  Unruh-DeWitt (UDW) detector that couples locally to a quantum field is an
important tool for operationally studying field properties. Here, we study the
response of an inertial UDW detector to a non-relativistic particle state of a
massive scalar field and we find that the transition probability of the
detector splits into the vacuum contribution and the matter contribution. We
show that the matter part oscillates with the interaction time duration and the
oscillation period depends on the difference between the mass of the particle
and the energy gap of the detector; a strong resonance pattern for the
transition probability is found when such difference equals to zero. We compare
the matter part contribution with the non-relativistic probability density of
the particle and find that they provide qualitatively similar description of
the particle.
