US2622208A

US2622208A - Radioactive particle counting

Info

Publication number: US2622208A
Application number: US150692A
Authority: US
Inventors: Bernstein William; Ballentine Robert
Original assignee: Individual
Current assignee: Individual
Priority date: 1950-03-20
Filing date: 1950-03-20
Publication date: 1952-12-16
Anticipated expiration: 1969-12-16

Description

1366- 16, 1952 w. BERNSTEIN ETAL RADIOACTIVE PARTICLE COUNTING 2 SHEETS-SHEET 1 Filed March 20, 1950 vFIG.I.

INVENTORS. WILLIAM BERNSTEIN ROBERT BALLENTINE 1952 w. BERNSTEIN ET AL 2,622,208

RADIOACTIVE PARTICLE COUNTING Filed March 20, 1950 2 SHEETS-SHEET 2 JNVENTOR'S. WILLIAM BERNSTEIN ROBERT BALLAENTINE Patented Dec. 16, 1952 UNITED STATES PATENT OFFICE RADIOACTIVE PARTICLE COUNTING Application March 20, 1950, Serial No. 150,692

6 Claims.

The present invention relates to a method and apparatus for counting particles emitted from a radioactive substance. The accurate counting of radioactive particles presents several diflicult problems. These difficulties are inherent in the properties of the particles and consideration of these properties is necessary in the design of the detection instrument.

Alpha particles are absorbed by the walls or window of the usual counter unless a very thin window is used. Thin windows are mechanically difficult to construct and usually let in a large amount of moisture as well as alpha particles. The present method and apparatus eliminates the need for using thin windows.

Beta particles are not emitted from the nucleus with a definite energy but are emitted in a continuous spectrum. The end point of the spectrum corresponds to the maximum energy of the nuclear decay but the particles are distributed with energies down to zero. The method and apparatus of the present invention counts beta particles throughout the entire spectrum with the same high accuracy.

The requirements of any accurate counter are stringent especially with respect to absolute counting. The wall and air absorption paths must be kept as low as possible particularly for low energy particles. Also it is necessary that no portion of a sample be lost in the radioactive source preparation for absolute counting. The sensitive counting volume of the apparatus of the present invention contains the sample whose radioactivity is to be measured. This reduces the wall and air absorption paths to essentially zero. It is also possible with the present invention to collect the radioactive substance to be measured directly in the counter so that no part of the sample can be lost in the preparation for counting. The physical configuration of the collected substance therefore remains the same from the time of its accumulation until after its radioactivity is measured.

It is an object of the present invention to provide an apparatus to count accurately the number of particles emitted from a radioactive substance.

It is another object of the present invention to provide a counter which can be accurately used for either relative or absolute counting.

It is still another object of the present invention to provide a counting apparatus with a very low background count.

It is a further object of the present invention to provide a counter which can be easily dismantled for decontamination purposes.

It is still a further object of the present invention to provide a counter which includes means for collecting the substance whose radioactivity is to be measured.

Another object of the present invention is to provide a counter which can be built inexpensively and still maintain a high counting accuracy.

Still another object of the present invention is to provide a method for measuring the number of particles emanating from a radioactive substance.

Other objects and advantages will be in part obvious and in part pointed out hereinafter.

More particularly, a preferred embodiment of the apparatus of the present invention comprises two electrodes mounted on a base plate and electrically insulated from one another. A counter envelope which surrounds both electrodes is detachably mounted on the same base plate. The counter envelope contains a support on which the radioactive substance to be measured can be collected. Means are also included for hermetically sealin the counter envelope and for introducing a counting gas into the envelope. Electronic measuring circuits and a source of high potential are connected across the electrodes to provide means for counting the particles and registering the counts.

The many objects and advantages of the present invention may best be appreciated by reference to the accompanying drawings the figures of which illustrate apparatus incorporating a preferred embodiment of the present invention and capable of carrying out the method of the invention. In the drawings:

Figure 1 is an isometric view of a counting apparatus embodying the invention with a portion of the counter wall broken away to show the internal construction. The apparatus is shown in relation to standard electronic amplifying circuits and a mechanical counting register.

Figure 2 is a vertical section of Figure 1 taken through the plane 2-2.

Referring to the drawings, particularly Figure 2, it is seen that base plate l0 supports connector E2 on a collar M. Collar I4 is attached to plate by means of screws I6. Connector I2 is fastened to collar I l by screws l8. Connector i2 is a standard high voltage electrical connector with a single centrally located terminal l3 (shown only in Figure 1). Soldered to terminal I3 is wire 20 which extends through hole 22 in plate 18. Surrounding wire 2|] for part of its length is a tube 24 which is sealed to and insulated from plate Hi by a glass-to-metal seal 26. Welded to wirev 23 at point 28 is a thin wire anode 30. The

inclusion of weld 28 within tube 24 shields any sharp points which may be present in the weld and thereby prevents spurious counts and eliminates sparking. Anode 3B terminates in an electrically conductive bead 32 for similar reasons. This bead may be glass with an aquadag coating.

Surrounding anode 38 is the cylindricalcathode of the counter made up of

sections

34 and 36. Section 3a is attached to plate ID by screws 38. than as and can slide on over section 34. sensitive counting volume 35, beneath'tube 2.4,

sections

34 and 36 form a cylinder with anode 30 located along its axis. The; purpose of. this. construction is to facilitate the dismantling-of the cathode for decontamination purposes.

Surrounding cathode sections 34 and counter envelope 4!) which is supported by support 42 and gasket 44. Support 42 is attached to se.t. screw #55 which ,in turn is threaded through a yoke, 4.8, The. yoke. (seeFigure 1). ispivotally mounted to plate to. by screws 53. Thevolume enclosed by counter envelope 40 may be rendered gas-tight by tightening screw cwhich. presses the envelope up against a. gasket fizinplate. l0. One end. of,en velope ifl-containsasupport 5.4 for a radioactive substance 56. Support 5.5 can be solid or it canbe porous for. filtration purposes. The entire counter assembly can be supported by means ofstand 5! (see Figure l) whichv is attached to plate It! by screws 53.

Penetrating collar l4. and plate it are. inlet and outlet tubes 58Iandi6i! respectively. Gas may be .admitted into the, counter. envelopev from a gas supply 6.2.through valve 6.4, bubble meter. 6% and inlet. tube 58.. To.regulate.thefoutwardflow'of air and gas throughtubefil] .a .valvefia is provided.

To operate the counting. apparatus a .high. voltage. supplylll is. connected between thev anode. and cathode. of. thecounter. The difference of. potential. between. anode 3B and cathode section 36 is,.pr0v.ided.from supply 7!) by means oileadTZ, terminal. l3, wire 20 to anode 39. The other end of-.the circuit is completedthrough lead M, screw ifiithroughcollar. Mend screw IE to base. plate I El. From baseplat h) the circuit. is carried through screw 38, cathode section 3llto cathode section 38.

Inorder to. amplify andregister the pulsesreceived at anode 39 during theoperation of the counting apparatus, standard electronic componentsconsistingof preamplifier 76, amplifier 13 and. counting. register 80. are connected. to the anode through terminal l3. Preamplifier 16 is used when itis desired. to eliminate the effect of 1 cable capacitanc between counter, anode 3i]. and amplifier. 2'3. When used in this manner preamplifier 15 is mounted relatively closeto terminal. i3. Amplifier l8. and register 66 can then be connectedto preamplifier it by any length of cable necessary.

One preferred embodiment of counter envelope 4%] is the sintered glass C-ooch crucible whichhas longbeen used'by chemists in gravimetric analyses. andsupport 56 is sintered glass. Themethod and apparatus of the present invention will be de scribed'using this embodiment.

Counter. envelope finnay be removed from the counter assembly by loosening screw 45 and swinging aside yokettl, The crucible can then be. used in. a, standard filtration procedure to accumulate the substance whose radioactivity is to .be measured. Whenthe filtration is complete the crucible 49 'can.be.removecl from the associ- Section 35 is of slightly larger diameter i In the Bois- In this embodiment envelope d is glass ated apparatus, with the radioactive substance 56 collected on sintered support 5 3, and mounted in gas-tight fashion between

gaskets

52 and 44 by means of screw 46 and yoke 48. At this stage the radioactive substance 56 has been introduced into the sensitive volume 35': of the: counter without. disturbing its physical. configuration from the time of its accumulation. In this Way there is no danger of loosing any of the substance in the transfer process. The radioactive particles emanating from substance 5% are now being emitted into thesensitive counter volume 35.

To obtain: the proper counting gas mixture within the counter, valves t4 and 68 are opened. Therate. of; gas? input can be measured on bubble meter. Theair. that is inside the counter envelope'will be emitted through outlet tube 6!) when valveEB; is open. The proper length of time for flushing the counter in this manner may easily b.e-determined .with .use.

Operationof the. counter. will. depend on the difference. in. potential between. anode: 30 and cathode-3.6., maintained by. high voltage supply it. In.thepreferred.embodiment a difference in potential. of, 3860. volts. using. methane as. the gasfwill cause. the counter to. operate as a proportional counter. That is, the height'of the detected pulse is dependent uponthe total ionization causedby the. radioactive particle passing through the sensitive,countervolume. The field intheimmediate vicinity of anode 3G issufiicient- 1y. intense. to cause-secondary. ionization yetnot enoughtocause the dischargeto spread through out the counter.

Because the particles: do, not have. to pass through-a window, absorption paths are. reduced to essentially zeroand extremely low energy parti'cles willproduce-suificient ionization to record counting impulses. High energy particles pass completely through the sensitive volume before theiiienergy is lost, and therefore produce a maximumdischargewhich is limited by thelength of the sensitive volume. These factorsplus the continuousrenewal of the counting gas results in excellent reproducibility f counts-over short and longtime. periods- The particles ionized by the passage. of. the radioactive particles are attracted. to anode 3i! and change the potential. of this wire whenthey arrive. This results inan output pulse appear.- ing at terminal l 30f the connector: Ilwhich pulse is. amplified by preamplifier i5, and amplifier l8 and subsequently counted on register. 8th The height of the output pulse depends on theamount of. the original ionization, the type. of gas used and. the. original difference in. potential. maintaineolLbetween anode 39 and-cathode 36.

To. determine when the proper. operation of counting gas has-been reached it is merely necessary to note the action of counting register 8%. When substantially all the air hasbeen'. emitted and the counter contains only thecounting gas the counts on register 83 willbe recorded in a regular manner. In. the preferredembodiment using a Goochcrucible as the counter envelope 46 it has been found that the proper proportion of counting gas is Qbtainedafter the counter is flushed at the rate of one bubble of counting per-second for a period of two minutes without priordryi'ng.

The difference in potential between anode 35 and cathode 35 may be. lowered so that the counter works as an ionization chamber. This difference in'potentialcan be raised above the Geiger threshold so that the counter operates in the Geiger region. The gas requirements are such that most any gas can be used for counting in the proportional region. When using gas the counter may be operated at pressures slightly above atmospheric to prevent the diffusion of air into the counter. However, it is not necessary that a counting gas be used at all as

tubes

58 and 50 may also be used for vacuating the counter. Using the counter in this way will still maintain the advantages of having the substance emit particles directly into the sensitive counting volume.

The apparatus of the present invention may also be used without the

cathode sections

34 and 36. This can be accomplished by making counter envelope 40 of an electrically conductive material. When used this way the substance 56 can be collected on the support 54 and the envelope 40 can be positioned on base plate l0 so that wire anode will be along the axis of the envelope. Electrical connection can easily be made between base plate In and envelope 40. In this manner, the envelope will act as the cathode of the radioactive particle counter.

By making envelope 40 of an electrically conductive material is also meant making an envelope that has this material coated on its inner surfaces. The coating when properly connected would then act as the cathode of the counter.

Since many embodiments of the present invention might be made and since many other changes may be made in the preferred embodiment described above it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sens except as required by the appended claims.

We claim:

1. A separable apparatus for counting radioactive particles which comprises in combination, a base plate, a fixed structure including a first and a second electrode fixedly mounted thereon, said first electrode being positioned axially within said second electrode and electrically insulated therefrom, a support plate adjustably attached to said base plate in spaced relation thereto, a removable counter envelope surrounding said second electrode and detachably afiixed to said base plate and to said support plate, a support for a radioactive substance fixedly mounted within said envelope to position said substance adjacent to one end of said first electrode, means for mounting and dismounting said counter envelope between said base plate and said support plate, means for hermetically sealing said counter envelope and means secured to said base plate for introducing and expelling a gas from said counter envelope.

2. Apparatus as recited in claim 1 together with electrical measuring means and a source of high potential, both connected between said first and said second electrode.

3. A separable apparatus for counting radioactive particles which comprises in combination, a fixed structure including a base plate, a first and second electrode fixedly mounted thereon, said first electrode being positioned axially within said second electrode and electrically insulated therefrom, a support plate adjustably attached to said, base plate in spaced relation thereto, a removable open-ended counter envelope surrounding said second electrode and detachably affixed to said base plate and to said support plate, a support for a radioactive substance fixedly mounted within said envelope to position said substance adjacent to one end of said first electrode, means for mounting and dismounting said counter envelope between said base plate and support plate, means for hermetically sealing said counter envelope, means secured to said base plate for introducing and expelling a gas from said counter envelope and electrical measuring means and a source of high potential both connected between said first and said second electrode.

4. A separable apparatus for counting radioactive particles which comprises in combination, a fixed structure including a base plate, a first and second electrode fixedly mounted thereon, said first electrode being positioned axially within said second electrode and electrically insulated therefrom, a support plate adjustably attached to'said base plate in spaced relation thereto, a removable open-ended counter envelope surrounding said second electrode and detachably affixed to said base plate and to said support plate, a support for a radioactive substance fixedly mounted within said envelope to position said substance adjacent one end of said first electrode, said support including a porous filter member suitable for deposition of said substance thereon by filtration, means for mounting and dismounting said counter envelope between said base plate and support plate, means for hermetically sealing said counter envelope, means secured to said base plate for introducing and expelling a gas from said counter envelope and electrical measuring means and a source of high potential both connected between said first and said second electrode.

5. A separable apparatus for counting radioactive particles which comprises in combination, a fixed structure including a base plate, an electrode fixedly mounted thereon, a support plate adjustably attached to said base plate in spaced relation thereto, a removable counter envelope made of an electrically conductive material surrounding said electrode, said envelope being detachably afiixed to said base plate and to said support plate and with said electrode positioned axially within said envelope and electrically insulated therefrom, a support for a radioactive substance fixedly mounted within said envelope to position said substance adjacent to one end of said electrode, means for mounting and dismounting said counter envelope between said base plate and said support plate, means for hermetically-sealing said counter envelope, means secured to said base plate for introducing and expelling a gas from said counter envelope and electrical measuring means and a source of high potential both connected between said electrode and said counter envelope.

6. A separable apparatus for counting radioactive particles which comprises in combination, "a fixed structure including a base plate, an electrode fixedly mounted thereon, a support plate adjustably attached to said base plate in spaced relation thereto, a removable open-ended counter envelope made of an electrically conductive material surrounding said electrode, said envelope being detachably affixed to said base plate and to said support plate and with said electrode positioned axially within said envelope and electrilope; meanssecured' to said" base:= plate for introdixoingand' expelling a 7 gas fromv said" counter enve1ope-andelectribal measuring means and a source'of highpotential' both connected between said electrode and said counter envelope.

WILLIAM BERNS'IEIN. ROBERT BALLENTINE.

REFERENCES CITED Thefd11owing'references'are of record in the file of 1 this" patent UNITED STATES PATENTS Number: Name Date 12,442,314? Reid' v May 25, 1948 2,452,524i Metten= Oct.. 26, 1948 2519,00? Wilson Aug. 15, 1950