Med Phys 3A03/3AA1

Med Phys 3A03/3AA1

Practical Health & Medical Physics Communications

D.R. Chettle, with D.F. MoscuTA: Helen Moise

ContaminationModule 3

November 12th, 13:30 – 14:20: introductionNovember 19th, 13:30 – 15:20: lab, groups A1 & A2November 26th, 13:30 – 15:20: lab groups B1 & B2December 3rd, 13:30 – 14:20: report back

Contamination

• Contamination of surfaces, tools, equipment

• Check integrity of sealed sourcs

Contamination

• Can it easily be removed and so transferred to another surface, area or person?– So, is it “non-fixed” wipeable?

• McMaster licence has “Contamination criteria”– two levels, controlled areas, all other areas

Contamination criteria – a)

• Non-fixed contamination in all areas where unsealed nuclear substances are used or stored

– (i) class A radionuclides 3 Bq cm-2

– (ii) class B radionuclides 30 Bq cm-2

– (iii) class C radionuclides 300 Bq cm-2

Averaged over area 100 cm2

Contamination criteria – b)

• Non-fixed contamination in all other areas

– (i) class A radionuclides 0.3 Bq cm-2

– (ii) class B radionuclides 3 Bq cm-2

– (iii) class C radionuclides 30 Bq cm-2

Averaged over area 100 cm2

What are class A, B, C radionuclides?

• Class A, eg: emitters, 18F, 24Na, 60Co

• Class B, eg: 131I, 90Sr, 198Au, 59Fe

• Class C, eg: 14C, 109Cd, 99mTc, 32P

• Classification appears to depend on emissions, half life, physical form and chemical and physical behaviour

Contamination criteria

• 0.3 Bq cm-2!?

• Compare 40K: 31.7 kBq kg-1 (natural K)• Natural uranium: 238U 12.3 MBq kg-1 (+ all

daughters[7, 6] if in secular equilibrium); 235U 0.568 MBq kg-1 (+ daughters)

• Natural thorium: 4.07 MBq kg-1 (+ daughters)

• Maybe think of it as 3 kBq m-2 (to 3 MBq m-2)

Measuring contamination

Measuring contamination

• Using “pancake” detector, as in module #1

• Scan over suspect area

• Careful not to allow detector to touch surface (or object)

• Contaminated detectors do not help!

Measuring contamination

Measuring contamination

• Remember criterion “averaged over area 100 cm2”

• Personal protective equipment?

• Properties of “swipe”?

Checking sealed sources

• “a radioactive nuclear substance in a sealed capsule or in a cover to which the substance is bonded, where the capsule or cover is strong enough to prevent contact with or the dispersion of the substance under the conditions for which the capsule or cover is designed”.

125I brachytherapy seed

238Pu-Be

• Double stainless steel encapsulation, meets drop test, fire test, water test

• What is decay mode?• Another name?• What happens to that substance?• How many years might this source be around?• (Also, what about 241Am-Be?)

Checking sealed sources

• Precautions as for swipe test of surface• Checking source directly will give dose• Sources are kept in appropriate containers• If source has retained its integrity, container

will not have any contamination

Checking sealed sources

• Swipe container and check as for contaminated surfaces

• If any suspect contamination, check whether it is same isotope as sealed source, may well need different type(s) of detector

Self-assessment/peer-assessment

• Module 1: received 12, missing 3

• Module 2: received 6, awaiting 9