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Part I Review of Fundamentals

Module 3 Interaction of Radiation withMatter

Session 3 Beta Particles

Session I.3.3Session I.3.3

IAEA Post Graduate Educational CourseIAEA Post Graduate Educational CourseRadiation Protection and Safe Use of Radiation SourcesRadiation Protection and Safe Use of Radiation Sources

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In this session we will discuss the following In this session we will discuss the following as they relate to beta particle interactions as they relate to beta particle interactions

Mechanisms of Energy TransferMechanisms of Energy Transfer BremsstrahlungBremsstrahlung Cerenkov RadiationCerenkov Radiation ShieldingShielding

OverviewOverview

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Ionizing radiation Ionizing radiation removes orbital removes orbital electrons from atomselectrons from atoms

This creates an ion This creates an ion pair – an electron and pair – an electron and the atom that has lost the atom that has lost an electronan electron

IonizationIonization

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Ionizing radiation Ionizing radiation includes photons, includes photons, but the result is the but the result is the same – an ion pair is same – an ion pair is producedproduced

This section focuses This section focuses on the electron on the electron interactionsinteractions

IonizationIonization

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Recall that unlike Recall that unlike photons, electrons photons, electrons have a charge (-) have a charge (-) and massand mass

ElectronsElectrons

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Electrons are much lighter than the nucleons Electrons are much lighter than the nucleons – the neutron and proton in the nucleus– the neutron and proton in the nucleus

ElectronsElectrons

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All of the photon interactionsAll of the photon interactions

photoelectric effectphotoelectric effect compton scatteringcompton scattering pair productionpair production

ElectronsElectrons

result in the productionresult in the production

of electrons. These are ionizing radiation just of electrons. These are ionizing radiation just like beta particle sourceslike beta particle sources

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Electron interactions are comparable to those of Electron interactions are comparable to those of other charged particlesother charged particles

More energetic electrons travel faster and so More energetic electrons travel faster and so create a lower ionization densitycreate a lower ionization density

Dose is the amount of energy deposited per Dose is the amount of energy deposited per mass of material (joules/kg)mass of material (joules/kg)

Energetic electrons deposit less energy so the Energetic electrons deposit less energy so the dose is lower until they slow downdose is lower until they slow down

ElectronsElectrons

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In addition to the energy of the electron, In addition to the energy of the electron, the stopping power depends on the the stopping power depends on the material in which the electron is material in which the electron is interactinginteracting

ElectronsElectrons

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BremsstrahlungBremsstrahlung

When an electron interacts When an electron interacts close to a nucleus, it close to a nucleus, it accelerates and changes accelerates and changes directiondirection

The result is that a photon is The result is that a photon is produced. This process is produced. This process is called “bremsstrahlung” which called “bremsstrahlung” which means ‘braking radiation’means ‘braking radiation’

Bremsstrahlung photons have a Bremsstrahlung photons have a continuous energy distributioncontinuous energy distribution

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BremsstrahlungBremsstrahlung

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The bremsstrahlung production of electrons The bremsstrahlung production of electrons is expressed as the mass radiative stopping is expressed as the mass radiative stopping powerpower

wherewhere

BremsstrahlungBremsstrahlung

dTdTdxdx

NNAAZZ22

AAdTdTdxdx

= = o o (E + m(E + moocc22)B)Brr

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00 = 5.8 x 10 = 5.8 x 10-28-28 cm cm22/atom/atom

NNA A = Avogadro’s number= Avogadro’s number

Z = atomic numberZ = atomic number A = atomic massA = atomic mass E = kinetic energy of the electron in MeVE = kinetic energy of the electron in MeV mm00 = rest mass of an electron = rest mass of an electron

c = speed of lightc = speed of light BBrr = a function of E and Z (approximately 16/3 = a function of E and Z (approximately 16/3

for E << 0.5 MeV; 6 for 1 MeV; 12 for 10 MeVfor E << 0.5 MeV; 6 for 1 MeV; 12 for 10 MeV

BremsstrahlungBremsstrahlung

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The fraction of electrons producing The fraction of electrons producing bremsstrahlung follows the relationship:bremsstrahlung follows the relationship:

F = 3.5 x 10F = 3.5 x 10-4-4 (Z)(E) (Z)(E)

Empirical RelationshipEmpirical Relationship

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Shielding for Beta SourcesShielding for Beta Sources

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Cerenkov RadiationCerenkov Radiation

Cerenkov radiation is the visible light that Cerenkov radiation is the visible light that is created when charged particles pass is created when charged particles pass through a material at a velocity greater through a material at a velocity greater than the velocity of light for that materialthan the velocity of light for that material

Cerenkov radiation is observable in spent Cerenkov radiation is observable in spent fuel pools of reactors and in irradiator fuel pools of reactors and in irradiator source storage poolssource storage pools

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Cerenkov RadiationCerenkov RadiationReactor Spent Fuel PoolReactor Spent Fuel Pool

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Cerenekov RadiationCerenekov RadiationIrradiator Source RackIrradiator Source Rack

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While no particle can exceed the speed of light While no particle can exceed the speed of light in a vacuum (3.0x10in a vacuum (3.0x1088 m/s), it is possible for a m/s), it is possible for a particle to travel faster than the speed of light in particle to travel faster than the speed of light in certain mediums such as watercertain mediums such as water

When the charged beta particle moves through When the charged beta particle moves through the water it tends to "polarize" (or orient) the the water it tends to "polarize" (or orient) the water molecules in a direction adjacent to its water molecules in a direction adjacent to its path thus distorting the local electric charge path thus distorting the local electric charge distribution distribution

Cerenkov RadiationCerenkov Radiation

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After the beta particle has passed, the molecules After the beta particle has passed, the molecules realign themselves in their original, random charge realign themselves in their original, random charge distributiondistribution

A pulse of electromagnetic radiation in the form of A pulse of electromagnetic radiation in the form of blue light is emitted as a result of this reorientationblue light is emitted as a result of this reorientation

When the speed of a beta particle is less than the When the speed of a beta particle is less than the speed of light, the pulses tend to cancel themselves speed of light, the pulses tend to cancel themselves by destructive interference, however, when the by destructive interference, however, when the speed of the beta particle is greater than the speed speed of the beta particle is greater than the speed of light (in water) the pulses are amplified through of light (in water) the pulses are amplified through constructive interferenceconstructive interference

Cerenkov RadiationCerenkov Radiation

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The phenomenon is analogous to the acoustic The phenomenon is analogous to the acoustic sonic boom observed when an object exceeds sonic boom observed when an object exceeds the speed of sound in airthe speed of sound in air

The intensity of the blue glow is directly The intensity of the blue glow is directly proportional to the number of fissions occurring proportional to the number of fissions occurring and the reactor power leveland the reactor power level

This property is utilized in Cerenkov detectors This property is utilized in Cerenkov detectors that measure the magnitude of Cerenkov that measure the magnitude of Cerenkov radiation produced in a detector made of luciteradiation produced in a detector made of lucite

Cerenkov RadiationCerenkov Radiation

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Although most of the Cerenkov radiation is in Although most of the Cerenkov radiation is in the ultraviolet region, it is visible to us with a the ultraviolet region, it is visible to us with a distinctive soft blue glowdistinctive soft blue glow

The blue glow persists for a short time after the The blue glow persists for a short time after the reactor has been shut downreactor has been shut down

This property may be used to inspect spent fuel This property may be used to inspect spent fuel to see if it is actually spent fuel or dummies used to see if it is actually spent fuel or dummies used to mask a diversion of materialto mask a diversion of material

Cerenkov RadiationCerenkov Radiation

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Where to Get More InformationWhere to Get More Information

Cember, H., Introduction to Health Physics, 3Cember, H., Introduction to Health Physics, 3rdrd Edition, McGraw-Hill, New York (2000)Edition, McGraw-Hill, New York (2000)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8Table of Isotopes (8thth Edition, 1999 update), Wiley, Edition, 1999 update), Wiley, New York (1999)New York (1999)

International Atomic Energy Agency, The Safe Use International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)IAEA, Vienna (1995)