Interaction of radiation with matter - 5 Neutrons Day 2 – Lecture 5

Objective To discuss about Neutron interaction, radiative capture, fission, neutron cross sections and neutron activation analysis

Content Neutron interaction mechanisms Neutron energy categories Radiative capture Charged particle emission Fission Elastic and inelastic scattering Neutron cross sections and removal Neutron activation analysis

Charged Particle Interactions Ionizing radiation is divided into two categories: Directly Ionizing alpha and beta Indirectly Ionizing photons and neutrons Since neutrons carry no electrical charge, their interaction mechanisms with matter are very different from alpha and beta radiation. Neutrons are said to be “indirectly ionizing” whereas alpha and beta particles are said to be “directly ionizing.” Gamma radiation (photons) are also indirectly ionizing. Gamma rays and neutrons release charged particles in matter which are themselves directly ionizing. Gamma rays and neutrons release charged particles in matter which are themselves directly ionizing.

Neutron Interactions Neutrons have no charge and thus are indirectly ionizing Always a “mixed” field of neutrons and gamma rays Biological effects of neutrons are strongly energy dependent Neutrons are arbitrarily divided into “slow” (thermal) or “fast” (energies of 1 MeV and above) All neutrons are “born fast” and then lose energy as they interact with matter in various ways that we will discuss in this session. The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). The probability of slow neutron interactions is very dependent on their energy. Neutron dosimetry is particularly challenging since it always involves both neutrons and gamma rays together. One needs to be able to distinguish between the two types of radiation to accurately measure and record radiation dose to humans.

Neutron Interactions All neutrons are “born fast” and then lose energy as they interact with matter The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). All neutrons are “born fast” and then lose energy as they interact with matter in various ways that we will discuss in this session. The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). The probability of slow neutron interactions is very dependent on their energy. Neutron dosimetry is particularly challenging since it always involves both neutrons and gamma rays together. One needs to be able to distinguish between the two types of radiation to accurately measure and record radiation dose to humans.

Slow Neutron Interactions Radiative Capture 1n + 1H  2H +  1H(n, )2H This reaction is important in neutron dosimetry and shielding. Hydrogen is a component of human tissue, so this reaction will produce radiation dose to humans. Also, this reaction makes materials containing hydrogen (concrete, water, parrafin, polyethylene, etc.) good shields for neutrons. This reaction is important in neutron dosimetry and shielding. Hydrogen is a component of human tissue, so this reaction will produce radiation dose to humans. Also, this reaction makes materials containing hydrogen (concrete, water, parrafin, polyethylene, etc.) good shields for neutrons. H has one proton and one electron. This reaction forms Note that this reaction produces a gamma ray which must be shielded against, since it can also produce dose to people. This gamma ray is fairly energetic, with an energy of 2.23 MeV. It will produce a whole body dose when people are exposed to neutron radiation, by virtue of the neutron interactions with hydrogen in the body. For neutrons of low energy, this interaction contributes much of the dose throughout the human body and for neutron energies up to 2.5 MeV, it contributes a major portion of the dose in deep layers, since the photons can penetrate over considerable distances prior to electron interactions.

Slow Neutron Interactions Radiative Capture 1n + 113Cd  114Cd +  113Cd(n, ) 114Cd This reaction is important in neutron shielding and is also used as the principal reaction for some neutron detectors. Note that this reaction also produces a gamma ray, which can cause dose to people. This reaction is important in neutron shielding and is also used as the principal reaction for some neutron detectors.

Charged Particle Emission Slow Neutron Interactions Charged Particle Emission 1n + 10B  7Li + 4He 10B(n,) 7Li This is why boron controls are used in nuclear power reactors, since it tends to reduce the number of neutrons present and therefore helps control the fission process. This reaction is important for both neutron instrument design (neutron measurement) and neutron shielding. It is also the basis for boron controls used in nuclear power reactors, since it tends to reduce the number of neutrons present and therefore helps control the fission process. Since the alpha particle and the Li-7 ion are very massive particles, they cannot travel very far in matter at all. They are therefore of no concern in terms of neutron shielding.

Charged Particle Emission Slow Neutron Interactions Charged Particle Emission 1n +14N  14C + p 14N(n, p)14C This reaction is important to neutron dosimetry, since N is a component of human tissue. If released in tissue, the proton and C-14 particles can cause dose. The proton and C-14 particles are not significant in terms of neutron shielding, since they will not travel very far in matter. The energy of the proton released in this reaction is 0.6 MeV. For thermal neutrons, this reaction is of particular importance in human tissue. The energy of the proton released in this reaction is 0.6 MeV.

Slow Neutron Interactions Fission 1n + 235U  fission products available for more fission A very important neutron interaction mechanism is fission, which is the basis for nuclear power reactors. More neutrons are relased in this reaction than are absorbed (the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.” the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.”

Fast Neutron Interactions Elastic scattering - neutrons interact with particles of approximately the same mass such as protons (billiard ball analogy) Occurs in materials rich in hydrogen such as water, wax, concrete Accounts for about 80% of fast neutron dose to tissue 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.

Elastic Scattering In collision with protons, neutrons lose half their energy on average. This reaction makes hydrogenous materials (materials rich in protons) good shields (e.g. concrete, wax, water, and various plastics). This reaction is very important from a health physics standpoint, since it is responsible for most of the tissue dose from fast neutrons. In collision with protons, neutrons lose half their energy on average.

Fast Neutron Interactions Inelastic scattering – neutrons interact with particles of much greater mass (e.g. iron) (analogy of ping pong ball striking bowling ball) 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. In human tissue, and for fast neutron energies in excess of 10 MeV, inelastic scattering and nuclear reactions (frequently with the emission of several particles) become comparable in frequency with elastic scattering. For fast neutrons of energies of about 1 MeV, inelastic scattering can become appreciable. Inelastic scattering occurs primarily with high-Z absorbers.

Inelastic Scattering This interaction is best described by the compound nucleus model, in which the neutron is captured, then re-emitted by that target nucleus together with the gamma photon. This is a threshold phenomenon; the neutron energy threshold varies from infinity for hydrogen (I.e. inelastic scattering cannot occur with H) to about 7 MeV for oxygen to less than 1 MeV for U. This reaction is not very significant from a tissue dose standpoint, since tissue is composed of relatively low-Z materials. Iron has a particularly strong probability of fast neutron inelastic scattering. The neutron is captured, then re-emitted by the target nucleus together with the gamma photon. It has lesser energy

Fast Neutron Interactions Why do you think fast neutrons interaction is not significant from a tissue dose point of view? 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. In human tissue, and for fast neutron energies in excess of 10 MeV, inelastic scattering and nuclear reactions (frequently with the emission of several particles) become comparable in frequency with elastic scattering.

Neutron Cross Sections Probability that neutron will interact with a given material Unit is “barn” where 1 barn = 10-24 cm2 Neutron cross sections are strongly energy dependent. The barn is an almost legendary unit in the history of neutron physics. The story says that when neutron cross sections were first being measured, the cross section for a given material (maybe boron?) was said to be “as big as a barn!” A “shed” is equal to 10-24 barns or 10-48 cm2. The shed is used for particle interactions where the cross section is extremely small, e.g. neutrino interactions.

Microscopic Cross Section Sum of separate cross sections for all processes which may occur with a given atom. Unit is cm2 total = scatter + capture + fission Neutrons can be removed from a beam by the absorber material generally in three ways: Scatter Capture Fission The total removal cross section is the sum of the three cross sections for these processes.

Macroscopic Cross Section Product of microscopic cross section and the total number of atoms per cm3 in the material. Unit is cm-1  total = N total where N = number of atoms/cm3 The macroscopic cross section is used for neutrons, in place of the linear or mass absorption coefficients that are used in photon attenuation. N can be calculated using the material density, atomic weight, and Avogadro’s Number. Note the macroscopic cross section is used for neutrons, in place of the linear or mass absorption coefficients that are used in photon attenuation.

Neutron Removal by an Absorber I = I0 e where I = neutrons passing through the absorber I0 = neutrons incident on the absorber N total = total macroscopic cross section x = absorber thickness - N total x Neutron removal is exponential.

Neutron activation analysis (NAA) NAA is a process used for determining the concentrations of trace elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The study spectra of the emissions of the radioactive sample allow s the identification of the element. In chemistry, neutron activation analysis (NAA) is a process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts. NAA can also be used to determine the activity of a radioactive sample

Neutron Capture in 23Na 23Na (n,) 24Na This example illustrates the principle of neutron activation of a stable isotope. In this case, stable Na-23 absorbs a neutron and becomes radioactive Na-24. A gamma ray photon is emitted in this reaction. Na-24 happens to be radioactive, with a half-life of about 15 hours. The short form notation for this capture reaction is shown below the figure. This reaction is known as “radiative capture.”

Examples of Importance of Neutron Activation Production of isotopes (for example 60Co, 192Ir, etc.) 5927Co + n → 6027Co → 6028Ni + e + gamma rays Accident dosimetry (for example 24Na in blood) Crime detection in forensic medicine (for example Napoleon’s hair) Neutron activation, with its resulting production of radioactive isotopes, is a very important phenomenon in health physics. Some of the important radioisotopes used in industry are made by this process. If a person has been exposed to a criticality accident (which involves a large release of gamma and neutron radiation), their tissues can become activated, which results in residual, induced radioactivity in the body. Na-24 is produced in the person’s blood and this can be measured and related back to the number of neutrons incident on the body. In this manner, neutron dose can be estimated, as well as the total number of neutrons released during the criticality. Acute neutron radiation exposure (e.g., from a nuclear incident) converts some of the stable 23Na in human blood plasma to 24Na. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed. 22Na is a positron emitting isotope with a remarkably long half life By neutron irradiation of objects it is possible to induce radioactivity; this activation of stable isotopes to create radioisotopes is the basis of Neutron activation analysis. One of the most interesting objects which has been studied in this way is the hair of Napolean’s head, which have been examined for their arsenic content. Naturally occurring arsenic is composed of one stable isotope75As. Neutron irradiation hair on Napoleon's head was examined for their arsenic content. Naturally occurring arsenic is composed of one stable isotope75As.

Examples of Importance of Neutron Activation Activation analysis for measurement of trace elements Activation products in a reactor are major sources of radiation exposure to workers (for example 60Co) They can expose the public (for example direct gamma radiation from 16N in steam in BWR is produced by neutron irradiation of water ) Neutron activation analysis is a very sensitive method for quantifying the amounts of trace elements in samples which have been irradiated by neutrons. Co-60, a neutron activation product, is responsible for about 75% of occupational radiation dose to workers in US commercial nuclear power plants. N-16, which has very energetic and highly penetrating gamma ray (7 MeV), is produced by neutron irradiation of water in a commercial BWR reactor core. This N-16 is carried over in the steam into the turbine building. The gamma rays from N-16 can travel far enough to cause exposure to members of the public living near the reactor site. The good news about N-16 is that it has only a 7 sec half-life, so that when the reactor is shut down (and the steam supply stops), the N-16 dose rate decays off very rapidly.

Examples of Importance of Neutron Activation Determination of fast neutron radiation component at Hiroshima Accelerator mass spectrometry of 63Ni (half-life = 100 years) produced by fast neutron activation of copper in building materials Reaction is 63Cu (n, p) 63Ni One final example is the recent effort to better quantify the fast neutron radiation component that was associated with the atomic bomb at Hiroshima, Japan. Neutrons released from the detonation of the atomic bomb irradiated and activated copper in buildings. The copper in the building materials was converted to radioactive Ni-63, which has a 100 yr half-life. Samples of the irradiated copper have been analyzed to measure Ni-63. In this manner, the neutron flux incident on the building can be estimated. Knowing the distance from the building to the hypocenter of the bomb, one can estimate the total number of neutrons released by the bomb. Good estimates of the neutron flux at Hiroshima are needed to determine neutron biological effects and the radiation quality factor for fast neutrons.

Summary Neutron interactions with matter were discussed Neutron interaction mechanisms, neutron energy categories, the concept of neutron cross section, and neutron removal from a beam and neutron activation were described

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)