The relative-efficiency curve of full-energy peak of various g-ray energies obtained by a NaI detector (f 76 mm 76 mm) and analytically pure KCl have been used for measuring activity of 24Na in water solutions(0.1-4Bq/cm3). The relative efficiency means the ratio of full-energy peak counting efficiency of differential energies to that of 0.8468 MeV. The method need not calibrate the measured solution by standard solution of any nuclide

of which the activity exactly has been known. The relative-efficiency curve was determined by least-square fitting of relative efficiencies of various g-ray抯 energies

E

to a function

f (E) = A + BE + CE2 + DE3 + UE4. These g energies (in MeV) included 0.8468

1.8108 and 2.113 from 56Mn

0.8438 and 1.0144 from 27Mg

1.0439

1.3175 and 1.4748 from 82Br

and 1.6422

and 2.1676 from 38Cl. These radioactive nuclides were produced by activation using neutrons from a 252Cf neutron source (of 130靏). The method may be used for measuring activity of radionuclides other than 24Na

and is more suitable to Ge detector than NaI when the relative-efficiency curve of full-energy peak is determined by g energies from 82Br only.