US2442314A - Geiger counter improvement - Google Patents

Description

May 25, 1948. A.

GEIGER COUNTER IMPROVEMENT F. REID Filed July 17, 1945 l nsoo l I I300 I100 VOLTAGE Fi g. 3

VOLTAGE mhzsoo 5T0 INDICATOR Fig.2

INVENTOR QLLEN F. RElD Fig.1

ATTORNEY Patented May 25, 1948 UNITED STATES PATEliIT OFFICE GEIGER COUNTER IMPROVEMENT Allen F. Reid, New York, N. Y., assignor to the United States or-Amerlea. as represented by the United States Atomic Energy Commission Application July 17, 1945, Serial No. 605,543

.1 7 Claims.

This invention relates to a Geiger counter and counting method and to an improved gas for the interior oi. the counter which iunctlons to become ionized and produce an electrical discharge when a high velocity charged particle enters the counter and also to quickly quench the electrical discharge so that the counter may respond accurately to a rapid succession of such particles.

The invention will be described with reference to the drawing in which:

Fig. 1 is a central vertical section of a Geiger counter and a, circuit therefor;

Fig. 2 is a top view of the counter;

Fig. 3 is an explanatory curve of an ideal relationship between the number of counts made per minute and the voltage applied to the counter; and

Fig. 4 is'a curve of experimental results of the same relationship.

The Geiger counter is an instrument which has been used extensively for measuring the activity of radioactive materials and one construction thereof is illustrated in Fig. 1. The counter comprises a generally bell-shaped glass container I, a nickel cylinder 2, a tungsten wire 3, and a mica window ll. The container has a flared end 5 and the cylinder similarly has a flared end 6 which fits under the end 5. The wire 3 extends axially within the container and within a glass bore 1. At one end the wire is sealed to the container at a, tapered central portion 8; and at the other end the wire is provided with a small ball 9. The cylinder 2 is in contact with a lead l which is sealed to the container at a tapered portion II.

The mica window is fused to the flared end of the container. The lead in connects the cylinder 2 to the negative pole of a voltage source 12 and the wire 3 is connected to the positive pole. A resistance I3 is connected between the wire and the positive pole of the voltage supply. The counter circuit includes an indicator (not shown) such as an electronic scaling unit and mechanical register for receiving and providing an indication of the electrical impulses produced in said circuit. When, for example, the activity of a radioactive material is to be measured, the material is placed adjacent the thin mica window t.

In order to more clearly understand the invention, an explanation of the operation of the counter is necessary. In general, the operation of the counter involves the ionization of the gas within the counter by the advent of a. high velocity charged particle and the causation of an electrical discharge. For example, in counting betarays emitted by a radioactive material, when a beta-ray enters the counter it collides with molecules of the gas present and in so doing detaches electrons from atoms of the gas molecules, or

in other words ionizes the molecules. These detached electrons, in a high field may in turn collide with other gas molecules and detach more electrons. The result of this chain-like sequence of events is to produce a large number of electrons which surge to the positively charged wire 8 and are discharged there, this event being indicated by the indicator.

It is very important that the counter be promptly restored to its initial condition in order to ensure that the next succeeding beta-ray, which may enter the counter in the next fraction of a second, will be counted. In other words, it is necessary that the electron discharge which is initiated by the entry of a beta-ray in the counter shall be promptly extinguished or quenched.

Quenching of the discharge may be accomplished by various methods. By one-method an extinguishing circuit is used which causes a drop in the high positive potential of the wire suflicient to quickly extinguish the discharge. By another method, which has the advantage of simplicity, the gas itself present in the counter in conjunction with the resistance l3 inthe counter circuit,

by decreasing the positive potential of the wire.

The gas molecules which have had an electron removed are positively charged ions and hence are attracted to the cylinder 2 which comprises the negative electrode. Since these positive ions are relatively slow moving owing to their large mass, they are not immediately discharged at the negative electrode but form a layer or sheath of positively charged particles between the wire 3 ary discharge of electrons suflicient to start the discharge again. This may be due to the fact that the positive ions recombine with electrons at the negative electrode to form a combination which is unstable and from which electrons again are split of! to start the discharge'anew. Double pulses and triple pulses or a continuous discharge may result from this incomplete 1 quenching. Also, gases of different chemical composition vary 'in the quantum of energy which is needed to ionize them. some will be I ionized only by beta-rays of relatively high entively high energy, representing a proportion of I those which enter the counter, cause discharges. As the voltage is further increased, the field becomes strong enough to count not only the high energy beta-rays but some of the lower energy ones and a range on the voltage axis is finally reached, seen as a plateau on the curve,

at which a number of counts per minute is observed which amounts to 100 per cent of the beta-rays entering the effective part of the counter. Above this plateau the curve rises indicating that some of the beta-rays are counted twice or three times. On further increasing the voltage there .is finally a continuous discharge owing to the fact that the applied voltage is so high that the potential difference between the wire and the sheath of ions is great enough to attract the electrons to the wire at a speed to cause suflicient secondary ionization to create a continuous discharge.

In the ideal curve of Fig. 3 the point at which 100 per cent of the beta-rays which enter the counter are counted will coincide with the plateau of the curve. In other words, this point on the count axis will be maintained over a range of applied voltages. The necessity for this is apparent when it is considered that even with a good voltage regulator there will always be some variation in the voltage applied to the counter. Therefore the curve should have a plateau which has an extent at least sufllcient to include the anticipated variations in voltage. The voltage range which should be included in the plateau will therefore depend on the precision 01 the voltage regulator used, or on the variation of the supply if no regulator is used. For example, a good voltage regulator will be accurate to within 56 of 1 per cent. If it is desired to apply 1400 volts there will then be a possible variation of 7 volts either way. The plateau should therefore include this range.

The ideal plateau of Fig. 3 is perfectly horizontal, that is, the count is up per cent along the entire plateau. Hitherto it has been found experimentally that the plateau actually has a slopewhich may be as much as 2 per cent of the counts per 100 volts. It is apparent that when the applied voltage varies slightl and the per.

centage of beta-rays counted varies with the applied voltage, it is not possible to maintain the counter at a voltage at which 100 per cent of the beta-rays are counted, this being the objective for accurate results. Thus, any slope of the plateau away from the perfectly horizontal will introduce an error which it is desirable to keep to a minimum.

It may be shown that the characteristics of the curve depend upon the nature of the quenching gas. Although all gases form ions when electrons are detached from atoms of the gas molecules, not all gases, when ionized, will form a sheath of ions which will effectively quench the discharge. In some gases there will be a seco dergy and others by beta-rays of relatively low energy. If the voltage applied to the counter is made high enough even those gases which require high energy beta-rays to ionize them may be'ionized by beta-rays of lower energy, but at the high voltages required only a continuous discharge in the counter will be obtained. 'It is thus apparent that the nature of the gas in the counter determines the nature or the curve obtained expressing the relationship between the percentage or beta-rays counted to the applied voltage. and that a desirable gas is one which will permit the attainment of a curve closely approximating the ideal, that is, a curve with a wide and horizontal plateau.

Preferably the gas should have a number of other characteristics which are quite important. The gas should not be poisonous so that it may be handled without special precautions. This is an objection to the use of lead tetra-methyl which has previously been used as gas for the counter. The gas should also not be sensitive to light inasmuch as the effect of light on a light-sensitive gas is to cause independent discharges so that a spurious count is registered. Therefore, when the gas is light-sensitive the counter must be carefully shielded from light. This may be disadvantageous when the obiect whose radioactivity is to be measured is very large or, as frequently happens, if a part of the human body is to be measured, since then the part to be measured must .be placed outside the shield with the result that some of the radioactivity does not get through the shield. Lead tetramethyl and also amargon-ethyl alcohol mixture which is a commonly used gas are both objectionable because of their light-sensitivity.

The gas should be one which is not quickly decomposed under the conditions existing in the Geiger counter. When an electron is split of! from an atom of a gas molecule the bond between the deprived atom and the rest of the molecule is weakened. If this bond is weakened suiiiciently the atom will split off from the molecule, i. e. the gas molecule will be chemically decomposed. It is desirable that this chemical decomposition of the gas molecule should take plate at a very slow rate so that the tube may be used over a long period of time without the neoessity of refilling. In other words, the molecule must be of such a nature that an electron may be split off from one of the atoms under the conditions at which the counter will be operated but this process should not result in rapid chemical decomposition of the gas. Furthermore, the decomposition products of the gas should not interfere with the proper functioning of the counter.

It is apparent, therefore, that a desirable gas for a Geiger counter must combine a large number of diverse characteristics in order to function in the required manner.

an object of the invention is therefore to proof a. curve expressing the relationship between the number of counts per minute and the applied voltage, in which there is a wide and horizontal plateau.

A further object of the invention is to provide a quenching s which may be used without any special precautions and over a long period of time with good results.

According to the invention, the alkyl monohalides of two to six carbon atoms in their vapor phase have been found to be excellent quenching gases for Geiger counters.

The alkyl halide used may have any of the possible isomeric forms such as, for example, the normal, the iso-, the secondary, or the tertiary form.

The halogen atom of the halide may be chlorine, bromine, iodine or fluorine, but only one halosen atom should be present in the molecule.

e halide may be an ethyl, propyl, butyl, any], or heal halide.

It has also been found that among the alkyl mono-des of two to six carbon atoms, there are individuals or groups of individuals which are particularly desirable with respect to a certain charristic. For example, it has been found the mono-halides of higher chain len give better Plateaus so that it an especial- 1y good plateau is desired for a measurement, one of the w halides or howl halides should be used. Also, as the atomic weight of the halogen atom decreases, the stability of the molecule increases and hence its lifetime increases. Thus a fluoride has greater stability and a longer life hrs-1 a chloride. A quenching gas which comblues a number of desirable properties is, for example, n-- I" l fluoride.

"1'; pressure of the gas in the counter will be usted with reference to the voltage one wishes to app y to the counter. For a given pressure, the plateau of one gas will occur at a diflerent voltage from that of another gas. However, the plateau oi a may be shifted to a higher voltage e by increasing the pressure of the gas.

Thus if one desires to operate at higher voltages, the pressure of the gas used is adjusted accordingly. At higher voltages better plateaus are obted. For the mono alkyl halides of two to six carbon atoms a useful range of pressures was 9 to 225 2-1 or of mercury.

Fig. s is a curve of experimental results obtain when the counter of Fig. l was filled with A 1 f (t the ideal c of 3. The plateau has range of 15004560 volts whichis a 1 1':

voltage variation tolerance. The slope of the plateau did not exceed $5 of 1 percent per volts as compared with 2 percent per 100 volts which is the best slope normally obtained with an argon-ethyl alcohol mixture. Atter one million counts the counter operated with the same accuracy. The gas was non-poisonous and was unafl'ected by light.

Although the invention has been described with particular reference to the measurement of betarays by a Geiger counter, it is apparent that it is equally applicable for the measurement of other high velocity charged particles such as positrons and secondary electrons. It will be apparent to those skilled in the art that a, Geiger counter may be operated in the proportional region which covers a comparatively lower voltage range and that the quenching gases of the present invention may be used for such application.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it

5. A Geiger counter including a quenching gas comprising n-amyl fluoride.

6. A Geiger counter including a quenching gas comprising a mono-alkyl halide of two to six carbon atoms at a pressure or 9 to 25 mm. of mercury.

7. A Geiger counter including'a quenching gas comprising n-amyl chloride at a pressure of 20 mm. oi. mercury.

. ALLEN F. REID.

amnncus crrnn The following references of, record. in the file of this patent:

. UNITED STATES PATENTS Number 7 Name Date 2,405,572 Fri Aug. 13, 1946 2,409,498 Keston 001;. 15, 1946 REFERENCES Handbook of Chemistry and Physics, 27th ed. 1943-1944, Chemical Rubber Publishing 00-, Mes 580, 581, 672-675; 816 and 817. (Copy in DM- sion 6.)

or two to six'

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