Transcript Med Phys 3A03/3AA1 - McMaster Faculty of Science

Med Phys 3A03/3AA1
Practical Health & Medical Physics
Communications
D.R. Chettle, with D.F. Moscu
TA: Helen Moise
Contamination
Module 3
November 12th, 13:30 – 14:20: introduction
November 19th, 13:30 – 15:20: lab, groups A1 & A2
November 26th, 13:30 – 15:20: lab groups B1 & B2
December 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
– (ii) class B radionuclides
– (iii) class C radionuclides
Averaged over area
3 Bq cm-2
30 Bq cm-2
300 Bq cm-2
100 cm2
Contamination criteria – b)
• Non-fixed contamination in all other areas
– (i) class A radionuclides
– (ii) class B radionuclides
– (iii) class C radionuclides
Averaged over area
0.3 Bq cm-2
3 Bq cm-2
30 Bq cm-2
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
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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