  We construct the detection rate for particle detectors moving along
non-inertial trajectories and interacting with quantum fields. The detectors
described here are characterized by the presence of records of observation
throughout their history, so that the detection rate corresponds to directly
measurable quantities. This is in contrast to past treatments of detectors,
which actually refer to probes, i.e., microscopic systems from which we extract
information only after their interaction has been completed. Our treatment
incorporates the irreversibility due to the creation of macroscopic records of
observation. The key result is a real-time description of particle detection
and a rigorously defined time-local probability density function (PDF). The PDF
depends on the scale $\sigma$ of the temporal coarse-graining that is necessary
for the formation of a macroscopic record. The evaluation of the PDF for
Unruh-DeWitt detectors along different types of trajectory shows that only
paths with at least one characteristic time-scale much smaller than $\sigma$
lead to appreciable particle detection. Our approach allows for averaging over
fast motions and thus predicts a constant detection rate for all fast periodic
motions.
