US2885584A - Gas target - Google Patents

Description

M y 1959 R. J. VAN DE GRAAFF 2,885,584

GAS TARGET 3 Sheets-Sheet 1 Filed Nov. 16, 1955 1/ [III y 5, 1959 R. J. VAN DE GRAAFF 2,885,584

GAS TARGET Filed Nov. 16, 1955 s Sheets-Sheet 2 FIG. 2

May 5, 1959 R. J. V-AN DE GRAAFF 2,885,584

I GAS TARGET Filed Nov 16, 1955 s Sheets-Sheet s United States Patent GAS TARGET Robert J. Van de Graaflf, Lexington, Mass., assignor to High Voltage Engineering Corporation, Cambridge, Mass, a corporation of Massachusetts Application November 16, 1955, Serial No. 547,185

11 Claims. (Cl. 313-61) This invention relates to the bombardment of a target by charged particles artificially accelerated, and in particular to a gaseous target which is adapted to be bombarded by such charged particles.

In the artificial acceleration of charged particles, the

'region in which the charged particles are accelerated is generally evacuated to such an extent that the mean free is of the order of .OS/p, where p is the pressure in mm.

Hg, a mean free path of at least 150 cm. requires a vacuum of at least 3 X mm. Hg. At such low pressures,

the nature of the surface of the material which forms the 'boundary of the evacuated region becomes extremely im- 'portant, and such surfaces must be carefully designed so as to minimize the evolution of gases or vapors.

It is therefore evident that if the target which is to I be bombarded by the charged particles is in the gas state,

the gas target must generally be separated from the evacuated region. In the case of high energy electrons, the gas target may easily be separated from the evacuated region by a thin aluminum foil, through which the electrons pass with little loss of energy. However, in the case of heavier charged particles, such an arrangement is apt to be min-- satisfactory, since too much of the energy of the charged particles would be absorbed by the toil. Thus, for example, the range of a 2m.e.v. electron in aluminum is about 4 mm., so that an aluminum foil .1 mm. thick would absorb only a fraction of the electrons energy. But the range of a 2-m .e.v. proton in aluminum is about .04 mm., so that the proton could not even penetrate an aluminum foil .1 mm. thick.

Neutrons are commonly produced by the bombardment of certain targets by heavy charged particles, such as protons, ordeuterons.

Targets, for this and many other purposes of this nature may be classified in two groups: thick targets and thin targets. A thick target is defined as a target which absorbs practically all of the energy of the incident particles. A thin target is defined as a target in which the energy absorbed by the target from the incident particles is small compared with their total energy.

For accurate analysis of the nuclear reaction involved,

' it is desired to measure the intensities and energies of the n particles and radiation given olf from the various target nuclei which undergo nuclear reactions. Obviously the accuracy of the measurement may be adversely affected if both the bombarding particles and the emitted particles .have to pass through thick portions of the target with resultant energy loss.

Hence the use of a thin target is desirable for accurate measurement.

A thin target, as presently constructed, may comprise a thin film supported on a backing, which may-be either a foil or a thick piece of material. The most common type' thus minimize diffusion of the gas into the evacuated acheat or radiation damage.

1 process.

atlon region terminates in a gas target.

of thin target comprises a thin plastic backing upon which suitable material is evaporated to form a thin film. Another type of thin target comprises a thin strip of foil or plastic. Still another type of thin target comprises a gas target which is contained by a thin metal foil or plastic. The thickness of such foils or sheets is often on the order of a wavelength of light, or about 5000 l()-- cm. This thickness is thousands of times as great as the diameter of the atoms involved.

A gas target has several advantages over other targets; in particular, damage due to heat and radiation is mini mized. A serious limitation of a solid target is its tendency to melt or evaporate, owing to the heat generated by the beam of charged particles. Since this tendency increases with beam current, it imposes a limitation on beam current. But a gas target cannot melt or evaporate, and so gasification trouble is avoided.

Moreover, both heat and radiation damage cause deterioration of the target, thus limiting target life. But a gas target, where the gas is in constant motion, is continually being replaced, so that the effect of target deterioration is negligible. Target deterioration may be minimized merely by using a large quantity of gas, which is continuously circulated, even if it were not purified. However, additional protection is obtained by purifying the gas at some part of its cycle.

Unfortunately, as hereinbefore stated, the gas target generally cannot be in direct communication with the evacuated acceleration region without spoiling the .vac-

-or other matter between the gas target and the interior of the acceleration region thus has the advantage of avoiding damage to such foil or other matter caused .by Moreover, organic vapors originating in the acceleration tube cannot form a layer on the gas target of the invention. Such vapors do form a layer on foils and solid targets, which layer is carbonized by the beam. The layer thus becomes a cumulative source of contaminants, which can give spurious disintegrations.

In accordance with this invention, a directed flow is imparted to the gas target material of sufficient velocity so. as to dominate the random motion of the gas molecules and celeration region.

Other objects and features of this invention may best be understood from the following detailed description in connection with the accompanying drawings. Since the bombardment of deuterium with deuterons afiords a source of neutrons used for many purposes, the invention will be described with particular reference to this However, this invention is not limited thereto, but includes any apparatus wherein an evacuated acceler- In the drawings: Fig. 1 is a diagram illustrating the principal features of this invention;

' invention;

Fig. 3 is a detail in vertical cross-section showing the target area of the apparatus of Fig. 2;

Fig. 4 is a vertical cross-section similar to .that .of Fig. 3 and showing a modified form of the target .area of the apparatus of Fig. 2; and

Fig. is a diagram illustrating certain features of a De Laval steam turbine which are similar to certain features of this invention.

The principles of my invention may best be understood by referenceto the diagram of Fig. 1, wherein a beam of charged particles is indicated by the broken line 1 and a material in the gas state which is to be bombarded is indicated by the full arrows 2. The beam of charged particles may be created by any of a. number of particle accelerators well known in the art, such as cyclotrons, linear accelerators, electrostatic accelerators, etc. Regardless of the particular particle accelerator used, however, the charged particles must be accelerated in a highly evacuated region. Since the nature of the accelerator used is immaterial to the present invention, it is indicated in the diagram of Fig. 1 merely as an evacuated region 3 wherein charged particles emitted by a suitable source 4 are accelerated in the form of a beam 1.

The acceleration region 3 communicates with a target region 5 through an aperture 6 which should be of small area, in order to assist in preventing molecules of the gas 2 from traveling therethrough, but which is sufficiently large to permit the beam 1 to issue therethrough into the target region 5. The target region 5 is an integral part of an enclosed system 7 which is completely sealed off from the atmosphere, so that essentially the only matter in said enclosed system 7 is the gas target material 2. Motion is imparted to the gas 2 in the direction of the arrows by means of a suitable pumping mechanism 8 and the gas 2 is caused to enter the target region 5 in the form of a jet directed away from the aperture 6. By imparting sufficient velocity to the gas 2 as it passes through the jet, the random motion of the gas molecules may be overcome to a sufficient extent, so that diffusion of gas molecules into the evacuated region 3 through the aperture 6 is greatly reduced; and the few gas molecules that do diffuse into the evacuated region 3 may be removed by suitable pumping apparatus.

Referring now to Figs. 2 and 3, therein is shown apparatus for bombarding a heavy-water vapor target with deuterons in accordance with my invention. Deuterons are accelerated to high energy in an evacuated acceleration tube 3 and enter a tube extension 9 as a well-de- 'fined beam 10. The deuterons travel through the tube extension 9 at high velocity, and enter a target region 5 through an aperture 6, which, as hereinbefore stated, should be of small area.

The target region 5 is an integral part of an enclosed system 7, which comprises a boiler 11, a vapor conduit 12, a nozzle 13 of annular cross-section, the target region 5, a cooling chamber 14, and a return conduit 15. Initially, the target region 5 and the interior of the acceleration tube 3 and tube extension 9 are evacuated; a conventional pumping system 16 associated with the acceleration tube 3 may be used for this purpose. Heavy water is then introduced into the boiler, as shown at 17.

At room temperature the vapor pressure of heavy water is on the order of 10 mm. Hg. Since the target region 5 has been evacuated to a pressure of on the order of 10- mm. Hg, the boiler 11 and the vapor conduit 12 quickly become filled with heavy water vapor due to evaporation from the surface of the heavy water 17.- Since such evaporation tends to reduce the temperature of the heavy water 17, a heating coil 18 is provided to maintain the boiler 11 at room temperature or thereabouts. The pressure in the boiler 11 soon reaches about 10 mm. Hg. Since the pressure in the target region 5 is initially about 10* mm. Hg, the vapor flows through the vapor conduit 12 and through the nozzle 13 into the target region 5.

The cross-section of the nozzle 13 is varied along its length in such a manner as to cause a maximum interchange of randomto directed velocity of the molecules of the vapor. Thus the vapor travels into the path of the beam 10 of deuterons as a jet which is directed away from the aperture 6 through which the beam 10 issues into the target region 5. Since the velocity of most of the vapor molecules in the jet has been directed away from the aperture 6, very few such molecules will diffuse back into the tube extension 9. Nearly all the Vapor molecules will travel on into the cooling chamber 14, in which they will acquire a more random motion through collisions with the walls of the cooling chamber 14 and with other vapor molecules. In order to minimize any tendency for these molecules to diffuse back into the target region 5, the walls of the cooling chamber 14 are cooled by water and Dry Ice, not shown, so that as the vapor molecules strike the walls of the cooling chamber 14, they lose much of their kinetic energy, so that the vapor condenses and is returned in liquid form to the boiler 11 through the return conduit 15.

By means of the heating coil 18 the pressure in the boiler 11 is maintained at about 10 mm. Hg, and by means of the water and Dry Ice the pressure in the cooling chamber 14 is maintained at about 10- mm. Hg. In this way a pressure difference is maintained across the vapor conduit 12 and the nozzle 13, which forces the vapor through the vapor conduit 12 and the nozzle 13 into the target region 5. As the vapor traverses this path, its potential energy (as measured by the vapor pressure) is reduced, and its kinetic energy (as measured bythevelocity of flow of the vapor) is increased. Alternatively, if one regards the vapor pressure as merely a manifestation of the random motion of the vapor molecules, ,one may say that as the vapor travels through the vapor conduit 12 and the nozzle 13, random motion of the vapor molecules is changed into directed motion thereof.

The nozzle 13 is designed so as to maximize this interchange of random to directed motion of the vapor molecules, and the nozzle 13 is oriented so that the resultant directed motion points away from the evacuated tube extension 9 as the vapor passes by the aperture 6. The critical factor in the proper design of the nozzle 13 is the manner in which the cross-sectional area thereof varies. Generally, this cross-sectional area should decrease in the direction of the vapor flow up to a point at or near the termination of the orifice. In Figs. 2 and 3, the point of minimum cross-sectional area, i.e. the throat of the nozzle 13 is at the termination of the nozzle 13. Fig. 4 shows a modified design of a nozzle 13' wherein the throat is a short distance upstream from the termination of the nozzle 13'.

Further details with regard to the proper design of the nozzle 13 may be derived from a study of steain turbine design, particularly the De Laval steam turbine. The type of jet desired for use in my invention is essentially the same as that desired for the De Laval steam turbine, although the reasons therefor are different. As pointed out hereinbefore, in accordance with my invention the gas or vapor molecules should emerge from the nozzle 13 with a maximum number of such molecules traveling in the same direction, in order. to minimize back diffusion into the tube extension 9.

Referring now to Fig. 5, in a De Laval steam turbine a steam jet 19 is directed tangentially against a turbine wheel 20 so as to strike the turbine buckets 21 as shown. Desirably all the steam molecules in the jet 19 should travel in the same direction, in the plane of the turbine wheel 20, so that when each molecule strikes a bucket 21, the molecule and the bucket 21 are traveling in the same direction. If the velocity of the gas stream is large, it

will deliver almost all of its kinetic energy to the wheel when its velocity, at the time it strikes a bucket 21, is twice that of the bucket 21. Since the wheel 20 is rotating very rapidly, the steam jet 19 should strike the buckets 21 a't a veryhighwelocity: for maximum efficiency.- Thusn'n th'e steam jet for-the turbine; .as in theF-jcttarget ofi my invention a high velocity directed-flow iszdesired;

Referring-now to Fig. 2'; althouglr the action:of.- the jet'raloneissufficient to prevent most ofthe'vapor molecules from entering thetube extension 9- bybackdiffusion through theaperture 6, a certain amount of vapor does escape to the tube extension. In order to prevent this vapor from reachingxthe, acceleration: tube 3- proper, a series of baifles 22 are' provided withinzthe'tube extension 9. Each baifie=22 comprises an apertured'disk 23 through the central aperture of which a restriction tube24 is fixed, the'restricn'on tubes-24 being alignedaso as to permitthe-beam to traveltherethrough'. The deuterons in'-th'e beam 10 'are'directedat high-velocity, so that'they travel through-the restriction-tubes 24 with ease. The gas molecules, however, are'moving-in, random fashion with relatively low velocity, and so:the-- restriction tubes 24 constitute' anxeflective obstructionto-the small amount of vapor which-does back-diffuse through the aperture 6.

Condensate traps 25 are annexed to the tube extension 9 on that side of each bafiie 22 which is nearer the target region 5', in order to removethevaporcollected by the baffles 22.

At each stage by far the largest part of the gas is removed transversely, so-that only asmall fraction of the gas-moves'up the-tube extension 9 to the next: stage. Since this happens over and over again, the amount of gas finally reaching the acceleration tube 3 is reduced exponentially, so that the remaining amount of gas is extremely small.

In the apparatus of Fig. 2, motion is imparted to the gas target material by maintaining the vapor in the boiler 11 at a pressure higher than that in the cooling chamber 14. The heating coil 18, the water and the Dry Ice thus serve as the pumping mechanism shown at 8 in Fig. 1. Since the gas target material is continuously circulated, it becomes possible to provide a target of a high degree of purity. The bombardment of a target may produce nuclear changes and other alterations in the target material, thereby changing its composition. The impurities thus introduced may be removed, as by providing a suitable filtering device in the return conduit 15; or a fresh supply of target material may be introduced into the boiler 11 from time to time, and the used target material removed from the system 7. Thus the target material may be purified without interrupting the bombardment of the target by the high-energy charged particles.

Moreover, the thickness of the target may be controlled merely by adjusting the pressure difference in the system 7. Thus, in the apparatus of Fig. 2, the pressure in the boiler 11 may be varied merely by varying its temperature through suitable adjustment of the heating coil 18. 'By thus varying the pressure in the boiler 11, the molecular density in the target region 5, and hence the target thickness, may be controlled.

A further advantage of my invention is the elimination of problems arising from overheating of the target. Since the gas target material is in constant motion through the charged-particle beam, the heat generated in the target region 5 is continuously being carried away by the gas target material itself. Since hydrogen is a very effective cooling medium, apparatus such as that described with reference to Fig. 2, wherein the target material includes an isotope of hydrogen, are especially adapted to the dissipation of heat generated in the target region 5.

Having thus described the principles of the invention together with a preferred embodiment thereof, 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.

I claim:

1. Apparatus for bombarding a gas target with charged particles comprising in combination: a particle accelerator having an. evacuated region-in ,which abean'n ofwcharged particlesis: produced: andv directed: into a target region which is: in open communication with said evacuated region; means for introducing gas target material: into said target region and into the pathof said beam; means, for removing said gas target material from said" target region; and means for imparting. a directed flow tosaid gas ta'rget material of sufficient velocity soas substantiallytoover come the random motion of the molecules-of" said. gas target. material, the direction of said directed flow being such as to minimize diffusion of said gastargetmaterial into said evacuated region.

2. Apparatus for bombardinga gas target with charged particles comprising in combination: a .particleaccelerator having an evacuated enclosure inwhich a beam :of charged particles is produced and out ofwhich said" beam issues through an aperture; and means for directing gas target material. into the path of said beam outside of said enclosure in a direction: substantially parallel to. the direction of travel of the charged particles; inv said beam, with sufiicient velocity so. as substantially to overcome the random motion. of the molecules of said gas'targetmaterial.

3. Apparatus for bombardinga gas target with charged particles comprising in combination: a particle, accelerator having an evacuated enclosure in which a beam of charged particles is produced and out of which said beam issues through an aperture; a source of gas target material; a conduit one extremity of which communicates with said source of gas target material and the other extremity of which terminates in an orifice in the vicinity of said aperture; the cross-sectional area of said conduit varying in such a manner as to give a maximum interchange of random to directed velocity of molecules of said gas target material as it travels from said source to said orifice; said conduit being so oriented that said gas target material issues through said orifice as a jet directed away from said aperture and into the path of said beam outside of said enclosure.

4. Apparatus for bombarding a gas target with charged particles comprising in combination: a particle accelerator having an evacuated enclosure in which a beam of charged particles is produced; a nozzle through which gas target material is introduced at the boundary of said evacuated enclosure, said nozzle having an annular cross-section and surrounding a portion of said evacuated enclosure through which said beam travels at high energy, the annular cross-section of said nozzle varying along the length of the nozzle in such a way as to provide a maximum interchange of random to directed velocity of the molecules of gas traveling therethrough; and means for introducing gas target material at the boundary of the evacuated region through said nozzle.

5. Apparatus for bombarding a gas target with charged particles comprising in combination: a particle accelerator having an evacuated region in which a beam of charged particles is produced and directed into a target region which is in open communication with said evacuated region; means for introducing gas target material into said target region and into the path of said beam; means for removing said gas target material from said target region; and means for imparting a directed flow to said gas target material of suflicient velocity so as substantially to overcome the random motion of the molecules of said gas target material, the direction of said directed fiow being such as to minimize diffusion of said gas target material into said evacuated region, and the rapid motion of the stream of gas comprising the target providing a cooling mechanism capable of carrying away large amounts of heat which might be generated by intense beams.

6. Apparatus for bombarding a gas target With charged particles comprising in combination: a particle accelerator having an evacuated enclosure in which a 'beam of charged particles is produced and out of which said beam issues through an aperture; and means for directing gas target material into the path of said beam outside of said enclosure in a direction substantially parallel to' the direction of travel of'the charged particles in said beam, with sufl'icient velocity so as substantially to overcome the random motion of .the molecules of said gas target material, and the rapid motion of the stream of gas comprising the target providing a cooling mechanism capable of preventing large temperature rises in the target region.

7. Apparatus in accordance with claim 1, wherein there is provided means to circulate said gas target material.

8. Apparatus in accordance with claim 1, wherein there is provided means to circulate and re-purify said gas target material, so as to maintain a pure target even under intense bombardment.

9. Apparatus in accordance with claim 3, wherein there is provided means for varying the amount of gas in the jet in order to produce a target of continuously controllable thickness.

10. Apparatus for bombarding a gas target with charged particles comprising in combination: a particle accelerator having an evacuated region in which a beam of charged particles is produced and directed into a target region which is in open communication with said evacuated region; means for introducing gas target material into said target region and into the path of said beam; means for removing said gas target material from said target region; and means for imparting a directed flow to said gas target material of suflicient velocity so as substantially to overcome the random motion of the molecules of said gas target material, the direction of said directedwflow being such as to minimize difiusion of said gas target material into said evacuated region, whereby the target reaches equilibrium in approximately the time required for the passage ofthe swift gas stream through the target region. r i a l l 1. Apparatus for bombarding a gas target with charged particles comprising incombination: a particle accelerator having an evacuated enclosure in which a beam of charged particles is produced and out of which said beam issues through an aperture; and means for directing gas target material into the path of said beam outside of said enclosure in a direction substantially parallel to the direc-L tion of travel of the charged particles in said beam, with sufiicient velocity so as substantially to overcome the random motion of the molecules of said gas target material, whereby the target is continuously being renewed so that it does not cumulatively change in composition in time.

References Cited in the file of this patent UNITED STATES PATENTS Kallmann et al. July 29, 1941

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