Step‑by‑Step Process
1. Radiation enters the detector
Incoming radiation (usually gamma or X‑rays) enters a special crystal, such as:
- NaI(Tl) (sodium iodide with thallium)
- CsI, LaBr, etc.
2. The crystal produces a flash of light (scintillation)
When radiation interacts with the crystal:
- It transfers energy to the material
- The crystal emits a tiny flash of light (a photon burst)
This is called scintillation
Key point:
- Higher radiation energy → brighter flash
3. Light is detected by a photomultiplier tube (PMT)
The light flashes hit a photomultiplier tube (PMT) or photodiode.
Inside the PMT:
- Light photons hit a photocathode → release electrons
- Electrons are amplified through a series of stages (dynodes)
- This creates a large electrical pulse
The PMT acts like an extremely sensitive “light amplifier”
4. The electrical signal is measured
The detector electronics:
- Count the number of pulses (how often radiation occurs)
- Measure pulse size (how much energy each event had)
This allows the detector to display:
- CPS / CPM → count rate
- Dose rate (µSv/hr or mR/hr)
- In advanced systems → energy spectrum
Simple Analogy
Think of it like this:
Radiation = invisible bullets
Crystal = target that glows when hit
PMT = night‑vision camera that sees and amplifies the glow
Electronics = counter + analyzer