Neutron protection

Question 1

How is neutron therapy done?

Answer 1

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

Question 2

How does photodisintergration occur?

Answer 2

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

Question 3

What is the energy threshold for photoneutrons?

Answer 3

10MeV

Question 4

What is the equation for mass-energy conservation for photodisintegration?

Answer 4

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

Question 5

Where do neutrons contribute to dose?

Answer 5

Contribute to patient dose
Contribute to hazard in the maze
Produce activated products in the treatment head

Question 6

What are the four types of neutrons? What are their energies?

Answer 6

Thermal neturons (<0.4eV)
Intermediate neutrons (0.4eV - 200keV)
Fast neutrons (200keV - 10MeV)
Relativistic neutrons (>10MeV)

Question 7

What are the 5 interaction processes neutrons undergo?

Answer 7

Elastic scattering
Inelastic scattering
Capture
Non-elastic reactions
Fission

Question 8

What is the process of elastic scattering?

Answer 8

Neutron shares initial KE with target nucleus which recoils in an excited state, total KE is constant - momentum conserved

Question 9

What is the process of inelastic scattering?

Answer 9

Fast neutrons scatter the nucleus, energy is lost, nucleus can either decay with a gamma photon or stay in a metastable state

Question 10

What is the process of neutron capture?

Answer 10

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

Question 11

What is the process of non-elastic reactions? Why do they make calculating a neutron spectrum difficult?

Answer 11

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

Question 12

What is the process of fission?

Answer 12

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

Question 13

Why is fission useful for neutron detection?

Answer 13

Fission fragments are easy to detect in a detector

Question 14

Why are neutrons so damaging for the body?

Answer 14

High LET (50x higher than x-rays)
RBE ~2x that of x-rays
Reduced hypoxia so potentially better tumour response

Question 15

What personal dosemiters are used for neutrons?

Answer 15

Thermoluminescent albedo dosimeters
Electrochemically etched plastic
Bubble dosimeters

Question 16

How do thermoluminescent albedo dosimeters work?

Answer 16

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

Question 17

What are the advantages and disadvantages of thermoluminescent albedo dosimeters?

Answer 17

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

Question 18

How do electrochemically etched plastics

work?

Answer 18

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

Question 19

What are the advantages and disadvantages of electrochemically etched plastics?

Answer 19

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

Question 20

How do bubble detectors work?

Answer 20

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

Question 21

What are the advantages and disadvantages of bubble detectors?

Answer 21

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

Question 22

What is the equation for the attenuation coefficient of a neutron beam?

Answer 22

attenuation coefficient = atoms per unit vol x cross section

Question 23

Why do small atoms have a large cross section?

Answer 23

Interaction occurs at the nucleus not the surrounding electron cloud

Question 24

What is the neutron interaction with boron?

Answer 24

Produces a Li-7 ion and an alpha particle

Question 25

How do Boron tri-fluoride detectors work?

Answer 25

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

Question 26

How is charge converted to dose?

Answer 26

Same as x-rays with conversion to dose at 10mm depth of ICRU sphere
Conversion accounts for backscatter, geometry, and RBE of 5-20

Question 27

What are the typical specifications for a BF3 detector?

Answer 27

Energy - +/-10% of ICRP
Range 1-100000uSv/hr
Linearity within a few %
Gamma rejection of ~10^-6
Lacks spectral data for accurate dosimetry

Question 28

What are bonner spheres?

Answer 28

Series of moderating polythene spheres with interchangable detectors
Each sphere modifies the neutron spectrum leaving thermal neutron signal at the centre

Question 29

What are the detectors used at the centre of bonner spheres?

Answer 29

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

Question 30

What is the equation for the detector signal in a bonner sphere?

Answer 30

Rj = sum(sigmai,j.phii)
where Rj = detector signal
sigmai,j = response of sphere in ith band
phii = neutron flux in ith band

Question 31

What are the issues with neutrons in practice?

Answer 31

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

Question 32

Why does metal shielding increase neutron dose?

Answer 32

High back scatter

Question 33

Is there an issue for neutrons outside the bunker?

Answer 33

No with 15MV concrete shielding

May need to line walls with litium salts that are high in hydrogen in maze with low z material

Question 34

What issues do activated products pose?

Answer 34

Dose rates can be in 10’s uSv/hr - decay within 30s

May be significant doses though if dismantling the head

Question 35

How do neutrons cause electronic failures of equipment?

Answer 35

Electrons have intensely ionising recoil ions