Effect of instrumental instability and detector parameters in positron annihilation Coincidence Doppler Broadening (CDB) measurements: A Monte Carlo simulation study

摘要

The momentum distribution of electrons around the site of positron annihilation is studied by Doppler broadening of annihilation radiation. The higher momentum region of Doppler broadening spectra are representative of the element irrespective of its chemical form. Conventional Doppler broadening suffers from poorer sensitivity in terms of elemental specificity due to large Compton background in the higher momentum region. Of late, this problem has been circumvented by the use of two-detector CDB technique. The difference spectra is taken with a gate on sum of the energies E_s to be equal to 2m_0C~2-Eb, where m_0 is the rest mass of electron and E_b is the binding energy of the electron. This reduces the background in the higher momentum region of the spectrum to a great extent, enabling the use of this technique to identify the element with which the positron is annihilating in a given chemical matrix. However, it is essential that the amplitude of the two electronic pulses corresponding to the energies E_(gamma 1) and E_(gamma 2). are exactly matched so that their difference would give the momentum distribution without parallax. Further, the actual annihilation gamma ray energy distribution is broadened by the detector resolution. Hence, it is important to understand the effect of the gain and resolution on the CDB spectra and corresponding information extracted from them.