Detection of the positron annihilation in flight process in condensed matter

Investigation of positron annihilation-in-flight using a digital coincidence Doppler broadening spectrometer

“…For the spectrum shown in figure 5(a), we obtained N 4γ /N 2γ ≈ 2.5 × 10 −5 . Using the known activity of the positron source A ≈ 0.6 MBq, the active area of detectors S det ≈ 25 cm 2 , the distance of detectors from the source r ≈ 27 cm, the measured absolute efficiency of detectors for 511 keV gamma rays η 1 ≈ 0.09, η 2 ≈ 0.10 and P 2γ ≈ 0.98 [16], we obtained τ ≈ 1940 ns, which agrees with the upper limit of 100 channels (1 channel = 2000 ns) imposed on the horizontal shift of a waveform by software in step (i) of the analysis. The ratio of the area of the 'four-photon' contribution (N 4γ ) filtered in the filtered spectrum to the area (N 4γ ) raw in the raw spectrum is equal to the ratio of time intervals for which the pulses are considered as a single event Figure 6 shows (N 4γ ) filtered plotted against (N 4γ ) raw for spectra measured in the same geometry using 22 Na and 68 Ge/ 68 Ga sources with various activities.…”

Positronium Physics and Biomedical Applications

“…A positron passing through matter may annihilate with electrons directly in flight (Hunt et al, 2001;Weber et al, 1999;Čížek et al, 2012). However, due to the fact that the cross-section of annihilation is inversely proportional to the positron velocity, these annihilations represent only about 1% (Dryzek et al, 2007;Harpen, 2004) of the total annihilation rate. At the end of the positron thermalization path, when its energy is small (in the order of tens of eV compared to its initial energy of MeV) the annihilation rate becomes large.…”