Monochromatizationof synchrotron radiation down to about 10 eV at an energy of 14.4 keV has been achieved by double nuclear Bragg diffraction from "Fe-yttrium iron garnet single-crystal films set for the electronically forbidden (200) reflection. The monochromatized y quanta have been analyzed with respect to time delay and capability of resonance absorption. By setting of appropriate energy and time windows a pure beam of resonant y quanta at a rate of about 1 Hz is available.PACS numbers: 76.80.+y, 07.85.+ n, 61.10.Fr y quanta with an energy width of 10 6 to 10 s eV, which is typical for low-lying nuclear states, have a coherence length of 0.2 to 20 m. Thus a highly brilliant beam of such quanta would open new perspectives to experimental y optics. Unfortunately, the usual radioactive sources are monochromatic but not brilliant whereas synchrotron radiation is very brilliant but white.Resonant nuclear diffraction of synchrotron radiation as has been first pointed out by Ruby' is a suitable method to achieve the desired beam, which in addition would have a well-defined time structure and polarization pattern. A first attempt to observe resonant nuclear excitation in Fe has been published by Cohen, Miller, and West.In order to maintain the outstanding brilliance of the synchrotron radiation only coherent reflections come into consideration, i.e. , either grazing-incidence reflec-tions3 or nuclear Bragg and Laue diffraction. 4 5 Obviously the 14.4-keV resonance of Fe is a good choice because the excitation energy is easily available from any synchrotron radiation source which covers the xray range and because the mean life v = 140 ns of the excited state allows the application of delayedcoincidence techniques. Furthermore, the chemistry and crystallography of iron compounds is well known, so that the needed large-size single crystals can be made.While present experience with grazing-incidence antireflection films indicates that four sequential reflections are required in order to obtain the desired suppression of the nonresonant prompt radiation, only two nuclear Bragg reflections will be sufficient if pure nuclear reflections are used. The greatest progress with this technique has been published by Chechin et al. , who reported an enhancement of delayed counts behind an o. -Fe203 single crystal positioned for the I777) pure nuclear reflection.

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The saturation magnetization of gallium-substituted yttrium iron garnet single crystals of composition Y3[Fe2−x Gax](Fe3−y Gay) O12 with 0≤x+y≤1.5 has been investigated in the temperature range 4.2≤T≤Tc. The magnetization values are below most data of previous investigations on this system. From the fit of the molecular field theory to the experimental results at T=4.2 K the fraction of the tetrahedral gallium ions ft=y/(x+y) could be deduced and are found to range between 1 and 0.91 for 0≤x+y≤1.5. The compensation temperature occurring for x+y>1 has been studied as a function of the gallium distribution.

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Ir4+, incorporated on the octahedral a-sites of Y3Ga50,,:Ir, has been investigated by electron spin resonance a t 9 and 35 GHz, and by optical absorption spectroscopy in the range 5000 to 6000 cm-l. For an unequivocal optical identification Zeeman measurements were performed. I n t h e same energy range fluorescence from Ir4+ could be observed. Analysis of the data obtained b y electron spin resonance and optical techniques reveals that the properties of the ,Tzg ground state of Ir4+ are partly determined by interaction with higher excited states.

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