Interaction of Radiation with Matter - 6 Neutron Shielding Day 2 – Lecture 6

Objective To learn about the principles of neutron shielding (as well as associated gamma radiation) To discuss health physics significance of neutron sources and shielding

Content Neutron energy categories Neutron shielding principles Shielding materials Gamma emission from neutron shields Common neutron sources

Neutron Energy Categories Neutron Name/Title Energy (eV) Cold Neutrons 0 < 0.025 Thermal Neutrons 0.025 Epithermal Neutrons 0.025 < 0.4 Cadmium Neutrons 0.4 < 0.6 Epicadmium Neutrons 0.6 < 1 Slow Neutrons 1 < 10 Resonance Neutrons 10 < 300 Intermediate Neutrons 300 < 1,000,000 Fast Neutrons 1,000,000 < 20,000,000 Relativistic Neutrons >20,000,000 Thermal neutrons are neutrons which have the same energy and move at the same velocity as a gas molecule does at a temperature of 20° C. The velocity of a thermal neutron is 2200 m/sec. As indicated previously, fast neutrons are those neutrons with energies of 1 MeV or greater. For purposes of this discussion, “very fast neutrons” will be defined as neutrons with energies greater than 10 MeV.

Neutron Shielding Complicated and not straightforward Choice of shielding is strongly dependent on neutron energy Neutrons are shielded with combinations of high-Z, low-Z and absorber materials In all cases, shielding materials must account for induced gamma rays Neutron shielding is more challenging than shielding for other kinds of radiation such as gamma and beta. Remember that “all neutrons are born fast.” That is, they have a lot of energy when they are emitted. Also, there is no such thing as a pure neutron radiation field. Gamma rays and even x-rays will also be present and will present exposure potential, as well as shielding issues. Having no electric charge the neutron cannot interact with material directly or via the coulomb force, instead relying mainly on its strong force with nuclei. There are fewer reactions due to the short range of the force, and the probability of a reaction is reduced further by the fact that a neutron must pass within 10-13 cm of the nucleus before an interaction can take place. The distance that a neutron can travel through matter without an interaction is known as its Mean Free Path. There are many types of neutron interactions depending upon the energy: highly energetic neutrons can produce many varied secondary particles, and these particles will change in character as the energy reduces.

Neutron Shielding For very fast neutrons with energies > 10 MeV use inelastic scattering (high-Z materials) For fast neutrons with energies > 1 MeV, use elastic scattering (moderation with H) For slow neutrons use absorbers Elastic scattering is the most likely interaction between fast neutrons and low-atomic numbered absorbers. This interaction is a “billiard ball” type collision, in which kinetic energy and momentum are conserved. Up to neutron energies of the order of 10 MeV, the most important interaction of fast neutrons with matter is elastic scattering. In inelastic scattering, kinetic energy and momentum are not conserved. Rather, some of the kinetic energy is transferred to the target nucleus which excites the nucleus. The excitation energy is then emitted as a gamma-ray photon. Inelastic scattering occurs primarily with high-Z absorbers. For fast neutrons of energies of about 1 MeV, inelastic scattering can become appreciable.

Shielding Very Fast Neutrons Use shielding materials in this order High Z (for example Pb or iron) or concrete - inelastic scattering for initial slowing down Followed by low Z (H) (for example polymers, water, concrete) – elastic scattering For neutrons with energies greater than 10 MeV, use inelastic scattering for very energetic neutrons first and then slow them down further by elastic scattering with hydrogenous materials.

Shielding Fast Neutrons Use low-Z materials (with H) (for example polymers, water, concrete) Takes advantage of elastic scattering with hydrogen Elastic scattering interaction predominates for neutrons with energies less than 10 MeV.

Fast Neutron Scattering From Heavy Nuclei Fast neutrons are not efficiently slowed down by scattering off heavy nuclei such as iron. Little energy is lost by the neutron in this type of interaction.

Fast Neutron Scattering From Hydrogen Nuclei Fast neutrons give up a lot of energy when they collide with hydrogen nuclei (i.e. protons) and thus are slowed down very efficiently.

Shielding Slow Neutrons Preferentially use absorbers to capture slow neutrons without gamma emission Use absorbers such as B or Li utilize (n,) reaction without emission of capture gamma rays Second choice is hydrogenous materials using 1H (n,) 2H reaction (emits a very energetic 2.23 MeV gamma ray which must be shielded against) Note that a gamma ray is emitted from an excited state of Li-7 following the (n, ) reaction with B-10. However, its energy (0.478 MeV) is less than that of the gamma ray emitted from the capture reaction with H (I.e. 2.23 MeV). The 10B (n, ) 7Li reaction does not of itself emit a gamma ray. Neither does the 6Li (n, ) 3H reaction. Thermal Neutrons can be virtually eliminated by the presence of high thermal neutron cross section materials such as Boron, Lithium, or Cadmium.

Shielding Neutrons of Mixed Energies Slow neutrons down, then absorb Use low Z materials – elastic scattering to moderate neutron energies Use absorber materials to remove moderated neutrons Use high Z materials to shield against the induced x and gamma rays The principle here is to slow down the neutrons, capture them, and shield against the associated gamma rays which are emitted.

Gamma Radiation Produced by Neutron Shielding Secondary gamma rays arise mostly from capture of thermal neutrons Neutron inelastic scattering also contributes somewhat Secondary gamma rays are less important for higher neutron energies

Neutron Capture Gamma Rays for Selected Materials Target Thermal Neutron Cross Section (barns) Highest Energy Gamma Ray (MeV) Al 0.235 7.724 B-10 3837 0.478 Cd 2450 9.046 C-12 0.0034 4.95 H 0.332 2.23 Si 0.160 10.599 N-14 0.075 10.833 The 0.478 MeV gamma ray from B-10 neutron capture comes from emission from an excited state of Li-7 following the 10B(n,)7Li reaction. Some relatively high energy gamma rays are emitted by thermal neutron capture, I.e. (n,) reactions. Recall that secondary gamma rays are less important for higher neutron energies.

Material Compositions for Common Neutron Shields Elements Contained Atoms/cm3 (x10-21) Borated polyethylene (8% B4C by weight) H C B-10 B-11 76.8 39.2 0.658 2.67 Water O 66.9 33.45 Concrete Al Si 13.75 45.87 1.743 20.15 The density of borated polyethylene (I.e. 0.97 g/cc) is very close to that of water (I.e. 1 g/cc). One material, polyethylene, has proven to be an especially useful neutron shield. Polyethylene is an exceptional base material for neutron shielding because of its outstanding nuclear, physical and chemical properties. Of all practical materials, it contains more hydrogen atoms in a given volume than any other substance. It also has excellent machining and fabrication characteristics. It is chemically inert, and can be obtained in a very pure form. Its physical properties permit uniform distribution of various additives throughout the material. These additives can be varied over a wide range to meet specific needs. Other base materials include: - Epoxy, Silicone, Urethane, Hydrocarbons, refractories, and cementitious materials. Additives include: - Boron, B-10, Lead, Li-6, Tungsten, Gadolinium, and Cadmium.

Average Neutron Energy (MeV) Neutron Sources Source Reaction Average Neutron Energy (MeV) Reactor Fission Fission spectrum 24Na + Be (,n) 0.83 Ra + Be (,n) 5.0 Po + Be 4.0 252Cf Spontaneous fission Pu + Be This slide shows some common neutron sources. Neutrons are produced in these sources by fission, photoneutron production (,n), or by alpha capture reactions (,n). Neutrons are also produced by accelerators. Some of the sources listed above produce neutrons of a single energy (I.e. monoenergetic sources) and some (e.g. fission sources and Cf-252) produce neutrons with a spectrum of energies.

Shielding of Po-Be Neutrons Material Half-Thickness (cm) Paraffin 6.6 Water 5.4 12% Borax in Water 5.3 Brass 4.9 Steel (cold roll) Lead 6.8 Aluminum 7.8 Note that PoBe neutron sources produce neutrons with an average energy of 4.0 MeV. The half-thickness of the material is that thickness required to reduce the initial neutron intensity by a factor of 2. Based solely on amount of absorber material required, for these neutrons, brass or steel appear to be the best shields.

Summary Principles of neutron shielding were discussed Students learned about neutron energy categories, shielding principles, commonly used shielding materials, associated gamma radiation, and common neutron sources

Where to Get More Information Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009) International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)