Higher Physics – Unit 3

3.5 Dosimetry and Safety

Activity of Radiation

The activity of a radioactive source is the average number of nuclei decaying per unit time.

tN

A activit

y

(Bq)

number of nuclei decaying

(no unit)

time

(s)

1 becquerel (Bq) = 1 decay per second

Absorbed Dose

When tissue is exposed to radiation, the absorbed dose is the energy absorbed per unit mass of tissue.

mE

Dabsorbed dose

(Gray, Gy)

energy

(J)

mass

(kg)

1 gray (Gy) = 1 joule per kilogram

The risk of biological harm to tissue depends on

• absorbed dose

• type of radiation

• body organ / tissue exposed.

Weighting Factor

The radiation weighting factor (WR) is a measure of the biological effect of the radiation.

Radiation Weighting Factor

α particle 20

fast neutrons 10

gamma rays 1

Weighting factor has no unit.

Equivalent Dose

The equivalent dose is a measure of the biological damage caused by radiation on living tissue.

Equivalent dose is the product of absorbed dose and radiation weighting factor.

RW DH equivalent dose

(Sievert, Sv)

weighting factor

absorbed dose

(Gray, Gy)

Equivalent Dose Rate

Equivalent dose rate is the dose per unit time.

tH

H.

equivalent dose rate

(Sv s-1, Sv h-1)

equivalent dose

(Sievert, Sv)

time

(s, mins, h)

Background Radiation

Every day we are exposed to small amounts of radiation known as background radiation.

The average annual effective dose for people in the UK is 2 mSv.

Background Radiation Effective Dose

Cosmic rays from space. 0.3 mSv

Radioactivity from rocks & soil

0.3 mSv

Radioactivity from human body

0.4 mSv

Inhaled radon gas 1.0 mSv

Exposure Limits

For the general public, the limit is 5 mSv per year in addition to background radiation.

Workers who are exposed to radiation during their employment, the limit is 50 mSv per year in addition to background radiation.

Half-Value Thickness

Experiment to measure half-value thickness of an absorber

lead discs

G-M tube

counter

γ source

Method

The count rate is measured with:

• no radiation source (background radiation)

• radiation source no absorber

• radiation source, one lead disc

• radiation source, two lead discs etc.

The count rate per second is found by measuring the count over two minutes and calculating an average.

The corrected count rate is the count rate less the background count.

Graph of Results

The corrected count rate is plotted against absorber thickness.

absorber thickness

corrected count rate

Conclusion

Half value thickness of an absorber is the thickness of absorber which reduces the intensity of radiation by half.

Half-Value Thickness Calculations

Example 1

The dose equivalent rate from a radioisotope is 64 μ Sv h-1.

A barrier which is 120 cm thick is placed in front of the radioisotope.

The dose equivalent rate falls to 8 μ Sv h-1.

Calculate the half value thickness of the barrier.

8163264

3 half value thicknesses

cm 120 sthicknesse value half 3

cm 40 thickness value half 1

Example 2

The half thickness of lead for a particular gamma source is 11 mm.

A 33 mm thick piece of lead is placed in front of the gamma source which has an unshielded dose equivalent rate of 60 μ Sv h-

1.

Calculate the dose equivalent rate of the shielded gamma source.

sthicknesse value half 3 mm 33

thickness value half 1 mm 11

7.5153060

-1h Sv μ 7.5 rate equivalent dose

Questions

1. A gamma source has a corrected count rate of 256 counts per second. When 30 cm of material is placed between the source and detector the count rate falls to 64 c.p.s.

Calculate the half value thickness of the material.

2. The half value thickness of water is 200 mm. A used fuel element has a dose equivalent of 40 Sv h-1.

Calculate the dose equivalent rate when 1 metre of water is placed between the fuel rod and the detector.

15 cm

1.25 Sv h-1

Reducing Exposure

The equivalent dose rate from a radioactive source is reduced by

• shielding (placing an absorber in the path of radiation)

• increasing distance from the source.

The reduction in equivalent dose rate depends on the material and thickness of absorber used.