Radio Frequency muSR

Radio frequency muSR resonance from the cyclohexadienyl radical.

Radio frequency muSR resonance from the cyclohexadienyl radical.
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Radio Frequency muSR, usually abbreviated to RF-muSR, is a technique similar to ESR or NMR. It is an extension of the muSR method which gives additional capabilities. It involves application of one or more radio frequency pulses to a sample, to stimulate either the implanted muons, or the sample nuclei, or both.

The additional capabilities provided by RF-muSR can be used for studies in a number of science areas.  These include studies of muonium reactions, free radicals, hydrogen behaviour in semiconductors and ionic conductors.

The RF µSR technique

The following sequence demonstrates the technique:

 

The following figures show the type of signals we might expect to observe.

 

An analysis of the envelope of each of the signals can give information about the local field distribution at the muon site.

 

Why is it needed?

ISIS is a pulsed muon source.  The pulse structure leads to an uncertainty in the arrival time of the muons.  The bandwidth is therefore limited to 6 MHz.  Standard mSR techniques cannot resolve precession frequencies greater than the bandwidth.

 

In order to obtain the 20 MHz signal shown above the direction of the muon polarisation was turned through 90°.  In addition,  the RF technique enables delayed state formation to be observed directly and their dynamics investigated.

 

How is this done?

As mentioned earlier, the strength and duration of the RF pulse determines the angle through which the muon's spin is rotated.  To understand this we need to look at the timing diagram for a typical RF experiment.

 

The following diagrams show the result on the detected signal.

 

 

 

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