Neutrons

Radiation Protection IIINUCP 2331

Neutrons

• No charge• Very penetrating• ~1 AMU -similar to proton• Stable in the nucleus• Free neutron T ½= about 12 min• Interacts by collision• Losses energy in multiple interactions• High H content material is good shielding

Neutron Energies

• The energies of neutrons are important in how they react

• Neutrons that have too much energy will not interact with the atom

• Capture cross section of the atom is dependant on the energies of the neutron

• U-235 will not interact with a fast neutron, needs to be low energy

Golf Analogy• Consider the uranium is a hole on a golf course and

the neutron a golf ball on the putting green. • If the ball is hit too hard, even if it hits the hole near

perfectly, it will likely hit the rim and be rejected.

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Golf Analogy

• If the ball is moving slowly enough, it is more likely to fall in the hole.

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Neutron Energies• Cold- 0-.025 eV• Thermal .025 ev• Epithermal .025-.4 eV• Cd .4-.6 ev• EpiCd .6-1 eV• Slow 1-10 eV• Intermediate 10ev -1MeV• Fast 1-20 MeV• Relativistic > 20 Mev

Neutron Production

• Only very heavy radionulcides emit neutrons as part of their normal decay Cf-252, Cm-254

• Need to be created in a accelerator or a neutron generator

• Sources have to be a two part source– Heavy alpha emitter– Low Z metal– Am-Be, Pu-Li, Pu-Be, Ra-Be

Neutron Production

• Heavy atom produces an alpha particle as part of its natural decay process

• The alpha particle interacts with the low Z metal

• This initiates and (alpha, n) reaction in the metal

• Neutron production is based on activity of the alpha emitter

Neutron Source

• Each source combo generates and number of neutrons /sec and of a certain energy

• Pu-Be 2.3 E 6 n/sec/Ci• Am-Be 2.2 E 6 n/sec/Ci• Compare this with Cf-252– 4.3 E 9 n/sec/Ci

Neutron Interactions

(Indirectly Ionizing Radiation)

Inelastic and Elastic Collisions

Nuclear Capture

Cross section

• Neutrons have to come in close proximity to another particle/nucleus in order to interact

• The area that the particle/nucleus can interact with the neutron is called the capture cross section of that particle/nucleus

• The cross section of interaction is expressed as barns

• 1 barn = 10-24 cm2

Cross Sections• Capture cross section-probability of nuclear

reaction with neutron• Total cross section takes into account all

reactions(scatter, capture, absorption, fission, etc)

• Can have specify cross sections for each interaction

• Probability determined by energy

Cross section

• Macroscopic cross section- ratio between neutron flux and reaction rate, – property of the material – reactions per volume

• Microscopic cross section- probability of interaction with individual atom – property of the nucleus– probability per area

Elastic Scattering

• This interaction is similar to the bouncing of a ball on the floor

• The amount of energy transferred to the other object depends on the size difference in the objects

• Neutron hitting a large nucleus will not loose too much energy

• Neutron hitting an object of similar size may transfer most or all energy to other object

Ping Pong Ball Analogy• Consider the neutron as a fast ping pong ball. • If the ping pong ball hits a larger more massive bowling ball,

the bowling ball won’t budge and the ping pong ball will scatter off of it at roughly the same speed.

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Moderators • If the ping pong ball hits another slower moving or stationary

ping pong ball of the same size, both balls will scatter off at roughly ½ the speed of the initial ping pong ball.

• Materials used to slow down neutrons are called moderators.

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Ineleastic scattering

• Inelastic interactions are where one of the colliding particles is composed of smaller units

• The neutron will interact with the other nucleus and transfer energy and the other atom will become energized

• The atom will then emit and photon or other particle to return to a ground state

Nuclear capture

Neutrons loose energy by elastic scattering through matter

When the neutron losses enough energy it will interact differently with atoms

If the neutron is the right energy it will be absorbed into the atom

This will add a neutron to the atomic mass and possibly making the atom unstable

Neutron Activation

• The process of a material becoming radioactive after being subjected to a field of neutrons

• Material such as magnets in accelerators get highly radioactive

• Material can be put in neutron field for elemental analysis

• Can be used in forensics

Activation• N=Aφσt• N= number of radioactive atoms produced in the

reaction• A= number of atoms in the sample• φ = neutron flux of the system• σ = capture cross section of the atom• T= time in the neutron field• Formula get s complicated if one need to take into

account decay while activation and counting

N-capture induced FissionN-capture induced Fission

Uranium-235Plutonium-239

Neutron

10n

10n

10n

Strontium-90

Xenon-144

Demonstration

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We begin with a fast neutron

The neutron passes through water, slowing down and transferring its energy to the water molecules.

The slow (thermalized) neutron is absorbed by a U-235 atom.

Demonstration

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The uranium becomes highly excited and begins to deform.

Eventually, the nucleus splits into two fission products and releases 2 or 3 neutrons

Neutron

Neutron

Neutron

Fission Product

Fission Product

Fission & Chain ReactionsFission & Chain Reactions

Neutron

Uranium-235Plutonium-239

10n

Neutron in reactor

• The amount of electricity that is produced is proportional to the amount of heat generated

• The amount of heat generated is proportional to the number of fissions taking place

• The number of fissions taking place is proportional to the free neutron population

• the change in the number of neutrons is related to the reactivity of the reactor

Reactivity

• Reactivity is defined as a reactor’s departure from criticality.

• It is a quantitative measure of the rate of change of fission neutron population.– We can not detect the number of fissions occurring,

but we can detect neutrons from fission to determine if they are increasing or decreasing in population.

– Mathematically, reactivity is described as the fractional change in neutron population.

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Explanation

• Reactivity can be positive, negative or zero.– A reactor with zero reactivity is critical by

definition.– A reactor with positive reactivity is supercritical.– A reactor with negative reactivity is subcritical.

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Neutrons• In the fission process there can be two

different types of neutrons• Prompt– Neutrons that are immediately released from the

fission process

• Delayed– Neutrons that are emitted from a high energy

beta decay from one of the fission fragments– Can be up to several minutes later– Needs to be taken into account when calculating

reactivity

Neutrons

• Flux- the number of neutrons passing through a space in a given time, Neutrons per area per time (n/cm2/sec)

• Fluence- number of neutrons passing through an area neutrons per area (n/cm2)

Neutron flux-density

• How many neutrons are passing through a square cm per second.

• in order to do this we need two pieces of info– How many neutrons are being emitted by the

source– How far away from the source are we

Neutron flux-density

• Total number of neutrons divided by total surface area that is at certain distance from the source.

• Number of neutrons/4πr2

• 10 Ci-Pu-Be source generates 2.3 E 6 n/sec/Ci• You are at 5 feet from source• Neutron flux –density= ????

Neutron dose

• We need the neutron flux in order to determine the dose generated by those neutrons

• Dose from the neutrons is based on the energy of the neutrons

• Compare the flux-density of the neutrons to the number needed to generate 1 Rem

Neutron dose

• 0.5 MeV neutrons• Flux density is 1.5 E 8 n/cm2

• What is the dose?

Questions?