GB2398381A

GB2398381A - Attenuation compensation of radioactive emission measurements

Info

Publication number: GB2398381A
Application number: GB0329751A
Authority: GB
Inventors: Karl Anthony Hughes
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 2002-12-24
Filing date: 2003-12-23
Publication date: 2004-08-18
Anticipated expiration: 2023-12-23
Also published as: GB0329751D0; GB2398381B; GB0230110D0

Abstract

A system to measure the emission of radioactive sources in a body of material comprises a method of correcting the detected emission level for attenuation in the body. The attenuation correction method uses transmission of radiation through a sample or slice of the body to determine the density of material in the sample The attenuation correction method further provides a weight-based correction for samples within the body for which the transmission-based correction is not valid. The weight-based correction method distributes the mass of material not accounted for by the transmission-based method amongst the uncorrected samples in order to calculate a density of material in those samples. The system is used to accurately measure gamma-ray emissions from materials such as radioactive waste where the density of material varies within the container.

Description

239838 1

IMPROVEMENTS IN AND RELATING TO APPARATUS

AND METHODS FOR MATERIALS INVESTIGATION

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 an improved 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.

As well as simply using the results obtained from the detectors, emission count rates for the radioactive materials in the sample under consideration, it is known to investigate to an extent the effects of the all the material in a sample, through their effect on the transmission of external emissions from an applied source through the sample to the detectors.

Unfortunately this type of transmission based technique does not achieve sufficient precision to be used in all cases, and where problems occur that body of material, and hence all the samples forming it, are considered using an alternative non-transmission based correction technique which is more robust but less accurate.

The present invention aims to provide the most accurate correction possible, based on density, for each sample of the body of material.

According to a first aspect of the invention we provide a method of investigating radioactive sources in a body of material including two or more 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 correction method including: the provision of an emission generator, passing at least a portion of the emissions of the generator into the sample, detecting emissions from the generator with the sample in place, and determining a factor relating to the amount of material in the given sample based on the relationship of the emissions detected with the sample in place relative to the emissions which would be detected without the sample in place, the determined factor being used in the correction of that sample when the detection meets the applied criteria for that sample, the correction method further providing that for samples whose detection does not meet the applied criteria, an alternative approach to determining a factor relating to the amount of material in the given sample is used, based on deducting the known amounts of material allocated to samples which do meet the detection criteria from the total amount of material in the body of material and the division of the remaining material amongst the samples for which the detection criteria were not met, the determined factor for a sample being used to give the corrected level from the detected level for that sample.

Preferably the relationship between the emissions detected with and without the sample in place employs measured transmission coefficients, and ideally comprises measured transmission coefficients. The measured transmission coefficients, most preferably all, may be provided according to the equation:

R

Trans. Coeff. = ---- = exp(-ppx) Ro where R is the rate of detected photons with the sample in place, Ro is the rate of photons which would be detected without the sample in place, is the mass absorption coefficient, is the matrix density and x is the matrix thickness. In the method preferably Ro and x are known, R is measured and an assumed value for is taken. Preferably p is determined by the calculation and is the factor.

The deduction of the known material from the total may be made by deducting the factor for that sample from the total equivalent factor for the body of material, but is preferably the deduction of the mass allocated to the sample from the total mass measured by weighing.

The division of the previously unallocated mass amongst the samples not meeting the criteria may be made equally.

Alternatively an unequal division/allocation may be made.

The allocation may be weighted according to the distribution of mass in the samples for which the criteria are met.

The criteria may be a level of statistical precision.

The criteria may be a given number of counts in a time period.

The factor and/or amount of material in a sample and/or density for a sample may be used in correcting the detected level to the corrected level for that sample.

Preferably the generator is provided on the opposing side of the sample to the detectors, most preferably in direct opposition.

One or more of the detectors for the sources may be used for detecting the generator emissions and/or vice-versa.

Preferably under the conditions of the investigation x and/or A, 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 is substantially constant due to the energy of the generator emissions. An energy of greater than 400keV, preferably greater than 1000 keV and ideally greater than 1300 keV may be used. Preferably the source is as detailed in WO00/42447 of British Nuclear Fuels Plc, the contents of which are incorporated herein by reference.

According to a second aspect of the invention we provide apparatus for investigating radioactive sources in a body of material, the body of material comprising a plurality of samples, the apparatus comprising: one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected; an investigating location into which the sample is introduced; signal processing means for relating the detector signals to a detected level for the sources; processing means providing a correction method for correcting the detected level for the sources to give a corrected level; the apparatus further comprising: a generator of radioactive emissions, at least of portion of the emissions entering the investigating location and, in use the sample; one or more detectors for detecting the generator emissions with the sample at the investigating location; processing means for determining a factor relating to the amount of material in the given sample based on the relationship of the emissions detected with the sample in place relative to the emissions which would be detected without the sample in place, the determined factor being used for the correction of that sample when the detection meets the applied criteria for that sample, the apparatus providing further processing means for correcting those samples whose detection does not meet the applied criteria, the alternative approach to determining a factor relating to the amount of material in a given sample being based on the deduction of the known amounts of material allocated to samples which do meet the detection criteria from the total amount of material in the body of material and the division of the remaining material amongst the samples for which the detection criteria were not met, the determined factor for each sample being used to give the corrected level from the detected level for that sample.

The processing means for one or more of the stages may be the same or different.

The first and/or second aspects of the invention may further include any of the features, options, possibilities and steps set out below.

The sources may be singular or plural in disposition and/or type. The sources may be one or more isotopes of one or more elements. The sources may be alpha and/or beta and/or gamma emitters, but are preferably gamma emitters at least.

One or more sources of the same type and/or of different types may be present in the sample. The sources may be homogeneously distributed, or more usually, unevenly distributed. The size and/or shape and/or mass of a source may be different from the size and/or shape and/or mass of another source in the sample, be they of the same or different types.

The sources may be investigated by detecting one or more of their emitting energies. Thus a characteristic energy of an isotope may be detected.

The sources may be investigated directly, for instance they contribute directly to the detected level, and/or the sources may be investigated indirectly, for instance they do not contribute directly to the detected level but are associated with sources which do.

The samples may be gaseous and/or liquid and/or solid.

The samples may contain one or more non-emitting or non- source materials. The materials may include one or more of metals, such as iron, steel, aluminum; wood; glass; plastics, such as Polythene, PVC; liquids, such as water.

The sample may be the whole or part of a body of material. The body of material may be free standing, but is preferably contained in a container. The sample may be a part of a body of material, including the part of the container associated with that part of the body of material.

The sample may be a segment or slice through a body of material. Preferably the segment is taken horizontally through the body of material. Preferably the segment has the same thickness throughout the body of material.

Other segments of the body of material may be investigated in subsequent repeats of the method.

The container preferably entirely encloses the body of material. The container may be of metal or of concrete or a combination of such materials. Drums are a particularly preferred container, such as right cylindrical drums.

The containers may be of one or more standard sizes.

The height and/or diameter of the containers may be standard.

Preferably the containers introduced to the investigating location one at a time. The containers may be introduced by conveying along a surface, preferably a horizontal surface. The surface may include or be formed of a plurality of rollers. Preferably the container is removed from the investigating location in a manner equivalent to its

introduction.

The investigating location is preferably provided in proximity to the emission detector or detectors. The investigating location may be provided in proximity to one or more radioactive sources. The sources are preferably intended to transmit radiation through the sample. Ideally the investigating location is provided between the detector(s) and the transmission source(s).

The sample, preferably the container for it, may be rotated at the investigating location. Preferably the rotation presents different portions of the sample in proximity to the detector(s) and/or transmission source(s).

the rotation may be continuous or stepped. The rotation may be provided at between 5 and 25 rpm.

Preferably the sample and/or body of material and/or container are weighed at the investigating location, for instance by the turntable used to rotate it.

The sample, preferably the container for it, may be raised and/or lowered at the investigating location.

Preferably the rasing and/or lowering presents different portions of the sample to the detector(s) and/or transmission source(s). the raising and/or lower may be continuous or stepped. Preferably investigations are performed as the sample is lower and raised.

The sample may be rotated and/or lower and/or raised.

A single detector may be used. Preferably a plurality of detectors, for instance three, may be used.

The detectors may be of the high purity germanium type.

Preferably the detectors are collimated to restrict their field of view to the body of material of which the sample is the whole or a portion thereof. Where the sample is less than the whole of the body of material, preferably the detectors are collimated to restrict their field of view to the sample only. The sample is preferably a slice or segment of the whole. The segments may be of the same thickness.

Preferably the detected level is obtained from a passive counting stage. Preferably the transmission based investigations are performed before and/or after the passive count stage.

Preferably the transmission source(s) is provided in opposition to the detector(s). Preferably the same number of transmission sources are provided as there are detectors. It is particularly preferred that the transmission source be provided according to the nature of the transmission source detailed in British Nuclear Fuels Plc's patent application One or more surface dosimeters may be provided.

Preferably the surface dosimeters are configured to investigate gamma emitting sources. Alpha and/or beta emissions may alternatively or additionally considered.

Preferably the correction method is used in correcting the detected level to the corrected level, for instance by established subsequent techniques, such as those set out in the Los Alamos primer, 2nd Edition, March 1991, ISBN0-16- 032724-5. - 9 -

Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawing in which: Figure 1 illustrates schematically an instrument suitable for implementing the present invention.

The instrument illustrated in Figure 1 is suitable for investigating gamma emission sources in a variety of situations and materials. The system 1 is particularly designed to investigate waste samples presented in drums 3 to investigating location 5.

The drums considered may be in a variety of sizes, but the instrument is readily adapted to consider 100, 200 and 500 litre drums. Masses of waste, such as 40kg to 550kg, can readily be accommodated.

The drums 3 may be provided with a barcode which can be examined by a barcode reader on the system 1 so as to permanently assign results obtained to that drum 3.

The system 1 features a conveying table 7 formed of a large number of parallel rollers 9, onto which the drums are lowered. Cranes, forklift trucks and other lifting and manoeuvring means can be used to this end.

The conveying table 7 leads to the investigating location 5 and then onward out the other side to a dismounting location 11 from which the drums 3 are lifted.

This set up allows the flow of samples for investigation through the system.

The drum 3 is supported at the investigating location 5 by a turntable 13 which again is formed by a number of parallel rollers 15 mounted on a moveable frame 17. The frame 17 can be rotated about a vertical axis, using a motor (not shown), so as to rotate the drum 3 about its longitudinal axis at the investigating location 5. A rotational rate of 12 rpm is preferred, but stepped rotation can be used. The frame 17 can also be provided with the option of being raised and lowered relative to the level of the surrounding conveying table 7, using electrical drives, hydraulics or other systems (not shown), so as to adjust the vertical positioning of the drum 3 within the investigating location 5. The lowering and rotating motions may be applied simultaneously.

The turntable 13 and frame 17 are configured to weigh the drum 3 whilst it is on the turntable 13.

On either side of the investigating location 5 are investigating assemblies 19, 21. In general one of these assemblies 19 acts as the, optional, transmission side and the other 21 as the detecting/receiving side for the investigations.

The detecting/receiving side 21 provides one or more detectors 23 for the emission type under consideration. The detectors 23 are collimated using shields 25 to give a restricted field of view into the investigating location 5 for each detector 23. Through the use of variable aperture collimators (not shown) the range of radioactivity level for the waste which can successfully be handled is increased.

The field of view is generally configured to be a slice through that investigating location 5, the slice being substantially parallel to the turntable 13 and/or perpendicular to the axis of the drum 3 under consideration.

For gamma rays the detectors may be of the germanium type, for instance high purity germanium type. LRGS or HRGS detectors may be used, with electrical or LN2 cooling for HRGS detectors.

The provision of more than one detector, collimated to different fields of view, allows a greater number of measurements to be taken simultaneously, hence increasing the throughput for the system.

The detectors 23 monitor gamma rays originating in their field of view in the drum 3 and in effect generate count rates. A count time of less than 30 minutes is generally employed.

The signals obtained from the detectors are fed to processing electronics 27 and hence to a CPU 29 and operator display 31. Operator control and inputs are facilitated through keyboard 33.

The processing electronics 27 are provided with error handling functions and diagnostic facilities, as well as providing the appropriate calibration functions.

Processing of the signals gives detailed information on the assay of waste material in the drum 3, the isotopic make- up of the waste. More details of these analyses are discussed below. The results can be used to classify waste according to the relevant disposal categories, including those below a deminimus level which can be characterized as non-radioactive. The results can be expressed the identification of fission products, activation products or MGA code. The results can also be combined with the "fingerprinting" technique to give non-measurable isotope determinations.

The results obtained can be improved using a variety of potential correction techniques. Correction based on weight and/or differential peak adsorption and/or use of a transmission source may be used. In this invention, the new correction techniques detailed in the same applicant's International patent applications W000/42447 and / or W000/42446 may also be used and the details of those techniques are fully incorporated herein by reference.

Through the use of a transmission source or sources 35 on transmission side 19 in combination with stepped rotation of the drum 3 tomographic style investigations of the drum can be made to give plots of density distribution and radioactive distribution for the drum 3.

Surface originating alpha and/or beta emissions, most preferably gamma emissions, for the drum 3 can also be measured using optional dosimeters 37. The dosimeters are normally provided on the transmission side 19 in proximity with the surface of the drum 3.

As discussed above three different correction techniques may be deployed. The principles and operation behind each of these is now discussed.

Weight based correction seeks to account for the attenuating effects of the body of material the sources are in by a factor based on the body of materials density. The total mass of the entire body of material is divided by the total volume of the body of material, more commonly the total volume of the container for the body, to give an overall density value for the entire body/container. This single value is then used in the correction of the detected count to account for the reduction arising due to attenuation.

Differential peak adsorption based correction again seeks to account for the attenuation effects of the material, but through a more direct investigation of attenuation. The gamma emissions from a source anticipated to be in the body of material at two characteristic energies are considered.

The ratio of the emissions at one energy to the emissions at the other energy is known (1:1, for instance, for Co-60) without attenuating materials, and this base ratio is compared with the actual ratio measured with the attenuating effects of the body of material present to give a factor relating to the attenuating effect and hence allowing correction.

Transmission source based correction comes in a variety of forms, but each is generally based on determining attenuation effects on emissions from within the body of material by measuring the attenuation on externally sourced gamma emissions. Emissions at a known energy from the transmission source are detected after their passage through the body. The ratio of the detected emissions with the body of material present is compared with the detected emissions which would occur without the material. The attenuation is corrected for based on this difference. The energy of the transmission source is selected to be close to the energy of the emissions from the sample which need correcting.

Unfortunately in certain situations transmission based correction does not provide an investigation of a sample which is sufficiently precise to be used. This is also potentially true for some of the samples considered under the technique set out in W000/42446. However, rather than revert to an alternative more robust but less accurate correction method for the whole of the body of material the present invention provides an intermediate correction method.

The technique of the present invention involves considering a body of material in terms of two or more samples. Correction specific to each of those samples is then made. This uses a transmission based correction for each of the samples where that is valid, with a modified weight based correction being performed on the samples forming the body of material for which transmission based correction is not valid.

For samples where the transmission based correction is applicable the actual correction is obtained from the measured transmission coefficients, which at the respective energies are:

R

Trans. Coeff. = ---- = exp(-ppx) Ro where R is the rate of detected photons, Ro is the rate of emitted photons from the source, is the mass absorption coefficient, p is the matrix density, x is the matrix thickness.

As Ro is known, x is known, R can be measured and can be given an assumed, but accurate, value, the equation can be solved to give a good density determination and hence correction for that sample alone. In combination with the known volume of that sample, this figure can be used to calculate a mass of material in that sample.

By repeating this process for all the samples for which the technique provides sufficient precision, a number of the samples can generally be determined and hence a proportion of the total mass of the body of material can be allocated to samples.

For the remaining samples a density value can be provided according to the principle used in the basic weight correction method. In effect the unallocated mass is divided equally between the equal volume samples and a density determination for the samples is made from their allocated proportion of the total.

This combination allows the better correction technique to be used for all samples within a body of material for which it works and also provides an appropriate form of correction for the other samples too. The overall effect is a correction method which is more accurate than reverting to an alternative correction method. The correction method is particularly suited to handling cases where part of the body of material is very dense (for which transmission based investigation might not work) and/or for part filled containers (for which the weight based correction is inappropriate due to the variation in density occurring in the body).

The correction method is exemplified as follows. The transmission based investigations give results for the ten samples of the body of material which are not statistically valid in three cases, but in the other seven proportions of the material are determined.

SEGMENT ONE TRANSMISSION OK 3% OF MASS SEGMENT TWO " " 3% SEGMENT THREE " 5% SEGMENT FOUR " 6% SEGMENT FIVE " 7% SEGMENT SIX " 9%

SEGMENT SEVEN NO MEASUREMENT UNKNOWN

SEGMENT EIGHT TRANSMISSION OK 17%

SEGMENT NINE NO MEASUREMENT UNKNOWN

SEGMENT TEN NO MEASUREMENT UNKNOWN

TOTAL 100% The unallocated mass, 50%, can then be allocated according to a division of the remaining mass between the 3 unknown segments (16.6% each for an equal division). Densities can then be determined for all of the segments and each of the segments can be independently corrected.

Claims

CLAIMS: 1. A method of investigating radioactive sources in a body of

material including two or more 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 correction method including: the provision of an emission generator, passing at least a portion of the emissions of the generator into the sample, detecting emissions from the generator with the sample in place, and determining a factor relating to the amount of material in the given sample based on the relationship of the emissions detected with the sample in place relative to the emissions which would be detected without the sample in place, the determined factor being used in the correction of that sample when the detection meets the applied criteria for that sample, the correction method further providing that for samples whose detection does not meet the applied criteria, an alternative approach to determining a factor relating to the amount of material in the given sample is used, based on deducting the known amounts of material allocated to samples which do meet the detection criteria from the total amount of material in the body of material and the division of the remaining material amongst the samples for which the detection criteria were not met, the determined factor for a sample being used to give the corrected level from the detected level for that sample.
2. A method according to claim 1 in which the relationship between the emissions detected with and without the sample in place employs measured transmission coefficients.
3. A method according to claim 2 in which the measured transmission coefficients provided according to the equation:

R

Trans. Coeff. = ---- = exp(-px) Ro where R is the rate of detected photons with the sample in place, Ro is the rate of photons which would be detected without the sample in place, is the mass absorption coefficient, is the matrix density and x is the matrix thickness.
4. A method according to claim 3 in which Ro and x are known, R is measured and an assumed value for is taken.
5. A method according to any proceeding claim in which density of the sample is determined by the calculation and is the factor.
6. A method according to claim 5 in which the deduction of the known material from the total is made by deducting the factor for that sample from the total equivalent factor for the body of material.
7. A method according to claim 5 in which the deduction of the known material from the total is made by the deduction of the mass allocated to the sample from the total mass measured by weighing.
8. A method according to any preceding claim in which the mass allocated to each of the samples when detection meets the criteria is deducted from the total mass of the body of material and the division of the previously unallocated mass amongst the samples not meeting the criteria is made equally.
9. A method according to any preceding claim in which the applied criteria is a level of statistical precision.
10. A method according to any preceding claim in which the criteria is a given number of counts in a time period.
11. A method according to any claim depending from claim 3 in which x is kept constant by fixing the generator, detector and sample positions relative to one another.
12. A method according to any preceding claim in which an energy of greater than lOOOkeV is used for the generator of emission.
13. A method according to any preceding in which the sample is a segment or slice through the body of material.
14. Apparatus for investigating radioactive sources in a body of material, the body of material comprising a plurality of samples, the apparatus comprising: one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected; an investigating location into which the sample is introduced; signal processing means for relating the detector signals to a detected level for the sources; processing means providing a correction method for correcting the detected level for the sources to give a corrected level; the apparatus further comprising: a generator of radioactive emissions, at least of portion of the emissions entering the investigating location and, in use the sample; one or more detectors for detecting the generator emissions with the sample at the investigating location; processing means for determining a factor relating to the amount of material in the given sample based on the relationship of the emissions detected with the sample in place relative to the emissions which would be detected without the sample in place, the determined factor being used for the correction of that sample when the detection meets the applied criteria for that sample, the apparatus providing further processing means for correcting those samples whose detection does not meet the applied criteria, the alternative approach to determining a factor relating to the amount of material in a given sample being based on the deduction of the known amounts of material allocated to samples which do meet the detection criteria from the total amount of material in the body of material and the division of the remaining material amongst the samples for which the detection criteria were not met, the determined factor for each sample being used to give the corrected level from the detected level for that sample.