  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 100 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.
