Cedo - (Section 6) Time, Distance, Shielding Flashcards
Explain how using “Time” can protect you from radiation.
Not staying near a radiographic source or exposure device any longer than necessary.
What are the three methods to keep you within the principles of ALARA?
Time, Distance and Shielding.
Explain how using “Distance” can protect you from radiation.
Moving as far away as possible from a radiographic source or exposure device.
Explain how using “Shielding” can protect you from Radiation.
Place yourself between shielding and a radiographic source or exposure device as much as possible.
What are two ways intensity be determined?
With a survey meter and by applying a output factor ( mathematical equation) to the activity of your radiographic source.
If the distance from a source is doubled what happens to intensity?
The intensity is reduce by 4.
What are some the prefixes, symbols and multiples used in conjunction with the SI system?
tera (T) - 10*(12) giga (G) - 10*(9) mega (M) - 10*(6) kilo (k) - 10*(3) centi (c) - 10*(-2) milli (m) - 10*(-3) micro (u) - 10*(-6) nano (n) - 10*(-9)
What is the intensity of Ir 192, Co 60 and Se 75 at 1 metre?
Ir 192 - 0.15 mSv/h per GBq @ 1m
Co 60 - 0.36 mSv/h per GBq @ 1m
Se 75 - 0.05 mSv/h per GBq @ 1m
What is the formula to determine intensity at 1 metre?
➡️ Intensity @1m =
source strength in GBq)✖️ (source output factor
Determine the intensity at 1 metre for a 2.2 TBq Ir 192 source.
➡️ Intensity @ 1m =
(source strength in GBq) ✖️ (source output factor)
➡️ convert
TBq to GBq (2.2 TBq = 2200 GBq)
➡️ int @ 1m = 2200 x 0.15
= 330 mSv/h
What is the formula to determine dose as a function of time?
➡️ Dose = Intensity ✖️ Time
- Time must be converted from minutes to parts of a hour i.e - 5 mins = 1/12 hr, 10 mins = 1/6 hr, 20 mins = 1/3 hr
What is the absorbed dose at 15m for 20 mins if intensity is 1.45 mSv/hr?
➡️ Dose = Intensity ✖️ Time
= 1.45 mSv/hr ✖️ (20 mins ➗ 60 mins) must convert mins to hrs
= 1.45 mSv/hr ✖️ 1/3 hr
= 1.45 ➗ 3
= 0.48 mSv
What is the absorbed dose at 13m for 5 mins if intensity is 1.93 mSv/hr?
➡️ Dose = Intensity ✖️ Time
= 1.93 mSv/hr ✖️ (5 mins ➗ 60 mins) must convert mins to hrs
= 1.93 mSv/hr ✖️ 1/12 hr
= 1.93 ➗ 12
= 0.16 mSv
What is the absorbed dose at 1m for 30 secs if intensity is 326.7 mSv/hr?
➡️ Dose = Intensity ✖️ Time
= 326.7mSv/hr ✖️ (30 secs ➗ 3600 secs) must convert secs to hrs
= 326.7 mSv/hr ✖️ 1/20 hr
= 326.7 ➗ 120
= 2.72 mSv
What is the absorbed dose at 2mm for 2 secs if intensity is 81 675 000 mSv/hr?
➡️ Dose = Intensity ✖️ Time
= 81 675 000 mSv/hr ✖️ (2 secs ➗ 3600 secs) must convert secs to hrs
= 81 675 000 mSv/hr ✖️ 1/1800 hr
= 81 675 000 ➗ 1800
= 45 375 mSv
If you know the intensity of a radiographic source at a given distance, you can determine the intensity at any other distance by applying what?
Inverse Square Law (intensity as a function of distance)
What is the formula to determine intensity as a function of distance using the Inverse Square Law?
➡️ Intensity = (I,1/I,2) ✖️ (D,2)’2/(D,1)’2
see page 45
Where: I,1 = Original intensity I,2 = New Intensity D,1 = Original Distance D,2 = New Distance
What generally determines how effective a material is at shielding Gamma rays?
The materials density.
What are 5 common materials used for shielding? List them in order of highest to lowest effectiveness.
- Depleted Uranium
- Tungsten
- Lead
- Steel
- Concrete
In Radiography the most practical means of shielding is achieved by the us of?
Collimators.
The decrease of intensity achieved by using Collimators is expressed in what?
Half-value layers (HVL) and Tenth-value layers (TVL)
The thickness of a specific absorbing material required to reduce (attenuate) the intensity to one half it’s original value is known as what?
Half-value layer (HVL)
The thickness of a specific absorbing material required to reduce (attenuate) the intensity to one tenth it’s original value is known as what?
Tenth-value layer (TVL)
What is the formula to determine HVL and TVL?
HVL - divide by 2 until desired number of HVL’s is reached or desired intensity.
TVL - divide by 10 until desired number of TVL’s is reached or desired intensity.
Pg 50-55
