8 Radiation Protection Flashcards
Thermoluminescent dosimeters
–Solid-state materials can store energy absorbed during x-ray exposure in electron traps.
–The stored energy is in the form of electrons trapped in high-energy imperfections in the crystal.
–Heating TLDs after exposure results in a light output that is proportional to the radiation air kerma incident on the material.
–The response of TLD does not depend on photon energy.
Ionization chambers
–Detect ionizing radiation by measuring the charge (electrons) liberated when x-ray photons ionize the gas inside.
–Need a positive voltage at the collecting electrode (anode), which attracts the liberated electrons.
–Accurate dosimetry devices.
–Not very sensitive, and would be useless for detecting small amounts of radioactive contamination in a nuclear medicine.
Geiger counter
–Ionization chamber with a very high voltage across the chamber.
–An incident photon interacting in this chamber produces a small number of free electrons.
–These electrons are accelerated by the large positive voltage and gain energy.
–These energetic electrons will cause more electrons to be ejected from gas atoms in the chamber, which are further accelerated and produce even more electrons.
–There is an electron avalanche corresponding to a large amplification of the initial charge liberated by the incident electron.
–Quenching gases are added to Geiger counters to improve stability.
–Sensitive and are used to detect low levels of radioactive contamination.
–Far too sensitive to measure diagnostic x-ray beams.
–It cannot differentiate between different types of radiation.
–Not accurate radiation dosimeters, provides “counts per minute”, not mGy/minute.
Pocket ionization chambers
–Pocket dosimeters are ionization chambers that look like large pens.
–A typical analog dosimeter uses a positively charged quartz fiber suspended in an air-filled chamber.
–X-rays incident on the chamber will produce ions that neutralize the charge and cause the fiber to move.
–The x-ray photon energy must exceed 20 keV to penetrate the wall of the dosimeter.
–The typical range of a pocket ionization chamber is 0 to 2 mGy up to 50 mGy.
–Easily recharged and reused.
Effective dose limit
20 mSv per year, when averaged over 5 years.
The ICRP recommends a whole-body dose limit for members of the public
1 mSv/year.
The ICRP recommends a limit of the declaration of a pregnancy by a radiation worker to the subsequent birth of a child.
–1 mSv.
–Monthly limit of 0.5 mSv
The dose limit to the eye lens of an occupational worker
–20 mSv per year.
The dose limit to the skin of a radiation worker
The hands and feet of radiation workers
–500 mSv per year.
Deterministic effects can be prevented by keeping doses below
– The threshold dose of ∼2 Gy.
The half-value layer of lead
∼0.1 mm at 60 kV,
∼0.2 mm at 80 kV,
and ∼0.3 mm at 130 kV.
workload (W)
–How often the machine is in operation (mA minute/week).
The use factor (U)
–The fraction of time that radiation points toward a specific barrier.
–For primary barriers, the use factor is 1 for the floor, 1/16 for the walls, and 0 for the ceiling.
The occupancy factor (T)
– The fraction of time people work on the other side of the barrier.
–Occupancy factors are 1 for offices and laboratories, 1/5 for corridors and employee lounges, and 1/20 for restrooms and storage areas.
Leakage radiation (mGy per hour) at 1 m from an x-ray tube must not exceed:
1.
