Momentum space valence region

When a positron enters a solid, it thermalizes in a time much shorter (few picoseconds) than its lifetime (few hundred of picoseconds). At the time of annihilation, the positron has diffused over a certain distance (few hundreds of nm). This diffusion process is critical to determine the state of the electron-positron pair at the time of annihilation the positron migrates in a region energetically favorable, namely the interstitial region of the lattice. The positron might also be trapped if it reaches a defect and experiences a longer lifetime. Otherwise, it annihilates with an electron in the valence or conduction band. Observing the annihilation radiation provides information on the electronic structure in momentum space. 

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