Lecture 10 Flashcards
What is radiographic film?
Film that contains radiation-sensitive silver halide (bromine) crystals that reduce to silver. After being processed this produces an image. It is a cheap method of imaging for long-term, low-level monitoring of radiation.
How are images formed on radiographic film?
X-rays are fired through the person and onto radiographic film where the image is produced. These are known as shadowgraphs.
What are film badges used for?
They are used by radiographers to determine radiation dose over a given time (for long-term and low-level radiation).
What is the range of sensitivities for a film badge?
0.1 - 10 mSv
The film response to radiation is __________________ with dose.
Non-linear
How can film badges be adapted for professions with different types/levels of radiation?
- Different metals can be used in the badge to act as filters for different types of radiation.
- The film can be produced in differing sensitivities.
Define fluorescence
High-energy photons excite atoms and, when these atoms return to their ground state, photons of lower energy are released (as fluorescent light). This is done using caesium iodide crystals.
What is fluorescence used for?
It is an image intensifier for film when forming X-ray images, hence, a greater sensitivity and lower doses of X-rays can be used.
What is thermo-luminescence dosimetry?
When a thermoluminescent crystal is irradiated, a very minute fraction of absorbed energy is stored in the crystal. Some of this energy can be recovered later as visible light if the material is heated.
What is a scintillation detector?
The most sensitive way to measure photons and radiation in general.
How do scintillation detectors work?
A gamma ray is passed through a sodium iodide crystal doped with thallium, generating bursts of light from the gamma-ray through the photoelectric effect. These bursts of light are greater than other spectroscopic scintillators. The number of photons detected is increased by a photomultiplier tube.
Why is thallium-doped sodium iodide used in scintillation crystals?
Large crystals can be produced using sodium iodide and intense bursts of light are produced due to the presence of thallium (which behaves as an impurity) and the photoelectric effect so the gamma ray can be detected more effectively.
What is the typical photon wavelength used for scintillation crystals?
410 nm
What are three common types of scintillator crystals?
- Sodium iodide (NaI)
- Bismuth germinate (BGO)
- Barium fluoride
How does a photomultiplier tube work?
An initial photon is fired into the photomultiplier tube and causes an increasing number of photoelectrons to be generated at each ‘dynode’ as the incident energy is greater than the work function. It works via the photoelectric effect.
