Flashcards in Neutron Protection Deck (43)
What are the sources of neutrons?
Radionuclides, accelerators (produced by ion beams or photonuclear reactions), nuclear reactions (prompt fission nuetrons, delayed neutrons, photoneutrons, gamma radiation), and nuclear fuel reprocessing plants.
Describe the properties of Beryllium.
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
What is the binding energy per nucleon of Beryllium, and why does this mean it is useful for neutron production?
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.
Describe the main principles of neutron therapy.
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
What is photodisintegration?
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).
What is the threshold photon energy to produce a photo-neutron?
What is the threshold photon energy to produce an alpha or proton from photodisintegration?
Much higher than 10 MeV.
What is the mass energy conservation equation?
hv + m(0)c^2 = m(1)c^2 + k(1) + m(n)c^2 + k(n)
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
What are the consequences of neutrons in a linac bunker?
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.
What are the 4 types of neutrons and at what energy ranges do they occur?
Thermal neutrons (<0.4 keV)
Intermediate neutrons (0.4-200 keV)
Fast neutrons (200 keV - 10 MeV)
Relativistic neutrons (>10 MeV)
Describe thermal neutrons.
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
Describe intermediate neutrons.
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
Describe fast neutrons.
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
Describe relativistic neutrons.
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.
What are the 5 interactions of neutrons with matter?
Elastic scattering: (n,n)
Inelastic scattering: (n,n'), (n,nγ)
Non-elastic reactions: (n,n), (n,p), (n,d), (n,α), (n,t), (n,αp), etc.
Describe elastic scattering for neutrons.
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.
Describe inelastic scattering for neutrons.
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
Describe neutron capture.
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.
Describe neutron non-elastic reactions.
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
Describe neutron fission.
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.
How does RBE change with neutron energy?
As energy increases, RBE increases.
Which personal dosimeters can be used for neutron dosimetry?
Thermoluminescent albedo dosimeters
Electrochemically etched plastics (CR-39)
Describe a thermoluminescent albedo dosimeter.
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
Describe electrochemically etched plastics.
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. )
What are the disadvantages of a thermoluminescent albedo dosimeter?
-relatively low response in fast neutron energy range
-high contribution of incident thermal neutrons on dose indication
What are the advantages of a thermoluminescent albedo dosimeter?
-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
What are the disadvantages of electrochemically etched plastics?
-Batch variations due to lack of dosimetry grade material
-Significant angular dependence
-Under-responds for certain neutron energy ranges
lower energy neutrons from reactors or high energy accelerator-produced neutrons
What are the advantages of electrochemically etched plastics?
-Fast neutron effective response
-Low neutron energy threshold
Describe a neutron bubble detector.
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.