PATENTS 6/1953 Young 250108 11/1956 Hiestand.
US. Cl. X.R. 250-105, 108