Abstract
Various counting geometries have been developed for whole-body measurements using a variety of detectors: ionization chambers (1, 2), Geiger-Müller counters (3), liquid and plastic scintillators (4, 5), and NaI (Tl) crystals singly and in multiple array (6, 7). The different measuring technics have been employed in an attempt to achieve high sensitivity in the detection and measurement of the radiation from different isotopes independent of body distribution. The liquid and plastic scintillators give high body coverage, and their sensitivity is therefore relatively high and somewhat independent of body distribution, but these detectors have poor energy resolution. Multiple NaI (Tl) crystals have been used by Rundo and others (7, 11), so positioned as to record a flat response for a radioisotope distributed along the center line of the body. This system has poor accuracy, though higher sensitivity has recently been claimed for it by Miller (8). Multiple arrangements further complicate electronics circuitry and daily check-up of energy calibration. The Argonne tilting chair with a single NaI (Tl) detector is in use in many wholebody counting facilities, but information is not available as to the response of the crystal with the distribution of radioactivity along the center line of the body. Dynamic systems in which many crystals surrounding the body are made to move along its length may meet the requirements of high sensitivity and independence of distribution of the radioisotope. The feasibility of these systems is still to be confirmed by experience. The single crystal detector system is a greatly simplified technic and with suitable geometry promises to yield the desired sensitivity and a flat response for all practical purposes. We shall present here information obtained with a single large NaI (Tl) crystal detector and the 50-cm.-arc chair in comparison with the Argonne tilting chair and 1-meter-arc geometries. Response to a Point Source at Various Positions around the Single Crystal The presentation of different solid angles by the large (8 in. diameter; 4 in. thickness) NaI (Tl) crystal detector in different directions and its positioning in close proximity to the human body to achieve higher sensitivity do not render the counting rate independent of the position of the source. In the 1-meter-arc technic of Evans (3) this has been achieved by use of a small detector at a comparatively long distance from the body. The 1-meter- or 1.75-meter-arc geometries are now employed by C. E. Miller for calibration and conversion factor purposes only when specific radionuclides, e.g., K42 (8), are administered. In practice it is not technically feasible to obtain conversion factors for all the radionuclides nor for each individual body build.
Published Version
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