Measurement of Spectral Distribution of Positron Flux in an Infinite Copper Medium ContainingCu64

The spectral distribution of the flux of positrons inside a beta radioactive medium was experimentally determined using an anthracene coincidence scintillation spectrometer. Positrons leaving a copper cavity or plane source containing ${\mathrm{Cu}}^{64}$ were absorbed in a thin anthracene crystal. The light pulses from the anthracene were analyzed and recorded if a NaI scintillation spectrometer nearby recorded simultaneously a count under the total absorption peak for annihilation radiation. Thus anthracene pulses due to absorption of negatrons and secondary electrons were not recorded. The spectra were corrected for the nonlinear pulse-height-energy characteristic of anthracene by using a semiempirical theory of Birks. This correction was shown to change an observed nonlinear Fermi plot of ${\mathrm{Pm}}^{147}$ to the accepted linear shape. The positron spectra were compared with the theoretical continuous slowing-down model. Over the range measured (20-650 keV) it was concluded that the continuous-slowing-down model gives the correct shape for the primary slowing-down spectrum for the cavity source, but that the plane source exhibited a deficiency of low-energy positrons.