The aerogel is a low density material (about 1/10 of the water density), where a positron emitted by the radioactive source can capture one electron and form positronium. The advantage of using such a material is that in the pores the positronium atom can decay almost freerly. In fact in dense material, the density of electrons is higher, thus the probability that the positron annihilates with one electron which is not is bounded partner is bigger .This effect is called pick-off annihilation (see below for more details) and is the basic principle used for spectroscopy with positrons.  

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