Radiant energy – Invisible radiant energy responsive electric signalling – With radiant energy source
Reexamination Certificate
2000-04-14
2003-03-04
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With radiant energy source
C250S363040
Reexamination Certificate
active
06528797
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and systems for determining depth distribution of radiation-emitting material located in a source medium and radiation detector system for use therein.
BACKGROUND ART
In principle, in situ gamma-ray spectrometry determines the quantities of radionuclides in some source medium with a portable detector. In comparison, the more established method of laboratory gamma-ray spectroscopy consists of taking small samples of the medium into the laboratory for gamma-ray analysis. In situ gamma-ray spectrometry characterizes a larger volume of material, requires less time to determine accurate radionuclide concentrations, and minimizes worker doses and the risk of radioactive contamination. The main limitation of in situ gamma-ray spectrometry lies in determining the depth distribution of radionuclides.
In general, radionuclide depth distributions aid conventional in situ gamma-ray spectrometry in determining accurate radionuclide inventories and surface does rates from individual radionuclides. Depth distributions also represent reliable data for radionuclide transport studies. Indications of neutron or energetic charged particle fluxes can result from determinations of the activation as a function of material depth. For decontamination and decommissioning activities, the radionuclide depth distribution determines the amount of material that must be remediated to satisfy the release limits.
To date, three in situ gamma-ray spectroscopic methods have been used to determine the depth distribution of radionuclides in soil and are presented hereinbelow. These three in situ methods are based on multiple photopeak responses, the photopeak-to-valley ratio, and the attenuation of a lead plate as illustrated in
FIGS. 1
a
and
1
b
. Each method requires a priori assumptions of the depth distribution function and uses a gamma-ray spectrometer. Spectrometers allow the users to decipher the energies of gamma-ray emissions, a necessity for determining the specific radioisotope present. In addition to usually assuming a uniform soil density with depth, all three approaches for determining depth distributions also assume a spatially uniform radionuclide distribution. All three in situ methods require a priori assumptions of the functional form for the depth distribution. The multiple photopeak and peak-to-valley methods only have the ability of determining a single depth parameter. An exception exists if the radionuclide of interest emits three or more significant gamma-rays, decently separated in energy, and if the spectrometer used has sufficient energy resolution to identify and separate each gamma-ray emission. In such cases, the multiple photopeak method could determine one fewer number of depth parameters than the number of significant gamma-rays emissions. The subsurface maxima exhibited by aged
137
Cs fallout in soil are best described by at least two depth parameters and can not be adequately characterized by a single depth parameter. Table 1 summarizes the advantages and disadvantages of the three in situ methods.
TABLE 1
GENERAL ADVANTAGES AND DISADVANTAGES OF THE THREE STANDARD
IN SITU METHODS FOR DETERMINING RADIONUCLIDE DEPTH DISTRIBUTIONS
Method
Advantages
Disadvantages
Multiple Photopeak
Requires a single measurement at each site
Requires at least two significant gamma-ray emissions
Gamma-ray emissions must have a large separation in
energy
Depth information limited by the gamma-ray decay
scheme of the radionuclide of interest
Multiple measurements at the same site yield no
additional depth information
Peak-to-Valley Ratio
Requires only one significant gamma-ray emission
Sensitive to interference in complex gamma-ray fields
Requires a single measurement at each site
Multiple measurements at the same site yield no
additional depth information
Lead Plate
Requires only one significant gamma-ray emission
Requires multiple measurements at each site
Multiple measurements at the same site yield additional
Adds weight to the portable system
depth information
In addition to the three in situ methods for determining depth distributions, spectroscopic measurements in boreholes have also been studied for applications in oil wells. Because boring itself qualifies as an invasive process, borehole measurements should be considered a quasi-in-situ approach. In addition to increased contamination risks, borehole measurements require boring equipment and custom fabricated detection equipment (extended cryostat lengths for HPGe detectors).
Three other imaging techniques include: pinhole collimation, coded aperture imaging, and Compton scatter imaging. The main limitation, common to all three of these imaging techniques, is the energy resolution of the detectors used. These other imaging techniques utilize position-sensitive detector arrays, which typically are large scintillation crystals with insufficient energy resolution for complex gamma-ray fields. For characterizing low levels of radioactivity, advancements in position-sensitive semiconductor detectors have not yet yielded devices that are large enough for adequate sensitivities or affordable enough for a rugged and portable in situ system.
U.S. Pat. No. 4,197,460 to Anger discloses a collimator assembly used to perform multi-angle nuclear imaging and the results are used to estimate relative depth of objects. Multi-angle display circuits divide the probe radiation image into different regions.
U.S. Pat. No. 3,979,594 to Anger discloses how relative positions of radiation sources at different depths are estimated via a focused collimator. Multiple-channel collimators are mentioned as an option to be used.
U.S. Pat. No. 5,429,135 to Hawman et al. discloses how a focusing collimator detects the depth of an organ in nuclear medicine.
U.S. Pat. No. 5,442,180 to Perkins et al. discloses an apparatus for determining the concentration of radioactive constituents in test samples (such as surface soil) by means of a real-time direct readout.
Other U.S. patents of a more general interest include: U.S. Pat. Nos. 4,394,576; 5,773,829; and 5,870,191.
The primary measurement problem which is not solved by the prior art is the in situ determination of the depth distribution of gamma-ray emitting radionuclides in source media. Contaminated soil and activated concrete are common examples of anthropogenic radionuclides in large area geometries. For these measurement situations, the gamma-ray spectrum tends to be complex due to the presence of multiple-radionuclides (natural or anthropogenic in origin). Therefore, the spectrometers used in the field must possess excellent energy resolution to minimize the deleterious effects of interfering gamma-ray emissions. Other practical issues are that an in situ detection system should be portable and rugged. Because it is not uncommon for low levels of anthropogenic radionuclides to be present in smaller quantities than natural radionuclides, it is important that the detection system also possess a sufficient gamma-ray detection efficiency for reasonable counting times.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein the invention can be used with respect to any source medium as long as its attenuation properties are insignificant, known, measurable, or estimable in any way.
Another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein in situ radiation measurements are performed such as gamma-ray spectrometry and offer a superior ability for characterizing complex depth distributions.
Still another object of the present invention is to provide a method and system for determining depth distribution of radiation-emitting material located in a source medium and a radiation detector system for use therein wherein the radiation-em
Benke Roland R.
Kearfott Kimberlee J.
McGregor Douglas S.
Gabor Otilia
Hannaher Constantine
The Regents of the University of Michigan
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