US2989662A - Radiographic device - Google Patents

Description

June 29,1961 B. R. LINDEN 2,989,662

RADIOGRAPHIC' DEVICE Filed Nov. 13, 1958 2 4Sheets-Sheet 1 mmm PJIIIIIIIIIIII'IIIIIA` I5 uTlLlzAfnoN crRouw I l l i F fg.

INVEN TOR. BERNARD ROBERT LINDEN ATTORNEYS June 20, 1961 B. R. LINDEN RADIOGRAPHIC- DEVICE 2 Sheets-Sheet 2 Filed Nov. 13, 1958 lll/lll UTILIZATION CIRCUIT l/ llfll/l/lllllzllfl/ Fig. 2

IN VEN TOR.

N E D W T R@ E Bf 0 R DM R A MQ. Y E BB E N w T T A.

the impinging X-rays to 'materiaL there would be negligibleconduchon, and neglinited States This invention relates to radiographic analysis, and in particular to such an analysis using X-rays or neutrons.

Since a stream of neutrons is called nuclear radiation,

the term radiation, as used herein, will be construed to include both X-rays and a stream of neutrons.

One frequently desires to study the internal condition of an object. One way to conduct such a study is known as radiography. 'Ihe basic principle of radiography is that when a composite structure is exposed to radiations, various elements of the structure absorb the radiation in different amounts. X-radiation is frequently used, and the emergent, unabsorbed radiation produces either a display on a iluoroscope or a picture on a lilm.

Both of the above systems have limitations. While the fluoroscope will show moving elements, it cannot show tine detail. Film will show more detail, but is blurred by movement. A ready solution to both problems is the use of television type equipment. This can picture moving elements, produce a large picture, and show ne detail, the amount of detail being determined by the pickup device.

One pickup devicey has a dielectric material which becomes conductive when X-rays impinge on it. This material has one or both sides thereof covered with a conductive X-ray transparent coating, and a potential is applied to the coating to set up an electric iield. 4In operation, the impinging X-rays free some of the electrons of the dielectric material. The freed electrons in the dielectric material come under the inuence of the electric eld, land migrate; thus causing the dielectric material to become somewhat` more conductive. The amountand pattern of impinging X-ray radiation determines where and to what extent the dielectric becomes conductive. It is imperative that the electric field-producing coating be X-ray transparent, otherwise it would absorb the X-'rays and prevent them from impinging on the dielectric material.

ceed the above energy limit.

Radiography is also used in industry to analyze thick or dense materials for the quality of welds, flaws, air bubbles, etc. In many industrial uses, the X-rays must have very high penetrating power, and the energy of the radiations may range up to 2000 kilovolts.

With the increasing use of industrial radiography, there is an increasing need for pickup devices capable of use with such high energy radiations. It has been suggested that prior art pickup devices be used or modified for use with high energy X-rays. [lf the unmodified previously described prior art device was used for such high energy X-ray radiation, it would not operate satisfactorily. In this high energy range, the previously used thickness of dielectric material makes it practically X- ray transparent, that is, it would not stop and absorb the X-rays. Since the operation of the device requires be absorbed by the dielectric gible output signal.

Modifying this device to absorb high energy X-rays by using a thicker layer of dielectric materlal is unfeas- Patented `iunie 20, 1961 ible for two reasons, even though the thicker dielectric material would stop the X-rays. Firstly, most of the freed electrons would be trapped in the thick material, and would not be available for producing an external signal. Secondly, in traversing the thick material, those electrons which are not trapped would be so widely scattered that they would emerge far olf their direct path. This diversion would produce a picture 0f poor resolution, i.e., the tine detail would be lost.

It is therefore one object of my invention to provide an improved radiographic system.

It is another object of my invention to provide a transducer which will transform neutron or high energy X- ray radiation into a television type signal.

The attainment of these objects and others will be realized from the following speciicat-ion, taken in conjunction with the drawings, in which,

FIG. l is a diagrammatic view of a pickup device constructed in accordance with the principles of my invention; and

FIG 2 illustrates a diierent embodiment of this device.

This application is a continuation-impart of my application Serial No. 498,992, filed April 4, 1955, now abandoned for X-ray Transducer.

The basic concept of my invention is to detect and produce a picture from neutrons or high energy X-rays. I use two materials in contiguous relationship, the first, or signal plate, is a material which is very eifective as an absorber of the impinging radiation. This signal plate absorbs the impinging radiation, and produces high velocity, electrically charged particles which are ejected from the signal plate. These ejected particles bombard the second, or target layer. Since the signal plate is a good absorber it is relatively thin, and the ejected particles in traversing it are not widely scattered or trapped.

The target or second layer changes its conductivity under particle bombardment, and is therefore known as a combardment induced conductivity material (BICM). The characteristics of this material will be more fully described hereinafter.

My invention will be best understood if it is rst described in terms of X-ray radiography. Referring no-w to FIG. 1, an evacuatedenvelope 10 forms a tube having afaceplate 12. I have found that glass is a suitable material for both the envelope yand faceplate, provided the faceplate portion is permeable to the impinging radiation.

Within theenvelope 10, and adjacent to faceplate 12., is asignal plate 14. This signal plate is formed of a material which is an effective X-ray absorber. It is preferably chosen from the classl of heavy metals which includes gold, platinum, tungsten, tantalum, osmium, cadmium, bismuth, palladium, rhenium, silver, rhodium, iridium, and tin. These metals are characterized by high absorption of X-rays, and, when struck by high energy X-rfadiation, they eject high velocity electrons. Of these metals, gold and platinum are particularly suitable because of their low vapor pressure and other physical properties. Materials other than those listed do not absorb as much X-radiation per unit of thickness, and therefore eject fewer electrons because of low X- ray absorption or electron trapping.

Signal plate 14 may be deposited directl on the inner surface offaceplate 12. An electrical lead attached tosignal plate 14 penetrates the glass wall of the envelope, and is connected to the ungrounded terminal of a pair of terminals.

The exposed surface of signal plate 14- is then covered with atarget material 16 which has the property of bombardment induced conductivity. Such material (exemplilied by selenium, silicon, and magnesium fluoride) are ordinarily non-conductive, that is, they exhibit an extremely high dark resistivity or, conversely stated, a low conductivity. When these BIC materials are Ybom'- barded by charged particles, the particles, due to their mass and charge, enter the structure of the material and cause an interaction with the electrons thereof. These interactions free additional electrons, and these, plus the bombarding charged particles, are available for current carrying duty. The bombarded material thus becomes conductive where bombarded, and remains nonconductive elsewhere.

BIC is a diierent phenomenon than X-r-ay excitation. X-rays carry no charge and therefore Itheir eticiency of absorption is much lower than that of a charged particle.

At the other end of envelope is positioned an electron gun structure 20l which produces a scanning beam ofelectrons 22. This beam is preferably of low velocity to minimize the production of secondary electrons and to allow easier deflection and focusing. Deflection means, not shown, causeselectron beam 22 to scantarget 16 in the form of a raster well known in the art.

Spaced fromtarget 16 is agrid 18 which acts as an accelerator electrode, and also as a collector of any secondary electrons which may be liberated.Potential source 24 serves to maintain the Various elements of the tube at the relative potentials indicated in FIG. 1. It will be understood that it is the cathode (not shown) ofelectron gun 20 which is maintained at the negative potential.

During quiescence, i.e., the absence of X-ray excitation,electron beam 22 sweeps acrosstarget 16, and deposits thereon suicient electrons to cover the surface uniformly, and to establish a potential substantially equal to that existent at the cathode. Due to the aforementioned high dark resistivity oftarget material 16, these electrons do not leak off. Succeeding electron beam scansions during quiescence therefore do not deposit any additional electrons and there is no change in the intensity ofelectron beam 22.

When an object is under examination, X-rays from a suitable source are directed through it. If the object is thick, or of particularly dense material, the source must be such as to supply X-rays of suiciently high energy for penetration. The unabsorbed or transmitted. X-rays for an image wherein the amount ofthe X-rays corresponds to the density variations of the object. A

As shown in FIG. l, high energy X-rays traveling in the direction indicated by arrow 2.6V impinge onsignal plate 14. The high energy X-raysare absorbed, and cau-se photoelectrons and Compton electronsY of high speed to be ejected fromsignal plate 14. These high speed electrons bombard adjacent areas oftarget matelial 16. Since this target material exhibits bombardment induced conductivity, the bombardment by these high velocity electrons induces local areas of conductivity. These areas form a resistivity pattern which corresponds to the aforementioned X-ray image.

Under the inuence of the existent .potential field, the electrons which hadV previously been deposited by electron beam 2,2 onto the surface oftarget material 16, now leak off to signalplate 14 through these local areas of low resistivity. The surface oftargetmaterial 16 there.- fore acquires a charge distributionV pattern which also corresponds` to the X-ray image of the object.

As electron beam Z2 scans acrosstarget 16, it encounters a charge distribution pattern in which adjacent local areas exhibit various degrees of electron depletion, and are thus at relatively different positive potentials. The beam therefore deposits suiiicient electrons to replace those which have leaked oif.

The instantaneous deposition of electrons on the proximal surface of thetarget material 16 produces a capacitive effect which displaces electrons from its distal surface intosignal plate 14. The displaced electrons ow through resistor 2.8` to ground, thereby producing a signal voltage atoutput terminals 15. The output signal therefore corresponds to the instantaneous position of the scanning beam, and its amplitude is determined by the intensity of the X-rays causing that particular portion of the X-ray image.

An alternative method of obtaining an output signal is shown in FIG. 2. Electron beam Z2 is here reeoted back toelectron multipliers 30 located near the base portion ofenvelope 10 in a manner similar to that used in an image orthicon pickup tube. It is possible with this arrangement to amplify the variations in the intensity of the electron beam. These amplified variations pass throughresistor 32, and the resultant voltage appears as the output signal atoutput terminals 34.

In both -forms described, the newly deposited electrons leak offtarget 16 between scansions, to establish a charge distribution pattern corresponding to the instantaneous X-ray image. If the object under study is stationary, the original image will be reestabiished. If motion is present, a new pattern will be produced, and this will be presented as the subsequent output signal.

The output signal derived by either method may be treated as a television type signal, and in a manner known to those versed in the art can be applied to various cathode ray tube receivers either in a closed circuit or through actual transmission through space via modulated radio frequency waves. Multiple viewing at remote stations is thus possible, and lthe use of large television screens permit clear enlarged pictures to be obtained. X-ray study of moving or of stationary objects is thereby possible, due to the utilization of the novel pickup tube disclosed in this application.

It was previously stated that radiography depended upon the fact that various materials absorbed the radiations to a different extent. Generally speaking, heavy metals and elements with high densities are most effective in stopping the passage of X-rays. Materials having similar `densities have similar X-ray absorption properties. Boron and carbon are two such materials with similar densities and similar X-ray absorption propertie. These cannot beY easily distinguished from each other by X-ray analysis.

The problem then arises how to distinguish between such closely paired elements. The answer is found in neutron radiography. Neutrons are uncharged particles obtainable in many ways, and these may be used instead of X-rays, for a particular type of radiography.

Materials, generally speaking, have different absorption characteristics for X-rays and for neutrons. This may be understood when it is recalled that an atom consists of a nucleus land a series of electrons which revolve around the nucleus. X-rays, carrying no charge, interact with the electrons as previously explained. If their energy is low, they cause the material to become conductive. High energy X-rays, when absorbed by a thin material of good absorbing properties, eject high velocity electrons. Neutrons, on the other hand, fall into .the nucleus of the atom, whereupon it disrupted. Charged particles of various sorts (electrons, alpha particles, protons, heavy nuclei, etc.) are ejected. These may be called nuclear particles. It may thus be seen that X- ray absorption is Ian atomic phenomenon, while neutron absorption is a nuclear phenomenon. Thus, the relative neutron absorption of two diiferent materials is entirely unrelated to their relative X-ray absorption. In one striking illustration of this fact, a piece of waxed string was placed behind a two inch thick slab of iron. An X-ray picture showed no contrast between the iron and the str-ing,V because the absorption of X-rays by the string was negligible compared with that of iron. On the other hand, a picture taken by means of neutrons .clearly showed the string as a white streak across the iron slab. This eifect wasdue to the factthat iron did not impede the passage of neutrons, while the hydrogen which was present in the wax did.

Another illustration of neutron radiography is in analyzing steel. Steels having a high carbon content or a high boron content absorb X-rays to a similar extent. Since neither carbon nor iron impede the passage of neutrons, a neutron picture of a high carbon steel would be uniform. Boron, on the other hand, is very effective in impeding neutrons. The neutron picture therefore shows light patches against a dark background, thus indicating that that sample contained boron. This picture would show an additional factor about the boron iron, namely the distribution of the boron; Whether it were evenly dispersed, or Whether it was present in patches throughout the iron.

To use my device for neutron radiography, the signal plate is made of a meterial which absorbs neutrons, i.e., its nuclear structure is such that neutrons are readily absorbed.

One characteristic of neutron absorption is that low energy neutrons (which have an energy rating of between -2 to 10+2 electron volts) land high energy neutrons (greater than 400 kilovolts) are best absorbed by different materials. For example, lithium, boron, nitrogen, chlorine, uranium 235, and plutonium 239, are extremely good 'absorbers of low energy neutrons. On the other hand, phosphorous 31,sulphur 32, thorium, uranium 23 8, and neptunium are most effective for absorbing high energy neutrons.

In the embodiment of my invention for using neutrons, the signal plate is made of a material which is an effective absorber for the particular energy range of neu- .trous being used. The optimum thickness of the absorbing signal plate depends upon a number of conditions. This thickness should be such at it permits the maximum number of particles to be ejected.

The ejected particles bombard the target, causing it to become conductive, as previously explained.

What is claimed is:

l. A pickup tube for low energy neutrons, comprising: means absorbing said neutrons to cause ejection of high speed charged particles, said means comprising a material from the class consisting of lithium, boron, nitrogen, chlorine, uranium 235, and plutonium 239; a target layer of material which exhibits bombardment induced conductivity, said target layer having a rst surface and a second surface, said first surface being positioned to be bombarded by said high energy particles; and an electron gun producing an electron beam, said electron gun being positioned so that said beam strikes said second surface of said target layer.

2. The device of claim 1 wherein said absorbing means is formed of boron, and said ejected particles are alpha particles.

3. A pickup tubeV for high energy neutrons; comprising: means to absorb said neutrons to cause ejection of high speed charged particles, said means comprising a material from the class consisting of phosphorous 31,sulphur 32, thorium, uranium 238, and neptunium; a target layer of material which exhibits bombardment induced conductivity, said target layer having a rst surface and a second surface, said rst surface being positioned to be Ibombarded `by said high energy electrons; and an electron gun producing an electron beam, said electron gun being positioned so that said beam strikes said second surface of said target layer.

References Cited in the tile of this patent UNITED STATES PATENTS 2,747,131 Sheldon May 22, 1956 2,776,387 Pensak Ian. l, 1957 2,879,400 Schneeberger Mar. 24, 1959 2,890,360 Jacobs June 9, 1959 2,900,555 Schneeberger Aug. 18, 1959

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