The United States Government has rights in this invention pursuant to contract no. DE-AC05-84OR21400 between the United States Department of Energy and Martin Marietta Energy Systems, Inc.

HELD OF THE INVENTION

The present invention relates to methods and apparatus for detecting small quantities of electronegative species, and more particularly to such methods and apparatus which rely on a proportional counter to measure electron attachment.

BACKGROUND OF THE INVENTION

Chlorofluorocarbon (CFC) heat exchanging compositions, referred to hereinafter as refrigerants, currently used in heat pumps, air conditioners, and refrigerators, are now known to have serious environmental effects due to destructive reactions with ozone in the Earth's atmosphere. It is believed that the chlorine atom in these refrigerants is the "culprit" which leads to the destruction of the ozone layer. Thus, refrigerant containing any chlorine atom as part of its chemical composition will likely be prohibited for use in air conditioners. Non- CFC refrigerants which have no Cl atom in their chemical structure, such as fluorocarbons, will likely be used to replace CFC refrigerants currently in use.

It is necessary to locate and repair minute leaks of refrigerants in heat exchanging equipment. Currently available refrigerant leak detectors are generally based on the detection of the chlorine constituent of the molecule. For example, a common CFC leak detector utilizes a small flame impinging upon a copper plate. When CFC is present, the flame will be green in color, due to the chlorine constituent in the CFC.

Other commonly-used detection methods include heated anode and corona suppression. Heated anode leak detectors employ a red-hot platinum and ceramic heater element which releases positive ions. These positive ions are collected on a negatively charged cylindrical cathode to provide a standing current. The presence of CFC in sampled gas increases the emission of positive ions which then triggers the leak signal. The corona suppression leak detector applies a voltage across a sensor element to produce a corona (spark) which generates a standing current. The presence of CFC in sampled gas inhibits the corona spark which triggers a leak signal. Because the use of red-hot elements or sparks raises the possibility of undesired ignition, these types of detectors are not suitable for use

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in areas where flammable or explosive gas is likely to be present. These detectors are not suitable for detecting chlorine free refrigerants. Also, no intrinsic leak signal amplification appears to be involved in those leak detectors.

A reliable, sensitive, and simple instrument which can be used to detect chlorine-free refrigerant leaks of less than one ounce per year from heat exchange equipment will soon be essential to the heat exchange industry.

Furthermore, awareness of the sensitivity of the environment to the presence of small amounts of hazardous, toxic, or otherwise undesirable materials has created a need for new methods and apparatus for detecting small quantities of these materials in suspect localities. Certain of these materials are comprised of electronegative species, which can form a basis for their detection in very low concentrations. Examples of these materials are polychlorinated biphenyls (PCB's), carbon tetrachloride, and trichloroethylene (TCE). A reliable, sensitive, and simple instrument which can be used to detect one part per million or less of these materials in the atmosphere is essential to the protection of the environment.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide new and improved methods for protecting the environment.

It is another object of the invention to provide a new and improved method and apparatus for detecting refrigerant leaks.

It is a further object of the invention to provide a new and improved method and apparatus for detecting electronegative species.

Further and other objects of the present invention will become apparent from the description contained herein.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the foregoing and other objects are achieved by apparatus for detecting an electronegative species which comprises an ionizing energy source in association with a proportional counter, said proportional counter being operable in ambient atmospheric conditions.

In accordance with another aspect of the present invention, apparatus for detecting an electronegative species comprises: a radioactive source for emitting radioactive energy for ionizing a component of a

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sample containing the electronegative species and an ionizable component; proportional electron detection means associated with the radioactive source for detecting electrons emitted from the ionized component; and, circuit means for measuring the electrons and determining the presence of the electronegative species by detecting a reduction in the number of available electrons due to capture of electrons by the electronegative species.

In accordance with another aspect of the present invention, apparatus for detecting an electronegative species comprises: an analysis chamber; admitting means communicating with the analysis chamber for admitting a sample containing the electronegative species and an ionizable component; a radioactive source within the analysis chamber for emitting radioactive energy for ionizing a component of the sample; proportional electron detection means within the analysis chamber for detecting electrons emitted from the ionized component; and, circuit means for measuring the electrons and determining the presence of the electronegative species by detecting a reduction in the number of available electrons due to capture of electrons by the electronegative species.

In accordance with a further aspect of the present invention, a method for detecting an electronegative species comprises the steps of: providing an apparatus which comprises an ionizing energy source in association with a proportional counter, the proportional counter being operable in ambient atmospheric conditions contacting the apparatus with a gaseous sample to determine the presence of the electronegative species.

In accordance with a further aspect of the present invention, a method for detecting an electronegative species comprises the steps of: providing a radioactive source for emitting radioactive energy; providing proportional electron detection means associated with the radioactive source for detecting electrons; providing circuit means for measuring the electrons; and,

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contacting the radioactive energy with a sample containing the electronegative species and an ionizable component so that the radioactive energy ionizes the component of the sample, the proportional electron detection means detects electrons emitted from the ionized component, and circuit means measures the electrons and creates data representing the presence of the electronegative species by detecting a reduction in the number of available electrons due to capture of electrons by the electronegative species.

In accordance with a further aspect of the present invention, a method for detecting an electronegative species comprises the steps of: providing an analysis chamber; providing admitting means communicating with the analysis chamber for admitting a sample containing the electronegative species and an ionizable component; providing a radioactive source within the analysis chamber for emitting radioactive energy for ionizing a component of the sample; providing proportional electron detection means within the analysis chamber for detecting electrons emitted from the ionized component; providing circuit means for measuring the electrons and determining the presence of the electronegative species by detecting a reduction in the number of available electrons due to capture of electrons by the electronegative species; and, introducing through the admitting means and into the analysis chamber the sample so that the radioactive energy ionizes the component of the sample, the proportional electron detection means detects electrons emitted from the ionized component, and circuit means measures the electrons and creates data representing the presence of the electronegative species by detecting a reduction in the number of available electrons due to capture of electrons by the electronegative species.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

Fig. 1 is a schematic sectional side view of an embodiment of the subject invention.

Fig. 2 is a graph representing data collected during a test of the subject invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawing.

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DETAILED DESCRIPΗON OF THE _I ^* TvΕNTION

Many refrigerants have fluorine atoms as part of their chemical compositions. Since compounds containing fluorine tend to have high electron affinity, most refrigerants can easily form negative ions by the process of electron attachment. A proportional counter can be used to detect a small number of electrons due to the amplification of a number of electrons when they are accelerated toward the counter wire which is biased at positive voltage. When an electro-negative species is introduced, most electrons are absorbed, or captured, by these molecules with high electron affinity. Thus, the counting efficiency of the proportional counter is reduced. From the reduction of the counting rate, molecules with high electron affinity, such as refrigerants, can be detected. Electronegative species are defined as compositions that exhibit electron affinity, and more particularly, compositions that are comprised of at least one halogen.

Proportional counters are generally considered to require the use of a counting gas, such as P10 gas (10% CH ₄ and 90% Ar), as a medium to achieve the amplification process. However, when a source of ionizing energy is associated with a proportional counter, the proportional counter can then be operated in an ambient atmosphere. A preferred embodiment of the invention utilizes a thin wire type proportional counter element attached at or close to the center of a radioactive source. Any radioactive source will produce electron ion pairs in a medium through the decay process. Other sources of ionizing energy may be used, but possibly at greater expense. Electrons are attracted to the wire, which is biased with positive voltage. When electrons are accelerated toward the wire, more electrons will be produced by electron atom collisions; most amplification is achieved in the region which is very close to the wire.

The basic reason proportional counters have heretofore not been considered suitable for use in ambient conditions is the concern of electron attachment to O ₂ to form O ₂ ". The large percentage of O ₂ in air exhibits significant response in attaching to free electrons; hence, intentionally locating the ionizing energy source very close to the wire overcomes the reduction of amplification by O ₂ sufficiently to allow highly sensitive measurements of compositions which exhibit higher electron affinity. Amplification can, however, be reduced if electron-ion pairs are produced too far from the wire.

Referring now to Fig. 1, a proportional counter adapted for detecting small amounts of electronegative species comprises a cylindrical analysis chamber 10 which houses a radioactive source 12, preferably a nominal 1 microcurie α source, at one end and a counter wire 14 suspended between the radioactive source 12 and the other end of the analysis

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chamber 10. The radioactive source 12 and counter wire 14 are supported by insulating supports 16, 18 attached to the ends of the analysis chamber 10. The counter wire 14 preferably runs centrally and parallel to the greatest cross section of radiation for maximum exposure to available electrons.

The counter wire 14 is a conductor, preferably a metal, and more preferably stainless steel; with a preferred, although not critical diameter of about .002 inch. The counter wire 14 is biased at a sufficient voltage, preferably about 100 to about 5000 V, more preferably about 1300 to about 3000 V, most preferably about 1000 to about 2000 V to detect electrons generated by α decay ionization of sample gasses. Higher bias voltages generally increase sensitivity, but also increase signal noise, while lower bias voltages generally decrease signal noise, but also decrease sensitivity; the bias voltage should be adjustable in order to optimize conditions for specific analysis situations. The bias voltage is supplied by a power supply 20 connected through a resistor 21, which may have a nominal value of about 23 MΩ, that value not being critical to any particular set of parameters.

A charge sensitive preamplifier 22 amplifies a signal emanating from a capacitor 23 connected to the counter wire 14. The capacitor may have a nominal value of about .002 μF, that value not being critical to any particular set of parameters. A count rate meter 24 converts the amplified signal into useful data indicating the number of available electrons within the analysis chamber 10.

An air pump 26 is preferably used to pump sample air or gas from a source through a probe 27 and thence through an inlet 28 and into the analysis chamber 10. An outlet 32 is provided to allow the sample air or gas to pass through the sample chamber 10. Pressures and flow rates are not particularly critical to the operability of the detector. A nominal 1 atm of air is quite satisfactory; there is no critical need for vacuum or high pressure equipment. The preferred method for using the apparatus involves the detection of electronegative refrigerant leaks from cooling systems.

EXAMPLE I Air was continuously sampled and pumped through the analysis chamber 10. The a source produced electrons and positive ions in the air inside the analysis chamber 10. Electrons produced were accelerated toward the counter wire 14, producing signal pulses which were subsequently amplified by the preamplifier 22 and converted into useful data by the count

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rate meter 24. A standard counting technique was used to obtain useful counting rate data. Air spiked with an amount of refrigerant R-12 representing a small leak was introduced into the analysis chamber 10. The counting rate dropped due to electron capture by refrigerant molecules, producing a reduced counts-per-second (CPS) signal as shown in Fig. 2. The data presented therein indicate the presence of the refrigerant.

EXAMPLE II In another experiment similar to that described in Example I, air spiked with a small amount of TCE was introduced into the analysis chamber 10. The counting rate dropped due to electron capture by TCE molecules, producing a negative signal, which indicated the presence of the TCE.

EXAMPLE III

Stack gases from a waste incinerator are continuously introduced into the apparatus. A drop in the counting rate indicates the presence of PCB's or other electronegative species escaping from the incineration process.

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Because the fluorine constituent is the most electronegative constituent in refrigerants, it has been demonstrated that the present invention is useful for detecting both chlorine containing chlorofluorocarbon molecules and non-chlorine containing fluorocarbon molecules. The subject apparatus and method are suitable for analyzing samples that contain flammable or explosive constituents because the apparatus and method do not rely on processes which require red-hot elements or sparks which could ignite flammable or explosive gases.

While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the inventions defined by the appended claims.

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