Method and system for activation analysis

Abstract

Described herein is a method for preparing a sample that is at least partially liquid for activation analysis. The method may include placing a sample containing at least one target element in a container, and curing the sample within the container. A method and system for performing neutron or gamma activation analysis is also described herein. The method may include providing an at least partially liquid sample containing at least one target element. Curing the sample. The cured sample is irradiated to activate at least one target element within the sample. The number of gamma rays emitted by the at least one target element may be measured. A value representative of a concentration of at least one target element in a sample may be determined using a measurement of gamma rays emitted by the at least one target element and a calibration determined from a solid sample of known composition.

Description

Technical Field

[0001] The present invention generally relates to a method or system for the activation analysis of liquid samples for determining the concentration of one or more target elements within the sample. The present invention also relates to a method for preparing a liquid sample. Background Art

[0002] Neutron activation analysis (NAA) and gamma activation analysis (GAA), also known as photon activation analysis, are methods for measuring the elemental composition of a sample of material of otherwise unknown composition. In particular, activation analysis is useful in determining the concentration of a desired target element within the sample. The sample is irradiated with a neutron beam or a high-energy X-ray beam generated by an X-ray source. Irradiation of the sample in this manner will initiate a nuclear transition within at least one target element, which can cause the formation of a radioactive isotope.

[0003] After a period of irradiation, the sample is placed proximal to one or more gamma-ray detectors. The radioactive isotopes initiated within the sample decay with characteristic half-lives and can emit gamma rays having one or more characteristic energies. The one or more gamma-ray detectors count the gamma rays emitted from the sample and measure their energies. The number of gamma rays emitted from the target element is proportional to the amount of that element within the sample.

[0004] For many applications including mineral exploration, mining, and mineral processing, for samples such as mineral ores, it is advantageous to measure large quantities of material to ensure that the sample is representative. Such large samples are particularly susceptible to spatial sensitivity correlations. To improve the sensitivity of the equipment, it is advantageous to position the irradiation source close to the sample during irradiation and to position the gamma-ray detector close to the sample during gamma-ray detection. For example, for the measurement of gold present in mineral ores at concentrations of parts per million or less, a 0.5 kg sample can be conveniently measured in a cylindrical container having a diameter of approximately 100 mm and a height of approximately 50 mm. The irradiation source and detector can be positioned as close as practicable to the outer surface of the can and thus close to the mineral ore sample.

[0005] The radiation flux from a point source (i.e., the radiation source) decreases with the square of the distance from the source, and due to attenuation of the incident beam by the material within the sample, the radiation flux is further reduced. Thus, the flux from a radiation source positioned proximal to the surface of a cylindrical sample will be highly non-uniform. Irradiating the sample from one side while rotating the sample can improve flux uniformity, but some regions of the sample will still be exposed to a higher flux of incident radiation compared to other regions. As a result, atoms of one or more target elements in the high-flux regions have a higher probability of being activated by the incident radiation compared to target atoms in the low-flux regions.

[0006] In a similar manner, the probability of detecting gamma rays emitted by radioactive isotopes formed during the activation process decreases approximately as the inverse square of the distance of the atom from the radiation detector. The detection probability is further attenuated by the absorption of gamma rays within the sample material. For lower energy gamma rays, the attenuation within the sample can be quite significant.

[0007] Then, the probability of detecting an atom of the target element is given by the product of the activation rate of each atom (related to the incident radiation flux at the position of the atom) and the gamma ray detection probability. The overall instrument sensitivity for a given target element is given by integrating the position-dependent detection probability over the volume of the sample material.

[0008] Calibration methods, such as using calibration software, can be used to calculate the concentration of one or more activated elements from the measured characteristic gamma ray emission intensities. Unless the sample size is very small, both the intensity of the activating neutrons or X-ray beam and the efficiency for detecting characteristic gamma rays vary with position over the entire sample volume. The calibration method or software must account for the spatial sensitivity correlation. Typically, calibration will utilize measurements of standard samples that have accurately known compositions and accurately known amounts of the target element.

[0009] Samples can take different physical forms, such as solid materials (including powders or fragments), liquids, or mixtures of solid materials and liquids (slurries). Solid samples can be assumed to be non-flowing. Thus, for a solid sample, the position of any given target atom is the same during irradiation and during measurement. Liquids and slurries are flowing samples because their elements are in motion and not static, including during the irradiation or detection process. For example, the entry of the sample into the irradiation device and its movement between the irradiation device and the detection device typically causes movement of the liquid. Additionally, significant heating that may occur when presenting a liquid sample to a strong incident radiation source can cause convection within the liquid sample.

[0010] Therefore, the instrument sensitivity for liquid or slurry samples will be different from that for solid samples, and calibration methods developed for non-flowing solid samples cannot be directly applied to the measurement of flowing samples such as slurries or liquids. That is, the movement of activated elements within the volume of a liquid or slurry sample during irradiation or between irradiation and measurement will affect the relationship between the measured characteristic gamma ray intensity and the element concentration. Thus, there are difficulties in obtaining calibrated measurements of liquid or slurry samples.

[0011] Variations in instrument sensitivity are inconvenient when they mean that separate calibrations must be maintained for different sample types (solid or liquid). Additionally, if liquid samples with different properties including density, heat capacity, or viscosity are to be measured, separate calibrations may be required for each variation in the liquid properties, and specifically, where the liquid moves differently during irradiation, measurement, or transfer between the two processes.

[0012] Although a separate method for obtaining calibrated measurements of elemental concentrations in each sample type can be designed, maintaining multiple calibrations is inconvenient, especially in a commercial laboratory environment, as it can be time-consuming and also requires separate quality control standards and validation. Additionally, it is inconvenient that for each analysis performed, the sample type must be correctly identified, including the relevant properties for liquid or slurry samples, which imposes further record-keeping and data quality checking requirements.

[0013] Burmistenko et al. describe a method for solving the current problem in SU464224. They propose forming the sample in the shape of a cylinder and irradiating it through its curved surface from one side. The sample is then rotated around the cylindrical axis during irradiation. During measurement, the sample is positioned between two cylindrical detectors such that the axes of the two detectors are the same as the axis of the sample. The intention of this configuration is to try and ensure that both the incident flux and the gamma-ray detection probability are as uniform as possible throughout the sample volume. If both the flux and the detection probability are perfectly uniform, then the probability of detecting target atoms will be independent of their position, and as a result, the overall instrument sensitivity will be the same for both flowing and non-flowing samples. However, using this method, only a limited uniformity of both the flux and the detection probability can be achieved, and it has been shown that this method is not ideal.

[0014] Another approach is to reduce the size of the sample. For example, when performing thermal neutron activation analysis, it is common practice to reduce the sample size to a minimum (a few grams or less), so that it can be assumed that the sample has no perturbing effect on the incident neutron flux. Similarly, a small sample size results in negligible internal attenuation of the gamma rays emitted from the sample. Additionally, if the sample is positioned at a distance from the gamma-ray detector that is greater than the sample size, the probability of gamma-ray detection over the entire sample volume becomes uniform. This method is not ideal as it relies on a very small sample size. Such small sample sizes do not conform to the requirements of large sample masses in many applications. Reasons for desiring a larger sample size include reduced sampling error and increased sensitivity to elements present at low concentrations within the sample. For example, in the case where the sample material is part of a mineral ore containing a low concentration of a precious metal such as gold, measuring a larger mass of the material reduces the sampling error and improves the accuracy. Introducing a large separation between the sample and the gamma-ray detector also reduces the sensitivity.

[0015] Another difficulty in measuring large liquid samples via NAA or GAA arises when the sample container breaks during the analysis. For example, the container may be dropped during loading, the operator may incorrectly cap the container lid, or an automated sample handling system may accidentally break the container. A broken container of liquid or slurry material is likely to spill, which not only renders the sample unusable for analysis but also poses several risks. First, the liquid may contaminate parts of the analysis equipment, potentially affecting the measurement of subsequent samples. Second, the sample may be toxic, corrosive, or acidic. For example, an aqueous solution containing gold used in mineral processing may also contain a certain amount of cyanide or have a very low pH. Third, the sample that spills after activation may be radioactive and may pose a health risk to anyone in the vicinity, including those responsible for cleaning up the spill.

[0016] Accordingly, it is desirable to provide a method or system that overcomes, mitigates, or provides a useful alternative to address at least one of the problems associated with the prior art. SUMMARY OF THE INVENTION

[0017] According to one aspect of the present invention, there is provided a method for performing neutron or gamma activation analysis, the method comprising: providing a sample that is at least partially liquid and contains at least one target element; solidifying the sample; irradiating the sample to activate at least one target element within the sample; detecting, in a measurement, the number of gamma rays emitted by at least one target element within the sample; and determining a value representing the concentration of at least one target element in the sample using the measurement of the gamma rays emitted by at least one target element and a calibration determined based on a solid sample of known composition.

[0018] According to an embodiment, the sample is cured using a gelling agent or a coagulant. The gelling agent or coagulant may comprise one or more of the following: superabsorbent polymers, polyacrylates or polyacrylamides. The gelling agent or coagulant preferably comprises sodium polyacrylate. According to an embodiment, the gelling agent or coagulant comprises one or more of the following: fumed silica, calcium sulfate, calcium sulfate hemihydrate or cement. According to other embodiments, the sample is cured by a freezing process.

[0019] According to an embodiment, the sample that is at least partially liquid is in liquid form, or is a liquid sample. According to an embodiment, the sample that is at least partially liquid may comprise a slurry or a process solution. The slurry or process solution may be from a mineral processing plant.

[0020] According to an embodiment, the sample is irradiated by an X-ray source. The X-ray source may be a solid metal target struck by an electron beam. The electron beam source may be a linear accelerator (LINAC). The solid metal target generates X-rays with an energy up to the energy of the electron beam. The LINAC may accelerate electrons at an energy higher than 5 MeV. The LINAC may accelerate electrons at an energy of about 8 MeV to about 14 MeV. The LINAC may accelerate electrons at an energy of about 8 MeV. The electron beam energy may be selected according to one or more target elements in the sample whose concentration is to be determined. The LINAC may include a solid metal target. The electrons may be rapidly decelerated by the solid metal target to produce a continuous energy spectrum of X-rays with a maximum energy corresponding to the energy of the electron beam.

[0021] According to another embodiment, the sample is irradiated by a neutron source. The neutron source may be a radioactive isotope source, such as californium-252, or a mixture of americium-241 and beryllium. The neutron source may be a sealed-tube neutron generator that can accelerate deuterium or tritium ions onto a target containing additional deuterium or tritium, thereby producing neutrons via DD or DT fusion reactions. The neutron source may be an ion beam accelerated to strike a metal target. The neutron source may be an electron beam from a linear accelerator that strikes a metal target, where the electron energy is greater than the neutron separation threshold of at least one isotope constituting the target. The neutron source may comprise a moderator to increase the thermal neutron flux in the sample. Elements in the sample may be activated by fast neutron reactions such as (n,n'), (n,2n) and (n,p) or by thermal neutron capture reactions (n,γ).

[0022] According to an embodiment, the sample is placed in a container before curing. According to an embodiment, when contained in the container, the cured sample is irradiated. According to an embodiment, the container is substantially cylindrical. The container can be a can. The container can be formed of a plastic or polymeric material. The container can include a lid. The lid can be detachably attached to the container body via a screw fastening mechanism. The container can have a diameter in the range of about 50 mm to about 100 mm. The container can have a height in the range of about 40 mm to about 70 mm. The container can have a volume of about 100 mL to 1 L, or about 200 mL to about 500 mL, or about 300 mL. The container can hold a sample mass of about 100 g to 1 kg, or about 300 g.

[0023] According to an embodiment, the method further comprises: irradiating a reference material to activate a reference element within the reference material, the reference element having a known concentration within the reference material; detecting the number of gamma rays emitted by the reference element; and normalizing the measurement of gamma rays emitted from at least one target element by using the detected number of gamma rays emitted by the reference element, wherein determining a value representing the concentration of the at least one target element utilizes the normalized measurement.

[0024] According to an embodiment, the sample and the reference material are irradiated simultaneously. The sample and the reference material can be positioned adjacent to each other before irradiation. According to an embodiment, the sample and the reference material can be placed in respective containers and positioned adjacent to each other.

[0025] According to an embodiment, the reference material takes the form of a disk or a circular wafer. The reference material can be in a cylindrical shape. The reference material can have a thickness of 0.1 mm to 3.0 mm. The reference material can have a diameter substantially similar to or less than the diameter of the container that holds the sample. The diameter of the reference material can be about 50 mm to about 100 mm. During irradiation and measurement, the reference material can be positioned on a flat surface of the container such that the central axis of the reference material coincides with the central axis of the container.

[0026] According to another aspect of the present invention, there is provided a method for preparing a sample that is at least partially liquid for neutron or gamma activation analysis, the method comprising: placing the sample that is at least partially liquid in a container, the sample containing at least one target element; and curing the sample within the container.

[0027] According to an embodiment, the sample is cured using a gelling agent or a coagulant. The gelling agent or coagulant may comprise one or more of the following: superabsorbent polymers, polyacrylates, or polyacrylamides. The gelling agent or coagulant preferably comprises sodium polyacrylate. According to an embodiment, the gelling agent or coagulant comprises one or more of the following: fumed silica, calcium sulfate, calcium sulfate hemihydrate (plaster of Paris), or cement. According to other embodiments, the sample is cured by a freezing process.

[0028] According to an embodiment, the sample that is at least partially liquid comprises a slurry or a process solution. The slurry or process solution may be from a mineral processing plant.

[0029] According to an embodiment, the container is substantially cylindrical. The container may be a can. The container may be formed of a plastic or polymeric material. The container may include a lid. The lid may be removably attached to the container body via a screw fastening mechanism. The container may have a diameter in the range of about 50 mm to about 100 mm. The container may have a height in the range of about 40 mm to about 70 mm. The container may have a volume of about 100 mL to 1 L, or about 200 mL to about 500 mL, or about 300 mL. The container may hold a sample mass of about 100 g to 1 kg, or about 300 g.

[0030] According to an embodiment, the sample holder may hold the sample. The sample holder may hold the container containing the sample. The sample holder may hold a reference material in a fixed relationship with the sample. The sample holder may be movable and / or operable to shuttle between an irradiation system and a detector system. The sample holder may move between a curing system where the sample is cured and the irradiation system.

[0031] Another aspect of the present invention provides a system for performing neutron or gamma activation analysis on a sample that is at least partially liquid, the system comprising: depositing the sample that is at least partially liquid in a container, the sample that is at least partially liquid comprising at least one target element; a curing station where the sample that is at least partially liquid is cured in the container to produce a cured sample; an irradiation station where the cured sample is irradiated and at least one target element undergoes activation; a detection station where the number of gamma rays emitted by one or more activated target elements in the cured sample is measured; and a computer system that uses the measured number of gamma rays emitted by one or more target elements and a calibration determined from a solid sample of known composition to determine values representing the concentration of each target element within the sample.

[0032] The above system may include any one or more features of the methods according to any other aspect or embodiment described herein.

[0033] According to an embodiment, the system includes a sample holder adapted to hold a sample container. The sample holder may hold a reference material in a fixed relationship relative to the sample.

[0034] According to an embodiment, the system includes a sample transport device configured to move a sample between a curing station and an irradiation station. According to an embodiment, the system includes a sample transport device configured to move a sample between the irradiation station and a detection station. According to an embodiment, the sample transport device is configured to move a sample between each of the curing station, the irradiation station, and the detection station. According to an embodiment, the sample holder holds the sample during movement by the sample transport device.

[0035] According to an embodiment, a sample that is at least partially liquid is cured by adding a gelling agent or a coagulant. The gelling agent or coagulant may include a superabsorbent polymer, polyacrylate, or polyacrylamide. A fixed mass or fixed volume of the gelling agent or coagulant sufficient to cure the liquid volume that makes up the sample may be added. The gelling agent or coagulant may include sodium polyacrylate.

[0036] According to an embodiment, the system further includes a reference material that includes a reference element. The reference material is positioned adjacent to the sample container when at the irradiation station and when at the detection station, and the reference material may be held by the sample holder in a fixed relationship relative to the sample. The computing system may utilize measurements of gamma rays emitted by the reference element when determining values representing the concentration of each target element within the sample that is at least partially liquid.

[0037] According to an embodiment, the system includes a computer configured to control the operation of at least one of the curing station, the irradiation station, the detection station, and the sample transport device. The computer may control the movement of the sample between the curing station, the irradiation station, and the detection station. The computer may control the electron beam emitted by a linear accelerator at the irradiation station. The computer may control at least one detector at the detection station. The computer may receive one or more measurements from one or more detectors. The computer may include a computing system. The computer may control the curing of a sample that is at least partially liquid. For example, the computer may control the addition of a fixed mass or fixed volume of a coagulant or gelling agent to the sample that is at least partially liquid. The computer may control waiting for a prescribed period of time for the coagulation or gelling process to complete. The computer may control securing a lid to the sample container.

[0038] According to an embodiment, the irradiation station includes: a linear accelerator that generates a high-energy electron beam, and the solid metal target that produces X-rays when the electron beam strikes the surface of the solid metal target.

[0039] According to an embodiment, the detection system includes at least one radiation detector. The at least one detector may be a high-resolution detector. The at least one detector may be a semiconductor detector. The at least one detector may be a scintillator detector. The detector may be a solid-state detector, for example, formed of ultra-pure germanium. The at least one detector may include two detectors that are configured to be positioned on opposite sides of a sample container in use. The detector or each detector may be cylindrical.

[0040] The methods provided herein may provide any one or more of the following advantages:

[0041] a) The measurement of the concentration of elements in a liquid sample or a sample that is at least partially liquid can be performed using the same systems, devices, and methods that have previously been designed for solid samples. This means that a single calibration can be applied to all types of samples, which can simplify the overall process and prevent the need for re-calibration of the instrument.

[0042] b) When measuring the concentration of a target element within a sample that is at least partially liquid by neutron or gamma activation analysis, the precision, accuracy, and convenience can be improved.

[0043] c) By solidifying one or more samples, spillage of one or more samples that are at least partially liquid, such as from a damaged or dropped container, can be avoided. This is particularly useful in cases where one or more samples may contain potentially toxic or harmful substances.

[0044] An intended application of the method according to the present invention is to analyze liquid samples or samples that are partially liquid using an instrument that is designed for measuring large solid samples. The proposed method is particularly useful for the measurement of gold, silver, copper, and other precious metals via gamma activation analysis in solutions, slurries, or liquids from mineral processing, waste recycling, and other extraction operations.

[0045] Definitions

[0046] As used herein, the term "liquid sample" is used to refer to any sample that is flowing and has a predominantly liquid composition. A liquid sample may have a low viscosity and may be highly fluid; alternatively, a liquid sample may be highly viscous and low in fluidity, although still a flowing substance. A liquid sample may have solid components, such as solid particles or precipitates within the liquid. In some embodiments, the liquid sample may be a slurry or a process solution, such as may be obtained from a mineral processing plant or a mining operation. The liquid sample may include a liquid body that contains at least one target element within the body. The reference herein to "a sample that is at least partially liquid" encompasses samples that are completely liquid, substantially liquid, and partially liquid.

[0047] As used herein, the terms "solidify", "solidification", or "solidified" refer to the act of converting a liquid substance into a substantially solid, non-flowing substance or into a final substance that has undergone such a conversion. The solidified substance will be non-flowing such that the elements within the solidified substance are not in motion and have a substantially fixed position that does not vary upon application of heat or movement of the substance as a whole. By way of example, the solidified substance can be a hardened gel substance or a coagulated substance. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the present invention will now be described with reference to the drawings. It should be understood that the embodiments are given by way of illustration only and the present invention is not limited by this illustration. In the drawings:

[0049] Figure 1 is a schematic diagram showing a device configured to perform gamma activation analysis of a sample; and

[0050] Figure 2 is a graph showing a comparison of the measured levels and the certified levels of gold for six samples prepared from a certified standard solution. DETAILED DESCRIPTION

[0051] According to an embodiment of the present invention, a liquid sample is obtained. According to some embodiments, the liquid sample can be a slurry or a process solution, such as can be obtained from a mineral processing plant or a mining operation.

[0052] According to the method of the present invention, the liquid sample is solidified prior to activation of one or more elements within the sample. Solidification of the sample will transform the liquid sample of the flowing material into a solidified sample of a non-flowing material. Solidification of the liquid sample can be performed by any desired method, some of which may be more preferred or advantageous than others. The act of solidifying the liquid sample can also be referred to as non-fluidization of the sample.

[0053] One method for solidification of the liquid sample is to freeze the sample. This method will involve subjecting the liquid sample to low temperatures to freeze and solidify the liquid. Freezing the liquid sample to non-fluidize the liquid sample can be a slow process and may take up to several hours to obtain the solidified frozen sample. Compared to freezing the sample, it is desirable to provide a method that solidifies the sample more quickly, which can non-fluidize the sample. Additionally, freezing the sample carries an additional risk that the frozen liquid will expand and the expansion of the sample within the container may cause damage or rupture of the container. Freezing the sample to achieve non-fluidization may not be the most ideal method to use, but can still overcome the problems of the prior art and can prevent the need to recalibrate the equipment in cases where the sample is initially flowing or liquid.

[0054] Other methods of immobilizing liquid samples involve adding a coagulant or gelling agent to the liquid sample to cause the sample to solidify. The coagulants used in this method can include cement, plaster of Paris (also known as calcium sulfate hemihydrate or gypsum plaster), or other similar gypsum materials such as calcium sulfate dihydrate. These types of coagulants may take at least several minutes and up to several hours to solidify. It may be preferred that the coagulant or gelling agent will solidify in less than a few minutes so that the sample can be sent for irradiation in the activation analysis process more quickly. Other gelling agents or coagulants suitable for the methods herein can include fumed silica or cement.

[0055] The preferred method of the present inventors is to use a gelling agent to immobilize the liquid sample before activation analysis. Fumed silica is one type of gelling agent that can be suitable for use in this method. The gelling agent can include superabsorbent polymers, polyacrylates, or polyacrylamides. Sodium polyacrylate can be added to the liquid sample and can solidify the sample in about a few seconds. Sodium acrylate is also non-toxic, relatively inexpensive, and readily available. As an example, adding 20 g of sodium polyacrylate to a 300 mL liquid sample causes the formation of a firm, non-flowing gel in less than a minute. The present inventors have confirmed that samples solidified in this manner can be exposed to intense X-ray radiation, heat, and vigorous movement without reverting to the liquid form. Once immobilized or gelled, the elements in the initially liquid sample can be subjected to activation analysis and easily measured using the same calibrations developed for solid materials. Only standard corrections for sample mass and radiation attenuation may be required.

[0056] It is desirable that any coagulant or gelling agent used to solidify the liquid sample does not contain any amount of one or more target elements that are the subject of the activation analysis. For example, the presence of target elements in the coagulant or gelling agent may cause an increase in the value of the concentration of that target element calculated by activation analysis compared to using a coagulant or gelling agent that does not contain the target element. It may also be desirable that the coagulant or gelling agent used does not contain any reference elements of the reference materials used to ensure that the normalization of the concentration of one or more target elements is not affected.

[0057] According to an embodiment of the present invention, a liquid sample is placed in a container. Then, the liquid sample is solidified / immobilized within the container. For example, a coagulant or gelling agent is added to the liquid sample in the container, such as discussed above, so that the sample will be solidified within the container. For the steps of activation by irradiation and then gamma-ray detection in the activation analysis process, the sample preferably remains in the same container.

[0058] As an example, the container can be a cylindrical plastic jar with a screw cap. A liquid sample can be placed into the container, and then the liquid sample can be immobilized, for example, by adding a coagulant or a gelling agent, and the cap can be screwed tightly onto the jar to contain the solidified liquid sample. Optionally, a sealing component, such as a pressure-sensitive disc or an induction-sealing foil, can be introduced under the screw cap, having a diameter substantially equal to the diameter of the mouth of the jar, to further contain the solidified sample. A jar with a volume of about 300 ml can contain up to about 500 g of a typical gold-bearing ore or about 300 g of a typical gold-bearing process solution. The diameter of the jar can be in the range of 50 mm to 100 mm, and the height of the jar can be in the range of 40 mm to 70 mm.

[0059] The PCT publication number WO2015 / 089580 (A1) of the present inventors relates to a method for rapid analysis of target elements in a sample via gamma activation analysis. The publication describes an improved accuracy in determining the concentration of a target element in a sample by irradiating the sample and a reference material containing a known amount of a reference element simultaneously with X-rays. Optionally, once solidified, a liquid sample according to the present invention can be subjected to an activation analysis process using the same or similar equipment as discussed in WO2015 / 089580 (A1). The entire content of WO2015 / 089580 (A1) is incorporated herein by reference.

[0060] The PCT publication number WO2022 / 047537 (A1) of the present inventors relates to improvements in gamma activation analysis measurements and describes methods and systems for determining the corrected concentration of one or more target elements in a sample by irradiating a reference material containing at least two reference elements simultaneously with X-rays. The variations in the activation rates of the at least two reference elements within a predetermined X-ray endpoint energy range are different from each other. Optionally, once solidified, a liquid sample according to the present invention can be subjected to an activation analysis process as described in WO2022 / 047537 (A1). The entire content of WO2022 / 047537 (A1) is incorporated herein by reference.

[0061] Figure 1 A schematic view of an apparatus 100 for gamma activation analysis of a sample 155 is shown. The sample 155 is a liquid sample that has been solidified, for example, by any suitable method as discussed above. According to this example, the target element is gold, but it can also be any other element. The apparatus includes a sample holder 120 for holding each sample 155.

[0062] The irradiation system 130 is used to irradiate the cured liquid sample 155, the detector system 140 is used to detect and quantify the intensity of characteristic decay products, and the transport system 150 is used to move the sample holder 120 between the irradiation system 130 and the measurement / detector system 140. The sample holder 120 holding the sample 155 is operable to shuttle or otherwise transport between the irradiation system 130 and the measurement / detector system 140. As noted above, the cured liquid sample 155 to undergo activation analysis can be packaged in a container, such as a cylindrical plastic jar with a screw cap.

[0063] The cured liquid sample material 155 is irradiated with X-rays. The sample 155 and / or the container holding the sample material 155 can have at least one flat surface. The processed material can be irradiated with X-rays through one of the flat surfaces. The irradiation system 130 includes a linear electron accelerator (LINAC) that is substantially enclosed within a radiation shielding device 110. The LINAC accelerates an electron beam to an energy of about 8 MeV, which then strikes a solid metal target 111 that converts the energy of the electrons into X-rays. The electrons then rapidly decelerate to produce a continuous energy spectrum of X-rays with a maximum energy corresponding to the energy of the electron beam. During the process of irradiating the sample, the position of the electron beam on the solid metal target can be scanned to maximize the uniformity of the X-ray flux passing through the sample container. The sample container is placed as conveniently close as possible to the outer surface of the X-ray conversion target.

[0064] The PCT publication number WO2018 / 232435 (A1) of the present applicant relates to an X-ray radiation shielding device. The shielding structures and devices described in WO2018 / 232435 (A1) can be utilized in the present method and system, including as the radiation shielding device 110. The entire content of WO2018 / 232435 (A1) is incorporated herein by reference.

[0065] When the sample 155 has been irradiated for a sufficient length of time, the irradiation system is turned off. Then, the sample holder 120 is rapidly moved to the detector system 140 for analysis by means of the transport system 150. The transport system 150 operates under the control of the control system 165. The control system 165, in turn, is under the control of a computer 180, which is also responsible for controlling the operation of the linear accelerator 130 and the gamma-ray detectors 170, 175.

[0066] Before activation, the liquid sample is solidified. The solidification of the liquid sample can occur at a solidification station (not shown). The sample can be held in a container, such as the cylindrical plastic jar mentioned above. Then, the liquid sample is solidified, such as by adding a coagulant or gelling agent in the manner previously mentioned. Then a lid or cap is fixed to the container. After a sufficient period of time for solidifying and immobilizing the liquid sample, the sample is moved to the irradiation system. The sample can be moved from the solidification system to the irradiation system by a transport system, which can be a part of or similar to the transport system 150 mentioned above. After solidification has occurred or during the solidification of the liquid sample, the sample can be held in a sample holder. The transport system 150 can be a shuttle system, which includes tracks and a carriage that travels on the tracks. Alternatively, the transport system 150 can utilize any suitable mobile device, including a pulley system or a conveyor belt system. According to other embodiments, the sample 155 can be solidified within the container and then manually or by a robot be manipulated into a position at the irradiation system, including being placed on the sample holder 120.

[0067] A pair of high-resolution detectors 170, 175 can be used to measure the activation of the sample 155. In other words, the detectors 170, 175 can measure the gamma rays emitted by the sample. The corresponding detectors 170, 175 can be cylindrical. Compared to the sample container, the detectors 170, 175 can have a similar or larger diameter and can be placed far enough apart to receive the sample container for measurements therebetween. In this embodiment, the detectors 170, 175 are large-area semiconductor devices with an FWHM resolution of 1.5 keV or better at 279 keV. It should be understood that other detectors known to those skilled in the art can be used, including but not limited to scintillation detectors. According to other embodiments, there can be a detector arrangement different from that shown in Figure 1 what is shown. For example, there can be only a single detector, or there can be more than two detectors, and / or the detectors can be in a shape other than cylindrical. In particular, the detector arrangement shown in Figure 1 with the detectors 170, 175 can be suitable for the detection of gamma rays emitted by both the sample 155 and the optional reference material 160. When the reference material 160 is not used, the detector arrangement utilized in the embodiments according to the present invention can be the same as or different from that shown in Figure 1 what is shown. When the reference material 160 is used, the detector arrangement utilized in the embodiments according to the present invention can also be the same as or different from that shown in Figure 1 what is shown.

[0068] To achieve high sensitivity, the sample can be conveniently measured for multiple cycles. Advantageously, the number of cycles can be chosen to be even, and the orientation of the sample container can be flipped by 180° between alternating cycles. Inevitably, the X-ray flux on the surface 155 of the sample closest to the target 111 is higher than the flux on the far side of the sample 155, and this causes a higher level of activation. Combining measurements made on the sample 155 in alternative orientations can improve accuracy by improving the uniformity of the measurement relative to the distribution of the target element (e.g., gold) within the sample 155.

[0069] The measurement, irradiation, and cooling times should be selected to give the highest possible accuracy within a given time. Simple analysis shows that this is achieved when the irradiation time and the measurement time are equal and the cooling time is as short as possible. Additionally, the accuracy shows a broad maximum when the measurement time and the cooling time are equal to approximately 2 or 3 times the half-life of the sample isotope. For gold, it is convenient to irradiate and measure the sample for approximately 15 to 20 seconds. The cooling time can be set by the rate at which the sample can be transferred from its corresponding position at the irradiation system to the detector / measurement system 140. Using a pneumatic or mechanical automatic transfer mechanism, this time can be shortened to approximately 2.5 seconds or less.

[0070] The device can be calibrated relative to a non-flowing and solid standard sample of accurately known concentration of the target element (e.g., gold). According to the present invention, the calibration value can be used for liquid samples of unknown concentration undergoing activation analysis without the requirement for recalibration of liquid standard samples.

[0071] The following description relates to embodiments using an optional reference material 160, such as is similarly described in WO2015 / 089580(A1). The following is generally non-limiting of the present invention, and it should be understood that the present invention is not limited to requiring simultaneous irradiation of the sample and the reference material.

[0072] The sample holder 120 can hold each sample 155 and an optional reference material 160. The optional reference material 160 contains at least one reference element. As described in WO2105 / 089580, it has been found useful to use bromine (Br) as the optional reference element in the case where gold is the target element in the sample. For example, compared with around 279 keV for gold, bromine has a gamma-ray peak at around 207 keV, which means there is no interference between the signal of bromine in the reference material and the signal of gold in the sample. Bromine has a half-life of 4.86 seconds, which is less than the half-life of gold at 7.73 seconds. Bromine is also relatively scarce in the earth's crust and is unlikely to be found in the sample. Conveniently, a stable bromide salt such as potassium bromide can be contained in an inert metal shell made of titanium, magnesium or similar materials. This means that the reference material can be reused repeatedly for a long time before replacement, thereby reducing the frequency at which the equipment needs to be recalibrated. Selenium (Se), erbium (Er) or iridium (Ir) can also be selected as the reference element.

[0073] The optional reference material 160 can take the form of a disc or circular wafer. The thickness of the reference material 160 can be from about 0.1 mm to 3.0 mm. In the case where the reference material 160 comprises a powder, it can be conveniently contained in a durable metal housing formed of a metal such as titanium or magnesium that does not undergo substantial activation reactions. The diameter of the reference material 160 can be less than or substantially similar to the diameter of the container that holds the sample. During irradiation and measurement, the reference material 160 can be positioned on a flat surface of the sample container such that the axis of the reference material 160 coincides with the axis of the container.

[0074] The sample holder 120 can be designed to hold the sample 155 and the reference material 160 in a releasable fixed relationship relative to each other. In this example, the optional reference material 160 has the form of a metal containing an appropriate amount of potassium bromide.

[0075] In an alternative embodiment, as Figure 1As shown, during the irradiation process, the reference material 160 is placed on the flat surface of the sample container facing the target 111. A pair of high-resolution detectors 170, 175 can be used to measure the activation of both the sample 155 and the reference material 160. In other words, the detectors 170, 175 measure the gamma rays emitted by both the sample and the reference material. The corresponding detectors 170, 175 can be cylindrical. Compared to the sample container, the detectors 170, 175 can have a similar or larger diameter and can be placed far enough away to receive the sample container and the reference material for measurements therebetween. In this embodiment, the detectors 170, 175 are large-area semiconductor devices and can have an FWHM resolution of 1.5 keV or better at 279 keV. It should be understood that other detectors known to those skilled in the art can be used, including but not limited to scintillation detectors.

[0076] The measurement of the intensity of the signal from the reference material 160 in the adjacent detector 170 provides a direct measurement of the number of gamma rays emitted by the reference element. The measurement of the intensity of the signal from the reference material 160 in the opposite detector 175 provides a measurement of the gamma ray attenuation in the sample, which can be used to supplement or replace the direct measurement of the mass of the sample, and a known function of the mass of the sample is required for the direct measurement of the mass of the sample to correct for the difference in attenuation of the reference element and the target element.

[0077] In another possible embodiment (not shown), during irradiation, an optional reference material 160 is placed on the flat surface of the sample container opposite the solid metal target 111. A single detector can be used to measure the activation of the sample material and the reference material. During the measurement, the sample 155 is positioned relative to the detector such that the reference material 160 is in close proximity to the detector. In this embodiment, it is necessary to correct for the attenuation of the reference material 160 before the main X-ray beam reaches it. This attenuation correction is small and mainly depends on the sample mass, and in a manner similar to calculating the function of the mass of the sample to correct for the difference in attenuation of the reference element and the target element, Monte Carlo or other computer codes can be used to estimate it.

[0078] There is a small correlation between the attenuation correction and the sample composition. In particular, samples such as copper concentrates containing high concentrations of heavy elements such as iron and copper attenuate higher-energy X-rays responsible for nuclear activation more strongly than light elements such as silicon and aluminum that form rocks. This correlation with the sample composition may introduce unwanted calibration biases.

[0079] However, with the reference material 160 positioned on the face of the sample 155 opposite the target 111, the X-rays activating the nuclei in the reference material must pass through the full thickness of the sample 155. In contrast, the X-rays exciting the nuclei in the sample 155 must pass through only about half of the sample thickness on average. If the reference material 160 is chosen such that the attenuation of the X-rays causing activation in said material varies less with sample composition than the attenuation of the X-rays causing activation in the sample, the correlation with sample composition can be eliminated. In particular, when the element being measured is gold and the reference element is bromine, for a wide range of sample compositions including carbon, silica, and high-grade copper concentrate, the relative activation rate of the sample 155 and the reference material 160 was found to vary by less than 0.2% with changes in composition. This may mean that a single calibration parameter can be applied to a wide range of different sample types. In any arrangement of the reference material 160 relative to the sample 155, if the diameter of the reference material 160 is substantially similar to or slightly less than the diameter of the sample, the normalized target element gamma-ray count rate and the reference signal also correct for the following: small displacements of the X-ray beam relative to the sample (due to variable positioning of the sample by the transport system or fluctuations in the operation of the LINAC 130), and displacement of the sample relative to one or more detectors during measurement. In essence, these displacements have a similar effect on both signals, and thus this potential source of error is also largely eliminated.

[0080] Furthermore, if the position of the reference material 160 is fixed relative to the sample 155, accidental displacements of the sample 155 and the reference material 155 relative to the target 111 or the detectors 170, 175 proportionally reduce the activation of both the reference element and gold in the sample. However, since the target element content is determined based on the ratio of activation levels, this reduction in activation is largely eliminated. In this way, the analysis can be made relatively less sensitive to inaccuracies in the positioning of the sample 155, and the accuracy can be improved and the requirements for the precision of the sample holder 120 and the transport system 150 can be reduced.

[0081] To achieve high sensitivity, it is convenient to measure the sample for multiple cycles. Advantageously, the number of cycles can be chosen to be even, and the orientation of the sample container can be flipped 180° between alternate cycles. Inevitably, the X-ray flux on the surface 155 of the sample closest to the target 111 is higher than the flux on the far side of the sample 155, and this causes a higher level of activation. Combining the measurements taken on the sample 155 in alternate orientations improves the accuracy by improving the uniformity of the measurement relative to the distribution of the target element (e.g., gold) within the sample 155.

[0082] The device is calibrated using immobile and solid standard samples with accurately known concentrations of the target element (e.g., gold). The signal of the target element in the sample of unknown concentration can be directly related to these standard calibration values via a constant signal from a reference material. It is expected that the same reference material can be used over a long period, limited only by eventual mechanical or radiation damage and possible loss of the reference element (e.g., bromine) from the reference material. When it becomes necessary to replace the reference material, the system can be recalibrated back to the immobile and solid standard samples. According to the present invention, the calibration values can be used for liquid samples of unknown concentration undergoing activation analysis without the requirement for recalibration of liquid standard samples.

[0083] According to an embodiment of the present invention, results can be rapidly obtained on-site for determination of the concentration of the target element within a liquid sample.

[0084] Those skilled in the art will appreciate that many variations and / or modifications can be made to the above embodiments without departing from the broad general scope of the present invention. For example, although the examples provided involve using gold as the target element, it should be noted that the present invention can also be used to determine the concentration of silver, copper, or any other valuable element in a sample.

[0085] The present invention can solve the problems of the prior art by adding a coagulant or gelling agent to the liquid sample before irradiation and measurement. By making the sample substantially solid so that the target atoms are immobile during irradiation, measurement, and sample transfer, any difficulties of the prior art can be avoided.

[0086] The overall activation rate and gamma-ray detection probability of the target element in the sample will depend on the macroscopic properties of the sample. That is, the density of the material and the attenuation cross-section for the incident radiation and the emitted gamma-ray radiation. According to the present invention, the instrument calibration developed for solid samples with given macroscopic properties can be applied to liquid samples with equivalent properties.

[0087] Furthermore, when the liquid sample has been solidified and immobilized, rupture of the container in which the sample is housed is less likely to result in spillage or loss of the contents. When containing any toxic, corrosive, acidic, or radioactive material in the liquid sample when it is solidified, this means it is less likely to cause contamination. A spilled solid sample may also be easier to detect and remove than a liquid sample.

[0088] During the selection of a suitable gelling or coagulating agent, one or more of the following properties can be considered:

[0089] a) The agent should not contain one or more of the target elements to be detected, or any element that will severely interfere with the measurement of one or more target elements during activation analysis;

[0090] b) The agent should have a high liquid fixation rate, which is defined as the mass of the liquid that can be gelled or solidified divided by the mass of the required agent;

[0091] c) The agent preferably should have a rapid solidification reaction rate, preferably such that it takes less than one minute or even more preferably a few seconds to solidify the sample;

[0092] d) The solidified or gelled sample should be stable and substantially unaffected by an increase in the sample temperature, radiation from an activation source, or any solutes present in the liquid sample;

[0093] e) The agent should be low-cost and easily obtainable;

[0094] f) The agent should have low or negligible toxicity; and / or

[0095] g) The solidified or gelled sample should have compatible physical properties with respect to ease of handling, disposal, and reuse in a container.

[0096] Example

[0097] According to the example, the concentration of gold in a liquid sample will be measured via gamma activation analysis. Gold exhibits an isomeric excitation reaction, which causes the formation of a short-lived metastable state within its nucleus, and this short-lived metastable state causes the emission of gamma rays when it relaxes to the ground state. The reaction of the gold nucleus can be excited using bremsstrahlung X-rays with endpoint energies in the range of 6 MeV to 9 MeV. The following sequence shows the effect of gold excited by high-energy X-rays:

[0098] 197 Au-> 197M Au(t 1 / 2 =7.73s)-> 197M Au + 279 keV gamma-ray (Au: gold, gamma-ray: gamma ray)

[0099] Adding 20 g of sodium polyacrylate to a 300 mL liquid solution of gold salt causes the formation of a firm gel that is substantially non-flowing. For gold concentrations up to 300 ppm and chloride ion concentrations up to 2 wt%, this proportion of sodium polyacrylate was found to produce an acceptable gel.

[0100] The chemical formula is (C3H3NaO2) nThe sodium polyacrylate contains carbon, hydrogen, sodium, and oxygen elements, and these elements do not undergo an excitation reaction from X-rays with endpoint energies in the range of 6 MeV to 9 MeV. The liquid fixation rate is 15, that is, one part of sodium polyacrylate to 15 parts of the liquid sample, which means that less mass of the gelling agent is required for each sample. The gelling reaction between the sodium polyacrylate and the liquid sample occurs rapidly, and the sample reaches the final cured state in less than one minute. The inventor has confirmed that the gel is stable at temperatures up to at least 60 °C, which is significantly higher than the temperatures that occur during the measurement process, and is also stable at X-ray doses up to 100 kGy, which is again significantly higher than the doses used during the irradiation process. Sodium polyacrylate is readily available, low-cost, and non-toxic. The gelled material can be easily removed from the sample container so that the container can be recycled or reused when needed.

[0101] Figure 2 The analytical results obtained for 6 liquid samples prepared from a certified standard gold solution are shown, that is, where the concentration of gold is accurately known. Samples with a volume of 300 mL were placed in cylindrical plastic cans and solidified using 20 g of sodium polyacrylate. Then, bremsstrahlung X-rays with an endpoint energy of 8.5 MeV were used as the excitation source, and the samples were measured via gamma activation analysis. A dual high-resolution germanium detector measured and counted the 279 keV gamma rays emitted by the activated gold nuclei in the samples. The gamma-ray measurements were calibrated using values taken from known solid samples. The measured values for the six different liquid samples were compared with the known gold concentrations, and the results are shown in Figure 2 . Excellent correlation and linearity were observed between the certified gold concentration and the measured gold concentration.

[0102] Although the invention has been described in connection with a limited number of embodiments, those skilled in the art will understand that, in light of the foregoing description, many alternatives, modifications, and variations are possible. Accordingly, the invention is intended to cover all such alternatives, modifications, and variations that may fall within the spirit and scope of the disclosed invention.

[0103] Any reference or discussion of any document, act, or item of knowledge in this specification is included solely for the purpose of providing context for the invention. No implication or representation is made that any one of these items, or any combination thereof, formed part of the common general knowledge at the priority date, or was known to be relevant to an attempt to solve any problem addressed by this specification.

[0104] In this specification, the terms "comprises", "comprising", "includes", "including", or similar terms are intended to mean non-exclusive inclusion, such that a method, system, or device that comprises a list of elements does not include only those elements but may also include other elements not listed.

Claims

1. A method for performing neutron or gamma activation analysis, comprising: providing a sample, at least part of which is liquid and contains at least one target element; solidifying the sample; irradiating the sample to activate the at least one target element within the sample; detecting, in a measurement, the number of gamma rays emitted by the at least one target element within the sample; and determining a value representing the concentration of the at least one target element in the sample by using the measurement of the gamma rays emitted by the at least one target element and a calibration determined based on a solid sample of known composition.

2. The method according to claim 1, wherein, Before solidifying, the sample is placed in a container, preferably where the sample is irradiated within the container.

3. The method according to claim 1 or 2, wherein The sample is irradiated by an X-ray source.

4. The method according to any one of the preceding claims, wherein, The at least partially liquid sample is solidified using a gelling agent or a coagulant.

5. The method according to claim 4, wherein The gelling agent or the coagulant comprises one or more of the following: superabsorbent polymer, polyacrylate, polyacrylamide, sodium polyacrylate, fumed silica, calcium sulfate, hemihydrate calcium sulfate, or cement.

6. The method according to any one of the preceding claims, further comprising: irradiating a reference material to activate a reference element within the reference material, the reference element having a known concentration within the reference material; detecting the number of gamma rays emitted by the reference element; and normalizing the measurement of the gamma rays emitted from the at least one target element by using the detected number of gamma rays emitted by the reference element, wherein determining the value representing the concentration of the at least one target element utilizes the normalized measurement.

7. The method according to claim 4, wherein The sample and the reference material are irradiated simultaneously, and / or wherein the sample and the reference material are positioned adjacent to each other before irradiation.

8. A method for preparing a sample that is at least partially liquid for neutron or gamma activation analysis, comprising: placing the at least partially liquid sample in a container, the sample containing at least one target element; and solidifying the sample within the container.

9. The method according to claim 8, wherein The at least partially liquid sample is solidified using a gelling agent or a coagulant.

10. The method according to claim 9, wherein, The gelling agent or the coagulant comprises one or more of the following: superabsorbent polymer, polyacrylate, polyacrylamide, sodium polyacrylate, fumed silica, calcium sulfate, hemihydrate calcium sulfate, or cement.

11. The method according to any one of the preceding claims, wherein, The at least partially liquid sample contains a slurry or a process solution from a mineral processing plant.

12. The method according to claim 2 or 8, wherein The container is substantially cylindrical and has a diameter in the range of about 50 mm to about 100 mm and a height in the range of about 40 mm to about 70 mm.

13. A system for performing neutron or gamma activation analysis on a sample that is at least partially liquid, the system comprising: depositing the at least partially liquid sample in a container, the at least partially liquid sample containing at least one target element; a solidification station at which the at least partially liquid sample in the container is solidified to form a solidified sample; an irradiation station at which the solidified sample is irradiated and the at least one target element undergoes activation; A detection station where the number of gamma rays emitted by one or more activated target elements in the cured sample is measured; and A computer system that uses the measured number of gamma rays emitted by the one or more target elements and a calibration determined from a solid sample of known composition to determine values representing the concentration of each target element within the sample.

14. The system according to claim 13, further comprising a sample holder adapted to hold the sample container.

15. The system according to claim 13 or 14, further comprising a sample transport device configured to move the sample between the curing station and the irradiation station and / or between the irradiation station and the detection station.

16. The system according to any one of claims 13 to 15, wherein, The at least partially liquid sample is cured by adding a gelling agent or a coagulant, preferably where the gelling agent or the coagulant comprises one or more of the following: superabsorbent polymers, polyacrylates, polyacrylamides, sodium polyacrylate, fumed silica, calcium sulfate, calcium sulfate hemihydrate, or cement.

17. The system according to any one of claims 13 to 16, further comprising a reference material comprising a reference element, the reference material being positioned adjacent to the sample container when at the irradiation station and when at the detection station, and wherein, The computing system utilizes measurements of gamma rays emitted by the reference element in determining the values representing the concentration of each target element within the sample 18. The system according to any one of claims 13 to 17, further comprising a computer configured to control the operation of at least one of the curing station, the irradiation station, the detection station, and the movement of the sample between each station.

19. The system according to any one of claims 13 to 18, wherein, The irradiation station includes: a linear accelerator that generates a high-energy electron beam, and the solid metal target that generates X-rays when the electron beam strikes the surface of the solid metal target.

20. The system according to any one of claims 13 to 19, wherein, The detection system includes at least one radiation detector, preferably where one or more of the following: the at least one detector is a high-resolution semiconductor detector or a scintillation detector; the at least one detector includes two detectors configured to be positioned on opposite sides of the sample container in use; and / or the detector or each detector is cylindrical.

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