Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-11-17
2001-10-02
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140VT, C313S532000, C313S541000
Reexamination Certificate
active
06297489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electron tube used as a photodetector for quantitatively measuring weak light and particularly having a sensing device such as a semiconductor device for multiplying photoelectrons emitted from a photocathode and outputting the electric signals.
2. Related Background Art
Conventionally, an electron tube which causes an electron lens to accelerate and focus photoelectrons emitted from a photocathode upon incidence of light and makes the photoelectrons incident on, e.g., a semiconductor device to obtain a high gain is known. This electron tube is disclosed in, e.g., U.S. Pat. No. 5,120,949, Japanese Patent Laid-Open No. 6-318447, U.S. Pat. No. 5,374,826 or 5,475,227. Particularly, U.S. Pat. No. 5,475,227 discloses a structure for preventing a phenomenon that ions generated from gas molecules adsorbed on the electron incident surface of the semiconductor device due to electrons incident on the semiconductor device are accelerated and fed back to the photocathode to result in a large degradation in photocathode. More specifically, a semicylindrical ion deflecting electrode is arranged immediately before the semiconductor device to bend the orbits of ions generated on the electron incident surface of the semiconductor device, thereby preventing the ions from returning to the photocathode.
SUMMARY OF THE INVENTION
The present inventors examined the prior arts and found the following problems. In the prior art disclosed in U.S. Pat. No. 5,472,227, ions generated from the semiconductor device are bent in orbit and prevented from being fed back to the photocathode. With this structure, although the photocathode can be prevented from degrading, the ions bent in orbit collide with the insulating side wall, so no stable operation can be obtained. This is because secondary electrons are emitted from the insulating side wall of the container upon collision of ions to charge the side wall to a positive potential, thus affecting the orbits of electrons propagating from the photocathode to the semiconductor device. Particularly, with the arrangement of each prior art, only a specific portion of the side wall of the container is charged upon collision of ions to make the electron lens asymmetric. Therefore, the orbits of electrons are largely bent. In addition, the secondary electrons generated upon collision of ions are incident on the semiconductor device to generate a pseudo signal or stray to produce a new unstable state.
An object of the present invention is to provide an electron tube having a structure for enabling a stable operation for a long time.
In accordance with the present invention, there is provided an electron tube comprising, at least, a photocathode arranged so as to emit photoelectrons in correspondence with incident light, a semiconductor device having an electron incident surface for receiving the photoelectrons from the photocathode, the electron incident surface being arranged so as to face the photocathode, and a confining mechanism arranged between the photocathode and the electron incident surface to confine orbits of the photoelectrons from the photocathode. Particularly, the confining mechanism has an opening which contributes to confine the spread of the photoelectrons (the photoelectrons from the photocathode pass through this opening and arrive at the electron incident surface of the semiconductor device). The area of the opening is set to be equal to or smaller than that of the electron incident surface of the semiconductor device. Therefore, the opening of the confining mechanism is arranged at a position close to the electron incident surface.
A container of the electron tube according to present invention can be selected from at least one of a pipe type having first and second openings, envelope type having one opening, and the like. In the pipe type container, the photocathode is arranged on the first opening side thereof, and a conductive stem is arranged on the second opening side thereof. The stem functions to define a distance between the photocathode and the electron incident surface of the semiconductor device. And the confining mechanism is positioned between the photocathode and the semiconductor device while being accommodated in the pipe type container. On the other hand, in the envelope type container, the photocathode is arranged on the opening thereof, and the semiconductor device is mounted on an inner bottom surface of the envelope type container.
The electron tube further comprises an electron lens constituted by a cathode electrode arranged so as to apply to the photocathode and having a through hole for passing the photoelectrons from the photocathode toward the electron incident surface, and an anode electrode arranged between the photocathode electrode and the electron incident surface of the semiconductor device. In the pipe type container, the cathode electrode is arranged on the first opening side of the container. In the envelope type container, the cathode electrode is arranged as a conductive film on an inner wall of the container. The anode electrode has a first surface facing the photocathode, a second surface opposing the first surface, and a through hole extending from the first surface to the second surface.
In this arrangement, the confining mechanism includes the anode electrode, and the opening of the confining mechanism corresponds to a second-surface-side opening of the through hole of the anode electrode. In other words, the opening having smallest area within the openings of the electron lens corresponds to the opening of the confining mechanism.
In this electron tube, external light is converted into electrons by the photocathode. The electrons (photoelectrons) emitted from the photocathode pass through the through hole of the anode electrode and then arrive at the electron incident surface of the semiconductor device. At this time, positive ions are generated on the electron incident surface. The anode electrode is set at a positive potential with respect to the electron incident surface of the semiconductor device. Since the anode electrode is reverse-biased with respect to the positive ions generated on the electron incident surface, the generated positive ions cannot return to the photocathode or case through the through hole of the anode electrode.
In this case, preferably, a cylindrical collimator portion extending toward the photocathode is arranged on the first surface of the anode electrode concentrically with the first-surface-side opening of the through hole of the anode electrode. When the collimator portion is arranged on the anode electrode in use of the semiconductor device (e.g., an avalanche photodiode: APD), extension of the electric field from the photocathode toward the semiconductor device through the through hole of the anode electrode can be minimized. Therefore, ion feedback can be effectively suppressed.
More preferably, a conductive mesh electrode is arranged in the through hole of the anode electrode. When the mesh electrode is arranged in the anode electrode in use of the semiconductor device (e.g., a photodiode: PD), extension of the electric field from the photocathode toward the semiconductor device through the through hole of the anode electrode can be minimized. Therefore, ion feedback can be effectively suppressed.
The electron tube according to the present invention may further comprise a collimator electrode supported by the anode electrode. The collimator electrode has a third surface facing the photocathode, a fourth surface opposing the third surface, and a through hole extending from the third surface to the fourth surface. The confining mechanism includes the collimator electrode, and the opening of the confining mechanism corresponds to a fourth-surface-side opening of the through hole of the collimator electrode. The orbits of the photoelectrons incident from the photocathode on the third-surface-side opening of the collimator electrode at a predetermined angle are collimated by the coll
Kimura Suenori
Morita Tetsuya
Saito Tetsuya
Suyama Motohiro
Hamamatsu Photonics K.K.
Lee John R.
Pillsbury & Winthrop LLP
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