The DUBBLE beamlines use gases and gas mixtures in a large number of experiments. Gas mixtures are used in many catalysis experiments and on the EXAFS beamline a special gas mixing facility has been installed [1]. This system can be used for most experiments including experiments requiring NOx mixtures. The notable exception is H2S which contaminates the gas lines. Experiments with H2S are still possible but experimenters are required to bring their own gas systems and seek approval of both the ESRF safety group and DUBBLE staff before the experiments. In general the required gas mixtures are made available free of charge on condition that they are available from suppliers and the staff is notified in time.

Another gas often used is He. This gas is released in experiments with superconducting magnets as a boil off product, and on an exceptional basis is used as a method to reduce X-ray absorption and parasitic scatter by air. Since He is a dwindling, non renewable resource He recovery systems have been installed on both beamlines. When using He experimenters are expected to use these recovery systems and make sure that the measured He recovery rate is not less than 95%. The design of the He-gas environment and connection to the recovery system is the responsibility of the users.

Experiments with a higher wastage will not be accepted for both financial as well as societal reasons. With the emphasis on the latter. In the case users insist on having the possibility of a higher He consumption this will not be stopped but they are expected to carry the costs of the He consumption.

On the http://henke.lbl.gov/optical_constants/ website one can find an easy tool to calculate the energy and path length dependent X-ray absorption in air and He atmospheres.

air_scatter.png

Absorption by air for a path length of 5 cm and 50 cm calculated with the help of the CXRO website

It will be clear for any experimenter that a reduction in intensity by 3% of the incoming X-ray beam, and the extra background generated by this will in X-ray scattering or spectroscopy not be the rate limiting step in an experiment. The effect in background reduction might be visible but will not mean the difference between a successful or non-successful experiment. See also: (https://en.wikipedia.org/wiki/Common_sense

[1] Vladimir  Martis,  Andrew M.  Beale,  Dirk  Detollenaere, Dipanjan Banerjee, Martine  Moroni,  Fabrice  Gosselin and  Wim  Bras, A high pressure and controlled flow gas system for catalysis research; J. Synch. Rad. (2014)  vol. 21(2);  online: http://journals.iucr.org/s/issues/2014/02/00/vv5066/vv5066bdy.html