US3288999A - Humidity insensitive proportional counter having a heating source connected to a wire anode - Google Patents

Description

Nov. 29, 1966 C.J. BORKOWSKI ETAL 8 HUMIDITY INSENSITIVE PROPORTIONAL COUNTER HAVING A HEATING SOURCE CONNECTED TO A WIRE ANODE 2 Sheets-Sheet 1 Filed Oct. 21, 1963 HEATING Nov. 1966 C.J. BORKOWSKI ETAL HUMIDITY INSENSITIVE PROPORTIONAL COUNTER HAVING A SOURCE CONNECTED TO A WIRE ANQDE 2 Sheets-Sheet 2 Filed Oct. 21, 1963 E O a 2 O 0 O O O 0 06 m m m s 4 P525 mwm wpznou APPLIED VOLTAGE 0 DRY AIR 100% RH-NO CURRENT E1 |OO% RH-CURRENT THROUGH WIRE mo2 wDOIF APPLIED VOLTAGE r w 00 m Tk r NFXG E 0 o F V W. R a H m Q o n C R A Y B 3 Wa m ATTORNEY.

United States Patent 3,288,999 HUMIDITY INSENSITIVE PRGPORTIONAL COUNTER HAVING A HEATING SOURCE CUNNECTED TO A WIRE ANODE Casirner J. Borlrowski, Angel M. R. Ferrari, and Richard J. Fox, Oak Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Oct. 21, 1963, Ser. No. 317,853 1 Claim. (Cl. 250-83.6)

The present invention relates to a proportional counter, for the detection of alpha or beta particles, that is not aifected by high relative humidity.

One of the simplest and least expensive of large-area alpha particle detectors is the air proportional counter, that is, a proportional counter using air as the counter gas in contrast to a gas of carefully controlled composition. The major disadvantage of such counters, and one that has limited the use of the same, is the detrimental effect humidity has upon operation of the counter. At higher relative humidity the counter generates spurious pulses that cannot be distinguished from actual alpha pulses.

In the prior art it has been shown that, if the counter is maintained at a temperature higher than that of the surface being monitored, the effect of high relative humidity is reduced. For example, it has been proposed to operate the counter at a temperature of about 20 F. above room temperature. This has been accomplished by installing a l2-watt heater adjacent to the cathode surface. This quantity of electrical power, however, requires attachment to a power line and thus substantially restricts the portability of the device. Also, the effect of relative humidity, even with such a heater, is not completely eliminated and still affects the overall accuracy of such a counter.

With a knowledge of the limitations of the prior art, as discussed above, it is a primary object of this invention to provide a proportional counter that is substantially completely humidity-insensitive.

It is another object of this invention to provide a proportional counter that is not only humidity-insensitive, but that is also portable,

These and other objects and advantages of this invention will become apparent upon a consideration of the following detailed specification and the accompanying drawings, wherein:

FIG.. 1 is a schematic diagram of a circuit for accomplishing the above objects;

FIG. 2 is a plot of data showing the response of the proportional counter of this invention to background radiation, as a function of applied voltage, for several conditions of relative humidity; and

FIG. 3 is a plot of data showing the response to a source of alpha particles, as a function of the applied voltage, for the same conditions of relative humidity as those for FIG. 2.

The above objects have been accomplished in the present invention by providing a small battery in series with the anode of the counter for heating the anode wire only. It has been found that this results in a counter that is substantially insensitive to humidity with an expenditure of only one-hundreth of the power as compared with the prior art. Thus, the battery is adequate and the counter does not require a power line connection to an external source of heating power.

In FIG. 1, a proportional counter 1 is provided with a cathode 5; an anode 6 which is supported from insulators, not shown, in the ends of the cathode 5; and guard rings 7, 7'. The cathode 5 may be a l /z-inch O.D brass tubing, for example, and a portion of the tubing is pro- "ice vided with a plurality of perforations 13 to admit alpha particles. The inside surface of the tubing is polished and gold plated, and the above-mentioned perforations are covered with a thin Mylar film 14 to exclude dust, etc. The anode 6 is a 0.001-inch diameter platinum Wire, for example.

A high voltage power supply 8 is connected to the anode 6 through resistors 10 and 11. The cathode 5 is grounded as shown. The guard rings 7, 7 at anode potential are utilized to reduce spurious pulses from leakage along the insulators and to act as an electrostatic shield between the anode 6 and the above-mentioned anode supporting insulators.

The means for heating the anode 6 is a current supply 9, which may be a battery, for example. The battery must be properly insulated and completely shielded. The current supply 9 is connected directly across the anode 6, as shown in FIG. 1.

The output of the counter 1 is coupled'by a condenser 12 to a linear amplifier and trigger circuit 2. The output of unit 2 is connected to a monitor scope 3, which in turn is connected to a sealer 4. The components 2, 3 and 4 may be any of several conventional units. One example of a linear amplifier and trigger circuit which may be used is described in an article by P. R. Bell and W. H. Jordon in Review of Scientific Instruments, vol. 18, No. 10, pp. 703-705, October 1947. The monitor scope 3 may be, for example, Model No. 535 made by Tektronix, Inc., Portland, Oregon, and the scaler 4 may be, for example, Model No. N-240 made by the Hamner Electronic Company, Princeton, New Jersey. The high voltage supply 8 may be. for example, Model No. 400 BDA made by the John Fluke Manufacturing Company, Seattle, Washington, or any other conventional well regulated supply.

In order to provide some operating data for the counter of FIG. 1, the counter was connected to a system, not shown, containing a conventional mechanical vacuum pump, a source of dry air and a source of water vapor and over a range of heating of the anode 6. For counter was determined under conditions varying from com-pletely dry air to air nearly saturated with water vapor and over a range of heating of the anode 6. For all tests the pressure and gas temperature were held constant (720 mm. Hg and 25 C., respectively), and the voltage applied between the anode 6 and cathode 5 was increased in SO-volt steps from 1800 volts to the point where corona breakdown occurred. The counting rate at each step was determined using, first, background radiation and, second, a plutonium-238 alpha source which was at the same position for each measurement. Operations with and without wire heating were compared and the data plotted in FIG. 2 and FIG. 3 for background radiation and for the alpha source, respectively. With saturated air and no anode heating, operation was erratic, the efficiency was lower and corona breakdown occurred at a lower voltage. This is typical of the results obtained in the prior art. However, when the anode was heated .sufficiently, the counter operated satisfactorily either with dry or water-vapor-saturated air, as can be clearly seen from the graphs of FIG. 2 and FIG. 3.

It has been determined that in the counter of FIG. 1 the minimum amount of current needed for heating the anode to sufiiciently suppress humidity effects is about 13 milliamperes. A range of 12 to 16 ma. heating current was investigated. With a current of 13 ma., the temperature of the anode is lower than 40 C. The power consumed at 13 ma. heating current is about milliwatts. It should be clear that a small battery is all that is required to meet the above heating requirements. Thus, the various components of the system of FIG. 1 can easily be assembled in a suitable portable case, when such is desired.

Although the present invention is particularly useful for alpha proportional counters, it similarly would be useful in certain beta particle detecting application in gases (other than air) having a high humidity. Such an application is the measurement of tritium. In the past, gases having an ionization potential lower than air have been used; With a counter les sensitive to humidity, a wider choice of counter gas is possible.

This invention has been described by Way of illustration rather than limitation and it should be apparent that this invention is equally ap able infields other than those described.

What is claimed is:

In an air proportional counter for detecting alpha particles including a tubular cathode, a center platinum wire anode, a source of air to be monitored and a high voltage source of potential connected between'said anode and said cathode, said cathode interior being gold plated and said cathode being provided with a pluralityiof apertures covered with a thin Mylar film, the improvement for substantially reducing the effect of all values of relative humidity on said counter comprising a small battery for supplying a source of heating current and being connected to opposite ends of said wire anode, said heating current having a minimum value of at least 13 milliarnperes, whereby said counter is substantially insensitive to said relative humidity effect.

References Cited by the Examiner UNITED STATES PATENTS 1,099,365 6/1914 Henderson 219-209 2,499,830 3/1950 Mallory 313-93 2,532,956 12/1950 Simpson 25083.6 2,819,614 1/1958 Sion 3247l RALPH G. NILSON, Primary Examiner. JAMES W. LAWRENCE, Examiner.

S. ELBAUM, Assistant Examiner.

Images