Electron multiplying device

Abstract: The electron multiplying device according to this invention comprises an electron multiplying unit (E) including dynodes arranged in a plurality of stages. The electron multiplying unit has an incidence opening (21) for an energy beam to be multiplied to enter through, and has the proximal end secured to a base (13). There is provided a casing (16) for housing the electron multiplying unit. The forward edge of the casing is secured to the base (13), and a space defined by the base and the casing houses the electron multiplying unit. The casing has an entrance window (26) formed at a position opposed to the incidence opening (21). Energy beams enter the electron multiplying unit through the entrance window (26), but the electron multiplying unit itself is housed in the casing to be protected from surrounding air flow and unnecessary energy beams not to be measured.

Description: This invention relates to an ion (electron) multiplier for detecting or measuring energy beams of electrons, ions, charge particles, ultraviolet rays, soft X-rays, etc. As schematically shown in FIG. 1, in an ion multiplying device, energy beams, as of electrons or others, impinge on dynodes of the ion multiplying unit to multiply and emit secondary electrons, and the collecting electrodes (anodes) A collect the emitted secondary electrons for detection. The ion multiplying units have various types. Conventional quarter-cylindrical dynodes are substantially alternately arranged in a direction of incidence of energy beams. The arrangement of FIG. 1 is the typical one which is the so-called box-and-grid-type. Resistors are inserted between the respective dynodes DY and their adjacent ones. The resistors equidivide a voltage applied between a first-stage dynode DY1 and a final-stage dynode DY16. This is the basic structure of the ion multiplying units. The general actual assembly of the ion multipliers is shown in FIGs. 2 and 3. In the ion multiplier of FIGs. 2 and 3, respective dynodes DY are supported, enclosed by respective support frames 1. Each support frame 1 is made of a conducting material and is electrically connected to the associated dynode DY. The ion multiplier further comprises two support rods 3 which are secured to a holder 2 of a thin steel plate and are parallel with each other. These support rods 3 are inserted in holes 4 of each support frame 1 to support the dynodes by the support rods 3. A gap between each support frame 1 and its adjacent one is retained constant by spacers 5 through which the support rods 3 are inserted. In this conventional ion multiplying device, resistors R are disposed in one row on one of the rows of the dynodes. Leads L of each resistor R are welded respectively to vertically adjacent ones of the support frames 1. For the measurement of energy beams, as of ions, the above-described ion multiplying device is installed in a vacuum vessel with an energy beam source built in. But it is a problem that when the holder of a thin steel plate is not strong enough to install the device in the vessel. In addition, the dynodes are exposed, and need careful handling. The installation of the ion multiplying device is followed by drawing air out of the vessel, But the dynodes, which are exposed in the vessel, are subjected to air streams when the air of the vessel is evacuated. Sometimes the air streams contain dust, and the dust sticks to the surfaces of the dynodes, which may cause erroneous measurements. This problem also occurs when, after measurements, the vacuum vessel is released, and air flows into the vessel from the outside. Also in operations in vacuum, oil used in a vacuum pump, sample solvents may be attached onto the surfaces of the dynodes, and as the result, gain of the multiplying device may be degraded. Furthermore, in some cases energy beams not to be measured, e.g., scattered energy beams, are incident on the sides of the ion multiplying device to enter the exposed dynodes. For analysis of ions of some kinds, plasmas are used, and in some cases, ultraviolet radiation from the plasmas are incident on the dynodes. These energy beams are a cause for noises. From US-A-3 272 984 an electron multiplier is known, comprising : a sequence of dynodes arranged to receive at one end an incident beam of radiation ; a base for supporting the sequence of dynodes ; and a housing mounted to the base and enclosing the sequence of dynodes. In view of these problems, this invention has been made. The invention aims to provide an ion multiplying device which has sufficient strength and is easy to handle, and can prevent the intrusion of unnecessary energy beams. According to the invention there is provided an electron multiplier comprising: a sequence of dynodes arranged to receive at one end an incident beam of radiation; a base for supporting the sequence of dynodes; and a housing mounted to the base and enclosing the sequence of dynodes; characterised in that: the sequence of dynodes is supported at the other end thereof by the base; and the housing has a window disposed therein for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes. It is preferable that the opening is of substantially the same shape as the window. The housing may be formed of a magnetic metal. Preferably the electron multiplier further comprises a pair of insulating support plates for supporting the sequence of dynodes, the support plates being secured to the base. The housing may have positioning slots formed therein; the support plates having tabs which are inserted in the slots when the housing is secured to the base. It is preferable that the electron multiplier further comprises an insulating filler plate disposed between said one end of the sequence of dynodes and the housing, the filler plate defining an opening aligned with the window for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes. The electron multiplier may comprise a baffle disposed between said one end of the sequence of dynodes and the housing, the baffle defining a hole aligned with the window for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes, the baffle absorbing or reflecting incident beams not directed toward said one end of the sequence of dynodes. It is preferable that the hole is larger than the window, the baffle comprising a plurality of black-colored spaced plates. The invention extends to a mass spectra analyser comprising a vacuum containing an electron multiplier as aforementioned. The present invention will become more fully understood from the detailed description of embodiments given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view explaining the principle of the ion multiplying device; FIG. 2 is a side view of the conventional general ion multiplying device; FIG. 3 is a perspective view of the ion multiplying device of FIG. 2 being assembled; FIG. 4 is a broken-down perspective view of the ion multiplying device according to one embodiment of this invention; FIG. 5 is a perspective view of the finished ion multiplying device of FIG. 4; FIG. 6 is a longitudinal sectional view of the ion multiplying device of FIG. 4; FIG. 7 is a sectional view of a modification of the casing used for the ion multiplying device of FIG. 4; FIG. 8 is a circuit diagram of a voltage dividing circuit used in the ion multiplying device of FIG. 4; and FIG. 9 is a block diagram of a vacuum vessel with the ion multiplying device installed in. Description of the Preferred Embodiments Preferred embodiments of the invention will be explained with reference to the drawings attached hereto. In the drawings the common members are represented by common reference numerals. In the following description, "vertically, or up to down", and "horizontally, or left to right" means "vertically, or up to down" and "horizontal, or left to right" as viewed in the drawings. As shown in FIGs. 4 to 6, the ion multiplying device according to one embodiment of this invention includes, as does the above-described conventional device, an ion multiplying unit E including a plurality of stages of dynodes DY (16 stages in this embodiment), and a collecting electrode (anode) for capturing electrons emitted from the final-stage dynode DY 16. The respective dynodes DY have a potential difference with respect to their downwardly adjacent ones so that they emit secondary electrons to the latter. To this end, the ion multiplying unit E includes a voltage dividing circuit of FIG. 8. Resistors R are inserted between the respective dynodes and their adjacent ones. A resistor R is inserted between the final-stage dynode DY 16 and the earth. In this embodiment, the resistors R, the dynodes DY and the collecting electrode A are mounted between two support plates 10a, 10b of ceramics which are parallel with each other. Each support plate 10a, 10b is substantially rectangular. A block 11 is secured to one ends of the support plates 10a, 10b between both support plates by bolts and nuts 12. The block 11 is secured by screws to the central portion of a substantially square base 13. Thus the support plates 10a, 10b are fixed to the base 13 in parallelism with each other. The base 13 is formed of a relatively thick stainless steel plate, and is so rigid that the base 13 is not deformed by normal uses. In each corner of the base 13 there are formed three holes 14, 15, 15. The hole 14 nearest to the corner is for mounting the ion multiplying device to, e.g., a vacuum vessel (not shown). The other holes 15, 15 are for mounting on the base 13 a casing which will be described later. As shown in FIG. 6, the dynodes DY are arranged substantially alternately between the support plates 10a, 10b in the longitudinal direction thereof. The first-stage to the third-stage dynodes DY1 ∼ DY3 which are relatively larger are arranged in the so-called box-and-grid-type arrangement, and the other smaller dynodes DY4 ∼ DY 16 are arranged in the so-called line focus-type arrangement or linear focus arrangement. In this arrangement, an energy beam enters along the longitudinal axis of the support plates 10a, 10b and impinges on the concave surface of the first-stage dynode DY1, and secondary electrons are emitted to multiply electrons. The secondary electrons are led to the concave surface of the second-stage dynode DY2. Thus secondary electrons are led to a next stage-dynode and finally to the last-stage dynode DY16, which is nearest to the base 13. The collecting anode A is disposed at a position where the anode A can receive the electrons emitted from the final-stage dynode DY 16. A plurality of recesses are formed at a set interval in the longitudinal edges to each support plate 10a, 10b. The resistors of the voltage dividing circuit are mounted between the support plates 10a, 10b by the recesses 17. A resistor R is disposed between a pair of the recesses at the same height and is secured by inserting the leads of the resistors in recesses of the pair with the forward ends of the leads welded to the forward ends of tabs of the associated dynode DY. In this embodiment, 9 resistors R are disposed on one side, and on the other side 7 resistors are disposed. The first-stage dynode DY1, the collecting electrode A and the final-stage dynode DY16 are connected to hermetic terminals 18 by a ceramic piped conductor 19. A metal plate 20 is mounted between the upper ends of the support plates 10a, 10b. In the metal plate there is formed an incidence opening 21 at a position opposed to an energy beam entrance of the first-stage dynode DY1. This metal plate 20 is connected to the first-stage dynode DY1 to have the same potential as the latter so that the metal plate has shielding function and also as a reinforcement of the ion multiplier assembly. The ion multiplier according to this embodiment further comprises a casing 16 for protectively housing the dynodes DY, etc. The casing 16 has a shape of an upside-down cup, and includes a cylindrical portion 16a surrounding the support plates 10a, 10b secured to the base 13 and the hermetic terminals 18, an outward flange 16b formed in one-piece on the lower end of the cylindrical portion 16a, and a top surface 16c closing the top of the cylindrical portion 16a. It is preferable that the casing 16 is made of a magnetic metal, Permalloy or others, for the protection from the influence of the magnetic field. The flange 16b has a substantially rectangular shape as the base 13. Three holes 23, 22, 23 are formed in each corner of the flange 16b. When the casing 16 is mounted on the base 13 at a set position, each corner of flange 16b and that of the base 13 agree with each other with the holes 23, 22, 23 and the holes 15, 14, 15 respectively aligned with each other. A vis 24 is inserted through the inner holes 15, 23 and is fastened with a nut 25 to thereby secure the casing to the base 13. An entrance window 26 is formed in the top surface of the casing 16. The entrance window 26 is for inletting energy beams and is brought into alignment with the incidence opening 21 of the metal plate 20 and with the energy beam entrance of the first-stage dynode DY1. In this embodiment, in the top surface of the casing 16 there are formed 4 slots 27 in addition to the entrance window 26. The slots 27 receive tabs formed upward on the upper edges of the support plates 10a, 10b when the casing 16 is mounted on the base 13 at the set position. The assembly of the slots 27 and the tabs 28 facilitate the positioning of the casing 16, and the alignment of the incidence opening 21 with the entrance window 26. The ion multiplying device according to this embodiment is secured by bolts to a mounting place, such as a vacuum flange or others, by means of the holes 14, 22 of the casing 13 and of the lange of the casing 16. The casing 13 has a rigidity sufficient to secure the ion multiplying unit E to the set position. Since the dynodes DY, etc. are housed in the casing 16, the fabricating operation can be made without paying special attention to their interference with the other members. FIG. 9 shows the ion multiplying device disposed in a vacuum vessel 40. The device of FIG. 9 is a Mass spectra analyzer. Inside the vacuum vessel 40 the ion multiplying device is disposed on the left end. A sample gas introduction chamber 41 is disposed opposed to the ion multiplying device for introducing sample gas into the vacuum vessel 40. In the vacuum vessel 40 there is provided an ion source 42 for ionizing the introduced sample gas and emitting ionized particles to the ion multiplying device. The ionized particles emitted from the ion source 42 take curved orbits when they pass through the ion analyzer 43, and only specific ones of the ionized particles selectively arrive at the ion multiplying device. Vacuum pumps 44, 45 are connected to the sample gas introduction chamber 41 and the vacuum chamber 40 respectively through a vacuum valves 46, 47 so that residual gas in their associated spaces are evacuated to maintain the interiors of the spaces at a vacuum atmosphere. In the case that the ion multiplying device is mounted in a vacuum vessel as in this case, the interior of the vessel is evacuated before a measurement, but the dynodes DY, etc., which are housed in the casing 16 are not exposed to the air flow. The risk of dust sticking to the dynodes DY is much reduced. Even when the dynodes DY are left in the air, the dynodes DY housed in the casing 16 are much less contaminated in comparison with those without the casing 16. Gain deterioration of the ion multiplying device due to backward diffusion of vacuum oil, sample solvents, etc. in an evacuating operation can be much reduced. The casing 16 shields off energy beams, as of neutrons, which might be irregularly reflected to adversely enter the ion multiplying device from the sides, and background ultraviolet radiation in mass analysis preventively from entering the dynodes DY. The casing 16 made of a magnetic metal, such as Permalloy, functions as an electromagnetic shield and prevents the influence due to magnetic fields and electric fields of incident energy beams. In FIGs. 4 and 6, reference numeral 30 represents an insulator (filler plate). The insulator 30 is disposed between the metal plate 20 and the top surface 16c of the casing 16. In the central part of the insulator 30 there is formed a passage opening 32 of the same shape as the entrance window 26 and the incidence opening 21. A gap is formed between the top surface 16c of the casing 16 and the metal plate 20. There is a very low possibility that dust and unnecessary energy beams which have entered through the entrance window 26 intrude into the casing 16 through the gap. But the insulator 30 can perfectly prohibit the intrusion of the dust, etc. The insulator 30 can have various forms. It is preferable for sealing the gap that is shown, the insulator 30 has a cylindrical shape having an outer diameter substantially equal to an inner diameter of the casing 16. In the case that the insulator 30 has such cylindrical shape, positioning slots 31 are formed in the insulator 30 so as to be into alignment with the slots 27 . The tabs 28 of the support plates 10a, 10b are inserted into the slots 31 and the slots 27, whereby the passage opening 32 of the insulator 30 for inletting energy beams is brought into alignment with the entrance window 26 of the casing 16, the incidence opening 21 of the meal plate 20 and the energy beam receiving surface of the first-stage dynode DY 1. FIG. 7 is a sectional view of another example of the casing 16. This example is different from the casing involved in the above-described embodiment in that in the former the upper end of a cylindrical portion 16a of the casing 16 is extended upward beyond a top surface 16c. A baffle (energy beam introducing members) 35 in the form of a plurality of rings is mounted on the inside peripheral surface of the extended portion 16d of the cylindrical portion 16a. The baffle 35 comprises a plurality of metal plates 36 each having both sides colored in black. Each metal plate 36 has an opening formed in central part thereof. The opening 36a is larger than the entrance window 26 below the metal plate 36. The openings of the respective metal plates define an energy beam introducing hole. The baffle 35 is for absorbing or reflecting energy beams entering from the sides, which are not to be measured so as to prohibit their entrance through the entrance window 26 of the casing 16. Because of the baffle 35, background ultraviolet radiation and neutral moleculed, etc. which are problems with mass analysis can be usefully reduced. The baffle 35 can have various shapes for preventing the intrusion of the background molecules and ultraviolet radiation. In the above-described embodiment, the dynodes DY housed in the casing 16 are mounted on the two support plates 10a, 10b, but this invention is also applicable to the structure of, e.g., FIG. 2. As described above, the ion multiplying device according to this invention includes the casing. The casing can protect the ion multiplying unit E including the dynodes and the resistors, etc. from unnecessary energy beams not to be measured and dust, the backward diffusion of vacuum oil and sample solvents, etc. Unnecessary energy beams cause noises, and to shield off the unnecessary energy beams improves achievement of the ion multiplying device. Dust, vacuum oil, etc. are hindered from sticking to the dynodes, whereby the deterioration of gains of the ion multiplying device can be precluded. The casing protects the dynodes, etc. from external forces, such as impacts, etc. The base has a sufficient rigidity which facilitates the handling the ion multiplying device. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the invention as defined in the following claims.

Claims: An electron multiplier comprising: a sequence of dynodes (DY1 to DY16) arranged to receive at one end an incident beam of radiation; a base (13) for supporting the sequence of dynodes; and a housing (16) mounted to the base and enclosing the sequence of dynodes; characterised in that: the sequence of dynodes (DY1 to DY16) is supported at the other end thereof by the base; and the housing has a window (26) disposed therein for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes. An electron multiplier as claimed in claim 1, further comprising a metal plate (20) at said one end of the sequence of dynodes (DY1 to DY16), the metal plate defining an opening (21) through which radiation can pass to the sequence of dynodes. An electron multiplier as claimed in claim 2, wherein the opening (21) is of substantially the same shape as the window (26). An electron multiplier as claimed in any preceding claim, wherein the housing (16) is formed from a magnetic metal. An electron multiplier as claimed in any preceding claim, further comprising a pair of insulating support plates (10a, 10b) for supporting the sequence of dynodes, the support plates being secured to said base (13). An electron multiplier as claimed in claim 5, wherein the housing (16) has positioning slots (27) formed therein; and the support plates (10a, 10b) have tabs (28) which are inserted in the slots (27) when the housing is secured to the base (13). An electron multiplier as claimed in any preceding claim, further comprising an insulating filler plate (30) disposed between said one end of the sequence of dynodes and the housing, the filler plate defining an opening (32) aligned with the window (26) for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes. An electron multiplier as claimed in any of claims 1 to 6, further comprising a baffle (35) disposed between said one end of the sequence of dynodes and the housing, the baffle defining a hole (36a) aligned with the window (26) for permitting passage of said incident beam of radiation to said one end of the sequence of dynodes, the baffle absorbing or reflecting incident beams not directed toward said one end of the sequence of dynodes. An electron multiplier as claimed in claim 8, wherein the hole (36a) is larger than the window (26), the baffle (35) comprising a plurality of spaced plates (36). An electron multiplier as claimed in claim 9, wherein the plates (36) are coloured black. A mass spectra analyser comprising a vacuum vessel (40) containing an electron multiplier as claimed in any preceding claim.

Inventors: MORITA TETSUYA, NAKAMURA MAKOTO, OKAMOTO TAKEHISA, SAKAI SHIRO, YAMAGUCHI HARUHISA, MORITA, TETSUYA, NAKAMURA, MAKOTO, OKAMOTO, TAKEHISA, SAKAI, SHIRO, YAMAGUCHI, HARUHISA
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: H01J43
IPC codes: H01J43