Summing coincidence errors using 152Eu lungs to calibrate a lung-counting system:: Are they significant?

The use of a lung phantom containing Eu-152/Am-241 activity can provide a sufficient number of energy lines to generate an efficiency calibration for the in vivo measurements of radioactive materials in the lungs. However, due to the number of energy lines associated with Eu-152, coincidence summing occurs and can present a problem when using such a phantom for calibrating lung-counting systems. A Summing Peak Effect Study was conducted at three laboratories to determine the effect or using an efficiency calibration based on a Eu-152/Am-241 lung phantom. The measurement data at all three laboratories showed the presence of sum peaks. While one of the laboratories found only small biases (<5%) when using the Eu-152/Am-241 calibration, the other facilities noted up to 30% positive bias in the 140 keV to 190 keV energy range that prevents the use of the Eu-152/Am-241 lung phantom for routine calibrations. Although manufactured by different vendors, the three facilities use similar types of germanium detectors (38 cm(2) by 25 mm thick or 38 cm(2) by 30 mm thick) for counting. These results underscore the need to evaluate the coincidence summing effect, which appear system dependent, when using a nuclide such as Eu-152 for the calibration of low-energy lung counting systems and highlight the problem of using a general calibration curve in place of specific nuclide calibration factors.