US2803766A

US2803766A - Radiation sources in charged particle accelerators

Info

Publication number: US2803766A
Application number: US312260A
Authority: US
Inventors: Malcolm H Hebb
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1952-09-30
Filing date: 1952-09-30
Publication date: 1957-08-20
Anticipated expiration: 1974-08-20

Description

M. H. HEBB Aug. 20, 1 957 RADIATION SOURCES IN CHARGED PARTICLE ACCELERATORS Filed Sept. 50, 1952 Inventor": Malcolm 1'1. Hebb,

His AttOTneH.

United States Patent RADIATIQN SOURCES IN CHARGED PARTICLE ACCELERATORS Malcolm H. Hebb, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application September 30, 1952, Serial No. 312,260 Claims. (Cl. 31362) The present invention relates to charged particle accelerator apparatus and, more particularly, to radiation sources in charged particle accelerator apparatus.

Apparatus for accelerating charged particles by means of magnetic induction effects is .shown and described in United States Patent Nos. 2,394,071, 2,394,072 and 2,394,073, all of which were patented February 5, 1946 by Willem F. Westendorp and assigned to the assignee of the present invention. Such apparatus can comprise a core of magnetic material including a pair of opposed, rotationally symmetrical pole pieces which define a toroidal gap wherein an evacuated container is positioned. The core is excited by means of windings that are energized by a source of time-varying voltage to produce a time-varying magnetic fiux which links an equilibrium orbit within the evacuated container and a time-varying magnetic guide field which traverses the equilibrium orbit. Charged particles, e. g. electrons injectedalong the equilibrium orbit from'an electron gun positioned adjacent to the orbit within the region of influence of the time-varying magnetic guide field, are accelerated to high energy levels,

by the time-varying magnetic flux during a great number of revolutions while the time-varying magnetic guide field constrains the particles to follow paths along the equilibrium orbit. After acceleration to a desired energy level, the charged particles can be diverted from the equilibrium orbit to a target for the generation of X-radiation.

A major problem in the utilization of magnetic induction accelerator apparatus and other forms of accelerator apparatus employing a time-varying magnetic guide field is that of obtaining so-called thick target X-radiation. Thick target radiation is that obtained from a target having a thickness in the direction of travel of impinging particles sufficientto stop substantially all ofthe particles and thus to convert a maximum fraction of the particle energy into radiation.- In diverting the charged particles from theequilibrium orbit of accelerator apparatus of the above-described forms, the particles are caused to spiralawayfrom the equilibrium orbit until their paths intersect the target. Since these paths are necessarily slow spirals, i. e. of small pitch, the particles succeed only in striking the leading edge of a thick target; whereby many particles are lost by scattering through the edges of the target without having a maximum fraction of their energy converted to X'radiation as desired. I

it istherefore a principal object of the present inventiontoprovide an efiicient means of obtaining thick target radiation from charged-particle accelerator apparatus of the forms described.

According to one aspect ofthe invention, a thin target, i. -e. a target whichcharged particlescan traverse without an appreciable loss in energy, is positioned adjacent to the equilibrium orbit in azimuthally spaced relationship with respect to a thick: target in charged particle accelerator apparatus. Bothof the targets are located on the same side of, the equilibrium orbit. The relative positions of thetwo targets permit the particles diverted fromthe 2,803,766 Patented Aug. 20, 1957 ICQ equilibrium orbit first to strike and be scattered by the thin target and subsequently to strike the thick target well within its edge, whereby thick target radiation is efii ciently generated.

The features of the invention desired to be protected herein are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the fol- Fig. 3 is a section view taken along lines 3--3 of Fig. 2; and

Fig. 4 is a section view taken along lines 44 of Fig. 2 Referring particularly now to Fig. 1, there is shown in exemplary fashion magnetic induction accelerator apparatus suitably embodying the invention. The apparatus comprises a magnetic core 1 which can be laminated to minimize the generation of eddy currents therein. Core 1 includes laminated, rotationally symmetrical, opposed pole pieces Zland 3 having generally outwardly tapered pole faces 4 and 5 for the provision of a magnetic guide field traversing an equilibrium orbit O, as will be more fully described hereinafter. Coaxial with pole pieces 2,

3 and disposed between pole faces 4, 5 is an evacuated annular container or envelope 6 of dielectric material,

which provides within its interior an annular chamber 7 wherein charged particles can be accelerated. The central portions of pole pieces 2, 3 are terminated respectively by fiat, surfaces 8, 9 between which are disposed laminated metallic disks (not shown) and dielectric support spacers 1t 11. The metallic disks serve the purpose of reducing the reluctance of the magnetic path in the region between surfaces 8 and 9.

Magnetic core 1 can be excited from a suitable source of time-varying voltage 12 connected as: indicated to series-connected energizing windings 14, 1S surrounding pole pieces 2, 3. To minimizethe current drawn from source 12, energizing

windings

14 and 15 can be resonated by poWer-factoncorrecting capacitors 16. Within chamber 7 adjacent to equilibrium orbit O and also within the region of influenced the time-varying magnetic guide field existing between pole faces 4, 5 during operation of the apparatus, there is provided a charged particle source 17 which is supported from a hermetically-sealed side arm 18 of envelope 6. More detailed illustration and description of electron gun structure suitable for present purposes can be found by reference to the above-mentionedpatents orby reference to the United States Patent No. 2,484,549 of J. P. Blewett, patented October 11, 1949 and assigned to the assignee of the present invention.

It is well understood by those familiar with magnetic induction accelerator apparatus that energization of

windings

14, 15 by the source of time-varying voltage 12 results in a time-varying magnetic flux which traverses magnetic core 1 and pole pieces .2, 3 to provide a timevarying magnetic flux that links equilibrium orbit O and a time-varying magnetic guide field that traverses the locus of equilibrium orbit 0 and the vicinity thereof between pole faces 4, 5. Electrons emitted by gun 17 at a desiredtimed instant near zero in the cycle of magnetic flux and field variations are continuously ac-.

celerated during the acceleration portion of the cycle as they execute repeated revolutions along and about equi librium orbit 0. As a consequence, the injected electrons can be caused to assume energies of many millions ofelectron volts. and then can be automaticallydiverted from the equilibrium orbit by means of pulsatingly energized orbit shift coils 19, 20 to produce X-radiation in a manner which will be more fully described hereinafter. Means including circuits for arranging the proper timed injection and subsequent diversion of the charged particles from the equilibrium orbit at or near the end of the acceleration cycle are disclosed in the aforementioned patents and additionally in the United States Patent No. 2,394,070 of D. W. Kerst, patented February 5, 1946 and assigned to the assignee of the present invention. As has been explained in D. W. Kerst Patent No. 2,297,305, patented September 29, 1942 and assigned to the assignee of the present invention, the time-varying magnetic flux linking the equilibrium orbit may be caused to produce centripetal forces which balance the centrifugal forces upon the charged particles undergoing acceleration at all times throughout the acceleration cycle, providing the following relationship is satified:

where A is the total change in flux linking the equihbrium orbit, R is the radius of the orbit and B0 is the flux density of the time-varying magnetic guide field at the equilibrium orbit. The condition specified by this relationship may be realized by making the reluctance for one unit area of cross section of the magnetic path of the time-varying fiux greater by an appropriate amount at the equilibrium orbit than its average reluctance for one unit area of cross section within the orbit.

The fulfillment of the foregoing condition, however,

only assures stable acceleration for those charged particles which are injected tangentially to their instantaneous circles or orbits. The instantaneous circle or orbit is the circular orbit along which a charged particle started at the proper position with the right energy will travel in a time-constant, radially symmetric magnetic field. With a time-varying magnetic flux as above specified, the loci of the instantaneous circles of all the charged particles approach and eventually essentially coincide with the equilibrium orbit during the latter portions of the acceleration cycles. Consequently, meeting the foregoing condition does not take into consideration the requirements for stable acceleration of charged particles which tend for one reason or another to deviate from their respective instantaneous circles or to deviate from the equilibrium orbit when their respective instantaneous circles coincide therewith. Nevertheless, by arranging the spatial distribution of the time-varying magnetic guide field in the vicinity of the equilibrium orbit as specified by the following relationship, both radial and axial focusing forces which tend to constrain deviating particles to their respective instantaneous circles or to the equilibrium orbit can be provided:

where H is the intensity of the time-varying magnetic guide field in the vicinity of the equilibrium orbit, r is the radius of a particular point under consideration and n is a parameter having a value lying between zero'and one. The outwardly directed taper of pole faces 4 and 5 as illustrated in Fig. 1 enables the utilization of the condition set forth in Equation 2. It is apparent from Equation 2 that the parameter n is a measure of the rate of decrease of the time-varying magnetic guide field with radius. Both radial and axial focusing forces exist if 0 n 1. For a uniform field, n=0 and no axial focusing of the particles can take place. In a field inversely proportional to the radius (11:1), there are no radial focusing forces. Since both radial and axial focusing forces are required to secure collimation of the particle beam during acceleration, the foregoing limits are placed upon the selected value of the n.

After the charged particles have been accelerated to a desired energy level under the foregoing conditions, they must be diverted from the equilibrium orbit to permit useful utilization of the energy which has been imparted to them. As has been stated above, diversion of the charged particles from the equilibrium orbit can be accomplished by supplying a properly timed pulse of current to orbit shift coils 19 and 20. The application of the current pulse to the orbit shift coils modifies the magnetic induction throughout the stable region surrounding equilibrium orbit O and causes the charged particles to spiral very slowly inwardly or outwardly from the equilibrium orbit, depending upon, the direction of the flux generated by the orbit shift coils with respect to the flux generated by

windings

14, 15. During their travel away from the equilibrium orbit the charged particles can be considered as following paths tangent to their respective instantaneous circles, the radii of which are gradually decreasing or increasing as the case may be. Since the charged particles do follow spiral paths of such small pitch, it is readily understood that the positioning of a thick X-ray generating target adjacent to the equilibrium orbit does not provide an opportunity for efiiciently generating thick target radiation. Essentially all of the charged particles diverted from the equilibrium orbit in this manner will strike the leading edge of the thick target, whereby many of the particles will be scattered through the edges of the target without a maximum fraction of their energy having been converted into X-radiation.

Accordingto the present invention thick target X- radiation may be efficiently obtained from charged particle accelerator apparatus by first causing the charged particles to be scattered by a thin target and subsequently causing the charged particles to strike a thick target. Referring specifically now to Figs. 2, 3 and 4, there is shown a thin target 21 and a thick target 22, both of which are positioned adjacent to and on the same side of equilibrium orbit 0. Thin target 21, which may comprise a thin strip of a low atomic number material such as beryllium, aluminum, magnesium, etc., is supported by a bent rigid rod 23 that is hermetically sealed into the wall of envelope 6 as illustrated. Rod 23 is extended downwardly shortly after it enters chamber 7 in order that charged particles will not impinge thereupon during their acceleration along equilibrium orbit 0. Thick target 22. which may comprise a relatively thick plate of a heavy material such as tungsten, molybdenum, copper, etc, is adjustably supported from a rigid rod 24 which is introduced into chamber 7 through a hermetically sealed bellows 25. The portion of rod 24 extending transversely of envelope 6 is likewise removed from the vicinity of equilibrium orbit O in order to avoid premature collision of the charged particles thereupon. The outer edge of thick target 22 is positioned more radially inward than the outer edge of thin target 21 so that charged particles spiraling inwardly from equilibrium orbit O first strike thin target 21. Preferably, the inner edge of thin target 21 is placed at essentially the same as or at a larger radius than the outer edge of thick target 22 when the charged particles are shifted inwardly from equilibrium orbit O by coils 19 and 20.

Now it will be understood that when orbit shift coils 19, 20 are energized with a pulse of current to cause the charged particles to spiral inwardly after they have been accelerated to a desired energy level as described in the aforementioned Kerst Patent No. 2,394,070, the charged particles first strike thin target 21. According to the invention, thin target 21 is so selected that it produces primarily a scattering of the charged particles which impinge upon it. In this manner the trajectories of the charged particles are changed abruptly, thus facilitating the generation of thick target radiation by the subsequent impingement of the scattered particles upon thick target 22. Thick target 22 is positioned farther from equilibrium orbit O and at a defined azimuthal spacing with respect to thin target 21 in order that a maxium number fields in the target.

saga-tree The considerations involved the selection .of the proper position of thick target 22 with respect tothin target 21 will be discussed presently.

When charged particles such as fast electrons pass through target foils such asthin target 21, several processes occur. First, some'of the energy of the charged particles is converted to radiation such as; X-radiation. This process chiefly involves interaction with the nuclear field of the target atoms and can be determined by the following relation:

where Wr is the quantity of energy radiated in millions of,

where W1 is the energy lost by ionization in millions of electron volts. Thirdly, the charged particles striking the target element are multiply scattered. This process is the charged particle deflection accumulated as the result of many successive collisions, mainly with the nuclear It transforms an initially collimated beam into one with a Gaussian distribution in space, the root-mean-square angle of scattering being t.,.=-.-, N As is explained above, the present invention contemplates the utilization of the scattering phenomenon in thin target 21 to change abruptly the trajectoriesof the charged particles impinging thereupon, whereby the particles can subsequently strike a thick target to generate radiation by the radiation process. The above equations show that the scattering process decreases with higher energies of the charged particles hence it is preferable that the charged particles not be accelerated to an excessively high energy. This effect is illustrated by the ratio of Equation 3 to Equation 5 which indicates that an adequate scattering target will generate a considerable amount of X- radiation at high energies and therefore will itself become a fairly thick target due to multiple passages of. the particles, however the energy at which this becomes obje'ctionable can be made higher by choosing a low atomic number for target 21. Since ionization loss in target 21 is undesirable in the present invention, it will be seen from Equations 4 and 5 that a minimum thickness should be selected for target 21.

After the charged particles have struck thin scattering target 21 and have passed therethrough, their trajectories are of course affected by the restoring forces of the abovedescribed time-varying magnetic guide field. Essentially none of the charged particles will be on its respective instantaneous circle, hence each will oscillate about its own instantaneous circle. The path followed by each particle about its own instantaneous circle has a radial fre quency equal to /1nf and an axial frequency equal to V17 where f is the equilibrium orbital revolution frequency. According to the present invention, it is preferable that thick target 22 be in azimuthally spaced relationship with respect to thin target 21 such that target 22,

will intercept the scattered charged particles at the position where the charged particle beam is most Widely dispersed. With thick target 22 placed in such a defined position it is possible to intercept nearly one-half of the scattered charged particles upon their first revolution. Some of the remainder of the scattered charged particles pass through thin target 21 upon subsequent revolutions and 7 are scattered, again. Many of these, re scattered particles;

strike thick target 22 thereafter, along withmany of the particles which did not strike target 22 upon their first revolution and were not re-scattered by target 21.

It has been found that the charged particle beam is radially most widely dispersed at a particular azimuthal angle with respect to thin target 21. This angle is defined by the following relations:

where at is the azimuthal angle between the maximum dispersion of the particles beam and thin target 21. Accordingly, when the charged particle beam is deflected radially from the equilibrium orbit, placement of thick target 22 approximately at an azimuthal angle 6; defined by Equation 6 produces maximum generation of thick target radiation. The essentially separation of the targets illustrated in Fig. 2 should be employed for an apparatus having a value of 0.75 for the parametern, as a substitution into Equation 6 shows.

It Will be understood that targets 21 and 22 can be positioned at greater radii than equilibrium orbit O and orbit expansion utilized to direct the charged particles to thin target 21. In addition, the targets may be positioned axially above or below equilibrium orbit O With thin target 21 located approximately at the radius of the equilibrium orbit. In this latter event the accelerated particle beam must be shifted axially as described in the aforementioned Westendorp Patent No. 2,394,072; furthermore, since the frequency of the axial oscillations of the charged particles about their instantaneous circles differs from the frequency of the radial oscillations, thick target 22 must be positioned at a difierent azimuthal angle with respect to thin target 21 in orderto secure the advantageuof obtaining the most widely dispersed particle beam. With axial orbit shift the azimuthal separation of the targets may be determined approximately from the equation:

where 6.. is the azimuthal angle between the two targets,

From the foregoing description it is readily. appreciated that the present invention makes possible the generation of thick target radiation with high efiicienoy in orbital charged particle accelerator apparatus. Thick target radi; ation is very desirable in many radiographic applications inasmuch as thick targets provide broader X-ray beams with a greater total quantity of radiation than do thin targets. The present invention is not limited to utilization in conjuncion with accelerator apparatus which employs magnetic induction phenomena alone, but can also. be used with synchrotron apparatus such as that disclosed in United States Patent No. 2,485,409, patented October 18, 1949 by Willem F. Westendorp and Herbert C. Pollock and assigned to the assignee of the present invention. Further application of the present invention can be made in connection with non-ferromagnetic accelerating apparatus, e. g. the apparatus disclosed in United States Patent No. 2,465,786, patented March 29, 1949 by I. P. Blewett and assigned to the assignee of the present invention. In general, the present invention has application in all types of orbital charged particle accelerating apparatus having a time-varying magnetic guide field essentially satisfying the condition of Equation 2.

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

l. A charged particle accelerating apparatus having an enclosure defining a stable accelerating region containing an equilibrium orbit and traversed by a time-varying magnetic guide field, a thin target positioned within said enclosure and at a smaller radius than the radius of the equilibrium orbit, and a thick target positioned Within said enclosure with the outer edge of said. thick target at a smaller radius than the outer edge of said thin target but not substantially smaller than the inner edge of said thin target so that charged particles emerging from the 7 equilibrium orbit first strike said thin target and subsequently strike said thick target.

2. A charged particle accelerating apparatus having an enclosure defining a stable accelerating region containing an equilibrium orbit and traversed by a time-varying magnetic guidefield, a thin target positioned within said enclosure and at a smaller radius than the radius of the equilibrium orbit with its nearest edge a predetermined distance from the equilibrium orbit, and a thick target positioned within the enclosure and in azimuthally spaced relationship with respect to said thin target and on the same side of the equilibrium orbit with its nearest edge approximately at the same radius as the farthest edge of said thin target, whereby charged particles emerging from the equilibrium orbit first strike said thin target and subsequently strike said thick target.

- 3. A charged particle accelerator apparatus having an enclosure defining a stable accelerating region containing an equilibrium orbit and traversed by a time-varying magnetic guide field essentially satisfying the relation d (log T) where H is the intensity of the time-varying magnetic guide field within the stable accelerating region, r is the radius of a particular point under consideration and n is a parameter having a value lying between zero and one, a thin target positioned within the enclosure and at a smaller radius than the radius of the equilibrium orbit, and a thick target positioned within the enclosure with the outer edge thereof at a smaller radius than the outer edge of said thin target and not substantially smaller than the radius of the inner edge of said target, the azimuthal separation of said targets being approximately determined by the relation where 0r is the azimuthal angle between the two targets,

whereby charged particles diverted radially inwardly from the equilibrium orbit strike first said thin target and subsequently said thick target.

d (log 1") Where H is the intensity of the time-varying magnetic guide field within the stable accelerating region, r is the radius of a particular point under consideration and n is a parameter having a value lying between zero and one, a thin target positioned within the enclosure'adjacent the equilibrium orbit with its nearest edge a predetermined distance radially from the equilibrium orbit, and a thick target positioned adjacent the equilibrium orbit with its nearest edge essentially at the same radius as the farthest edge of said thin target, the azimuthal separation of saidtargets being approximately determined by the relation an equilibrium orbit and traversed by a time-varying mag netic guide field satisfying the relation d (log r) where H is the intensity of the time-varying magnetic guide field within the stable accelerating region, r is the radius of a particular point under consideration and n is a parameter having a value lying between zero and one, a thin target positioned within the enclosure adjacent the equilibrium orbit with its nearest edge a predetermined distance axially from the equilibrium orbit, and a thick target positioned within the enclosure adjacent the equilibrium orbit with its nearest edge essentially at the same axial distance from the equilibrium orbit as the farthest edge of said thin target, the azimuthal separation of said targets being approximately determined by the relation where 0., is the azimuthal angle between the two targets, whereby charged particles diverted axially from the equilibrium orbit strike first said thin target and subsequently said thick target.

References Cited in the file of this patent UNITED STATES PATENTS