Which of the following best describes most well counters used for measuring radioactivity?

Most well counters used for measuring radioactivity are designed with a sodium iodide crystal as the primary detection material. Sodium iodide crystals are effective scintillation detectors that convert ionizing radiation into visible light. When radioactive particles interact with the sodium iodide crystal, they excite the atoms within the crystal, leading to the emission of light. This light is then detected by photomultiplier tubes (PMTs), which convert the light into an electrical signal that can be quantified.

Sodium iodide is advantageous for well counters due to its relatively high atomic number, which increases the probability of interactions with gamma photons, making them highly sensitive to gamma radiation. This sensitivity makes them suitable for various applications in nuclear medicine, including the measurement of radioactivity in samples such as blood or other biological fluids.

In contrast, while silver halide layers and thermoluminescent crystals are also used in radiation detection, they serve different purposes and are less common in the context of well counters. Silver halide layers are traditionally used in photographic films for capturing radiation, and thermoluminescent crystals measure radiation exposure by storing energy and releasing it as light upon heating. The presence of an applied voltage from anode to cathode is a characteristic of some types of electronic detection