  A novel technique for remote, non-contact, in situ monitoring of the protein
crystal temperature has been developed for the new I23 beamline at the Diamond
Light Source, a facility dedicated to macromolecular crystallography (MX) with
long-wavelength X-rays. The temperature is derived from the
temperature-dependant decay time constant of luminescence from a minuscule
scintillation sensor (0.05 mm3) located in very close proximity to the sample
under test. In this work we present the underlying principle of cryogenic
luminescence lifetime thermometry, discuss the features of the detection
method, the choice of temperature sensor and demonstrate how the temperature
monitoring system was integrated within the viewing system of the end-station
used for the visualisation of protein crystals. The thermometry system was
characterised using a Bi4Ge3O12 (BGO) crystal scintillator that exhibits good
responsivity of the decay time constant as function of temperature over a wide
range (8-270 K). The scintillation sensor was calibrated and the uncertainty of
the temperature measurements over the primary operation temperature range of
the beamline (30-150 K) was assessed to be +-1.6 K. It has been shown that the
temperature of the sample holder, measured using the luminescence sensor,
agrees well with the expected value. The technique was applied to characterise
the thermal performance of different sample mounts that have been used in MX
experiments at the I23 beamline. The thickness of the mount is shown to have
the greatest impact upon the temperature distribution across the sample mount.
Altogether these tests and findings demonstrate the usefulness of the
thermometry system in highlighting the challenges that remain to be addressed
for the in-vacuum MX experiment to become a reliable and indispensable tool for
structural biology
