3/2003 Rev 1 I – slide 1 of 20 Part I Review of Fundamentals Module 3Interaction of Radiation with Matter Sessions 1-2Heavy Particles Session I IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

3/2003 Rev 1 I – slide 2 of 20  Upon completion of this section, the student should have an understanding of the following interactions for particles:  Bragg curve  Stopping power  Shielding Overview

3/2003 Rev 1 I – slide 3 of 20  When a charged particle interacts with an atom, it may:  traverse in close proximity to the atom (called a “hard” collision)  traverse at a distance from the atom (called a “soft” collision)  A hard collision will impart more energy to the material Particle Interactions

3/2003 Rev 1 I – slide 4 of 20 Bragg Curve - Alpha Alpha Particle Beta Path

3/2003 Rev 1 I – slide 5 of 20 Bragg Curve - Proton

3/2003 Rev 1 I – slide 6 of 20  Absorbed dose is energy imparted per unit mass of material:  The unit of absorbed dose is the Gray (Gy) (1 Gray = 1 joule/kg)  To calculate the dose from charged particles, we need to determine the amount of energy deposited per gram of material Absorbed Dose

3/2003 Rev 1 I – slide 7 of 20  The amount of energy deposited will be the sum of energy deposited from hard and soft collisions  The “stopping power,” S, is the sum of energy deposited for soft and hard collisions  Most of the energy deposited will be from soft collisions since it is less likely that a particle will interact with the nucleus Stopping Power

3/2003 Rev 1 I – slide 8 of 20  The stopping power is a function of the charge of the particle, the energy of the particle, and the material in which the charged particle interacts Stopping Power

3/2003 Rev 1 I – slide 9 of 20  Stopping power has units of MeV/cm – the amount of energy deposited per centimeter of material as a charged particle traverses the material  It is the sum of energy deposited for both hard and soft collisions. S = = + S = = + Stopping Power dEdx Tot dE s dx dE h dx

3/2003 Rev 1 I – slide 10 of 20 Mass Stopping Power  Often the stopping power is divided by the density of the material,   This is called the “mass stopping power”  The dimensions for mass stopping power are MeV – cm 2 g

3/2003 Rev 1 I – slide 11 of 20 Mass Stopping Power m 0 c 2 is the rest mass of the charged particle in MeV 3727 MeV for an alpha particle and MeV for an electron or beta particle MeV for an electron or beta particle S  = (0.3071) Zz 2 A2A2A2A ln -  2 – ln I 2222 (1-  ) 2 moc2moc2moc2moc2 E ½  = = = =

3/2003 Rev 1 I – slide 12 of 20 Stopping Power  Z = atomic number  z = charge of the particle (  = 2,  = 1)  m 0 = rest mass of the particulate radiation  c = speed of light (3 x cm/s)  I = the mean excitation potential of an atom of the absorbing material (2.16 x ) (Z)

3/2003 Rev 1 I – slide 13 of 20 Stopping Power Stopping power is used to determine dose from charged particles by the relationship: D =  in units of MeV/g, where  =the particle fluence, the number of particles striking an object over a specified time interval dE  dx

3/2003 Rev 1 I – slide 14 of 20 Stopping Power Converting this to units of dose results in the relationship: D =  (1.6 x ) Gy dE  dx

3/2003 Rev 1 I – slide 15 of 20 Tissue Equivalent Stopping Power for Electrons Energy (MeV) Mass Stopping Power, S/  (MeV-cm 2 )/g

3/2003 Rev 1 I – slide 16 of 20 Stopping Power Example Calculate the dose from a 37,000 Bq source of 32 P spread over an area of 1 cm 2 on the arm of an individual for 1 hour D =  (1.6 x ) Gy Assume that 50% of the particles on the skin interact with the skin (2  geometry) dE  dx

3/2003 Rev 1 I – slide 17 of 20  = (½)(37,000 Bq)(1 dis/s/Bq)(1 hr)(3600 s/hr) = 6.67 x 10 7 dis 32 P has a 0.6 MeV beta particle (average energy) For tissue equivalent plastic and a beta particle with an energy of 0.6 MeV, the stopping power is 1.96 MeV-cm 2 /g Stopping Power Example

3/2003 Rev 1 I – slide 18 of 20 D = (6.67 x 10 7 dis)(1.96 MeV-cm 2 )(1.6 x ) D = Gy D =  (1.6 x ) Gy dE  dx 1 cm 2 g Stopping Power Example

3/2003 Rev 1 I – slide 19 of 20 Shielding

3/2003 Rev 1 I – slide 20 of 20 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)