The arrangement and characteristics of the x-gamma ray coincidence-counting system and the method of standardizing the disintegration rate of
88Y and
54Mn sources were previously reported. In the present paper, the absolute disintegration rate of a
55Fe source is determined by modifying the x-gamma ray coincidence-counting method. The absolute disintegration rate of a
55Fe source emitting only x-rays has, ordinarily, been measured by the 4πX counting method. In this report, the calibration of a
55Fe source by the x-gamma ray coincidence method is attempted. The prin-e ciple of this idea is as follows: a known amount of
57Co or
54Mn isotope, which emitts both x- and gamma-rays, is added to a
55Fe source separately, the radioactivity of each mixed source is counted with x-gamma ray coincidence equipment, and then the absolute disintegration rate of the
55Fe isotope is calculated by analyzing the counting results, The 55Fe solution used in this experiment was a standard solution supplied by Radio-Chemical Center (U. K.). The
57Co and
54Mn isotopes, were produced by irradiating highly-pure nickel sheet with a high energy gamma ray beam from the linac at JAERI, and by irradiating electrolytic iron with fast neutrons in JRR-3, respectively. The absolute disintegration rates of
57Co and 54Mn sources were well defined by the x-gamma ray coincidence method decribed in previous papers, and then either
57Co or
54Mn master solution was, mixed with the standard solution of
55Fe. The counting sources were prepared by dropping and evaporating the solution on a thin mylar film, and the radioactivity of these sources were measured with the x-gamma ray coincidence counting system. It is found that the agreement between
57Co. and
54Mn tracer method of absolute counting for
55Fe was good.
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