A new precise instrument for determination of the absolute disintegration of β

α emitter solutions by the method of liquid scintillation counting is described. The instrument includes a triple phototube liquid scintillation detector and three output counting channels which are used for the number of triple coincidences Nc

the number of sum of double coincidences N

D

and the number of sum of single phototube counting N

E

seperately. This instrument is based on the principle of extrapolating the triple to double coincidence ratio (TDCR) to unity.

Examples of application of the instrument for

3

H and

14

C are given.

The instrument has two main advantages: 1. Higher accuracy. The overall measurement uncertainty(3σ)

for

3

H

63

Ni

35

S and

14

C etc. is in 0.5—1.5%. 2. Easy to operate. The measurement time is much less than usual metrology instrument.