PURPOSE: To prevent the lowering in sensitivity caused by the adhesion of an impure gas to the surface of a detector, by providing such constitution wherein a collimator is provided between the detector and plasma and cooled prior to cooling the detector.
CONSTITUTION: A cold fin 110 supports a detector 106 and, at the same time, directly supports a collimator 107 while the opposite sides of the detector 106 and the collimator 107 are accommodated in a jar 111 and the detector and, by liquid nitrogen 112, the detector 106 and the collimator 107 are cooled to a low temp. in the vicinity of the b.p. thereof. At this time, because the collimator 107 is directly supported by the cold fin 110 while the detector 106 is supported by the cold fin 110 through an insulator 109, a mechanism wherein the collimator 107 is previously cooled timewise is formed. That is, an impure gas other than neutral particles generated from plasma is adsorbed and removed by the previously cooled collimator 107 and prevented from being invaded to the side of the detector 106.
KIMURA HIRONOBU
MIURA IWAO
