Reflection mode dynode

Abstract

A device configured to convert or amplify a particle, the conversion or amplification being reliant on the impact of a particle on a surface of the device causing emission of one or more secondary electrons from the same surface. The device includes a carbon-based layer capable of secondary electron emission upon impact of a particle. The surface may be used to convert, for example, an ion into an electron signal, or an electron signal into an amplified electron signal, such as in conversion or amplification dynodes.

Claims

1. An ion detector comprising: a space to admit a particle for detection, and a device configured to convert or amplify an admitted particle in reflection mode, the conversion or amplification being reliant on an impact of the admitted particle on a surface of the device causing emission of one or more secondary electrons from the same surface, wherein the device comprises a polycrystalline carbon-based layer capable of secondary electron emission upon impact of a particle, wherein the polycrystalline carbon-based layer comprises a dopant and is hydrogen terminated on its surface, and wherein the device provides a secondary electron yield of at least 5 electrons for incident energies of at least about 200 eV with secondary electron yield increasing over incident energies between about 200 eV and about 600 eV.

2. The ion detector of claim 1, wherein the device is a conversion dynode or an amplification dynode or a microchannel wafer or a microchannel plate.

3. The ion detector of claim 1, wherein the carbon-based layer is a diamond layer or a diamond-like carbon layer.

4. The ion detector of claim 1, wherein the dopant is boron.

5. The ion detector of claim 1, wherein the substrate is a metal.

6. The ion detector of claim 5, wherein the metal is a transition metal.

7. A mass spectrometer comprising: a mass analyzer; and an ion detector comprising: a space to admit a particle for detection, and a device configured to convert or amplify an admitted particle in reflection mode, the conversion or amplification being reliant on an impact of the admitted particle on a surface of the device causing emission of one or more secondary electrons from the same surface, wherein the device comprises a polycrystalline carbon-based layer capable of secondary electron emission upon impact of a particle, wherein the polycrystalline carbon-based layer comprises a dopant and is hydrogen terminated on its surface, and wherein the device provide a secondary electron yield of at least 5 electrons for incident energies of at least about 200 eV with secondary electron yield increasing for incident energies between about 200 eV and about 600 eV.

8. A method comprising: providing a device configured to convert or amplify an admitted particle in reflection mode, the conversion or amplification being reliant on an impact of the admitted particle on a surface of the device causing emission of one or more secondary electrons from the same surface, wherein the device comprises a polycrystalline carbon-based layer capable of secondary electron emission upon impact of a particle, wherein the polycrystalline carbon-based layer comprises a dopant and is hydrogen terminated on its surface, and operating the device at an incident energy of at least about 200 eV to provide a secondary electron yield of at least about 5 electrons.

9. The method of claim 8, wherein the carbon-based layer is a diamond layer or a diamond-like carbon layer.

10. The method of claim 8, wherein the dopant is boron.

11. The method of claim 8, comprising operating the device at an incident energy of at least about 400 eV to provide a secondary electron yield of at least about 8 electrons.

12. The method of claim 8, comprising operating the device at an incident energy of at least about 600 eV to provide a secondary electron yield of at least about 10 electrons.

13. The method of claim 8, comprising operating the device at an incident energy of at least about 800 eV to provide a secondary electron yield of at least about 11 electrons.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a highly schematic diagram illustrating the steps involved in the fabrication of a reflection mode dynode of the present invention.

(2) FIG. 2 is a series of scanning electron micrographs showing the surface of a boron-doped polycrystalline diamond grown on a molybdenum substrate. Panels (a) and (b) are micrographs taken at 0 tilting angle. Panels (c) and (d) are micrographs taken at 45 tilting angle. Scale bar=10 m.

(3) FIG. 3 is a series of atomic force microscopy images of a boron-doped polycrystalline diamond grown on a molybdenum substrate. Two different regions were imaged. Panels (a) and (b) are directed to a first region, and panels (c) and (d) to a second region. The panels (b) and (d) are three dimensional representations of the topographic images of panels (a) and (c) respectively. Scale bar=5 m.

(4) FIG. 4 is a graph comparing the secondary electron yield of a device of the present invention (produced in accordance with the illustrative embodiment described herein) compared to a prior art device. Both devices were operated in reflection mode.

(5) FIG. 5 is a highly schematic diagram showing the generation of a secondary electron avalanche in a prior art electron multiplier.

(6) FIG. 6 is a highly schematic block diagram showing a typical arrangement whereby a gas chromatography instrument is coupled to a mass spectrometer, the mass spectrometer having an ion detector/electron multiplier having at least one dynode having a carbon-based layer functioning as an electron emissive material.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) The invention will be further described by reference to a highly preferred embodiments with regard to the device and method used to fabricate the device. It is emphasised that the embodiments in this section are strictly non-limiting in scope.

(8) With regard to the highly preferred method, reference is made to FIG. 1 which illustrates highly diagrammatically the fabrication of a dynode (10) operable in reflection mode.

(9) The first step is the provision of a substrate (15) which in this embodiment is molybdenum having a planar upper surface of dimension 50 mm50 mm and a thickness of 1 mm. The substrate was cleaned by sonication followed by exposure to oxygen plasma. After cleaning, the planar upper surface was spin coated with nanodiamond particles.

(10) The second step is the commencement of growing a boron-doped polycrystalline diamond film (20) on one face of the substrate (15). The film was deposited by a plasma assisted chemical vapour deposition method (PA-CVD), at a pressure of 70 Torr, with a microwave power of 900 Watts, and at a temperature of 700 C. to 1000 C. During growth, gases were introduced into the pressurised chamber at predetermined flow rates as tabled below.

(11) TABLE-US-00001 Time Pressure Heater Power H.sub.2 CH.sub.4 B(CH.sub.3).sub.3 O.sub.2 Step (min) (Torr) (Deg. C.) (Watts) (SCCM) (SCCM) (SCCM) (SCCM) 1 0.5 10 0 0 100 1 0 0 2 0.5 10 300 0 100 1 0 0 3 0.2 10 300 400 100 1 0 0 4 0.2 10 300 500 100 1 0 0 5 0.2 10 300 600 150 2 0 0 6 0.5 20 300 700 200 2.5 0 0 7 0.5 30 300 1000 250 3 0 0 8 0.5 30 400 900 300 3 0 0 9 5 60 500 900 300 3 0 0 10 480 70 500 900 100 4 5 0.3 11 480 70 600 900 100 4 5 0.3 12 240 70 600 900 100 4 5 0.3 13 180 70 600 900 100 4 5 0.3 14 360 70 600 900 100 4 5 0.3 15 480 80 600 900 100 4 5 0.3 16 480 90 700 1200 300 2.5 0.15 0 17 10 90 700 1100 300 2.5 0.15 0 18 2 40 700 1000 300 0 0 0 19 2 40 0 0 250 0 0 0 20 2 30 0 0 250 0 0 0 21 2 20 0 0 150 0 0 0 22 2 10 0 0 100 0 0 0 23 25 80 0 0 300 0 0 0

(12) With reference to the above table, growth of the diamond layer occurs from step 10 to step 15.

(13) Growth of the film (in terms of depth) continues until the required depth is achieved, as shown in the third step. In this embodiment, a film thickness of about 10 m was used. Thicknesses between about 5 m and about 30 m are contemplated to be useful in the context of this preferred method.

(14) On the exposed surface of the deposited film (20) are shown surface carbon dangling bonds and carbon-carbon unsaturated bonds (collectively marked 25). These bonds will be present on the surface of the diamond film (25) as it is growing, but shown only at the third step for clarity.

(15) The fourth step shows the result of hydrogen termination of the surface bonds (25). This termination is achieved by the introduction of molecular hydrogen into the pressurized chamber once the required film depth is achieved. Ionized hydrogen is generally present in the reaction chamber to inhibit growth of graphitic carbon. Upon completion of growth the hydrogen concentration is increased while the surface is still exposed to the plasma.

(16) The boron-doped diamond film produced by the method described above was analysed by scanning electron microscopy. The resultant micrographs shown in FIG. 2 reveal a surface consistent with an underlying polycrystalline structure.

(17) The film was also analysed by atomic force microscopy, the micrographs being present in FIG. 3. Again, a surface consistent with a polycrystalline structure is revealed.

(18) From the micrographs shown in FIG. 2 and FIG. 3, the grain sizes of the diamond film are estimated to range from about 1 m, to about, to about 20 m.

(19) The dynode produced according the method described above was placed in a test rig to determine secondary electron yield. Electrons at increasing energy were directed at the hydrogen-terminated surface of the diamond film, with secondary electron yield being measured at a range of incident electron energies. The experiment was repeated under identical conditions for a standard dynode having an Al.sub.2O.sub.3-based material as the electron emissive material. The results of this comparative study are shown in the graph of FIG. 4. It will be noted that the yield from the dynodes is substantially the same for energies up to about 100 eV. For higher energies, the diamond film reflection dynode continues to increase substantially, before reaching an apparent maximum at a yield of 13 electrons.

(20) By contrast, the standard reflection mode dynode rises at a lower rate with increasing energy, and peaks at a yield of about 4.5 electrons at an energy of around 400 eV. After the peak, yield gradually declines.

(21) FIG. 4 demonstrates clearly the significant increase in yield when the boron-doped and hydrogen terminated polycrystalline diamond film as produced according this embodiment is used as an electron emissive material in a reflection dynode, as compared with a standard dynode. As will be appreciated from the Background section, increases in sensitivity of reflection mode dynodes have be sought after for decades, with prior artisans having limited success in that regard.

(22) Those skilled in the art will appreciate that the invention described herein is susceptible to further variations and modifications other than those specifically described. It is understood that the invention comprises all such variations and modifications which fall within the spirit and scope of the present invention.

(23) While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art.

(24) Accordingly, the spirit and scope of the present invention is not to be limited by the foregoing examples, but is to be understood in the broadest sense allowable by law.