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 15 th, 13:30 – 14:20: introduction October 22 nd, 13:30 – 15:20: lab, groups A1 & A2 October 29 th, 13:30 – 15:20: lab groups B1 & B2 November 5 th, 13:30 – 14:20: report back

Field radiation surveys What radiation fields? What survey instruments?

Field radiation surveys γ-ray radiation field: 226 Ra t ½ 1600 y 226 Ra→ 222 Rn +  ( 4 He), 5.5% to excited state, which de-excites emitting γ-ray (186 keV, 3.3%) t ½ 3.82 d 222 Rn→ 218 Po + , 99.9% to ground state t ½ 3.05 m 218 Po→ 214 Pb + , 99+% to ground state t ½ 26.8 m 214 Pb→ 214 Bi +  -, various levels, many γ-rays (352 keV, 37%) t ½ 19.7 m 214 Bi→ 214 Po +  -, various levels, many γ-rays (609 keV, 46%) Balance of γ-rays depends on whether chain decay is in equilibrium, governed by t ½ of daughters

Equilibrium? N b λ b = λ b (1 – e -(λb – λa)t ) N a λ a (λ b – λ a ) λ a = ln(2)/(1600x365) λ b = ln(2)/3.82 λ c to λe > = λ b At 0.3 day, x equilibrium At 3 days, x equilibrium At 30 days, x equilibrium

PGNAA of Cd Based on thermal neutron capture and detection of prompt gamma ray emitted – 238 Pu-Be neutron source – 2000 s live time measurement – 0.8 mSv/hr skin dose rate ReactionNatural abundance Thermal neutron capture cross- section Elemental cross-section Prompt gamma 113 Cd(n,  ) 114 Cd 12.2 %20,600 b2500 b559 keV

Neutron Source for PGNAA 238 Pu-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 System optimized to lower the MDL of Cd in the liver and kidneys Phantoms filled with known concentrations of Cd Top view of PGNAA experimental apparatus (not to scale).

Field radiation surveys Neutron radiation field – Pu/Be 238 Pu → 234 U +  + Q (= MeV) 4 He + 9 Be → 12 C + 1 n + Q (= MeV) 4 He + 9 Be → 12 C + 1 n + E ex + Q (= MeV) E γ = MeV Neutron energies up to ~10.3 MeV

Neutron source At McMaster we have: Reactor – not suitable, E n > 2 MeV 238 Pu/Be source: – uses reaction 4 He + 9 Be  12 C + n – Not suitable, E n > 2 MeV Accelerator: – Uses reaction 7 Li + 1 H  7 Be + n – Threshold at 1.88 MeV E p – At E p = 2.3 MeV, E n = 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 7 Li(p, n) 7 Be reaction

Irradiation/Shielding Cavity An irradiation/shielding cavity has been designed to maximise activation of 28 Al while minimising radiation dose to the subject

Experimental Setup

Field radiation surveys Neutron radiation field – 2 Tandetron accelerator Reaction 7 Li(p,n) 7 Be ( 7 Li + 1 H → 1 n + 7 Be) Q = MeV, E p(th) = MeV Accelerate protons to 2.3 MeV, E n = 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 cm 3 Sliding shield to eliminate (most)  -particles

Field radiation surveys Survey instrument – 2 Neutron detector, Ludlum 42-31H Proportional counter filled with 3He Reaction n + 3 He → 3 H + 1 H + Q (Q = MeV) Thermal neutron reaction, less probable at higher energies Slow down some of the neutrons before they reach the 3 He counter 3 He 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