US2994773A - Radiation detector - Google Patents

Description

Aug. 1, 1961 E. J. STERNGLASS I 9 RADIATION DETECTOR Filed Feb. 20, 1956 IO H 4 18 Source of 33 3 Radiation 5 5 24 E El 12 5 14 Fig.l. ii 49 Q I Fig.2. 'w ;50Radiation 54 Secondary Electrons Delta Ray Insulating Layer Low Atomic Scattering Layer Number Layer 60 f Salrce ofa x 66 70 Ra iafion if 64 ea 72 t 5 IOI '84 sa 92 '96 Fig.3.

vW WQF VQN" WITNESSES INVENTOR Ernest J. Srernglass ATTORNEY United States Patent 2,994,773 RADIATION DETECTOR Ernest J. Sternglass, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 20, 1956, Ser. No. 566,467 16 Claims. (Cl. 250-833) This invention pertains to electrical discharge devices and, more particularly to radiation detectors.

A widely used method for counting all types of radiation including nuclear particles when short time resolution is desired is a scintillation type of counter. Such a counter employs a phosphor which emits light flashes when radiation or nuclear particles impinge thereon, and a photomultiplier tube to convert the light flashes to electrical pulses and amplify the electrical pulses so they may be detected by well-known amplifier and counting circuits. When organic compounds are used as the phosphors, the time resolution of the counter can be reduced to the order of a few millimicroseconds. This interval is satisfactory for many purposes, but for other purposes, such as a study of nuclear processes associated with fision and fusion reactions, it is desirable to shorten the interval even further.

Another problem which arises in the use of scintillation counters is the need for an extremely sensitive photocathode in the photomultiplier tube. This problem is further complicated by the present trend toward even larger photocathode surfaces. At present, photocathode surfaces can only be prepared by a complex series of critical manufacturing steps which result in a very high manufacturing cost for the photocathode surface and thus a high cost for the scintillation counter.

Accordingly, it is an object of this invention to provide a radiation detector having a cathode which converts radiation directly into a substantial number of slow electrons which can be multiplied by various electron multiplying means.

Another object of this invention is to provide a cathode which converts radiation directly into a substantial number of slow electrons and which has a very short resolution time.

Another object of this invention is to provide a cathode which converts radiation directly into a substantial num ber of slow electrons which can be multiplied by means of a transmission type of electron multiplier.

These and other objects and advantages of this invention will be more easily understood by those skilled in the art from the following detailed description of a preferred embodiment thereof when taken in conjunction with the attached drawing, in which:

FIGURE 1 is a longitudinal section of a radiation detector constructed in accordance with this invention employing a transmission type of electron multiplier;

FIG. 2 is an enlarged cross section of a portion of the cathode shown in FIG. 1; and

FIG. 3 is a longitudinal section of another embodiment of this invention using a conventional type of electron multiplier.

The radiation detector of this invention consists generally of an envelope with acathode 16 mounted adjacent one end of the envelope. Thecathode 16 converts the radiation directly to slow electrons which are multiplied by means of an electron multiplier which is also "ice enclosed within theenvelope 10. When the word radiation is used herein, it is meant to include alpha and beta particles, as well as electrons, mesons and other forms of nuclear radiation. Neutrons may also be detected by adding boron or barium compounds, such as lithium borate, to thecathode 16. The evacuatedenvelope 10 may be formed of any desired material such as glass or metal and is preferably tubualr in shape withend portions 12 and 14 at each end thereof. Theend portion 12 should be formed of a material through which the radiation which is to be detected can easily pass, such as glass or aluminum, while theend 14 may be formed of the same material as the tubular portion of the envelope. Thecathode 16 is mounted on the inner surface of theend 12 with an electricallyconductive layer 18, such as a thin film of aluminum which will allow radiation to pass through, interposed between thecathode 16 and the inner surface of theend portion 12. The exact construction of thecathode 16 will be described in greater detail below. Placed behind thecathode 16 are a series ofdynodes 20, '22 and 24 of the transmission type. These dynodes are capable of receiving primary electrons on the surface facing thecathode 16 and emitting a larger number. of secondary electrons from the other side of the dynode. In this manner, electrons emitted from thecathode 16 are multiplied so that the signal transmitted by the radiation detector is sufiiciently large that it can easily be amplified and counted by well-known amplifying and counting circuits. This type of electron multiplier which is referred to as a transmission type electron multiplier is more particularly described in the applicants copending application entitled Electron Discharge Device, Serial No. 434,467, filed June 4, 1954, now US. Pat. No. 2,905,844, issued Sept. 22, 1959, and assigned to the same assignee as this invention, and which is incorporated in this application by reference.

Thecathode 16 as well as thedynodes 20, 22 and 24 should be provided with a source of positive potential which increases in magnitude between thecathode 16 and thedynode 20 and each succeeding dynode so that the electrons which are emitted from thecathode 16 will be accelerated to thedynode 20 and each succeeding dynode. This source of potential is provided in FIG. 1 by abattery 26, the negative terminal of which is connected to theconductive layer 18 by means of alead 28. A plurality of series connectedresistors 32, 34, 42 and 44 are connected between the positive and negative sides of thebattery 26 as shown in FIG. 1. Thedynode 20 is connected to a point betweenresistors 32 and 34 by means of alead 30, while thedynode 22 is connected to a point between theresistors 42 and 44 by means of alead 40. The dynode oranode 24 is connected to theresistor 44 by means of alead 49 and aresistor 46. The positive side of thebattery 26 is connected to the junction of theresistors 44 and 46 by means of alead 48, and alead 50 connects the positive side of thebattery 26 to ground. The signal from the radiation detecting device is transmitted to the associated amplifying and counting equipment (not shown) by means of a lead 51 which is connected to thelead 49 between thedynode 24 and theresistor 46. In this manner,cathode 16, dynodes 2t), 22 and 24 have progressively increasing steps of positive potential, with respect to thecathode 16 so as to accelerate electrons fromcathode 16 to the first dynode 2G and each succeeding dynode. Although equal steps .travel a shorter distance.

. V 3 of volta-ges between thecathode 16 and thedynodes 20, 22 and 24 are shown, it may be desirable to operate some of the dynodes at higher voltages.

Shown in FIG. 2 is an enlarged cross section of thecathode 16 consisting of threelayers 52, 54 and 56 which are in contact with each other. Thelayer 52 is a layer of material having a low atomic number, so as not to scatter the delta rays which are formed therein, such as graphite or beryllium, and it should have a thickness no greater than the range of the delta rays which are released when radiation impinges on thecathode 16. This thickness will vary between to 10- cm. in thickness depending on the type of radiation being detected.

The thickness of thelayer 52 should be no greater than the range of the fastest delta ray released because a greater thickness would stop those delta rays formed in the portion of the layer which exceeds their range. A thickness less than the range of the fastest delta ray could be used but fewer delta rays would be formed by the radiation impinging on the cathode since it would Thus, the thickness of thelayer 56 should be equal to or less than the range of .the fastest delta ray formed by the radiation because this thickness will give the best results while absorbing .a minimum of the radiation being detected. For example, if it is desired to detect 300 million electron volt (mev.) protons which will form .5 mev. delta rays when they strike thelayer 52, the layer should have a thickness equal to approximately .200 gram/per cm. Thelayer 54 is formed of a material having a high atomic number which is also a conducting material such as platinum or gold, so that it will scatter the delta rays which are formed in thelayer 52 and also serve as a grid in case it is desired to omitlayer 18. Thelayer 54 should be relatively thin on the order of 10* to 10- cm. so that the delta rays can easily pass through it and not be stopped. Of course, the thickness of thelayer 54 should be matched to the energy of the delta particles released by the particles being detected, that is, for higher energy delta particles thelayer 54 should be thicker than for lower energy delta rays. Thefinal layer 56 is composed of an insulating material such as an alkali-halide, alkali earth oxides or metal oxides, which has the property that slow secondary electrons can travel large distances in it before losing their ability to escape.

In order to have this property thelayer 56 should have a long mean free path for secondary electrons and a large energy gap between the filled valence band and the conduction band of each atom. This will result in a large yield of secondary electrons which can diffuse through a substantial thickness oflayer 56 without losing a large amount of energy. Thus, a large number of secondary electrons will be formed which can easily escape from the back side oflayer 56. The material should also have good crystallinity and be easily fabricated into thin films by evaporation or other means.

Some examples of materials having these properties are potassium chloride, sodium chloride, aluminum oxide, magnesium oxide and magnesium fluoride. Thelayer 56 should have a thickness equal to a few diffusion lengths of the secondary electrons in the particular material used and is approximately 10* to 10- cm. in thickness. Thevarious layers 52, 54 and 56 may be fabricated by any well known method such as the depositing of the layers, one on the other, as described in the application referred to above.

The cathode described above will convert the radiation which impinges on it to a substantial number of slow electrons which will be emitted from the back or the right hand side of thelayer 56 as shown in FIG. 2. When the radiation strikes thecathode 16, thelayer 52 will convert it to a few fast delta rays which will easily escape from thelayer 52. When these delta rays strike thelayer 54, they will be scattered and escape into thelayer 56 of insulating material at a large angle. The

insulating material will, in turn, convert the delta rays to a large number of slow electrons which can easily escape from the back of thelayer 56. For example, when 300 mev. protons strike thelayer 52, approximately 1 to 10 slow electrons, but averaging three, are emitted from the back side of the insulatinglayer 56. These three electrons will be greatly multiplied by the transmission type of electron multiplier placed behind thecathode 16 shown in FIG. 1 so that the pulses transmitted from the detecting device will correspond to 10 to 10 electrons atanode 24.

Since the number of delta rays per cm. of path is inversely proportional to the square of the velocity of-the particle striking thelayer 52, the yield of slow electrons will be much greater for particles having a lower velocity than the 300 mev. proton example given above. In addition to this, the yield of slow electrons is directly proportional to the square of the charge carried by theparticle striking layer 52. Thus, alpha particles of the same velocity as the proton example above will yield four times the number of slow electrons as are yielded by the proton because of the higher charge carried by the alpha particles.

Shown in FIG. 3 is a modification of the radiation detecting device shown in FIG. 1. In this embodiment, the electrons emitted by thecathode 16 are multiplied by the conventional type of electron multiplier in which the primary electrons impinge on one surface of the dynodes and the secondary electrons are emitted from the same surface and transmitted to the next dynode. This device utilizes an evacuatedenvelope 60 similar in construction to the envelope It in FIG. 1 with thecathode 16 mounted adjacent one end thereof. Also mounted inside theenvelope 60 is a plurality of dynodes 62-72. These dynodes are provided with a source of progressively increasing positive potential so that slow electrons emitted from thecathode 16 will be accelerated from the successive dynodes to thefinal dynode 72. This positive potential is provided by abattery 76, the negative terminal of which is connected to thecathode 16 by means of alead 74. A plurality of series connectedresistors 78, 82, 86, 90, 94 and 98 are connected between the positive and negative terminals of thebattery 76. Thedynode 62 is connected to a point between theresistors 78 and 82 by means of a lead and each succeeding dynode is connected to a similar point between the remainder of the resistors by means ofleads 84, 88, 92 and 96. The last dynode oranode 72 is connected to one end of theresistor 98 through alead 101 and aresistance 100. The positive terminal of thebattery 76 is connected to the common junction ofresistors 98 and by means of a lead 162 and to ground by alead 104. The signal from the detector is transmitted by a lead 103 which is connected .between thedynode 72 and theresistance 100.

The operation of this embodiment of the invention is the same as that described for the embodiment shown in FIG. 1. The only difference is in the type of electron multiplier used for multiplying the electrons emitted from thecathode 16. The embodiment of the detector shown in FIG. 3 utilizes a multiplier in which the secondary electrons are emitted from the same side of the dynode from the primary electrons impinged. The secondary electrons are accelerated from one dynode to the next dynode by means of progressively increasing steps of positive potential between the various dynodes.

In addition to the use of this invention as a radiation detection device as described above, the embodiment of the invention shown in FIG. 1 could easily be modified to serve as an imaging device. In order to modify this embodiment to serve as an imaging device, it would only be necessary to substitute a phosphor screen in place of thelast dynode 24. The phosphor screen should be mounted directly on the inner surface of theend closure 14, which should preferably be transparent. Thus, the exact shape of the radiation striking thecathode 16 could be viewed on the phosphor screen.

While only two embodiments of this invention are shown, many modifications and additional embodiments thereof will occur to those skilled in the art within the broad spirit and scope of this invention, and, accordingly, it should be limited only as required by the prior art.

I claim as my invention:

1. A radiation detector comprising an evacuated envelope, a cathode mounted in said envelope, said cathode including a first material capable of directly producing delta rays therein upon impingement of radiation, a second material capable of scattering said delta rays, and a third material capable of converting each of said delta rays to at least one slow electron emitted by said third material, said materials being disposed adjacent one another, respectively, and electron-multiplying means mounted within said envelope.

2. A radiation detector comprising an evacuated envelope, a cathode mounted within said envelope adjacent a surface thereof, said cathode comprising a first material disposed adjacent said surface and capable of converting radiation when impinging thereon directly into delta rays, a second material capable of scattering said delta rays and disposed inwardly of said first material in contact therewith, and a third material capable of converting said delta rays into emitted electrons and disposed inwardly of said second material in contact therewith; and electronmultiplying means mounted within said envelope.

3. A radiation detector comprising an evacuated envelope, an electron-multiplying means mounted within said envelope, and a cathode mounted within said envelope and juxtaposed to said multiplying means, said cathode including a first material arranged for impingement by radiation and capable of converting said radiation directly into delta rays, a second material capable of scattering said delta rays, and a third material capable of converting said delta rays into electrons having an energy level capable of being multiplied by said electron-multiplying means, said materials being adjacent to one another, respectively.

4. A radiation detector comprising an evacuated envelope, electron-multiplying means mounted within said envelope, and a radiation-receiving cathode mounted within said envelope and juxtaposed to said electronmultiplying means, said cathode including means for converting said radiation directly into delta rays and additional means for scattering said delta rays and for converting said delta rays into electrons capable of being multiplied by said electron-multiplying means.

5. A cathode for a radiation converting, electronic discharge device, said cathode comprising a laminated structure having a first material capable of converting said radiation directly into delta rays, a second material for scattering said delta rays, and a third material capable of emitting electrons when exposed to said delta rays.

6. A cathode for a radiation converting, electronic discharge device, said cathode comprising means for receiving said radiation and for converting said radiation directly into delta rays, additional means for receiving and scattering said delta rays, and an electron-emitting means for receiving said scattered delta rays and for converting said delta rays into emitted electrons.

7. A cathode for an electronic discharge device arranged for converting radiation into electronic emission, said cathode comprising a first material selected from the group including carbon and beryllium, a second material selected from the group including platinum and gold, and a third material selected from the group including alkali halides metal oxides and alkali oxides, said materials being disposed adjacent one another, respectively.

8. A cathode for an electronic discharge device arranged for converting radiation into electronic emission, said cathode comprising a first material including an element of relatively lower atomic weight disposed to receive said radiation, said first layer being capable of 6 converting said radiation directly into delta rays, a sec ond layer including an element having a relatively higher atomic weight disposed to scatter said delta rays, and an electron-emitting third material disposed to convert said delta rays into electronic emission, said materials being disposed adjacent to one another, respectively.

9. A radiation detector comprising an evacuated envelope, electron-multiplying means mounted within said envelope, and an electron-emitting cathode mounted within said envelope in juxtaposed relation to said multiplying means, said cathode enclosing a first layer having an element of relatively lower atomic weight disposed to receive said radiation and capable of converting said radiation directly into delta rays, a second material having an element of relatively higher atomic weight for scattering said delta rays, and an electron-emitting third material capable of converting said delta rays into electronic emission, said materials being disposed adjacent one another, respectively.

10. A radiation detector comprising an evacuated envelope, an electron-multiplying means mounted within said envelope, and an electron-emitting cathode mounted within said envelope in juxtaposed relation to said multiplying means, said cathode having a first material capable of converting said radiation directly into delta rays, a second material for scattering said delta rays, and an electron-emitting third material capable of converting said delta rays into emitted electrons and of multiplying the electrons thus formed, said materials being disposed adjacent one another, respectively.

11. A neutron detector comprising an evacuated envelope, a cathode mounted in said envelope, a neutronreactive material supported adjacent said cathode, said material being capable of emitting charged radiation upon impingement of said neutrons, said cathode including a first material capable of converting said radiation when impinging thereon directly into delta rays, a second material capable of scattering said delta rays and a third material capable of converting each of said delta rays into at least one slow electron emitted by said third material, said materials being disposed adjacent one another respectively, and electron multiplying means mounted within said envelope.

12. A cathode for a neutron sensitive electronic discharge device, said cathode comprising a material capable of emitting charged radiation on impingement of neutrons and a laminated structure having a first material capable of converting said radiation directly into delta rays, a second material for scattering said delta rays and a third material capable of emitting electrons when exposed to said delta rays.

13. A cathode for a radiation-converting, electronic discharge device, said cathode comprising means for receiving said radiation and converting said radiation directly into electrons, and means for directly multiplying said converted electrons into electrons of lower energy.

14. A cathode for a radiation-converting, electronic discharge device, said cathode comprising means for receiving said radiation and converting said radiation directly into electrons, and means for converting said electrons into emitted electrons of lower energy, said lastmentioned means also being formed to directly multiply said emitted electrons.

15. A cathode for a radiation-converting, electronic discharge device, said cathode comprising means for receiving said radiation and converting said radiation directly into electrons, means for converting said electrons into emitted electrons or" lower energy, and means for directly multiplying said emitted electrons.

16. A cathode for a neutron sensitive electronic dis charge device and capable of emitting electrons in response to neutron excitation, said cathode comprising means for receiving and converting said neutrons into References Cited in the file of this patent UNITED STATES PATENTS Sheldon Mar. 16, 1954 Victoreen Feb. 13, 1940 Teal Apr. 9, 1940 Kallmann et a1. Jan. 20, 1942 8 Kallmann Sept. 29, 1942 Kallrnann Mar. 14, 1944 Sheldon Oct. 17, 1950 Sheldon June 5, 1951 Marshall et a1. Sept. 30, 1952 Sheldon Dec. 14, 1954 Sheldon May 22, 1956 Botden et a1 May 29, 1956 Jacobs et al. June 9, 1959

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