The instrument described is of simple and rugged design. The anode is a wire loop, and the cathode includes a grounded line wire which electrically shields the sample. The line wire, when properly placed, provides greater stability and freedom from spurious effects due to the accumulation of a charge on the sample. The geometry is essentially 2&pgr;. The instrument may be used for the counting of alpha‐ as well as soft beta‐particles. It is self‐shielded.