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Y3: Radiation Safety in Radiotherapy > Neutron Protection > Flashcards

Neutron Protection Flashcards

1
Q

What are the sources of neutrons?

A

Radionuclides, accelerators (produced by ion beams or photonuclear reactions), nuclear reactions (prompt fission nuetrons, delayed neutrons, photoneutrons, gamma radiation), and nuclear fuel reprocessing plants.

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2
Q

Describe the properties of Beryllium.

A

Large nucleus; small atom
Close packed crystal structure
Can be formed into high purity metal components
Almost transparent to x-rays
High cross section for high energy alphas or protons
Neutrons discovered by irradiating Be with alphas
e.g. Be-9 + He-4 = C-12 + n-1
Be-9 binding energy per neucleon: 6.46 MeV

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3
Q

What is the binding energy per nucleon of Beryllium, and why does this mean it is useful for neutron production?

A

6.46 MeV
Beryllium is used as has a low binding energy, so is easy to release neutrons from nucleus as less energy required to overcome binding energy. So high energy alphas or protons produce high energy neutrons.

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4
Q

Describe the main principles of neutron therapy.

A

Cyclotron-produced protons (26-66 MeV) onto Be target (more common).
Deuterium (12.5-14 MeV) onto Tritium target (less common).
Therapeutic beams are mixture of x-rays and neutrons

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5
Q

What is photodisintegration?

A

A photon passes through an electron cloud and interacts with a nucleus. This produces a neutron (photoneutron) or possible a proton or alpha particle, and an ionising recoil nucleus with a changed mass number and/or atomic number.

I.e: Photon hits a nucleus, if energy>B.E = recoil nucleus (with changed mass/atomic number) + photoneutron (or maybe proton/alpha).

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6
Q

What is the threshold photon energy to produce a photo-neutron?

A

10 MeV

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7
Q

What is the threshold photon energy to produce an alpha or proton from photodisintegration?

A

Much higher than 10 MeV.

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8
Q

What is the mass energy conservation equation?

A

hv + m(0)c^2 = m(1)c^2 + k(1) + m(n)c^2 + k(n)
where:
hv is energy of incoming LINAC photon
m0c2 is the initial mass-related energy of the nucleus
m1c2 is the final mass-related energy of the nucleus
mnc2 is the final mass-related energy of the neutron
k1 and kn are final kinetic energies of nucleus and neutron

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9
Q

What are the consequences of neutrons in a linac bunker?

A

Contribute to patient dose
Contribute to a radiation hazard at the maze
Present an additional radiation hazard at the treatment as a result of activated products.

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10
Q

What are the 4 types of neutrons and at what energy ranges do they occur?

A 
Thermal neutrons (<0.4 keV)
Intermediate neutrons (0.4-200 keV)
Fast neutrons (200 keV - 10 MeV)
Relativistic neutrons (>10 MeV)
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11
Q

Describe thermal neutrons.

A

Energy distribution same as atoms and molecules of surrounding medium
Maxwellian distribution of velocities
(Epithermal neutrons: 0.4 eV – 100 eV)
Most common interaction with matter is neutron capture but other reactions may also occur

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12
Q

Describe intermediate neutrons.

A

Produced as result of elastic scattering of fast neutrons in materials with low atomic number e.g. carbon or hydrogen or in the human body
Interact with matter by elastic scattering

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13
Q

Describe fast neutrons.

A

Interact with light nuclei primarily through elastic scattering
Although inelastic scattering predominates with higher energies
fast-neutron monitoring instruments become insensitive and inaccurate below 200 keV

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14
Q

Describe relativistic neutrons.

A

Inelastic scattering more important than elastic
elastic cross-section negligible for high atomic number materials
main form of non-elastic collision ejection of protons or neutrons from target nucleus
At very high energies energy appearing as gamma radiation negligible c.f. energy transferred to cascade protons, neutrons and other nuclei.

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15
Q

What are the 5 interactions of neutrons with matter?

A 
Elastic scattering: (n,n)
Inelastic scattering: (n,n'), (n,nγ)
Capture: (n,γ)
Non-elastic reactions: (n,n), (n,p), (n,d), (n,α), (n,t), (n,αp), etc.
Fission: (n,f)
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16
Q

Describe elastic scattering for neutrons.

A

Neutron shares initial kinetic energy with the target nucleus
Target nucleus suffers only recoil and is not left in an excited state
Total kinetic energy in system remains constant.
Momentum is conserved.

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17
Q

Describe inelastic scattering for neutrons.

A

only possible with fast neutrons
Total energy of scattered neutron and recoil nucleus < incident neutron
nucleus left in excited state
In (n,nγ) process, excitation energy released by nucleus via emission of prompt gamma ray
In (n,n’) process, nucleus remains in a metastable state

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18
Q

Describe neutron capture.

A

Thermal neutron capture possible in nearly all nuclides
Target nucleus captures incident neutron
Forms compound nucleus left in excited state
Excitation energy may be emitted in one or more gamma rays
Some elements have a high cross-section (proportional to 1/v) for thermal neutron capture
Others (e.g. gold) show resonance capture

Some detectors make use of this – measures the gamma emissions from the material that is activated by the neutrons & infers what neutron flux was to create those emissions.

19
Q

Describe neutron non-elastic reactions.

A

Incident neutron captured by the target nucleus
particles (protons, deuterons, alpha particles, tritons, etc.) emitted
(n, 2n) reaction can occur at incident energies above 10 MeV
These reactions are not usually uniform with incident energy but show resonances which make calculations of the interactions of a complex neutron spectrum difficult

20
Q

Describe neutron fission.

A

Interactions with fissile nuclei cause formation of a compound nucleus
compound nucleus then splits into two fission fragments and one or more neutrons.
May occur in isotopes of Th, U, Np, Pu and higher actinides
Can use these materials as neutron detectors because relatively simple to detect the fission products
Occurs at practically all neutron energies but cross-section for fission considerably higher for 233U, 235U and 239Pu when thermal neutrons incident
In 232Th and 238U practically no fissions take place at neutron energies < 1MeV.

21
Q

How does RBE change with neutron energy?

A

As energy increases, RBE increases.

22
Q

Which personal dosimeters can be used for neutron dosimetry?

A

Thermoluminescent albedo dosimeters
Electrochemically etched plastics (CR-39)
Bubble dosimeters

23
Q

Describe a thermoluminescent albedo dosimeter.

A

Albedo is to do with reflection rather than incident measurement of neutrons. Neutrons entering human body are moderated & back-scattered which creates neutron fluence at body surface. (especially in thermal- and intermediate-energy ranges)

LiF TLD chip designed to detect thermal neutrons

24
Q

Describe electrochemically etched plastics.

A

Nuclear track detector - widely used
Neutrons detected by path of damaged molecules in material where the tracks are detected by suitable etching process: chemical etching (CE), electrochemical etching (ECE), or both combined.
Calibration of track density / neutron dose equivalent required.
(Etches away anything that hasn’t been damaged = only left with tracks from neutrons = can count number of tracks. )

25
Q

What are the disadvantages of a thermoluminescent albedo dosimeter?

A

Disadvantages:

  • relatively low response in fast neutron energy range
  • high contribution of incident thermal neutrons on dose indication
26
Q

What are the advantages of a thermoluminescent albedo dosimeter?

A

Advantages:

  • no energy threshold for detection
  • extended dose range from 0.1 mSv to about 10 Sv
  • low fading for longer monitoring periods
  • small influence of body size on dosimeter reading
  • low dependence on direction of incident radiation if at least two dosimeters have been worn on the front and on the rear of the body
  • acceptable gamma dose discrimination
27
Q

What are the disadvantages of electrochemically etched plastics?

A

Disadvantages:
-Batch variations due to lack of dosimetry grade material
-Significant angular dependence
-Sensitivity
-Under-responds for certain neutron energy ranges
lower energy neutrons from reactors or high energy accelerator-produced neutrons

28
Q

What are the advantages of electrochemically etched plastics?

A

Advantages:

  • Fast neutron effective response
  • Low neutron energy threshold
  • Photon insensitivity
29
Q

Describe a neutron bubble detector.

A

A small container (measuring several cm.) filled with superheated Freon droplets within an elastic clear polymer.
Recoil protons are produced by neutron interactions with the polymer.
Protons striking Freon droplet may vaporize it, which remains trapped as a visible bubble in the polymer.
Count number of bubbles to determine neutron fluence.
Recharging is accomplished by pressuring the polymer container above vapor pressure of Freon gas mixture
reforms bubbles to liquid droplets = all becomes clear again.

30
Q

What are the advantages of a bubble detector?

A

Advantages:

  • very sensitive
  • detection limit of few microSv
  • neutron energy threshold of standard option 100 keV
  • need no electronics or power to operate
  • cannot be disturbed by electromagnetic interference
  • insensitive to photons
31
Q

What are the disadvantages of a bubble detector?

A

Disadvantages:
-strong temperature dependence
-temperature correction can be used to minimise effect
severe physical shock sensitivity – impact number of visible bubbles
-sharp impact causes superheated drops to vaporise & form bubbles like neutron- induced ones
-cannot be used to accumulate dose information for longer than few days: over time, bubbles spontaneously reform into liquid.

32
Q

What is the neutron attenuation coefficient equal to?

A

Attenuation coefficient =

Atoms per unit volume x cross section

33
Q

Does hydrogen have a large or small neutron cross section?

A

As neutrons only interested in nucleus – smaller cross section = more interaction.
Hydrogen – large cross section as nucleus in comparison to atom is much bigger.

34
Q

Describe the principles of a boron tri-fluoride detector.

A

A neutron interacts with boron which produces Li-7 plus an alpha particle. Both are highly charged and can be collected and counted.
Elemental boron is not a gas but BF3 and doubles as a quenching media in the detector.
The detector voltage is set in the proportional range to enable lower gamma peaks to be discounted.

Area monitors have a single axial BF3 detector surrounded by large polythene to moderate most energetic neutrons to thermal energies (this makes up the bulk of the detector). A perforated boron or cadmium insert is used to improve energy response. Approx 2500V is within the proportional range.
Modern versions have He-3 fill instead to comply with air carriage regulations.

35
Q

What are the specifications of a boron tri-fluoride detector?

A

Energy – around +/-10% of ICRP
Range 1-100,000 µSv per hour
Linearity – within a few percent
Gamma (X-ray) rejection about 10-6
Not good enough for main beam LINAC use
Lacks spectral data for accurate dosimetry
Only gives single figure for neutron energy

36
Q

Describe Bonner spheres.

A

Series of polythene moderating spheres - each one a different size.
Central interchangeable detector (passive or active) - either TLD, Li-6 scintillation detector, or Gold foils.
Each sphere uniquely modifies the neutron spectrum - so can quantify the neutron spectrum.
Residual thermal neutron signal at centre.

The data is ‘unfolded’: the detector signal = sum of nuetron flux in specific band x response of sphere in same band.

37
Q

Describe the possible detectors used at the centre of a Bonner sphere.

A

Thermo-luminescent dosimeters:
Li6F and Li7F have different thermal neutron sensitivities but similar gamma/x-ray sensitivities so together reasonable gamma rejection; limited accuracy

Lithium6 iodide scintillation detectors:
Connected to PM tube via light pipe; can energy discriminate gammas from thermal neutrons but cannot be used with LINACS or other RF equipment

Gold foils Au197(n,γ) Au198:
Activation counted (T½ = 2.7 days); neutron specific so excellent gamma rejection and reproducible
38
Q

What is the typical neutron dose at the patient plane?

A

Range 10-6 Sv per Monitor Unit at 15MV; 2.6x10-5 at 20MV

39
Q

What could increase the neutron dose to the patient?

A

High neutron back-scatter if metal shielding used in primary barrier (higher for lead than steel than concrete).

40
Q

Up to which energy is normal concrete and steel barriers appropriate for neutron shielding?

A

15 MV

41
Q

What is a good wall lining agent against neutrons, and why?

A

Lithium salts, as it has a high hydrogen content.

42
Q

Why is it necessary to leave a gap after exposures (approx 30 minutes) before working on a Linac head?

A

Neutron activation may be present. Especially at or above 10MV.
Dose rates immediately after beam off can be 10’s of µSv/hr.

43
Q

Other than significant doses, what can neutron dose also cause?

A

Electronic failure of equipment. Mechanism may be intensely ionising recoil ions.

Y3: Radiation Safety in Radiotherapy (6 decks)

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