US3432665A - Irradiation device for providing a constant dose rate - Google Patents

Description

March 11, 1969 H. E. KIMBLE IRRADIATION DEVICE FOR PROVIDING A CONSTANT DOSE RATE Filed Feb. 15, 196? III-l llll lluillvl ATTORNE Y.

United States Patent 5 Claims ABSTRACT OF THE DISCLOSURE A constant level dose rate from a radiation source is provided over a period of time which may correspond to a duration substantially greater than the halflife of the source. The source size is increased by as much as several factors over the size of a source required for the desired dose rate. A sufficiently large portion of this bigger source is then exposed at a window in a housing containing radiation moderating material to provide the desired dose rate and, as the dose rate decreases due to natural decay, a greater portion of the source is moved to the window to provide and maintain the constant level dose rate.

The present invention relates generally to irradiation devices and more particularly to an irradiation device wherein the radiant energy emanating from a radiation source may be maintained at a constant dose rate over a period of time corresponding to several half-lives of the source by selectively and progressively exposing greater proportions of the source at a window in a shielded housing. This invention was developed in the course of, or under, a contract with the US. Atomic Energy Commission.

Radioisotopes have for some time proven to be invaluable as sources of radiation for use in research and industrial areas such as radiography, chemistry, sterilization or purification of organics and inorganics, preservation of food products, medicine, biological studies, etc.

Of the various types of radiation, perhaps the most wide- 7 1y employed for such uses is gamma radiation due to the highly penetrative nature and intensity of gamma rays. There are presently available several radioisotopes which are capable of providing adequate gamma photon emission to accomplish the desired result. For example, some of the available longer half-life gamma photon emitters which have sufliciently high specific activity (curies per gram) include cobalt 60, cesium 137, and americium 241.

Some shortcomings and drawbacks are inherently present when using gamma radiation emitters since, besides the very strict shielding and handling requirements for this type radiation emitter, the emitter material is constantly undergoing a natural decaying process which renders it virtually impossible to provide uniform dose rates over an extended period of time. For example, with Co having a half-life of approximately 5.2 years, the activity of the source material decreases about 5 percent every 5 or 6 months. Consequently, the source activity is normally recalculated about every 6 months until the activity diminishes to an excessively low level which then necessitates the replacement of the source by an expensive procedure.

Efforts to compensate for the decay of the source material to maintain a somewhat uniform activity over a greater period of time than naturally provided for by the source include the use of a shielding arrangement at the window that may be readily varied in thickness. Thus, as the source decays, less shielding is used to provide a dose rate or flux density similar to that provided Patented Mar. 11, 1969 at some earlier stage of decay. While this arrangement may provide the desired uniform flux densities, it cannot provide a uniform source spectrum in that the energy level of the photons is directly affected by the use of the various shielding thicknesses. In other words, as the source decay progresses and thinner shielding is employed the energy level of the photons reaching the target is less attenuated than when a thicker shield is used even though the flux density of the photons may be substantially the same.

It is the aim of the present invention to overcome or substantially minimize the above and other shortcomings or drawbacks by providing a new and improved irradiation device wherein both the photon energy and flux densities of radiation sources are maintained at essentially constant levels over a period of time which may correspond to as much as several half-lives of the source. Generally, this unique feature is obtained by using a radiation source greater in size by several factors over that normally used in an irradiation device for providing a particular dose rate and then exposing increasingly larger portions of the source at a window as the source decays. Thus, by selectively moving the source material to expose more and more of the source as it decays, the intensity of the photons and the dose rate are maintainable at virtually uniform levels. Further, by selectively positioning predetermined portions of the source, an arrangement is provided whereby the photon emission may be readily increased or decreased for a particular purpose.

An object of the present invention is to provide a new and improved irradiation device.

Another object of the present invention is to provide an irradiation device wherein the radiation activity is maintainable at a virtually constant output over a period of time which may correspond to one or more half lives of the source.

A- still further object of the present invention is to provide an irradiation device wherein the time period between the periodic replacements of radiation source material due to decay is substantially extended.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description. The preferred embodiment illustrated is not intended to be exhaustive or to limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated.

In the accompanying drawing:

FIG. 1 is a sectional view of a preferred form of the novel irradiation device of the present invention together with a mechanism for selectively positioning the source material at the window; and

FIG. 2 is a fragmentary sectional view showing the radiation emitter of the FIG. 1 embodiment in greater detail.

With reference to the drawing, the irradiation device of the present invention is shown comprising a housing 10 within which a radioactive mass may be maintained in a shielded or safe position or moved to an exposed position for permitting the emanation of radiation from the radioactive mass to a point of use. The housing 10 may comprise an envelope 12 of stainless steel or the like filled with a lead mass 14 or any other material exhibiting satisfactory radiation absorption properties. As shown, the housing is formed of two portions or sections 16 and 18, each containing a portion of a throughgoing bore or passageway 20 and which may be connected or joined together by a bolting arrangement such as shown at 22 or by any other suitable coupling mechanism. This passageway may be provided with a liner 24 of a material such as stainless steel or the like to form a suitable conduit for facilitating the movement of a radioactive source-carrying package 26 therethrough, as will be described in greater detail below. As shown, the housing section 18 is provided with a suitable port or window 28 which projects through the housing envelope and shielding as Well as the passageway liner for exposing the passageway and the radiation source contained therein when the latter is selectively positioned in the window opening. The window 28 is preferably of an inwardly converging or coneshaped configuration so as to permit the use of a relatively small opening adjacent the passageway for permitting greater control over the quantity of source material exposed at the window while simultaneously providing for the irradiation of relatively large areas outside of the housing. For example, a window with the side walls thereof inclined at an angle of degrees with respect to the plane of the passageway provides a l20-degree beam port which is satisfactory for the irradiation of large surface areas. Of course, if desired, the side wall inclination may be much steeper than 30 degrees to permit a greater concentration of the radiation beam emanating from the source. The window 28 may be covered with a dust cover 29 which may be formed of aluminum or any other suitable material.

The source-carrying package 26, which is adapted to be moved through the passageway 20, may comprise a tubulation 30 of a diameter slightly less than the inner diameter of the passageway 20 and formed of a material such as stainless steel or the like. The tubulation 30, in turn, houses a source capsule assembly 32 which may comprise an elongated mass 34 of radioactive material encapsulated in a suitable sealed container 35 and disposed in a pocket or bore 36 formed within a metal rod 38. The radioactive mass or source 34 may be sealed within the bore 36 by placing and securing a suitable end cap 40 in the open end of the bore 36. The source-containing metal rod 38 may be rigidly supported in the tubulation in a plane parallel to the plane of the passageway by placing a plug 42 of a material such as lead or the like within the tubulation and securing the rod 38 to the plug in any suitable manner such as, for example, a thread-type coupling such as shown.

The source capsule assembly 32 preferably occupies only a minor portion of the tubulation 30 so as to permit the use of shielding in the passageway 20 at both ends of the source for preventing the escape of radiation through the ends of the passageway. This shielding preferably comprises a pair of elongated lead blocks or bodies 44 and 46 disposed within the tubulation 30 as shown. The tubulation 30 may then be sealed with a pair of metal end caps 48 and 50 which may be pressed-fit, brazed, or otherwise secured to the tubulation 30. This arrangement assures that the passageway except for the window communicating therewith does not provide an escape path for radiation.

In order to move the source package 26 through the passageway 20 to expose the source 34 at the window or to move it into a safe position within the housing section 16, the source package may be secured to a plunger or piston rod 52 of a conventional double-acting pneumatic piston and cylinder assembly 54. This assembly includes a cylinder 56 containing a piston 58 coupled to the piston rod 52 and is connected to a suitable source of compressed air (not shown) through lines 60 and 62 which may be provided with suitable control valves 64 and 66, respectively. The stroke of the piston 58 should be sufficiently great so as to reciprocally move the entire source capsule assembly from the safe position within housing section 16 through the passageway to a location in housing section 16 where the source is in registry with the window 28.

In the event the housing 10 is oriented in such a manner that the passageway 20 is vertically oriented, the source package 26 may be moved through the passageway by utilizing a stream of compressed air and gravitational forces. In this arrangement the source package may serve as a piston against which a stream of compressed air may be directed to move the source towards the window. When the need for the radiation is completed, the source may be returned to a safe position in housing section 16 by terminating the flow of compressed air so as to allow gravitational forces to move the source package back through the passageway.

Suitable guides 68 and 70 of nylon or the like may be placed at opposite ends of the tubulation 30 to facilitate the movement of the source package through the passageway. Also, a switch 71 may be placed in registry with the passageway for providing a signal indicative of the position of the source package in the passageway 20 with respect to safe and possibly unsafe positions.

As briefly mentioned above, the source 34 is positioned at the window 28 in such a manner that the radiation emanating from the source through the window is maintainable at a constant level over a period of time which may be greater than the half-life of the source and, in any event, greater than the duration naturally provided by the source. This feature is achieved by using a source 34 of substantially greater curiage (preferably at least by a factor of two or more) than required for the desired activity or dose rate and selectively positioning increasingly greater lengths of the source at the window 28 as the source decays. The preferred configuration of the source 34 which is capable of providing the best results is in the form of an elongated rod as shown, since this configuration permits greater control over the quantity of source material exposed at the window.

The selective positioning of the source 34 at the window 28 to maintain the desired dose rate may be accomplished by placing an adjustable stop assembly 72 at the end of the passageway 20 remote to the pneumatic assembly 54 or the compressed air supply. This stop assembly may comprise a simple, manually adjustable screw 74 extending into the passageway for abutting against the end of the source package 26 when it is propelled through the passageway by the pneumatic assembly or the flow of compressed air against the source package. Thus, when a particular quantity of radiation is desired, the pneumatic assembly may be actuated or the flow of compressed air initiated to provide a constant bias against the source package for continually urging the latter against the end of the screw 74 and then selectively adjusting the screw to obtain and maintain the desired dose rate. Of course, if desired, the manual adjustment may be replaced by an automatic source-adjusting mechanism for maintaining the desired radiation output. For example, this mechanism may comprise a scintillation counter disposed at a spaced location from the window opening for providing a signal to a suitable control circuit capable of actuating an electric motor, servo, or other drive means for moving an adjustable device to control the exposure of the source 74 at the window 28 in response to radiation intensity.

In a typical operation with a cobalt 60 source increased in size by a factor of four, the useful life of the source is increased to about twenty years and by controlling the exposure thereof a dose rate corresponding to that of a one-curie source of cobalt 60 may be maintained at a constant level for a period of about ten years.

It will be seen that the present invention sets forth a unique irradiation device whereby substantial monetary savings may be realized due to substantially extending the time period between the expensive source replacements required by the previous irradiation devices. Further, the source in the present device is controlled in such a manner that the radiation emanating therefrom through the window is rnaintainable at a virtually constant level over a period of time substantially greater than that previously afforded. The use of the two-piece housing also affords another advantage in that the window-carrying section may be readily removed for facilitating maintenance work or other tasks without subjecting the technician to radiation.

As various changes may be made in the form, construction, and arrangement of the parts herein without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An irradiation device for providing a constant level of radiation over a period of time greater than the duration naturally afforded by a radiation source useable in the device, comprising source housing means containing radiation absorbing material, a passageway within said housing means and encircled by said material, said housing means having an aperture therein in registry with said passageway and projecting to an outer surface thereof, elongated means disposed in said passageway for carrying a radiation source of a greater curiage than that required for said constant level of radiation emission, means for moving the elongated means through said passageway, and adjustable means for interrupting the movement of the elongated means to position predetermined portions of said radiation source at a location in said passageway in registry with said aperture.

2. The irradiation device claimed in claim 1, wherein said radiation source when in said passageway is of an elongated configuration, the greater dimension of the elongated source is disposed in a plane substantially parallel with the axis of the passageway, and wherein increasingly greater portions of said greater dimension of the source are positioned at said location by actuating the adjustable means for maintaining said constant level of radiation emission through said aperture as the over-all source activit decreases due to decay.

3. The irradiation device claimed in claim 2, wherein the elongated means comprises a tubulation, the elongated radiation source is disposed in said tubulation at a location intermediate the ends thereof, radiation absorbing material is disposed in said tubulation between said source and each end of the tubulation, and wherein support means are disposed in said tubulation for maintaining said radiation source in said plane.

4. The irradiation device claimed in claim 3, wherein the means for moving the elongated means includes an elongated structure connected to one end of the tubulation for reciprocally moving the elongated means through said passageway.

5. The irradiation device claimed in claim 1, wherein said adjustable means comprises a selectively movable abutment disposed adjacent one end of and in axial alignment with said passageway, and wherein said abutment is adapted to bear against said elongated means for preventing said moving means from positioning more than said predetermined portions at said location.

References Cited UNITED STATES PATENTS 2,872,587 2/1959 Stein 250-106 2,936,378 5/1960 Jensen 250-108 3,048,701 8/1962 Thomas 250-106 ARCHIE R. BORCHELT, Primary Examiner.

US. Cl. X.R. 250-108

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