Measurement of 90Sr activity with Cherenkov radiation in a silica aerogel

A method of determination of 90Y (Eβmax = 2.27 MeV, t12 = 64 h), the daughter of 90Sr (Eβmax = 0.546 MeV, t12 = 28.5 yr), on the basis of Cherenkov radiation of the β particles in a silica aerogel has been tested. The relatively high end-point energy of the 90Y β-spectrum, together with a suitably chosen aerogel refractive index and thus electron threshold energy, permits elimination or at least a considerable reduction of the interference of other β emitters with lower end-point energies. The interference of γ emitters is reduced by using a thin, transmission multiwire proportional chamber in coincidence with the aerogel Cherenkov detector. Any possible contribution of other radionuclides to the count rate may be monitored by the shape of the Cherenkov pulse height spectrum, which depends sensitively on the electron energy. With an aerogel refractive index of 1.055, which corresponds to an electron threshold energy of 1.09 MeV, we have obtained an efficiency of 5 × 10−3 for 90Y. The background counting rate of 15 × 10−3 s−1 is stable (σ = 1 × 10−3s−1) and allows the determination of 1 Bq of 90Y activity in a few hours of measurement of a thin sample. If the ratio of 90Y to 90Sr activity is known (e.g. in equilibrium), the activity of 90Sr may be obtained.