On the doppler broadening of two-photon positron annihilation radiation

Abstract

Doppler broadenings of positron annihilation radiation have been measured with a Li-drifted Ge-detector having a resolution of 1.76 keV FWHM for the 482 keVγ-line of181Hf. From the observed shapes of the photopeaks of the annihilation radiation in various metals, the functional form of the momentum distribution of the annihilating pair has been obtained. The least-square analysis of the experimental curves gives values for the FWHM of the unfolded pair momentum distribution curves as 2.7 keV for Cu, and 2.5 keV for Zn. These widths are consistent with angular correlation measurements on the annihilation quanta. Results obtained in the electric-field quenching in gaseous O2 are presented. A double-subtraction method is presented and its utility is discussed in determining the Doppler broadening of the radiation originating from the positron annihilation with bound atomic electrons. A search for the one-photon (E = (4/3)mc2) decay has been made. The upper limit for the relative cross-section of the one-photon to two-photon free annihilation in potassium metal is ⩽7.0·10−5. The relative merits of Doppler broadening and conventional techniques are compared.