Neutron protection Flashcards
How is neutron therapy done?
Cyclotron produced protons (26-66MeV) batter a Be target
or deuterium (12.5-14MeV) batter a tritium target
Beams are a mix of x-rays and neutrons
How does photodisintergration occur?
Photon has enough energy to pass through electron cloud - interacts with target nucleus
Depending on energy either causes a photoneutron to be ejected or recoil charged particle
What is the energy threshold for photoneutrons?
10MeV
What is the equation for mass-energy conservation for photodisintegration?
hv + m0c^2 = m1c^2 + k1 + mnc^2 + kn
where hv is the energy of the incoming photon
m0c^2 is the initial mass related energy of the nucleus
m1c^2 is the final mass related energy of the nucleus
mnc^2 is the final mass related energy of the neutron
k1 and kn are final kinetic energies of the nucleus and neutron
Where do neutrons contribute to dose?
Contribute to patient dose
Contribute to hazard in the maze
Produce activated products in the treatment head
What are the four types of neutrons? What are their energies?
Thermal neturons (<0.4eV) Intermediate neutrons (0.4eV - 200keV) Fast neutrons (200keV - 10MeV) Relativistic neutrons (>10MeV)
What are the 5 interaction processes neutrons undergo?
Elastic scattering Inelastic scattering Capture Non-elastic reactions Fission
What is the process of elastic scattering?
Neutron shares initial KE with target nucleus which recoils in an excited state, total KE is constant - momentum conserved
What is the process of inelastic scattering?
Fast neutrons scatter the nucleus, energy is lost, nucleus can either decay with a gamma photon or stay in a metastable state
What is the process of neutron capture?
Possible in nearly all nuclides for thermal neutrons
Target nucleus absorbs neutron and is left in an excited state - energy emitted as gamma rays
Cross section proportional to 1/v
Gold shows resonance capture
What is the process of non-elastic reactions? Why do they make calculating a neutron spectrum difficult?
Incident neutron captured by target nucleus with particles emitted
Can occur in reactions >10MeV
The reactions are not uniform with incident energy and have resonances
What is the process of fission?
Nuclei split into two fission fragments and neutrons after interaction with neutron
Can occur at all energies but cross section is higher when thermal neutrons >1MeV are incident
Why is fission useful for neutron detection?
Fission fragments are easy to detect in a detector
Why are neutrons so damaging for the body?
High LET (50x higher than x-rays)
RBE ~2x that of x-rays
Reduced hypoxia so potentially better tumour response
What personal dosemiters are used for neutrons?
Thermoluminescent albedo dosimeters
Electrochemically etched plastic
Bubble dosimeters
How do thermoluminescent albedo dosimeters work?
Neutrons entering human body are moderated and backscattered
This creates neutron fluence at body surface - especially for thermal and intermediate energy ranges
Detected by LiF TLD chip
What are the advantages and disadvantages of thermoluminescent albedo dosimeters?
Adv: no energy threshold, dose range of 0.1mSv-10Sv, low fading, small influence of body size, low dependence on direction as long as 2 worn on front and back, acceptable gamma dose discrimination
Disadv: low response in fast neutron range, high contribution of incident thermal neutrons on dose
How do electrochemically etched plastics
work?
Ploymeric nuclear track detectors have a path of damaged molecules in the material
Tracks detected by etching process: chemical or electrochemical
Needs a calibration of the track density/neutron dose equivalent
What are the advantages and disadvantages of electrochemically etched plastics?
Adv: fast neutron effective response, low neutron energy threshold, photon insensitivity
Disadv: batch variations due to lack of dosimetry grade material, significant angular dependence, sensitivity, under-responds for certain energies - very low and very high
How do bubble detectors work?
Small container with elastic clear polymer with super heated freon droplets in
Recoil protons are made from interactions with neutrons
Protons vaporise the droplets which stays trapped as a visible bubble
Can recharge by pressuring polymer container above vapour pressure of freon gas mixture to remake droplets
What are the advantages and disadvantages of bubble detectors?
Adv: very sensitive, detection limit of few uSv, neutron energy threshold of 100keV, no electronics, no EM interference, insensitive to photons
Disadv: strong temperature dependence, severe shock sensitivity, sharp impact causes bubble formation, bubbles disipate over the course of days
What is the equation for the attenuation coefficient of a neutron beam?
attenuation coefficient = atoms per unit vol x cross section
Why do small atoms have a large cross section?
Interaction occurs at the nucleus not the surrounding electron cloud
What is the neutron interaction with boron?
Produces a Li-7 ion and an alpha particle
How do Boron tri-fluoride detectors work?
Polythene surround BF3 to slow neutrons to thermal energies
Boron or cadmium rod inserted to improve energy response
HT set to 2500V
Charged particles are collected and counted
He-3 fill in modern versions
How is charge converted to dose?
Same as x-rays with conversion to dose at 10mm depth of ICRU sphere
Conversion accounts for backscatter, geometry, and RBE of 5-20
What are the typical specifications for a BF3 detector?
Energy - +/-10% of ICRP Range 1-100000uSv/hr Linearity within a few % Gamma rejection of ~10^-6 Lacks spectral data for accurate dosimetry
What are bonner spheres?
Series of moderating polythene spheres with interchangable detectors
Each sphere modifies the neutron spectrum leaving thermal neutron signal at the centre
What are the detectors used at the centre of bonner spheres?
Thermoluminescent dosimeters - Li6 and Li7 together give reasonable gamma rejection as have different thermal neutron sensitivities
Li6 iodide scintillation detector - connected to PM tube via light pipe - can discriminate gammas but can’t be used with linacs and RF equipment
Gold foils - activation counted - neutron specific
What is the equation for the detector signal in a bonner sphere?
Rj = sum(sigmai,j.phii)
where Rj = detector signal
sigmai,j = response of sphere in ith band
phii = neutron flux in ith band
What are the issues with neutrons in practice?
Neutron dose at patient plane is 10^-6Sv/MU @ 15MV, 2.6x10^-5Sv/MU @ 20MV
May be significant contributor to concomitant dose
Depends on geometry, collimator setting
May be enhanced for certain treatments - TBI
Risk assessment required
Why does metal shielding increase neutron dose?
High back scatter
Is there an issue for neutrons outside the bunker?
No with 15MV concrete shielding
May need to line walls with litium salts that are high in hydrogen in maze with low z material
What issues do activated products pose?
Dose rates can be in 10’s uSv/hr - decay within 30s
May be significant doses though if dismantling the head
How do neutrons cause electronic failures of equipment?
Electrons have intensely ionising recoil ions
