THE GAS ELECTRON MULTIPLIER (GEM)

We describe operating principles and results obtained with a new detec tor element: the Gas Electrons Multiplier(GEM) [1]. Consisting of a th in composite sheet with two metal layers separated by a thin insulator , and pierced by a regular matrix of open channels, the GEM electrode, inserted on the path of electrons in a gas detector, allows the trans fer of charge with an amplification factor approaching ten. Uniform re sponse and high rate capability are demonstrated, Coupled to another d evice, multiwire or micro-strip chamber, the GEM electrode permits hig her gains or less critical operation; separation of the sensitive (con version) volume and the detection volume has other advantages: a built -in delay (useful for triggering purposes), and the possibility of app lying high fields on the photo-cathode of ring imaging detectors to im prove efficiency. Multiple GEM grids in the same gas volume allow larg e amplification factors to be achieved in a succession of steps, leadi ng to the realization of an effective gas-filled photomultiplier.