Radio Frequency muSR
25 Aug 2009
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Radio Frequency muSR, usually abbreviated to RF-muSR, is a technique similar to ESR or NMR. 

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​​Radio frequency muSR resonance from the cyclohexadienyl radical. ​

 

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 stimulat​e 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.

In a B0 field of magnitude 1476 G a bare muon will precesses at the Lamor frequency of 20 MHz.

The 0.2 MHz Lamor frequency of a bare muon as it precesses about a B1 field of magnitude 15 G.

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.



A few ms after the ISIS extract trigger, the muon pulse enters the sample. The RF pulse can be triggered relative to this extract trigger such that its duration results in the muon polarisation being rotated through 90°.

 

The following diagrams show the result on the detected signal.

 


In a longitudinal B0 experiment the result of a 90° RF pulse is loss of polarisation
 

 

However, in a transverse B0 field experiment the 90° RF pulse results in the build-up of polarisation. Data can now readily be taken above the ISIS bandpass.

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