Patent Publication Number: US-11640902-B2

Title: Ion detector and mass spectrometer each including multiple dynodes

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     At least one aspect of the present invention relates to an ion detector. Another aspect of the present invention relates to a mass spectrometer. 
     2. Description of Related Art 
     Known ion detectors detect positive or negative ions (see, for example, Japanese Unexamined Patent Publication No. S63-276862 and Japanese Unexamined Patent Publication No. H4-233151). The ion detector disclosed in Japanese Unexamined Patent Publication No. S63-276862 includes a dynode that emits a secondary electron due to collision of the positive ion, a dynode that emits the secondary electron due to collision of the negative ion, a scintillator on which the secondary electron is incident, and a photomultiplier tube that detects light generated by the scintillator. The ion detector disclosed in Japanese Unexamined Patent Publication No. H4-233151 includes a first conversion dynode that generates a positive ion in response to incidence of the negative ion, a second conversion dynode that converts the positive ion from the first conversion dynode into an electron, and a secondary electron multiplier tube that detects the electron from the second conversion dynode. 
     SUMMARY OF THE INVENTION 
     In order to extend a life-span of the ion detector, it is desirable to realize an ion detector including at least two configurations. That is, it is desirable to realize an ion detector including a configuration in which the ion detector includes a scintillator and a photomultiplier tube that detects light emitted from the scintillator, and a configuration in which an electric potential given to the scintillator is possibly set low. Therefore, it is desirable for the ion detector to realize an ion detector including a configuration in which, regardless of whether an ion to be detected is a positive ion or a negative ion, the ion to be detected is converted into an electron and light converted from the electron by the scintillator is detected by the photomultiplier tube. 
     In order to extend the life-span of the ion detector, it is also desirable to realize an ion detector including another configuration. That is, it is desired that the ion detector is provided with a diode that possibly withstands long-term use. Therefore, it is desirable for the ion detector to realize an ion detector including a configuration in which, regardless of whether the ion to be detected is a positive ion or a negative ion, the ion to be detected is converted into an electron and the converted electron is detected by the diode. 
     Japanese Unexamined Patent Publication No. S63-276862 discloses the scintillator and the photomultiplier tube, but does not disclose a configuration in which a positive ion converted from a negative ion to be detected is converted into an electron. Japanese Unexamined Patent Publication No. H4-233151 does not disclose the scintillator and the photomultiplier tube. Neither Japanese Unexamined Patent Publication No. S63-276862 nor Japanese Unexamined Patent Publication No. H4-233151 discloses a diode as an ion detector. 
     An object of the first to third aspects of the present invention is to provide an ion detector having a long life-span. An object of the fourth aspect of the present invention is to provide a mass spectrometer including an ion detector having a long life-span. 
     An ion detector according to the first aspect is an ion detector that detects an incident ion, and includes a first dynode configured to emit a charged particle in response to the incidence of the ion, a second dynode configured to be given a negative potential and emit a secondary electron in response to incidence of the charged particle from the first dynode, a scintillator including an electron incident surface arranged to receive the secondary electron from the second dynode, and configured to convert the secondary electron into light, a conductive layer disposed on an electron incident surface, and a photomultiplier tube configured to detect the light from the scintillator. 
     According to the first aspect, the ion detector includes the scintillator and the photomultiplier tube configured to detect the light emitted from the scintillator. The ion detector includes the first and second dynodes. The first dynode emits the charged particle in response to the incidence of an ion. The second dynode emits the secondary electron in response to the incidence of the charged particle from the first dynode. The secondary electron from the second dynode is incident on the scintillator. The scintillator converts the incident secondary electron into light even when the given electric potential is low. Since the potential given to the scintillator is possibly set low, the life-span of the ion detector is extended. 
     In the first aspect, the scintillator may include a light exit surface arranged to emit light. The photomultiplier tube may include a light incident window arranged to receive the light from the light exit surface. The light exit surface may be disposed in close proximity to the light incident window. 
     In this case, optical loss of light incident on the photomultiplier tube from the scintillator is reduced. Even in a case the electric potential given to the photomultiplier tube is low, photodetection sensitivity in the photomultiplier tube is ensured. 
     In the first aspect, the first dynode may be configured to be given a negative potential to convert a positive ion into the secondary electron, and the second dynode may be configured to allow the secondary electron from the first dynode to be incident on the electron incident surface of the scintillator, in the ion detector configured to detect the positive ion. 
     In this case, the positive ion incident on the ion detector is converted into the secondary electron by the first and second dynodes. The converted secondary electron is incident on the scintillator. The scintillator reliably converts the incident secondary electron into light even in a case the given potential is low. 
     In the first aspect, the first dynode may be configured to be given a positive potential to convert a negative ion into a positive ion, and the second dynode may be configured to convert the positive ion from the first dynode into the secondary electron and allow the secondary electron to be incident on the electron incident surface of the scintillator, in the ion detector configured to detect the negative ion. 
     In this case, the negative ion incident on the ion detector is converted into the secondary electron by the first and second dynodes. The secondary electron from the second dynode is incident on the scintillator. The scintillator reliably converts the incident secondary electron into light even in a case the given potential is low. 
     In the first aspect, the scintillator may be configured to be given a negative potential. The second dynode may be configured to be given the negative potential whose magnitude is larger than a magnitude of the negative potential given to the scintillator. 
     In this case, the scintillator is given an electric potential lower than the magnitude of the negative potential given to the second dynode. 
     In the first aspect, the second dynode may be configured to be given a negative potential whose magnitude is between a magnitude of the negative potential given to the first dynode and a magnitude of the negative potential given to the scintillator, in the ion detector configured to detect a positive ion. 
     In this case, the second dynode is given an electric potential lower than the magnitude of the negative potential given to the first dynode. 
     In the first aspect, the photomultiplier tube may include a side tube configured to be given a cathode potential. The conductive layer may be electrically connected to the side tube. 
     In this case, the electric potential of the scintillator is approximately the same as the cathode potential of the photomultiplier tube. A single power source may supply electric power to the scintillator and the photomultiplier tube. The number of power supplies is reduced. 
     The first aspect may include a cover covering the second dynode. The cover may include a first passage port arranged to allow the charged particle from the first dynode to pass therethrough and a second passage port arranged to allow the secondary electron from the second dynode to pass therethrough. 
     In this case, the secondary electron emitted from the second dynode is more reliably directed to the scintillator. 
     The first aspect may include a mesh covering the first passage port and being configured to be given a negative potential. 
     In this case, the mesh reduces that the secondary electron passes through the first passage port and is directed from the second dynode to the first dynode. The secondary electron emitted from the second dynode is more reliably directed to the scintillator. 
     In the first aspect, the first dynode may be disposed to be spaced apart from a virtual plane including the second dynode, the second passage port, and the electron incident surface of the scintillator. The first dynode may be configured to allow the charged particle from the first dynode to be incident on the second dynode from a direction intersecting the virtual plane. 
     In this case, the secondary electron emitted from the second dynode tends not to be directed to the first dynode. The secondary electron emitted from the second dynode more reliably tends to be directed to the scintillator. 
     An ion detector according to the second aspect is an ion detector that detects an incident ion, and includes a first dynode configured to emit a charged particle in response to the incidence of the ion, a second dynode configured to be given a negative potential and emit a secondary electron in response to incidence of the charged particle from the first dynode, and a diode including an electron incident surface arranged to receive the secondary electron from the second dynode, and configured to detect the incident secondary electron. 
     According to the second aspect, the ion detector includes the diode. The first dynode emits the charged particle in response to the incidence of the ion. The second dynode emits the secondary electron in response to the incidence of the charged particle from the first dynode. The secondary electron from the second dynode is incident on the diode. Since the diode possibly withstands long-term use, the life-span of the ion detector is extended. 
     In the second aspect, the first dynode may be configured to be given a negative potential to convert a positive ion into the secondary electron, and the second dynode may be configured to allow the secondary electron from the first dynode to be incident on the electron incident surface, in the ion detector configured to detect the positive ion. 
     In this case, the positive ion incident on the ion detector is converted into the secondary electron by the first and second dynodes. The converted secondary electron is incident on the diode. The diode reliably detects the incident secondary electron and outputs an electric signal. 
     In the second aspect, the first dynode may be configured to be given a positive potential to convert a negative ion into a positive ion, and the second dynode may be configured to convert the positive ion from the first dynode into the secondary electron and allow the secondary electron to be incident on the electron incident surface, in the ion detector configured to detect the negative ion. 
     In this case, the negative ion incident on the ion detector is converted into the secondary electron by the first and second dynodes. The secondary electron from the second dynode is incident on the diode. The diode reliably detects the incident secondary electron and outputs an electric signal. 
     The second aspect may include a cover covering the second dynode. The cover may include a first passage port arranged to allow the charged particle from the first dynode to pass therethrough and a second passage port arranged to allow the secondary electron from the second dynode to pass therethrough. 
     In this case, the secondary electron emitted from the second dynode is more reliably directed to the diode. 
     The second aspect may further include a mesh covering the first passage port and being configured to be given a negative potential. 
     In this case, the mesh reduces that the secondary electron passes through the first passage port and is directed from the second dynode to the first dynode. The secondary electron emitted from the second dynode is more reliably directed to the diode. 
     In the second aspect, the first dynode may be disposed to be spaced apart from a virtual plane including the second dynode, the second passage port, and the electron incident surface. The first dynode may be configured to allow the charged particle from the first dynode to be incident on the second dynode from a direction intersecting the virtual plane. 
     In this case, the secondary electron emitted from the second dynode tends not to be directed to the first dynode. The secondary electron emitted from the second dynode is more reliably directed to the diode. 
     The second aspect may include a substrate on which the diode is disposed and a drive circuit configured to drive the diode. The drive circuit may include an electrical resistance element including one end electrically connected to an anode of the diode, and another end configured to be grounded. The electrical resistance element may be spaced apart from the diode and the substrate. 
     In this case, since the electrical resistance element is disposed to be spaced apart from the diode and the substrate, heat generated in the electrical resistance element tends not to be transferred to the diode. A gain of the diode tends not to decrease. 
     An ion detector according to the third aspect is an ion detector that detects an incident ion, and includes a first dynode configured to emit a charged particle in response to the incidence of the ion, a second dynode configured to be given a negative potential and emit a secondary electron in response to incidence of the charged particle from the first dynode, and a detection unit including an electron incident surface arranged to receive the secondary electron from the second dynode, and configured to detect the incident secondary electron. 
     According to the third aspect, the ion detector includes the detection unit that detects the incident secondary electron. The first dynode emits the charged particle in response to the incidence of the ion, and the second dynode emits the secondary electron in response to the incidence of the charged particle from the first dynode. The secondary electron from the second dynode is incident on the detection unit. Since the detection unit possibly include a configuration that withstands long-term use, the life-span of the ion detector is extended. 
     The mass spectrometer according to the fourth aspect includes an ionization unit configured to ionize a sample, a mass spectrometer unit configured to allow only an ion to be detected to pass among ions from the ionization unit, and the above-mentioned ion detector configured to detect the ion to be detected from the mass spectrometer unit. 
     According to the fourth aspect, the mass spectrometer includes an ion detector having a long life-span. The life-span of the mass spectrometer is extended. 
     The present invention will become more fully understood from the detailed description given hereinafter 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 embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and 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 illustrating a configuration of a mass spectrometer according to an embodiment; 
         FIG.  2    is a perspective view illustrating an ion detector; 
         FIG.  3    is a diagram illustrating a support; 
         FIG.  4    is a diagram illustrating a cross-sectional configuration of a scintillator and a photomultiplier tube; 
         FIG.  5    is a diagram illustrating a cross-sectional configuration of a second dynode and a cover; 
         FIG.  6    is a diagram illustrating the ion detector; 
         FIG.  7    is a diagram illustrating a first modification of the ion detector; 
         FIG.  8    is a diagram illustrating a second modification of the ion detector; 
         FIG.  9    is a diagram illustrating the second modification of the ion detector; 
         FIG.  10    is a diagram illustrating a third modification of the ion detector; 
         FIG.  11    is a diagram illustrating a fourth modification of the ion detector; 
         FIG.  12    is a diagram illustrating an equivalent circuit of a drive circuit of a diode; 
         FIG.  13    is a diagram illustrating an equivalent circuit of the drive circuit of the diode; 
         FIG.  14    is a diagram illustrating an equivalent circuit of the drive circuit of the diode; and 
         FIG.  15    is a diagram illustrating a fifth modification of the ion detector. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same elements or elements having the same functions are denoted with the same reference numerals and overlapped explanation is omitted. 
     A configuration of a mass spectrometer  1  according to the present embodiment will be described with reference to  FIGS.  1  to  5   .  FIG.  1    is a schematic view illustrating the mass spectrometer according to this embodiment.  FIG.  2    is a perspective view illustrating an ion detector.  FIG.  3    is a diagram illustrating a support.  FIG.  4    is a diagram illustrating a cross-sectional configuration of a scintillator and a photomultiplier tube.  FIG.  5    is a diagram illustrating a cross-sectional configuration of a second dynode and a cover. 
     As illustrated in  FIG.  1   , the mass spectrometer  1  includes a sample introduction unit  2 , an ionization unit  3 , a mass spectrometer unit  4 , an ion detector  5 , and a signal processing unit  6 . The sample introduction unit  2  introduces a sample P 1  into the ionization unit  3 . The ionization unit  3  is configured to ionize the sample P 1  introduced from the sample introduction unit  2 . The ionization unit  3  introduces an ionized sample P 2  into the mass spectrometer unit  4 . The mass spectrometer unit  4  is configured to allow only an ion to be detected to pass among ions from the ionization unit  3 . The mass spectrometer unit  4  includes, for example, a quadrupole analyzer, and allows only an ion P 3  to be detected to pass through. The ion P 3  to be detected is incident on the ion detector  5 . The ion detector  5  detects the incident ion P 3 . The ion detector  5  is configured to detect the ion P 3  to be detected from the mass spectrometer unit  4 . The signal processing unit  6  processes a detection signal SG 1  from the ion detector  5 . 
     The mass spectrometer  1  includes a housing  7 . The ionization unit  3 , the mass spectrometer unit  4 , and the ion detector  5  are contained in the housing  7 . In this embodiment, the housing  7  includes a vacuum chamber. The mass spectrometer  1  includes a power source unit  8 . The power source unit  8  supplies electric power EP 1  to the ion detector  5 . The power source unit  8  includes, for example, an assembly of a plurality of power sources. 
     As illustrated in  FIGS.  2  and  3   , the ion detector  5  includes a first dynode  10 , a second dynode  20 , a detection unit  30 , and a support  60 . The detection unit  30  includes a scintillator  40  and a photomultiplier tube  50 . The first dynode  10  is configured to emit a charged particle P 4  in response to the incidence of the ion P 3  to be detected. The second dynode  20  is configured to emit a secondary electron P 5  in response to the incidence of the charged particle P 4  from the first dynode  10 . The detection unit  30  is configured to detect the secondary electron P 5  that is incident from the second dynode  20 . In this embodiment, the charged particle P 4  includes a positive ion or a secondary electron. In  FIG.  2   , the support  60  is not illustrated. 
     In the detection unit  30 , the scintillator  40  converts the secondary electron P 5  from the second dynode  20  into light. The scintillator  40  emits the converted light toward the photomultiplier tube  50 . The photomultiplier tube  50  is configured to detect the light from the scintillator  40 . The photomultiplier tube  50  includes a plurality of electrodes  58 . Some of the plurality of electrodes  58  transmit the detection signal SG 1  of the photomultiplier tube  50  to the signal processing unit  6  (see  FIG.  1   ). Of the plurality of electrodes  58 , other electrodes transmit the electric power from the power source unit  8  to the detection unit  30 . The scintillator  40  and the photomultiplier tube  50  may be disposed to be spaced apart from each other, or may have a configuration in which they are integrally coupled to each other. The scintillator  40  is made of, for example, an organic material or an inorganic material. The organic material is, for example, plastic. The inorganic material is, for example, gadolinium oxysulfide, zinc oxide, or gallium nitride. 
     The detection unit  30  is spaced apart from the second dynode  20  in a second direction D 2 . A distance between the detection unit  30  and the second dynode  20  is relatively small so that the secondary electron P 5  from the second dynode  20  is more reliably incident on the scintillator  40 . The distance between the detection unit  30  and the second dynode  20  in the second direction D 2  is, for example, 4 mm. In  FIGS.  2  and  3   , an example of each path through which the ion P 3 , the charged particle P 4 , and the secondary electron P 5  move is illustrated with a solid line and a broken line. The ion P 3 , the charged particle P 4 , and the secondary electron P 5  are schematically indicated with arrows. The arrows indicating the ion P 3 , the charged particle P 4 , and the secondary electron P 5  are illustrated to be spaced apart from the above-mentioned paths in order that each arrow can be seen well on the drawing. 
     The support  60  supports the first dynode  10 , the second dynode  20 , and the detection unit  30 . The support  60  includes a base  62  in which an inlet  61  is formed, and supports  64 ,  66 , and  68  coupled with the base  62 . In this embodiment, the first dynode  10  is positioned opposite side of the second dynode  20  and detection unit  30  with the base  62  being sandwiched therebetween in the first direction D 1 . The base  62  is made of, for example, stainless steel. The electric potential of the base  62  is set to a ground potential. 
     The support  64  supports the first dynode  10  to the base  62 . The first dynode  10  is supported by the support  64  to emit the charged particle P 4  in the first direction D 1 . The charged particle P 4  that have passed through the inlet  61  are directed to the second dynode  20 . The support  64  includes an insulating material. The support  64  electrically insulates the first dynode  10  from the base  62 . 
     The support  66  supports the second dynode  20  to the base  62 . The second dynode  20  is supported by the support  66  so that the charged particle P 4  that has passed through the inlet  61  is incident. The second dynode  20  emits the secondary electron P 5  in response to the incidence of the charged particle P 4 . The support  66  includes an insulating material. The support  66  electrically insulates the second dynode  20  from the base  62 . A distance between the first dynode  10  and the second dynode  20  in the first direction D 1  is, for example, 20 to 40 mm. In this embodiment, the distance between the first dynode  10  and the second dynode  20  in the first direction D 1  is 23 mm or 35 mm. 
     The support  68  supports the detection unit  30  to the base  62 . The secondary electron P 5  from the second dynode  20  travels in the second direction D 2  and is incident on the scintillator  40  of the detection unit  30 . The scintillator  40  is disposed so that a surface on which the secondary electron P 5  is incident faces the second direction D 2 . The support  68  includes an insulating material. The support  68  electrically insulates the detection unit  30  from the base  62 . The insulating material contained in the supports  64 ,  66 , and  68  is made of, for example, ceramics or PEEK (polyetheretherketone). 
     In the ion detector  5 , the first dynode  10  is given a negative or positive potential by the power source unit  8  depending on whether the incident ion P 3  to be detected is a positive ion or a negative ion. When the ion P 3  to be detected is a positive ion, the first dynode  10  is configured to be given a negative potential by the power source unit  8 . The first dynode  10  given a negative potential attracts a positive ion. The first dynode  10  converts the attracted positive ion into the secondary electron. The converted secondary electron is incident on the second dynode  20 . When the ion P 3  to be detected is a negative ion, the first dynode  10  is configured to be given a positive potential by the power source unit  8 . The first dynode  10  given a positive potential attracts a negative ion and converts the attracted a negative ion into a positive ion. The converted positive ion is incident on the second dynode  20 . The positive and negative ions as ion P 3  are incident on a surface  10   a  of the first dynode  10  approximately perpendicular to the surface  10   a . The charged particle P 4  emitted from the first dynode  10  is emitted in an approximately perpendicular direction from the surface  10   a  of the first dynode  10 . The first dynode  10  is, for example, an electrode made of a metal material. In this embodiment, the first dynode  10  is made of aluminum, stainless steel, or a Cu—Be alloy. The first dynode  10  has, for example, a plate shape. 
     The second dynode  20  is configured to be given a negative potential by the power source unit  8 . When the ion P 3  to be detected is a positive ion, the second dynode  20  emits the secondary electron from the first dynode  10  toward the scintillator  40 . When the ion P 3  to be detected is a negative ion, the second dynode  20  attracts the positive ion from the first dynode  10 . The second dynode  20  converts the attracted positive ion into the secondary electron P 5 . The converted secondary electron P 5  is incident on the scintillator  40 . The second dynode  20  is, for example, an electrode made of a metal material. In this embodiment, the second dynode  20  is made of aluminum, stainless steel, or a Cu—Be alloy. The second dynode  20  has, for example, a plate shape. 
     The scintillator  40  is given a negative potential by the power source unit  8 . When the ion P 3  is a positive ion, as described above, the secondary electron converted from the positive ion by the first dynode  10  is incident on the scintillator  40 . The scintillator  40  is configured to convert the secondary electron from the first dynode  10  into light. When the ion P 3  is a negative ion, as described above, the secondary electron P 5  converted from the positive ion by the second dynode  20  is incident on the scintillator  40 . The scintillator  40  converts the secondary electron P 5  incident from the second dynode  20  into light. 
     As illustrated in  FIG.  4   , in the ion detector  5 , the scintillator  40  includes an electron incident surface  42  and a light exit surface  44 . The ion detector  5  includes a conductive layer  46  disposed on the electron incident surface  42 . A negative potential is given to the conductive layer  46  by the power source unit  8 . The secondary electron P 5  from the second dynode  20  is incident on the conductive layer  46  and passes through the conductive layer  46 . The secondary electron P 5  that has passed through the conductive layer  46  is received by the electron incident surface  42  of the scintillator  40 , and enters the scintillator  40  from the electron incident surface  42 . The electron incident surface  42  is arranged to receive the secondary electron P 5 . The scintillator  40  converts the secondary electron P 5  into light. The light converted by the scintillator  40  is emitted from the light exit surface  44  toward the photomultiplier tube  50 . The light exit surface  44  is arranged to emit the light converted by the scintillator  40 . In this embodiment, the electron incident surface  42  and the light exit surface  44  oppose each other in the second direction D 2 . The conductive layer  46  is provided on the electron incident surface  42 . The conductive layer  46  is, for example, a vapor deposition film made of a metal material. In this embodiment, the conductive layer  46  is made of aluminum. 
     The photomultiplier tube  50  detects the light from the scintillator  40 . The photomultiplier tube  50  includes a side tube  54  in which an opening  52  is formed. The opening  52  is formed at one end of the side tube  54 . The side tube  54  is disposed in the scintillator  40  so that the opening  52  opposes the scintillator  40 . The photomultiplier tube  50  includes a light incident window  55 . The light from the scintillator  40  passes through the opening  52  and is incident on the light incident window  55 . The light from the light exit surface  44  is incident on the light incident window  55 . The light incident window  55  is arranged to receive the light from the light exit surface  44 . The light incident window  55  is disposed in the opening  52 . The photomultiplier tube  50  converts the light incident on the light incident window  55  into electron. The photomultiplier tube  50  multiplies the photoelectrically converted electron. A negative potential is given to the photomultiplier tube  50  by the power source unit  8 . The light incident window  55  is disposed in close proximity to the light exit surface  44  of the scintillator  40 . The expression “in close proximity to” as used herein includes, for example, the following two aspects: The light incident window  55  is optically coupled to the light exit surface  44  via silicone oil or the like. A distance between the light incident window  55  and the light exit surface  44  is small. 
     The side tube  54  is configured to be given a cathode potential of the photomultiplier tube  50 . The conductive layer  46  is electrically connected to the side tube  54 . In this embodiment, a connecting body  56  made of an electrically conductive paste electrically connects the conductive layer  46  and the side tube  54 . The connecting body  56  is provided to cover a boundary between the scintillator  40  and the photomultiplier tube  50 . In the configuration in which the conductive layer  46  and the side tube  54  are electrically connected, the electric potential of the scintillator  40  is approximately the same as the potential of the side tube  54 . In this embodiment, the scintillator  40  and the photomultiplier tube  50  constitute the detection unit  30  integrated by the connecting body  56 . The emission surface  44  and the light incident window  55  are optically coupled to each other. 
     In the ion detector  5 , the power source unit  8  changes a polarity of the electric potential given to the first dynode  10  and adjusts a magnitude of the electric potential given to the first dynode  10 , the second dynode  20 , and the scintillator  40 , depending on whether the incident ion P 3  to be detected is a positive ion or a negative ion. When the ion P 3  to be detected is a positive ion, the potential given to the first dynode  10  is, for example, about −12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. The potential given to the scintillator  40  is set, for example, in a range of 0 kV to −1 kV. In this embodiment, the magnitude of the negative potential given to the second dynode  20  is a magnitude between the magnitude of the negative potential given to the first dynode  10  and the magnitude of the negative potential given to the scintillator  40 . The magnitude of the negative potential given to the second dynode  20  is larger than the magnitude of the negative potential given to the scintillator  40 . As used herein, the “magnitude of negative potential” means an absolute value of the magnitude of the negative potential. For example, the expression “the magnitude of the negative potential given to the second dynode  20  is larger than the magnitude of the negative potential given to the scintillator  40 ” means that “the absolute value of the negative potential given to the second dynode  20  is larger than the absolute value of the negative potential given to the scintillator  40 ”. 
     When the ion P 3  to be detected is a negative ion, the electric potential given to the first dynode  10  is, for example, about 12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. The potential given to the scintillator  40  is set, for example, in a range of 0 kV to −1 kV. In this embodiment, even when the ion P 3  to be detected is a negative ion, the magnitude of the negative potential given to the second dynode  20  is larger than the magnitude of the negative potential given to the scintillator  40 . The power source unit  8  supplies electric power to the first dynode  10 , the second dynode  20 , and the scintillator  40 , and also supplies electric power to the photomultiplier tube  50 . In this embodiment, the power source unit  8  includes an assembly of four power sources. 
     As illustrated in  FIGS.  2  and  5   , the ion detector  5  includes a cover  70  that covers the second dynode  20 . The cover  70  includes a side wall  71   a , a side wall  71   b , a pair of end walls  72   a  and  72   b  opposing each other, and a bottom wall  73 . In this embodiment, the second dynode  20  is located in the bottom wall  73  and is integrated with the cover  70 . A structure ST 1  in which the second dynode  20  and the cover  70  are integrated has, for example, a hollow triangular prism shape. In the structure ST 1 , a bottom portion  20   b  and the bottom wall  73  of the second dynode  20  constitute one side surface of the hollow triangular prism. Each of the side walls  71   a  and  71   b  constitutes another side surface of the hollow triangular prism. Each of the end walls  72   a  and  72   b  constitutes one bottom surface of the hollow triangular prism. 
     As illustrated in  FIG.  2   , in the structure ST 1 , the side wall  71   a  extends in a third direction D 3  intersecting the first direction D 1  and the second direction D 2 , and couples the pair of end walls  72   a  and  72   b  each other. A first passage port  75  is formed in the side wall  71   a . The first passage port  75  is located, for example, in a central region of the side wall  71   a . The first passage port  75  is arranged to allow the charged particle P 4  from the first dynode  10  to pass therethrough. In the structure ST 1 , the side wall  71   b  extends in the third direction D 3  and couples the pair of end walls  72   a  and  72   b  each other. A second passage port  76  is formed in the side wall  71   b . The second passage port  76  is located, for example, in a central region of the side wall  71   b . The second passage port  76  is arranged to allow the secondary electron P 5  from the second dynode  20  to pass therethrough. The charged particle P 4  from the first dynode  10  passes through the inlet  61 . The charged particle P 4  that has passed through the inlet  61  passes through the first passage port  75  and is incident on the second dynode  20 . The secondary electron P 5  from the second dynode  20  passes through the second passage port  76  and is incident on the detection unit  30 . 
     The ion detector  5  includes a mesh  77  that covers the first passage port  75 . The mesh  77  is given a negative potential. The electric potential given to the mesh  77  is, for example, the same potential as the potential given to the second dynode  20 . The potential given to the mesh  77  is, for example, about −5 kV. Since the potential of the base  62  is set to the ground potential, the potential given to the mesh  77  is lower than the potential of the base  62 . The mesh  77  is made of, for example, a metal material. In this embodiment, the mesh  77  is made of stainless steel. 
     As illustrated in  FIG.  5   , the second dynode  20  is disposed to intersect the first direction D 1 . The charged particle P 4  from the first dynode  10  is obliquely incident on a surface  20   a  of the second dynode  20 . The incident angle T 1  of the charged particle P 4  on the surface  20   a  is defined as an angle formed by an incidence direction of the charged particle P 4  and a normal direction Nx 1  of the surface  20   a . In this embodiment, the incident direction of the charged particle P 4  is the first direction D 1 . The incident angle T 1  is, for example, about 22.5 degrees. In  FIG.  5   , an example of each path through which the charged particle P 4  and the secondary electron P 5  move is illustrated by a solid line. The charged particle P 4  and the secondary electron P 5  are schematically indicated with arrows. The arrows indicating the charged particle P 4  and the secondary electron P 5  are illustrated to be spaced apart from the above-mentioned paths in order that each arrow can be seen well on the drawing. 
     Next, a layout of the ion detector  5  according to the embodiment will be described with reference to  FIGS.  2  and  6   .  FIG.  6    is a layout diagram of the ion detector  5  when viewed in the second direction D 2 . As illustrated in  FIGS.  2  and  6   , the charged particle P 4  from the first dynode  10  is incident on the second dynode  20  in the first direction D 1 . The secondary electron P 5  from the second dynode  20  is incident on the detection unit  30  in the second direction D 2 . In  FIG.  6   , a virtual plane V 1  is illustrated by a chain double-dashed line, and the virtual plane V 1  is defined as a plane including the second dynode  20 , the second passage port  76 , and the electron incident surface  42 . In this embodiment, the first direction D 1  and the second direction D 2  are included in the virtual plane V 1 . The first dynode  10 , the inlet  61 , and the first passage port  75  are located in the virtual plane V 1 . The charged particle P 4  from the first dynode  10  passes through the inlet  61  and the first passage port  75  in this order along the virtual plane V 1 . The charged particle P 4  that has passed through the first passage port  75  is incident on the second dynode  20 . The secondary electron P 5  from the second dynode  20  is incident on the detection unit  30  along the virtual plane V 1 . In  FIG.  6   , the charged particle P 4  is schematically illustrated with an arrow. An example of the path of movement of the charged particle P 4  corresponds to a chain double-dashed line displaying the virtual plane V 1  when viewed in the second direction D 2 . The arrow indicating the charged particle P 4  is illustrated to be spaced apart from the chain double-dashed line displaying the virtual plane V 1  in order that the arrow can be seen well on the drawing. 
       FIG.  7    is a layout diagram of an ion detector  5   p  according to a first modification when viewed in the second direction D 2 , and corresponds to the layout diagram of  FIG.  6   . In  FIG.  7   , the incident direction of the secondary electron P 5  from the second dynode  20  to the detection unit  30  coincides with the incident direction of the secondary electron P 5  illustrated in  FIG.  6   . Even in the ion detector  5   p , the second dynode  20  and the detection unit  30  are disposed on the virtual plane V 1 . However, in the ion detector  5   p , positions of a first dynode  10   p , an inlet  61   p , and a first passage port  75   p  are different from the positions in  FIG.  6   . The ion detector  5   p  also does not include a mesh that covers the first passage port  75   p . In the description of this modification, a reference numeral in which “p” is added to the reference numeral used in the above-described embodiment is used for the element having the same configuration or function as the element provided in the ion detector  5 , and the description is omitted as much as possible. 
     In this modification, the first dynode  10   p , the inlet  61   p , and the first passage port  75   p  are spaced apart from the virtual plane V 1 . The first dynode  10   p  is disposed in a direction D 1   p  intersecting the virtual plane V 1 . The inlet  61   p  is provided between the first dynode  10   p  and the second dynode  20  and located in the direction D 1   p . The first passage port  75   p  is located in the side wall  71   a  in the direction D 1   p . The first passage port  75   p  is formed, for example, in a peripheral region of the side wall  71   a . The charged particle P 4  from the first dynode  10   p  passes through the inlet  61   p  and the first passage port  75   p  in this order. The charged particle P 4  that has passed through the first passage port  75   p  is incident on the second dynode  20  in the direction D 1   p . In  FIG.  7   , the charged particle P 4  is schematically illustrated with an arrow. An example of the path of movement of the charged particle P 4  corresponds to a dashed line displaying the direction D 1   p . The arrow indicating the charged particle P 4  is illustrated to be spaced apart from the dashed line displaying direction D 1   p  in order that the arrow can be seen well on the drawing. 
     In the ion detector  5   p , none of the first dynode  10   p , the inlet  61   p , and the first passage port  75   p  are located in the virtual plane V 1 . The first passage port  75   p  is not formed in the side wall  71   a  located in the virtual plane V 1 . The secondary electron P 5  from the second dynode  20  tends not to be affected by the ground potential of the base  62 , and are incident on the detection unit  30  along the virtual plane V 1 . In the ion detector  5   p , the mesh does not have to be placed at the first passage port  75   p . Even if the mesh is not disposed at the first passage port  75   p , the secondary electron P 5  tends not to pass through the first passage port  75   p . In the modification, an angle T 2  formed by the direction D 1   p  and the virtual plane V 1  is about 45 degrees. The ion detector  5   p  may include a mesh that covers the first passage port  75   p.    
     As described above, in the present embodiment and the modification, the ion detectors  5  and  5   p  include the scintillator  40  and the photomultiplier tube  50  configured to detect the light emitted from the scintillator  40 . The ion detectors  5  and  5   p  include the first and second dynodes  10 ,  10   p , and  20 . The first dynodes  10  and  10   p  emit the charged particle P 4  in response to the incidence of the ion P 3 . The second dynode  20  emits the secondary electron P 5  in response to the incidence of the charged particle P 4  from the first dynodes  10  and  10   p . The secondary electron P 5  from the second dynode  20  is incident on the scintillator  40 . The scintillator  40  converts the incident secondary electron P 5  into light even in a case the given electric potential is low. Since the potential given to the scintillator  40  is possibly set low, the life-span of the ion detector  5  is extended. 
     In the ion detectors  5  and  5   p , the scintillator  40  includes the light exit surface  44  arranged to emit light. The photomultiplier tube  50  includes the light incident window  55  arranged to receive the light from the light exit surface  44 . The light exit surface  44  is disposed in close proximity to the light incident window  55 . 
     In this case, optical loss of the light incident on the photomultiplier tube  50  from the scintillator  40  is reduced. Even in a case the electric potential given to the photomultiplier tube  50  is low, photodetection sensitivity in the photomultiplier tube  50  is ensured. 
     In the ion detectors  5  and  5   p , the first dynodes  10  and  10   p  are configured to be given a negative potential to convert a positive ion into the secondary electron P 5 , and the second dynode  20  is configured to allow the secondary electron P 5  from the first dynodes  10  and  10   p  to be incident on the electron incident surface  42  of the scintillator  40 , in the ion detectors  5  and  5   p  configured to detect the positive ion. 
     In this case, the positive ion incident on the ion detectors  5  and  5   p  is converted into the secondary electron P 5  by the first and second dynodes  10  and  20 . The converted secondary electron P 5  is incident on the scintillator  40 . The scintillator  40  reliably converts the incident secondary electron P 5  into light even in a case the given electric potential is low. 
     In the ion detectors  5  and  5   p , the first dynodes  10  and  10   p  are configured to be given a positive potential to convert a negative ion into a positive ion, and the second dynode  20  is configured to convert the positive ion from the first dynodes  10  and  10   p  into the secondary electron P 5  and allow the secondary electron P 5  to be incident on the electron incident surface  42  of the scintillator  40 , in the ion detectors  5  and  5   p  configured to detect the negative ion. 
     In this case, the negative ion incident on the ion detectors  5  and  5   p  is converted into the secondary electron P 5  by the first and second dynodes  10 ,  10   p , and  20 . The secondary electron P 5  from the second dynode  20  is incident on the scintillator  40 . The scintillator  40  reliably converts the incident secondary electron P 5  into light even in a case the given electric potential is low. 
     In the ion detectors  5  and  5   p , the scintillator  40  is configured to be given a negative potential. The second dynode  20  is configured to be given the negative potential whose magnitude is larger than a magnitude of the negative potential given to the scintillator  40 . 
     In this case, the scintillator  40  is given an electric potential lower than the magnitude of the negative potential given to the second dynode  20 . 
     In the ion detectors  5  and  5   p , the second dynode  20  is configured to be given a negative potential whose magnitude is between a magnitude of the negative potential given to the first dynode  10  and a magnitude of the negative potential given to the scintillator  40 , in the ion detectors  5  and  5   p  configured to detect a positive ion. 
     In this case, the second dynode  20  is given an electric potential lower than the magnitude of the negative potential given to the first dynodes  10  and  10   p.    
     In the ion detectors  5  and  5   p , the photomultiplier tube  50  includes the side tube  54  configured to be given a cathode potential. The electrically conductive layer  46  is electrically connected to the side tube  54 . 
     In this case, the electric potential of the scintillator  40  is approximately the same as the cathode potential of the photomultiplier tube  50 . A single power source may supply electric power to the scintillator  40  and the photomultiplier tube  50 . The number of power supplies is reduced. 
     The ion detectors  5  and  5   p  include covers  70  and  70   p  covering the second dynode  20 . The covers  70  and  70   p  include the first passage ports  75  and  75   p  arranged to allow the charged particle P 4  from the first dynodes  10  and  10   p  to pass therethrough and the second passage port  76  arranged to allow the secondary electron P 5  from the second dynode  20  to pass therethrough. 
     In this case, the secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the scintillator  40 . 
     The ion detector  5  includes the mesh  77  covering the first passage port  75  and being configured to be given a negative potential. 
     In this case, the mesh  77  reduces that the secondary electron P 5  passes through the first passage port  75  and is directed from the second dynode  20  to the first dynode  10 . The secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the scintillator  40 . 
     In the ion detector  5   p , the first dynode  10   p  is disposed to be spaced apart from the virtual plane V 1  including the second dynode  20 , the second passage port  76 , and the electron incident surface  42  of the scintillator  40 . The first dynode  10   p  is configured to allow the charged particle P 4  from the first dynode  10   p  to be incident on the second dynode  20  from a direction D 1   p  intersecting the virtual plane V 1 . 
     In this case, the secondary electron P 5  emitted from the second dynode  20  tends not to be directed to the first dynode  10   p . The secondary electron P 5  emitted from the second dynode  20  more reliably tends to be directed to the scintillator  40 . 
     The mass spectrometer  1  includes the ion detectors  5  and  5   p  having a long life-span. The life-span of the mass spectrometer  1  is extended. 
       FIG.  8    is a diagram illustrating an ion detector  5   q  according to a second modification, and corresponds to  FIG.  3    illustrating the ion detector  5 . The ion detector  5   q  includes the first dynode  10 , the second dynode  20 , a detection unit  80 , and a support  60   q . The detection unit  80  is configured to detect the incident secondary electron P 5 , and in this modification, the detection unit  80  includes a diode  81 . The diode  81  is configured to capture an emitted electron and generate an electric signal (detection signal SG 1 ) from the acquired electron. In this modification, the diode  81  is an avalanche diode. The diode  81  may be a diode other than the avalanche diode. For example, the diode  81  may be a normal diode that does not utilize avalanche multiplication. The ion detector  5   q  differs from the ion detector  5  in terms of the configuration of the detection unit  80  and the support  60   q . Hereinafter, differences between the ion detector  5  and the ion detector  5   q  will be mainly described. 
     The diode  81  includes an electron incident surface  82  arranged to receive the secondary electron P 5  from the second dynode  20 . The diode  81  is configured to detect the secondary electron P 5  that is incident on the electron incident surface  82 . The detection unit  80  includes a substrate  83  and a coaxial connector  84 . The diode  81  is disposed on the substrate  83 . A drive circuit  85  that drives the diode  81  is disposed on the substrate  83 . The drive circuit  85  is disposed, for example, on the coaxial connector  84  side of the substrate  83 . The drive circuit  85  is disposed, for example, in a portion of the substrate  83  closer to the coaxial connector  84 . The drive circuit  85  may be disposed on the diode  81  side of the substrate  83 . The drive circuit  85  may be disposed, for example, in a portion of the substrate  83  closer to the diode  81 . The substrate  83  is made of, for example, epoxy glass. In this modification, the epoxy glass includes FR-4 (Flame Retardant Type 4). The detection signal SG 1  generated by the diode  81  is transmitted to the signal processing unit  6  via the coaxial connector  84  (see  FIG.  1   ). 
     The detection unit  80  is spaced apart from the second dynode  20  in the second direction D 2 . A distance between the detection unit  80  and the second dynode  20  is relatively small so that the secondary electron P 5  from the second dynode  20  is more reliably incident on the electron incident surface  82 . The distance between the detection unit  80  and the second dynode  20  in the second direction D 2  is, for example, 1 to 10 mm. An effective aperture of the electron incident surface  82  has a diameter of, for example, 0.5 to 5 mm. 
     The support  60   q  supports the first dynode  10 , the second dynode  20 , and the detection unit  80 . Of the support  60   q , the base  62  and the supports  64  and  66  have the same configuration and the same material as those in the present embodiment. The first dynode  10  is positioned opposite side of the second dynode  20  and detection unit  80  with the base  62  being sandwiched therebetween in the first direction D 1 . A support  68   q  supports the detection unit  80  to the base  62 . In this modification, the support  68   q  is connected to the substrate  83 . The detection unit  80  is disposed so that the electron incident surface  82  of the diode  81  faces the second direction D 2 . The support  68   q  includes an insulating material. The support  68   q  electrically insulates the detection unit  80  from the base  62 . The material of the support  68   q  is the same as the material of the support  68 . 
     In the ion detector  5   q , when the ion P 3  to be detected is a positive ion, the electric potential given to the first dynode  10  is, for example, about −12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. The potential given to the electron incident surface  82  of the diode  81  is set, for example, in a range of −1 kV to +15 kV. When the ion P 3  to be detected is a negative ion, the potential given to the first dynode  10  is, for example, about 12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. In the configuration in which the diode  81  is the avalanche diode, the potential given to the electron incident surface  82  of the diode  81  is set, for example, in the range of −1 kV to +15 kV. The power source unit  8  (see  FIG.  1   ) supplies electric power to the first dynode  10 , the second dynode  20 , and the diode  81 . The power source unit  8  supplies electric power to the diode  81  via the drive circuit  85 . In the configuration in which the diode  81  is the normal diode described above, the potential given to the electron incident surface  82  of the diode  81  is set, for example, in the range of −1 kV to +15 kV even when the ion P 3  to be detected is either a positive ion or a negative ion. In this modification, a positive potential can be given to the diode  81 . In this case, focusing properties of the secondary electron P 5  incident on the electron incident surface  82  are improved. 
       FIG.  9    is a layout diagram of the ion detector  5   q  when viewed in the second direction D 2  and corresponds to  FIG.  6    illustrating the layout of the ion detector  5  when viewed in the second direction D 2 . In  FIG.  9   , the ion detector  5   q  differs from the ion detector  5  in terms of the configuration of the detection unit  80 . In the ion detector  5   q , the first dynode  10 , the inlet  61 , and the first passage port  75  are located in the virtual plane V 1 . The virtual plane V 1  is defined as a plane including the second dynode  20 , the second passage port  76 , and the electron incident surface  82 . The charged particle P 4  from the first dynode  10  passes through the inlet  61  and the first passage port  75  in this order along the virtual plane V 1 . The charged particle P 4  that has passed through the first passage port  75  is incident on the second dynode  20 . The secondary electron P 5  from the second dynode  20  is incident on the detection unit  80  along the virtual plane V 1 . 
       FIG.  10    is a layout diagram of an ion detector  5   r  according to a third modification when viewed in the second direction D 2 , and corresponds to the layout diagram of  FIG.  7   . The ion detector  5   r  differs from the ion detector  5   p  of  FIG.  7    in terms of the configuration of the detection unit  80 . Except for the detection unit  80 , the configuration of the ion detector  5   r  is the same as the configuration of the ion detector  5   p . In the description of this modification, for the element having the same configuration or function as the element provided in the ion detector  5   p , the reference numeral “p” used for the description in the ion detector  5   p  described above is changed to a reference numeral “r”, and the description is omitted as much as possible. A first dynode  10   r  is disposed to be spaced apart from the virtual plane V 1 . The charged particle P 4  from the first dynode  10   r  is incident on the second dynode  20  from a direction D 1   r  intersecting the virtual plane V 1 . The second dynode  20  and the detection unit  80  are disposed in the virtual plane V 1 . 
       FIG.  11    is a diagram illustrating an ion detector according to a fourth modification. An ion detector  5   s  according to this modification includes the first dynode  10 , the second dynode  20 , the detection unit  80 , and a support  60   s . The ion detector  5   s  differs from the ion detector  5   q  in terms of the configuration of the support  60   s . Hereinafter, differences between the ion detector  5   q  and the ion detector  5   s  will be mainly described. 
     The support  60   s  supports the first dynode  10 , the second dynode  20 , and the detection unit  80 . The support  60   s  includes a base  62   s  and supports  64   s ,  66   s , and  68   s  connected to the base  62   s . In this modification, the first dynode  10 , the second dynode  20 , and the detection unit  80  are located on the same side with respect to the base  62   s  in the first direction D 1 . The electric potential of the base  62   s  is set to a ground potential. No inlet is formed in the base  62   s.    
     The support  64   s  supports the first dynode  10  to the base  62   s . The first dynode  10  is supported by the support  64   s  to emit the charged particle P 4  in the first direction D 1 . The charged particle P 4  is directed to the second dynode  20 . The support  64   s  includes an insulating material. The support  64   s  electrically insulates the first dynode  10  from the base  62   s.    
     The support  66   s  supports the second dynode  20  to the base  62   s . The second dynode  20  is supported by the support  66   s  so that the charged particle P 4  from the first dynode  10  is incident. The second dynode  20  emits the secondary electron P 5  in response to the incidence of the charged particle P 4 . The support  66   s  includes an insulating material. The support  66   s  electrically insulates the second dynode  20  from the base  62   s . The distance between the first dynode  10  and the second dynode  20  in the first direction D 1  is, for example, 1 to 10 mm. 
     The support  68   s  supports the detection unit  80  to the base  62   s . In this modification, the support  68   s  is connected to the substrate  83 . The secondary electron P 5  from the second dynode  20  travels in the second direction D 2  and is incident on the diode  81  of the detection unit  80 . The detection unit  80  is disposed so that the electron incident surface  82  faces the second direction D 2 . The support  68   s  includes an insulating material. The support  68   s  electrically insulates the detection unit  80  from the base  62   s . The materials of the base  62   s  and the supports  64   s ,  66   s , and  68   s  are the same as the materials of the base  62  and the supports  64 ,  66 , and  68 , respectively. 
     In the ion detector  5   s , when the ion P 3  to be detected is a positive ion, the electric potential given to the first dynode  10  is, for example, about −12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. The potential given to the electron incident surface  82  of the diode  81  is set, for example, in a range of −1 kV to +15 kV. When the ion P 3  to be detected is a negative ion, the potential given to the first dynode  10  is, for example, about 12 kV. The potential given to the second dynode  20  is, for example, about −5 kV. In the configuration in which the diode  81  is the avalanche diode, the potential given to the electron incident surface  82  of the diode  81  is set, for example, in the range of −1 kV to +15 kV. In the configuration in which the diode  81  is the normal diode described above, the potential given to the electron incident surface  82  of the diode  81  is set, for example, in the range of −1 kV to +15 kV even when the ion P 3  to be detected is either a positive ion or a negative ion. In this modification, a positive potential can be given to the diode  81 . In this case, focusing properties of the secondary electron P 5  incident on the electron incident surface  82  are improved. 
     As illustrated in  FIG.  12   , the drive circuit  85  includes, for example, the diode  81 , a resistor  86   a , a capacitor  87   a , and the coaxial connector  84 .  FIG.  12    is a diagram illustrating an equivalent circuit of the drive circuit of the diode. The coaxial connector  84  includes an SMA (Subminiature version A) jack. In the example of the equivalent circuit illustrated in  FIG.  12   , the coaxial connector  84  includes the SMA (Subminiature version A) jack. The drive circuit  85  receives power supply from the power source unit  8 . An anode of the diode  81  is electrically connected to the power source unit  8  via the resistor  86   a . The diode  81  includes the anode on the electron incident surface  82  side. A cathode of the diode  81  is electrically connected to a signal output terminal TR 1 . The signal output terminal TR 1  is electrically connected to the signal processing unit  6  (see  FIG.  1   ). The potential of the power source unit  8  is, for example, −350 V. In a case the magnitude of the negative potential given to the second dynode  20  can be increased, the detection unit  80  tends to detect the secondary electron P 5 . An electrical resistance value of the resistor  86   a  is, for example, 1 kΩ. 
     In the drive circuit  85 , a node N 1  is electrically connected to a side surface of the coaxial connector  84  via the capacitor  87   a . The node N 1  is located between the diode  81  and the resistor  86   a . The side surface of the coaxial connector  84  is grounded. The node N 1  constitutes a return path. The capacitor  87   a  and the diode  81  are electrically connected in parallel. The return path is formed between the electron incident surface  82  of the diode  81  and the side surface of the coaxial connector  84 . In the capacitor  87   a , in a case the detection signal SG 1  is a high-speed signal, the high-speed detection signal SG 1  returns to the diode  81  with low impedance via the return path. A capacity of the capacitor  87   a  is, for example, 10 nF. The drive circuit  85  in the configuration in which the avalanche diode is used as the diode  81  and the drive circuit  85  in the configuration in which the above-mentioned ordinary diode is used as the diode  81  have the same equivalent circuit. The resistor  86   a  and the capacitor  87   a  constitute a low-pass filter. In a case an AC component from the power source unit  8  includes ripple noise, the ripple noise may deteriorate the detection signal SG 1  output from the diode  81  to the signal output terminal TR 1 . The low-pass filter formed by the resistor  86   a  and the capacitor  87   a  removes the AC component including the ripple noise. The low-pass filter formed by the resistor  86   a  and the capacitor  87   a  reduces the deterioration of the detection signal SG 1 . 
     As illustrated in  FIG.  13   , the drive circuit  85  includes, for example, the diode  81 , a Zener diode  88 , resistors  86   a ,  86   b , and  86   c , capacitors  87   a  and  87   b , and the coaxial connector  84 .  FIG.  13    is a diagram illustrating an equivalent circuit of the drive circuit of the diode. The anode of the diode  81  is electrically connected to the power source unit  8  via the Zener diode  88  and the resistor  86   a . The diode  81  includes the anode on the electron incident surface  82  side. The potential of the power source unit  8  is, for example, 10.35 kV. The cathode of the diode  81  is electrically connected to the power source unit  8  via the resistor  86   b . The diode  81  and the Zener diode  88  are electrically connected in parallel. The cathode of the diode  81  is electrically connected to the signal output terminal TR 1  via the capacitor  87   b . The signal output terminal TR 1  is electrically connected to the signal processing unit  6 . 
     The Zener diode  88  gives, for example, an electric potential difference of 350 V between the anode and cathode of the diode  81 . In the drive circuit  85  including the equivalent circuit illustrated in  FIG.  13   , for example, the potential of the anode of the diode  81  is 10 kV, and the potential of the cathode is 10.35 kV. The drive circuit  85  including the equivalent circuit illustrated in  FIG.  13    can increase the potential of the anode of the diode  81  in a positive direction, thus increasing a gain of the detection signal SG 1 . An electrical resistance value of the resistor  86   a  is, for example, 1 kΩ. The electrical resistance value of the resistor  86   b  is, for example, 100 kΩ. 
     As illustrated in  FIG.  13   , the node N 1  is electrically connected to the side surface of the coaxial connector  84  via the capacitor  87   a . The node N 1  is located between the diode  81  and the resistor  86   a . The side surface of the coaxial connector  84  is grounded. The node N 1  is disposed to constitute a coupling capacitor. The cathode of the diode  81  is electrically connected to the signal output terminal TR 1  via the capacitor  87   b . The capacitor  87   a  and the capacitor  87   b  are electrically connected in parallel. The capacitors  87   a  and  87   b  constitute a coupling capacitor. The capacitors  87   a  and  87   b  enable the current (detection signal SG 1 ) from the diode  81  to flow to the signal output terminal TR 1  while maintaining the high electric potential of the diode  81 . Even in a case the detection signal SG 1  is a high-speed signal, the capacitors  87   a  and  87   b  can effectively transmit the AC component of the detection signal SG 1 . The capacity of the capacitors  87   a  and  87   b  is, for example, 150 pF. A node N 2  is electrically connected to the grounded resistor  86   c . The node N 2  is located between the Zener diode  88  and the resistor  86   b . The resistor  86   c  is electrically connected to the anode of the diode  81  via the resistor  86   a . The electric potential at one end of the resistor  86   c  is the same as the potential at the anode of the diode  81 . Another end of the resistor  86   c  is grounded. For example, a current of 100 μA flows through the resistor  86   c  under a potential of 10 kV. The electrical resistance value of the resistor  86   c  is, for example, 100 MΩ. The resistor  86   c  generates, for example, 1 W of heat. The drive circuit  85  in the configuration in which the avalanche diode is used as the diode  81  and the drive circuit  85  in the configuration in which the above-mentioned ordinary diode is used as the diode  81  have the same equivalent circuit. For example, the resistor  86   c  constitutes an electrical resistance element. 
     As illustrated in  FIG.  14   , the drive circuit  85  includes, for example, the diode  81 , an n-Channel Metal-Oxide Semiconductor (NMOS)  89 , resistors  86   a ,  86   b ,  86   c ,  86   d , and  86   e , the capacitors  87   a  and  87   b , and the coaxial connector  84 .  FIG.  14    is a diagram illustrating an equivalent circuit of the drive circuit of the diode. The NMOS  89  is an example of field effect transistor (FET). The anode of the diode  81  is electrically connected to the power source unit  8  via the resistor  86   a  and the NMOS  89 . A source of the NMOS  89  is electrically connected to the resistor  86   a . A drain of the NMOS  89  is electrically connected to the power source unit  8 . A gate of the NMOS  89  is electrically connected to the power source unit  8  via the resistor  86   d  and grounded via the resistor  86   e . The diode  81  includes the anode on the electron incident surface  82  side. The potential of the power source unit  8  is, for example, 10.35 kV. The cathode of the diode  81  is electrically connected to the power source unit  8  via the resistor  86   b . The diode  81  and the NMOS  89  are electrically connected in parallel. The cathode of the diode  81  is electrically connected to the signal output terminal TR 1  via the capacitor  87   b . The signal output terminal TR 1  is electrically connected to the signal processing unit  6 . 
     The NMOS  89  creates an electric potential difference of, for example, 350 V between the anode and cathode of the diode  81 . In this modification, the potential of the anode of the diode  81  is 10 kV, and the potential of the cathode is 10.35 kV. In this modification, since the potential of the anode of the diode  81  can be increased, the gain of the detection signal SG 1  is increased. An electrical resistance value of the resistor  86   a  is, for example, 1 kΩ. The electrical resistance value of the resistor  86   b  is, for example, 100 kΩ. The electrical resistance value of the resistor  86   c  is, for example, 100 MΩ. The electrical resistance value of the resistor  86   d  is, for example, 35 MΩ. The electrical resistance value of the resistor  86   e  is, for example, 1 GΩ. 
     In this modification, the node N 1  is electrically connected to the side surface of the coaxial connector  84  via the capacitor  87   a . The node N 1  is located between the diode  81  and the resistor  86   a . The side surface of the coaxial connector  84  is grounded. The node N 1  is disposed to constitute a coupling capacitor. The cathode of the diode  81  is electrically connected to the signal output terminal TR 1  via the capacitor  87   b . The capacitor  87   a  and the capacitor  87   b  are electrically connected in parallel. The capacitors  87   a  and  87   b  constitute a coupling capacitor. The capacitors  87   a  and  87   b  enable the current (detection signal SG 1 ) from the diode  81  to flow to the signal output terminal TR 1  while maintaining the high electric potential of the diode  81 . Even in a case the detection signal SG 1  is a high-speed signal, the capacitors  87   a  and  87   b  can effectively transfer the AC component of the detection signal SG 1 . The capacity of the capacitors  87   a  and  87   b  is, for example, 150 pF. A node N 2  is electrically connected to the grounded resistor  86   c . The node N 2  is located between the NMOS  89  and the resistor  86   b . One end of the resistor  86   c  is electrically connected to the anode of the diode  81  via the resistor  86   a . The potential at one end of the resistor  86   c  is the same as the potential at the anode of the diode  81 . Another end of the resistor  86   c  is grounded. For example, a current of 100 μA flows through the resistor  86   c  under a potential of 10 kV. The resistor  86   c  generates, for example, 1 W of heat. An electrical resistance value of the resistor  86   a  is, for example, 1 kΩ. The electrical resistance value of the resistor  86   b  is, for example, 100 kΩ. The electrical resistance value of the resistor  86   c  is, for example, 100 MΩ. The drive circuit  85  in the configuration in which the avalanche diode is used as the diode  81  and the drive circuit  85  in the configuration in which the above-mentioned ordinary diode is used as the diode  81  have the same equivalent circuit. 
       FIG.  15    is a diagram illustrating a fifth modification of the ion detector, and illustrates a modification of the ion detector  5   q  illustrated in  FIG.  8   . An ion detector  5   t  according to the fifth modification differs from the ion detector  5   q  in terms of the position where the resistor  86   c  is disposed. In the ion detector  5   t , the resistor  86   c  is spaced apart from the diode  81  and the substrate  83 . That is, in the ion detector  5   t , the resistor  86   c  is physically spaced apart from the diode  81  and the substrate  83 , and is thermally spaced apart from the diode  81  and the substrate  83 . In this modification, the resistor  86   c  is electrically connected to the base  62  and is grounded. Also, in the ion detector  5   s  illustrated in  FIG.  11   , in a case the drive circuit  85  includes the resistor  86   c , the resistor  86   c  may be disposed to be spaced apart from the diode  81  and the substrate  83 . 
     As described above, the ion detectors  5   q ,  5   r ,  5   s , and  5   t  include the diode  81 . The first dynodes  10  and  10   r  emit the charged particle P 4  in response to the incidence of the ion P 3 . The second dynode  20  emits the secondary electron P 5  in response to the incidence of the charged particle P 4  from the first dynodes  10  and  10   r . The secondary electron P 5  from the second dynode  20  is incident on the diode  81 . Since the diode  81  possibly withstands long-term use, life-spans of the ion detectors  5   q ,  5   r ,  5   s , and  5   t  are extended. 
     In the ion detectors  5   q ,  5   r ,  5   s , and  5   t , the first dynodes  10  and  10   r  are configured to be given a negative potential to convert a positive ion into the secondary electron P 5 , and the second dynode  20  is configured to allow the secondary electron P 5  from the first dynodes  10  and  10   r  to be incident on the electron incident surface  82 , in the ion detectors  5   q ,  5   r ,  5   s , and  5   t  configured to detect the positive ion. 
     In this case, the positive ion incident on the ion detectors  5   q ,  5   r ,  5   s , and  5   t  are converted into the secondary electron P 5  by the first and second dynodes  10 ,  10   r , and  20 . The converted secondary electron P 5  is incident on the diode  81 . The diode  81  reliably detects the incident secondary electron P 5  and outputs the electric signal. 
     In the ion detectors  5   q ,  5   r ,  5   s , and  5   t , the first dynodes  10  and  10   r  are configured to be given a positive potential to convert a negative ion into a positive ion, and the second dynode  20  is configured to convert the positive ion from the first dynodes  10  and  10   r  into the secondary electron P 5  and allow the secondary electron P 5  to be incident on the electron incident surface  82 , in the ion detectors  5   q ,  5   r ,  5   s , and  5   t  configured to detect the negative ion. 
     In this case, the negative ion incident on the ion detectors  5   q ,  5   r ,  5   s , and  5   t  is converted into the secondary electron P 5  by the first and second dynodes  10 ,  10   r , and  20 . The secondary electron P 5  from the second dynode  20  is incident on the diode  81 . The diode  81  reliably detects the incident secondary electron P 5  and outputs the electric signal. 
     The ion detectors  5   q ,  5   r ,  5   s , and  5   t  further include covers  70  and  70   r  covering the second dynode  20 . The covers  70  and  70   r  include first passage ports  75  and  75   r  arranged to allow the charged particle P 4  from the first dynodes  10  and  10   r  to pass therethrough and the second passage port  76  arranged to allow the secondary electron P 5  from the second dynode  20  to pass therethrough. 
     In this case, the secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the diode  81 . 
     The ion detectors  5   q ,  5   s , and  5   t  further include the mesh  77  covering the first passage port  75  and being configured to be given a negative potential. 
     In this case, the mesh  77  reduces that the secondary electron P 5  passes through the first passage ports  75  and  75   r  and is directed from the second dynode  20  to the first dynode  10 . The secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the diode  81 . 
     In the ion detector  5   r , the first dynode  10   r  is disposed to be spaced apart from the virtual plane V 1  including the second dynode  20 , the second passage port  76 , and the electron incident surface  82 . The first dynode  10   r  is configured to allow the charged particle P 4  from the first dynode  10   r  to be incident on the second dynode  20  from a direction D 1   r  intersecting the virtual plane V 1 . 
     In this case, the secondary electron P 5  emitted from the second dynode  20  tends not to be directed to the first dynode  10   r . The secondary electron P 5  emitted from the second dynode  20  more reliably tends to be directed to the diode  81 . 
     The ion detector  5   t  includes the substrate  83  on which the diode  81  is disposed and the drive circuit  85  configured to drive the diode  81 . The drive circuit  85  includes the resistor  86   c  including one end electrically connected to an anode of the diode  81 , and another end configured to be grounded. The resistor  86   c  is spaced apart from the diode  81  and the substrate  83 . 
     Depending on the value of the current flowing through the resistor  86   c , a calorific value of the resistor  86   c  may increase. If the heat generated in the resistor  86   c  is transferred to the diode  81 , a gain of the diode  81  may decrease. In the ion detector  5   t , as described above, the resistor  86   c  is spaced apart from the diode  81 . Therefore, the heat generated in the resistor  86   c  tends not to be transferred to the diode  81 . As a result, even in a case the calorific value of the resistor  86   c  increases, the gain of the diode  81  tends not to decrease. 
     The ion detectors  5   q ,  5   r ,  5   s , and  5   t  include the first dynodes  10  and  10   r  configured to emit the charged particle P 4  in response to the incidence of the ion P 3 , the second dynode  20  configured to be given a negative potential and emit the secondary electron P 5  in response to the incidence of the charged particle P 4  from the first dynodes  10  and  10   r , and the detection unit  80  including the electron incident surface  82  arranged to receive the secondary electron P 5  from the second dynode  20 , and configured to detect the incident secondary electron P 5 . 
     The ion detectors  5   q ,  5   r ,  5   s , and  5   t  include the detection unit  80  that detects the incident secondary electron P 5 . The first dynodes  10  and  10   r  are configured to emit the charged particle P 4  in response to the incidence of the ion P 3 , and the second dynode  20  is configured to emit the secondary electron P 5  in response to the incidence of the charged particle P 4  from the first dynodes  10  and  10   r . The secondary electron P 5  from the second dynode  20  is incident on the detection unit  80 . Since the detection unit  80  possibly include a configuration that withstands long-term use, life-spans of the ion detectors  5   q ,  5   r ,  5   s , and  5   t  are extended. 
     The mass spectrometer  1  includes the ion detectors  5   q ,  5   r ,  5   s , and  5   t  having a long life-span. The life-span of the mass spectrometer  1  is extended. 
     Although the embodiment and modification of the present invention has been described above, the present invention is not necessarily limited to the embodiment, and the embodiment can be variously changed without departing from the scope of the invention. 
     The ion detectors  5 ,  5   p ,  5   q ,  5   r ,  5   s , and  5   t  may be provided in an apparatus other than the mass spectrometer  1 . 
     The conductive layer  46  does not have to be electrically connected to the side tube  54 . In the configuration in which the electrically conductive layer  46  is electrically connected to the side tube  54 , the number of power sources is reduced as described above. 
     The mass spectrometer  1  (ion detectors  5 ,  5   p ,  5   q ,  5   r ,  5   s , and  5   t ) does not have to include the covers  70 ,  70   p , and  70   r  that include the first passage ports  75 ,  75   p , and  75   r  and the second passage port  76 . In the configuration provided with the covers  70 ,  70   p , and  70   r  that include the first passage ports  75 ,  75   p , and  75   r  and the second passage port  76 , as described above, the secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the scintillator  40  or the diode  81 . 
     The mass spectrometer  1  (ion detectors  5 ,  5   p ,  5   q ,  5   r ,  5   s , and  5   t ) does not have to include the mesh  77 . In the configuration provided with the mesh  77 , as described above, the secondary electron P 5  emitted from the second dynode  20  is more reliably directed to the scintillator  40  or the diode  81 .