Med Phys 3A03/3AA1

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 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 • 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 • 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 – 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  12C + n • Not suitable, En > 2 MeV • Accelerator: • Uses reaction 7Li + 1H  7Be + n • Threshold at 1.88 MeVEp • 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 • 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 keVγ-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 • 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

Med Phys 3A03/3AA1