The decays of 56Co and 66Ga and precise gamma-ray intensities

66Ga: a standard for high-energy calibration of Ge detectors

“…It is now generally recognized that Ge detector efficiency curves do not exhibit the form assumed by Camp et al in the 2500 to 5000 keV energy region. This conclusion was reached in 1974 by McCallum and Coote [4] who, having calibrated their detector efficiency up to 11,588 keV using γ-ray pairs from several (p,γ) resonances, measured emission probabilities for 66 Ga and 56 Co and observed that the values reported by Camp et al [2] were systematically lower, by as much as 30% for the 4806-keV γ ray of 66 Ga. Also, they found that above 2500 keV their detector efficiency did not decrease linearly with energy on a log-log scale, but it decayed much more rapidly, indicating a lower capture rate in the detector for the higher-energy γ rays. Therefore, the emission probabilities for energies above about 2500 keV reported by Camp et al [2] are systematically too low, a fact that has recently been corroborated by Schmid et al [5].…”

“…Clearly, a linear extrapolation (on a log-log plot) beyond 2000 keV would, for this detector, result in an efficiency which is much too high by 5000 keV. It was this type of extrapolation by Camp et al [2] which led to incorrect values for the relative emission probabilities of γ rays from 66 Ga decay in the 3000-5000 keV energy range.…”

“…Table 1 shows the 66 Ga relative γ-ray emission probabilities determined in the present measurements, along with those those from Camp. et al [2] and from Raman et al [8]. We report here only those γ rays with emission probabilities greater than 1% of that of the 1039-keV γ ray, since these are the most useful for Ge-detector calibration.…”

“…The relative γ-ray emission probabilities of 66 Ga were measured with good statistical precision in 1971 by Camp et al [2] and a more extensive, but interrelated, measurement was reported in 1994 by Endt and Alderliesten [3] who normalized their emission probabilities for the strongest γ rays to the values deduced by Camp et al [2]. Camp et al determined their detector efficiency up to 2750 keV using radioactive sources, then assumed an almost linear extrapolation on a log-log plot of the efficiency curve between 1000 keV and 2000 keV to deduce the detector efficiency between 2500 keV and 5000 keV.…”

“…McCallum and Coote [4] presented factors to correct the results from Camp et al [2], and Trzaska [6] and Bhat [7] have taken these into account in their data evaluations when recommending γ-ray emission probabilities for 66 Ga. However, since the proposed correction factors lack explicit uncertainties, all of this effort produced new emission probability values, but without associated experimental uncertainties; thus, 66 Ga remained unsuitable as a calibration standard.…”