click on figure for more detailsThe stroboscopic method demands for certain experiments specifications matched by D20; a high flux to collect data in a reasonable number of cycles and a multidetector to observe a whole Bragg peak or even the complete powder pattern simultaneously. Currently the time resolution is about 50 µs for 2.4 Å neutrons, and about 15 µs for a neutron wavelength of 0.8 Å
The crystal was set up in Bragg geometry. In a first part of the experiment the frequency response of the coupled transducer-silicon sandwich was measured by tracing the 111 peak intensity as a function of applied rf-frequency. The figure below shows the results obtained.
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Frequency response of a vibrating Si crystal
The resonance frequency shows up as a shows up as a maximum reduction of extinction at 15.21 MHz. A second intensity maximum is seen at 15.63 MHz caused probably by a second parasitic resonant mode of the transducer-silicon system. Such a reduction of extinction in vibrating crystals is a well known phenomenon.
A second part of the experiment was interested in the time response of the crystal to an applied field. The quartz transducer was excited with an rf-pulse of 2500 µs duration and a repetition rate of 5000 µs corresponding to one synchro-cycle.
In this case the stroboscopic unit was used such that two slices of length T(3) = 20 µs separated by T(1) = 2500 µs were shifted over the transition "non-vibrating/vibrating crystal" by the use of a variable delay time T(3). The total counting time for a peak was t = 0.5 s corresponding to n = 25000 cycles. The top part of the figure below shows the silicon 111 peak against the delay time. In the bottom part of the figure below we present schematically the time position of the applied rf-pulse as well as the two time slices shifting over the ultrasonic pulse.
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Time-response of a Si crystal to a pulsed ultrasonic field.
The response of the crystal to the sound excitation is almost instantaneous. Only one intermediate intensity value between the vibrating and non-vibrating state is seen. We conclude that the rise time of the Si 111 peak due to the pure reduction of extinction by vibrating the sample is smaller than the current time resolution of the experimental set up. The result is in agreement with the fact that the transducer should vibrate resonantly after approximately 3 rf-frequency periods corresponding to 3 µs at 15 MHz. The method therefore allows for a precise measurement of the time resolution of the detector.
This experiment was a first test of the time resolution achievable on D20 for stroboscopic measurements. This information is needed for further experiments and for the interpretation of a recent experiment on a vibrating magnetic erbium iron garnet at low temperature. Here we observed what we interpret as a sound spin reorientation at about 15 K. Without the agitation of a sound wave polarised along the (111) direction of the sample the reorientation takes place at a temperature between 60K and 80K. Interpreting the results obtained one has to be extremely careful, because the application of the sound field also heats the sample; nor can pure domain effects be excluded.
Here time resolved measurements could help to distinguish between instantaneous sound induced effects, heating and domain relaxation, because of the different characteristic time behaviour.