Consider a non-relativistic quantum particle with wave function inside a region $Omegasubset mathbb{R}^3$, and suppose that detectors are placed along the boundary $partial Omega$. The question how to compute the probability distribution of the time at which the detector surface registers the particle boils down to finding a reasonable mathematical definition of an ideal detecting surface; a particularly convincing definition, called the absorbing boundary rule, involves a time evolution for the particles wave function $psi$ expressed by a Schrodinger equation in $Omega$ together with an absorbing boundary condition on $partial Omega$ first considered by Werner in 1987, viz., $partial psi/partial n=ikappapsi$ with $kappa>0$ and $partial/partial n$ the normal derivative. We provide here a discussion of the rigorous mathematical foundation of this rule. First, for the viability of the rule it plays a crucial role that these two equations together uniquely define the time evolution of $psi$; we point out here how the Hille-Yosida theorem implies that the time evolution is well defined and given by a contraction semigroup. Second, we show that the collapse required for the $N$-particle version of the problem is well defined. Finally, we also prove analogous results for the Dirac equation.
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