  We investigate a fully quantum mechanical spin model for the detection of a
moving particle. This model, developed in earlier work, is based on a
collection of spins at fixed locations and in a metastable state, with the
particle locally enhancing the coupling of the spins to an environment of
bosons. Appearance of bosons from particular spins signals the presence of the
particle at the spin location, and the first boson indicates its arrival. The
original model used discrete boson modes. Here we treat the continuum limit,
under the assumption of the Markov property, and calculate the arrival-time
distribution for a particle to reach a specific region.
