Shedding Light on Temperature

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Diamond scientists have developed, in collaboration with University of Oxford, a non-contact luminescence lifetime cryothermometry technique to measure the sample temperature on the Long Wavelength Macromolecular Crystallography (MX) beamline (I23) at Diamond Light Source. Monitoring the sample temperature is critical for in vacuum protein crystallography to ensure cryogenic temperatures are maintained during experiments.

A recent publication in the Journal of Synchrotron Radiation reports how this technique is integrated into the I23 beamline and can measure sample temperature under vacuum in the operation range of 30 to 150 Kelvin (-243 to -123°C) with an accuracy of 0.6 Kelvin (0.6°C). It has also been used to characterise the thermal performance of MX sample mounts. This technique demonstrates its usefulness in MX experiments and in the improvement of investigation design. 

The importance of temperature

Temperature is a fundamental parameter in experimental science and thus its accurate determination is often required. In harsher experimental environments, such as under vacuum in which I23 operates, temperature measurement can be a significant challenge. There are several techniques available and often they require a sensor to be attached to the sample. Heat leaks to the sample may occur through wires, which can influence the accuracy of the measured temperature. Using a wired sensor has additional problems on I23 due to the small size of the protein crystal samples and the vacuum environment. Ensuring a wired sensor is always attached to a sample, which should be regularly changed, can be a “next to impossible task” said I23 Beamline Scientist Dr Vitaliy Mykhaylyk. He continued, “if there is a way of determining temperature without having contacts, this would always be advantageous”.

Figure 1: A photograph of the BGO scintillator on a sample mount (a) pre and (b) luminescing post photoexcitation; (c) the luminescence decay time constant of the BGO scintillator as a function of temperature.

Luminating features of non-contact luminescence lifetime cryothermometry

There are several non-contact methods for temperature measurement. Temperature information is acquired by measuring a selected physical property and its variation with temperature. The technique developed on I23 is a non-contact method that exploits the property of luminescence and, unlike for other non-contact techniques such as infrared thermometry, temperature information can be obtained at the cryogenic operating temperatures of I23. The technique works by measuring the luminescence of a scintillator sensor as a function of time, following excitation. A luminescence decay constant is derived from which the important temperature information is found – the decay constant is temperature dependent.

In developing this technique for use on I23, the beamline team worked with Professor Hans Kraus at the University of Oxford. Previous work by Dr Mykhaylyk into the thermal properties of scintillators also led to the selection of the scintillator BGO (Bi4Ge3O12) found to be sensitive in the I23 beamline operation range (30 to 150 Kelvin; -243 to -123°C) and is excitable using ultraviolet (UV) radiation from LEDs. This is the first application of scintillation materials in luminescence lifetime cryothermometry.

Facile system integration

A significant driver in the success of the non-contact technique on I23 was the ease at which it could be integrated onto the beamline. Dr Mykhaylyk reported “practically integration of this idea into the environment of the beamline went very smoothly and didn’t require significant effort of development”. The current sample viewing system has been simply modified with an addition of a motorised mirror allowing luminescence light to be directed towards a photodetector whilst a previously known method for luminescence data collection and analysis is used to determine decay characteristics.

All-encompassing sample temperature information

In validating the non-contact luminescence lifetime cryothermometry technique an important scintillator sensor calibration step was first carried out off-line (without X-rays) to ensure reliable temperature reading. Following calibration, the uncertainty of the temperature measurements using the BGO scintillator was found to be 0.6 Kelvin (0.6°C) over the primary beamline operation temperature range (30 to 150 K; -243 to -123°C). A second crucial step confirmed that the non-contact technique could operate under the vacuum conditions of I23 and yield temperature information within error of the expected value.

The non-contact thermometry technique can also characterise the thermal performance of different sample mounts revealing effects of mount thickness or material on temperature. This knowledge is important for developments of the MX sample holder assembly. The cryothermometry technique will additionally benefit those where samples are changed frequently and is the “way into future because this will eliminate many mistakes, error and enable this experiment to be swift and easy” said Dr Mykhaylyk.

There are plans for further developments of the non-contact thermometry technology on I23 including to make the system more user friendly and to make it operable in real-time, where currently temperature information is available retrospectively and without X-rays. Dr Mykhaylyk went on to say “I am confident that there are quite exciting possibilities to develop this technique and make real time measurements and to make it compatible with the operation of the beamline and X-ray radiation.”

Figure 2: schematic of the operation of luminescence lifetime cryothermometry.

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