Neutron protection Flashcards
How can neutrons be liberated from Be-9? Why is Be-9 a good neutron liberation candidate?
- Alpha or proton irradiation.
- Be-9 has a low binding energy per nucleon.
How are neutron therapy beams typically created? What else needs to be considered aside from the neutron beam created?
- Cyclotron-produced protons are directed towards a Be target.
- OR Deuterium is directed onto a Tritium target.
- Gamma rays from activation products need also to be considered.
What is photodisintegration? What does it depend on?
- A high energy addition to other photon interactions (Rayleigh scattering, photoelectric effect, Compton scattering and pair production). A high energy photon of sufficient energy (~10 MV) passes through the electron cloud, interacting with the target nucleus and causing a photoneutron (or possibly a proton or alpha particle at higher energies) to be ejected. The recoiling nucleus, therefore, has a changed mass number and/or atomic number.
- A higher energy incident photon results in a higher energy photoneutron. The energy of the photoneutron also depends on the target material.
What hazards do neutrons present in a linac room?
Neutrons and activation products can contribute directly and indirectly towards patient dose. They can present hazards in the vicinity of the treatment head and the linac entrance maze.
What are the different kinds of neutrons at different energies? What are their energy ranges? How do each most commonly interact with mater?
Energy from low to high:
- Thermal (<0.4 eV) - neutron capture.
- Intermediate (0.4 eV - 200 keV) - Elastic scattering.
- Fast (200 keV - 10 MeV) - Elastic scattering with inelastic scattering at increased energies.
- Relativistic (> 10 MeV) - Inelastic scattering resulting in ejection of protons/neutrons.
How to neutrons interact with matter?
- Elastic scattering.
- Inelastic scattering.
- Capture.
- Non-elastic reactions.
- Fission.
How to neutron elastic and inelastic scattering differ?
- Elastic scattering: Target nucleus recoils after interaction with incident neutron but is not left in an excited state and the total energy of the system remains constant.
- Inelastic scattering: Total energy of scattered neutron and recoil nucleus is less than the incident neutron and the nucleus is left in an excited state. The nucleus may decay via gamma ray emission or remain metastable.
What is neutron capture?
A target nucleus captures an incident neutron and forms a compound nucleus left in an excited state. Excitation energy may be released in the form of one or more gamma rays. Some elements have a high cross-section for thermal neutron capture and others show resonance capture.
What are neutron non-elastic reactions?
A target nucleus captures an incident neutron causing the emission of a variety of particles (e.g. protons, deuterons, alpha particles etc.).
How do neutrons cause fission?
Interactions of neutrons with fissile nuclei cause the formation of compound nuclei which can split into two fission fragments and one or more neutrons.
Why do neutrons have an increased biological effect when compared to photons?
Neutrons have a high linear energy transfer meaning more energy is deposited per unit length. This results in more double strand breaks and less repair.
What is the radiation weighting factor for neutrons?
The radiation weighting factor for neutrons is variable depending on the neutron energy.
What options are available for neutron personal dosimetry?
- Thermoluminescent albedo dosimeters.
- Electrochemically etched plastics.
- Bubble dosimeters.
How does a thermoluminescent albedo neutron dosimeter work? What are some advantages and disadvantages?
- Neutrons entering the human body are moderated and backscattered creating neutron fluence at the body surface (typically in the thermal and intermediate ranges). These can be detected by a LiF TLD chip designed to detect thermal neutrons. The dose measured depends on the shape/size of the dosimeter encapsulation and detector distance from the body.
Advantages:
- No energy threshold.
- Large dose range.
- Little influence of body size on on dosimeter reading.
- Acceptable gamma dose discrimination.
- Low dependence on direction of incident radiation.
Disadvantages:
- Low response to fast neutrons.
- High contribution to thermal neutrons.
How do electrochemically etched plastic neutron dosimeters work? What are some advantages and disadvantages?
- Neutrons cause a path of damaged molecules in the material. The tracks can then be detected by some etching process. This requires a calibration of the track density to neutron dose equivalent.
Advantages:
- Effective fast neutron response.
- Low neutron energy threshold.
- Photon insensitivity.
Disadvantages:
- Results can vary by dosimeter batch.
- Significant angular dependence.
- Sensitivity.
- Under responds for certain energy ranges (best at the lower and higher ends).
