A low background beta measuring instrument is described here. This instrument consists of a 20mm Au-Si surface barrier semiconductor detector and an anticoincidence scintillation counter with a plastic scintillator and a lead chamber with a wall 8cm thick which is used as a shield.

Under these conditions the background is equal to 0.12±0.04 cpm. The detection efficiency is about 48% for

90

Sr-

90

Y.