  Low gain avalanche detectors can measure charged particle fluences with high
speed and spatial precision, and are a promising technology for radiation
monitoring and dosimetry. A detector has been tested in a medical linac where
single particles were observed with a time resolution of 50 ps. The integrated
response is similar to a standard ionising chamber but with a spatial precision
twenty times finer, and a temporal precision over a million times better, with
the capability to measure the charge deposited by a single linac pulse. The
unprecedented resolving power allows the structure of the $\sim 3\ \mu$s linac
pulses to be viewed and the 350 ps sub-pulses in the train to be observed.
