1. Charged Particle Radiation
Interaction of radiation with matter - 2
Charged Particle Radiation
(Beta Particles)
Day 2 – Lecture 2

2. Objective
To discuss the following as they relate to beta particle interactions
Mechanisms of Energy Transfer
Bremsstrahlung
Cerenkov Radiation
Shielding

3. Ionization Ionizing radiation removes orbital electrons from atoms
This creates an ion pair – an electron and the atom that has lost an electron

4. Ionization
Ionizing radiation includes photons, but the result is the same – an ion pair is produced
This section focuses on the electron interactions

5. Electrons
Recall that unlike photons, electrons have a charge (-) and mass

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

7. Electrons All of the photon interactions result in the production
photoelectric effect
Compton scattering
pair production
result in the production
of electrons. These are ionizing radiation just like beta particle sources

8. Electrons
Electron interactions are comparable to those of other charged particles
More energetic electrons travel faster and so create a lower ionization density
Energetic electrons deposit less energy so the dose is lower until they slow down
Dose is the amount of energy deposited per mass of material (joules/kg)

9. Electrons
In addition to the energy of the electron, the stopping power depends on the material in which the electron is interacting

10. Bremsstrahlung
When an electron interacts close to a nucleus, it accelerates and changes direction
The result is that a photon is produced. This process is called “Bremsstrahlung” which means ‘braking radiation’
Bremsstrahlung photons have a continuous energy distribution

11. Bremsstrahlung

12. Empirical Relationship
The fraction of electrons producing Bremsstrahlung follows the relationship:
F = 3.5 x 10-4 (Z)(E)
Note that the value of “E” is the maximum energy for beta particles
Beta particles that have higher energy will have a greater fraction of Bremsstrahlung photons created
Remind student that Z is the atomic number, that is, the number of protons in the nucleus. The relationship states that electrons interacting with atoms with a higher atomic number will produce more bremsstrahlung photons. Beta particles that have higher energy will have a greater fraction of bremsstrahlung photons created. Note that the value of “E” in this equation is the maximum energy for beta particles, not the average energy, and is in units of MeV. “F” is simply a fraction and was empirically derived. Caution the student not to try and attempt a dimensional analysis of this relationship.
Bremsstrahlung radiation is also called the “radiative yield” stopping power.
Because the bremsstrahlung fraction increases with increased atomic number of the shielding material, it is preferable not to shield a beta emitting isotope, particularly one that has a high energy, with a material that has a high atomic number. Note that low energy beta emitting nuclides, such as carbon-14 and tritium (hydrogen-3), are not likely to produce bremsstrahlung due to their low energy beta particles of MeV and MeV, respectively. Conversely, a higher energy beta emitting nuclide, like phosphorous-32, is very likely to create bremsstrahlung photons due to it’s 1.7 MeV beta particle.

13. Bremsstrahlung F = 3.5 x 10-4 (Z)(E)
Carbon-14, phosphorous- 32 are tritium (hydrogen-3) are all beta emitters but only one of these presents a radiation hazard due to bremsstrahlung radiation. Which radionuclide and Why do you think this is the case?
Remind student that Z is the atomic number, that is, the number of protons in the nucleus. The relationship states that electrons interacting with atoms with a higher atomic number will produce more bremsstrahlung photons. Beta particles that have higher energy will have a greater fraction of bremsstrahlung photons created. Note that the value of “E” in this equation is the maximum energy for beta particles, not the average energy, and is in units of MeV. “F” is simply a fraction and was empirically derived. Caution the student not to try and attempt a dimensional analysis of this relationship.
Bremsstrahlung radiation is also called the “radiative yield” stopping power.
Because the bremsstrahlung fraction increases with increased atomic number of the shielding material, it is preferable not to shield a beta emitting isotope, particularly one that has a high energy, with a material that has a high atomic number

14. Bremsstrahlung
Carbon-14 and tritium (hydrogen-3), are not likely to produce bremsstrahlung due to their low energy beta particles of MeV and MeV, respectively.
Conversely, a higher energy beta emitting nuclide, like phosphorous-32, is very likely to create bremsstrahlung photons due to it’s 1.7 MeV beta particle.
Remind student that Z is the atomic number, that is, the number of protons in the nucleus. The relationship states that electrons interacting with atoms with a higher atomic number will produce more bremsstrahlung photons. Beta particles that have higher energy will have a greater fraction of bremsstrahlung photons created. Note that the value of “E” in this equation is the maximum energy for beta particles, not the average energy, and is in units of MeV. “F” is simply a fraction and was empirically derived. Caution the student not to try and attempt a dimensional analysis of this relationship.
Bremsstrahlung radiation is also called the “radiative yield” stopping power.
Because the bremsstrahlung fraction increases with increased atomic number of the shielding material, it is preferable not to shield a beta emitting isotope, particularly one that has a high energy, with a material that has a high atomic number

15. Shielding for Beta Sources
Often plastic is used to shield beta particles to avoid creating bremsstrahlung photons which are more difficult to shield.

16. Cerenkov Radiation
Cerenkov radiation is the visible light that is created when charged particles pass through a material at a velocity greater than the velocity of light for that material
Cerenkov radiation is observable in spent fuel pools of reactors and in irradiator source storage pools

17. Reactor Spent Fuel Pool
Cerenkov Radiation
Reactor Spent Fuel Pool

18. Irradiator Source Rack
Cerenkov Radiation
Irradiator Source Rack

19. Cerenkov Radiation
While no particle can exceed the speed of light in a vacuum (3.0x108 m/s), it is possible for a particle to travel faster than the speed of light in certain mediums such as water
When the charged beta particle moves through the water it tends to "polarize" (or orient) the water molecules
From:

20. Cerenkov Radiation
After the beta particle has passed, the molecules realign themselves in their original, random charge distribution
A pulse of electromagnetic radiation in the form of blue light is emitted as a result of this reorientation
The intensity of the blue glow is directly proportional to the number of fissions occurring and the reactor power level

21. Cerenkov Radiation
Although most of the Cerenkov radiation is in the ultraviolet region, it is visible to us with a distinctive soft blue glow
The blue glow persists for a short time after the reactor has been shut down
This property may be used to inspect spent fuel to see if it is actually spent fuel or dummies used to mask a diversion of material

22. Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)