Tandem continuous channel electron multiplier

A channel electron multiplier including a single channel CEM for receiving an input particle. A multi-channel CEM is positioned after the single channel CEM for receiving emissions from the single channel CEM. An electron collector is positioned after the multi-channel CEM for generating a pulse current in response to emissions from the multi-channel CEM.

BACKGROUND OF THE INVENTION

Channel electron multipliers (CEMs) are used to amplify charged particle, photon, or energetic neutral particle signals. CEMs are used to detect photons, charged particles both positive and negative, and energetic neutral particles. They are used as detectors in mass spectrometers as well as in surface analyzers such as auger and x-ray/ultraviolet photoelectron spectrometers, and are also employed in electron microscopes. In addition, they can also be used for electron multiplication in a photon multiplier application.

The CEM makes use of an emissive surface to generate electron multiplication. The emissive surface will emit secondary electrons when struck by a charged particle, or energetic neutral particle, or photon, with sufficient energy. This process is repeated and generates an electron avalanche down the length of the channel. An electron collector, such as a Faraday cup, at the end of the channel collects the electrons and converts them into an electrical pulse.

Typical CEMs are tubular in nature and have an integral funnel cone attached to the input beam end to increase input beam profile detection. CEMs having single and multiple channels in one body have been commercialized. A single channel CEM has a shorter output current dynamic range than a multiple channel CEM having the same channel resistance per channel. The stability and lifetime of CEMs depend on the active emissive surface area. Therefore, a single channel CEM lifetime is shorter. In addition, the single channel CEM high output current operation is less stable than a multiple channel electron multiplier. However, a multiple channel electron multiplier suffers losses in detection efficiency due to an inactive area between the channels at the input beam end. In a single channel electron multiplier, the detection efficiency is maximized due to a smooth transition between the funnel cone and the channel.

There is a need in the art for a CEM providing high detection efficiency and increased lifetime.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the invention is a channel electron multiplier including a single channel CEM for receiving an input particle. A multi-channel CEM is positioned after the single channel CEM for receiving emissions from the single channel CEM. An electron collector is positioned after the multi-channel CEM for generating a pulse current in response to emissions from the multi-channel CEM.

DETAILED DESCRIPTION

FIG. 1shows an exemplary conventional CEM. Most CEMs include an input funnel cone101, a curved tubular channel102, and a Faraday cup103as shown inFIG. 1.FIG. 2shows an alternate CEM having an input funnel201, spiraled, tubular channel202and Faraday cup203.FIG. 3Adepicts a conventional CEM having an input funnel301, multiple curved channels302and Faraday cup303.FIG. 3Bis a front view of the CEM inFIG. 3A.

FIG. 4is an enlarged view of a portion ofFIG. 2illustrating the operation of the CEM. The CEM inside channel wall401is prepared with an electron emissive layer402, most commonly SiO2, on top of a semi-conducting layer403, most commonly reduced lead-oxide glass. The funnel cone201attached to the tubular channel202increases detection sensitivity due to a larger incoming particle beam profile acceptance.

When a charged particle, photon, or energetic neutral particle strikes the surface of the input end of a CEM, secondary electrons are generated which are then propelled into the channel by an applied electric field. This electric field drives the secondary electrons farther into the channel and the electrons again collide with the wall channel, further producing a large number of secondary electrons. This process repeats several times and creates an electron avalanche along the channel. A Faraday cup203at the output end collects the electrons and converts them into an electric pulse406which is fed into electronic circuitry for further signal processing. Electrons ejected from the channel walls of the multiplier are replenished by the electrical current404through the semi-conducting glass. The electric current404illustration shows the electric current pointing from the more positively biased back end of the CEM, the Faraday cup end, toward the more negatively biased front end of the CEM, the funnel cone end, and is in keeping with generally accepted convention. However, it is commonly understood that the electrons are flowing from the front end toward the back.

Multiple-channel CEMs have been manufactured, such as illustrated inFIG. 3; however, in this configuration, detection efficiency is decreased. Even with the presence of an input funnel cone301, most of the secondary electrons generated by incoming particles that strike the area304between the channel openings and do not enter the channels. Thus, no electron multiplication is generated by particles striking area304and the ion signal is considerably weakened. In contrast, the single-channel CEM, as illustrated inFIGS. 1and2, does not exhibit loss of particle detection due to the smooth structural transition between the funnel cone101,201and the channel102,202. Over time, charged particle bombardment of the channel walls causes a gradual degradation of the wall surface. Thus, the CEM lifetime is directly proportional to the total surface area of the channel walls. However, the multi-channel CEM ofFIG. 3Aoffers a stable and wider dynamic range of the output current, due to channel multiplicity (provided that there exists equal resistance, per channel, or equal bias strip current, per channel) when compared to single-channel units ofFIGS. 1 and 2. The multi-channel CEM ofFIG. 3Aalso has a longer lifetime, due to an active channel wall emissive layer402surface area that is larger than that found in single-channel CEM designs.

Embodiments of the invention constitute a performance improvement over existing CEMs, of both the single-channel and multi-channel varieties, by way of a tandem configuration that joins a single-channel with a multi-channel CEM configuration. The single-channel is positioned at the particle beam input end, followed by the multi-channel CEM arrangement at the electron avalanche output end.

FIG. 5Aillustrates a tandem CEM configuration501in embodiments of the invention. The tandem CEM501includes a single channel CEM502, and a multiple channel, single body CEM503. The multiple channels in the single body CEM503may be tubular channels, and curved as shown inFIG. 3A. The incoming beam of charged particles508is received at a funnel (not shown) and triggers an electron avalanche along the single channel CEM502. The output electron emission from the single channel CEM502is received at the multiple channels of the single body CEM503. The electron emission is collected at an electron collector (e.g., Faraday cup)520.

FIG. 5Billustrates a tandem CEM configuration504in alternate embodiments of the invention. Tandem configuration504includes a single channel CEM502followed by multiple, single body CEM units505. CEM units505may be single-channel or multiple-channel devices. The incoming beam of charged particles508is received at a funnel (not shown) and triggers an electron avalanche along the single channel CEM502. The output electron emission from the single channel CEM502is received at the CEM units505. The electron emission is collected at electron collectors (e.g., Faraday cup)520each positioned at the output end of the CEM units505.

FIG. 5Cillustrates a tandem CEM configuration506in alternate embodiments of the invention. Tandem configuration506includes a single channel CEM502followed by multiple, single body CEM units505. CEM units505may be single-channel or multiple-channel devices. A third stage array of CEM units507is positioned after CEM units505. CEM units507may be single-channel or multiple-channel devices. The incoming beam of charged particles508is received at a funnel (not shown) and triggers an electron avalanche along the single channel CEM502. The output electron emission from the single channel CEM502is received at the CEM units505. The output electron emission from the CEM units505is received at the CEM units507. The electron emission of CEM units507is collected at electron collectors (e.g., Faraday cup)520each positioned at the output end of the CEM units507. The tandem CEM configurations inFIGS. 5A-5Cprovide CEM configurations capable of delivering stable, high current output, and long lifetime, with high particle detection efficiency.

In the tandem configures ofFIGS. 5A-5C, a distance is needed between the single channel CEM and the multi-channel CEM. The workable distance between the two depends on the inter channel distance on the multi-channel CEM input end. In exemplary embodiments, the inter channel distance is 0.1″ center to center and the workable distance between the single channel CEM and the multi channel CEM is 0.1″ to 0.75″. In general, the spacing between the single channel CEM output end and the multi-channel CEM input end needs is determined based on the inter channel spacing of the channels in the multi-channel CEM. In embodiments of the invention, the single channel CEM and subsequent multi-channel CEM(s) are contained in a common housing, in a monolithic construction.

In above-described tandem configurations of a single-channel CEM followed by a multi-channel CEM, the single-channel CEM provides high detection efficiency, and the multi-channel CEM gives stable, high output current and longer lifetime. The electron avalanche produced by an input particle at the single-channel CEM end spreads all over the input surface area of the multi-channel. Even though some electrons are lost at the multi-channel input region, due to detection inefficiency in the area between channels, enough electrons are propelled into the channels to sufficiently generate an electron avalanche through the multi-channel CEM, which then arrives at the Faraday cup. In this case, the information of a single particle, input into the overall detector configuration, is preserved.

FIG. 6is a graph of deflection voltage versus relative output current. InFIG. 6, plot A represents the output signal from a 3-channel CEM only, resulting from a horizontal input beam scan, by means of voltage controlled deflection plates, across the input cone of the CEM, through a plane containing the beam axis. The beam axis is co-linear with the axis of the 3-channel CEM. In plot A, the scan intersects two of the three input channels, which intersections are represented by the two peaks at approximate deflection positions −75V and +75V. The sharp decrease in the output current between these two peaks is a direct result of the losses occurring from the input beam being scattered off the space between the channels. Plot B is a plot of the output from the tandem CEM configuration, using the same input beam scan scheme as for the multi-channel CEM. Plot B shows there are essentially no losses across the tandem configuration input cone in the region where the losses are greatest in the multi-channel CEM. This is a clear indication the use of a single-channel input multiplier, in a tandem CEM configuration, eliminates the losses that would otherwise occur were the beam to be introduced directly into a detector consisting of only a multiple-channel CEM.

FIG. 7is a graph of relative UV intensity versus output current for single-channel CEM and a tandem CEM. Plot B shows output current for the single channel CEM. Plot A shows output current for the multi-stage, tandem CEM. As illustrated inFIG. 7, the tandem CEM provides higher dynamic range for the output current. In this graph the tandem CEM output current response, as a function of signal input (relative UV intensity), is higher than that of a single channel CEM, even though the bias strip current per channel, in the tandem, is 1.6 times lower than that of a single channel CEM. It is clearly seen that the single channel CEM saturates at about 45 microAmps. This is a direct verification that channel multiplicity in the tandem continuous CEM is capable of producing an output current larger than that of single-channel CEMs operating at a relatively comparable gain.

FIG. 8is a graph of time versus normalized relative output current. Plot A corresponds to a tandem CEM and plot B corresponds to a single-channel CEM.FIG. 8confirms that the tandem CEM configuration is capable of sustaining expectedly higher output currents for longer periods of time than a single-channel CEM working under comparable operating conditions. Operating continuously over a period of 3.5 days the tandem CEM configuration remains at 74% of its initial output, compared to only 46% of initial output by the single-channel CEM. This indicates that a detector lifetime increase, by a factor of 1.6, has been achieved through use of the tandem CEM configuration. This is a result of the larger total channel surface area due to using the multiple channels.

Embodiments of the invention overcome difficulties inherent in both single channel multipliers, and multiple channel multipliers, by fabrication of a tandem configuration which incorporates both a single channel at the input beam end and multiple channels at the output end. The single channel electron multiplier contributes to high detection efficiency and the multi-channel electron multiplier maintains high output current (dynamic range) and output current stability, as well as yielding a long lifetime. In tandem configurations, the electron avalanche produced by an input particle at the single channel end spreads over the entire input area of the multiple channel input cone. Even though some electrons are lost on the surface area between the input ends of the multiple channels a quantity of electrons, more than sufficient to start electron avalanches in the multi-channel stage, do enter the multiple channels. Therefore, the information of a single input particle is preserved.

In this tandem configuration, the input beam end of the single channel is biased electrically so it is negative with respect to the output end of the multi-channel. Biasing is accomplished by the application of voltage across the overall length of the tandem configuration. The input end and the output end of the electron multiplier incorporate an electrical contact so that voltage can be applied to the channel. An input beam at the electron multiplier input end generates secondary electrons. These electrons, under the influence of the applied electric field, travel toward the output end of the multiple channels. Along the way they undergo repeated wall collisions and, overall, generate an electron avalanche that is then collected by a Faraday cup.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claims.