It can be more easily detected in quantitative fashion by observing the 511-k.e.v. annihilation quanta produced when the positron and a free electron mutually annihilate. To simplify the determination of the counting geometry, it is necessary to ensure that all positrons are stopped in the immediate vicinity of the source.Niteogen -14(d, «)Oxygen-15. The nuclide oxygen-15 has an inconveniently short life of 2.1 minutes. In order to get a reproducible counting rate of oxygen-15 from a given nitrogen sample, the deuteron beam must not vary appreciably in intensity. As this is sometimes difficult to achieve with a'Van de Graaff, another reliable means of obtaining reproducibility has been used. The charge on the capacitor in the beam current integrator is caused to decay at the same rate (through a leak resistor) as the nuclide under investigation. Thus a