The Frisch grid is named after the Austrian–British physicist Otto Robert Frisch. Basically it is a grid made of a conductive material that is typically placed between the cathode and anode in ionization chambers and is commonly on ground potential or weakly charged.
The Frisch grid brings two advantages: The height of a pulse measured in a ionization chamber depends on the location of the primary ionization. So detectors that cover great dihedral angles will have a bad signal quality. By deflecting the anode with the Frisch grid the dependence of the location of the primary ionization is dramatically reduced; As a second advantage of a Frisch grid is the improvement of the resolution of time. The pulse rises after the electrons have passed the Frisch grid so the rise time will be shorter than the rise time without a Frisch grid. As mentioned before the rise time is shorter and the peak is higher, so the Frisch grid improves the quality of the measured signal.
A Frisch grid is designed to measure the induced charge primarily from electrons. A gamma-ray interaction occurring in the main volume of the detector excites the electron-hole pairs. An externally applied electric field drifts the carriers in opposite directions, in which the electrons drift through the grid and into the measurement region of the device. From the Shockley-Ramo theorem, the induced charge produced at the anode results from charge carriers moving between the grid and the anode and not from charge motion between the cathode and the grid. As a result, the detector is primarily sensitive to only the electron charge carriers.
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