Patent Application
20050279933
Aspects of the invention generally relate to mass spectrometers and methods of increasing dispersion between ion beams.
Isotopic analysis of materials provides increased amount of information relative to information generated by traditional chemical analyses. Although qualitative and quantitative structural analyses identify the chemical composition of a compound or individual molecules of the compound, isotopic analysis provides additional information regarding the source, origin and formation of such compounds and molecules.
Mass spectrometers are well known and are used for wide ranging applications, such as isotope ratio monitoring, chemical analysis ranging from environmental analysis (e.g., detection of poisons) to the analysis of petroleum products, tracing of metals and biological materials. Mass spectrometers produce charged particles (e.g., ions) from chemical substances that are to be analyzed. After producing the ions, the mass spectrometers use electric and magnetic fields to measure the mass of the ions for isotope ratio monitoring.
Mass spectrometers are generally described in U.S. Pat. No. 4,638,160 to Soldzian et al. and U.S. Pat. No. 5,194,732 to Bateman, both of which are incorporated herein by reference. Mass spectrometers manufactured by Cameca are disclosed at www.cameca.fr, mass spectrometers manufactured by GV Instruments are disclosed at www.gvinstruments.co.uk, and mass spectrometers manufactured by Thermo Electron Co. are disclosed at www.thermo.com
Design and construction of a mass spectrometer with high sensitivity to measure isotope ratios require compromises in design and construction. High absolute sensitivity and high abundance sensitivity are required to make isotope ratio measurements of elements with wide (e.g., 108) isotope ratios. In order to make such measurements with an extremely small sample, it is necessary to simultaneously measure the isotopes.
For example, a wide dynamic range is required to determine weapon yield using ratios of 242Pu and 244Pu to 239Pu, and tailing from the major peak at 239 onto the small peaks must be limited (high abundance sensitivity) in order to make a meaningful measurement.
Samples having smaller sizes may produce signals with meaningful intensities for only a short period of time (e.g., minute or less). Signal intensity typically changes rapidly under such circumstances. Scanning mass spectrometers that can only measure one isotope at a time are at a disadvantage under these circumstances, since the signals from the isotopes of interest may have to be interpolated to obtain isotope ratios.
Prior mass spectrometers manufactured by such entities as Thermo Finnegan and GV Instruments use arrays of Faraday cups and are configured with miniaturized channeltron multipliers for pulse counting. Such channeltron multipliers have high background counts and no more than 70% efficiency. The high background counts tend to limit sensitivity. Mass spectrometers made by the above-noted entities do not have sufficient dispersion between adjacent isotopes to accommodate full-sized multipliers that have 100% efficiency and background levels of about 3 counts/minute.
Instruments used for isotope ratio measurements typically had a single magnetic sector. Such instruments operated in the scanning or peak stepping mode and were not practical to set up to collect an entire U or Pu spectrum simultaneously.
Prior approaches necessitate use of miniaturized detectors that are less than 100% efficient and have a high background noise level. Individual ion beams cannot readily be separated far enough apart to allow use of full sized Faraday cups or discrete dynode pulse counting detectors for each separated beam with existing approaches.
a shows a schematic of a prior art commercial isotope ratio mass spectrometer having an ion source 202 that generates a beam of ions 204 that are dispersed by a magnetic sector 206 into a plurality of beams B according to their mass to charge ratios. Beams B are simultaneously focused by the magnetic sector 206 into multiple miniature faraday cup collectors C, with one of the beams being focused into a miniature electron multiplier C1.
Aspects of the invention generally relate to high dispersion mass spectrometers and methods of increasing dispersion between adjacent ion beams. Aspects of the invention relate to a mass spectrometer having sufficient dispersion to accommodate full-sized discrete dynode multipliers for simultaneously measuring adjacent isotopes.
Aspects of the invention also relate to a mass spectrometer configured to separate individual ion beams by multiple centimeters to enable the use of high efficiency and low-noise detectors.
In one aspect, a mass spectrometer includes a magnetic sector configured to separate a plurality of ion beams, and an electrostatic sector configured to receive the plurality of ion beams from the magnetic sector and increase separation between the ion beams, the electrostatic sector being used as a dispersive element following magnetic separation of the plurality of ion beams. The dispersive element herein after referred to as the electrostatic dispersion lens (EDL).
In another aspect, a mass spectrometer includes a first device configured to separate a plurality of ion beams of a sample, and a second device configured to receive the plurality of ion beams from the first device and to increase separation between the ion beams for simultaneously measuring the plurality of ion beams, the increased separation enabling a plurality of isotopes of the sample to be simultaneously measured.
In yet another aspect, a mass spectrometer for measuring isotope ratios of elements of a sample includes an ion source configured to produce a plurality of ion beams from the sample, a magnetic sector having an exit, and having an entrance positioned to receive the plurality of ion beams from the ion source. The magnetic sector is configured to separate the plurality of ion beams using magnetic separation into individual ion beams, one of the individual ion beams being separated from a second one of the individual ion beams at the exit of the magnetic sector by a first distance. The mass spectrometer also includes an electrostatic sector having an exit, and having an entrance configured to simultaneously receive the plurality of ion beams from the magnetic sector. The electrostatic sector is configured as an EDL to produce an increased separation between the adjacent ion beams, one of the ion beams being separated from another one of the ion beams by a second distance, greater than the first distance, following the exit of the electrostatic sector. The electrostatic sector is used as a dispersive element, following the magnetic separation of the plurality of ion beams, to achieve the increased separation. The mass spectrometer also includes a plurality of deflection electrostatic sectors individually configured to receive a separated ion beam from the electrostatic sector and to further increase the separation between the adjacent ion beams, and a plurality of detectors, each of the detectors associated with a respective deflection electrostatic sector of the plurality of deflection electrostatic sectors. Each of the plurality of ion beams enters the electrostatic sector at a different physical location and wherein the beams are dispersed at different angles upon exiting the electrostatic sector. The electrostatic sector produces increased dispersion of each of the plurality of ion beams exiting the electrostatic sector for simultaneously measuring isotopes of the sample.
In a further aspect, a method of increasing separation between ion beams in a mass spectrometer includes receiving a plurality of ion beams of a sample, magnetically separating the plurality of ion beams, simultaneously receiving the magnetically separated ion beams in an electrostatic sector, and increasing the separation between the ion beams using the electrostatic sector, the electrostatic sector being used as a dispersive element following the magnetic separation of the ion beams.
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
a show schematics of prior art mass spectrometers.
a-5f illustrate dispersion between mass separated beams as a function of separation of the beams at the entrance of the electrostatic sector in accordance with various embodiments of the invention.
This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).
The ion source 302 is configured to provide stable ion currents. The beams of ions 304 generated by the ion source 302 are focused and accelerated using an ion gun (e.g., univoltage ion gun). For example, the ion source 302 may comprise Li-zeolite powder that is pressed into a platinum tube (not shown) which is spot welded to small diameter rhenium wire mounts configured to serve as a heater. Heating of the platinum tube results in emission of a beam of Li ions (e.g., beams 304). Alignment of the beams of ions 304 with the magnetic sector 306 may be accomplished mechanically. Such details are not relevant to the invention and are therefore not discussed in detail here.
The magnetic sector 306 includes a magnet whose included angle results in a magnetic field that maintains stigmatic focusing of the beams of ions 304. In one example, the included angle of the magnetic sector 306 may be 54 degrees. In one case, the inventors have observed that for a magnetic field strength of about 4.15 kG, the magnetic sector 306 mass separated 6Li+ from 7Li+ at an energy of about 1600 electron volts.
The magnetic sector 306 has non-normal entrance and exit shims to provide Z-focusing. For example, if a plutonium sample is used, the ion beams having 238-244 isotopes may be generated and the magnetic sector 306 mass separates the 238-244 isotopes of the plutonium sample. A physical beam slit “S” (
The electrostatic sector 308 is configured as an EDL to provide magnified angular dispersion for the mass separated ion beams 305, 307 that are received from the magnetic sector 306. The electrostatic sector 308 includes electrodes (e.g., two at least generally right-cylinder shaped electrodes) held at opposite potentials. Further details of the electrostatic dispersion lens 308 are described with reference to
The Faraday cup collector 312 includes a secondary electron suppression grid and ground shield (not shown) and is used to measure beam current of the ion beams 309, 310 exiting the electrostatic sector 308.
A multichannel plate detector may be coupled to the screen 314 (e.g., phosphor screen), that retains spatial information, via a fiber optic bundle. In one example, the inventors have conducted measurements by adjusting the voltage of the ion source 302 such that both the 6Li+ from 7Li+ ion beams were visible on the screen 314. The beam current was measured with the Faraday cup detector 312 as a function of lateral position. Such measurement enables both the individual width of the beams (e.g., 309, 310) and their relative spacing (e.g., dispersion) to be determined. The Faraday cup measurements were made using a Keithley model electrometer connected to a computer system 316 having a processor 318 and a memory or storage device 320. A data acquisition program embodied in the computer system 316 was used to record the electrometer signals as a function of the Faraday cup position. Typical ion currents for 7Li+ were observed to be in the range of 50-100 pA. Residual gas pressure during the measurement was observed to be 3×10−6 Torr.
In the embodiment of
The ion beams 305, 307 after passing through the slit “S” simultaneously enter the electrostatic sector 308 at different spatial positions. As the ion beams 305, 307 enter the electrostatic sector 308 at different spatial positions, they follow different trajectories through the electrostatic sector 308 and are further dispersed (e.g., separated relative to adjacent ion beams) on exiting the electrostatic sector 308. The dispersed ion beams are shown using reference numerals 309, 310. As noted above, the angular dispersion between the ion beams 309, 310 that exit the electrostatic sector 308 is greater than the angular dispersion between the ion beams 305, 307 that enter the electrostatic sector 308.
The dispersion of the ion beams 309, 310 increases with distance as the beams move away from the electrostatic sector 308. At a predetermined distance “d” from the exit portion of the electrostatic sector 308, the space between the ion beams 309, 310 increases to a point where each of such ion beams can be deflected using a deflection electrostatic sector (e.g., 402, 404) to be received by a discrete-dynode multiplier. Such further dispersion provides sufficient space for configuring individual detectors (e.g., 403, 405) for each isotope of a sample and an additional filter against scattered ions to maintain high abundance sensitivity while permitting simultaneous detection of all of the isotopes of the sample.
The deflection sectors 406, 408 may be configured as miniature versions of the electrostatic sector 308, the details of which have been described above with reference to
The number of deflection sectors (e.g., 406, 408) and the detectors (e.g., 410, 412) are shown to be merely exemplary. As such, more or less number of deflection sectors and detectors are possible and such may be configured to be proportional to the number of ion beams generated by the ion source 302. The embodiment of the mass spectrometer shown in
The electrostatic sector 308 acts as a dispersing lens, rather than a focusing energy filter, in order to magnify or increase the separation between adjacent ion beams (e.g., ion beams 305, 307). The magnified dispersion enables the individual ion beams (e.g., ion beams 305, 307) to be deflected to individual detectors (e.g., 410, 412) thereby enabling such individual ion beams to be separately measured with increased precision.
The position and the included angle of the electrostatic sector 308 may be varied to increase the performance of the mass spectrometer 400. The mass spectrometer 400 may be used with other samples (e.g., Uranium) by changing the magnetic field (e.g., to move the mass from 239 to 233 with other masses moving proportionally). If the masses are sufficiently similar, then the spacing between the collectors (e.g., detectors 410, 412) may be left unchanged. For example, in the case of adapting the mass spectrometer from Pu to U, the spacing between the collectors (e.g., 410, 412) may not have to be altered.
a-5f illustrate dispersion between mass separated beams, in the electrostatic sector shown in
Referring to
Since the individual ion beams are diverging after the focal plane of the magnetic sector (e.g., broadened) as well, in some embodiments it is preferred to place the electrostatic sector 308 in a position where the ratio of the ion beam separation to ion beam width is the greatest and the ion beam angular divergence is low. In some embodiments, the ion beams focus just prior to the entrance 502 to the electrostatic sector 308.
In some embodiments, for a constant gap width between plates 504 and 506 of the electrostatic sector 308, the radius of the electrostatic sector 308 formed by the plates 504, 406 is inversely proportional to the dispersion, for a given separation between the ion beams (e.g., 305, 307)
Referring to
The gap width “w” between the ion beams was found to have an effect on the beam width when the ion beams (e.g., 305, 307) entering the electrostatic sector 308 have an angular divergence and focus prior to their entry into the electrostatic sector 308. Such is demonstrated in
As shown in
Other features that are relevant to the design of the electrostatic sector 308 include height-to-width ratio of the gap width “w”. For example, for an electrostatic sector that having a height-to-width ratio of 5, and a beam height to gap ratio of 1/10, the electrostatic sector may be offset by +/−1 beam height with no significant distortion. Thus, in some embodiments, the ability to align the electrostatic sector's vertical centerline is evaluated in order to configure it at a height that would accommodate the expected beam size and positioning accuracy.
Aspects of the invention offer various advantages, which in some embodiments include using a Z-focusing magnet, simultaneous detection of multiple isotopes with full-sized, high efficiency multipliers that are fully shielded in separate chambers, high transmission efficiency from the ion source to the detector chambers, high abundance sensitivity, and high sensitivity. Other advantages include ability to employ the total evaporation method without any peak jumping, and the ability to make the measurements with a small sample.
Advantages of the wide dispersion design of the mass spectrometer as described above in some embodiments and applicable to scanning triple sector instruments include simultaneous detection of all relevant isotopes. For example, if there are six isotopes being measured, simultaneous ion counting of all six isotopes provides more than six times sensitivity corresponding to the time expended in measuring individual isotopes. The sensitivity enhancement is more than six due to the settling time required between peak steps.
Advantages of various other aspects of the invention as applied to large magnet multi-sector instruments include providing adequate space for complete shielding between individual dynode multipliers in order to minimize stray ions and electrons from interfering with the measurement of minor isotopes. In prior approaches, such stray ions and electrons were found to decrease the abundance sensitivity of the instruments. The wide dispersion design of the various aspects of the invention provides a relatively short flight path coupled with the energy filtering inherent in the small electrostatic sector at the entrance to each detector chamber, thereby providing abundance sensitivity on the order of 106, for example.
In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.
This invention was made with Government support under Contract No. DE-AC07-991D13727 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
