Radiant energy – Invisible radiant energy responsive electric signalling – With radiant energy source
Reexamination Certificate
2001-10-10
2004-05-25
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With radiant energy source
C250S394000, C382S128000
Reexamination Certificate
active
06740886
ABSTRACT:
This invention is concerned with improvements in and relating to apparatus and methods for materials investigations. The invention is particularly, but not exclusively, concerned with investigating gamma ray emissions from materials. The invention is still more particularly, but not exclusively, concerned with correction techniques in such investigations and the provision of a new correction technique.
In a variety of situations it is necessary to investigate emissions from radioactive sources in or on materials to form a basis for a variety of subsequent decisions, actions or further considerations. The investigations of the samples may relate directly to the emission, for instance the emission source, or indirectly, for instance the consideration of associated non-emitters or emitters which are not directly measurable. The emissions of interest are in particular gamma ray emissions, but other emission forms may be considered additionally or alternatively.
Emission investigation is particularly important in waste evaluation cases. For a given waste sample it is desirable to be able to determine a variety of unknowns. The unknowns may include, but are not limited to, one or more of the level, type, constituents, nature and distribution of the emissions, emission sources, associated materials or associated factors.
When taking measurements of the radioactive waste contained in a sample using prior art techniques it is necessary to make a number of assumptions to gain a solvable system. One such assumption for certain correction techniques is that the materials are homogeneously distributed within a body or container and, in particular that the density profile of the material within the container is even. Given the various materials encountered and the varying size and shape of the materials potentially making up the waste this is not a truly valid assumption. Further variations occur in practice as materials settle or move over time, for instance during transportation.
Relevant correction techniques accept this assumption and are qualified by an error level as a result. For safeguard reasons the level of radioactive sources in a sample should be determined as accurately as possible. In safety cases the error requires that the level be placed at the worse case level. As a consequence the level of radioactive sources is frequently over estimated, with consequential complexity of further handling or storage, as well as cost penalties.
The present invention seeks, amongst other aims, to provide a technique in which greater account of variations or potential variations in density are taken. The improved account may be used in accounting for the effects of the material on the emissions as detected.
According to a first aspect of the invention we provide a method of investigating radioactive sources in a body of material provided at an investigation location, the body of material comprising a plurality of samples, the method comprising detecting a portion of the emissions arising from a sample, the detected portion relating to a detected level, the detected level being corrected according to a correction method to give a corrected level, the method being repeated for one or more of the other samples, the correction method for one or more of the samples comprising providing a generator of radioactive emissions and detecting the radioactive emissions from the generator with the sample at the investigating location, the relationship of the emissions detected with the sample at the investigating location to the emissions which would be detected with the sample absent from the investigating location determining a characteristic of the sample, the determined characteristic being employed as a factor in the correction method used for that sample to obtain the corrected level.
The body of material may be free standing, but is preferably contained in a container. The sample may include the part of the container associated with that part of the body of material.
Preferably the sample is a portion of the body of material which extends from one side of the body through to the other. The sample may be a segment or slice through a body of material, preferably a segment or slice which extends outwards to the limits of the body of material in two dimensions. Preferably the sample is taken horizontally through the body of material. Preferably the sample has substantially the same thickness throughout the body of material. Preferably the thickness of the sample is its depth.
The samples may be investigated in order, for instance from one end of the body of material to the other.
Preferably the generator of emissions is a radioactive source provided externally of the position occupied by the container and/or body of material in use. Preferably the generator is provided in opposition to the detectors therefore.
The relationship of the emissions detected with the sample at the investigating location to the emissions which would be detected with the sample absent the investigating location may be the ratio of the respective count rates for the detectors. Preferably the emissions leaving the generator count rate is determined in the absence of the sample. The absence may be an absence of any body of material in the investigating location. The relationship may be based on the ratio R/R
o
, where R is the rate at which the emissions are detected with the sample in place and R
o
is the rate of emissions which would be detected without the sample in place. Preferably the relationship is based on, and ideally equates to −ln(R/
o
).
Preferably the characteristic determined is a function of the density of the sample and preferably the density of the sample. The characteristic may be a function of the effective amount of material in the sample. The characteristic may relate to the effective amount of material determined to be in a sample relative to one or more other samples.
The determination of the characteristic for one or more or all of the samples may be based on the interrelationship of a plurality of potential variables. Preferably the variables include one or more of, and most preferably all of:
I) the relationship of the emissions which would be detected without the sample present to the emissions detected with the sample in place, more preferably the relationship of R and R
o
, where R is the rate at which the emissions are detected with the sample in place and R
o
is the rate of emissions which would be detected without the sample in place;
ii) the attenuation effects of the sample, more preferably &mgr;, where &mgr; is the mass attenuation coefficient;
iii) the path of the emissions through the sample, more particularly, x, where x is the thickness of the sample (between source and detector);
iv) the density of the sample, &rgr;.
Preferably the determination of the characteristic is based on the equation:
R=R
o
exp(−&mgr;&rgr;
x
)
where the symbols have the meanings referred to above.
Preferably under the conditions of the investigation x and/or &mgr;, and most preferably both are substantially constant. Preferably x is kept constant by fixed relative generator, detector and sample positions (the sample may rotate without affecting this). Preferably &mgr; is substantially constant due to the energy of the generator emissions. An energy of greater than 400 keV, preferably greater than 1000 keV and ideally greater than 1300 keV may be used. Preferably the source is as detailed in the British Nuclear Fuels PLC UK Patent Application no. 9900449.1 filed 11 Jan. 1999 and referenced P17454, the contents of which are incorporated by reference. In particular we may provide a method of investigating radioactive sources in a sample, the method comprising detecting a portion of the emissions arising from the sample, and further comprising the provision of a radioactive generator, passing at least a portion of the emissions of the generator into the sample, detecting at least a portion of the emissions from the generator leaving the sample, the radioactive generator emissions being of at least
Beyer Weaver & Thomas LLP
British Nuclear Fuels PLC
Gabor Otilia
Hannaher Constantine
LandOfFree
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