US2499830A - Air proportional counter - Google Patents

Description

March 7, 1950 E. w. MOLLOY 2,499,830

AIR PROPIORTIONAL COUNTER Filed Nov. 21, 1946' 4 v A v AMPLIFIER Sgt-5S SCALER HIGH 46 SELECTOR VOLTAGE o J F 4 INVENTOR.

' Z/erez'f afi/yalfqy BY Maw Patented Mar. 7, 1950 AIR PROPORTIONAL COUNTER Everett W. Molloy, Pasadena, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application November 21, 1946, Serial No. 711,321

8 Claims.

This invention relates to an improvement in apparatus for the measurement of radioactivity. More specifically, the invention relates to an improved proportional counter for the measurement of alpha particle emission from radioactivesamples.

In the counting of alpha particles emitted from a radioactive sample it is necessary to discriminate between the ionization caused by the alpha particles under measurement and ionization caused by beta particles which may likewise be emitted from the sample. This problem is particularly difficult where the beta background emanating from the sample is very high. An effective instrument for the measurement of alpha particle emission in the presence of a high beta background is the proportional counter. The ionization caused by a single alpha particle is much greater than that caused by a single beta particle. tion induced by a charged alpha or beta particle causes further ionization by collision of the ions and electrons primarily induced with other molecules of" the gas present in the counter. The total amount of ionization thus primarily and secondarily produced by a charged particle within the counter is proportional to the number of ions primarily produced. The ratio of the total amount of charge, primarily and secondarily produced by the charged particle traversing the counter and collected by the electrodes of the proportional counter, to the charge primarily produced is known as the gas amplification factor.

Persons skilled in the art will readily under- 3 stand that the term proportional, as used herein, does not apply to amplification factors of unity; when the voltage applied between the ion-collecting electrodes is sufficient to impart to the ions created by the impinging particle veloc" ities adequate to assure that recombination of the ions will not occur before collection, but not adequate to cause secondary ionization by the ions so primarily created, the number of ions collected by the electrodes is equal to the number of ions caused directly by the impinging particle. A device operated in such a voltage region is known as a saturated ionization chamber. The operating voltage of a proportional counter is much higher than that of a similar ionization chamber since it is necessary for proper operation of the former that the ions primarily induced be sufficiently accelerated to cause secondary ionization before being collected by the collecting electrodes.

In the proportional counter the ionizarounding this fine Wire electrode.

In the proportional counter, at least one U1 the electrodes is commonly in the form of a fine wire. The purpose of this design is to procure a region of extremely high field gradient sur- By having such a region of high electric field gradient it is possible to achieve a long chain or series of secondary ionizations (commonly called an avalanche) from the primary ionizing event, thus greatly increasing the gas amplification factor.

The gas amplification factor of a proportional counter may be very high, of the order of 10 or 10''. The gas amplification thus introduced is extremely advantageous in that the voltage signals produced by the counter are much more readily discernible from such spurious signals as microphonics, vacuum-tube noise, and stray electric fields than they are when such amplification within the gas is not present, as in an ionization chamber.

In the proportional counters heretofore used, employing gases at pressures much lower than atmospheric, for example, 6 to 9 centimeters of mercury, it has been necessary to make the counter vacuum-tight. In sealing a counter so as to render it vacuum-tight, it is extremely difiicult at the same time to render it possible for alpha particles from a sample to be counted to enter into the active portion of the counter. As is well known in the art, alpha particles are readily absorbed by even the smallest thicknesses of almost any known material. Therefore, in the past it has usually been necessary to place the sample under measurement within the proportional counter and to evacuate and fill the proportional counter each time that a sample Was to be counted. It is readily seen that such a procedure is not suitable for operations involving the counting of samples as a routine matter.

It is, therefore, the principal object of the present invention to provide a proportional counter adapted to measure alpha particle emission from a radioactive sample without requiring the evacuation and filling of the counter for each sample to be counted.

It is a further object of the invention to provide a proportional counter adapted to be used with a radioactive sample disposed outside the counter.

Generally the above objects are achieved by the construction of a proportional counter wherein the gas filling within the active portion of the counter is air at atmospheric pressure. In this manner there is achieved a counter which may be completely permeable to the surrounding atmosphere and thus may freely admit alpha particles placed external thereto.

For a more complete understanding of the invention, reference is made to the description below and to the drawing, in which Fig. l is a central longitudinal, cross sectional view of a proportional counter; Fig. 2 is a perspective view of the proportional counter of Fig. 1 together with the mounting thereof and the mounting of a radioactive sample to be counted; Fig. 3 is a cross-sectional view taken along the line 3-3 of Fig. 2 in the direction indicated by arrows; and Fig. 4 is a schematic block-diagram of the electrical connections of the proportional counter and the electronic system associated therewith.

Referring first to Fig. 1, the counter, generally designated by the numeral II], hastwo electrodes. The outer electrode I2 is a tube of coarse wire mesh. The inner electrode or center wire I4 is .a wire of a metal adapted to be sealed into glass, forexample, Kovar. The center wire I4 is supported at each end by sealing into glass end pieces I6 which are made from glass tubing having the same coefficient of thermal expansion as the center wire I4, for example, Kovar glass. The inner end of each glass end piece I6 fits tightly into a cup-shaped cavity I8 bored into the outer end of a polystyrene insulator 20.

At the inner end of each of the polystyrene insulators .29 is neck portion 22 adapted to fit tightly into the outer electrode I2. Axially of each insulator is an aperture 24, through which passes the center-wire I4.

In assembling the deviceof Fig. 1, one end of the center wire I4 is first sealed into one of the end pieces I6. The corresponding insulator 20 is then forced over the unsealed end of the end piece I6. Thecenter wire I4 then protrudes through the axial aperture 24. The outer electrode I2 is then forced over the neck portion 22. The other end of the counter is then assembled similarly in reverse order. As the last operation, the center wire I'4-is'carefully centered so as to be axial of the outer electrode I2. With the center wire I4 held rigidly inthis axial position, the outer end of the second glass end piece I6 is sealed. The counter is thus held rigidly together by the center wire I4 and the forced fits mentioned above.

In Fig. 2, the counter II! is supported by the end pieces I6. The supports '25 are rectangular blocks, preferably of an insulating plastic such as methyl methacrylate. secured to the blocks 25 by clamps 28 secured by screws 39. Below the counter ID is a sample holding table 32. The sample holding table 32 is likewise preferably of an insulating'plastic. Atop the sample holding table'32 is a sample holder 34. The sample holder 34 comprises no part of the present invention and, therefore, need not be described, being merely a plate, usually of metal, used as a carrier for the radioactive sample under measurement which is placed onthe upper surface thereof.

As more fully illustrated in Fig. 3, the upper surface of the sample holder 34, and thus the sample under measurement, is spaced a small distance from the outer electrode I2, preferably not more than 1 centimeter. As is well known in the art, the path'of an "alpha particle in air is totally only about 3 to '5 centimeters. It is, of course, desirable that the maximum amount of this total pathshould be within the. active The end pieces I6 are p being 10.6 centimeters.

portion of the counter ID in order to maximize the amount of charge collected by the electrodes I2 and I4 as a result of ionization caused by the alpha particles sought to be counted.

As illustrated in Fig. 2, the center wire I4 is externally connected to a lead wire 40. Another lead wire 42 is connected, for example, by a solder joint, to the outer electrode I2. The apparatus is preferably enclosed within an electrostatic shield (not shown in the drawing) having an opening adapted to allow the placing and removal of the sample holder 34.

In Fig. 4 is illustrated a typical electronic circuit for use with such a proportional counter. The electronic circuit is conventional and is presented here merely for purposes of illustration. The outer electrode I2 of the counter I0 is connected by the lead wire 42 to the negative terminal of a high-voltage power supply 44. The center wire I4 is connected by the lead wire 40 to one terminal ofa resistor 46, the other terminal of which is grounded, as is the positive terminal of the power supply 44. The power supply 44, the counter I 0, and the resistor 46 are thus in series. When a pulse of ionization'occurs within the counter II], a momentary current flows through this seriescircuit, thus creating a negative voltage pulse across the resistor 46. The amplitude of this voltage pulse is proportional to the amount of ionization occurring within the counter I8 inducing the pulse. As is well known in the art, alpha particles are very heavily ionizing and produce voltage pulses across the resistor 46 which are much larger than the voltage pulses produced by beta particles. All the pulses are transmitted through a condenser 48 to a conventional amplifier 50. After amplification the pulses are transmitted to a pulse height selector 52. As is well known in the art, a pulse height selector transmits only pulses of an amplitude greater than a given preset level. Thus the pulse-height selector, appropriately preset, acts to discriminate between the alpha pulses and the beta pulses, the former being greater in amplitude than the latter. The beta pulses are suppressed in the pulse-height selector 52 and the alpha pulses are transmitted to a sealer 54 whereby they are counted.

The dimensions of the counter ID are not critical. However, as is well known in the art, the voltage required for proper operation is a function of the spacing of the outer electrode I2 and the center wire I4 and also of the diameter of the center wire I4, which is largely determinative of the field strength in the vicinity of the axis of the counter II). In one structure made in accordance with the illustration of Fig. l of the drawing there has been used as the outer electrode I2 a tube of 30 mesh bronze screen, the diameter of the tube being 9.5 millimeters and the length The insulators 20 may be, for example, 5 centimeters in length, the diam eter of the neck portion 22 being 9.5 millimeters to correspond with the inner diameter of the outer electrode I2. The cup-shaped cavities I8 may have a diameter of 8 millimeters, corresponding to the outer diameter of the glass end pieces I6, which are, for example, '7 centimeters in length. The diameter of the center wire I4 may be 5 mils.

With the dimensions mentioned above it has been found that the proper operating voltage for the counter I9 and thus the required voltage value of the power supply 44 is approximately 3300 volts. Exact adjustment of the operating voltage, together with adjustment of the gain of the amplifier 50 and the setting of the pulse height selector 52 are well known in the art. As is likewise well-known, if the voltage is set too high, discrimination between beta particles and alpha particles cannot be achieved as under these circumstances, operation takes place in the Geiger region, wherein the amplitude of the pulse is no longer proportional to the ionization caused directly by the incident particle, all pulses being substantially equal in size.

It will, of course, be understood that the scope of the present invention is in no way limited by the specific embodiment illustrated in the drawing and described above. Persons skilled in the art will readily utilize the teachings of this invention to construct devices for the measurement of radioactivity equivalent to that herein disclosed.

What is claimed is:

1. Proportional counter apparatus comprising, in combination, an anode, a cathode, an ionizing medium between said anode and said cathode, a voltage supply having positive and negative terminals, and means for connecting said positive and negative terminals to said anode and cathode, respectively, wherein said ionizing medium is air at atmospheric pressure and said voltage supply is of a value adapted to produce ionization in said ionizing medium proportional to the direct ionization caused by incident charged particles, and wherein there are provided passages for air and for alpha particles between the anode-cathode space and the surrounding atmosphere exterior to the counter.

2. A proportional counter having an anode, a cathode and an ionizing medium comprising air at atmospheric pressure, wherein there are provided passages for air and for alpha particles between the anode-cathode space and the surrounding atmosphere exterior to the counter.

3. In proportional counter apparatus having a proportional counter comprising a center wire electrode, a second electrode spaced apart from said center wire electrode, and an ionizing medium between said electrodes, and a voltage supply adapted to produce ionization in said ionizing medium proportional to the direct ionization caused by incident charged particles, an ionizing medium comprising air at atmospheric pressure and passages for air and for alpha particles connecting the interelectrode space with the surrounding atmosphere exterior to the counter.

4. Proportional counter apparatus comprising in combination, an anode, a cathode, air at atmospheric pressure between said anode and said cathode, and a voltage supply adapted to produce ion collection by said anode and cathode,

the amount of said ion collection being proportional to the ionization caused by incident charged particles, wherein there are provided passages for air and for alpha particles between the anode-cathode space and the surrounding atmosphere exterior to the counter.

5. Proportional counter apparatus comprising, in combination, an anode electrode, a cathode electrode, an ionizing region between said electrodes filled with air at atmospheric pressure, an air passage aerostatically connecting said ionizing region to the ambient atmosphere exterior to the counter, and means for maintaining between said electrodes a potential difference adapted to produce ionization in said region proportional to the ionization directly caused. by an incident particle.

6. A proportional counter comprising, in combination, an unenclosed tubular electrode of conducting wire mesh, a straight center wire electrode, and means for supporting said center wire axially of said tubular electrode, said supporting means being adapted to insulate electrically said electrodes and the space between said electrodes containing air at atmospheric pressure.

7. A proportional counter comprising, in combination an unenclosed tubular electrode, a straight wire electrode, and means for supporting said straight wire electrode axially of said tubular electrode, said tubular electrode containing air at atmospheric pressure and having apertures therein adapted to transmit freely to the region enclosed by said tubular electrode the air of the ambient atmosphere.

8. Proportional counter apparatus comprising, in combination, an outer electrode enclosing an ionizing region containing air at atmospheric pressure, an inner electrode supported within said ionizing region and electrically insulated from said outer electrode, and means for maintaining between said electrodes an electrical potential adapted to cause in said ionizing region ionization proportional to the ionization directly caused by incident particles, said outer electrode having apertures therein adapted to transmit to said ionizing region alpha particles and the air of the ambient atmosphere exterior to the counter.

EVERETT W. MOLLOY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,197,453 I-Iassler Apr. 16, 1940 2,397,071 Hare Mar. 19, 1946 2,472,365 Borkowski June 7, 1949 OTHER REFERENCES Brown et al., Review of Scientific Instruments, vol. 16, N0. 5, May 1945, pp. 125-129.

Brubaker and Pollard, Review of Scientific Instruments, vol. 8, July 1937, pp. 254-258.

Korfl, Electron and Nuclear Counters, D. Van Nostrand Co., Inc., April 1946, pp. 34, 35, 74-77, -83.

Geiger and Muller, Article in Physlkalische Zeitschrift, vol. 30, August 15, 1929.

Images