pdf 3.5 the magic of transmutation

7/28/2019 PDF 3.5 the Magic of Transmutation

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Nuclear Reactionsand Radiation

3.5 The magic of transmutation

We cant see, hear, taste orsmell radiation. How do I

detect it?

L. R. Foulke


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Fissionable and Fissile Nuclides in the Actinide period are classified by their

potential to undergo fission events when their nucleus is

struck by a neutron

A nuclide is said to be Fissionable if neutron-inducedfission is possible in the nuclide.

All nuclides with atomic number Z > 89 are fissionable. Fissionable nuclides are further classified as

Fissile, if fission can be caused by neutrons with any amountof kinetic energy (Typically even-odd, odd-even, or odd-odd)

Non-Fissile, if fission is a threshold reaction that can only becaused by high energy neutrons with a certain amount of

kinetic energy (Typically even-even nuclides)


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Fissile and Non-Fissile Fissile nuclides are most effective in a nuclear

chain reaction because any neutron can cause

an additional fission. (e.g., U-235)

Non-Fissile nuclides can only fission duringreactions with high-energy neutrons. (e.g.,

U-238)

In some cases, the neutron absorption can change anon-fissile nuclide into a fissile nuclide through

transmutation.


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TransmutationNeutron Capture (n,)

Radioactive Decay ()

Naturally occurring

Non-Fissile Nuclide

Artificially created

Fissile Nuclide

Fission (Energy + fission products)

Image Source: See Note 1


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Pu Buildup



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Actinide Transmutation

Non-Fissile nuclides that can be converted to fissilenuclides by a neutron absorption reaction are referred to

as fertile nuclides.

232Th and 238U are the most common examples offertile nuclides The process of converting fertile nuclides into fissile

nuclides is called conversion or breeding.

232Th + n 233U 238U + n 239Pu


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Fissionable Nuclidesn Major Fissionable Nuclidesn Fissile 235U 233U 239Pu 241Pu

n Fertile 232Th 238U 240PuConversion

or Breeding


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Radiation Detection Radiation cannot be seen, heard, smelled, tasted orfelt directly.

Radiation can only be detected indirectly, after itundergoes interactions with atoms in a material.

In order to detect the presence of radiation, we need adevice that can measure the rate and energy of

interactions.


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Radiation Detectors Most modern radiation detectors rely

on the creation of ion pairs.

As ionizing radiation passes through adetector, the interactions can splitneutral atoms into pairs of positive and

negative ions.

In the presence of an electric field The positive ions (nuclei) will migrate

towards the negative pole (cathode)

The negative ions (electrons) willmigrate towards the positive pole

(anode)

Gas Atom

Nucleus (pos. ion)Free Electron (neg. ion) ray

ElectricPote

ntial(Volts)

Anode (pos.)

Cathode (neg.)

before

after



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Basic Gas Detector Design

+_

Ammeter

Cathode (neg.)

Anode (pos.)

Battery

Gas Atoms

Sealed, gas filled

container



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Ion Chamber Operation

Gas Atom

Nucleus (pos. ion)Free Electron (neg. ion)

n Ion Chambern Lower Voltagen Only collects ion pairs formed directly by

radiation.

n Pulse or current mode. Good for highradiation levels

ray

Image Source: See note 2


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Neutron Detectors Since neutrons do not directly cause ionization, they cannot bedetected by traditional detectors.

However, it is possible to modify a proportional gas detector toserve as a neutron detector

This is done by filling the detector (or lining the outside) with amaterial that is susceptible to neutron interactions (typicallyUranium or Boron).

When neutrons strike this target material they interact and excitethe constituent atoms.

The excited atoms decay (or fission) and eject charged particles orgamma rays into the gas filled detector

Neutron detectors are useless once the neutron target material hasbeen depleted.


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Neutron Detector Operation

Gas Atom

Nucleus (pos. ion)Free Electron (neg. ion)

n Neutron Detectorn Proportional detector coated with neutron sensitive

material (Uranium). Referred to as Fission Chamber.

n Neutron interactions in coating creates chargedparticles or gamma rays which enter gas chamber.

n Alternatively, detector may be filled with BF3 gas.Free Neutron



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1. Adapted with permission from the American NuclearSociety. Nuclear Engineering Theory andTechnology of Commercial Nuclear Powerby RonaldAllen Knief, 2nd Edition. Copyright 2008 by the

American Nuclear Society, La Grange Park, Illinois.Figure 2-16 (slide 4) and 6-2 (slide 5).

2. Reprinted with permission from David Griesheimer,University of Pittsburgh.

3. Public domain:http://science.ksc.nasa.gov/payload/missions/cassini/images/captions/KSC-97EC-0903.html

4. Creative Commons: User:Mrcomputerwiz,http://creativecommons.org/licenses/by/3.0/,http://en.wikipedia.org/wiki/

File:CivilDefenseVictoreen720_underside.jpg

Image Source Notes

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