The half-life of 18F.

The half-life of the positron-emitter (18)F has been measured by following the decay rate with three systems: ionization chambers, Ge detectors and coincidence with fast scintillators. The decay rate was measured for periods of time up to 9 half-lives. The combination of the results obtained with the three measuring systems gives a value of T(1/2)=1.82871 (18)h, in good agreement with recommended data and with an estimated uncertainty lower than any other previously reported value.

Calculation of electron deposition in proportional counters.

Efficiency extrapolation is a common technique in the standardization of radionuclides by the technique of 4pibeta-gamma coincidences. It can be achieved in two ways: adding thin films onto the radioactive source or by electronic discrimination in a pressurized 4pibeta-gamma system. The last case is the most extensively used actually in metrology laboratories. There is a need to know the electron energy loss inside the proportional counter as a function of the electron energies and working pressures. Calculations of electron energy loss have been performed for electron energies in a range from a few keV to 2 MeV in a mixture of Ar(90%) CH4(10%) and for pressures ranging from 100 to 2000 kPa in a 5 cm radius proportional counter. Wall effects, such as electron backscattering, are included in the calculations. Results are provided in the form of graphs and some recommendations are given about the optimal values of the energies that can be used to perform the extrapolation in the standardization of some radionuclides.