  We consider the problem of computing, for a detector surface waiting for a
quantum particle to arrive, the probability distribution of the time and place
at which the particle gets detected, from the initial wave function of the
particle in the non-relativistic regime. Although the standard rules of quantum
mechanics offer no operator for the time of arrival, quantum mechanics makes an
unambiguous prediction for this distribution, defined by first solving the
Schr\"odinger equation for the big quantum system formed by the particle of
interest, the detector, a clock, and a device that records the time and place
of detection, then making a quantum measurement of the record at a very late
time, and finally using the distribution of the recorded time and place. This
leads to the question whether there is also a practical, simple rule for
computing this distribution, at least approximately (i.e., for an idealized
detector). We argue here in favor of a rule based on a 1-particle Schr\"odinger
equation with a certain (absorbing) boundary condition at the ideal detecting
surface, first considered by Werner in 1987. We present a novel derivation of
this rule and describe how it arises as a limit of a "soft" detector
represented by an imaginary potential.
