Analysis of materials containing radioactive sources

Abstract

The invention provides a technique for correcting gamma ray intensities detected to account for variation in attenuation effects with energy. The corrected intensity values enable more accurate isotopic analysis to be conducted and render such techniques applicable to low level emission cases. The technique is particularly useful in investigate waste materials with a gamma emitting content which needs to be determined. The attenuation is corrected for using a bi- modal function to account for the attenuation effects arising from low and high atomic mass components of the material in which the emitters are present.

What is claimed is:  
     
         1 . A method for investigating the level of one or more gamma emitting materials in a sample, the method comprising: 
 obtaining an intensity signal at a plurality of gamma ray energies, the plurality of energies including a plurality of energies characteristic of an isotope, the intensity at the plurality of energies for the isotope being corrected for variation in attenuation with energy; and    calculating levels for the isotope and/or one or more other isotopes from the corrected intensities for the plurality of energies for the isotope.    
     
     
         2 . A method according to  claim 1  in which the attenuation correction involves the calculation of a level for an isotope derived from the intensity at one characteristic energy, corrected according to a factor, the calculated level being used together with a calculated level derived from the intensity at another characteristic energy.  
     
     
         3 . A method according to  claim 2  in which the calculated levels for a set of energies obtained using the factor are compared against the calculated levels for a set of energies obtained using the factor, the factor being varied between sets.  
     
     
         4 . A method according to  claim 3  in which the standard deviation of the calculated level for one set is compared against the standard deviation of the calculated level for another set, the comparison being repeated with sets corrected using different factor values, the factor values are adjusted to minimise the difference in calculated levels and/or their standard deviation.  
     
     
         5 . A method according to  claim 4  in which the factor includes two or more variable components, one component in the correction factor relating to the attenuation effect of lower atomic mass elements, those less than 30, and another component relating to the attenuation effect of high atomic mass components, those greater than 30, in the sample.  
     
     
         6 . A method according to  claim 4  in which the factor is defined by:  
         G ( E   gam )= e   (−K1.f1)   *e   (−K2.f2)    
       where K1 and K2 are attenuation correction fit parameters and f1 and f2 are the “low Z” and “high Z” functions of gamma energy.  
     
     
         7 . A method according to  claim 4  in which the attenuation correction is provided together with detector efficiency correction and together with gamma line emission rate correction.  
     
     
         8 . A method according to  claim 1  in which an overall correction is applied according to the equation:  
       
         
           
             
               
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       to give the mass of isotope x in grams; where R y@y  is the count rate of the gamma peak for isotope x at energy y; ξ y@y  is the efficiency of the detector system at energy y; A yx@y  is the specific activity of isotope x at energy y; and e −f1K1 e −f2K2  is a two-material attenuation model.  
     
     
         9 . A method according to  claim 1  in which an attenuation co-efficient for the desired energy spectrum is determined from the attenuation correction and the attenuation co-efficient is used to correct all the gamma intensity values used in the subsequent calculations.  
     
     
         10 . A method according to  claim 9  in which the subsequent calculations calculate the isotopic masses for the materials present in the sample.  
     
     
         11 . A method for investigating the level of one or more gamma emitting materials in a sample, the method comprising: 
 obtaining an intensity signal at a first gamma ray energy, the intensity at the first energy being corrected for variation in attenuation with energy;    obtaining an intensity signal at a second gamma ray energy, the intensity at the second energy being corrected for variation in attenuation with energy;    the first and second energies being energies characteristic of a first isotope; and    calculating levels for the first isotope and/or one or more other isotopes from the corrected intensities for the first energy and second energy.    
     
     
         12 . A method according to  claim 11  in which the method comprises the calculation of a level for an isotope derived from the intensity at one energy, corrected according to a factor, the calculated level being used together with a calculated level derived from the intensity at another energy, corrected according to a factor.  
     
     
         13 . A method according to  claim 12  in which the calculated levels for the different energies are combined in a weighted manner.  
     
     
         14 . A method according to  claim 13  in which combining the levels for the energies gives rise to a standard deviation value.  
     
     
         15 . A method according to  claim 12  in which the calculated levels obtained using the factor for a first set of energies are compared against the calculated levels obtained using the factor for a second set of energies, the factor being varied between sets.  
     
     
         16 . A method according to  claim 15  in which the same energies are considered in each set.  
     
     
         17 . A method according to  claim 15  in which the standard deviation of the calculated level for one set is compared against the standard deviation of the calculated level for another set, the comparison being repeated with sets corrected using different values for the factor, the values for the factor being varied adjusted to minimise the difference in calculated levels and/or their standard deviation.  
     
     
         18 . A method according to  claim 17  in which the factor includes two or more variable components, one component in the correction factor relating to the attenuation effect of lower atomic mass elements, those less than 30, and another component relating to the attenuation effect of high atomic mass components, those greater than 30, in the sample.  
     
     
         19 . A method according to  claim 17  in which the factor is defined by:  
         G ( E   gam )= e (K 1 .f 1 ) *e   (−K2.f2)    
       where K1 and K2 are attenuation correction fit parameters and f1 and f2 are the “low Z” and “high Z” functions of gamma energy.  
     
     
         20 . A method according to  claim 11  in which the attenuation correction is provided together with detector efficiency correction and together with gamma line emission rate correction.  
     
     
         21 . A method according to  claim 11  in which an overall correction is applied according to the equation:  
       
         
           
             
               
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       to give the mass of isotope x in grams; where R y@y  is the count rate of the gamma peak for isotope x at energy y; ξ y@y  is the efficiency of the detector system at energy y; A yx@y  is the specific activity of isotope x at energy y; and e −f1K1 e −f2K2  is a two-material attenuation model.  
     
     
         22 . A method according to  claim 11  in which an attenuation co-efficient for the desired energy spectrum is determined from the attenuation correction and the attenuation co-efficient is used to correct all the gamma intensities used in the subsequent calculations.  
     
     
         23 . A method according to  claim 22  in which the attenuation co-efficient is used to correct the gamma intensities of the isotope and one or more other isotopes.  
     
     
         24 . A method according to  claim 20  in which the subsequent calculations calculate the isotopic masses for one or more of the materials present in the sample.  
     
     
         25 . A method according to  claim 24  in which the calculations of isotopic masses include a calculation of gamma emitting and/or non-gamma emitting isotopes.  
     
     
         26 . A method according to  claim 25  in which the level of non-gamma emitting isotopes is determined through isotopic correlation and/or is derived from a ratio relative to determine levels for one or more gamma emitters.  
     
     
         27 . A method for investigating the level of one or more gamma emitting materials in a sample, the method comprising: 
 obtaining an intensity signal at a first gamma ray energy, the intensity of the first energy being corrected for variation in attenuation with energy, calculating an apparent isotope mass for the first energy;    obtaining an intensity signal at a second gamma ray energy, the intensity of the second energy being corrected for variation in attenuation with energy, calculating an apparent isotope mass for the second energy;    combining the first energy apparent isotopic mass and second energy apparent isotopic mass in a weighted manner to give a first and second energy apparent isotopic mass combination;    obtaining an intensity signal at a further gamma ray energy, the intensity at the further energy being corrected for variation in attenuation with energy, calculating an apparent isotopic mass for the further energy;    obtaining an intensity signal at a still further gamma ray energy, the intensity at the still further energy being corrected for variation in attenuation with energy, calculating an apparent isotopic mass for the still further energy;    combining the further energy apparent isotopic mass and still further energy apparent isotopic mass in a weighted manner to give a further and still further energy apparent isotopic mass combination;    the correction for variation in attenuation with energy involving a factor, the factor having a different value for the first and second energies to the value used for the further and still further energies; and    comparing the standard deviation for the first and second energy apparent isotopic mass combination with the standard deviation of the further and still further energy apparent isotopic mass combination; the combination giving the lowest standard deviation giving the values for the factor used to correct for variation in attenuation with energy to be used in correcting intensities at other gamma ray energies;    the ratio between pairs of corrected energies of an isotope being used to calculate the level of that isotope in the sample.