US3021445A

US3021445A - Electron lens

Info

Publication number: US3021445A
Application number: US43559A
Authority: US
Inventors: Wideroe Rolf; Sempert Max
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1959-07-24
Filing date: 1960-07-18
Publication date: 1962-02-13
Anticipated expiration: 1979-02-13

Description

Feb- 13, 1962 R. WIDEROE ETAL 3,021,445

ELECTRON LENS Filed July 18, 1960 NVENTORS Rolw WI er'e By Max Sempert 1 m & PM!

I AHiornegs 3,021,445 ELECTRON LENS Rolf Wideriie, Nussbaumen, and Max Sempert, Ennetbaden, Switzerland, assignors to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Filed July 18, 1960, Ser. No. 43,559

Claims priority, application Switzerland July 24, 1959 3 Claims. (Cl. 313-84) This invention relates to lenses and more particularly I High-speed electron beams which are produced, for

example, by means of electron accelerators such as those of the magnetic induction type, one of which is known as a betatron, have such great penetrating power that they can be used for radiation therapy in treating points located deeply within the human body. Thus, for example, a beam of high-speed electrons having a value of 30 mev. and a diameter of 10cm. impinging upon the human body produces, at'a depth of 8 cm. within the body, a dose which is about 90% of the dose effected at the surface of the body. I

An increase in the depth of dose can be achieved with the help of a converging electron lens which is arranged in the path of the electron beam between the source of the beam and the body to be irradiated and which effects a concentration of the electrons at the desired depth within the body. It is of particular advantage to arrange such a lens directly upon the surface of the body because the ratio of depth-dose to surface-dose is then particularly great. This requires use of a lens having a comparatively large relative aperture; as a rulethe focal length of the lens will be of the order of the diameter of the aperture. Magnetic fields of very great intensity are required to influence these high-energy electron beams in a directional manner and the magnetic fields must extend,

overv a rather large cross sectional area transverse'to the axis of the electron beam. In order to bring about the necessary converging action of the electron beam in the lens with a smallermagnetic field, it has been suggested to limit the magnetic field to an annular zone which is arranged at the outer portion of the electron beam, while the central portion of the electron beam remains unaffected by the magnetic field. Such an arrangement is disclosed in,-a co-pending application, Serial No.-758,787,

filed September 3, 1958, which is now US. Patent No. 2,952,791 granted September 13, 1960 in the name of Rolf Wideroe, who is a coapplicant of the present invention, and is particularly suitable for use with a betatron, the winding on the lens which produces the necessary magnetic, field being energized with short impulses which are synchronized with the impulses which effect periodic issuance of the highly accelerated beams of electrons from the accelerator structure. This periodic energization of the lens winding results in a reduction in the power consumed by the winding but in many cases, the current impulse source required for feeding such a lens and the means for obtaining the necessary synchronization constitute a very large expenditure. On the other hand, the lens construction in accordance with the present invention has the advantage in that it can be energized with direct current, 'or it can even be provided with permanent type magnets to produce the necessary magnetic field.

The object of the invention is therefore to provide an electron lens which is more simple in construction than lenses previously developed for this purpose and which can be operated in a less costly manner. The improved lens is characterized by a diaphragm type electron shield interposed in the path of the electron beam perpendicular United States. Patent least one aperture which extends substantially along at least one radius of the beam, and a magnet carried by the diaphragm which produces at an :air gap aligned with the diaphragm aperture, a magnetic field which causes the a electrons passing through the aperture and air gap to be deflected toward the beam axis. That part of the diaphragm aperture influenced by the magnetic'field is located somewhat remote from the beam axis while the remainder of the aperture extending radially inward to the beam axis is uninfluenced by the magnetic field. Consequently, the electrons in the beam located closer to the beam axis will be uninfluenced by the lens and hence their direction will remain parallel with the beam axis as they pass through the aperture in the diaphragm. On the other hand, the electrons located more remotely from the beam axis will be directionally influenced by the magnetic field as they pass through the diaphragm aperture so as to converge on the beam axis at a predetermined distance beyond the diaphragm. The diaphragm, with the magnet thereon is caused to rotate at high speed about the beam axis as a center of rotation so that the concentrating effect of the magnetic lens is caused to sweep in a circular path. I

k The foregoing objects and advantages of the invention will become more apparent from the following detailed description of one practical embodiment thereof and y from the accompanying drawings wherein:

to the beam axis, this diaphragm being provided with at spective body surface.

FIG. 1 is a diagrammatic view illustrating the optics of the improved lens; 7 a

FIG. 2 is a side view of the diaphragm showing the radially extending aperture therein and the flux lines of the magnetic field;

FIG. 3 is a view of the opposite side of the diaphragm in FIG. 2 with the magnet structure assembled thereon; and

FIG. 4 is aview of the diaphragm and magnet structure taken at a right angle to FIGS. 2 and 3.

With reference now to FIG. 1, the electron beam which is to be concentrated by the lens is indicated by letter S and has a diameter equal to D. Beam S impinges from the side L on the surface of the body K which is to be irradiated. By means of the rotatable apertured diaphragm B, in accordance with the invention, which is placed transverse to the beam and intercepts the latter prior to entering the body K, except forthat continuously changing portion which is .passed through the aperture and deflected toward the beam axis x-x as .the diaphragm is rotated about this axis, a depth dose is achieved in the zone Z of the body K to be irradiated which is much greater than the dose on the surface of the body. In the particular embodiment to be described, the diaphragm is provided with two apertures which extend radially outward from the center thereof which is co-centered with the beam axis, and these apertures extend along radii located apart, i.e. along a diameter of the diaphragm. Moreover, only the radially outer portions of the apertures are under the influence of the magnetic field. Consequently, one of these apertures establishes an ab-axial magnetic field zoneAl and the diametrically opposite aperture establishesa diametrically opposite ab-axial magnetic field zone A2. The radially inner portions of these two apertures represented by the zone M are unaffected by the magnetic fields.

The assembled apertured diaphragm and magnetic structure can be rotated in either direction about the beam axis x-x so that the surface dose is distributed practically uniformly during the period of irradiation over the re- A relatively high speed of rotation of the apertured diaphragm so that a relatively large number of revolutions takes place during the irradiation will assure a sufliciently uniform distribution of the electrons even though the number of revolutions is not a whole number. The optimum shape of the aperture in the diaphragm will be determined by taking into account the electron beam field, the uniformity of the distribution and the possible air gap length under the conditions given by the beam energy and the focal'length of the lens.

FIGS. 2, 3 and 4 show the constructonal details of the improved electron lens. In FIG. 2, the diaphragm is seen to be comprised of a circular plate 1 which is made of a material that is opaque to the electron beam S. While a single aperture construction may be utilized, there are two apertures in the plate which is illustrated. These apertures are diametrically opposite and are indicated by 2 and 2'. Each aperture begins at the center of the plate, which is coincident with the beam axis xx, and extends radially outward. The edges of the apertures diverge from the center outward so as to become increasingly wider and then proceed parallel to a diameter of the plate and terminate at a distance from the center equal to D/2. The magnetic field in the zone A1 related to aperture 2 is indicated by a series of short arrows, and the magnetic field in zone A2 related to aperture 2 is similarly indicated by short arrows. It will be noted that these short arrows do not appear in those portions of the apertures closer to the center, as represented by the zone M.

The magnetic structure is shown in FIG. 3. While these magnets may be of the so-called permanent type, the ones illustrated in the present embodiment are of the electro-magnetic type, the magnetic field being produced by energizing a coil which is wound upon a magnetic core. The

cores

3, 3 have the configuration of a horseshoe and these are mounted on the diaphragm plate 1 with the

pole pieces

3a, 3a in confronting spaced relation so as to establish air gaps 4, 4' therebetween, these air gaps being in alignment with the apertures 2 and 2' respectively. For purposes of clarity, the magnetic field zones A1 and A2 have also been indicated on FIG. 3. Magnetic core 3 is provided with a coil 5 and magnetic core 3' is similarly provided with a coil 5'. The two coils 5 and 5' are energized with direct current in such direction that the magnetic field extends in a closed path through the core material and air gaps in a clockwise direction as viewed in FIG. 3.

In accordance with the invention, the diaphragm plate 1 and magnetic structure mounted thereon are rotated on the axis xx of the electron beam S. This may be eifected in any suitable manner. In the illustrated embodiment, the periphery of the plate 1 constitutes a ring gear 6 which meshes with a driving pinion 7 that is driven by a motor 8. Current to the energizing coils 5 and 5' can be provided by conventional slip rings and brushes, not illustrated.

If desired, variation of the magnetic field strength produced at zones A1 and A2, which influences the focal length of the lens with a given strength of electron beam energy, can be eifected by varying the exciting current applied to

coils

5 and 5 and the distance, respectively, between the pole pieces of the magnets, or by placing magnetic shunts in parallel with the air gaps between the confronting pole pieces of the two magnetic cores.

It is also advisable to make the diaphragm plate 1 easily exchangeable, so that a diaphragm with an optimum aperture configuration can be used for each special case.

In conclusion, it will be understood that while one practical embodimentof the invention has been described, various modifications of the arrangement of component parts may be made without, however, departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1. In an electron lens for concentrating electrons of a high-speed electron beam, the combination comprising a diaphragm arranged in the path of the electron beam, said diaphragm being disposed transverse to the beam axis and including at least one aperture for electrons extending along a radius of the beam from the center of the diaphragm which is coincident with the beam axis, a magnet mounted on said diaphragm, said magnet including an air gap in alignment with a radially outer portion only of said aperture and across which a magnetic field is produced which deflects the electrons passing through said aligned aperture and air gap in the direction of the beam axis while those electrons passing through the radially inner portion of said aperture are uninfluenced by said magnetic field, and means for rotating said diaphragm on the beam axis.

2. An electron lens as defined in claim 1 wherein said diaphragm includes a pair of apertures extending outwardly from the center of the diaphragm along radii disposed apart.

3. An electron lens as defined in claim 2 wherein said magnet is comprised of two horseshoe-shaped magnet elements arranged with their pole pieces in confronting relation and which establish air gaps therebetween in alignment respectively with the radially outer portions only of said apertures.

References Cited in the file of this patent UNITED STATES PATENTS