Thiacrystal scintillation counter.

Abstract

The preoperative detection and identification of many types of lesions such as mediastinal tumors (1) have depended upon the use of the Geiger-Muller counter. The conventional scintillation counter has been generally inapplicable due to its size and weight. The necessity for collimation with a thick crystal has required increased distance which more than offsets the scintillation counter's greater sensitivity for gamma rays (usually by a factor of 2 or more), and until recently (2, 3) this instrument has had no significant sensitivity to the soft x-rays from a lesion activated with P32. For these reasons experiments were begun about eighteen months ago using thin NaI and CsI crystals of various diameters. In counting for the purpose of detecting, locating, and outlining a lesion (in contrast to pulse-height analysis, for example) only the first few millimeters of the crystal are necessary. A thin crystal requires a negligible shielding weight and has an inherent spatial sensitivity similar to a good, properly shielded G-M tube with many times the sensitivity of the tube. Detailed theoretical and practical considerations will be given elsewhere (4), and further work is in progress. It appears that a crystal about 3 mm. in thickness and 25 mm. in diameter is optimum. A beryllium window has not proved helpful. For the past six months a practical thin-crystal counter has been in use and has proved highly advantageous for many types of diagnostic procedures such as the preoperative localization of malignant brain tumors. The present counter is shown in Figure 1. It has a conventional PM-PA chassis, a 3 × 25-mm. NaI (T1) crystal, with an 0.10-mm. Al window. Shielding consists of a Pb ring 12.5 mm. thick surrounding the crystal and extending 15 mm. behind it around the magnetic shield. The entire instrument weighs 1,180 gm. (2.6 lb.). Response curves (Fig. 2) of this counter to two phantom “lesions,” each under a slab of Lucite 25 mm. thick are reproduced in Figure 2: (a) 0.35 μc I131, 5 mm. in diameter; (b) 6.0 μc P32, 2.0 cm. in diameter. The G-M tube is still the reference instrument, but the thin-crystal counter has equal versatility and as much as fifty times the sensitivity. As in the surgical probe counter (5), the relative sensitivity and background can be varied to suit the problem by varying the operating voltage. Student physicians are currently learning to use the new instrument.