What is exposure time in RT?
Simply putting Exposure Time (ET) is the duration usually in seconds or minutes when the radiographic film is exposed to the ionizing radiations, either in X-rays or Gamma-rays inspection. More technically definition will be the time needed by the X-ray or gamma radiation to ionize the RT film emulsion to the best density after film processing. Selection of the appropriate exposure time often involves trial and error methods as well as mathematical calculations. Various factors can affect the final RT film density result, such as:
- The spectrum of energy generated by the x-ray machine
- Voltage potential (KeV) used in a x-ray machine.
- Amperage (mA) used in a x-ray machine.
- RT exposure time.
- Source to film distance (SFD).
- Type of material to be radiographed.
- Material thickness via the radiation will pass during the test.
- The amount of scattered radiation reaching the film.
- Type of RT film.
- Condition of chemicals used for film processing, its concentration & time in contact.
Formula for Exposure time calculation
The formula for Radiographic Testing exposure time calculation in minutes is given below.
Here, FF: is Film Factor
SFD: Source to film distance
RHM: Rotegen per hour at 1 meter for the source being used.
Film Factor in Radiography Testing (RT)
Film Factor is the amount of Exposure (R) per unit area of the Film required to produce a desired optical density in the radiographic testing.
The Film Factors for IR-192 Gamma Source as given below. These factors shall change according to the energy of radiation sources.
1. D-7 ——– 2.2R
2. D-5 ——– 2.7R
3. D-4 ——– 3.5R
4. D-2 ——– 8.0R
Source to Film Distance (SFD)
Source to film distance or simply called SFD is the distance measured between the radiation source and the RT film being exposed by the radiation and measured towards the direction of the radiation beam. SFD is the sum of Source to object distance (SOD) plus Object to film distance (OFD).
Half Value thickness (HVT)
half Value Thickness or HVT is the thickness of a material, generally known as an absorber, required to minimize the radiation intensity to half of its initial value. HVT of various media used in RT is given in the below table.
Curie (Ci) of the Source
Curie is the unit of the radioactivity of the source (decay of source). Curie value can be obtained from the decay chart of the radiation source provided with the source or calculated using the half-life of the radioactive source.
Present Activity (A) = Ao/ 2
Where Ao = Initial Activity, n= Time / HVT
Half Life (HL) of radiation source
Half-life (applicable only for gamma radiation) is the time period for the radiation source or isotope is the length of time by which half (50%) of the radioactive isotope has decayed into a stable element – two half-lives mean the source has only 25% of its original strength, three half-lives 12.5%. The half-life of various RT sources is given in the below table.