US2931941A - Apparatus for the efficient use of ionizing radiation produced by microwave linear accelerators - Google Patents

Description

April 5, 1960 D- R. DEWEY ll, EI'AL APPARATUS FOR THE EFFICIENT USE OF IONIZING RADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORS Filed Jan. 31, 1955 3 Sheets-Sheet 1 4 FIG.

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2,931,941 RADIATION RATORS 3 Sheets-Sheet 2 DEWEY ll, ETAL IENT USE OF IONIZING MICROWAVE LINEAR ACCELE FIC Apnl 5, 1960 APPARATUS FOR THE EF PRODUCED BY Filed Jan. 31, 1955 MOD.

Apnl 5, 1960 D. R. DEWEY u, ETAL 2,931,941 APPARATUS FOR THE EFFICIENT USE OF IONIZING RADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORS Filed Jan. -31, 1955 3 Sheets-Sheet 3 FIG. 4

United States Patent APPARATUS FOR THE EFFICIENT USE OF IONIZ- ING RADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORS Davis R. Dewey II, Lincoln, and John C. Nygard, Waltham, Mass., assignors to High Voltage Engineering Corporation, Cambridge, Mass., a corporation of Massachusetts Application January 31, 1955, Serial No. 484,940 2 Claims. (Cl. 315-39) This invention relates to microwave linear accelerators, and in particular to novel apparatus for the eflicient use of the ionizingradiation, including high-energy electrons and X-rays, produced by microwave linear accelerators. I 1

High energy electrons are one of the major forms of ionizing radiation used for sterilization of food, drugs,

and other materials, and for the promotion of chemical reactions. The major limitation attendant upon highenergy electrons is the relatively short range of electrons in matter. Thus a 2-m.e.v. electron has a range of about 1 cm. in water. The range is approximately proportional to the energy of the electrons, so that increased penetration in the matter irradiated is obtained by increasing the electron energy.

Owing to the high energies attainable therewith, microwave linear accelerators are a preferred source of electrons for these purposes. However, despite the fact that electron energies of 50 m.e.v. and higher may be obtained from microwave linear accelerators, the cost of a microwave linear accelerator increases roughly in proportion to the square root of the power output. For eflicient operation at higher electron energies, more power must be delivered to the microwave linear accelerator than at lower electron energies, and the cost of the accelerator per m.e.v. of electron energy increases with increasing electron energy.

Moreover, at electron energies in excess of a certain energy, nuclear reactions may be produced in the matter irradiatedj The energy above which such reactions may I occur varies, depending upon the particular matter being Accordingly, one ob ect of our invention is to increase the efiective penetration in matter obtainable with electrons of a given energy, through the use of a plurality of essentially opposed electron beams, and our invention comprehends novel apparatus for the production of a Qplurality oi electron beams by means of microwave linear r accelerator components.

Another object of our invention is to increase the efficiency of operation of microwave linear accelerators Ewhich are used for the production of X-rays for the treatment-of patients in cancer therapy and the like.

Our invention may best be understood from the following description with reference to the accompanying drawings in which, since the construction of microwave linear accelerators is well-known in the art, the various components of microwave linear accelerators are merely diagrammatically illustrated.

velocity of the wave.

2,931,941 Patented Apr. 5, 196i) In the drawings: a

Fig. l is a diagram illustrating the major components of a conventional microwave linear accelerator;

Fig. 2 is a diagram illustrating apparatus for the production, in accordance with our invention, of two electron beams by means of conventional microwave linear accelerator components;

Fig. 3 is a diagram illustrating a modification of the apparatus of Fig. 2, in which three electron beams are produced by means of conventional microwave linear accelerator components;

Fig. 4 is a diagram illustrating the irradiation of tubular material with four electron beams; and

Fig. 5 is a diagram illustrating apparatus for the treatment of patients with X-rays produced by conventional microwave linear accelerator components, but wherein said components are arranged, in accordance with our invention, so as to increase efiiciency of operation.

Referring to the drawings, and first to Fig. 1 thereof, in a microwave linear accelerator an electron beam 1 is created by an electron injector 2 and injected axially into one end of a waveguide 3 in which a traveling electromagnetic wave is produced by means of highfrequency power fed into the waveguide 3 from a highfrequency oscillator 4 such as a magnetron or klystron or other high-frequency oscillator. The apparatus is designed so that the electromagnetic wave has an axial electric field component, and the waveguide 3 is iris-loaded so that the phase velocity of the traveling wave is very nearly equal to the velocity of light in vacuo, or 3 x 10. chm/sec. The electrons are injected into the waveguide 3 at a velocity which is almost equal to the velocity of light, so that the electrons start traveling down the waveguide 3 with falrnost the same velocity as the phase Some of the electrons will find themselves in an accelerating electric field which raises their velocity to a value very nearly equal to the velocity of light. These electrons can then gain only a negligible amount of additional velocity, and so they remain in the I L E +1; EdL electron volts where E is the energy of the electrons at injection into the waveguide 3.

The function of the oscillator 4 is to convert D.-C. power into high-frequency power. Owing to the high power involved, the D.-C. power is not applied continuously to the oscillator 4, but rather in the form of square-wave pulses which are produced by a modulator S. The modulator 5 is thus the basic power supply for the microwave linear accelerator and furnishes a pulsed D.-C. output. The oscillator 4 converts the pulsed D.-C. output of the modulator 5 into high-frequency power, and in the waveguide 3 the high-frequency output of the oscillator 4 is converted into kinetic energy of the electrons injected into the waveguide 3 by the injector 2. Since the traveling electromagnetic wave exists in the waveguide 3 only during the intervals in which the modulator 5 is delivering a square-wave pulse, the injector 2 is usually pulsed in synchronism with the modulator 5 in order to conserve the power required for the injector 2.

At the present time, a representative modulator 5 may give an output of square-wave pulses about 1 to 10 or more microseconds in duration with a pulse repetition rate of about 1 to 1000 pulses per second. Since the frequency of the electromagnetic oscillations produced in the waveguide 3 by the oscillator 4 is usually, at the present time, about 3000 megacycles, the pulse length is seen to be very long compared with the period of the high-frequency oscillations. In describing the operation of microwave linear accelerators, reference is made throughout the description herein to conditions during the pulse. Thus, for example, at the present time, magnetrons'can be constructed to give a power output of up to 5 megawatts (during the pulse) and klystrons can be constructed to give a power output of up to megawatts (during the pulse) 5 but the average output of such magnetrons and klystrons is less than the figures mentioned. V I

The power output of fa microwave linear accelerator is limited principally by the oscillator 4. Thus, as stated, the power output of a magnetron oscillator is limited to about 5 megawatts at the present time, and the power output of a klystron oscillator is limited to about 10 megawatts at the present time. The lr gh-frequency power output of the oscillator may beincreased to means of one or more amplifiers, such as klystrons. In general, only one oscillator is used, owing to the difficulty "of properly pha'sing the outputs of more than one oscillater in order to produce the desired traveling wave in the waveguide 3. V

As stated, the basic power source for the microwave linear accelerator is the modulator 5, and in general the modulator 5 may be designed "to give 'sufiicient power for the operation of any present day oscillator, with or without amplifiers, i v v c p The frequency of the high-frequency power delivered by the oscillator 4 to the waveguide 3 should be as high as. possible. A frequency as high as 10,000 megacycles would be desirable, but the power tubes currently available limit the frequency attainable. At the present time, a frequency of 3,000 megacycles may be taken as representative, which corresponds to a wavelength of 10 cm. for an unbounded wave in vacuo.

The length of the waveguide 3 is determined by the electron energy desired. For a given waveguide 3 of length L there is a maximum electron energy attainable for a given power input which'is greater the greater value of L. However, increas ng the length L of the waveguide 3 decreases the 'efiiciency of the microwave linear accelerator, Hence the-length L of the waveguide 3 should be great enough to provide thedsired electron energy, but no greater.

Moreover, the electron energy decreases as the electron beam current increases, and the maximum electron energy just referred to is the electron energy approached as the electron beam current approaches zero. Therefore, the length L of the waveguide 3 must be great enough to provide the desired electron energy at the desired electron beam current.

The foregoing discussion is summarized by the following approximate equation:

-1 is the'electron ssamf -c ags linainpe'rs') L is the length of the waveghide fincm.)

a is the attenuation of thewaveguide (in -nepers7n1i) P is the high-frequency power delivered to 'the waveguide by the oscillator (in watts) A is a constantfor agiven waveglideconstructionand a given frequency (in Vohms/m'.)

a It is apparent from the foregoing equation that, as the electron beam current I increases, the electron energy decreases linearly from a maximum of It is also apparent that for a given length L of the waveguide, the power P =l V in the electron beam is a maximum, and hence the efficiency of the microwave 10 linear accelerator is a maximum, when The foregoing equations are only approximate, and in practice the microwave linear accelerator is usually operated with a beam current somewhat lower than that given by Equation 4. These equations show that, for a given length L of the Waveguide S and when V and I 5 have the values given by Equations 3 and 4, the electron 56m nqiiati'e'n an a parent that the eflicienc'y appreaches 100% as 'aL becomes very -small, and that the 40 as aL becomes very large. Evidentlyit is desirable to minimize the length 'L ofthe waveguide, "as hereinbefore stated. From Equation 3 it appears that the lower limit of L for ".ag'iven microwave linear acceleratoris imp'o'sed'by the upper limit of the power available. Hence,

if the available power is limited, the efficiency is also 7 Mareever, Equations '3 and 5 show that'if the electroh energy V 'is increased, the power input P must increase in proportion to V? or 'else the effi'cien'cy is lowered. 'Ifthe power input P is not increased in proportion "to V the efficiency is lowered because either the energy V'will not have its most efiicient value (i.e. /.,.V or else 'L must be increased.

The'efiiciencies given by Equation 5 for various values of aL are shown in the following table:

Table] uL Eiliclency L if a='.004 nepers/cm.

Percent cm.

NorErfAt'the present time,-waveguide lengths of less'fihan' 50 cm; are 2 not practical owing to the excessive amount gi power required toproduce the desiredielectron energy. Moreover, even if the very high power required were attainable, the microwave linear accelerator would ha've (F0. operate at yeryh gh currents in iqrder to operate.efliciently. This would create difficult problems or electron emission and bemlocusing.

'to the object 6 is proportional to the electron beam current I. While increasing the beam current I decreases the length of time required to deliver the desired dose, the magnitude of the beam current I is less critical than that of the energy V, which obviously must be sufliciently great or else part of the object 6 receives no dose at all.

It is well-known in the art that the electron energy required for full penetration of a given object may be reduced by irradiating the object from opposite aspects with two opposed electron beams. Using this doublebombardment technique, the energy required is at most half that required with single bombardment, and usually the energy. required in double bombardment is less than half that required in single bombardment, owing to the distribution in depth of the ionizing energy of the electron beam.

Our invention comprehends novel apparatus for the production of two (or more) electron beams for the purpose of irradiating objects with high energy electrons using double-bombardment techniques. Our invention provides advantages over single-bombardment techniques which are common to other double-bombardment techniques. If an object of thickness t is irradiated with single-bombardment techniques, the electrons must have sufficient energy so that an adequate ionizing dose is delivered at the depth t. However, in order that enough electrons may reach the depth t, the beam energy must be so high that some of the electrons travel through the object, with the result that some of the beam power is wasted. Using double bombardment, this excess beam power is delivered to the interior of the object, and is not wasted. Hence double bombardment makes more efiicient use of the available beam power.

By our invention, not only is the increased efiiciency inherent in double-bombardment techniques obtained, but the conversion of high-frequency power to electron beam power is also more eflicient than in the single-bombardment case, as will appear hereinafter.

Referring now to Fig. 2, therein is shown apparatus for producing two opposed electron beams in accordance with our invention. Electron beams 7 and 7 are created by electron injectors 8 and 8' and injected into waveguides 9 and 9' respectively, and after being accelerated the electron beams 7 and 7 impinge upon the object 6 from opposing aspects. The high-frequency power for the waveguides 9 and 9 is derived from the oscillator 4, which is driven by the modulator 5. The high-frequency output of the oscillator 4 is divided equally by a microwave power divider 10, so that one-half of the power output of the oscillator 4 is delivered to each waveguide 9, 9'.

In order to compare the apparatus of Fig. 2 with that of Fig. 1, it will be assumed that the oscillator 4 and the modulator 5 are the same in both cases, that the electron injectors 8 and 8 are each identical to the electron injector 2, that the object 6 is the same in both cases, and that the waveguides 9 and 9 are each identical to the waveguide 3 except for their lengths.

Referring first to Fig. 1, the electron energy V required to irradiate the object 6 adequately is determined by the effective thickness t of the object 6. Using the double bombardment arrangement of Fig. 2, the electron energy V required of each beam 7, 7 to irradiate the same object 6 is, as hereinbefore stated, therefore somewhat less that /2\',, but for simplicity of discussion it will be assumed that V /2V If P is the power output of the oscillator 4, the power delivered to the waveguide 3 of Fig. 1 will be P, and

the power delivered to each of the waveguides 9 and 9' of Fig. 2 will be /2P. The appropriate lengths of the waveguides are then given by Equation 3, as follows:

For waveguide 3 of Fig. 1:

From Equation 8 it is seen that the length L of each of the waveguides 9 and 9 is less than the length L of the waveguide 3. Consequently, the conversion of highfrequency power to electron beam power is more efficient in the case of the apparatus of Fig. 2 than in the case of the apparatus of Fig. 1. This means that the beam current 1 delivered by each of the waveguides 9, 9' of Fig. 2 is greater than the beam current I delivered by the waveguide 3 of Fig. 1. v A

The foregoing may be illustrated by a specific example. Suppose that, for both the apparatus of Fig. 1 and that of Fig. 2:

a=.004 neper s/cm. A=60 /ohm/cm. P=4 megawatts If the thickness t of the object 6 is such that the energy of the electrons produced'by the apparatus of Fig. 1 must be 10 mev., Equation 6 shows that the length L of the waveguide 3 will be 275 cm. Table I shows that the efiiciency at thisllength is about 50%, and the beam current I, will therefore be .2 ampere.

From Equation 8, it is seen than the length L of each of the waveguides 9 and 9 will be cm. Table I shows that the efficiency at this length is about 70%, and the beam current I will therefore be .56 ampere.

From Table I it is apparent that, as greater lengths are reached, the efiiciency becomes greatly reduced. From Equation 3 it is apparent that, for a given energy, the limitation upon the shortness of the waveguide is the maximum power available. As stated previously, the power limit is imposed mainly by the oscillator, although amplifiers may be used to increase the power output of the oscillator. Owing to the power limit, the efficiency becomes increasingly less as higher electron energies are required. As stated, one of the advantages of our invention is the fact that this reduction in efiiciency at the higher energies is overcome.

In some cases, such as where particularly thick products are to be irradiated, more than two electron beams may be desired, and our invention also comprehends the use of more than two waveguides to produce the desired number of electron beams. In Fig. 3, three electron beams 11, 11', 11" are created by electron injectors 12, 12', 12 and injected into waveguides 13, 13', 13" respectively, and after being accelerated the three electron beams 11, 11', 11 simultaneously bombard an object 14 from essentially opposing aspects.

Where greater power is required than is available from a single oscillator, the output of the oscillator may be amplified, as hereinbefore stated, by amplifiers. Alternatively, more than one oscillator may be employed. Thus, in the apparatus of Fig. 3, a separate oscillator 15, 15, 15 is used for each waveguide 13, 13', 13" and a single modulator 16 drives all three oscillators 15, 15, 15". Generally the output stage of the modulator 16 comprises a pulse transformer which transforms a relatively lowvoltage pulse into the desired high-voltage pulse. In the apparatus of Fig. 3, wherein three oscillators are employed, it is preferable to provide a pulse transformer 17, 17', 17" ror each oscillator, the pulse transformers 17,

7 17', 17" beingplaced near their respective oscillators 15, 15', 15". The relatively low-voltage output of the modulator 16 is then divided by a suitable power divider 18 and applied to the 3 pulse transformers .17, 17, 17". This arrangement minimizes the undesirable effect of capacitance between the relatively long leads 19, 19', 19" and ground, by sending low-voltage pulses rather than high-voltage pulses along these'long leads. Since each oscillator drives a difieren't waveguide, there is no problem of phasing their outputs.

The irradiation of an object by a plurality of electron beams is advantageous, not only for the purpose of irradiating thick objectsQbut also for the purpose of irradiating tubular material, particularly if the tubular material to be irradiated encloses a high-density material which need not be irradiated but which would absorb much of the ionizing-energy of a single electron beam, since the electron beam would have to pass through the high-density material in order fully to irradiate the tubular material. For example, it may be desired to irradiate a length of extruded polyethylene tubing (or other extruded plastic tubing) with high energy electrons in order to cause cross-linking of the polyethylene (or other plastic), or it may be desired to irradiate a length of cable, comprising a conductor such as copper surrounded by a sheath of polyethylene (or other plastic), for the same purpose. In either event, it is desired to irradiate a layer of annular cross-section. This may conveniently be done by using several electron beams, as shown in Fig. 4, wherein four electron beams 20, 20, 20", 20" are sirnultaneouslydi- .rected upon the circumferential surface of a length of :26, 26', wherein patients, situated as at 27, 27', receive X-ray treatment in cancer therapy or the like. paratus of Fig. .is similarto that of Fig. 2, except that The apthe output of the high-frequency power unit is applied only to one waveguide'at a time, by means of a suitable -microwa've power switch 28.

In X-ray installations for treatment of patients, the X-ray apparatus is not operated during the time in which each patient is properly positioned in the treatment room. Generally the patient-preparation time is about twice the actual irradiation time. Hence, a given installation is actually operated only about one-third of the available time.

By means of the arrangement shown in Fig. 5, the

patient-treating capacity of a microwave linear accelerator installation is doubled at an increase in capital cost which is only a small fraction of the total cost of a complete new installation. While our invention includes the use of a single high-frequency power unit to drive alternately more than two waveguides in a manner similar to that shown in Fig. 5, it would probably not be advantageous toprovide .more than three waveguides for a singlejpower unit, since, as hereinbefo-re stated, each waveguide would be in operation about one-third of--the available time. v

In the foregoing description, particular reference is made to traveling-wave linear accelerators, since such linear accelerators arein common use. However, it is evident that the-advantagesof our invention apply equally well to standing-wave or resonant-cavity linear accelerators, and that our invention is not limited to any particular type of microwave linear accelerator.

Having thus described several illustrative embodiments of our invention, his to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims:

We claim:

1. Apparatus for irradiating matter with high-energy electrons comprising in combination a plurality of electron accelerator units, each unit including a waveguide and means'for injecting electrons into the waveguide, each unit being adapted to accelerate an electron beam upon excitation of the waveguide by a high-frequency power source; a high-frequency power source whose power out- .put is suitable for the excitation of said waveguides;

means for delivering approximately half of the power output from said high-frequency power source simultaneously to each of said waveguides; and means for directing the electron beams produced by said waveguides onto matter to be irradiated from essentially opposing aspects.

2. Apparatus for irradiating matter with high-energy electrons comprising in combination a plurality of electron accelerator assemblies, each assembly including a waveguide adapted to accelerate electrons so as to produce an electron beam upon being excited by high-frequency power, means for injecting electrons into the wave, guide, and a high-frequency oscillator adapted to convert D.-C. power delivered thereto into high-frequency power and to deliver such high-frequency power to the waveguide; a power source providing a DC. power output suitable for energizing said oscillators; means for delivering approximately half of the power output from said power source simultaneously to each of said oscillators; and means for directing the electron beams produced by said waveguides onto matter to be irradiated from essentially opposing aspects.

References Cited in the file of this patent UNITED STATES PATENTS

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