オリフィス

orifice

[目次：装置]

一般には流体を流す小さな穴のことであるが、電子顕微鏡の場合には、直結する高真空と低真空の空間（部屋）の間で、電子ビームを通してなおかつ圧力の差を保つための絞り（小穴）のこと。
低真空SEMでは、対物レンズから上のレンズ室や電子銃室は高真空（低圧力）に、対物レンズから下の試料室は低真空（高圧力）に保たなければならない。高真空側と低真空側を別々の排気系で排気（差動排気）することによって、レンズより上の室と試料室の間に絞り（小穴）を置いても、4～5桁の圧力差が保たれている。

An orifice is generally a small hole through which fluid flows. In the case of electron microscopes, the orifice is defined to be a small aperture that allows to pass an electron beam but maintains the pressure difference between the high vacuum chamber and low vacuum chamber.
In a low-vacuum SEM (LVSEM), using the aperture, the electron gun chamber and the lens chamber above the objective lens must be kept at a high vacuum (a low pressure), and the specimen chamber below the objective lens must be kept at a low vacuum (a high pressure). The high vacuum gun chamber and lens chamber and low vacuum specimen chamber are evacuated independently by different evacuation systems (differential pumping), thereby the pressure difference of 4 to 5 orders of magnitudes being maintained.