Determining a presence of a beta-emitting radionuclide

Abstract

Embodiments of the present invention provide a computer-implemented method of determining a presence of a β-emitting radionuclide in a migratory environment, the method comprising determining (210) an indication of energy distribution of ionizing radiation incident upon a scintillation detector proximal to the migratory environment, determining (220) a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy, and determining (230) the presence of the β-emitting radionuclide in the migratory environment in dependence on the ratio.

Description

P370180GB Determining a Presence of a Beta-emitting Radionuclide

[0001] This invention relates to methods and apparatus for determining one or more attributes of a source of radiation. In particular, some embodiments of the present invention relate to methods and apparatus for determining a presence of a beta- emitting radionuclide. BACKGROUND

[0002] It is known that environments can be contaminated with by radioactivity, such as associated with nuclear facilities. For example, environments such as structures, the ground, ponds and sumps may be contaminated with radioactivity from leaking nuclear waste stores and silos. Common radioactive nuclides in such environments are caesium-137 (137Cs), which causes emission of gamma ^ rays of 661.7keV, and beta-emitting nuclides e.g. strontium-90 (90Sr) and yttrium-90 (90Y). A dose risk associated with the decay of137Cs is predominantly external whereas 90Sr etc. is a concern via ingestion, with it being known that such nuclides can be captured by human bones. Furthermore, strontium and caesium also behave differently in such wet environments: caesium is mobile, forming salts and hydroxides readily; strontium, whilst similar in migratory terms, presents alongside 90Y (half-life ~ 2.7 days) given sufficient time has elapsed for this to grow in. Strontium is expected to remain in solution due to its relatively high ionic potential (16 nm-1) with, for example, changes in its abundance in groundwater being largely indicative of changes at its source. Conversely, it is known that yttrium as a trivalent rare earth forms insoluble carbonate complexes in neutral and alkaline aqueous environments that, being prone to precipitation, inhibit its mobility arising as colloids or suspended particles.

[0003] It is therefore desired to determine whether beta-emitting radionuclides are present in environments, particularly those which allow migration of radioactivity through the environment. It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art. BRIEF SUMMARY OF THE DISCLOSURE

[0004] In accordance with the present inventions there is provided a method, apparatus and computer software as set forth in the appended claims.

[0005] According to an aspect of the present invention there is provided method and apparatus as set forth in the appended claims.

[0006] According to an aspect of the present invention there is provided a computer- implemented method of determining a presence of a β-emitting radionuclide in aP370180GB migratory environment, the method comprising determining an indication of energy distribution of ionizing radiation incident upon a scintillation detector proximal to the migratory environment, determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy, and determining the presence of the β-emitting radionuclide in the migratory environment in dependence on the ratio.

[0007] According to an aspect of the present invention, there is provided a computer- implemented method of determining a presence of a β-emitting radionuclide in an aqueous environment, the method comprising determining an indication of energy distribution of ionizing radiation incident upon a scintillation detector proximal to the aqueous environment, determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy, and determining the presence of the β-emitting radionuclide in the aqueous environment in dependence on the ratio.

[0008] The β-emitting radionuclide may be determined to be present in the aqueous environment in dependence on the ratio being above a predetermined threshold.

[0009] The determining the ratio of the proportion of the energy distribution of ionizing radiation in the first energy range to the proportion of the energy distribution at the predetermined energy may comprise fitting a model to the indication of energy distribution to determine the proportion of the energy distribution at the predetermined energy.

[0010] The model may be of the form:^^ = ^^^^^ + ^^^^^ + ^^^^^ + ^wherein ^^^^^ is a Gaussian peak function, ^^^^^ is indicative of a second source of radiation, ^^^^^ is a tail function and ^ is a constant representing residual background counts.

[0011] The first energy range optionally comprises an energy range of around 60 to 800 keV. The predetermined energy may be around 662keV.

[0012] Determining the proportion of the energy distribution at the predetermined energy may comprise calibrating a model to the indication of energy distribution of ionizing radiation, the model representative of a photopeak of ionizing radiation at the predetermined energy.P370180GB

[0013] The scintillation detector may be located in contact with the aqueous environment.

[0014] The β-emitting radionuclide may comprise90Sr. The β-emitting radionuclide may comprise90Y.

[0015] According to an aspect of the present invention, there is provided an apparatus for determining a presence of a β-emitting radionuclide in an aqueous environment, the apparatus comprising a detector for detecting ionising radiation, a memory for storing an indication of an energy distribution of radiation detected by the detector, and a processor arranged to execute computer readable instructions to perform a method comprising steps of determining an indication of energy distribution of ionizing radiation incident upon the detector proximal to the aqueous environment, determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy, and determining the presence of the β-emitting radionuclide in the aqueous environment in dependence on the ratio.

[0016] According to an aspect of the present invention, there is provided computer software which, when executed by a computer, is arranged to perform a method as described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 is an illustration of a system according to an embodiment of the invention; Figure 2 illustrates a method according to an embodiment of the invention; Figure 3 illustrates a locating system according to an embodiment of the invention; Figure 4 illustrates data recorded by a detector in the presence of ionizing radiation; Figure 5 illustrates counts against detector distance for different energies; and Figure 6 illustrates a ratio of proportions of radiation detections. DETAILED DESCRIPTION

[0018] Figure 1 shows an illustration of a system 100 according to an embodiment of the invention.

[0019] The system 100 comprises a control unit 110 and a detector 120 which is communicably coupled to the control unit 110. The detector 120 may be in wireless or wired communication with the control unit 110. In some embodiments, the detector 120 communicates with the control unit 110 via a wired interface 130P370180GB which advantageously provides improved communication, particularly when the detector 120 is located at depth within a borehole which may hamper wireless signals. Furthermore, in some embodiments, the wired interface may be a network communication protocol, such as an IP-based communication protocol. In one embodiment the wired interface may be Ethernet. Such network communication protocols may provide improved long-distance communication over, for example, USB communication. As will be explained, the detector 120 may comprise signal processing functionality to locally process and analyse signals from the detector 120. The detector 120 may be connected to a flexible support member such as a cable, for example, a metal cable, or chain from which the detector 120 may be suspended within the borehole. The control unit 110 is arranged to record an indication of energy of ionizing radiation experienced by the detector 120 e.g. within the borehole.

[0020] The detector 120 comprises a radiation detector 121 for detecting ionizing radiation. The detector may be a scintillator 121 such as in the form of a scintillation crystal. The scintillation crystal may be cerium bromide (CeBr3) in some embodiments. The detector 120 may comprise a signal processing module 122 for processing signals from the scintillator 121. The detector 120 may record counts each corresponding to an interaction of ionizing radiation with the detector in one of a plurality of channels each corresponding to energy. In this way an energy distribution of ionizing radiation interacting with the detector 120 may be determined. As noted above, in some embodiments the detector 120 and the control unit 110 communicate over a wired interface 130 such as Ethernet. A detector 121 may be obtained from Scionix ® of the Netherlands. The detector 120 may comprise a photomultiplier and preamplifier to process signals from the scintillation crystal. The signal processing module 122 may be arranged to perform pulse height analysis on signals output by the detector 121. The signal processing module 122 may be arranged to output data indicative of a number of counts of ionizing radiation detected by the detector in each of a plurality of channels corresponding to respective energy bands over a period of time. The detector 120 is mounted in a metal housing, which may be at least partly aluminium. In some embodiments the detector is housed in a generally cylindrical metal can, such as an aluminium can.

[0021] The control unit 110 may comprise a processor 111 and memory 112 for storing data therein. The memory 112 may store computer readable code or instructions for execution by the processor 111. The control unit 110 may further comprise aP370180GB communication module 113 for receiving data from the detector 120, such as via the wired interface 330. The communication module 113 may be a network interface, such as an Ethernet network interface. The received data may be stored in the memory 112 of the control unit 310. The processor 111 is arranged to determine a presence of a beta-emitting radionuclide using the stored data.

[0022] Figure 2 illustrates a method 200 according to an embodiment of the invention. The method 200 is a computer--implemented method of determining a presence of a β-emitting radionuclide, such as in a migratory environment. The method 200 may be performed by a computer or the control unit 110 of the system 100 illustrated in Figure 1. Thus embodiments of the method 200 are performed by a processor executing computer-readable instructions stored in a memory such as the memory 112.

[0023] It will be understood that a migratory environment is one which allows migration of a contaminant through the environment. For example, the migratory environment may be a substantially solid environment e.g. a man-made structure, such as comprising concrete and / or metals. The migratory environment may be a porous media such as concrete etc. The migratory environment may be the ground, such as comprising one or more of soil, sand or vegetation matter. In such cases a borehole or other aperature may facilitate placement of a detector in relation to the structure. The migration may be under the influence of gravity, particularly where contamination is from an elevated position. Migration may also be due to drift of an ionic species. The migration may be according to Fick’s law according to electrical potential. The contamination may comprise actinides which form insoluble micro particles (often as oxides). The contamination may be a liquid such as oil or water comprising actinides. Where the contamination is a liquid it may migrate through a porous media, such as concrete. The contamination may enter a liquid environment such as an aqueous environment. The contaminant may migrate through the liquid environment e.g. via dispersion, distrubances or currents in the liquid. Thus it will be understood that a migratory environment is one which allows migration of contaminants therethrough.

[0024] The migratory environment may be an aqueous environment. An aqueous environment is one which comprises water. In some instances the aqueous environment may be substantially water i.e. the environment may comprise other components such as compounds or materials dispersed or dissolved in the water, or other liquids such as oils dispersed within or upon the water. The migratoryP370180GB environment may be an oil-based environment e.g. a liquid formed substantially of one or more oils.

[0025] The method comprises a block 210 of determining an indication of an energy distribution of ionizing radiation incident upon the detector 120. Block 120 may comprise arranging the detector 120 proximal to the migratory environment, such as close to or in contact with the structure or liquid environment e.g. the aqueous environment. For example, block 210 may comprise locating the detector in contact with liquid, which may be predominately water such as potentially contaminated water, at a nuclear facility e.g. in a pond or sump, although embodiments of the present invention are not restricted in this respect. Embodiments of the invention will be described with reference to a borehole containing a liquid as illustrated in Figure 3 with it being appreciated that this is merely for understanding of the invention. Block 210 may comprise lowering, or otherwise placing, the detector 120 into the aqueous environment e.g. into water.

[0026] Figure 3 illustrates a locating system for locating the detector 120 in the aqueous environment e.g. lowering into water 320 within a borehole, sump or other container 310. The locating system 300 comprises an apparatus for controllably inserting the detector 120 into the aqueous environment. In the illustrated example the apparatus is arranged to controllably suspend the detector 120 at a determinable or known depth within the borehole 310 e.g. with the detector 120 suspended in the water 320. The locating system 300 may comprise a winch 340 which is arranged to withdraw or extend a flexible support member 345 from which the detector 120 is suspended. In the illustrated example the winch 340 is mounted upon a support frame 350 such that it is generally located above the borehole 310 in use. However it will be appreciated that other arrangements may be envisaged. For example, the winch 340 may not be located upon the support frame 350. In another embodiment the winch 340 may be located at a lower level, such as ground level, and a pulley or guide may be located upon the support frame 350 over which the flexible support member 345 runs to allow a change of direction. The winch 340 may be controlled by the control unit 110 to controllably raise and lower the detector 120 within the borehole 310. The control unit 110 may instruct the winch 340 to raise or lower the detector 120 by an instructed distance e.g.0.1m, 0.2m etc. In other embodiments, the control unit 110 may instruct the winch 340 to raise or lower the detector 120 until instructed to stop.

[0027] When the detector 120 is e.g. in contact with the water 320, a number of counts of ionizing radiation is recorded in each of a plurality of channels. Each channelP370180GB corresponds to a respective energy or energy band. Thus the channels are indicative of the energy distribution of ionising radiation encountered by the detector 120. An example of data recorded by the control unit 110 from the detector 120 is shown in Figure 4. The data may be stored in the memory 112 of the control unit 110. Figure 4 illustrates two sets of data, a first 410 with the detector 120 in contact with the water 320 and a second set of data 420 with the detector away from any sources of radiation e.g. a background set of data 420, illustrating the influence of ionising radiation on the detector 120.

[0028] Embodiments of the present invention allow the presence of a presence of a beta- emitting radionuclide to be determined alongside a source of gamma radiation such as from137Cs. Embodiments of the invention are particularly useful in aqueous environments. Embodiments of the invention utilise a recognition that beta particles have a relatively short range, particularly in water which is around 1cm, whereas gamma radiation from137Cs has a much longer range. Furthermore, it has been appreciated that when the detector 120 housing is in contact with the aqueous environment, such as water, beta particles from90Sr and90Y dispersed in the aqueous environment, are able to interact with the metal housing to yield bremsstrahlung X-rays which are measured by the detector 120.

[0029] The method 200 comprises a block 220 of determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy. It has been appreciated that when a beta-emitting nuclide is present in the aqueous environment, or is too distant from the detector 120, such that only137Cs is present within a range of the detector 120, a spectral response measured by the detector approximates that of only the137Cs with no measured augmentation by bremsstrahlung. However, when both137Cs and 90Sr and / or90Y are present in combination near to the detector 120, a greater detected proportion of bremsstrahlung is recorded.

[0030] In block 220, a number of detections or counts corresponding to, or originating from, the137Cs is determined. The determination is based on determining a detection of ionising radiation at or around the predetermined energy. The predetermined energy corresponds to around 662keV, or, more specifically, 661.7keV. The predetermined energy corresponds to a137Cs gamma-ray full- energy peak response energy which is caused by a decay of137Cs to137Ba ground state with the emission of a 661.7keV gamma photon which is detected by the detector 120. The number of counts may be determined in a predetermined energyP370180GB band or range around the predetermined energy e.g.661-663keV or other energy band or range as may be envisaged.

[0031] In embodiments of the invention, the control unit 110, in particular the processor 111 thereof executing computer-readable instructions stored in the memory 112, is arranged in block 220 to fit a model to the data stored in the memory 112 i.e. the count data corresponding to the plurality of channels. The model comprises a component corresponding to the full-energy peak or photopeak. The model comprises components corresponding to a baseline (b), a step S(Ep), tail T(Ep) and a Gaussian G(Ep) corresponding to the full-energy peak or photopeak.

[0032] The model f1 may be of the form defined in Equation 1 of: ^^ = ^^^^^ + ^^^^^ + ^^^^^ + ^Equation 1 Wherein Epis indicative of energy or channel number. A first part G(Ep), as defined in Equation 2, is representative of ionizing radiation:^^^ ^^^ = ^^^^^Equation 2

[0033] It will be appreciated that the full-energy peak is a region of the energy distribution spectrum caused by complete absorption of gamma rays by the scintillator of the detector 120. G(Ep) is a Gaussian peak function, which may represent a full- energy line, A is the amplitude of the Gaussian function, μ its centroid and σ the standard deviation.

[0034] The model f1comprises a second part S(Ep) which may be indicative of a second source of radiation, such as a background source of radiation causing detection events, whereas detections from137Cs are considered to be primary radiation. The second part S(Ep) may comprise one or both of second and third functions, as defined by Equations 3 and 4, respectively.^^^^^ = ^^ erfc!√2Equation 3P370180GB

[0035] S(Ep) is a step function which represents a step discontinuity that may appear in the continuum below the Gaussian peak towards its low energy side. S(Ep) may be produced by detection of Compton scattering photons into the detector 120 from surrounding materials, folded with Gaussian noise. B is the step function amplitude expressed as a fraction of A amplitude of G(Ep), and erfc(Ep) is a complementary error function.^^^ ^ = ^%^ &^ ^erfc ^ ^ −+1 ^!√2 (√2$Equation 4

[0036] T(Ep) is a tail function which represents an exponential-type discontinuity that may appear in the continuum below the Gaussian peak towards its low energy side. T(Ep) may represent the effect of incomplete charge collection in the detector volume, which is modelled by an exponential decaying distribution of counts below the peak, folded with Gaussian noise. C is the amplitude of T(Ep) expressed as a fraction of A, and T the slope of the exponential. In some embodiments b is a constant representing residual background counts.

[0037] Further details of the above equation may be obtained from Elísio, S. C., Bala, A., Bandala, M., Graham, J., Grievson, A. and Joyce, M. J., ‘Point-spread Analysis of^^-ray / depth Spectra for Borehole Monitoring Applications’, IEEE Trans. Nuc. Sci., 70 (11) 2506 – 2514 (2024), which is herein incorporated by reference for all purposes.

[0038] Thus, in a portion of block 220, the number of detections or counts corresponding to, or originating from, the137Cs is determined as the detection of ionising radiation by the detector 120 at or around the predetermined energy i.e. at or around 662keV. The number of detections is determined by fitting a model to the detection data from the detector 120 as explained above.

[0039] Block 220 comprises determining a portion of the energy distribution detected by the detector 120 in the first energy range. The first energy range may have lower and upper energy bounds or limits which, in some embodiments, are 60 and 800 keV, respectively, although other limits may be envisaged. The upper energy bound may be higher than the predetermined energy e.g. in some embodiments higher than 662keV, whilst the lower energy bound may be lower than the predetermined energy. In some embodiments, block 220 comprises determining aP370180GB number of counts at the detector 120 corresponding to detections in the first energy range e.g. between 60-800keV in one embodiment.

[0040] As noted above, block 220 comprises determining the ratio of the proportion of the energy distribution of ionizing radiation in the first energy range to the proportion of the energy distribution at the in Equation 5:=%+,-.^567686593:67Equation 5 Where low and high are the respective lower and higher energy bounds for the Count of detections there-between and Countpredeterminedis the number of counts at the predetermined energy. Thus, in one embodiment, Equation 5 may be as in Equation 6:)(*) = ^@A%?Equation 6

[0041] In order to demonstrate the use of Equation 6 in determining the presence of a bet- emitter such as90Sr in the presence of137Cs or separate from one another, an experiment was performed using an apparatus similar to that illustrated in Figure 3.

[0042] A sealed137Cs source (activity: 304 kBq) was maintained at a constant distance from a detector probe (5.1 cm). A90Sr source (370 kBq) was moved to a plurality of predetermined distances from the detector probe to effect a change in its range (and therefore bremsstrahlung yield from interaction with a metal blind-tube lining) from the detector in the probe, as per Table I. Here B is the distance from the top of the laboratory-based, blind-tube testbed to the position of the90Sr source, * is the distance of the90Sr source from the bottom of the probe metal can and !Cis the uncertainty in the * measurement determined via error propagation. Measurement Source Position Distance !C / index (d ± (x) / cm cm 0.5) / cm 0137Cs 5.1 4.8 0.1 1137Cs 5.1 4.8 0.1 90Sr 5.1 4.8 0.1 2137Cs 5.1 4.8 0.1 90Sr 6.2 5.1 0.2P370180GB 3137Cs 5.1 4.8 0.1 90Sr 7.3 5.6 0.3 4137Cs 5.1 4.8 0.1 90Sr 8.2 6.1 0.3 5137Cs 5.1 4.8 0.1 90Sr 9.2 6.8 0.4 6137Cs 5.1 4.8 0.1 90Sr 10.2 7.5 0.4 7137Cs 5.1 4.8 0.1 90Sr 15.2 11.8 0.5 8137Cs 5.1 4.8 0.1 90Sr 20.2 16.5 0.5 9137Cs 5.1 4.8 0.1 90Sr 25.2 21.3 0.5 10137Cs 5.1 4.8 0.1 90Sr 30.2 26.2 0.511137Cs5.1 4.8 0.1 90Sr 35.2 31.2 0.5P370180GB

[0043] The counts at each energy were recorded for each position, or distance, of the of the90Sr source from the detector. Results in different energy ranges are illustrated in Figures 5(a)-(d). Figures 5(a)-(d) presents a sum of counts versus90Sr-probe distance * for: a) 60-350 keV, b) 60-800 keV, c) 350-450 keV and d) the 662 keV full-energy peak region, with the response for137Cs-only at *=4.8 cm indicated at 510 in each Figure, with ^1^ uncertainties.

[0044] A comparison of the sum of counts within defined energy ranges yields the response achievable as the90Sr-detector distance, *, is increased. This is shown, for example, in Figures 5a, 5b and 5c for the data for the ranges 60-350 keV, 60- 800 keV and 350-450 keV, respectively. As * is reduced and hence the90Sr contribution to the spectrum from bremsstrahlung from the blind tube increases, the sum of counts, #counts, increases too, such that #counts → ∞, * → 0 whereas, for* → ∞, #counts tends to the 137C3-only baseline (see Fig. 6d). The greater numberof counts in the wider energy range of 60-800 keV, compared to those in the 350- 450 keV region, is evident in the smaller uncertainties in the former. When90Sr is too distant from the probe to be detected, #counts is consistent with that for * → ∞,i.e., there being no 90Sr, and when both sources are detectable, #counts → ∞ as* → 0. Thus it is demonstrated that presence of the 90Sr source affects the countsrecorded by the probe or detector in different energy ranges, with the wider range 60-800keV advantageously providing greater certainty.

[0045] As noted above, block 220 comprises determining the ratio of the proportion of the energy distribution of ionizing radiation in the first energy range, such as 60-800 keV to the proportion of the energy distribution at the predetermined energy such as 662keV, as in Equation)(*) = ^@A%?

[0046] Block 230 of the method 200 comprises determining the presence of the β-emitting radionuclide in the aqueous environment in dependence on the ratio of Equations 5 & 6.

[0047] Using the proportion )(*)determined as above, it can be determined whether the 90Sr and137Cs are present together or separate from one another in space, such as in the aqueous medium. Referring to Figure 6, an empirical fit 610 is shown to a respective ratio )(*)620 at each of a plurality of distances of the90Sr source from the probe, where the fit is of the form in Equation 7: )(*) = H^^IC + ;P370180GB Equation 7

[0048] In the example, H, ^ and ; were found to be (49 ± 11), (0.68±0.04) cm-1and (28.31±0.03), respectively, and χ^K=0.01. For * → 0, )(0) = H + ;, i.e., ~77. It willbe appreciated that for different detectors i.e. having different constructions, a calibration process as in the experiment described above may be performed to determine the above coefficients of Equation 7. Thus the values of the coefficients described above are merely exemplary.

[0049] As described above, by determining a ratio of the proportion of detections at the predetermined energy to the proportion of detections in the predetermined energy range, a distance of the beta emitter from the detector 120 can be determined using a model relating the ratio to the distance of the beta emitter, such as in Equation 7.

[0050] In block 230 the presence of the β-emitting radionuclide in the aqueous environment in dependence on the ratio of Equations 5 & 6 is determined. In one embodiment the presence of the β-emitting radionuclide in the aqueous environment is determined in block 230 by using a predetermined ratio threshold. When the ratio is determined to exceed the threshold it is determined in block 230 that there is a β-emitting radionuclide in the aqueous environment. In another embodiment, the presence of the β-emitting radionuclide in the aqueous environment is determined in block 230 by using a predetermined gradient threshold. The gradient threshold may be applied to the ratio )(*) such that if the gradient of the ratio )(*)exceeds the gradient threshold then it is determined that the β-emitting radionuclide is present in the aqueous environment.

[0051] Block 230 may comprise outputting an indication of whether the β-emitting radionuclide is present in the aqueous environment. For example, an output may be provided when it is determined that the β-emitting radionuclide is present in the aqueous environment to alert a user i.e. to provide a warning. The output may be one or both of audible and visual to alert the user.

[0052] Advantageously, embodiments of the present invention provide an output indicative of a presence of a β-emitting radionuclide, such as in the aqueous environment. The presence of the β-emitting radionuclide may be determined even in the presence of gamma radiation. It will be appreciated that embodiments of the present invention provide a qualitative indication of the presence of the β-emitting radionuclide. If, however, a relative quantification is required then the system mayP370180GB be calibrated in advance with fluid mixtures of known radioactivity to allow fitting to resulting spectra.

[0053] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0054] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0055] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

P370180GB CLAIMS 1. A computer-implemented method of determining a presence of a β-emitting radionuclide in a migratory environment, the method comprising: determining an indication of energy distribution of ionizing radiation incident upon a scintillation detector proximal to the migratory environment; determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy; and determining the presence of the β-emitting radionuclide in the migratory environment in dependence on the ratio.

2. The method of claim 1, wherein the β-emitting radionuclide is determined to be present in the migratory environment in dependence on the ratio being above a predetermined threshold.

3. The method of any preceding claim, wherein the determining the ratio of the proportion of the energy distribution of ionizing radiation in the first energy range to the proportion of the energy distribution at the predetermined energy comprises fitting a model to the indication of energy distribution to determine the proportion of the energy distribution at the predetermined energy.

4. The method of claim 3, wherein the model is of the form:^^ = ^^^^^ + ^^^^^ + ^^^^^ + ^wherein ^^^^^ is a Gaussian peak function, ^^^^^ is indicative of a second source of radiation, ^^^^^ is a tail function and ^ is a constant representing residual background counts.P370180GB 5. The method of any preceding claim, wherein the first energy range comprises an energy range of around 60 to 800 keV.

6. The method of any preceding claim, wherein the predetermined energy is around 662keV.

7. The method of any preceding claim, wherein determining the proportion of the energy distribution at the predetermined energy comprises calibrating a model to the indication of energy distribution of ionizing radiation, the model representative of a photopeak of ionizing radiation at the predetermined energy.

8. The method of any preceding claim, wherein the scintillation detector is located in contact with the migratory environment.

9. The method of any preceding claim, wherein the β-emitting radionuclide comprises90Sr.

10. The method of any preceding claim, wherein the β-emitting radionuclide comprises90Y.

11. The method of any preceding claim, wherein the migratory environment is a liquid environment.

12. The method of claim 11, wherein the liquid environment is an aqueous environment.

13. The method claim 11, wherein the liquid environment comprises an oil.

14. The method of any of claims 1 to 10, wherein the migratory environment is a substantially solid environment.

15. The method of claim 14, wherein the solid environment comprises the ground.

16. An apparatus for determining a presence of a β-emitting radionuclide in an migratory environment, the apparatus comprising: a detector for detecting ionising radiation;P370180GB a memory for storing an indication of an energy distribution of radiation detected by the detector; and a processor arranged to execute computer readable instructions to perform a method comprising steps of: determining an indication of energy distribution of ionizing radiation incident upon the detector proximal to the migratory environment; determining a ratio of a proportion of the energy distribution of ionizing radiation in a first energy range to a proportion of the energy distribution at a predetermined energy; and determining the presence of the β-emitting radionuclide in the migratory environment in dependence on the ratio.

17. Computer software which, when executed by a computer, is arranged to perform a method according to any of claims 1 to 15.

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