PDF 3.2 Neutrons Are Special

7/28/2019 PDF 3.2 Neutrons Are Special

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Nuclear Reactionsand Radiation

3.2 Neutrons are special !

L. R. Foulke


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Neutron Interactions Fission

As the fission products de-excite and decay overtime they release

rays particles, particles, and neutrons from

radioactive decay

These emissions are referred to as delayed.


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Neutron Interactions

Fission

Fission fragments (some unstable)

Pre-Collision

proton

neutrons

Electrons (beta particles)

ray

Image Source: See Note 1 for Ionized nucleus


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Nuclear Reactions Reactions between a subatomic particle (neutron) and

a nucleus are of particular interest.

Nuclear reactions are most easily caused by neutrons Charged particles cant reach nucleus due to

electrostatic forces

For convenience we use a shorthand notation todescribe nuclear reactions

Like everything else in nature, nuclear reactionsmust obey balance equations.


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Nuclear Reactions(n,n)

(n,n)

(n,)

(n,2n)

(n,f)

After formation, unstablecompound nuclei may

stabilize through one of

several decay mechanisms.

n Not all neutron interactionshave unique outcomes.

Image Source: See Note 2


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Nuclear Reactionsn Neutron Induced Reactionsn Elastic Scattering (n,n)

n Inelastic Scattering (n,n)

n (Radiative) Capture (also called n, reactions)

nH)*D(nH 101

1

2

1

1

0

1

1 ++

2656Fe+

0

1n (

26

57Fe)*

26

56*Fe+

0

1n+

0

0

++0

0

236

92

236

92

1

0

235

92 U)*U(nU

++0

0

24

11

24

11

1

0

23

11 Na)*Na(nNa


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Nuclear Reactionsn Neutron Induced Reactionsn Charged Particle (n , ) (n,p) (n,d) (n,2p) . . . .

n Multiple Neutron (n,2n) (n,3n) . . . .

n Fission (n,f)

4

2

7

3

*11

5

1

0

10

5 LiB)(nB ++

nUUnU1

0

232

92

*234

92

1

0

233

92 2)( ++

0

0

1

021

*236

92

1

0

235

92 )( ynxFFUnU ++++


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Nuclear Reactions

nOther Nuclear Reactions (Neutron as a Product)nAlpha ( ,n)

n Gamma ( ,n)nCC)*(Be

1

0

12

6

13

6

4

2

9

4 ++

nHH)*(D 1011210021 ++

Neutron source

Neutron source

Referred to as a photoneutron reaction


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Nuclear Fission Of all the nuclear reactions wehave considered, fission is the

most interesting

Occurs only in the heaviest,least stable atoms

Can occur as a natural decaymode (spontaneous fission),

or due to a nuclear interactionby another particle (a neutron)

n Only the Actinides (Z 89) are large enough to allow fissionn Only 2 elements are naturally occurring thorium (Th) and Uranium (U)n Only 3 total naturally occurring isotopes 232Th, 235U, and 238Un Other isotopes can be artificially produced by neutron bombardment

Image Source: See Note 3


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Energy Released During Fissionn Sensible energy released during fission

MeV %

Fission Fragments (Kinetic Energy) 168 84.0

Neutrons (Kinetic Energy) 5 2.5

Prompt Gamma Rays 7 3.5

Delayed Radiations

Beta Particles* (Kinetic Energy) 8 4.0

Gamma Rays 7 3.5

Radiative Capture Gammas 5 2.5

TOTAL 200 100

Fissioning one uranium atom providesnearly 100,000,000 times as much energy

as combusting one carbon atom.

*Neutrinos account for another

12 MeV that cannot be detected

7.5 %Max. Decay

Heat


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Energy Released During Fission

Fission

Delayed Energy Release

Prompt

Energy Release

Image Source: See note 2


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Prompt Energy Release Accounts for90% of energy released in fission

Kinetic energy of fission fragments ( 84%) Kinetic energy of free neutrons (2.5%) Gamma rays emitted during fission (3.5%)

Random number of free neutrons produced in fission Between 0 and 5 neutrons produced per fission Average (denoted ) between 2-3 produced per fission

Prompt energy is divided between fission fragmentsand neutrons and gamma rays

Free neutrons can be produced with a wide range of initialvelocities (kinetic energy).


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Neutron Energy Spectrum

Energy Range: 0.1 10 MeV

Most ProbableEnergy:

0.7 MeV Average Energy

2 MeV

Fission Neutron

Energy Distribution

Image Source: See note 2


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Delayed Energy Release Accounts for10% of energy released in fission

Radioactive decay of unstable fission products ( 10%) Energy may be released over hundreds of years as harmful

gamma radiation.

Explains why spent nuclear fuel is difficult to handle. The type and amount of delayed radiation depends

heavily on what daughter nuclei are produced during

fission. Fission splits original nucleus randomly into two pieces Fission fragments are typically not the same size


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1. Public domain:http://en.wikipedia.org/wiki/File:Alpha_Decay.svg

2. Reprinted with permission from theAmerican Nuclear Society. NuclearEngineering Theory and Technology ofCommercial Nuclear Powerby Ronald AllenKnief, 2nd Edition. Copyright 2008 by the

American Nuclear Society, La Grange Park,Illinois. Figure 2-4 (slide 6), 2-7 (slide 13),2-8 (slide 15).

3. Creative Commons: Adapted fromhttp://en.wikipedia.org/wiki/File:Periodic Table Radioactivity.svg

Image Source Notes

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