ABSOLUTE STANDARDIZATION OF RADIOACTIVE NEUTRON SOURCES: II. THE USE OF THE F19(α, n)Na22 REACTION

Abstract

Fluorine has only one stable isotope, F¹⁹. If neutrons are produced by the F¹⁹(α, n)Na²² reaction the neutron output can be calculated from the yield of the resulting radioactive Na²². The growth of Na²² (half-life, 2.58 years) has been measured in a neutron source consisting originally of 1.6 curies Po²¹⁰ mixed with CaF₂ powder. Since Na²² is a positron emitter, discrimination against γ-rays from Po²¹⁰ and from nuclear reactions could be achieved by detecting the two positron annihilation quanta in coincidence. The Na²² growth has been followed over 20 months and is in agreement with the theoretical growth curve. Comparison with a calibrated Na²² source yielded a neutron emission rate of (10.70 ± 0.25) × 10⁴ sec⁻¹. This resulted in a neutron emission rate of (3.16 ± 0.10) × 10⁶ sec⁻¹ for the Ra-α-Be source of the National Research Council, in good agreement with (3.22 ± 0.05) × 10⁶ sec⁻¹ obtained by a neutron thermalization method.