Abstract:
The invention relates to a device for collecting ions, in particular in an isotope mass spectrometer. At least one secondary electron multiplier (SEM) is provided, this being constructed in the manner of a card, namely box-like and with a thickness which is low in comparison with its length and width. The SEM is held in a frame. In addition, a flat flexible printed circuit board with a plurality of parallel conductors is connected to the SEM. The flexible printed circuit board permits the positioning of the SEM in order to match the ion paths to be sampled.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims priority to German Patent Application Serial No. 102 38 347.2, filed on Aug. 16, 2002, the disclosure of which is hereby incorporated by reference in its entirety.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0002]    The invention relates to a device for collecting ions, in particular in a mass spectrometer, having at least one secondary electron multiplier (SEM), the SEM being formed in the manner of a card, specifically substantially box-like with a low thickness in relation to the length and width.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a brochure entitled “Triton Neptune, Multicollector Mass Spectrometers for High Precision Isotope Ratio Determination” published by Thermo Finnigan MAT GmbH, Bremen, Germany, 2001, various constituent parts of mass spectrometers are explained. Inter alia, a multicollector as an ion collector having a large number of Faraday cups is illustrated. The Faraday cups are of rectangular and flat disc-like design, so that a plurality of Faraday cups can be arranged beside one another at short intervals in the ion path. For measurements of only low ion currents, in particular in connection with isotope ratio determination, SEM are preferably needed. These must be matched as well as possible to the existing system.  
           [0004]    SEM having approximately the size and the dimensions of a cheque card are known. The SEM are somewhat smaller than the Faraday cups.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0005]    It is an object of the present invention to provide a device for collecting ions in a mass spectrometer, which can be used and adapted as flexibly as possible. The device according to the invention is characterized in that the SEM is held in a frame. The frame can have approximately the dimensions of the Faraday cup, which would be otherwise used. The SEM is insertable and removable from the frame. Accordingly, the frame is preferably somewhat thicker than the SEM. As a result of using the frame, the external dimensions of the SEM are matched to the dimensions of other constituent parts of the device. At the same time, the frame permits easy replacement of the SEM. In this way, the SEM can be held in the frame merely by being wedged in.  
           [0006]    In one embodiment of the invention, provision is made for the SEM to have an entry opening for the ions on a narrow side, for the frame to have a passage opening for the ions on a narrow end side, the inlet opening and the passage opening being aligned with each other, and for the SEM to be at least partially inserted into the frame on a further narrow side of the latter which is located transversely with respect to the narrow end side. The frame is upright with large-area, upright walls. The SEM is inserted into the frame from above. The passage opening of the frame is located at its front end side, while the large-area upright walls of the frame are aligned substantially parallel to the ion flight path. The greatest length of the frame likewise extends parallel to the ion flight path. By contrast, the greatest dimension of the SEM extends parallel to the insertion direction (insertion of the SEM into the frame).  
           [0007]    Preferably, and independently of the inventive features mentioned previously, a flat flexible printed circuit board having a plurality of parallel lines is connected to the SEM. The flexible printed circuit board is also referred to as a strip conductor or flexible conductor track. Similar flexible conductor tracks are used in ink jet printers for the electrical drive of the print head. In the present case, the flexible printed circuit board initially extends in the plane of the SEM. In this plane, the individual lines including signal lines are located beside one another and parallel to one another at intervals. An end of the flexible printed circuit board opposite to the SEM is connected to electrical contacts, which lead onward. By means of the use of the flexible printed circuit board in the manner described, the space available for the electrical connections is optimally utilized. The influence on the signals resulting from high voltages, which are naturally present is minimized. If a plurality of SEM are arranged beside one another, nevertheless far-reaching possible adjustments for the individual SEM are provided for matching to the ion paths to be sampled. The flexible printed circuit boards are also preferably insulated electrically on only one side, for cost reasons, namely by the base material. Applied to the latter are individual conductor tracks, preferably without additional insulation. Adjacent flexible printed circuit boards cannot short-circuit one another since there is always an insulated side opposite a side provided with electrical conductors. Electric lines of adjacent SEM never come to lie opposite one another.  
           [0008]    One channel input of the SEM is preferably earthed. Accordingly, the channel output is connected to high voltage, in particular to about 2000 V.  
           [0009]    The frame can preferably be connected to a holder by its narrow underside. The frame is held in a defined position by the holder or can be inserted into the holder in a defined position. The holder itself can be adjusted transversely with respect to the ion flight path, so that the frame can be set to a defined ion flight path.  
           [0010]    The frame preferably has holding means on its narrow upper side to connect the frame to a guide means. This results in further possible adjustment in interplay between the holding means and the guide means (or a plurality of guide means).  
           [0011]    According to a further embodiment of the invention, a plurality of SEM are provided with frames, the frames being held on at least one common guide means and being capable of being positioned relative to one another at defined intervals on the latter. Accordingly, the guide means permit a plurality of frames to be combined into a group, it being possible for defined positions to be assumed within the group. At least one frame of the group is preferably provided with a holder, so that the result is that a group with a plurality of SEM (in corresponding frames) is held only by one frame. The guide means provided are preferably two rods, onto which the individual frames are threaded with appropriate holding means.  
           [0012]    In addition, at least one Faraday cup is preferably provided, whose external dimensions correspond to those of the frame. The Faraday cup is a specific ion collector, which can be provided in addition to the SEM.  
           [0013]    According to a further embodiment of the invention, groups are formed which contain either at least one Faraday cup and otherwise at least one SEM, or which contain more than one SEM, at least one Faraday cup or SEM in a group being connected to a holder by its narrow underside, and the Faraday cups and SEM within the same group being arranged on one or more common guide means via holding means on the upper side and being capable of being positioned relative to one another. Accordingly, by adjusting one holder, an entire group can be displaced, in particular transversely with respect to the running direction of the ion beam. Adjustments of the positions within a group are carried out by moving the holding means along the guide means.  
           [0014]    A mass spectrometer is also a constituent part of the invention, in particular an isotope mass spectrometer, having one or more devices according to the invention, preferably having a multicollector.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Further features of the invention emerge from the claims and otherwise from the description. Advantageous embodiments of the invention wall be explained in more detail below in connection with the drawings, in which:  
         [0016]    [0016]FIG. 1 shows a secondary electron multiplier (SEM) inserted into a frame, in side view;  
         [0017]    [0017]FIG. 2 shows an illustration similar to FIG. 1, but cut open and with a flexible printed circuit board connected to the SEM;  
         [0018]    [0018]FIG. 3 shows the view taken along the line III-III of FIG. 2 showing one end of the frame with inserted SEM;  
         [0019]    [0019]FIG. 4 shows an illustration corresponding to FIG. 1, but with a flexible printed circuit board on the SEM;  
         [0020]    [0020]FIG. 5 shows an illustration of the frame corresponding to FIG. 1 (without SEM) and on a holder on the underside;  
         [0021]    [0021]FIG. 6 shows a plan view of the frame with holder according to FIG. 5;  
         [0022]    [0022]FIG. 7 shows a plan view of a plurality of frames threaded onto guide means; and  
         [0023]    [0023]FIG. 8 shows a plan view of a group of SEM with Faraday cup on a holder. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 shows a secondary electron multiplier (SEM)  11  inserted into a frame  10 . The frame is of substantially rectangular design with a length LR in the direction of a longitudinal mid-axis  12  and a width BR extending transversely with respect thereto. The frame  10  has a thickness DR which is very low as compared with the width BR or length LR and which is oriented at right angles to the plane of the figure. Corresponding directional indicators for the aforementioned dimensions comprising length LR, width BR and thickness DR are likewise shown in FIG. 1. Overall, the dimensions of the frame are comparable with those of a somewhat thicker cheque card.  
         [0025]    The frame  10  has a narrow end side  14  provided with a passage opening  13 , a narrow end side  15  opposite the former, a narrow underside  16 , a narrow upper side  17  and large-area side walls  18 ,  19  lying parallel to the plane of the figure, see also FIG. 6. The designations upper side and underside also relate to the envisaged arrangement of the frame  10  in an isotope mass spectrometer.  
         [0026]    The SEM  11  likewise has a format similar to a cheque card, namely with a length LS along a longitudinal mid-axis  20 , a width BS and a thickness DS. The latter extends at right angles to the plane of the figure. The longitudinal mid-axis  20  runs in the plane of the figure but at right angles to the longitudinal mid-axis  12  of the frame  10 . The thickness DS is somewhat smaller than the thickness DR of the frame  10 .  
         [0027]    The SEM  11  is plugged into the frame  10  from above, that is to say, in the area of the narrow upper side  17 . A corresponding receiving opening in the frame  10  is designated by the number  21 . The SEM  11  inserts into the frame  10  over about ⅔ of its length and has a bell-like inlet opening  22 , a meandering channel  23  which adjoins the said inlet opening  22 , an insulating ceramic body  24  and electrical terminals  25 . The construction of such an SEM of cheque-card size with a continuous dynode is known in principle. The ions entering the inlet opening  22 , each time they collide with walls of the channel  23 , knock out secondary electrons which, in turn, separate further secondary electrons during the collision. There is an amplification of about 10 7  at a channel output  26  here. In the present case, a high voltage of about 2000 V is applied to the channel output  26 , while a channel input  27  is earthed. The inlet opening  22  of the SEM  11  is aligned with the passage opening  13  of the frame  10  and is arranged on a narrow long side  28  of the SEM  11 .  
         [0028]    The receiving opening  21  in the frame  10  is provided on the inside with a lower supporting surface  29  and two lateral supporting surfaces  30 ,  31  and a compression spring  32 . The compression spring  32  acts on a further narrow long side  33  of the SEM  11 , opposite the narrow long side  28 . A narrow lower end side  34  of the SEM  11  comes to lie on the lower supporting surface  29 , which is considerably smaller for this purpose. The narrow long side  28  rests on the lateral supporting surfaces  30 ,  31 , above and below the inlet opening  22 . Overall, the SEM  11  is positioned as accurately as possible after being inserted into the frame  10 .  
         [0029]    Opposite the lower end side  34 , the SEM has an upper narrow end side  35 . In the area of the same, a flexible printed circuit board  36  is connected to the SEM  11 . On the flexible printed circuit board  36 , four lines are printed beside one another on a carrier  37  made of flexible plastic, or applied in another way, see FIG. 4. The individual conductors  38 ,  39 ,  40 ,  41  are provided for different voltages. The outer conductor  38  carries the secondary electron voltage. The conductor  39  lying closest thereto is earthed. The following conductor  40  carries the ion signal. The last, outer conductor  41  is connected to a high voltage.  
         [0030]    At an end opposite the SEM  11 , the flexible printed circuit board  36  has a connecting piece  42  having four contacts  43 ,  44 ,  45 ,  46 , which are associated with the conductors  38 ,  39 ,  40 ,  41 .  
         [0031]    The SEM  11  with frame  10  is a constituent part of a multicollector, not specifically shown, for a mass spectrometer. In order to record an ion distribution (or for other reasons), a plurality of frames  10  with SEM  11  are provided beside one another. In this case, various types of groups can be formed. Firstly, the frames  10  with SEM  11  can be arranged with their undersides  16  on specific holders  47 . FIG. 5 shows a frame  10  on a holder  47 . The latter has a cylindrical receptacle  48  for the frame  10 . The receptacle  48  is provided, in a manner not specifically shown, with a specifically configured upper side, so that recesses  49  on the underside  16  can be placed on the holder  17  in an exactly reproducible position.  
         [0032]    The holder  47  is arranged on a carriage  50  which, in a manner not shown specifically, can be displaced with a movement component transverse to the ion beam. Given a plurality of carriages  50  each having a frame  10 , the individual SEM can be positioned independently of one another. An arrangement having a plurality of carriages (but with Faraday cups) is shown in more detail in the company brochure from Thermo Finnigan MAT GmbH cited in the introduction to the description. Reference is made to the entire disclosure of the company brochure.  
         [0033]    [0033]FIG. 7 shows a plurality of frames  10  arranged with their large-area side walls  18 ,  19  adjacent. The said frames are threaded onto two guide means  51 ,  52  constructed as rods. For this purpose, each frame  10  has holding means  53 ,  54  on its narrow upper side  17 . In practical terms, these are hooks with inner contact surfaces  55  and locking screws  56  opposite the latter. By means of the aforementioned guide means  51 ,  52  and holding means  53 ,  54 , it is possible to connect a plurality of frames  10  to one another to form a group  57  and, at the same time, to define the relative arrangement of the frames  10  within the group  57  exactly. One of the frames  10  is mounted on the holder  41  shown in FIGS. 5 and 6 and can be adjusted with the carriage  50 . Accordingly, the entire group  57  can be moved with the carriage  50 . Mobility is also favoured by the aforementioned flexible printed circuit boards  36  on each frame  10 .  
         [0034]    Instead of the frame  10  with SEM  11 , the Faraday cups shown in the aforementioned company brochure can also be arranged on the carriage  50  or holders  47 . Their construction can be seen in the company brochure and, in addition, is illustrated in the German Laid-Open Specification DE 198 38 553. In the present case, holding means corresponding to the holding means  53 ,  54  illustrated in FIG. 5 are additionally provided.  
         [0035]    The external dimensions of the frames  10  and of the Faraday cups should largely correspond to one another. This makes it possible to form groups which contain only frames  10  with SEM  11  or else at least one Faraday cup and at least one frame  10  with SEM  11 , the individual members of a group being held together by the guide means and holding means already mentioned. A group  57  with a Faraday cup  58  and four frames  10  is shown in plan view in FIG. 8, in a similar way to FIG. 6. The Faraday cup  58  is connected electrically by a conductor  59  to the carriage  50  but not to the SEM  11 . In addition, only the Faraday cup  58  is situated on the holder  47 . Otherwise, the group is held together by the holding means and guide means  51 ,  52 . Finally, the Faraday cups can also have flexible printed circuit boards for an electrical connection.