Performance of an array of large-volume germanium detectors for whole-body counting

An array of four large-volume coaxial germanium detectors was installed in a shielded room for the in vivo measurement of gamma emitters in the body. All four detectors are in a single 50-L dewar of liquid nitrogen and are positioned underneath the supine subject with the axes of the detector elements parallel with the length of the body. This orientation gives approximately 15% more counting efficiency than having the axes of the detectors perpendicular to the body length. The detectors have resolutions that are less than 2 keV with an average of 1.86 keV for the 60Co 1.33-MeV gamma-ray peak. The efficiencies of the four detectors range from 59% to 77% of a 7.62- x 7.62-cm (3- x 3-in.) NaI (Tl) detector. The detectors are interfaced to a multichannel analyzer such that spectra from the detectors are stored both as a sum of all counts from the four detectors and as a spectrum from each detector. The sensitivity of the germanium array for measuring radioactivity in the body exceeds that of multiple large NaI (Tl) detectors for measuring single radionuclides in the body and is much superior for measuring internal depositions of radionuclide mixtures. Another major advantage of this system compared to NaI(Tl) systems is that the results can be calculated by computer analysis with minimal human interpretation. The sensitivity of the array configuration is described and applications of the array in other configurations, such as that used for measuring the very small quantities of 24Na induced in the body during air travel, are also discussed.