Conversion electron Mossbauer spectroscopy (CEMS) is a surface-sensitive technique because of the shallowness of the escape depth of conversion electrons. The proportional counter with the sample built-in is usually used for CEMS with a flow of He mixed with a quenching gas. This flow-type proportional counter with a mixed gas, however, is not suited to high-temperature measurements. For high-temperature operation, we fabricated a sealed-off proportional counter operated above 850 K for CEMS, which is filled with pure He at a pressure of 1-8x10(4) Pa. The distribution of the output pulse voltage revealed an inflection point suggesting that two multiplication mechanisms are operative. The multiplication factor of the proportional counter was estimated to be about 10(4)-10(5). By means of the fabricated proportional counter, we investigated the magnetic phase transition of Fe3O4, whose Curie temperature is 858 K. We successfully observed the ferromagnetic-paramagnetic phase transition without the reduction of the sample.
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