Cedo - (Section 6) Time, Distance, Shielding

Question 0

Explain how using “Time” can protect you from radiation.

Answer 0

Not staying near a radiographic source or exposure device any longer than necessary.

Question 1

What are the three methods to keep you within the principles of ALARA?

Answer 1

Time, Distance and Shielding.

Question 2

Explain how using “Distance” can protect you from radiation.

Answer 2

Moving as far away as possible from a radiographic source or exposure device.

Question 3

Explain how using “Shielding” can protect you from Radiation.

Answer 3

Place yourself between shielding and a radiographic source or exposure device as much as possible.

Question 4

What are two ways intensity be determined?

Answer 4

With a survey meter and by applying a output factor ( mathematical equation) to the activity of your radiographic source.

Question 5

If the distance from a source is doubled what happens to intensity?

Answer 5

The intensity is reduce by 4.

Question 6

What are some the prefixes, symbols and multiples used in conjunction with the SI system?

Answer 6

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)

Question 7

What is the intensity of Ir 192, Co 60 and Se 75 at 1 metre?

Answer 7

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

Question 8

What is the formula to determine intensity at 1 metre?

Answer 8

➡️ Intensity @1m =

source strength in GBq)✖️ (source output factor

Question 9

Determine the intensity at 1 metre for a 2.2 TBq Ir 192 source.

Answer 9

➡️ 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

Question 10

What is the formula to determine dose as a function of time?

Answer 10

➡️ 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

Question 11

What is the absorbed dose at 15m for 20 mins if intensity is 1.45 mSv/hr?

Answer 11

➡️ 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

Question 12

What is the absorbed dose at 13m for 5 mins if intensity is 1.93 mSv/hr?

Answer 12

➡️ 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

Question 13

What is the absorbed dose at 1m for 30 secs if intensity is 326.7 mSv/hr?

Answer 13

➡️ 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

Question 14

What is the absorbed dose at 2mm for 2 secs if intensity is 81 675 000 mSv/hr?

Answer 14

➡️ 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

Question 15

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?

Answer 15

Inverse Square Law (intensity as a function of distance)

Question 16

What is the formula to determine intensity as a function of distance using the Inverse Square Law?

Answer 16

➡️ 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

Question 17

What generally determines how effective a material is at shielding Gamma rays?

Answer 17

The materials density.

Question 18

What are 5 common materials used for shielding? List them in order of highest to lowest effectiveness.

Answer 18

• Depleted Uranium
• Tungsten
• Lead
• Steel
• Concrete

Question 20

In Radiography the most practical means of shielding is achieved by the us of?

Answer 20

Collimators.

Question 21

The decrease of intensity achieved by using Collimators is expressed in what?

Answer 21

Half-value layers (HVL) and Tenth-value layers (TVL)

Question 22

The thickness of a specific absorbing material required to reduce (attenuate) the intensity to one half it’s original value is known as what?

Answer 22

Half-value layer (HVL)

Question 23

The thickness of a specific absorbing material required to reduce (attenuate) the intensity to one tenth it’s original value is known as what?

Answer 23

Tenth-value layer (TVL)

Question 24

What is the formula to determine HVL and TVL?

Answer 24

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

Question 25

The Nuclear Safety and Control Regulations require that collimators have a minimum shielding factor (SF) of what?

Answer 25

100.

Question 26

What federal publication contains information concerning the operation and safety of personnel dealing win radioactive isotopes?

Answer 26

Nuclear Safety and Control Act.

Question 27

The shielding factor (SF) is dependent on what?

Answer 27

The number of half-value or tenth-value layers of a collimators material.

HVL - SF= 2n
TVL - SF= 10n