College Park, Maryland      June 6 - 10 , 2004

WP25: Application of high frequency ultrasound for the simultaneous upgrade intensity and resolution of the double crystal and Bonse - Hart diffractometers

E. Iolin, L. Rusevich (Institute of Physical Energetic, 21 Aizkraukles, LV-1006 Riga, Latvia), M. Strobl, W. Treimer (Hahn-Meitner-Institut Berlin; University of Applied Sciences (TFH) Berlin), P. Mikula (Nuclear Physics Institute, 25068 Rež near Prague, Czech Republic)

It is known that neutron Double Crystal (DCD) and Bonse-Hart (BHD) diffractometers are used for the ultra small angle scattering research. However, this technique is flux limited. It is also known that h.f. ultrasound leads to the increasing of the difracted beam intensity at the perfect s.c. Unfortunately this increasing is accompanied by a loss of the momentum resolution. Some time ago it was argued (E.Iolin, Zh.Tekhn.Fiz.Pis’ma 15, 52,1989) that it should be possible to improve simultaneously resolution and intensity for the case of DCD. It could be achieved by the excitation of the h.f ultrasound with the same frequency in monochromator AND analyser. Such a perspective is especially interesting for the case of BHD. Ultrasonic satellites exist as additional zones of the almost total reflections for the case of symmetrical Bragg scattering. Reflectivity between phonon satellites is suppressed due to the multiply reflections inside BHD. Each satellite is narrow, but they are numerous. The whole effective diffracted neutron beam phase space volume is increased that’s led to the upgrade of the diffractometer parameters.

Here we report the first results of corresponding experiments. We excited longitudinal and transversal (LAW, TAW) acoustic waves with frequency 70.27 MHz in Si(111) plate and analysed the reflected beam by means of similar Si plate (DCD regime) or by triple-bounces BH analyser, respectively. The neutrons wavelength was 0.523 nm. Using the triple bounce BH analyser we observed a strong improvement of phonon satellites contrast with moderate, ~20%, loss of the main peak intensity. This was due to the suppression of of the wings of the Darwin curve. In this case BH analyser ”allows” to observe up to ±3 ultrasonic phonon satellites.

We studied DCD with simultaneously ultrasonically excited monochromator (LAW) AND analyzer (TAW). We observed ~25% narrowing of the main peak, its intensity increasing up to 70% and simultaneously the ultrasonic phonon satellites contrast is strongly improved in this “double-sound” case. These results were observed for the first time. It seems that our approach can be also applied for the improvement of the neutron imaging for the case of the refraction contrast.

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