1. 3/2003 Rev 1 I.3.3 – slide 1 of 23 Part I Review of Fundamentals Module 3Interaction of Radiation with Matter Session 3Beta Particles Session I.3.3 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

2. 3/2003 Rev 1 I.3.3 – slide 2 of 23  In this session we will discuss the following as they relate to beta particle interactions  Mechanisms of Energy Transfer  Bremsstrahlung  Cerenkov Radiation  Shielding Overview

3. 3/2003 Rev 1 I.3.3 – slide 3 of 23 Ionizing radiation removes orbital electrons from atoms This creates an ion pair – an electron and the atom that has lost an electron Ionization

4. 3/2003 Rev 1 I.3.3 – slide 4 of 23 Ionizing radiation includes photons, but the result is the same – an ion pair is produced This section focuses on the electron interactions Ionization

5. 3/2003 Rev 1 I.3.3 – slide 5 of 23 Recall that unlike photons, electrons have a charge (-) and mass Electrons

6. 3/2003 Rev 1 I.3.3 – slide 6 of 23 Electrons are much lighter than the nucleons – the neutron and proton in the nucleus Electrons

7. 3/2003 Rev 1 I.3.3 – slide 7 of 23 All of the photon interactions  photoelectric effect  compton scattering  pair production Electrons result in the production of electrons. These are ionizing radiation just like beta particle sources

8. 3/2003 Rev 1 I.3.3 – slide 8 of 23  Electron interactions are comparable to those of other charged particles  More energetic electrons travel faster and so create a lower ionization density  Dose is the amount of energy deposited per mass of material (joules/kg)  Energetic electrons deposit less energy so the dose is lower until they slow down Electrons

9. 3/2003 Rev 1 I.3.3 – slide 9 of 23  In addition to the energy of the electron, the stopping power depends on the material in which the electron is interacting Electrons

10. 3/2003 Rev 1 I.3.3 – slide 10 of 23 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. 3/2003 Rev 1 I.3.3 – slide 11 of 23 Bremsstrahlung

12. 3/2003 Rev 1 I.3.3 – slide 12 of 23 The bremsstrahlung production of electrons is expressed as the mass radiative stopping power where BremsstrahlungdT  dx NAZ2NAZ2NAZ2NAZ2A dT =  o (E + m o c 2 )B r

13. 3/2003 Rev 1 I.3.3 – slide 13 of 23  0 = 5.8 x 10 -28 cm 2 /atom  N A = Avogadro’s number  Z = atomic number  A = atomic mass  E = kinetic energy of the electron in MeV  m 0 = rest mass of an electron  c = speed of light  B r = a function of E and Z (approximately 16/3 for E << 0.5 MeV; 6 for 1 MeV; 12 for 10 MeV Bremsstrahlung

14. 3/2003 Rev 1 I.3.3 – slide 14 of 23 The fraction of electrons producing bremsstrahlung follows the relationship: F = 3.5 x 10 -4 (Z)(E) Empirical Relationship

15. 3/2003 Rev 1 I.3.3 – slide 15 of 23 Shielding for Beta Sources

16. 3/2003 Rev 1 I.3.3 – slide 16 of 23 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. 3/2003 Rev 1 I.3.3 – slide 17 of 23 Cerenkov Radiation Reactor Spent Fuel Pool

18. 3/2003 Rev 1 I.3.3 – slide 18 of 23 Cerenekov Radiation Irradiator Source Rack

19. 3/2003 Rev 1 I.3.3 – slide 19 of 23  While no particle can exceed the speed of light in a vacuum (3.0x10 8 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 in a direction adjacent to its path thus distorting the local electric charge distribution Cerenkov Radiation

20. 3/2003 Rev 1 I.3.3 – slide 20 of 23  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  When the speed of a beta particle is less than the speed of light, the pulses tend to cancel themselves by destructive interference, however, when the speed of the beta particle is greater than the speed of light (in water) the pulses are amplified through constructive interference Cerenkov Radiation

21. 3/2003 Rev 1 I.3.3 – slide 21 of 23  The phenomenon is analogous to the acoustic sonic boom observed when an object exceeds the speed of sound in air  The intensity of the blue glow is directly proportional to the number of fissions occurring and the reactor power level  This property is utilized in Cerenkov detectors that measure the magnitude of Cerenkov radiation produced in a detector made of lucite Cerenkov Radiation

22. 3/2003 Rev 1 I.3.3 – slide 22 of 23  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 Cerenkov Radiation

23. 3/2003 Rev 1 I.3.3 – slide 23 of 23 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)