US3017514A

US3017514A - Radioactive isotope combination

Info

Publication number: US3017514A
Application number: US563295A
Authority: US
Inventors: Robert B Costello
Original assignee: MW Kellogg Co
Current assignee: MW Kellogg Co
Priority date: 1956-02-03
Filing date: 1956-02-03
Publication date: 1962-01-16
Anticipated expiration: 1979-01-16

Description

Jan. 16, 1962 Filed Feb. 3, 1956 FIG.I

2 Sheets-Sheet .1

| I I I I Q IRIDIUM ANTIIriNV SfigINDIUM |92\ TANTALUM 60 U- m I O E 40 -d) in: I 3 A NTIMONY V) o .2 4 s ,5 L0 1,2 L4 L6 L8 2.0 MEV TRADIATION ENERGY (MILLION ELECTRON voLTs) F'IG.2

T NTALUM SCANDIU ANTIMO IRIDIUM ANTIMONY a '4 Lo 13 |,4 L0 L6 2.0 MEV TRADIATION ENERGY (MILLION ELECTRON VOLTS) INVENTOR ROBERT B. COSTELLO ATTORNEYS v I 3,017,514 RADIOACTIVE ISOTOPE COMBINATION Robert B. Costello, Railway, N.J., assignorto The W. Kellogg Company, Jersey City, N.J., a corporation of Delaware Filed Feb. 3, 1956, Ser. No. 563,295 Claims. (Cl. 250106) This invention relates to radiators formed of radioactive isotopes and more particularly 'to novel radiators for use in radiography, medicine, indicating and control instruments, the chemical and processing arts, etc., which comprise combinations of two or more radioactive isotopes.

The radioactive isotopes found in nature have always been in very limited supply and very expensive. Consequently, their use has always been severely restricted. The availability and price of the natural radioactive isotopes have always been such that industry and the arts found it cheaper and more expedient to produce the effects producible by the emanations from the natural radioactive isotopes by means of electrical, X-ray equipment. Initially the requirements'of industry and the arts were satisfied by X-ray equipment of comparatively low energy capacity; such X-ray equipment was small, easily portable and manipulatable, and comparatively cheap. However, as the energy requirements increased, the X- ray equipment rapidly increased in size and cost so that at present even at energy discharge capacities which are considered comparatively modest, the X-ray installation is very expensive and is so large in size to be in a practical sense immovable.

The use of radioactive isotopes as radiators in various applications has increased enormously since the advent of the nuclear reactor as with such a reactor it is possible to produce a large number of radioactive isotopes not previously available and of a wide range of properties and characteristics. These reactor isotopes can be produced with total energy contents undreamed of in connection with the natural radioactive isotopes at costs that are only a very small fraction of the cost of the natural radioactive isotopes. The radiators produced in the atomic reactor are of small bulk. Even when their energy content is rated at thousands of curies their volume is only in the order of a fraction of a cubic inch. While some of the isotopes produced in the nuclear reactor have very short half lives and find extensive use in medicine and as tracers, many are available having sufficient length of half lives to render their use commercially economical for radiography and similar applications.

The main problem involved in the transportation and use of these reactor produced radioactive isotopes is that of proper shielding. Even with radiators of the high energy rating above mentioned, containers weighing only a few hundred pounds have been designated by the Atomic Energy Commission as satisfactory for transport of the radioactive radiators in common carriers. Projectors for the use of these radiators which meet the requirements of the Atomic Energy Commission are available which range in weight from one that can be easily carried by a man, in the case of radiators of small curie rating, to only a few hundred pounds and generally less than 1000 pounds in the case of radiators of large curie rating. The transport containers and use projectors are generally of very simple, endurable construction so that there is essentially no repair or maintenance involved in their prolonged use. For these considerations these newly available radioactive isotopes have displaced X-ray equipment in many fields especially where high energy levels are required coupled with mobility and versatility.

The results produced when the radioactive isotopes are 3,017,514 Patented Jan. 16, 1'95;

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employed are generally comparable to those produced by X-ray apparatus and are satisfactory and acceptable. Thus, for instance in metal radiography, the radiographs produced by means of the radioactive isotopes are accepted and approved by the various societies 'and committees that set up radiographic examination procedures. However, the quality of the results produced by X-rays is in some instances at least, superior to that of the results produced by the radioactive isotopes. This is particularly true when the isotopes are employed in connection with chemical processes involving irradiation and w in metal radiography. In the chemical processes, X-rays produce more uniform results, while in radiography, they produce radiograms of superior contrast.

The X-rays, emitted by the electrical X-ray machines, and the gamma rays, emitted by the radioactive isotopes, are not radically different as they are both electro-rnagnetic radiations; actually they cover overlapping bands of wave lengths with the isotopes providing the shorter or higher energy waves, or particles in the case of quantum theory, and the X-rays, the longer and lower energy waves, or particles. There is one important difierence however. Each radioactive isotope emits energy at a single potential, measured as electron volts, or at a plurality of different more or less widely spaced potentials. In other words, the radioactive isotopes emit radiation of a single, specific wavelength or a plurality of relatively widely spaced specific wave lengths While X-ray apparatus on the other hand, emits energy over a very wide and substantially continuous wave band. It for a moment we were to consider the X-rays and the gamma rays as visible light it could be stated that the X-ray machine is an emitter of panchromatic light, whereas the radioactive isotopes are emitters of monochromatic light. This difference is important and partially explains the superior quality results obtained by the use of X-rays in the apirradiated by a high energy source of electromagnetic radiation such as an X-ray machine, a certain proportion of the radiation will be absorbed by the material of the plate as this certain proportion of radiation does not possess sufficient energy to penetrate the full thickness of the plate. However, if the plate density is not constant throughout by reason of slag inclusions, gaspockets, etc., less than said certain proportion of the radiation will be absorbed. This non-absorbed radiation will continue through the plate to expose the film to a greater degree so as to create an image of the defect. In the scattering process we suppose radiation at a sufficiently high energy level to readily penetrate the whole thickness of the plate. However, this energy in going through the plate has some of the particles or waves therefrom scattered away from the original path due to collision and other effects with the atoms constituting the plate material. The thus scattered or deflected energy penetrates the plate and exposes the film, If the density of the plate is not uniform by reason of slag inclusions, gas pockets, etc., the scattering will be diminished where these defects occur and a greater percentage of the energy will go unobstructed to the film and cause an enhanced exposure of the film in the area of the defect. With a source of emanation-s such as an X-ray machine and by reason of the substantially continuous wave band of its radiation, both of these physical processes are always in evidence. In the case of radioactive isotopes however, with only a limited number of spaced energy levels of radiation, one or the other of these processes and their efiects is diminished. For example the radioactive isotope cobalt 60 has only two energy levels of radiation but both are of sufficiently high level to pass through many inches of steel. With this isotope the effect of absorption is virtually lost for plate thicknesses up to as great as 1 /2 inches. The radioactive isotope iridium 192 on the other hand, has about 12 energy levels of radiation, all are low as compared with those of cobalt 60. Iridium 192 produces radiographs in plates of a thickness in the order of 1 inch which are highly satisfactory. The reason being that with plates of a thickness up to that stated both physical processes, that is absorption and scattering, are effective. However, if the plate thickness is increased, that is to the order of 2 inches, virtually all of the radiation is absorbed and a satisfactory radiograph is not possible. There is no presently known radioactive isotope which will produce radiograms with thick plates comparable to those which iridium 192 will produce with thin plates.

It is a principal object of this invention .to provide a radiator of electro magnetic energy composed of two or more radioactive isotopes which emits a plurality of radiations spaced stepwise along the whole range encompassed by the radiations of the component isotopes.

It is a further principal object of the invention toprovide a radiator of electro magnetic energy composed of two or more radioactive isotopes which emits radiations of progressively higher energy intensity stepwise between the lowest intensity radiation and the highest intensity radiation of the component isotopes.

It is a further principal object of this invention to provide a radiator of electro magnetic energy composed of two or more radioactive isotopes which emits a plurality of radiations spaced stepwise along the whole range encompassed by the radiations of the component isotopes, the total energy content of each of said spaced radiations being such as to provide a predetermined energy distribution over said range.

It is a further important object of this invention to provide a radiator of electro magnetic energy composed of two or more radioactive isotopes which emits a plurality of spaced radiations of progressively increasing intensity along the whole radiation range encompassed by the component isotopes, said component isotopes being of such half lives that the energy variations in the emission of said radiator-due to the decomposition of the component isotopes thereof goes forward in a predetermined manner.

The further objects, features, and advantages of this invention will become apparent from a consideration of the following description taken with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation illustrating the gamma radiation energy distribution of a composite radiator in accordance with my invention; and

FIGS. 2, 3, and 4 show plots similar to that of FIG.

1 illustrating the results that may be obtained by adjusting the proportions of the energy content of the component radioactive isotopes of the composite radiator.

The electro magnetic wave radiators of the invention are of general application and may be used in medicine, indicating and control instruments, examination instruments, the chemical and processing arts and elsewhere, including those applications wherein X-ray apparatus and which includes a plurality of radiations of progressively greater intensity more or less evenly spaced along the full range of the radiation band encompassed by the radiations of the constituent radioactive isotopes. The radioactive isotopes chosen may be in the form of the elemental substances or they may be in the form of chemical compounds thereof, the isotopes furthermore, may

be in the form of alloys or other elemental mixtures or quasi compounds, The radioactive isotopes making up the novel radiator when in the elemental state may be in the form of separate cylindrical or disc-like bodies, or they may be in the form of particles of irregular form and size. In the case of chemical compounds, only one or both radicals of the compounds may be radioactive.

By reason of the large number of reactor produced radio-active isotopes now possible, the number exceeds a thousand, the radiation range of the radiators of the invention may be as wide or as narrow as desired and required by the service. Thus the radiation range may lie between .01 mev. and 4.59 mev. (million electron volts) or within any desired or required limits within the mev. range just set forth. The more usual useful range is from .07 to 2.0 mev. With specific reference to radiography, the general useful range is from .07 to 2.0 mev. For instance, with light gauge ferrous metal, one-half inch or less in thickness, a range of from .07 to 0.3 mev. is satisfactory while with the same material of a thickness of from one-half to one inch, a range of from 0.2 to 0.6 mev. is preferred and with the same ferrous material of a thickness of from one to one and one-half inches 0.3 to 0.8 mev. is suitable, etc. For extremely thick material 2.0 mev. may be the top of the range. In other uses the energy intensity range which the novel radiator is designed for will be determined by the necessities of the use. The present upper end of the useful range for metal radiography is 2.0 mev. This radiation is emitted by the radioactive isotope antimony 124.

The spacing between emanations or energy levels, of the novel radiators along the chosen range should be more or less regular within the limitations of the radioactive isotopes available as satisfactory for the particular intended use. It is not possible to obtain an exact constant spacing between emanations other than accidentally in exceptional cases, consequently a reasonable approximation must be made. For this end the emanation range is broken up into a number of equal portions or bands and the isotopes are chosen such that there is one or more radiations in each of these bands. Thus if We suppose the range to be from 0.1 to 2.0 mev., this range may be subdivided into a plurality of bands of a width of 0.2 mev. The isotopes are chosen such that one or more emanations thereof occur in each of of the 0.2 mev. bands. Thus a spacing of emanations is obtained which gives a sufficiently uniform energy distribution for most purposes. Of course, when specific wave lengths are required, isotopes will be chosen having the proper emanations to satisfy the requirements.

The half life of the isotope is an important factor in determining the one chosen for the novel radiator. Usually, the isotopes chosen for a particular radiator will have approximately the same half lives so that in use the properties of the radiator will remain substantially constant during decay. However, it is possible to provide a radiator made up of isotopes of radically different half lives so that the properties of the radiator will change in a predetermined manner during decay. It is also necessary to consider the total energy content of the isotopes chosen, as by a proper choice of the energy content of each component radioactive isotope, a predetermined ideal energy curve may be approximated.

In further explanation of the invention, reference is had to the novel composite radiator composed of the plurality of radioactive isotopes as set forth below which was developed as an all purpose radiator for metal radiography and particularly for thick steel. This novel composite radiator produced radiograms with steel of a thickness in the range of A" to 6" of a quality closely approximating that of radiographs produced by 2 mev. X-ray machines. To achieve these stated ends it was determined that the radioactive isotope mixture should exhibit radiation over a radiation band extending from 0.25 mev. to 2.0 mev., the total quantity of the radiation emitted by the radiator should peak at 1.1 mev. and the tion level are tabulated as follows.

radioactive isotopes chosen to constitute the radiator should have about the same half life so that the plot of radiation intensity versus total energy would retain its characteristic form throughout at least one half life of the radioactive isotope mixture. The requirements set forth above were found to be satisfied by a radiator made up of the following radioactive isotopes:

Iridium 192 Half life 74 days. Antimony 124 Half life 60 days. 'Scandium 46 Half life 85 days. Tantalum 182 Half life 111 days.

The various gamma rays, in mev., given off by each of the chosen radioactive isotopes and the approximate percentage of the total quantity of radiation at each radia- (The percentage values given are approximate values and are based on the best information available at this time.)

Energy Percentage of Total (mev.)

Radiation Iridium 192 ooooooo machete-coco HOKDMOEHO Antimony 124 scandium 4s Most intense.

}Percentages have not been determined.

Tantalum 182 FIG. 1 is a graphic representation of the novel radiator set forth above illustrating the relation of 7 radiation quantity (in arbitrary units) versus 7 radiation energy (expressed in million electron volts) for a mixture of equal curie strengths of IR 192, Sb 124, Sc 46, and Ta 182, this mixture obviously does not peak at 1.1 mev. FIG. 1 is particularly useful in illustrating radiation distribution along the radiation spectrum of the novel composite radiator. It is to be noted that '7 radiation energy expressed in million electron volts, is broken up into a plurality of bands of uniform widths as it is convenient from the point of view of graphically illustrating the principles underlying this invention to break up said spectrum mixture into radiation band widths. This is purely a device to illustrate the principles and it has nothing whatsoever to do with reality, because the isotopes do not radiate continuously over the particular band width which they happen to be in but give off one or more specific wave lengths of gamma radiation in said band width as explained heretofore.

From the above listing of the radioactive isotopes chosen it can be seen why it is convenient to break down the radiation into band widths rather than to treat each discrete energy level separately. To this end the radiation spectrum has been broken down into band widths of 0.3 mev., starting with 0.25 mev. and concluding with 2.05 mev. At the band width from 0.25 mev. to 0.55 mev. we find that practically 100% of the radiation from iridium 192 is found since the 4.0% at 0.23 mev. is practically at the low limit of the band. From 0.5 mev. to 0 .85 mev., 58% of the radiation of antimony 124 is found, from 0.85 mev. to 1.15 mev. the radiation of 100% of scandium 46 is found, from 1.15 to 1.45 mev. the radiation of 100% of tantalum 182 is found, from 1.45 mev. to 1.75 mev. the radiation of 38% of antimony 124 is found, and in the band width from 1.75 mev. to 2.06 mev., 4% of the radiation of antimony 124 is found. Thus while the discrete radiations are not spaced apart 6 in an exact and regular manner the bands in which they occur-are uniformly spaced apart.

The radiator graph of FIG. 2 is composed of the same radioactive isotopes as that of FIG. 1 but the curie strength of the component isotopes have been chosen so as to obtain a quantity of radiation peaking at 1.1 mev. Since the peak is at approximately the middle of the radiation spectrum of the composite radiator, the quality radiation curve, designated as curve 1, conveniently approximates a parabola in shape similar to the spectrum obtained from an X-ray machine. The composite radiator of FIG. 2 is a mixture of 12% Ir 192, 39% Sb 124, 25% So 46 and 24% Ta 182 (the percentage is based on the total curie strength. It can be seen from FIG. 2 that there are limitations as to what can be done by mixing the various isotopes and that it is impossible in all cases to adjust the various quantities so as to conform to any curve whatsoever. However, as shown, a mixture of radioactive isotopes has been created which approximates the chosen ideal desired curve. FIG. 2 represents a band width spectrum of a radiator which does not exist in nature. Radiographs have been taken through steel plate of various thickness ranging from 1" to 4" cmploying the radiator of FIG. 2; the radiograms thus proare again based on the total curie strength). Here wecan see that the band width spectrum more nearly conforms to the ideal curve than that of FIG. 2. FIG. 4 shows the adjustment in percentage to obtain a radiator peaking at 1.3 mev. and with a quantity of radiation curve approximating the ideal curve designated as curve 3. The radiator of FIG. 4 is composed of 11.4% of Ir 192, 37.2% of Sb 124, 24% of Sc 46 and 26.8% of Ta 182 (the percentages are based on total curie strength). When referring to percentage of the various isotopes making up a particular composite radiator it is not intended to refer to weight percent or volume percent but rather to radiation percent or percent of the total curie strength that is supplied by the particular isotope in question. The actual physical quantities of the materials making up the isotope mixtures will vary widely from the radiation percent indicated and cannot be actually predicted unless further information is provided. This is because each different isotope presents a different cross-sectional area to the activating neutron flux in the pile, and the particular pile used to make the isotope differs from all others in the density of its neutron flux. Moreover, within the pile itself the density varies.

As can readily be seen with the same group of isotopes there are numerous possibilities of providing composite radiators whose quantity of radiation curves approach to a greater or lesser degree forms of ideal curves that may be arbitrarily drawn. By approach to the ideal curve is meant that as illustrated the band widths indicate a plurality of bars or steps whose upper ends approach or are intersected by the ideal curve and not a smooth flowing line. Further illustrating the invention, in producing the radiator illustrated in FIG. 3 with a total curie strength of 1 0 curies, 1.2 curies are supplied by the iridium 192, 3.9 curies are supplied by the antimony 124, 2.5 curies are supplied by scandium 46 and 2.4 curies are supplied by tantalum 182. This material was all received during the same week and was all in the form of metal pellets and small aluminum cylinders containing isotopes in granular form. The radioactive isotope material could have been provided in the form of metal powder and mixed together before encapsulation. When discs are employed it is generally preferable to encapsule them in such a manner that 7 the material having thegreater density or greater absorption coefiicient is furthest from the end of the capsule which in use is directed at the object to be irradiated. It may in some cases be advisable to readjust the percentages to allow for absorption by the other isotopes of the mixture so that the resulting emenging radiation spectrum can approach the ideal spectrum as closely as possible.

All three of the composite radiators of FIGS. 2, 3, and 4 produced radiographs of X-ray quality, that of FIG. 2 was designed for thickness of steel ranging from 2" to 4", that of FIG. 3 from 1" to 2", while that of FIG. 4 from 4" to 7".

As indicated heretofore iridium 192 produces satisfactory results in radiography with materials of a medium thickness, that is, up to about 1 /2 thickness of steel. However, the half life of iridium 192 is only 74 days. Difiiculty is experienced by reason of the increasing length of exposure required in the use of iridium as a source in radiography for any extended length of time. In accordance with the teachings of this invention a composite radiator comparable to iridium 192 is produced having a relatively extremely long half life so that it may be used with only minor changes in exposure factors for long periods. The long life iridium 192 type composite radiator is made up of:

While this combination of radioactive isotopes does not have all of the specific radiations of iridium 192 it has substantially the same characteristics since if the spectrum of iridium 192 is broken up into bands of a width of 0.2 mev. and beginning at 0.15 mev. on the radiation spectrum it will be found that approximately 84% of the total quantity of radiation is emitted in the band ranging from 0.15 to 0.35 mev., about 14% in the band ranging from 0.35 to 0.55 mev., and about 2% in the band ranging from 0.55 to 0.75 mev. In the composite equivalent radiator the barium 133 supplies 84% of the total radiation energy at 0.32 mev., krypton 85 supplies 14% of the total 7 radiation at 0.51 mev., and cesium 137 will supply 2% of the total 7 radiation at 0.66 mev. It will be noted that the characteristics of this composite radiator will remain substantially unchanged for a period of almost years. This novel composite radiator with a 10 curie rating contains 8.4 curies of barium 133, 1.4 curies of krypton 85 and 0.2 curies of cesium 137. In connection with the krypton, normal krypton gas in an amount to provide the 1.4 curies of krypton 85 under predetermined irradiation conditions will be sealed in a suitable steel or other suitable metal containers and the container placed in the nuclear pile for irradiation. This novel composite source produces radiographs indistinguishable from those produced when iridium 192 alone is employed.

Cobalt 6Qvwhich emits gamma radiations at 1.1 and 1.2 mev., is capable of penetrating metal of extreme thickness but it does not produce radiographs of high quality. A composite radiator in accordance with the invention having the penetration capacity of cobalt 60 but which produces radiograms of a quality comparable to that of X-rays can be made up of:

For a composite radiator of. 10 curie rating the radio.- active isotopes are chosen so that yttrium 88 provides 1.2 curies, sodium 22 provides 1.8 curies and cesium 134 provides 7 curies. This composite radiator will thus be an iridium type source in the cobalt 60 range and will have radiation spread along the spectrum on both sides of the specific emanations of cobalt 60. The quality of the radiographs produced by this composite radiator is superior to that of cobalt 60 and is of the same order'as those obtained 'by the use of X-rays.

The use of the invention in connection with chemical compounds is exemplified-by sodium bromide which is derived by conventional reaction between sodium 24 and bromine 82. Sodium 24 and bromine 82 emit both ,8 and *y radiations. Sodium 24 Whose half life-is 15 hours, has B emanations of 1.39 and 4.2 mev. and-'y emanations at 1.4, 2.7 and 3.7 mev. Bromine 82 whose half life is 36 hours has a 3 radiation of 0.5 mev.; its 7 radiations occur at 0.55, 0.61, 0.69 and 0.76 mev. This radioactive chemical compound thus has a wide spectrum which ranges from a region of comparatively low energy to a region of extremely high energy and is consequently adapted for wide usage as an irradiator, a tracer element, etc.

Although many changes can be made by those skilled in the art without departing from the scope of the invention, it is intended that all matter contained in the above description and appended claims and shown in the accompanying drawings shall be interpreted as illustrative and not limitative.

I claim:

1. A source of electro magnetic radiations consisting essentially of a plurality of radioactive isotopes, said source emitting radiations ofprogressively higher energy intensity stepwise along the composite radiation spectrum of the constituent radioactive isotopes thereof, said constituent isotopes having half lives in the same order of duration.

2. A source of electro magnetic radiations consisting essentially of a plurality of radioactive isotopes, said source having at least one of its specific radiations of major energy content distributed in each band of a plurality of bands of uniform Width subdividing the full length of the radiation spectrum thereof, said bands sufficient in number that said specific radiations vary in intensity progressively in stepwise fashion from one end to the other of said spectrum.

3. The electro magnetic radiation source defined in claim 2 in which, the component radio isotopes have half lives of the same general order of duration and each such that the decomposition of said source during at least onehalf life of said sources does not essentially alter the properties of said source.

4. A source of electro magnetic radiations consisting essentially of a plurality of radioactive isotopes, said source having its specific radiations distributed in bands of uniform width subdividing in full length of the radiation spectrum thereof, and sufficient in number to secure quasi-uniform distribution of said radiations along said spectrum, the total energy content of each isotope of said plurality of isotopes adjusted to provide a predetermined total energy distribution pattern along said spectrum.

5. A source of electro magnetic radiations as defined in claim 4 in which, the total energy content of each isotope of said plurality of isotopes is adjusted to provide a total energy distribution pattern which approaches a curve having a peak intermediate its ends.

6. A source of electro magnetic radiations consisting essentially of a radioactive isotope having at least one specific radiation of intensity sufficient for the required service and a second radioactive isotope having specific radiation of intensity approximating that of said at least one specific radiation to provide a plurality of spaced specific radiations which approximate in effect a band of non-specific radiations covering the same portion of the electro magnetic radiation spectrum.

7. A source of electro magnetic radiations consisting essentially of a plurality of radioactive isotopes having half lives of the same order of duration, said plurality of radioactive isotopes emitting a plurality of specific radiations distributed stepwise along a continuous portion of the electro magnetic radiation spectrum, said specific radiations of not substantially less energy than 0.01 mev. and not substantially more energy than 4.59 mev.

8. A source of electro magnetic radiations as defined in claim 7 in which, said distributed specific radiations range in energy level from about 0.07 mev. to about 2.0 mev. and are spaced sufficiently closeto approximate in effect a band of nonspecific radiation covering the same portion of the electro magnetic radiation spectrum.

9. A source of electro magnetic radiations consisting essentially of the radioactive isotopes iridium 192, antimony 124, scandium 46 and tantalum 182.

10. A source of electro magnetic radiations as defined in claim 9 in which, said constituent isotopes are included with total energy contents relatively proportioned to provide a total energy distribution which peaks intermediate the ends of the radiation spectrum of said source.

11. A source of electro magnetic radiations consisting essentially of the radioactive isotopes krypton 85, barium 133 and cesium 137, said radioactive isotopes included with total energy contents relatively proportioned to provide a major portion of the total energy content of the radiations of said source in the radiation range of barium 12. A source of electro magnetic radiations consisting essentially of the radioactive isotopes yttrium 88, sodium 22 and cesium 134, said radioactive isotopes included with total energy contents relatively proportioned to provide a major portion of the total energy content of the radiations of said source in the radiation range of cesium 134.

13. The method of irradiating material which comprises, positioning the material to be irradiated in an irradiation zone and subjecting said material to a plurality of specific radiations distributed stepwise along a continuous portion of the electromagnetic spectrum, said specific radiations sufficrently closely spaced to approximate a band of non-specific radiations covering the same portion of said spectrum and emitted by a source consisting essentially of a plurality of radioactive isotopes.

14. In the method of radiographically examining a metal body in which a sensitive element is exposed to the electromagnetic radiations passing through said metal body, the step comprising, interposing the metal body to be examined between a source of electromagnetic radiations and an element sensitive to said radiations, said source consisting essentially of a plurality of radioactive isotopes and emitting a plurality of specific radiations distributed stepwise along a continuous portion of the electromagnetic spectrum, said specific radiations sufficiently closely spaced to approximate in effect a band of non-specific radiations covering said portion of said spectrum, said portion of said spectrum including such energy level range that at least one of said specific radiations can easily penetrate the full thickness of said metal body, the energy level range of said portion of said spectrum beginning at not substantially less than 0.07 mev. and ending at not substantially more than 2.0 mev.

15. The method of irradiating material as defined in claim 13 in which, said specific radiations are emitted by a source consisting essentially of sodium 124 and bromine 82.

References Cited in the file of this patent UNITED STATES PATENTS 2,412,174 Rhoades Dec. 3, 1946 2,831,122 Brucer Apr. 15, 1958 2,847,581 Clark Aug. 12, 1958 OTHER REFERENCES Radioisotopes in Industry, edited by I. R. Bradford, Reinhold Pub. Corp., copyright 1953, chapters 2 and 13, pp. 13 to 27; 259 to 283.

Beta-Ray-Excited Low-Energy X-Ray Sources, by L.

Reiffel, from Nucleonics, March 1955, pp. 22-24.

Claims

1. A SOURCE OF ELECTRO MAGNETIC RADIATIONS CONSISTING ESSENTIALLY OF A PLURALITY OF RADIOACTIVE ISOTOPES, SAID SOURCE EMITTING RADIATIONS OF PROGRESSIVELY HIGHER ENERGY INTENSITY STEPWISE ALONG THE COMPOSITE RADIATION SPECTRUM OF THE CONSTITUENT RADIOACTIVE ISOTOPES THEREOF, SAID CONSTITUENT ISOTOPES HAVING HALF LIVES IN THE SAME ORDER OF DURATION.