BaF$_2$ calorimeters are radiation-hard and have good energy resolution, but the time resolution can be a problem at high rate. Indeed, the scintillation light has both a fast (decay time 0.9 ns at 220 nm) and a larger, slow (decay time 650 ns at 300 nm) component. The slow component can worsen the excellent time resolution available from the fast component.
In order to take advantage of the very fast scintillation component, it is necessary to have a
fast photosensor with high efficiency in the UV-region that is also
able to discriminate against the slow scintillation component.
A Caltech/JPL/RMD consortium has been formed to develop a modified
RMD large area APD into a delta-doped superlattice APD.
This device incorporates an atomic layer deposition antireflection
filter that will provide 60 % quantum efficiency at 220 nm
and 0.1 % efficiency at 300 nm to take full advantage
of the fast decay time component of BaF$_2$ while suppressing the slow one.
To cope with the higher rate expected for the Mu2e experiment upgrade and to reject pileup events,
This sensor will make possible to build a radiation-hard and extremely
fast BaF$_2$ based calorimeter having good energy and time resolution
and an extreme high rate capability that can cope with the
increased rates expected at upgrades for Mu2e or other experiments
with similar demands.