US2305452A - Method and device for depicting the intensity distribution in a beam of slow neutrons - Google Patents

Description

H. l. KALLMANN ET AL 2,305,452

METHOD AND DEVICE FOR DEPICTING THE INTENSITY DISTRIBUTION IN A BEAM OF SLOW NEUTRONS Filed Sept. 28, 1940 2 Sheets-Sheet l l. KALLMANN ET AL Dec. 15, 1942.

2,305,452 METHOD AND DEVICE FOR DEPICTING THE INTENSITY DISTRIBUTION IN A BEAM OF SLOW NEUTRONS 2 Sheets-Sheet 2 Filed Sept. 28, 1940 a nu end Mo W MM r 1 u e I n I n 1 I u v I 4 v n u I i l nten'leel 315,, race earl @LWW n'nturnons Hartmut Kernel liinllmann, herllmtllharlottenhurg, and Ernst Kuhn, Berlin, @ermany; vested in the Alien Property Eustodlnn Application September 28 lln Gerrnony den invention relates to improvements in methods of and devices for depicting the intensity distribution in a beam of slow neutrons, and as particularly concerned with efiecting separa- ;ion oi uuiclt neutrons from slow neutrons and naking use only of the slow neutrons for the lepictlon.

It has been proposed to use an intermediate caution tor the depicting of the intensity disrlbution in a beam of slow neutrons in which; meter the influence of the slow neutrons, charged iarticles or gamma rays are produced which exlte a fluorescent screen or directly or indirectly ilaclsen a photographic layer. It has further ieen proposed to use these neutron reactions in Milli, Serlnl No. 355,938

when 2%, 11939 neutron image converter for the production of luorescent screen images or photographic imges. It is very difficult to produce a beam conaining only slow neutrons. Mostly quick neurons occur at the same time with the low neutrons even with the best screening. lostly, also, a gamma radiation occurs at the sine time which also excites the fluorescent.

:reen and directly or indirectly blackens the hotographic plate. When the opening for the Kit of the slow neutrons amounts to only a very nail traction of the total surface of the source t neutrons, the quick neutrons which had not con absorbed become disturbingly perceivable wing to the many times greater surface ofthe )UI'CB of quick neutrons. It is therefore necestry to count with the fact that a beam of slow eutrons also contains the undesired quick neuoils which act, although if only in a less strong agree, upon the means for depicting the slow eutrons.

in the device according to the invention a sepration of the slow neutrons from the quick neu ons involves the intermittent production of the eutrons, the intervals of neutron production be- .g comparatively briei'. At agreater distance cm the source of neutrons, a spatial separation the quick neutrons from the desired slow neuons then takes place. According to the invenm the depicting. is carried out at such a place it by means or a device the operation of which nnot be timely influenced, but rather by means a neutron image converter according to the ct t the source of neutrons works only durshort intervals of time. The quick neutrons first an then the slow neutrons upon a ctlon layer of this image converter, in [oh l the impinging neutrons produce l icles or gs. rays, which are used 1' the production or the fluorescent Il'd lien:

screen image or for the blackening of the photographic plate. riccog to the invention care is taken by corresponding controlling of the voltages of the neutron image converter, that this converter operates only if no quick neutrons pass through it. The neutron image converter is actuelly put into operation only when slow neutrons pass through it; As the quick neutrons, as already explained, owing -;to their greater speed, pass sooner through the neutron image converter than the slow neutrons, only the slow neutronscome into edect in the image converter according to this method.

The quick neutrons and the gamma rays act, however. if only in a very slight measure, also directly upon the fluorescent screen or upon the photographic plate. As during the time when these quick neutrons and gamma rays pass through the image converter this converter is inomrati'le to accelerate electrons, its intensifying eflect is suppressed for these quick neutrons and gamma rays, so that the remaining direct effect of the quick neutrons and gamma rays upon the fluorescent screen or upon the photographic layer does not become disturbingly perceivable.

If the duration of the production of neutrons is short enough, and if the distance between the image converter and the source of neutrons is great enough, the neutrons of different speed pass successively through the converter. In this device it is therefore possible by adjusting the time difierence between the end of the production of the neutrons and the starting of the neutron image converter to allow only neutrons below a certain speed to come into cflect; The upper limit of the speed of the nutronsat a certaln adjusting of the phase shifting between the end of the production of neutrons and the starting of the neutron image converter is given by the distance of the image converter from the source of neutrons. The greater the distance of the neutron image converter from the source of neutrons is, thelower is at given phase shifting'the upper limit oi the speed of the neutrons coming into eflect.

In general it is not of advantage to extend longer the production of neutronsthan up to the moment at which the first desired slow neutrons of maximum speed just impinge on to the neutron image converter.

The invention will nowbe described in greater detail, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic sectional view, taken views illustrating two furthernwdiiications oi the apparatus of Fig. 1.

According to Fig. 1, the ion beam I produced in the discharge tube impinges on a substance 2, for instance lithium, in which quick neutrons are produced, which are slowed down by the surrounding medium i, such as water. The slow neutrons pass through the water-free aperture at t. Quicker neutrons pass, but with less intensity, through the same aperture. For obtaining a suitably narrow beam of slow neutrons two diaphragms h and t are provided, of which the one, for instance 0, serves for the absorption of slow neutrons, whereas the other diaphragm t absorbs the X-ray radiation. The beam l contains slow and quick neutrons which, in the diagrammatic illustration, are designated by and l i. These neutron traverse partly the body it to be examined, so that behind this body the distribution of intensity in the beam is different from that in front of the body. Fluorescent screen images or photographic images are produced by the neutron image converter Ill as lon as it is in operation and when the neutrons impinge on the same.

In the layer 20, which consists, for instance, of lithium, heavy charged particles are released by the impinging slow neutrons of ray bundle l,

and these charged particles for instance release slow electrons from the adjacent layer 2 I. These electrons are depicted on a fluorescent screen It, that is coated on its surface, suitably, with a conducting layer 23; the depicting being accomplished in the first place with the aid of an electric held that can be produced by means of the cylinder 22, and then by means of an electronoptical depicting system, for instance, a magnetic lens it. This fluorescent screen image is observed through the wall Id of the neutron image converter from the outside or reproduced photographically.- The depicting of the slow electrons emitted from the layer 2! is accomplished with this arrangement only as long as there is an electric field between the parts ii and 22 that accelerates the electrons.

The controlling of the operating voltage of the neutron image converter and of the source of neutrons takes place in such a rhythm, that the undesired quicker neutrons it have already left the image converter when the latter is put into operation. This is attained, in the constructional example shown in Fig. 1, in that the neutron image converter i0 is set into operation only a certain time after the cessation of production of neutron rays. The two switche ll, 25 that are connected together by a common actuating device it are used for this purpose. The switching in and out of the source of ions with the aid of switch I! is accomplished as follows:

The ions formed in the space between the anode l2 and the cathode It are collected into a beam by the shape of the anode it, this beam passing through the opening of the electrode it ascents and being accelerated by the potential V: maintained between this electrode and electrode It. They enter into the tube of electrode i5 in the direction of arrow I with great speed and impinge at its lower end on a substance 2--for instance, lithium-in which quick neutrons are produced by the impinging ions. The electrodes it, It and It are separated from each other by the insulators it, iii.

Ions are'produced with this arrangement as long as there is maintained the voltage V1 between the electrodes It and It. This comparatively small voltage of 20,000 volts for instance is controlled by the switch i'l shown by a symbol. This arrangement gives the advantage that switching of the very high accelerating voltage V: about 1000 kV., is avoided. By connecting switches ii and 26 together, in the described way the voltage Va is laid on the accelerating electrode of the neutron image converter only after the switch is open. In this way the neutron image converter becomes operative only a certain time after the production of neutrons has ceased. The adjustment of the period of the production of neutrons and the time difierence between the switching out the neutron ray production and the starting of the neutron image converter is accomplished in a simple way by shifting the contacts of the switches ll, 26 and regulating the speed of the coupling device It. The duration of the neutron ray production to be adjusted and the time differential between the switching-out of the neutron ray production and the starting of the neutron image converter is obtained in a simple manner from the times which are required by the desired slow neutrons to pass through the distance A, and which the slowest of the undesired quicker neutrons require for passing along the distance B.

In the gamma rays have a still greater speed than the quickest neutrons, the eflect of the gamma rays is not intensified according to this method by the neutron image converter. In a similar manner as the neutrons of undesired speed belonging to the beam I are prevented from acting on the image converter the undesired quick neutrons proceeding from the remaining portion of the device are separated out.

The source of neutrons can be switched in again as soon as the last of the desired slow neutrons hrve acted in the image converter. As the quick neutrons and the gamma rays have an extraordinarily high speed, so that they pass alon the distance A in a very short time, the image converter must be cut out before the production of the neutrons begins again, in order that it is inoperative when the first gamma rays oi the succeeding series reachit.

The controlling of the source of neutrons and of the image converter is preferably carried out periodically, in order to obtain a summation of the eflects. The coupling of the engaging and disengaging means of the source of neutrons is preferably carried out electrically or mechanically in a manner known per se. A periodical voltage of similar frequency can, for instance, be fed to the source of neutrons and to the neutron image converter. The service voltage of the image converter must lag behind the service voltage of the source of neutrons.

The periodic control of the source of neutrons and of the image converter can be accomplished by the coupling device it Figure 1 or for instance by the arrangement shown in Fig. 2. In the arrangement according to Fig. 2 two pairs oi contacts, for instance slip-brushes it, it are connected together periodically by segments of conducting material til, as that are embedded in disks 3d, 3!! of insulating material while the disks are turned by a common shaft 32. The length oi the conducting segments 2? and 29 and the relative position of these segments in relation to each other and the spacing of the contacts of the two pairs 26, 28 from each other and their relative position can be adjusted easily so that the desired switching sequence can be adhered to.

If the distance of the image transformer from the source of neutrons is about 2 to 3 metres, and if the object is merely to separate out the quick neutrons from the slow neutrons and from those of median speed, it is advisable to put into service the source of neutrons every time for about A 0 second. this time to pass through the distance A of 2 to 3 metres. Directly after the cutting out of the source of neutrons the neutron image converter is put into operation. so that the neutrons of desired speed come into action successively. As soon as the last of the slow neutrons have entered into action in the image converter, this converter is put out of operation and the source of neutrons is put into operation again. In this example all slow neutrons must have passed through the distance A before the source of neutrons is switched in again. The service frequency of the source of neutrons and of the image con verter amounts therefore to about 500 oscil lations per second, i. e., 500 Hertz.

It may, under certain conditions be advisable to limit the observation of the fiuorescentscreen image, for instance by mechanical means, to the time in which the fluorescent screen is energized by the desired slow neutrons by means of the radiation proceeding from the intermediate reaction layer. With this object in view a mechanically actuated diaphragm liberating the observation path with the frequency of the image converter can be provided, for instance in the ray path of an optical system producing a real or virtual image of the fluorescent screen. For this purpose, for instance, as shown in Fig. 3, the fluorescent screen is can be reproduced actually on a screen or on a photographic plate 35 by means of an optical system 33, 33. In the ray r path of this optical system, a rotating disk 35 for instance is provided that has one or more openings it that free the beam path periodically when the disk rotates on the shaft 38. When an arrangement according to Fig. 2 is employed for controlling the source of neutron rays and the neutron image converter, it is advisable to couple the shafts it and 32 to each other directly. It is then possible in a very simple way to couple the periodic sequence of the switching in and out of the source of neutron rays and of the neutron image converter with the freeing oi the beam path for producing the picture on the screen 35. The same arrangement can be used when, with the aid of the optical system it, ti l not a real but a? virtual picture is produced, that can be examined with the eye. A corresponding device may also be used in the photo graphic depicting of the fluorescent screen image, provided that the fluorescent screen is not arranged in direct neighbourhood of the photographic layer. These methods possess the advantage that the unintensified effect of the quick neutrons and gamma rays is practically absolutely excluded. Merely the extraordinarily The slow neutrons require slight efiect remains, which results from the re maining fluorescence of the fluorescent screen substance.

In controlling the source of neutrons it is often necessary to influence comparatively large efiiciencies if the controlling has to be carried out by cutting in and cutting out of the service voltage. It is therefore sometimes preferable to conduct the beam of ions during the pauses in operation of the source of neutrons so that it does not impinge on a substance emitting neutrons. With this object in view either the beam of ions may be deflected or the substance capable of emitting neutrons may be removed from the path of the beam of ions. The deflection of the beam of ions may be efiected in the usual manner by electric or magnetic means or mechanically. The removing of the substance capable of emitting neutrons is attained in the simplest manner by rotation of a body, the parts of which coated with a substance capable of emitting neutrons are actually brought into the path of the beam of ions'only during the times when the sources of neutrons has to operate.

Thus, for the purpose above stated, the ion beam i as shown in diagram in Fig. 4, is deflected by a magnetic field so that temporarily it does not impinge on the substance 2. For the sake of simplicity, only the electrode it of the dis charge tube for producing the quick neutrons is shown. The deflection is accomplished by a magnet coil 3% placed preferably outside the tube i5, this coil being fed by the battery V5 as long as the contact dd is closed. The contact iii can be switched in and out in the manner presented in the explanation of Fig. l, by a coupling device id, the left part of which is'shown in Fig. 4. As soon as the current flows through the coil 39, the ion beam 5 is deflected so that it impinges on the wall at and therefore cannot release quick neutrons from the substance 2. Since in this constructional example likewise the production of neutrons must be interrupted during operation of the neutron image converter. the contacts to and 25 must be closed simultaneously so that the magnet coil 39 is energized only during the time of operation of the neutron image converter. Instead of the magnet field, an electrical field could likewise be used for deflecting the ion beam, this being controlled in analogous manner.

The periodical interruption of neutron ray production could likewise be accomplished by temporarily removing the substance capable of emitting neutrons from the path of the ion beam.

Fig. 5 shows a constructional example of an arrangement of this kind. In this case likewise only the electrode it of the discharge tube for producing quick neutrons is shown, and at the lower end of this electrode, on the rotating rod at, an application of the material 2 is made'at one or more places. The rotation of therod 32 on the shaft 33 can be accomplished by electric motors arranged inside the receptacle for the production of neutron rays or, as shown in Fig. 5, by magnetic action through the receptacle walls. In this embodiment, the rod t2 consists preferably of a magnetic material, for instance iron, while the wall of the apparatus for producing neutrons consists of a non-magnetic materiai, so that the rod B2 is set into rotation under the influence of the magnetic field of the magnet rod t5 rotating outside the receptacle on the shaft Qt. With the use of a sufllciently strong magnet rod, it is possible to keep the slip tribution in a beam of which comprises producing between the movements of the rods i2 and ts so small that an exact adjustment of the time at which the ion beam impinges on the layer 2' and leaves it again is possible. The shaft M can likewise be coupled to the shaft 32 or the coupling device i8 in the manner described in connection with the other constructional examples, so that the substance Zcapable of emitting neutrons is brought into the path of the ion beam temporarily only during the desired times of operation of the source of neutron rays.

A special advantage of the arrangement according to the invention is, that the photographic plate or the fluorescent screen, although they are at rest, are not exposed to the action of neutrons of undesired speed. n

We claim:

1. A method for depicting the intensity distribution in a beam of slow neutrons produced by impinging a beam of ions upon a substance capable of emitting neutrons which beam of slow neutrons contains also a radiation of larger speed, especially for the depicting of objects, which comprises producing the neutrons intermittently, carrying out the depicting of the intensity distribution by means of a neutron image converter provided at a place at which the slow neutrons and the radiation of larger speed W9 ing to the intermittent operating or the source of neutrons are spatially separated the one from the other by their difference in moving-time, and

controlling the service voltages of the neutron image converter so that this converter depicts only during the time intervals in which no radiation of large speed pass through it.

2. Method as specified in claim 1, in which the source of neutrons is controlled by altering the service voltage, especially by the switching in and outof the same.

3. Method as specified in claim 1, in which the beam of ions serving for the production of neutrons is guided during the intervals of service so that it does not impinge on the substance capable of emitting neutrons.

4. Method as specified in claim 1, in which the substance capable of emitting neutrons is removed from the beam of ions for interrupting the production of neutrons.

5. A method for depicting the intensity -distribution in a beam of slow neutrons produced by impinging a beam of ions upon a substance capable of emitting neutrons which beam of slow neutrons contains also a radiation of larger speed, especially for the depicting of objects, which comprises producin odically in time, and carrying out the depicting of the intensity distribution by means of a neutron image converter provided at a place at ,which the slow neutrons and the radiation of larger speed owing to periodical operation'of the source of neutrons are spatially separated the one from,

the other by their difference in moving-time, and controlling the service voltages of the neutron images converter so that this converter depicts only during the time periods in which no radiation of large speed pass through it, so that a summation of the eifects of the individual operations takes place.

9. A method for depicting the intensity disslow neutrons produced by impinging a beam of ions upon a substance capable of emitting neutrons which beam of slow neutrons contains also a radiation of larger speed, especially for the depicting of objects,

the neutrons in sevthe neutrons perieral time intervals, carrying out the depicting of the intensity distribution by means of a neutron image converter provided at a place at which the slow neutrons and the radiation of larger speed owing to the intermittent operating of the source of neutrons are spatially separated the one from the other by their difference in moving-time, controlling the service voltages of the neutron image converter so that this converter depicts only during the time intervals in which no radiation of large speed pass through it and providing a diaphragm in the ray path of an optical system which produces a real or virtual image of a fluorescent screen in the path of emanation of an observation ray from the neutron image converter, which diaphragm is moved at the frequency of the neutron image converter, said diaphragm mechanically liber-- ating the observation ray only during the time in which the neutron image converter is operating.

7. Apparatus for depicting the intensity distribution in a beam of slow neutrons, especially for the depicting of objects, which comprises an ion source; a neutron source adapted to emit neutrons upon impingement thereon of ions from a said ion source, a neutron image converter in operative alignment with said neutron source but spatially removed therefrom at a preselected distance of the order of a few meters, said neutron image converter being adapted to be operated intermittently; means for depicting an observation ray from said neutron image converter; and means for interrupting emission of neutrons from said neutron source whenever the neutron image converter is being operated and for interrupting operation of the neutron image converter whenever neutrons of undesired speed are passing through the same.

8. Apparatus for depicting the intensity distribution in a beam of slow neutrons, especially for the depicting of objects, which comprises an ion source; a neutron source adapted to emit neutrons upon impingement thereon of ions from said ion source, a neutron image converter in operative alignmentwith said neutron source but. spatially removed therefrom at a preselected distance of the order of a few meters, said neutron image converter being adapted to be operated intermittently; means for depicting an observation ray from said neutron image converter; and means for interrupting emission of neutrons from said neutron source whenever the neutron image converter is being operated and for interrupting operation of the neutron image converter whenever neutrons of undesired speed are passing through the same, said last named means including means for intermittently deflecting a beam of ions passing from said ion source to said neutron source.

9. "Apparatus for depicting the intensity distribution in a beam of slow neutrons, especially for the depicting of objects, which comprises an ion source; a neutron source adapted to emit I neutrons upon impingement thereon of ions from said i source, a neutron image converter in oper ive alignment with said neutron source but spatially removed therefrom at a preselected distance of the order of a few meters, said neutron image converter being adapted to be operated intermittently; means for depicting an observation ray from said neutron image converter; and means for interrupting emission of neutrons from said neutron source whenever the neutron image converter is being operated and for interrupting asooaoa oneration oi the neutron image converter vvhenever neutrons oi' undesired speed are passing through the same, said last named means ineluding means for intermittently moving said neutron source out of range oi a beam of ions from said ion source.

lid. Apparatus for depicting the intensity distribution in a beam of slow neutronsespecially tor the depicting of objects, which comprises an ion source; a neutron source adapted to emit 1 neutrons upon impingement thereon of ions from said ion source, a neutron image converter in operative alignment with said neutron source tout spatially removed therefrom at a preselected distance oi the order or a few meters, said new tron image converter being adapted to be operated intettently; means for depicting an ohsired speed are passing through the same.

sen-ration rav from said neutron image converter said means including an optical system, in the ray path from the neutron image converter, for producing a visual image of the intensity distribution, a diaphragm in the ray path between said neutron image converter and said optical system and means for moving the diaphragm into and out of ray-obstructing position at the frequency of operation of the neutron image converter; and means for interrupting emission of neutrons from said neutron source whenever the neutron image converter is being operated and for interrupting operation of theneutronimage converter whenever neutrons of unde- Yll Q

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