US2270373A - Neutron image converter - Google Patents

Description

.Fan. 20, 1942. H. 1. KALLMANN ETAL NEUTRON IMAGE CONVERTER Filed May 20, 1940 ?atented .Fan. 20, 1942 NEUTRON IMAGE CONVERTER Hartmut Israel Kallmann, Berlin- Charlottenburg, and Ernst Kuhn, Berlin, Germany, assignors to I. G. Farbenindustrie Aktiengesellschaft, Frankfort-on-the-Main, Germany, a corporation of German Application May 20, 1940, Serial No. 336,311 In Germany June 24, 1939 11 Claims.

This invention relates to devices for producing images with the aid of neutrons as depicting radiation, and is more particularly concerned with a neutron image converter as more specifically described hereinafter.

It has already been suggested to irradiate an object with a beam of neutrons and to utilize the neutrons unabsorbed by the object for producing an image of the distribution of intensity in the residual beam. For this purpose, it has been proposed to-cause these neutrons to impinge upon a neutron-reactive layer provided on the side of the object remote from the source of the neutrons, said layer being of such a kind that neutrons liberate heavy charged particles or electrons from it or produce radio-active nuclei therein. In the previously suggested device the heavy particles or electrons or gamma radiation were then caused to energize an adjacent fluorescent screen or to blacken a photographic plate. The images produced in this way are, however, in many cases not rich in contrasts and not very intense. An increase of the neutron-radiation is, on principle, possible but requires a considerable technical expenditure, as very high potentials are necessaryto produce the neutrons.

It is an object of the present invention to provide improved means of depicting the intensity distribution, which means shall obviate this difllculty. This and other inventive objects are attainedby a device comprising, in combination, a vacuum tube, which contains a neutron reactive intermediate layer, from a surface of which slow electrons are liberated, an accelerating electricfield, an electron-optical depicting system and fluorescent screen or a photographic plate. When the objects to be investigated are irradiated with neutrons, the neutrons unabsorbed by the objects are caused to impinge upon the neutron reactive neutron-reactive layer, from which they liberate heavy charged particles or electrons. These particles or electrons in their turn are caused to liberate slow electrons from the surface of the neutron-reactive intermediate layer itself or of an adjacent layer. These slow electrons are then accelerated by the electric-field and collected by the electron optical depicting system on the fluorescent screen or the photographic plate, where. they produce an image and show a representation of the intensity of the beam of neutrons impinging on the intermediate layer. This additional device can be operated with by. far lower potentials than the source of neutrons so that, in spite of the additional expenditure a practically important advantage is obtained, as

the image produced by the accelerated electrons is, if the accelerating fields are properly chosen, considerably richer in contrasts and more intense than the image obtained with the formerly proposed method.

The material to be used for the neutron-reactive layer may be lithium or boron or a substance containing atoms of the one or the other of said elements. In connection therewith it is desirable to choose the distribution of the isotopes in such a manner that the sort of isotope active in the neutron-converting reaction is enriched relative to the natural distribution of the isotopes of the respective element. The neutron-reactive layer may, however, also consist of uranium or of atoms thereof. The neutrons liberate from these latter heavy particles particularly rich in energy and excellently suited for the production of slow electrons. Furthermore, the neutron-reactive layer may, under circumstance, especially if quick neutrons are used as depicting radiation, be formed advantageously by a layerloaded with hydrogen, this layer emitting slow electrons under the action of the recoil protons energized by the quick neutrons.

By quick neutrons or rapid neutrons" are meant neutrons whose wavelengths are longer than the diameter of the bombarded atom nucleus: with the atoms concerned, this requirement is met by neutrons with speeds corresponding to a few hundred thousand volts energy. By recoil protons are meant protons formed by repulsion. In the present; connection it is noted that the recoil protons are given an acceleration due to impact by the impinging neutrons, so that they move with a speed corresponding to the absorbed energy.

The neutron-reactive layer or the layer adja-- cent to it may also contain beryllium or magnesium or atoms of these substances to increase the production of slow electrons under the action of the heavy charged particles or of the quick electrons liberated by the neutrons out of the neutron-reactive layer.

The image on the fluorescent screen may be subsequently intensified either only once or several times by means of an electron-optical image amplifier. 1

It is, under certain circumstances, advantageous to cause the heavy charged particles or the electrons liberated from the neutron-reactive layer by the neutrons to act upon an adjacent In order to increase the intensity of the light falling upon the photosensitive layer a, reflecting layer may be provided between the neutronreactive layer and the fluorescent substance. This layer reflects the radiation emitted by the fluorescent mass towards the side of the neutronreactive layer back to the photosensitive layer. The reflecting layer must be chosen such that it permits the heavy charged particles or electrons liberated by the neutrons to pass through it unimpededly.

It has been discovered that also the use of a layer containing gadolinium atoms is in many cases particularly suited for the purpose in view, as thenumber of electrons liberated per incoming neutron is rather large.

In the case of gadolinium the electrons liberated by the neutrons have an energy of about 70,000 volts. These electrons when passing through the surface of the neutron-reactive layer or of an adjacent layer generate slow electrons which are employed for producing an image in the above described way.

The invention is illustrated diagrammaticallyand by way of example on the accompanying drawing on which Figure 1 shown an axial section through a device designed according to this invention, and Figure 2 shows a modified concharged particles or electrons is diiferent from place to place and corresponds to the intensity ofthe radiation of neutrons as it is transmitted by the object or body to be depicted. The heavy charged particles or electrons liberate slow electrons either from the neutron-reactive layer 5 itself or from a layer 6 located adjacent thereto, which electrons are emitted into the vacuum space I and are there accelerated, for instance by an electric field V1 existing between the electrode 8, or the layer 5, or 6 respectively. By means of an electron-optical depicting system, consisting, for instance, of said accelerating electric fleld and 01' an additional magnetic or electrostatic lens 9 said slow electrons are gathered upon a fluorescent screen or upon a photographic layer It], where they form an image, which shows a representation, rich in contrasts, of the object or body 3, as it depends on the absorption relations for neutrons in the object or body undergoing investigation.

There is provided in the constructional form shown in Fig. 1 another device intended to increase the intensity and the contrast, in case the layer l should constitute a fluorescent screen. The radiation proceeding from this screen acts upon the adjacent layeril which is sensitive to light and emits photo-electrons under the action of light which photo-electrons are accelerated by the electric field V3 existing between the layer II and the electrode I2 and collected upon the fluorescent screen I by means of an electronoptical depicting system, for instance a magnetic cent screen ll can again be amplified in intensity by a second image-amplifier. It is, under certain circumstances, desirable to design the device in such a manner that the vacuum spaces 1 and i5 communicate with one another. It is further desirable to provide the fluorescent screens i0 and II with reflecting layers l6 and I! on the sides remote from the observer. These layers. which must be permeable by the impinging electrons, serve to reflect the light emitted by the screens back to the observer.

If the device has been designed such that the heavy charged particles or electrons liberated from the neutron-reactive layer by the neutrons act upon an adjacent fluorescent mass [8, the radiation of which liberates slow electrons from a layer l9 sensitive to light, as is illustrated in Fig. 2, it is desirable to provide likewise a reflecting layer 20 between the neutron-reactive layer 5 and the fluorescent substance ll. 1

It may under certain circumstances be suitable to separate the photo-cathode H from the fluorescent screen by a transparent p'art H of the wall of the vessel: such separation comes into consideration in a case where the photo-cathode and the fluorescent screen might otherwise have an unfavorable eflfect upon each other. The two layers need not lie in this case on the same potential. It is under certain circumstances even advantageous to maintain between them a .poten-v 'tial different from 'zero by means of an outer source of potential Va.- For the absorption of the X-rays mostly produced simultaneously with the neutrons it is desirable to provide in the path of the beam of neutrons, for instance in front of the entrance side of the device, a filter 22 capable of absorbing the X-rays, for instance a filter of lead, which is permeable to neutrons.

The images obtained with the present improved device can be made considerably sharper than the images produced with the aid of the former devices, because the slow electrons utilized for making the images proceed only from an extraordinarily thin surface layer of the neutron-reactive layer 5, or of the special layer 6 delivering the slow electrons.

As the energy of the electrons emitted from the layer containing gadolinium is comparatively large, it is advantageous to utilize them not di-' rectly for the liberation of slow electrons from the neutron-reactive layer itself or from an adjacent layer, but to cause them to act upon a fluorescent mass which is either mixed with the substance of the neutron-reactive layer or is provided in its proximity. The radiation emitted by the fluorescent mass under the action of the electrons is employed for the liberation of slow electrons out of an adjacent layer, which electrons are used for the production of the image.

Another advantage of the neutron-reactive layer consisting of gadolinium or containing gadoiinium atoms is that it reacts practically only with neutrons of terminal energy (that is to say.

slow neutrons that have, by their impacts, en-

tered into equilibrium with the surrounding atomic nuclei). The gadolinium-containing neutron-reactive layer dlflers in this respect advantageously for instance from neutron-reactive layers containing lithium or boron atoms. This property is especially favorable in the case that, as is the rule, the source of neutrons emits neutrons of unhomogeneous energy so that there are, besides the neutrons of thermal energy, also neutrons with a considerably larger energy which are absorbed by the bodies under investigation olinium it is suitable to choose the thickness of the neutron-reactive layer nearly equal to the range of the electrons liberated from that substance, because no electrons reach the surface of the neutron-reactive layer (and pass from the latter into the image converter) from those parts of the layer which are farther away from the surface thereof than the width of range of the electrons. If, on the other hand, the neutron-reactive layer containing gadolinium is too thin, the output is reduced, as a part of the neutrons passes through such a thin layer without being caught by gadolinium atoms.

We claim:

1. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a neutron-reactive layer in which charged particles are generated by impinging neutrons, an electric field and an electron-optical depicting system for accelerating slow electrons dissolved by said charged particles, and an electron-sensitive system on which said slow electrons after having been accelerated are collected to form an image which shows a representation of the intensity of the incoming beam of neutrons.

2. A device as defined in claim 1 in which the neutron-reactive layer contains atoms se-. lected from the group consisting of boron, lithium, uranium and gadolinium.

3. A device as defined in claim 1 in which the electron-sensitive system includes a fluorescent substance.

4 A device as defined in claim 1 in which the electron-sensitive system includes a photographic layer.

5. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a neutron-reactive layer in which charged particles are generated by impinging neutrons, another layer adjacent to the neutron-reactive layer from which second named layer said charged particles dissolve slow electrons, an electric field and an electron-optical depicting system for accelerating said slow electrons, and an electron-sensitive system on which said slow electrons after having been accelerated are collected to form an image which shows a representation of the intensity of the incoming beam of neutrons.

6. A device as defined in claim 5 in which the layer from which the slow electrons are liberated contains atom selected from the group consisting of beryllium and magnesium.

7. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a neutron-reactive lay-- er in which charged particles are generated by impinging neutrons, an electric field and an electron-optical depicting system for accelerating slow electrons dissolved by said charged particles, a fluorescent substance on which said slow electrons after having been accelerated are collected to form an image, and a photographic layer adjacent to said fluorescent substance on which the radiation excited in the fluorescent substance produces a photographic representation of the intensity of the incoming beam of neutrons.

8. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a neutron-reactive layer in which charged particles are generated by impinging neutrons, an electric field and an electron-optical depicting system for accelerating slow electrons dissolved by said charged particles, at fluorescent substance on which said slow electrons after having been accelerated are collected to form an image, and a second image-amplifier which amplifies the intensity of said image.

9. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a neutron-reactive layer in which charged particles are generated by impinging neutrons, a fluorescent substance adjacent to said neutron-reactive layer, a photosensitive layer from which radiation excited in said fluorescent substance by said charged particles dissolves slow electrons, an electric field and an electron-optical depicting system for accelerating said slow electrons, and an electron-sensitive system on which said slow electrons after having been accelerated are collected to form an image which shows a representation of the intensity of the incoming beam of neutrons.

10. Adevice as defined in claim 9 in which a reflecting layer permeable by the charged particles generated by the neutrons is inserted between the neutron-reactive layer and said fluorescent substance,

' 11. A device for producing images with the aid of neutrons as depicting radiation comprising a vacuum tube containing a layer containing hydrogen atoms which are energized to high speed by impinging neutrons by collision, an electric field and an electron-optical depicting system for accelerating slow electrons dissolved by said high speed hydrogen atoms, and an electronsensitive system on which said slow electrons after having been accelerated are collected to form an image which shows a representation of the intensity of the incoming beam of neutrons.

HAR'I'MUT ISRAEL KALLMANN. Formerly known as Hartmut Kallmann.

ERNST KUHN.

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