Ionising Radiation: Measurements And Standards Flashcards
Who is the father of radiology
Wilhelm Conrad Rontgen
We cannot detect ionizing radiation ______
Directly
What are 2 methods commonly used to detect ionizing radiation
Film
Electroscope
In order to detect ionizing radiation
The radiation must interact with something else
We need to know the dose rate asap because
We can calculate the accumulated dose faster in time to give a warning if the dose rate is very high
What is an instrument that measures ionizing radiation
Dosimeter, it measures the amount of radiation you’ve been exposed to
Monitoring equipment are often calibrated in terms of
Dose rate can also be measured using an
Ionization chamber
Dose rate is
The dosage absorbed per unit time
Ionization chamber advantages
- more accurate
- less affected by radiation
- can measure high dose rates which would saturate pother monitors
Name 2 instruments to monitor dose/ dose rate that can be worn on the body
Pocket dosimeters
G-M tube instruments with a dose rate alarm
The cheapest and most commonly used personal monitors use
Thermoluminescent dosimetry (TLD)
4 Advantages of film dosimeters
- film badge are simple and inexpensive
- provides a permanent record
- very reliable
- measures and records radiation exposure due to gamma rays, X-rays and beta particles
3 Disadvantages box film dosimeters
- cannot be read on site instead they have to be send away for developing
- one time use only
- exposures less than 0.2 mSv (20 milligram) of gamma radiation are not measured accurately
4 advantages of TLDs dosimeters
- measure a greater range of doses in comparison with film badges
- doses are easily obtained
- can be read on the spot
- easily reusable
2 disadvantages of TLDs
- each dose cannot be read out more than once
- the readout process effectively “zeroes” the tld
What are electronic personal dosimeters?
They are high range, alarming, active dosimeters, designed for occupational radiation workers in planned exposure sites.
- displays does AND dose rate
- high level of radiation sensitivity
An electric current is simply a flow of
Electrons or ions
Electricity cannot flow through air because there are no free electrons or ions to carry the current, however;
If some of the atoms in the are are IONIZED, then free electrons are produced and an electric current can flow
Ionized atoms = free electrons
Free electrons = electrical current flow
Inside an ionization chamber, the _____ radiation ____ electrons into the air
Ionizing, frees
The nose free electrons fill the chamber and allow
A current to flow
The chamber may be ___ or _____ to the atmosphere
Sealed or open
When the chamber is exposed to ionizing radiation what happens besides the freeing of electrons
Positive and negative ions are produced.
The positively charged ions are attracted to the
Negative plate
The negative ions are attracted to the
Positive plate.
THE IONS ATTRACTIONS TO THE OPPOSITELY CHARGED PLATES IS WHAT ALLOWS A CURRENT TO FLOWW THROUGH THE CHAMBER.
The current flow in an ionizing chamber is measured by a
Sensitive ammeter
The current is measured at
10^-9 A (6 x 10^9 electrons per second)
Ionization chambers are used to
- Measure the ionizing radiation output of therapeutic and diagnostic ionizing radiation generators
- make accurate measurements of patient radiation dose
What is the very sensitive form of ionization chamber
The G-M tube “Geiger muller tube. SO SENSITIVE THAT IT CAN DETECT SINGLE IONIZING PARTICLES WHICH ENTER THE TUBE.
How does the G-M type differ from the ionisation chamber
It’s filled with gas such as argon or neon rather than air
The gas in an ionization chamber is about ____- of atmospheric pressure
One-fifth
How does the G-M tube work
- incident ionizing radiation will produce free electrons within the tube
- the free electrons will be attracted to the central electrode which is held at positive potential (larger potential than in the ionization chamber)
- the electrons attracted to the central anode are accelerated by the potential
- they therefore gain a lot of energy to cause further ionization = a chain reaction occurs
- when all electrons produced hit the central anode they can cause photons (visible light or ultraviolet radiation) to be emitted
- these photons cause more ionization in the gas of the chamber
- the original ionizing radiation can produce 10^5 electrons in the chamber (a pulse of current)
- the tube takes long to recover from the recorded pulse because of the positive ions in the tube “dead time”
The dead time limits the number of events which can be recorded each second
A G-M tube is used for the detection and measurement of all types of radiation like
Alpha, beta and gamma radiation
The use of G-M is not recommended for
diagnostic radiology
G-M counters are designed to detect
Isotope emissions
The two main difficulties in diagnostic radiology for the GM counters are
- response time of several seconds
- strong energy dependence at low photon energies
