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

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

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

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

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

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

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

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)

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

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

Bremsstrahlung

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 0.156 MeV and 0.018 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.

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

Bremsstrahlung Carbon-14 and tritium (hydrogen-3), are not likely to produce bremsstrahlung due to their low energy beta particles of 0.156 MeV and 0.018 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

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

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

Reactor Spent Fuel Pool Cerenkov Radiation Reactor Spent Fuel Pool

Irradiator Source Rack Cerenkov Radiation Irradiator Source Rack

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: http://nova.nuc.umr.edu/~ans/cerenkov.html

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

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

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)