Figure 1

¹⁸F energy spectra measured with the 2480 Wizard² gamma counter. When the source was located in the conventional position, inside the well (b), a peak at 511 keV and a coincidence sum peak at 1,022 keV were seen (a). When the source was positioned (for illustrative purposes) just outside the well (d), it was not possible for corresponding annihilation photons to be simultaneously measured and, as a result, the 1,022-keV coincidence sum peak was absent from the spectrum (c). Note that the photographs were taken with the shielding removed to show the two different positions of the (white) source holder.