Complex sentence: But for teletherapy sources the radioactive density (Ci in a given volume) needs to be very high, as a result it is not possible with any of the insoluble caesium compounds to create the source.
Split into simpler sentences: But for teletherapy sources, the radioactive density (Ci in a given volume) needs to be very high. As a result it is not possible with any of the insoluble caesium compounds to create the source.