Photodetector

Abstract

To provide a photodetector capable of protecting a semiconductor photodetector element from reflected electrons generated inside a vacuum enclosure.SOLUTION: A photodetector 1 includes a vacuum housing 2 having a window 12, a photocathode (first photoelectric conversion layer) 3 that emits electrons E in response to incidence of light L1, a phosphor layer (second photoelectric conversion layer) 4 that generates light L2 in response to incidence of the electrons E emitted from the photocathode 3, and a semiconductor photodetector element 5 that is disposed on the opposite side of the photocathode 3 with the phosphor layer 4 in between and detects the light L2 generated in the phosphor layer 4. The semiconductor photodetector element 5 is disposed at a distance from the phosphor layer 4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to photodetectors. [Background technology]

[0002] Photodetectors capable of detecting single photons are used in high-energy physics experiments such as cosmic ray detection experiments and accelerator experiments. Conventional photodetectors include so-called hybrid photodetectors (HPDs), which contain a photocathode and a semiconductor element such as an avalanche photodiode inside a vacuum enclosure. In hybrid photodetectors, electrons emitted from the photocathode are directly detected by the semiconductor element, but the gain required to detect single photons is high, which requires the semiconductor element to be driven at a high voltage.

[0003] In order to drive a semiconductor photodetector element at a lower voltage, a photodetector has been considered in which electrons emitted from a photocathode are first converted into light and then the converted light is detected by a semiconductor photodetector element. An example of such a photodetector is the pixel sensor described in Patent Document 1. In this conventional photodetector, a photocathode and a semiconductor photodetector element having a phosphor layer on its surface are arranged inside a vacuum housing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2003-507870 Summary of the Invention [Problem to be solved by the invention]

[0005] In a photodetector such as that described in Patent Document 1, some of the electrons emitted from the photocathode may be reflected inside the vacuum housing and become reflected electrons. An electric field is formed inside the vacuum housing such that the electrons emitted from the photocathode are focused on the phosphor layer. Therefore, it is conceivable that reflected electrons that deviate from the trajectory toward the phosphor layer may travel back toward the phosphor layer. If the reflected electrons deflect from the phosphor layer and collide with the semiconductor photodetector element or surrounding wiring, there is a risk of damaging the semiconductor photodetector element.

[0006] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a photodetector that can protect a semiconductor photodetector element from reflected electrons generated inside a vacuum enclosure. [Means for solving the problem]

[0007] The gist of the present disclosure is as follows.

[0008] [1] A photodetector comprising: a vacuum enclosure having a window portion through which light to be detected is incident; a first photoelectric conversion layer disposed inside the vacuum enclosure and emitting electrons in response to the incidence of the light; a second photoelectric conversion layer disposed inside the vacuum enclosure and generating light in response to the incidence of the electrons emitted from the first photoelectric conversion layer; and a semiconductor photodetector element disposed inside the vacuum enclosure on the opposite side of the first photoelectric conversion layer with the second photoelectric conversion layer in between, and detecting the light generated in the second photoelectric conversion layer, wherein the semiconductor photodetector element is disposed at a distance from the second photoelectric conversion layer.

[0009] In this photodetector, the semiconductor photodetector element is disposed at a distance from the second photoelectric conversion layer. When some of the electrons emitted from the first photoelectric conversion layer are reflected inside the vacuum enclosure to become reflected electrons, even if the reflected electrons deviate from the trajectory toward the second photoelectric conversion layer and travel again toward the phosphor layer due to the electric field inside the vacuum enclosure, the possibility of the reflected electrons colliding with the semiconductor photodetector element can be reduced. Therefore, in this photodetector, the semiconductor photodetector element can be protected from reflected electrons generated inside the vacuum enclosure.

[0010] [2] The photodetector according to [1], wherein an inner housing surrounding the semiconductor photodetector element is disposed inside the vacuum housing. In this case, the inner housing can physically block reflected electrons heading toward the semiconductor photodetector element. Therefore, it is possible to more reliably prevent the reflected electrons from colliding with the semiconductor photodetector element.

[0011] [3] The photodetector according to [2], wherein the inner housing defines the distance between the second photoelectric conversion layer and the semiconductor photodetector element. In this case, the distance between the second photoelectric conversion layer and the semiconductor photodetector element can be formed as designed. Therefore, the possibility of reflected electrons colliding with the semiconductor photodetector element can be more reliably reduced.

[0012] [4] The photodetector according to [2] or [3], wherein the inner housing has a hole that connects the inner space of the vacuum housing to the inner space of the other housing. In this case, when the vacuum housing is evacuated, the inner housing can be evacuated simultaneously through the hole. This makes it possible to suppress deterioration of the first photoelectric conversion layer due to residual gas inside the vacuum housing and the inner housing.

[0013] [5] The photodetector according to [4], wherein the semiconductor photodetector element is disposed at the bottom of the internal housing, and the hole is disposed around the semiconductor photodetector element at the bottom. With this configuration, the hole of the internal housing and the semiconductor photodetector element can be brought sufficiently close to each other. Therefore, even if the internal housing has a hole, it is possible to prevent reflected electrons from passing through the hole and colliding with the semiconductor photodetector element.

[0014] [6] The photodetector according to [4] or [5], wherein the bottom of the vacuum housing is provided with a tip tube that serves as an exhaust port for the vacuum housing and an inlet for introducing alkali used to alkalize the first photoelectric conversion layer, and the tip tube is positioned so as not to overlap with the inner housing when viewed from the front of the window. This configuration allows for a sufficient gap to be formed between the hole in the inner housing and the tip tube. Therefore, even if the inner housing has a hole, it is possible to prevent alkali introduced from the tip tube from passing through the hole and adhering to the semiconductor photodetector element. This contributes to suppressing noise (such as dark current) caused by alkali adhering to the semiconductor photodetector element.

[0015] [7] The photodetector according to any one of [2] to [6], wherein the inner housing is made of a conductive material. In this case, the inside of the inner housing can be made an electric field-free environment, and the formation of an electric field that would direct reflected electrons toward the semiconductor photodetector element can be suppressed. Therefore, even if reflected electrons travel inside the inner housing, the possibility of the reflected electrons colliding with the semiconductor photodetector element can be sufficiently reduced.

[0016] [8] The photodetector according to any one of [2] to [7], wherein a metal layer is disposed on the surface of the second photoelectric conversion layer facing the first photoelectric conversion layer. In this case, the component of the light generated in the second photoelectric conversion layer that is directed toward the first photoelectric conversion layer can be reflected by the metal layer and propagated to the semiconductor photodetector element. This improves the amount of light detected by the semiconductor photodetector element.

[0017] [9] The photodetector according to [8], wherein the metal layer is electrically connected to the internal housing. In this case, power can be supplied to the metal layer together with the internal housing. By supplying power to the metal layer, the metal layer can function as an electrode that forms an electric field that guides electrons emitted from the first photoelectric conversion layer to the second photoelectric conversion layer.

[0018]

[10] The photodetector according to any one of [1] to [9], wherein the area of ​​the first photoelectric conversion layer is larger than the area of ​​the second photoelectric conversion layer, and the area of ​​the second photoelectric conversion layer is larger than the area of ​​the semiconductor photodetector element. In this case, by reducing the area of ​​the semiconductor photodetector element, the response characteristics of the semiconductor photodetector element can be improved. Reducing the area of ​​the semiconductor photodetector element requires ensuring a sufficient area for the first photoelectric conversion layer to improve light collection efficiency. However, by making the area of ​​the second photoelectric conversion layer interposed between the first photoelectric conversion layer and the semiconductor photodetector element larger than the area of ​​the semiconductor photodetector element, it becomes unnecessary to focus electrons emitted from the first photoelectric conversion layer into a minute region. Eliminating the need for precise control of electron trajectories can avoid complex design of the photodetector.

[0019]

[11] The photodetector according to any one of [1] to

[10] , wherein the vacuum housing is made of a conductive material, a focusing electrode is disposed inside the vacuum housing to focus the electrons emitted from the first photoelectric conversion layer onto the second photoelectric conversion layer, and the focusing electrode has the same potential as the vacuum housing. With this configuration, the electrons emitted from the first photoelectric conversion layer can be efficiently guided toward the second photoelectric conversion layer.

[0020]

[12] The photodetector according to any one of [1] to

[11] , further comprising a light-guiding member disposed between the second photoelectric conversion layer and the semiconductor photodetector element, for guiding the light generated in the second photoelectric conversion layer to the semiconductor photodetector element. In this case, the light generated in the second photoelectric conversion layer can be efficiently guided toward the semiconductor photodetector element. Furthermore, even if ions are generated by collisions between residual gas inside the vacuum enclosure and electrons, the light-guiding member can physically block the ions from passing through the second photoelectric conversion layer toward the semiconductor photodetector element. Therefore, the semiconductor photodetector element can be protected from ion collisions. [Effects of the Invention]

[0021] According to the present disclosure, the semiconductor photodetector element can be protected from reflected electrons generated inside the vacuum enclosure. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a configuration of a photodetector according to an embodiment of the present disclosure. [Figure 2] 10(a) and 10(b) are diagrams showing the results of an evaluation test of the efficiency of electron incidence from the photocathode to the phosphor layer. [Figure 3] 10(a) and 10(b) are diagrams showing the results of an evaluation test of the efficiency of electron incidence from the photocathode to the phosphor layer. [Figure 4] 10(a) and 10(b) are diagrams showing the simulation results of the trajectories of reflected electrons generated on the surface of the phosphor layer. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, preferred embodiments of a photodetector according to one aspect of the present disclosure will be described in detail with reference to the drawings.

[0024] Fig. 1 is a schematic cross-sectional view showing the configuration of a photodetector according to an embodiment of the present disclosure. The photodetector 1 shown in Fig. 1 is configured as a photodetector capable of detecting single photons in high-energy physics experiments such as cosmic ray detection experiments and accelerator experiments. As shown in Fig. 1, the photodetector 1 includes a vacuum housing 2. Inside the vacuum housing 2, a photocathode (first photoelectric conversion layer) 3, a phosphor layer (second photoelectric conversion layer) 4, a semiconductor photodetector element 5, an inner housing 6, a light-guiding member 7, and a focusing electrode 8 are arranged.

[0025] The vacuum housing 2 is composed of, for example, a side tube 11, a window 12, and a stem 13. An internal space S1 of the vacuum housing 2 is defined by the side tube 11, the window 12, and the stem 13 and is maintained in a vacuum state. The degree of vacuum in the internal space S1 is, for example, 10 -6 Pa~10 -5 It is about Pa.

[0026] The side tube 11 is a base of the vacuum housing 2. The side tube 11 has, for example, a cylindrical shape with both axial ends open. Examples of materials that can be used for the side tube 11 include iron-nickel (FeNi) alloy, stainless steel (FeCrNi) alloy, Kovar (FeCoNi) alloy, ceramic, and glass. The side tube 11 may be made of a conductive material. When the photodetector 1 is used in a high-energy physics experiment and it is necessary to eliminate the presence of radioactive isotopes, it is preferable to use an iron-nickel (FeNi) alloy that does not contain cobalt (Co). In this embodiment, the side tube 11 is made of an iron-nickel (FeNi) alloy, which is a conductive material.

[0027] The window 12 is a portion that allows light L1 to be detected to enter the vacuum housing 2. The window 12 is provided in a disk shape so as to close one axial end of the side tube 11. Examples of materials that can be used to form the window 12 include glass materials such as synthetic quartz. The window 12 and the side tube 11 are joined together using, for example, an aluminum (Al) seal or brazing material. When the window 12 is made of synthetic quartz, it is preferable to perform thermocompression bonding using an aluminum (Al) seal.

[0028] The stem 13 constitutes the bottom of the vacuum housing 2. The stem 13 is disk-shaped and covers the other axial end of the side tube 11. The stem 13 may be made of, for example, Kovar metal. A plurality of stem pins 14 are inserted into the stem 13. The stem pins 14 include a feed pin 14A that supplies power to the vacuum housing 2 (photocathode 3), a feed pin 14B that supplies power to the semiconductor photodetector element 5, an output pin 14C that extracts a signal from the semiconductor photodetector element 5, a ground pin 14D that grounds the inner housing 6 (described below), and a feed pin 14E that supplies power to the focusing electrode 8 (described below). The stem pins 14 and the stem 13 are electrically insulated by an insulating material 15 such as glass.

[0029] The stem 13 is also provided with a tip tube 16. The tip tube 16 is used as an exhaust port for the vacuum housing 2 and as an inlet for introducing alkali used to alkalize the photocathode 3 during the manufacturing process of the photodetector 1. The tip tube 16 is located outside the position where the stem pin 14 is located and closer to the side tube 11. As a result, the tip tube 16 is located at a position where it does not overlap with the inner housing 6 when viewed from the front of the window portion 12. Note that if the size of the photodetector 1 is small, the tip tube 16 may be omitted.

[0030] The photocathode 3 is a portion that emits electrons E in response to incidence of light L1. The photocathode 3 is disposed inside the vacuum housing 2. In this embodiment, the photocathode 3 is provided in a circular shape on the inner surface of the window portion 12 (the surface facing the internal space S1). The photocathode 3 is a transmissive alkali photocathode. Examples of alkalis used for the photocathode 3 include potassium (K), cesium (Cs), and sodium (Na).

[0031] The phosphor layer 4 is a portion that generates light L2 in response to the incidence of electrons E emitted from the photocathode 3. Examples of materials for forming the phosphor layer 4 include compound semiconductors such as GaN and ZnO. A metal layer 18 is disposed on the surface of the phosphor layer 4 that faces the photocathode 3. Examples of materials for the metal layer 18 include aluminum (Al). The metal layer 18 has the function of reflecting the component of the light L2 generated by the phosphor layer 4 that is directed toward the photocathode 3, thereby allowing the component to travel toward the semiconductor photodetector element 5. Furthermore, by supplying power to the metal layer 18, the metal layer 18 can also function as an electrode that forms an electric field that guides the electrons E emitted from the photocathode 3 to the phosphor layer 4.

[0032] The semiconductor photodetector element 5 is a part that detects the light L2 generated in the phosphor layer 4. Examples of the semiconductor photodetector element 5 include a silicon photomultiplier (SiPM), an avalanche photodiode (APD), and a photodiode (PD). The semiconductor photodetector element 5 outputs a signal according to the amount of the detected light L2 to the outside of the photodetector 1 via the output pin 14C.

[0033] In this embodiment, the area of ​​the photocathode 3 is larger than the area of ​​the phosphor layer 4, which is larger than the area of ​​the semiconductor photodetector element 5. That is, when the area of ​​the photocathode 3 is P1, the area of ​​the phosphor layer 4 is P2, and the area of ​​the semiconductor photodetector element 5 is P3, the relationship P1 > P2 > P3 is satisfied. Reducing the area P3 of the semiconductor photodetector element 5 improves the response characteristics of the semiconductor photodetector element 5. Reducing the area P3 of the semiconductor photodetector element 5 requires ensuring a sufficient area P1 of the photocathode 3 to improve the collection efficiency of the light L1. However, by making the area P2 of the phosphor layer 4, which is interposed between the photocathode 3 and the semiconductor photodetector element 5, larger than the area P3 of the semiconductor photodetector element 5, it is not necessary to focus the electrons E emitted from the photocathode 3 into a minute region. Eliminating the need for precise control of the electron trajectory can avoid complex design of the photodetector 1.

[0034] The internal housing 6 is a housing provided inside the vacuum housing 2 so as to surround the semiconductor photodetector element 5. The internal housing 6 is disposed in the internal space S1 of the vacuum housing 2, at the center in the axial direction of the side tube 11, near the stem 13. The internal space S2 of the internal housing 6 is separated from the internal space S1 of the vacuum housing 2 by the internal housing 6. The internal housing 6 is made of, for example, a conductive material. As with the side tube 11 of the vacuum housing 2, examples of the conductive material include an (FeNi) alloy. When the photodetector 1 is used in a high-energy physics experiment and it is necessary to eliminate the presence of radioactive isotopes, it is preferable to use an iron-nickel (FeNi) alloy that does not contain cobalt (Co).

[0035] The inner housing 6 is composed of, for example, a side tube 21, a window portion 22, and a bottom portion 23. The side tube 21 has a cylindrical shape with a diameter slightly larger than that of the semiconductor photodetector element 5. The window portion 22 is provided at one end of the side tube 21 (the end facing the photocathode 3). An inward-facing tubular portion 24 that protrudes toward the inside of the side tube 21 is provided in the center of the window portion 22. The tubular portion 24 is formed so that, for example, its diameter gradually decreases toward the inside of the side tube 21. Both one end (the end facing the photocathode 3) and the other end (the end facing the semiconductor photodetector element 5) of the tubular portion 24 are open. The size of the opening at one end of the tubular portion 24 is, for example, approximately the same as the size of the semiconductor photodetector element 5. When the tubular portion 24 is viewed from the photocathode 3 side, the semiconductor photodetector element 5 can be seen through the tubular portion 24.

[0036] The phosphor layer 4 described above is bonded to the outer surface of the window portion 22 of the inner housing 6 so as to close the opening at one end of the cylindrical portion 24. Furthermore, the metal layer 18 provided on the phosphor layer 4 protrudes outward beyond the edge of the phosphor layer 4 and is bonded to the window portion 22 of the inner housing 6. A conductive adhesive, for example, can be used to bond the phosphor layer 4 to the window portion 22 and the metal layer 18 to the window portion 22. Therefore, the phosphor layer 4 and the inner housing 6 are electrically connected to each other, and the metal layer 18 and the inner housing 6 are electrically connected to each other.

[0037] The bottom 23 is disk-shaped and covers the other end of the side tube 21 (the end facing the stem 13). The power supply pin 14B and output pin 14C described above are inserted through the bottom 23. The stem pin 14 and the bottom 23 are electrically insulated by an insulating material 25 such as glass. The ground pin 14D described above is also electrically connected to the bottom 23. The semiconductor photodetector element 5 described above is disposed on the inner surface of the bottom 23, spaced apart from the phosphor layer 4 in the axial direction of the side tube 11. The semiconductor photodetector element 5 is bonded to the center of the inner surface of the bottom 23 so as to face the phosphor layer 4 via the cylindrical portion 24. The semiconductor photodetector element 5 is electrically connected to the power supply pin 14B and output pin 14C inserted through the bottom 23.

[0038] The distance T between the phosphor layer 4 and the semiconductor photodetector element 5 is determined by the internal housing 6. In the example of FIG. 1 , the distance T is the distance between the surface of the phosphor layer 4 facing the semiconductor photodetector element 5 and the surface of the semiconductor photodetector element 5 facing the phosphor layer 4, and is determined mainly by adjusting the axial length of the side tube 11. An inorganic adhesive, for example, can be used to bond the semiconductor photodetector element 5 to the bottom 23. The adhesive may be either a conductive adhesive or a non-conductive adhesive. An example of an adhesive is a nanometal paste, such as nanosilver paste. By applying the nanometal paste to the bonding area and baking it, the solvent evaporates, leaving behind metal nanoparticles, which then aggregate and fuse together, thereby bonding the materials to be bonded.

[0039] The inner housing 6 has holes 26 that communicate the inner space S1 of the vacuum housing 2 with the inner space S2 of the inner housing 6. The holes 26 ensure that the degree of vacuum in the inner space S2 of the inner housing 6 is approximately the same as the degree of vacuum in the inner space S1 of the vacuum housing 2. In this embodiment, the holes 26 are arranged around the semiconductor light-detecting element 5 on the bottom 23. There is no particular limitation on the number of holes 26, and multiple holes 26 may be arranged around the semiconductor light-detecting element 5 at a predetermined phase angle. The holes 26 are located, for example, closer to the center than the insertion positions of the feed pin 14B and the output pin 14C. As a result, the holes 26 are located closer to the semiconductor light-detecting element 5 than the insertion positions of the feed pin 14B and the output pin 14C when viewed from the front of the window 12.

[0040] The light-guiding member 7 is a portion that guides the light L2 generated in the phosphor layer 4 to the semiconductor photodetector element 5. The light-guiding member 7 is disposed between the phosphor layer 4 and the semiconductor photodetector element 5. In this embodiment, the light-guiding member 7 is disposed inside the cylindrical portion 24 of the inner housing 6. Various configurations may be applied to the light-guiding member 7. For example, the light-guiding member 7 may be made of synthetic quartz having a light-reflecting film such as an aluminum (Al) film deposited on its surface, or may be made of a metal reflector plate formed on the inner wall surface of the cylindrical portion 24. If a sufficient amount of light L2 from the phosphor layer 4 travels to the semiconductor photodetector element 5, the light-guiding member 7 may be omitted. In this case, the cylindrical portion 24 may be omitted, and a simple opening may be provided in the window portion 22.

[0041] The focusing electrode 8 is a component that forms an electric field for focusing electrons E emitted from the photocathode 3 onto the phosphor layer 4. The focusing electrode 8 has, for example, a circular frame shape and is arranged to surround the phosphor layer 4 at a position slightly closer to the photocathode 3 than the phosphor layer 4 in the window portion 12. The above-mentioned power feed pin 14E is electrically connected to the focusing electrode 8. Examples of materials that can be used for the focusing electrode 8 include iron-nickel (FeNi) alloy, stainless steel (FeCrNi) alloy, and Kovar (FeCoNi) alloy. If the size of the photocathode 3 is not significantly larger than the size of the phosphor layer 4, the focusing electrode 8 may be omitted. In this case, the power feed pin 14E can also be omitted.

[0042] When detecting light L1 in the photodetector 1, a negative high voltage (for example, about −3 kV) is applied to the vacuum housing 2 (photocathode 3) via feed pin 14A, and a drive voltage (for example, about several tens to several hundreds of kV) is applied to the semiconductor photodetector element 5 via feed pin 14B. Also, a negative high voltage similar to that of the vacuum housing 2 (photocathode 3) is applied to the focusing electrode 8 via feed pin 14E. The inner housing 6, phosphor layer 4, and metal layer 18, which are electrically connected to ground pin 14D, are all at ground potential.

[0043] In this state, when light L1 to be detected is incident on the window portion 12 of the photodetector 1, electrons E are emitted from the photocathode 3 in response to the light L1. The electrons E emitted from the photocathode 3 are guided to the phosphor layer 4 by an electric field formed by the cooperation of the focusing electrode 8 and the metal layer 18. When the electrons E enter the phosphor layer 4, light L2 is generated in the phosphor layer 4 in response to the electrons E. The light L2 generated in the phosphor layer 4 is guided by the light-guiding member 7 and enters the semiconductor photodetector element 5. The semiconductor photodetector element 5 generates a signal corresponding to the amount of detected light L2. This signal is output to the outside of the photodetector 1 via the output pin 14C. Note that the polarity of the voltage applied to the photodetector 1 is not limited to this, and any voltage may be applied so long as the electrons E emitted from the photocathode 3 are directed toward the phosphor layer 4. For example, the potential of the phosphor layer 4 may be positive relative to the photocathode 3.

[0044] During operation of the photodetector 1, some of the electrons E emitted from the photocathode 3 may be reflected inside the vacuum housing 2 (for example, on the surface of the phosphor layer 4) and become reflected electrons Er. Within the vacuum housing 2, an electric field is formed by the cooperation of the focusing electrode 8 and the metal layer 18 such that the electrons emitted from the photocathode 3 are focused onto the phosphor layer. For this reason, it is conceivable that the reflected electrons Er that have deviated from the trajectory toward the phosphor layer 4 will travel back toward the phosphor layer 4. If the reflected electrons Er are deflected from the phosphor layer 4 and collide with the semiconductor photodetector element 5 or surrounding wiring, there is a risk that the semiconductor photodetector element 5 will be damaged.

[0045] In contrast, in the photodetector 1, the semiconductor photodetecting element 5 is disposed at a distance from the phosphor layer 4. This reduces the possibility that, when some of the electrons E emitted from the photocathode 3 are reflected inside the vacuum casing 2 to become reflected electrons Er, the reflected electrons Er deviating from the trajectory toward the phosphor layer 4 may travel again toward the phosphor layer 4 due to the electric field inside the vacuum casing 2, the reflected electrons Er will collide with the semiconductor photodetecting element 5. Therefore, in the photodetector 1, the semiconductor photodetecting element 5 can be protected from the reflected electrons Er generated inside the vacuum casing 2.

[0046] In this embodiment, an inner housing 6 that surrounds the semiconductor photodetector element 5 is disposed inside the vacuum housing 2. This allows the inner housing 6 to physically block the reflected electrons Er heading toward the semiconductor photodetector element 5. This makes it possible to more reliably prevent the reflected electrons Er from colliding with the semiconductor photodetector element 5.

[0047] In this embodiment, the distance T between the phosphor layer 4 and the semiconductor photodetector element 5 is defined by the inner housing 6. With this configuration, the distance between the phosphor layer 4 and the semiconductor photodetector element 5 can be formed as designed. Therefore, the possibility of reflected electrons Er colliding with the semiconductor photodetector element 5 can be more reliably reduced.

[0048] In this embodiment, the inner housing 6 has a hole 26 that connects the inner space S1 of the vacuum housing 2 with the inner space S2 of the inner housing 6. This allows the inner housing 6 to be evacuated via the hole 26 at the same time as the vacuum housing 2 is evacuated. This makes it possible to suppress deterioration of the photocathode 3 due to residual gas inside the vacuum housing 2 and the inner housing 6.

[0049] In this embodiment, the semiconductor photodetector element 5 is disposed on the bottom 23 of the internal housing 6, and the hole 26 is disposed around the semiconductor photodetector element 5 on the bottom 23. With this configuration, the hole 26 of the internal housing 6 and the semiconductor photodetector element 5 can be brought sufficiently close to each other. Therefore, even if the internal housing 6 has the hole 26, it is possible to prevent reflected electrons Er from passing through the hole 26 and colliding with the semiconductor photodetector element 5.

[0050] In this embodiment, the bottom 23 of the vacuum housing 2 is provided with a tip tube 16, which serves as an exhaust port for the vacuum housing 2 and an inlet for introducing alkali used to alkalize the photocathode 3. The tip tube 16 is positioned so as not to overlap with the inner housing 6 when viewed from the front of the window 12. With this configuration, a sufficient gap can be formed between the hole 26 of the inner housing 6 and the tip tube 16. Therefore, even if the inner housing 6 has the hole 26, it is possible to prevent the alkali introduced from the tip tube 16 from passing through the hole 26 and adhering to the semiconductor photodetector element 5. This contributes to suppressing noise (such as dark current) caused by the adhesion of alkali to the semiconductor photodetector element 5.

[0051] In this embodiment, the inner housing 6 is made of a conductive material. This makes it possible to create an electric field-free environment inside the inner housing 6, and to suppress the formation of an electric field that would direct the reflected electrons Er toward the semiconductor photodetector element 5. Therefore, even if the reflected electrons Er travel inside the inner housing 6, the possibility that the reflected electrons Er will collide with the semiconductor photodetector element 5 can be sufficiently reduced.

[0052] In this embodiment, a metal layer 18 is disposed on the surface of the phosphor layer 4 facing the photocathode 3. This allows the component of the light generated in the phosphor layer 4 that is directed toward the photocathode 3 to be reflected by the metal layer 18 and travel toward the semiconductor photodetector element 5. This improves the amount of light detected by the semiconductor photodetector element 5.

[0053] In this embodiment, the metal layer 18 is electrically connected to the inner housing 6. This allows power to be supplied to the metal layer 18 as well as the inner housing 6. Supplying power to the metal layer 18 allows the metal layer 18 to function as an electrode that forms an electric field that guides electrons E emitted from the photocathode 3 to the phosphor layer 4.

[0054] In this embodiment, the area P1 of the photocathode 3 is larger than the area P2 of the phosphor layer 4, which in turn is larger than the area P3 of the semiconductor photodetector element 5. In this case, by reducing the area P3 of the semiconductor photodetector element 5, the response characteristics of the semiconductor photodetector element 5 can be improved. Reducing the area P3 of the semiconductor photodetector element 5 requires ensuring a sufficient area P1 of the photocathode 3 to improve the collection efficiency of the light L1. However, by making the area P2 of the phosphor layer 4, which is interposed between the photocathode 3 and the semiconductor photodetector element 5, larger than the area P3 of the semiconductor photodetector element 5, it becomes unnecessary to focus the electrons E emitted from the photocathode 3 into a minute region. Since precise control of the electron trajectory is not required, the design of the photodetector 1 can be simplified.

[0055] In this embodiment, the vacuum housing 2 is made of a conductive material. A focusing electrode 8 is disposed inside the vacuum housing 2 to focus the electrons E emitted from the photocathode 3 onto the phosphor layer 4, and the focusing electrode 8 has the same potential as the vacuum housing 2. This configuration allows the electrons E emitted from the photocathode 3 to be efficiently guided toward the phosphor layer 4. Furthermore, this technical effect can be achieved with a simple configuration. Note that the focusing electrode 8 and the vacuum housing 2 do not necessarily need to be at the same potential, and different potentials may be applied to them. In this case, more precise control of the electron trajectory becomes possible.

[0056] In this embodiment, a light-guiding member 7 is disposed between the phosphor layer 4 and the semiconductor photodetector element 5 to guide the light L2 generated in the phosphor layer 4 to the semiconductor photodetector element 5. By using the light-guiding member 7, the light L2 generated in the phosphor layer 4 can be efficiently guided toward the semiconductor photodetector element 5. Furthermore, even if ions are generated by collision between the residual gas inside the vacuum housing 2 and the electrons E, the light-guiding member 7 can physically block the ions from passing through the phosphor layer 4 toward the semiconductor photodetector element 5. Therefore, the semiconductor photodetector element 5 can be protected from collisions with the ions.

[0057] Ions generated by collisions between the residual gas and electrons E can cause malfunctions in the photodetector 1. For example, if negatively charged ions (negative ions) are generated, they may collide with the semiconductor photodetector element 5 on the same trajectory as the electrons E emitted from the photocathode 3 toward the phosphor layer 4. In this case, the phosphor layer 4 or the semiconductor photodetector element 5 may be damaged, resulting in a deterioration of their characteristics. Even if they do not ionize, if water, hydrocarbons, or the like adhere to the semiconductor photodetector element 5 as residual gas, their characteristics may be reduced. Therefore, by physically protecting the semiconductor photodetector element 5 with the light-guiding member 7, deterioration of the characteristics of the semiconductor photodetector element 5 can be suppressed.

[0058] Next, an example of the present disclosure will be described.

[0059] Figures 2(a), 2(b), 3(a), and 3(b) show simulation results for electron trajectories from the photocathode toward the phosphor layer. Figures 2(a) and 2(b) show the probability of electrons emitted from the photocathode impinging on the phosphor layer for a photodetector with a configuration similar to that of the photodetector 1 described above. The photocathode diameter was set to 3 inches, and the phosphor layer diameter was varied. The voltage applied to the photocathode and focusing electrode was set to -2 kV, and the phosphor layer was held at ground potential. The results showed that when the phosphor layer diameter was 6 mm, the probability of electrons impinging on the phosphor layer was less than 75%, but when the phosphor layer diameter was 8 mm or greater, the probability of electrons impinging on the phosphor layer exceeded 90%. In Figures 3(a) and 3(b), the voltage applied to the photocathode and focusing electrode was set to -3 kV, and the other conditions were the same as those in Figures 2(a) and 2(b). As a result, when the diameter of the phosphor layer was 6 mm, the probability of electrons entering the phosphor layer was less than 75%, but when the diameter of the phosphor layer was 8 mm or more, the probability of electrons entering the phosphor layer was over 90% in all cases.

[0060] These results show that when electrons emitted from the photocathode are converted into light by the phosphor layer, electrons can be made to enter the phosphor layer with high efficiency at an applied voltage of about -2 kV to -3 kV. When electrons are directly focused onto a minute region of the semiconductor photodetector, the gain required to detect a single electron is 1 x 10 5 The voltage required to be applied to the semiconductor detection element is about 10 kV, but in the configuration in which a phosphor layer is interposed between the photocathode and the semiconductor photodetector element as in the present disclosure, it is possible to operate it at a lower voltage.

[0061] 4(a) and 4(b) are diagrams showing the results of a simulation of the trajectories of backscattered electrons generated on the surface of the phosphor layer. In this simulation, the trajectories of backscattered electrons generated on half of the phosphor layer (the right half in FIG. 4(a)) were calculated. Here, the initial trajectories of backscattered electrons were set as follows, as shown in FIG. 4(a): backscattered electrons A (propagating perpendicularly from the surface of the phosphor layer 4), backscattered electrons B (propagating at an angle of 30° from the direction perpendicular to the surface of the phosphor layer 4), and backscattered electrons C (propagating at an angle of 60° from the direction perpendicular to the surface of the phosphor layer 4).

[0062] As shown in Figure 4(b), reflected electrons A traveled toward the center of the photocathode, then reversed their path toward the phosphor layer, taking a path primarily toward the left half of the phosphor layer. Reflected electrons B traveled toward the edge of the photocathode, then reversed their path toward the phosphor layer, taking a path primarily toward the right half of the phosphor layer. Reflected electrons C, which traveled at an angle 60° to the right from the direction perpendicular to the surface of phosphor layer 4, returned to the phosphor layer in a convex curve toward the photocathode, taking a path primarily toward the right half of the phosphor layer and the right side of the inner casing. Reflected electrons C, which traveled at an angle 60° to the left from the direction perpendicular to the surface of phosphor layer 4, took a path primarily toward the left half of the inner casing and the back surface.

[0063] These results confirmed that the configuration in which the semiconductor photodetector element is spaced apart from the second photoelectric conversion layer reduces the possibility of reflected electrons colliding with the semiconductor photodetector element. Also, the configuration in which the inner housing surrounding the semiconductor photodetector element is placed inside a vacuum housing physically blocks reflected electrons heading toward the semiconductor photodetector element, thereby providing suitable protection for the semiconductor photodetector element. [Explanation of symbols]

[0064] 1...photodetector, 2...vacuum housing, 3...photocathode (first photoelectric conversion layer), 4...phosphor layer (second photoelectric conversion layer), 5...semiconductor photodetector element, 6...internal housing, 7...light-guiding member, 8...focusing electrode, 16...chip tube, 18...metal layer, 23...bottom, 26...hole, L1...light to be detected, L2...light generated in the phosphor layer, T...separation length, S1...internal space of the vacuum housing, S2...internal space of the internal housing.

Claims

1. a vacuum housing having a window through which light to be detected is incident; a first photoelectric conversion layer disposed inside the vacuum enclosure and emitting electrons in response to the incidence of the light; a second photoelectric conversion layer disposed inside the vacuum enclosure and configured to generate light in response to the electrons emitted from the first photoelectric conversion layer; a semiconductor photodetector element disposed inside the vacuum enclosure on the opposite side of the first photoelectric conversion layer with the second photoelectric conversion layer interposed therebetween, the semiconductor photodetector element detecting light generated in the second photoelectric conversion layer; The photodetector, wherein the semiconductor photodetecting element is disposed at a distance from the second photoelectric conversion layer.

2. 2. The photodetector according to claim 1, wherein an inner housing surrounding said semiconductor photodetector element is disposed inside said vacuum housing.

3. 3. The photodetector of claim 2, wherein the inner housing defines a distance of the semiconductor photodetector element from the second photoelectric conversion layer.

4. 3. The photodetector according to claim 2, wherein the internal housing has a hole that communicates the internal space of the vacuum housing with the internal space of the internal housing.

5. the semiconductor light-detecting element is disposed on the bottom of the internal housing; 5. The photodetector according to claim 4, wherein the hole is disposed around the semiconductor photodetector element at the bottom.

6. a tip tube is provided at a bottom of the vacuum enclosure, the tip tube serving as an exhaust port of the vacuum enclosure and an inlet port of an alkali used to alkalizate the first photoelectric conversion layer; The photodetector according to claim 4 , wherein the tip tube is disposed at a position where it does not overlap with the inner housing when viewed from the front of the window portion.

7. The photodetector of claim 2 , wherein the inner housing is made of a conductive material.

8. The photodetector according to claim 2 , wherein a metal layer is disposed on a surface of the second photoelectric conversion layer facing the first photoelectric conversion layer.

9. The photodetector of claim 8 , wherein the metal layer is electrically connected to the inner housing.

10. an area of ​​the first photoelectric conversion layer is larger than an area of ​​the second photoelectric conversion layer; 10. The photodetector according to claim 1, wherein the area of ​​the second photoelectric conversion layer is larger than the area of ​​the semiconductor photodetector element.

11. the vacuum enclosure is made of a conductive material, a focusing electrode that focuses the electrons emitted from the first photoelectric conversion layer onto the second photoelectric conversion layer is disposed inside the vacuum enclosure; 10. The photodetector according to claim 1, wherein the focusing electrode has the same potential as the vacuum enclosure.

12. The photodetector according to any one of claims 1 to 9, wherein a light-guiding member is disposed between the second photoelectric conversion layer and the semiconductor photodetector element, the light-guiding member guiding the light generated in the second photoelectric conversion layer to the semiconductor photodetector element.

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