Big science –literally- on ID31 #weekendusers

01-06-2018

This is no ordinary experiment. With a huge detector in tow and a team of 15 scientists from Goethe University in Frankfurt (Germany), it is probably as big as science gets -literally.

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A 4-metre-long lorry arrived at the ESRF with a precious load: a so-called COLTRIMS Reaction Microscope. The chamber is so big that it requires a crane to fit it into the experimental hutch of ID31. And lots of manpower to set the experiment up. The aim: to image the momentum distribution of one of the two electrons in the Helium atom without averaging over the momentum distribution of the other, offering the most complete and detailed view on electron correlation.

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The installation of the COLTRIMS Reaction Microscope. Credits: C. Argoud.


The COLTRIMS technique allows the team to measure event by event the initial state momentum of a Compton scattered electron of a Helium atom and, in coincidence with this, they measure the second electron’s momentum as it is shaken off.

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The experiment set-up was not the easiest and needed a very "hands-on" approach. Credits: M. Kircher.

The experiment couldn’t just take place anywhere. “We need a beamline like ID31, with a high photon flux, because the Compton cross-section is very small. In order to get a statistically meaningful amount of data, the experiment required 18 shifts. And finally, we could only do it during the 16-bunch mode, as we use a time-of flight technique, which requires more than 100 nanoseconds between photon bunches”, explains Max Kircher, main proposer of the experiment.

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The team was in high spirits throughout the two-week duration of the experiment. Credits: M. Kircher.

 

It took the team a whole good week to set up the experiment. But the local contact and beamline responsible, Veijo Honkimäki, was familiar with it all. “This is the second part of an experiment they did 22 years ago”, he explained. Professor Dörner, who was a post-doctoral researcher in those days, reunited with Honkimäki, also a post-doctoral researcher, after all this time. “A very nice publication came out from this in those days. We’ll see what happens now”.

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Dörner (left) and Honkimäki (right) met again after 22 years since their first experiment. Credits: M. Kircher. 

With such a big group of scientists and two weeks of experimental time, ID31 has become a second house to the team: cards to play, lots of soft drinks, sweets and even a beach lounger for those long nights.

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The beamline control room was heaving during the experiment (left). On the right, the lorry the team used to transport their detector. Credits: M. Kircher.

So is it all worth it? “Definitely”, asserts Kircher. “We hope our experiment can be taken to larger molecules in the gas phase. So far, though, we are acquiring great data”, he concludes with a grin.

 

Text by Montserrat Capellas Espuny

Reference:

L. Spielberger et al. Phys. Rev. A 59 (1998) 371-379

Top image: View of the set-up on the beamline. Credits: M. Kircher.