Many of the applications described in this booklet require accurate
knowledge of the exact location of the radioactive source within the
body. Commonly a detecting tube is used having a collimating shield so
that it accepts only that radiation that strikes it head-on. A
motor-driven carrier moves the counter linearly at a slow rate.
Radiation is counted and whenever the count reaches the predetermined
amount—from one count to many—an electric impulse causes a synchronously
moving pen to make a dot on a chart. The scanner, upon reaching the end
of a line moves down to the next line and starts over, eventually
producing a complete record of the radiation sources it has passed over.