Method of surface surveying for radioactive sources
First Claim
1. A method of surveying a surface to detect a source of radioactivity, said survey being performed with a detector having predetermined characteristics, comprising:
- determining a background count rate for said detector;
determining an expected source count rate from passing said detector over said source;
selecting a first detector scan velocity, and determining a first distribution of counts probability for said detector as a function of said background count rate at said first velocity, and a second distribution of counts probability for said detector as a function of the combination of expected source and background count rates at said first velocity;
obtaining from said first probability distribution a background decision probability distribution as a function of counts, and obtaining from said second probability distribution a combination decision probability distribution as a function of counts;
presenting said decision probability distributions, and determining from them whether the source activity can be identified with a predetermined degree of confidence;
iteratively selecting one or more detector scan velocities, and for each selected velocity repeating the above steps of determining distributions of counts probability and decision probability distributions and determining therefrom the degree of confidence of identifying source activity corresponding to each selected velocity,selecting, based on said determinations of degree of confidence, an optimum scan velocity at which source activity can be found with about said predetermined degree of confidence; and
surveying said surface at about said optimum detector velocity.
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Accused Products
Abstract
A method of surveying a surface to determine the existence of a radiation source, e.g., a radioactive source, providing a procedure for determining an optimum scan velocity for scanning past a source so as to provide the required degree of confidence to detect the presence of radiation sources while avoiding false source detection. A first scan velocity is selected based upon detector information and the nature of the survey surface, following which spreadsheet-type calculations are made based upon the expected maximum source count rate, background rate and the selected velocity. The spreadsheet calculations provide an indication of the confidence of distinguishing source activity from background radiation at the selected scan velocity. The indicated confidence is compared to the required confidence level, and when the indicated confidence is not optimally close to the required confidence, another scan velocity is selected and the spreadsheet calculations are repeated. These steps are repeated iteratively until an indicated confidence is found which is optimally close to the predetermined confidence, following which the last selected scanned velocity is used in actually carrying out the surface survey.
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Citations
34 Claims
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1. A method of surveying a surface to detect a source of radioactivity, said survey being performed with a detector having predetermined characteristics, comprising:
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determining a background count rate for said detector; determining an expected source count rate from passing said detector over said source; selecting a first detector scan velocity, and determining a first distribution of counts probability for said detector as a function of said background count rate at said first velocity, and a second distribution of counts probability for said detector as a function of the combination of expected source and background count rates at said first velocity; obtaining from said first probability distribution a background decision probability distribution as a function of counts, and obtaining from said second probability distribution a combination decision probability distribution as a function of counts; presenting said decision probability distributions, and determining from them whether the source activity can be identified with a predetermined degree of confidence; iteratively selecting one or more detector scan velocities, and for each selected velocity repeating the above steps of determining distributions of counts probability and decision probability distributions and determining therefrom the degree of confidence of identifying source activity corresponding to each selected velocity, selecting, based on said determinations of degree of confidence, an optimum scan velocity at which source activity can be found with about said predetermined degree of confidence; and surveying said surface at about said optimum detector velocity. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A method of determining a surveying velocity for scanning a detector past a source of radiation, comprising:
- repetitively selecting respective scan velocities;
for each selected scan velocity, performing spreadsheet calculations of (a) the probability PB (i) of a detector count of i or greater being a true background count and (b) the probability PS+B (i) of a detector count of i or less being a true source+background count;
determining from said probabilities a decision probability confidence that a selected velocity the source+background count can be distinguished from the background count; and
selecting, on the basis of said determined confidences, a scan velocity that yields a confidence of a least a predetermined degree and is as low as is consistent with assuring said predetermined degree of confidence.
- repetitively selecting respective scan velocities;
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11. A method of surveying a surface which comprises scanning said surface with a detector at a selected velocity to determine the existence of a source of radiation from within said surface, said source having an expected maximum source count rate and said detector having a predetermined background count rate, said method including determining an optimum velocity for scanning said surface with said detector past said source to provide a predetermined confidence of distinguishing said source from said background radiation, said velocity determining step further comprising:
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selecting a first scan velocity;
making spreadsheet type probability calculations based upon said expected source rate, said background rate and said selected velocity;providing from said calculations an indication of the confidence of distinguishing source from background radiation at said selected velocity;
comparing the indicated confidence to a predetermined confidence; and
when said indicated confidence is not optimally close to said predetermined confidence, selecting another scan velocity and repeating said making, providing and comparing steps iteratively until a said indicated confidence is optimally close to said predetermined confidence, and using the scan velocity corresponding to said indicated optimally close confidence in said surveying. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19, 20)
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21. A method of surveying a site for a radiation source, comprising scanning said site with a detector at a scan velocity, said method comprising:
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selecting a first scan velocity, and for said first velocity making a set of calculations which indicate the probability that a detected radiation count represents source radiation and not just background radiation; selecting at least one more scan velocity and making said set of calculations for each selected scan velocity; choosing a scan velocity of about a selected scan velocity for which a said set of calculations indicates an acceptable said probability; and scanning said site with said detector at said chosen velocity. - View Dependent Claims (22, 23, 24, 25, 26)
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27. A computer system for determining whether scanning a predetermined surface with a given radiation detector at a selected detector scan velocity provides a predetermined degree of confidence of distinguishing a source plus background count from a background count, comprising:
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first means for determining for a probability PB (i) of a detector count of i or greater being a true background count for said selected detector scan velocity; second means for determining a probability PS+B (i) of a detector count of i or less being a true source background count for said selected detector scan velocity; third means for determining a degree of confidence that for said selected detector scan velocity a source plus background count can be distinguished from a background count; and fourth means for determining whether said determined degree of confidence provides said predetermined degree of confidence. - View Dependent Claims (28, 29, 30, 31, 32, 33, 34)
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Specification