Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
2000-03-01
2002-02-12
Sugarman, Scott J. (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S375000
Reexamination Certificate
active
06346709
ABSTRACT:
FIELD OF INTEREST
The invention relates to radiation sensors and, more particularly, to the measurement of radioactive contaminants in ambient air, soil or other types of media.
BACKGROUND OF THE INVENTION
It would often be desirable to quickly, accurately and inexpensively monitor the concentration of radionuclides found in the atmosphere, soil, or other places. Usually such contaminants consist of naturally occurring radionuclides in the atmosphere, namely, progenies of Radon-222 and Radon-220 (Thoron). In places where radioactive materials are stored, processed, or used, other radioactive contaminants may be present. The need for quick, accurate and inexpensive monitors for measuring radionuclides found in the air, soil, or attached to structures is important for such locations. In addition, homeowners and real estate purchasers are concerned about radon concentrations in the home. The National Research Council recently identified Radon in the home as an important public health problem, causing as many as 21,800 deaths annually (New York Times Feb. 20, 1998, page A 13).
The current commercial Radon measurement technique is to collect Radon in a charcoal filter that is sent to a laboratory where the gamma ray emission is counted and correlated with the equivalent number of picoCuries per liter (pCi l
−1
) of alpha particles in ambient air. Such commercial procedures require placing a charcoal canister within a home or commercial building for a week, then sending the sample to a laboratory for testing and waiting for the test results. One significant disadvantage with this approach is that such measurements are not made in real time. There is a critical and long-felt need for an improved Radon progenies measurement technique.
However, Radon and Thoron cannot be directly correlated with potential damage to lung tissue. Radon itself, a noble gas, is in a neutral electronic state, not attached to aerosols, and therefore, being chemically inert it is inhaled and exhaled without consequence to the lungs. The progeny
214
Po is also excluded because of its very short half-life of 10
−6
min.
EPA regulations are based on radon concentrations. Radon concentrations are used this way because this is a quantity that can be readily measured. However, radon concentration is proportional to potentially harmful alpha emitters only in cases where secular equilibrium of radon with its daughter products exists, or is at least a fixed fraction. In practice, this is never the case, because this equilibrium varies between 10% and 90% depending on location and time.
The total energy of alpha particles emitted by inhaled air is the quantity, which is most strongly connected to the potential health hazard to lung tissue. Currently, the two most frequently used measurement units for radon inhalation standards are the Working Level (WL) and the pCi
−1
of radon. One WL represents the total energy emitted by alpha particles from radon daughters (
218
Po,
214
Pb and
214
Bi), and presumably absorbed by lung tissue as a consequence of one person breathing ambient air containing 100 pCi l
−1
of radon. Under these conditions, the total energy, including all the energy emitted by alpha particles trapped in the lungs after enough half-lives have passed to bring their activity to virtually zero, was calculated as 1.275×10
5
Mev per liter of air. See J.N. Standard Radioactivity and Health, edited by R. W. Baalman, Jr., published by the Office of Scientific and technical Information, Springfield, Va., October 1988. This definition of WL does not include Radon-222 and Po-214.
The present invention can measure either the concentration of harmful alpha emitters or the total energy delivered by them. This last parameter can be measured by making the height of the chamber depicted in
FIG. 2
equal to the range in air of the biggest alpha energy in question, which is 5.5 mm, rather than 27.7 mm.
SUMMARY OF THE INVENTION
The present inventors have answered the long-felt need for an improved low cost and simple measurement technique of radioactive contaminants in the air, soil, or in buildings without suffering from the disadvantages and limitations of current commercial practices and instrumentation. The radiation monitor of the present invention comprises a series of three radiation windows on top of a pancake-shaped conductive plastic chamber, with two of the windows being removable, and a microscope and a carbon fiber electrometer protruding through the chamber's side wall, as depicted in FIGS
1
and
2
. The radiation windows further comprise a thin aluminized polyester electrically conductive polyethylene terephthalate film (mylar) window to admit alpha particles, which can be combined with a second thicker mylar window to eliminate alpha particles, as well as a thicker plastic window which eliminates alpha and beta particles but admits gamma rays, as depicted in FIGS
3
-
5
. These three windows, or means for covering, can be advantageously combined or used alone for measuring the different forms of contamination. Thus one can observe preferentially the contributions to the measurement of alpha, beta, and gamma radiation, beta and gamma radiation, or only gamma radiation. The present invention's configuration allows the concentration of each type of radiation to be independently determined. The covering means could also include other windows for admitting or eliminating different forms of radioactive contamination.
Using an adequate air filter paper and an air sampler, one can collect air borne contaminants and provide a sample for the measurement. All (100%) radioactive aerosol particles and positively charged radioactive ions become absorbed in the filter paper. This filter paper is then placed on top of the window of the pancake-shaped monitor, which has been charged to read zero, and the discharge time, expressed in scale units per minute of discharge time, is then observed. This discharge time is inversely proportional to the amount of radioactive material captured on the filter. In our measurements, filter paper in the shape of a disk with a diameter of 4.7 cm was used. The chamber walls of the initial prototype monitor were constructed with Vectra™, which is a tissue equivalent plastic. The filter paper used was Millipore pore size 0.8 &mgr;m, which is available in their catalog.
To better understand and appreciate the operation of the present invention, some details of the physics involved in measuring the alpha emissions from progeny of Radon-222 are described. The source of the Radon is Uranium, naturally occurring in the earth's crust. The decay portion of the Uranium-Radium series, comprising Radon-222 and leading to Lead-210 is as follows:
An analogous decay series describes the decay of Radon-220 (Thoron) into its progeny.
The decay process creates positive ions that attach to aerosol particles and these particles can be collected on filter paper by passing air through the filter for a measured period of time.
FIG. 6
depicts the theoretical alpha particle decay curve of the Radon progeny, RaA+RaC′, which are Polonium 218 and 214, respectively, as a function of time after the collection stops and the monitor begins discharging as the alpha particles from the filter paper source enter the monitor's chamber.
The results of the measurements are shown in FIG.
7
. The curves in the figure are the voltage discharge curves of the monitor, that is, scale readings versus time for several samples collected in different locations in central New Jersey, including a private home, the inventors' laboratory and outdoors. The monitor was calibrated in units of alpha particle disintegrations per liter of air. A calibration factor of 0.458 dpm l
−1
of air per divisions/min. was obtained. Noting that the Environmental Protection Agency (EPA) limit is 8.8 dpm l
−1
of air, the
FIG. 7
alpha particle concentrations varied from a minimum level of 0.77 in an outside location to a maximum level of 24.4 dpm l
−1
in a private hom
Brucker George J.
Horne Steven A.
Kronenberg Stanley
Hasan Mohammed
Sugarman Scott J.
Tereschuk George B.
The United States of America as represented by the Secretary of
Zelenka Michael
LandOfFree
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