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==Measuring principle ==
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PAC uses radioactive probes, which have an intermediate state with decay times of 2 ns to approx. 10 μs, see example <sup>111</sup>In in the picture on the right. After electron capture (EC), indium transmutates to cadmium. Immediately thereafter, the <sup>111</sup>cadmium nucleus is predominantly in the excited 7/2+ nuclear spin and only to a very small extent in the 11/2- nuclear spin, the latter should not be considered further. The 7/2+ excited state transitions to the 5/2+ intermediate state by emitting a 171 keV γ-quantum. The intermediate state has a lifetime of 84.5 ns and is the sensitive state for the PAC. This state in turn decays into the 1/2+ ground state by emitting a γ-quantum with 245 keV. PAC now detects both γ-quanta and evaluates the first as a start signal, the second as a stop signal.
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