US2712088A - Whitman - Google Patents

Description

June 28, 1955 Filed Dec. 10, 1952 RELATIVE COUNTING RATE A. WHITMAN RADIATION COUNTER AND FILL GAS 2 Sheets-Sheet l coum'ms CURVES FOR FLOW COUNTER 111'! [1| |1| 1||1| |o00 H00 1200 I300 I400 I500 I600 VOLTAGE FIG. 2

INVENTOR ALFRED WHITMAN ATTORNEY June 28, 1955 w N 2,712,088

RADIATION COUNTER AND FILL GAS Filed Dec. 10, 1952 2 Sheets-Sheet 2 COUNTING CURVES OF GEIGER-MUELLER TUBE f 30 II A D E E x 3 20 o LU z 4 l 5 I0 I; I I I I! ll H I II l l l l l I l l I l I I000 I200 I400 I600 ISQO 2000 VOLTAGE v m/vs/vrw? ALFRED WHITMAN BYWZM A TTOR/VE' Y United States Patent fiiice Patented June 28, 1%55 RADIATION COUNTER AND FILL GAS Alfred Whitman, Oxford, Ohio, ,assignor to Tracerlab, Inc., Boston, Mass, a corporation of Massachusetts Application December 10, 1952, erial No. 325,132

5 Claims. (Cl. 31393) This invention relates to improvements in Geiger- Mueller counters and more particularly to improved gas fillings therefor for providing self-quenching operation.

The Geiger-Mueller tube (hereafter referred to as G-M tube) is now a well-known instrument for the detec tion of radiations. Fig. l of the accompanying drawings diagrammatically illustrates a typical G-M tube which includes a cylindrical cathode electrode and a centrally located anode wire 11. The electrodes are enclosed in an envelope 13 formed of glass or metal, as the application may require, containing a suitable ionizable gas. The anode wire 11 is maintained usually at a positive potential relative to the cathode 10, this potential being nearly but not quite high enough to cause a discharge to take place between the electrodes. When an ionizing event such as is produced by radiations takes place within the sensitive volume of the counter, an electric discharge develops through the gas with a current of the order of a few microampe'res. This causes a relatively large voltage drop across a high resistance 14 connected in series with a high voltage supply 15 and anode l1, and by suitable amplification, a scaling circuit or other pulse registering apparatus may be actuated.

Geiger-Mueller tubes are usually of the form just described, but in certain cases where it is desired to detect alpha particles and weak beta particles, the presence of the envelope, however thin, may prevent these radiations from entering the sensitive volume of the chamber, in which case another type of counter, known as a windowless flow counter, is utilized. A flow counter is a low background radiation counter consisting essentially of a shielded counter tube in which a solid radioactive sample is inserted directly, and through which a constant gas flow is maintained to prevent air contamination. This counter has a cathode and anode center wire and corresponds generally to the conventional G-M tube, the only major differences in construction being the absence of a closure between the counter and sample and the constant flow of gas through the counter. This constant removability of counting gas eliminates the effect of aging on the counter characteristics, and, in

addition, insures freedom from drift, spurious pulses,

and hysteresis, thereby to improve the reproducibility of the counting data.

The combination of anode and cathode electrodes in the gaseous medium of either the filled or flow counter will continue in a state of electric conduction through the gas space once the operating voltage has been raised sufliciently high to initiate it. To be capable of detecting individual ionizing events, it is necessary that the discharge be extinguished after it has existed long enough to manifest itself as a measurable current impulse. It has been found that when certain gas fillings are used, the discharge is automatically extinguished by the tube itself without the use of external quenching circuits, tubes having this characteristic, being known in the art as self-quenching. In addition to being self-quenching, it is desirable that the counter have a good counting plateau, self-quenching properties at reasonable operating voltages, a short dead time, and negligible temperature dependence. The plateau is the curve which demonstrates the variation in counting rate when only the operating voltage is varied. This characteristic of the counter is expressed as the slope of the curve over what is considered the operating range of voltages for a particular counter. it is desirable that the plateau be rather long and fiat in order that reproducible counting be obtained in spite of reasonable variations in the high voltage applied to the counter.

The principal determining design factor in obtaining the foregoing desirable characteristics is the nature of the gas filling used in the tube. Many gasesand mixtures of gases have been employed which provide quenching operation and a reasonable counting plateau. For example, many polyatomic organic gases or vapors such as xylol, ethyl alcohol, amyl acetate, methane, ethane, propane, butane, and ethylene, to name a few, have been used with an inert gas such as helium or argon in various proportions. Halogens, for example chlorine, iodine, or bromine, mixed with neon and/ or argon have also been used in self-quenching counters. V

This invention is based on a study of the properties of counters, of both the filled and flow counter types, using such known vapors and a search for other gases having improved characteristics and advantages. More specifically, the operating characteristics of a commonly used mixture, namely, helium and isobutane, where the latter gas constitutes about one percent of the total gas pressure were carefully studied. Because of erratic operation and the inability to obtain reproducible results with this mixture, particularly When used in the abovedescribed flow counter, an investigation was made to discover a mixture with more desirable properties.

As a first step in the investigation, a counter of the flow type was operated using a mixture of helium and isobutane and an average counting plateau obtained to serve as a basis of comparison. In the graph of Fig. 2, in which counting rate is plotted against voltage, curve A is the counting curve of a flow counter in which the gas wasa mixture consisting mainly of helium and isobutane in an amount equal to .85% of the total gas pressure, which was essentially 760 mm. of mercury.

This curve is an average of several counting runs, and

it should be pointed out that this curve was diflicult to reproduce from one counting run to the other and from counter to counter. Apart from this difficulty, the gas mixture does not afford a counting curve which meets specifications which ordinarily require a plateau having a slope less than 2% per volts and a plateau length of about 300 volts. The slope of curve A in its flattest region, i. 'e., at 1350:50 volts, is of the order of 5%, and it will be noted that this slope does not obtain over a range of anywhere near 300 volts.

Curve B of Fig. 2 is an average counting curve of a flow counter of the type described in which the gas was a three-constituent mixture of helium, isobutane and butadiene, where the helium was the principal constituent and in which the isobutane constituted .85 of the total gas pressure and the butadiene constituted .l% of the gas pressure of the isobutane, the total gas pressure being essentially atmospheric. The threshold voltage (that voltage necessary to produce any counts in the counter) using this mixture was 1060 volts and the counter had a plateau which was essentially fiat over the range from 1200 to 1500 volts, as shown. As the voltage was increased beyond 1500 volts, the counting rate decreased as shown by the dotted extension of the curve, in contradis tinction with the two-constituent mixture of helium and isobutane where the counting rate continued to increase with increases in applied voltage. In either case, of

ture are flatter and considerably longer.

course, the counting rate is ultimately reduced to zero with increases in voltage because the voltage across the electrode becomes sutficiently high to cause a continuous discharge through the gaseous medium, once it is initiated, but the voltage at which continuous discharge occurs is much higher with the two-constituent mixture. In summary, the curves of Fig. 2 clearly illustrate the improved operating characteristics and advantages of the gas mixture of the present invention over that which was heretofore available.

Following the foregoing tests on the fiow counter, a similar comparison was made between two groups of G-M tubes of the filled type, one group being filled at substantially atmospheric pressure with helium and .85% isobutane, and the other group filled at substantially atmospheric pressure with helium, .85% isobutane and butadiene in an amount equal to .l% of the gas pressure of the isobutane. An average counting curve was obtained for each of the two groups of counters, the plateaus of the two-constituent mixture being shown as curve A of Fig. 3, and the plateau of the threeconstituentmixture being shown as curve B. Both mixtures provide comparable starting voltages, about 1100 volts, but the plateaus in the case of the threeconstituent mix- Curve A has an acceptable slope and plateau length in the region between .1250 and 1550 volts, but with further increases in voltage the counting rate increases rather sharply. Curve B, on the other hand, indicates a 600 volt plateau from 1250 to about 1850 volts, before increases in voltage causev an increasein counting rate. Thus, the gas mixture of the present invention also greatly improves the operating characteristics of the filled counter.

While the plotted data is for a mixture of gases wherein-the butadiene constituted .l% of the gas pressure of theisobutane, similar improved results were obtained where the butadiene constituted from about 05% to .4% of the gas pressure of the isobutane.

' Although the butadiene constitutes a seemingly small fraction of the total gas mixture, the curves of Figs. 2

and 3 clearly indicate the vitalizing efiect of its addition to the mixture of helium and isobutane. The exact manner in which the butadiene functions to provide this manifest improvement is not known at present, but it is believed that the butadiene having conjugated double bonds can assume higher energy levels of excitation than olefim'c or saturated hydrocarbons and therefore can more effectively absorb energy without decomposiventionally employed in organic quench-noble gas mixtures does not lead to a satisfactory counting mixture which would seem to indicate that the isobutane and the small amount of butadiene work in concert to give a much improved counting mixture which prevents the production of spurious pulses, the isobutane being effective in absorbing radiations of some energies and the butadiene being effective with other energies. This result is substantiated by the curves of Figs. 2 and 3, the extra counting rate of curve A in the plateau region indicating that when helium and isobutane are used, counts are produced by secondary electrons emitted from the cathode by positive ion bombardment, while when butadiene is added, spurious counts in the corresponding region disappear, as shown by curve B.

In summary, the gas mixture of the present invention provides a counter having a long and flat plateau when used in both filled G-M tubes and fiow counters. Isobutane and butadiene both having a relatively low boiling point, a counter using the present gas mixture is not temperature dependent over a large range of temperatures, and tests have indicated that counters filled with the mixture have a short dead time and a life comparable with those filled with a two-constituent mixture of helium and isobutane.

What is claimed is:

l. A Geiger-Mueller counter tube comprising an envelope, a pair of electrodes therein, and a gaseous filling therefor consisting of helium, isobutane and butadiene, where the isobutane constitutes about .85% of the total gas pressure and the butadiene constitutes from .05 to .4% of the gas pressure of the isobutane.

2. A self-quenching Geiger-Mueller counter tube com: prising an envelope, a pair of electrodes therein, and a gaseous filling therefor at substantially atmospheric pressure consisting of helium, isobutane and butadiene, where" the isobutane constitutes about .85 of the total gas pressure and the butadiene constitutes from .05 to .4% of the gas pressure of the isobutane,

3. A self-quenching radiation counter comprising a envelope, a pair of electrodes therein, and a gaseous filling therefor at substantially atmospheric pressure consisting of helium, isobutane and butadiene, where the isobutane constitutes .85 of the total gas pressure and the butadiene constitutes .l% of the isobutane.

4. A gas mixture for the filling of Geiger-Mueller counter tubes for the purpose of providing such tubes with self-quenching properties at substantially atmospheric pressure comprising helium, isobutane and butadiene where the isobutane constitutes .85 of the total gas pressure and the butadiene constitutes .l% of the gas pressure of the isobutane.

5. A gas mixture for a radiation counter of the Geiger- Mueller type for producing self-quenching operation at substantially atmospheric pressure consisting of helium, isobutane and butadiene, where the isobutane constitutes about .85% of the total gas pressure and the butadiene constitutes from about .05% to about 0.4% of the gas pressure of the isobutane.

References Cited in the file of this patent UNITED STATES PATENTS 2,449,697 Graves et a1. Sept. 21, 1948 2,519,864 Weisz Aug. 22, 1950 2,606,296 Simpson, Jr. Aug. 5, 1952

Claims (1)

1. A GEIGER-MUELLER COUNTER TUBE COMPRISING AN ENVELOPE, A PAIR OF ELECTRODES THERIN, AND A GASEOUS FILLING THERFOR CONSISTING OF HELIUM, ISOBUTANE AND BUTADIENE, WHERE THE ISOBUTANE CONSTITUTES ABOUT .85% OF THE TOTAL GAS PRESSURE AND THE BUTADIENCE CONSTITUTES FROM .05 TO .04% OF THE GAS PRESSURE OF THE ISOBUTANE.

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