UK and Switzerland: An anvil cell for high pressure muon spectroscopy?

There is great interest in developing the ability to perform µSR studies under applied pressure as they could reveal exciting new and perhaps exotic physical properties of materials. So, as well as the piston-cylinder pressure cell optimized at PSI, SINE2020 also wanted to explore the possibility of using anvil cells for high pressure µSR studies.

However, designing an anvil cell adapted to muon techniques is challenging. Current Paris-Edinburgh anvil cells are typically optimized for small samples (smaller than those required for muon experiments) so unfortunately the anvils themselves are likely to get in the way of a muon beam and the experimental decay products we need to detect. Current cells are also too big to fit into current muon instruments.

For the Sample Environment work package, investigations and simulations on the concept of using anvil cells for muon techniques were performed by a team of people at PSI (Switzerland) and ISIS (UK). It was determined that a significant factor in the feasibility of developing this type of pressure cell was the type of muon source being used.

Muon sources come in two types: continuous (as at PSI) and pulsed (as at ISIS). Unfortunately, with a pulsed source, simulations determined that most of the muons would end up in the anvil pressure cell walls (about 98% and therefore only about 2% of the muons would contribute to the sample signal). On the other hand, there appears to be a much better chance of success at a continuous source, where 37% of the muons would theoretically end up in the sample.

Preliminary tests and simulations have taken the concept further for a continuous beam, however, as the overall dimensions of the anvil cell will need to be limited to fit within the µSR instruments, this is likely to put an upper limit on the pressure of ~4-5 GPa.

The overall conclusion is that while an anvil cell may not be a feasible option at a pulsed muon source, it may eventually be possible at a continuous facility. However, the time and money investment required to complete the development is not possible within the SINE2020 project. This Task has come to an end, for now!

Acknowledgements: Stephen Cottrell, STFC and Rustem Khasanov, PSI