1. Radiation Protection III NUCP 2331
Neutrons
Radiation Protection III
NUCP 2331

2. 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

3. 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

4. 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.
Animation 1 click
Ball does not fall in hole

5. Golf Analogy
If the ball is moving slowly enough, it is more likely to fall in the hole.
Animation 1 click
Ball falls in hole

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

7. 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

8. 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

9. 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

10. Neutron Interactions
(Indirectly Ionizing Radiation) Inelastic and Elastic Collisions Nuclear Capture

11. 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 = cm2

12. 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

13. 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

14. 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

15. 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.
Animation - 1 click
Ping pong ball strikes bowling ball and bounces

16. 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.
Animation - 1 click
Ping pong ball strikes other ball and both bounce off

17. Elestic scatter of neutrons is similar to the way billiard balls bounce off of each other. how energy each neutron has and the angles involved determines how much energy is transferred to the other body, protons.

18. 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

19. 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

20. In this case the neutron is slowed down by an elastic scatter and absorbed into a nucleus byt a inelastic scastter

21. Neutrons that have the right energy can be absorbed into the nucleus of an atom, this will upset the ratio between neutrons and protons and therefore possibly make the atom unstable and hence radioactive. Neutron radiation is the only type of radiation that can make other non radioactive material radioactive.

22. 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

23. 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

24. N-capture induced Fission
Xenon-144
10n
10n
10n
Neutron
Fission is a process in which very heavy atoms, such as uranium and plutonium, after “absorbing” a neutron becomes so unstable that they literally split into two pieces accompanied by a large release of energy.
The energy released per fission is approximately 200 megaelectronvolts, equivalent to about 3.2x10-11 J of energy, (a very small number). However this energy release is over two million times larger than that given up in most chemical reactions, for example in the burning of coal.
If 1 gm of uranium-235 were to completely fission, it would require burning approximately 4900 pounds of coal to result in an equivalent energy release.
To generate 1 J of energy 3.1x1010 fissions must occur. Most modern reactors operate at about 3800 megawatts of power which is 3,800,000,000 joules per second. To create this power level, 1.18x1020 fissions must occur in the reactor each second.
Uranium-235
Plutonium-239
Strontium-90

25. Demonstration
The neutron passes through water, slowing down and transferring its energy to the water molecules.
Animation – 2 clicks
Zoom out on neutron
Neutron moves thru water and hits uranium
The slow (thermalized) neutron is absorbed by a U-235 atom.
We begin with a fast neutron

26. Demonstration The uranium becomes highly excited and begins to deform.
Eventually, the nucleus splits into two fission products and releases 2 or 3 neutrons
Fission Product
Neutron
Neutron
Animation – 1 click
Uranium gets excited and fissions
Neutron
Fission Product

27. Fission & Chain Reactions
Neutron
A chain reaction is the process where neutrons produced from fission go on to cause fissions in surrounding uranium atoms. Some of the neutrons produced will escape from the reactor core, while others will interact with other core materials other than fuel. These neutrons are “lost” and reduce the chain reaction process.
If the number of fissions in the first generation equal the number of fissions in a second generation, the chain reaction is said to be “critical”.
If the number of fissions is increasing between generations, the reaction is said to be supercritical, and the power level of the reactor will increase exponentially.
If the number of fissions in decreasing between generations, the reaction is “subcritical” and the power will drop away dramatically.
A nuclear reactor is operated so it is “critical” at some established power level, usually about 3800 megawatts thermal. To increase power the reactor is temporarily made supercritical, while to decrease power the reactor is made temporarily subcritical.
Uranium-235
Plutonium-239

28. 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

29. 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.

30. 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.

31. 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

32. 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)

33. 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

34. 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= ????

35. 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

36. Neutron dose 0.5 MeV neutrons Flux density is 1.5 E 8 n/cm2
What is the dose?

37. Questions?