US2892946A

US2892946A - Method of and apparatus for the more efficient use of high-energy charged particles in the treatment of gasphase systems

Info

Publication number: US2892946A
Application number: US548877A
Authority: US
Inventors: Ii Davis R Dewey; Alfred J Gale
Original assignee: High Voltage Engineering Corp
Current assignee: High Voltage Engineering Corp
Priority date: 1955-11-25
Filing date: 1955-11-25
Publication date: 1959-06-30
Anticipated expiration: 1976-06-30

Description

June 30, 1959 D. R. DEWEY 1|, E1- AL 2,892,946

METHOD OF AND APPARATUS FOR THE MORE EFFICIENT USE OF HIGHENERGY CHARGED PARTICLES IN THE TREATMENT OF GAS-PHASE SYSTEMS 2 SheetsSheet 1 Filed NOV. 25, 1955 [ll/ l FIG. 3

SE OF IN THE TREATMENT OF GAS-PHASE SYSTEMS ET AL 2,892,946

MORE EFFICIENT II 11111 1959 D. R. DEWEY ll METHOD OF AND APPARATUS FOR THE! HIGH-ENERGY CHARGED PARTICLES Filed Nov. 25, 1955 2 Sheets-Sheet 2 FIG. 5

United States Patent LMETHOD OF AND APPARATUS FOR THE MORE EFFICIENT USE OF HIGH-ENERGY CHARGED PARTICLES IN THE TREATMENT OF GAS- PHASE SYSTEMS Application November 25, 1955, Serial No. 548,877 3 Claims. (Cl. 250-495) This invention relates to the bombardment of gas-phase systems by high-energy charged particles, and in particular to a method of and apparatus for the more eificient use of such charged particles in the treatment of such gas-phase systems. Briefly stated, our invention comprehends directing high-energy charged particles into the chamber containing the gas-phase system to be bombarded, and subjecting the charged particles to the deflecting action of a magnetic field within the chamber, the strength-of the magnetic field being suflicient to confine the charged particles within the chamber. Examples of gaseous products include ethylene (which may be polymerized to form polyethylene under irradiation), methane, ethane, and propane.

The invention is particularly useful in the electron irradiation of gas-phase systems for the purpose of causing chemical reactions such as ethylene polymerization, etc., and for other purposes. Certain practical difliculties areencountered in the irradiation of gas-phase systems with electrons of sufficiently high energy to permit eflicient passage through an electron-permeable window. Electron-permeable windows usually comprise thin metal foil, and electrons must have energy of at least 0.5 m.e.v. for eificient passage therethrough. In the case of a 2-m.e.v. electron beam, for example, the range of the beam in gas at atmospheric pressure is of the order of 25 feet, compared with less than /z-inch for the same beam in water or material of water density. This means, in the first place, that special, cumbersome and large apparatus must be used to handle the gaseous product being irradiated if a high percentage of the beam is to be absorbed inthe gas, and, in the second place, that the ionization density in the gas is low (i.e., there are few ion pairs produced per cc.).

In accordance with our invention, the electron beam passes through a Window into the gas-phase system, and shortly thereafter, before it is too much enlarged by scattering, passes between the pole faces of a magnet (which may be in the reaction chamber or outside its walls), this magnet having field strength suflicient to cause the electron beam to bend in a full circle, ending up in a closing spiral and thereby completing its entire range in a very small gas volume.

' In the case of low-pressure gas-phase systems the invention is advantageous because it reduces the resultant electron path length to a value within the dimensions of thereaction chamber. The invention is also advantageous'in the irradiation of high-pressure gas-phase systems, even when the pressure is sufiiciently high to provide a gas density great enough to stop the electrons in a short distance, 'asa result of the fact that the walls of the reaction chamber in which the high-pressure gas-phase system is confined must be thick enough to support the high pressure. In order to get the electrons through the walls of the reaction chamber or vessel containing the gas, the electrons must be given much greater energy than that required to penetrate the relatively dense gas alone. Hence these electrons still have much of their energy ice after passing through the gas in the vessel, and the invention provides means for preserving this energy by causing the electrons to stay in the gas and there to expend their remaining energy in a useful manner.

The invention is not limited to use with electron beams, but includes positive-ion applications as well. For example, in the production of neutrons by bombarding a gaseous isotope of hydrogen with high-energy deuterons, the invention serves to confine the deuterons within a relatively small volume of target material despite the low density of the latter.

The magnetic field used to confine the high-energy charged particles should be substantially constant during the treatment time. In general, an accelerator giving a continuous beam of charged particles, such as an electrostatic belt-type accelerator, would require a magnetic field of approximately constant field strength, such as might be provided by a permanent magnet or an ironcore electromagnet. On the other hand, an accelerator giving a pulsed beam of charged particles, such as a microwave linear accelerator, might have a pulsed magnetic field whose field strength is approximately constant during each pulse, such as might be provided by an air-core electromagnet driven by a suitable pulsing circuit which is synchronized with the accelerator in some appropriate manner. Very high field strengths may be obtained in this manner and the use of an iron core, which owing to saturation would unduly limit the field strength, should be avoided. Such use of a pulsed magnetic field to produce very high field strengths is an important feature of the invention, for it not only facilitates the irradiation of gas-phase systems within treatment chambers of relatively small size with high-energy electrons, but it also permits a beam of ions to be delivered to a gas target in such a manner as to minimize energy loss, by confining the ions to the volume enclosed by the chamber contain-- ing the gas target. In order to confine an ion beam within a given volume, a much larger magnetic field strength must be used than is required with an electron beam of equivalent energy, owing to the much greater mass of the ions. The use of a pulsed magnetic field in; accordance with the invention thus not only facilitates the ion bombardment of gas targets, but also enables a beam of ions to be delivered to a very small volume of space, in the event that it should be desired to concentrate the energy of the bombarding ions upon a very small portion of the gaseous target material.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which:

Figure 1 is a perspective view of one embodiment of the invention;

Figure 2 is a vertical section of the apparatus of Figure 1;

Figure 3 is a diagram showing the magnetic field pattern produced by the apparatus of Figure 1;

Figure 4 is a diagrammatic view of another embodiment of the invention; and

Figure 5 is a diagrammatic view of the embodiment of the invention shown in Figure 4, but taken at right angles to the view of Figure 4. The view of Figure 5 is not drawn to the same scale as the view of Figure 4.

Referring to the drawings, and first to Figures 1, 2 and 3 thereof, the gas-phase system to be treated is confined within a reaction chamber 1 which is separated from the evacuated region of an electron accelerator 2 by a suitable electron window 3 comprising, for example, a strip of aluminuru foil. The electron accelerator 2 may be of any conventional type, and in Figures 1, 2 and 3 the electron accelerator 2 is provided with means for rapidly scanning the electron beam 4 in accordance with the teachings of U.S. Patent No. 2,602,751 to Robinson.

However, an unscanned beam may also be used, and the invention is not limited to any particular type of electron beam.

The reaction chamber 1 may be completely closed off, or, as shown in Figure l, a suitable conduit 5 may be employed through which the gas to be treated may be fed continuously. The reaction chamber 1 may be of any conventional type.

In accordance with the invention, a magnetic field is created within the reaction chamber 1 by suitable means, such as the permanent magnet 6 shown in Figures 1 and 3. The permanent magnet 6 is shown in Figures 1 and 3 as being outside the reaction chamber 1, but the magnet may equally well be placed inside the reaction chamber 1 and an electromagnet, either solid-core or air-core, may be substituted for the permanent magnet 6, all without departing from the spirit and scope of the invention.

Where, as in Figures 1-3, the electron beam 4 is scanned so as to enter the reaction chamber 1 in planar form, the direction of the magnetic field in the reaction chamber lshould be perpendicular to the plane of the beam 4, so that the electrons are deflected as shown in Figure 2. The effect of the magnetic field is to cause the electrons to travel in a circle whose radius is proportioned to the momentum of the electrons. As the electrons collide with the gas molecules in the reaction chamber 1, they lose energy and momentum with each collision, so that the radius of the circular path travelled by each electron keeps diminishing, resulting in the spiral path shown in Figure 2.

While the embodiment of the invention shown in Figures l3 is equally well adaptable to continuous or pulsed beams of charged particles, the use of a pulsed beam permits one to pulse the magnetic field, thereby attaining very high field strengths. An embodiment of the invention suitable for use with pulsed beams of charged particles is shown in Figures 4 and 5.

Referring to said Figures 4- and 5, a charged-particle accelerator providing a pulsed beam is shown at 7. The invention applies to positive as well as negative charged particles, but for illustrative purposes only, the accelerator 7 is assumed to deliver a pulsed beam of electrons 8 through an electron window 9 into a reaction chamber 10 containing the gas to be irradiated. For example, the accelerator 7 may be a microwave linear accelerator of conventional design. In Figures 4 and 5, the electron beam 8 is shown as unscanned, and the reaction chamber 10 is sealed off. However, as previously noted, the invention is not limited to such an arrangement.

The magnetic field is provided by a coil 11 into which the reaction chamber 10 is fitted, as shown. Alternatively, two separate coils could be used, one on either side of the reaction chamber 10. The coil 11 is air-cored because ferro-magnetic material would saturate at a field strength less than that otherwise obtainable. One possible circuit for pulsing the coil 11 is shown in Figure 4, but other circuit arrangements will readily suggest themselves to those skilled in the art.

In the circuit shown in Figure 4, the signal which triggers the pulse circuit is derived from the electron beam 8 itself, which, although it passes through the electron window 9 to a large extent, nevertheless loses some electrons to the electron window 9. The electron window 9 is insulated from the rest of the accelerator 7 as shown at 12, and the negative charge collected by the electron window 9 leaks oif to ground through a resistor 13. Thus, the arrival of a pulse at the electron window 9 reduces the potential of the point A. The point A is connected to the negative terminal of a bias-voltage source 14 whose positive terminal is connected to the cathode 15 of a thyratron tube 16 whose grid 17 is grounded. The plate 18 of the thyratron tube 16 is connected to one end of the coil 11, and the other end of the coil 11 is connected to the positive terminal of a power source 19 through a resistance 20. The point B is connected to the cathode 15 via a capacitance 21.

Between pulses of the electron beam 8, the bias-voltage source 14 prevents current from flowing through the thyratron 16 by keeping the cathode 15 positive with respect to the grid 17. When an electron pulse hits the electron window 9, the point A becomes negative with respect to ground and the potential of the cathode 15 is lowered sufiiciently to fire the thyratron 16. This releases the electric charge stored in the capacitance 21 and results in the delivery of a high current pulse through the coil 11, whereby a magnetic field of high field strength is created within the reaction chamber 10.

The invention is not limited to any particular type of pulsing circuit, and other types of circuits equally suitable for practicing the invention will readily suggest themselves to persons skilled in the electronic art. One of the important features of the invention is the provision of magnetic fields in the treatment chamber of very high intensity through use of a pulse technique such as that just described. The higher intensity of the magnetic field permits the more eflicient use of higher energy electrons for a given treatment chamber; or, alternatively, the more efiicient use of a smaller treatment chamber for a given electron source. The higher intensity of the magnetic field is even more advantageous where a beam of ions is to be confined thereby within a treatment chamber or other relatively small volume. Since the lightest ion, the proton, has a rest mass which is 1837 times the rest mass of the electron, the strength of the magnetic field which is required to confine an ion beam of a given energy within a given treatment chamber is at least 40 times as great as thestrength of the magnetic field which is required to confine an electron beam of the same energy within the same treatment chamber. Using the high magnetic field strengths attainable by means of the pulse technique of the invention, a beam of ions may be delivered to a very small volume of gaseous target material.

Having thus described the method of the invention, together with several embodiments of apparatus for practicing the method, it is 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 delivering a beam of high-energy charged particles to a volume of matter of which volume the dimensions are much smaller than the range of said charged particles in said matter, comprising in combination: a treatment chamber bounding said volume, means for creating such a beam of charged particles and directing the same into said treatment chamber, and means for creating a magnetic field within said treatment chamber whose intensity in a direction perpendicular to the direction of incidence of said beam into said treatment chamber is sufiicient to confine the charged particles so that their energy is expended substantially entirely within said treatment chamber.

2. Apparatus for delivering a beam of high-energy electrons to a voltune of matter of which volume the dimensions are much smaller than the range of said electrons in said matter, comprising in combination: a treatment chamber bounding said volume, means for creating such a beam of electrons and directing the same into said treatment chamber, and means for creating a magnetic field within said treatment chamber whose intensity in a direction perpendicular to the direction of incidence of said beam into said treatment chamber is sufficient to confine the electrons so that their energy is expended substantially entirely within said treatment chamber.

3. Apparatus for delivering a beam of high energy charged particles to a volume of matter of which volume the dimensions are much smaller than the range of said 5 charged particles in said matter, comprising in combination: means for creating a pulsed beam of charged particles and directing the same into said volume, means for creating a magnetic field within said volume whose intensity in a direction perpendicular to the direction of incidence of said beam into said volume is sufiicient to confine the charged particles so that their energy is expended substantially entirely Within said volume, said means for creating a magnetic field including at least one coil for the creation of said magnetic field by the passage of current therethrough, and means for deliver- References Cited in the file of this patent UNITED STATES PATENTS Birkeland Oct. 18,

Andriessens Apr. 8,

Slepian Oct. 11,

FOREIGN PATENTS Great Britain Oct. 30,

Great Britain Apr. 2,