Med Phys 3A03/3AA1
Practical Health & Medical Physics
Communications
D.R. Chettle, with D.F. Moscu
TA: Helen Moise
Field radiation surveys
Module 2
October 15th, 13:30 – 14:20: introduction
October 22nd, 13:30 – 15:20: lab, groups A1 & A2
October 29th, 13:30 – 15:20: lab groups B1 & B2
November 5th, 13:30 – 14:20: report back
Field radiation surveys
• What radiation fields?
• What survey instruments?
Field radiation surveys
• γ-ray radiation field: 226Ra
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t½ 1600 y
226Ra→222Rn
+ (4He), 5.5% to excited state, which de-excites emitting γ-ray
(186 keV, 3.3%)
t½ 3.82 d
222Rn→218Po
+ , 99.9% to ground state
t½ 3.05 m
218Po→214Pb
+ , 99+% to ground state
t½ 26.8 m
214Pb→214Bi
+ -, various levels, many γ-rays (352 keV, 37%)
t½ 19.7 m
214Bi→214Po
+ -, various levels, many γ-rays (609 keV, 46%)
• Balance of γ-rays depends on whether chain decay is in
equilibrium, governed by t½ of daughters
Equilibrium?
• Nbλb =
λb (1 – e-(λb – λa)t)
Naλa (λb – λa)
λa = ln(2)/(1600x365)
λb = ln(2)/3.82
λc to λe > = λb
At 0.3 day, 0.053 x equilibrium
At 3 days, 0.420 x equilibrium
At 30 days, 0.996 x equilibrium
PGNAA of Cd
• Based on thermal neutron capture and detection of prompt
gamma ray emitted
Reaction
113Cd(n,
)114Cd
Natural
abundance
Thermal neutron
capture crosssection
Elemental
cross-section
Prompt
gamma
12.2 %
20,600 b
2500 b
559 keV
– 238Pu-Be neutron source
– 2000 s live time measurement
– 0.8 mSv/hr skin dose rate
Neutron Source for PGNAA
•
238Pu-Be
–
–
–
–
source (17 Ci, 4 MeV average neutron energy)
Beryllium premoderator softens the high energy neutron
spectrum
Neutrons collimated by steel and graphite cylinders and by
layers of polyethylene and steel
Blocks of lead, hevimet and bismuth shield the detector
Assembly encased by borated resin plates
PGNAA Detection System
Top view of PGNAA
experimental apparatus
(not to scale).
• System optimized to lower the MDL of Cd in the liver
and kidneys
• Phantoms filled with known concentrations of Cd
Field radiation surveys
• Neutron radiation field – 1
• 238Pu/Be
• 238Pu → 234U + + Q (= 5.593 MeV)
• 4He + 9Be → 12C + 1n + Q (= 5.702 MeV)
• 4He + 9Be → 12C + 1n + Eex+ Q (= 1.263 MeV)
Eγ = 4.439 MeV
Neutron energies up to ~10.3 MeV
Neutron source
At McMaster we have:
• Reactor – not suitable, En > 2 MeV
• 238Pu/Be source:
– uses reaction 4He + 9Be
– Not suitable, En > 2 MeV
12C
+n
• Accelerator:
– Uses reaction 7Li + 1H 7Be + n
– Threshold at 1.88 MeV Ep
– At Ep = 2.3 MeV, En = 0.55 MeV (max) << 2 MeV
Neutron Activation Source
• There are three main components to the
in vivo neutron activation analysis
system
• The Tandetron accelerator provides the
source of neutrons via the 7Li(p, n)7Be
reaction
Irradiation/Shielding Cavity
• An irradiation/shielding cavity has been
designed to maximise activation of 28Al
while minimising radiation dose to the
subject
Experimental Setup
Field radiation surveys
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Neutron radiation field – 2
Tandetron accelerator
Reaction 7Li(p,n)7Be
(7Li + 1H → 1n + 7Be)
Q = -1.643 MeV, Ep(th) = 1.878 MeV
Accelerate protons to 2.3 MeV, En = 0.575 MeV
Also get 478 keV γ-ray from Li target from
7Li(p,p´)7Li reaction
Field radiation surveys
• Survey instrument – 1
• Ion chamber, Victoreen 451B
• Measures:
-particles E > 7.5 MeV;
-particles E > 100 keV
Photons E > 7 keV
• Air ionisation chamber, 349 cm3
• Sliding shield to eliminate (most) -particles
Field radiation surveys
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Survey instrument – 2
Neutron detector, Ludlum 42-31H
Proportional counter filled with 3He
Reaction n + 3He → 3H + 1H + Q (Q = 0.764 MeV)
Thermal neutron reaction, less probable at higher
energies
• Slow down some of the neutrons before they
reach the 3He counter
• 3He detector surrounded by a Cd loaded
polyethylene sphere, 22.9 cm diameter
Field radiation surveys
• Maximum permitted dose rates
Nuclear energy workers 25 Sv h-1
General public 2.5 Sv h-1
(2.5 mrem h-1/0.25 mrem h-1)
Field radiation surveys
• Warning notices:
If dose rate exceeds 2.5 Sv-1 , unless
• Controlled area then
If dose rate exceeds 25 Sv-1