Patent Grant
10748753
This application claims priority to European Patent Application Serial No. 18177774.9, filed Jun. 14, 2018, tilted “Accelerator Mass Spectrometry System and Associated Method,” the entire contents of which application are incorporated herein by reference.
The present invention relates to an accelerator mass spectrometry system for measuring an isotopic ratio of a chemical element in a sample. Furthermore, the present invention relates to a method for measuring an isotopic ratio of a chemical element in a sample.
Since the early development of accelerator mass spectrometry in 1977, accelerator mass spectrometers developed into a mature analytic tool for the measurement of extremely low concentrations of long-lived radio-isotopes. Best examples of radio-isotopes that found their application in research fields like archeology, geoscience, environmental science and biomedicine, include 3H, 10Be, 14C, 26Al, 36Cl, 41Ca, 59Ni, 129I and 239Pu.
Historically, the vast majority of accelerators used in accelerator mass spectrometry are tandem accelerators. Although other types of accelerators are sometimes applied, including single-ended accelerators, cyclotrons and linear accelerators, tandem accelerators are the first choice because of their widespread availability, versatility and the high precision that can be achieved with these accelerators.
Known accelerator mass spectrometry systems that use a tandem accelerator comprise an ion source, in which atoms from solid or gaseous sample material are negatively ionized. The negative ions are extracted from the ion source in an electrostatic field region and form a stream of ions, referred to as “ion beam”. The extracted negative ion beam contains the various isotopes of the element of interest. Usually, one of these isotopes is a long-lived radioisotope having an extremely low natural abundance, typically 1010 to 1016 times lower than the stable isotopes. For some elements, including carbon, several stable isotopes are present. Such stable isotope(s) are usually measured in a Faraday cup(s) with intensities of nano-amperes or micro-amperes. However, the concentration of radioisotopes is usually so low that individual ions of these isotopes are commonly counted one after another using a suitable particle detector.
Current systems available on the market include dedicated systems tailored for only one radioisotope (mostly 14C) as well as versatile systems that are capable of analyzing a variety of different radioisotopes with the same instrument. Despite the fact that these systems are carefully designed (they often comprise multiple filter stages, each separating specific interferences from the radioisotope of interest), it is very well possible that unintentionally other particles than the radioisotope of interest make it into the particle detector where they mimic the radioisotopes and increase the background level of the instrument. Since the concentration ratio between a radioisotope and a stable isotope can be as low as 10−16, it is required that current systems perform very well in term of analysis precision and background level reduction.
Reducing the background level of the instrument can be realized by adding one or more filter stages to the system. However, by doing so, costs, complexity and footprint of the accelerator mass spectrometry system are significantly increased.
It is therefore an object of the invention to provide an accelerator mass spectrometry system that effectively reduces background level in an efficient and cost-effective manner.
According to a first aspect, the invention provides hereto an accelerator mass spectrometry system according to the preamble of claim 1, characterized by an additional analyzer located in between the charge stripping section and the second accelerating section, which is configured to receive positive ions that have exited the charge stripping section and to separate positive ions having a charge state corresponding to a predetermined charge-state value from positive ions having a charge state not corresponding to the predetermined charge-state value, so as to transmit ions with different charge states in mutually different directions such that only ions having a charge state corresponding to the predetermined charge-state value are transmitted towards the particle detector. In this way, the additional analyzer serves as a charge-state selector, in which interfering ions, i.e. background-level increasing ions, that have a different charge-state than that of the first and second isotopes, i.e. the isotopes of interest, have a different bending radius and are consequently swept out of the path of the isotopes of interest. Because the relative difference between the two charge states, e.g. 1+ versus 2+, or 2+ versus 3+, et cetera, is large, the bending angle of additional analyzer may be relatively small in comparison to other analyzers in the accelerator mass spectrometry system, while still providing the required background reduction. The reason for this is that the other analyzers of the accelerator mass spectrometry system need to resolve one isotope from the other, which is a relative small difference and therefore requires a larger bending angle compared to mere charge-state selection. The location of the additional analyzer between the charge stripping section and the second acceleration section is optimal, since this is the only location where the interfering ions that need to be suppressed can be relatively easily discriminated from the first and second isotopes, i.e. the isotopes of interest. As a result, the additional analyzer, being a small analyzer having a small bending angle located in between the charge stripping section and the second accelerating section, is capable of effectively reducing the background level of the accelerator mass spectrometry system, while keeping an increase in costs, complexity and footprint of the system small.
In a preferred embodiment, the accelerator mass spectrometry system further comprises a Faraday cup configured to measure a quantity of ions of only one of the first and second isotopes, said one of the first and second isotopes being different from the one of the first and second isotopes that enter the particle detector. In this way, an isotopic ratio between a radioisotope and a stable isotope of a chemical element of interest in a sample can be determined by measuring ions of the stable isotope in the Faraday cup with intensities of nano-amperes or micro-amperes, while measuring individual ions of the radioisotope with the particle detector. The isotopic ratio can subsequently be determined from the measurement data of the Faraday cup and the measurement data of the particle detector. Preferably, the Faraday cup is arranged after the second mass analyzer.
In a preferred embodiment, the additional analyzer comprises a magnetic analyzer or an electrostatic analyzer. Essentially, both magnetic and electrostatic versions of the additional analyzer will be equally suitable as long as they provide different bending radii for the different charge-states. Preferably, the magnetic analyzer comprises a dipole magnet and the electrostatic analyzer comprises an electrostatic deflector.
In a preferred embodiment, the dipole magnet and the electrostatic deflector have a bending angle of 30 degrees or less. In the case that the additional analyzer is in the form of a dipole magnet, the additional analyzer will have slight different bending radii for the different isotopes of interest, which asks for a careful design of the second mass analyzer and the subsequent Faraday cups. Since the required bending angle of the additional analyzer can be relatively small and at the same time eliminates the dominant contribution to the background, it provides an efficient and cost-effective manner to surpass limitations on background interference of existing instrumentation. Preferably, the bending angle is between 5 and 20 degrees. More preferably, the bending angle is approximately 12 degrees.
In a preferred embodiment, the accelerator mass spectrometry system is configured for a measurement of a quantity of radiocarbon in the sample, wherein the additional analyzer is configured to select charge state 1+. In this specific configuration adapted to the measurement of radiocarbon (i.e. 14C) using charge state 1+, the bending angle can be smaller than 30 degrees, or even as small as approximately 12 degrees, so that the accelerator mass spectrometry system is eminently suitable for such measurement. Such bending angles are sufficiently large to effectively remove the otherwise interfering ions. The accelerator mass spectrometry system therewith provides an efficient and cost-effective tool to surpass limitations on background interference of existing instrumentation.
According to a second aspect, the invention provides a method according to the preamble of claim 10, characterized by subjecting the positive ions that have exited the charge stripping section to an additional analyzer, located in between the charge stripping section and the second accelerating section, to separate positive ions having a charge state corresponding to a predetermined charge-state value from positive ions having a charge state not corresponding to the predetermined charge-state value, so as to transmit ions with different charge states in mutually different directions such that only ions having a charge state corresponding to the predetermined charge-state value are transmitted towards the particle detector. In this way, the additional analyzer serves as a charge-state selector, in which interfering ions, i.e. background-level increasing ions, that have a different charge-state than that of the first and second isotopes, i.e. the isotopes of interest, have a different bending radius and are consequently swept out of the path of the isotopes of interest. Because the relative difference between the two charge states, e.g. 1+ versus 2+, or 2+ versus 3+, et cetera, is large, the bending angle of additional analyzer may be relatively small in comparison to other analyzers in the accelerator mass spectrometry system, while still providing the required background reduction. The reason for this is that the other analyzers of the accelerator mass spectrometry system need to resolve one isotope from the other, which is a relative small difference and therefore requires a larger bending angle compared to mere charge-state selection. The location of the additional analyzer between the charge stripping section and the second acceleration section is optimal, since this is the only location where the interfering ions that need to be suppressed can be relatively easily discriminated from the first and second isotopes, i.e. the isotopes of interest. As a result, the additional analyzer, being a small analyzer having a small bending angle located in between the charge stripping section and the second accelerating section, is capable of effectively reducing the background level of the accelerator mass spectrometry system, while keeping an increase in costs, complexity and footprint of the system small.
In a preferred embodiment, the method further comprises a step of measuring a second quantity of ions of only one of the first and second isotopes, said one of the first and second isotopes being different from the one of the first and second isotopes that enter the particle detector. In this way, an isotopic ratio between a radioisotope and a stable isotope of a chemical element of interest in a sample can be determined from a measurement of a quantity of ions of the stable isotope and a measurement of a quantity of ions of the radioisotope.
In a preferred embodiment, the step of measuring the second quantity is performed after the second mass analyzer. Essentially, both magnetic and electrostatic versions of the additional analyzer will be equally suitable as long as they provide different bending radii for the different charge-states. Preferably, the magnetic analyzer comprises a dipole magnet and the electrostatic analyzer comprises an electrostatic deflector.
In a preferred embodiment, the step of separating positive ions having a predetermined charge state from positive ions having a charge state corresponding to a predetermined charge-state value from positive ions having a charge state different from the predetermined charge-state value comprises deflecting particle trajectories of the positive ions over a bending angle of 30 degrees or less. In the case that additional analyzer is in the form of a dipole magnet, the additional analyzer will have slight different bending radii for the different isotopes of interest, which asks for a careful design of the second mass analyzer and the subsequent Faraday cups. Since the required bending angle of the additional analyzer can be relatively small and at the same time eliminates the dominant contribution to the background, it provides an efficient and cost-effective manner to surpass limitations on background interference of existing instrumentation. Preferably, the bending angle is between 5 and 20 degrees. More preferably, the bending angle is approximately 12 degrees.
In a preferred embodiment, the method steps are configured to measure a quantity of radiocarbon in the sample, wherein the step of separating positive ions having a charge state corresponding to a predetermined charge-state value from positive ions having a charge state different from the predetermined charge-state value is configured to select charge state 1+. In this specific configuration adapted to the measurement of radiocarbon (i.e. 14C) using charge state 1+, the bending angle can be smaller than 30 degrees, or even as small as approximately 12 degrees, so that the method is eminently suitable for such measurement. Such bending angles are sufficiently large to effectively remove the otherwise interfering ions. The method therewith provides an efficient and cost-effective manner to surpass limitations on background interference of existing instrumentation.
The present invention is further elucidated on the basis of the following figures which show preferred embodiments thereof and are not intended to limit the scope of protection of the invention in any way, wherein:
Referring to
After the charge-exchange in the stripper, the positive ions are accelerated back towards ground potential in a second acceleration section 10, after which they leave the tandem accelerator towards a second analyzer section 18, comprising a second mass analyzer 11 and an electrostatic analyzer 13. The second mass analyzer 11 directs e.g. the stable isotope ions into individual Faraday cups 12, whereas e.g. the radioisotope ions are subsequently further analyzed in the electrostatic analyzer 13. The electrostatic analyzer 13 serves to exclusively transmit those ions with the correct energy-to-charge ratio, thereby blocking those interfering ions that have unintentionally passed the second mass analyzer. Finally, the ions enter a particle detector 14 that discriminates, within limits, between bona-fide radioisotopes of interest and interfering ions. However, its resolving power is limited and therefore some interfering ions may be incorrectly identified as bona-fide radioisotope particles.
Finally, the data acquisition of the accelerator mass spectrometry system collects the data from the Faraday cups 12 and the particle detector 14 and from this the isotopic ratio is determined.
In contrast to the instrument of
The additional analyzer 15 serves as a charge-state selector, in which interfering ions 16 that have a different charge-state than that of an isotope of interest, such as a radioisotope, have a different bending radius and are consequently swept out of the path of the isotopes of interest 17.
In particular, the additional analyzer 15 enables improving accelerator mass spectrometry system performance in terms of background level in measurements of radiocarbon quantities. A known and cumbersome background source in the measurement of radiocarbon (i.e. 14C) quantities results from the injection of the interfering molecule of 13CH− into the tandem accelerator 4, along with the wanted 14C−. In the stripping process, the 13CH− is dissociated and charge-exchanged into a positive charge state. A background problem arises when the charge state of the 13C is one unit higher (for example 2+, instead of the 1+) than that of the 14C of interest and a subsequent unintended charge-exchange from 2+ to 1+ of the 13C ions takes place at a very specific location inside the second acceleration section 10. This may result in an energy of the 13C ions that is a little higher than the reference 14C ions so that the 13C interfering ions follow essentially the same path in the second mass analyzer 11 as the 14C of interest. Although the electrostatic analyzer 13 that follows is designed to block such interferences that have incorrect energy, its resolving power is insufficient to suppress the interfering 13C completely. It is known that by just one single scattering event on one of the electrodes of the electrostatic analyzer 13 a very small but noticeable fraction of the interfering 13C, in the order of 10−5, can reach the particle detector 14, adding to the background level. A comparable interference arises when the unintended charge-exchange from 2+ to 1+ of the 13C takes place at a different, but again a very specific location inside the second acceleration section 10, in which case it is the second mass analyzer 11 that has insufficient resolving power to suppress the interfering 13C completely.
The class of background described above is characterized in that it originates from particles that leave the stripper 8 having a charge state that is one unit higher than that of the radioisotope of interest in combination with a charge-exchange of the corresponding particle in the second acceleration section 10. It is generally accepted that this class of background is the main contributor to the overall background level of today's 14C analyzing accelerator mass spectrometry systems. The additional analyzer 15 directly behind the stripper 8 enables sweeping out interfering 13C ions that have a different charge-state than that of 14C from the path of the 14C ions due to the bending radius of the 13C ions being different from the bending radius of the 14C ions, because of their different charge state.
Because the relative difference between the two charge states, e.g. 1+ versus 2+, or 2+ versus 3+, et cetera, is large, the bending angle of additional analyzer 15 may be relatively small in comparison to other analyzers in the accelerator mass spectrometry system, while still providing the required background reduction. The reason for this is that the other analyzers need to resolve one isotope from the other, which is a relative small difference and therefore requires a larger bending angle compared to mere charge-state selection. The location of the additional analyzer 15 between the stripper 8 and second acceleration section 10 is optimal, since this is the only location where the interfering ions that need to be suppressed can be relatively easily discriminated from the isotope of interest. As a result, a compact additional analyzer 15 having a small bending angle located in between the stripper 8 and the second accelerating section 10 is capable of effectively reducing the background level of the accelerator mass spectrometry system, while keeping an increase in costs, complexity and footprint of the system small.
The additional analyzer 15 can be in the form of a magnetic analyzer or dipole magnet, or alternatively in the form of an electrostatic analyzer. Both magnetic and electrostatic versions of the additional analyzer 15 will be essentially equally suitable as long as they provide different bending radii for the different charge-states. The bending angle of the additional analyzer 15 should be sufficiently large to effectively remove the otherwise interfering ions 16. In practice and in the case of an accelerator mass spectrometry system that is specifically designed for 14C analysis using charge state 1+, it turns out that the bending angle can be smaller than 30 degrees, or even as small as approximately 12 degrees. In the case that additional analyzer 15 is in the form of a dipole magnet, the additional analyzer 15 will have slight different bending radii for the different isotopes of interest 17, which is also indicated in
The present invention is by no means limited to the above described preferred embodiments thereof. The rights claimed are determined by the appended claims within the scope of which many modifications are conceivable.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18177774 | Jun 2018 | EP | regional |
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| Number | Date | Country | |
|---|---|---|---|
| 20190385830 A1 | Dec 2019 | US |
