Side-on type photomultiplier comprising an envelope having an opening, a lens element, and a lens positioning structure

A side-on type photomultiplier comprising:

• an envelope having an opening for transmitting therethrough light to be detected;
• a photocathode for emitting photoelectrons in response to said light to be detected reaching said photocathode after being transmitted through the opening of said envelope, said photocathode having a light receiving surface with a predetermined area, said photocathode being accommodated in said envelope and supported by an electrode member so as to be inclined with respect to a direction of incidence of said light to be detected;
• a lens element for guiding onto a predetermined region in said photocathode said light to be detected, said lens element covering the opening of said envelope so as to constitute a part of said envelope;
• and
• a positioning structure for defining an installed position of said lens element, said positioning structure having a supporting section which is provided on an outer peripheral surface of said envelope so as to surround the opening of said envelope and extending from the outer peripheral surface of said envelope along a direction opposite to the direction of incidence of said light to be detected, said supporting section including a reference surface for supporting said lens element and for defining a distance between said lens element and said photocathode.