**Core Concept**
A scintillation counter detects radioactive emissions by converting ionizing radiation into visible light, which is then measured by a photomultiplier tube. This process relies on scintillating materials (e.g., sodium iodide) that emit flashes of light when exposed to radiation.

**Why the Correct Answer is Right**
In a scintillation counter, radioactive decay interacts with a scintillation crystal, producing flashes of light (scintillations). These light signals are detected and amplified by a photomultiplier tube, converting them into electrical pulses that can be quantified. This method is highly sensitive and widely used in nuclear medicine and radiology for detecting low-level radioactivity.

**Why Each Wrong Option is Incorrect**
Option A: A Geiger counter detects radiation but does so through ionization of gas, not light conversion. It produces electrical pulses, not visible light.
Option B: Photographic film records radiation exposure through chemical changes, not by converting emissions to light in real time. It is a passive, non-electronic detector.
Option D: A radiotracer is a radioactive substance used for imaging, not a detection device. It does not convert emissions into light.

**Clinical Pearl / High-Yield Fact**
Scintillation counters are preferred in nuclear medicine for their sensitivity and ability to detect low concentrations of radiotracers—especially in PET and SPECT imaging—making them essential in diagnostic radiology.

✓ Correct Answer: C. Scintillation counter