Radiation Physics PHY471 A.M. El-Khayatt A. M. El-Khayatt, PHY464 Autumn 2014 Radiological physics is the science of ionizing radiation and its interaction with matter, with special interest in the energy thus absorbed James E. Turner :Atoms, Radiation, and Radiation Protection

Introduction Radiological physics studies ionizing radiation and its interaction with matter. Began with discovery of x-rays, radioactivity and radium in 1890s. Special interest is in the energy absorbed in matter Radiation dosimetry deals with quantitative determination of the energy absorbed in matter. A. M. El-Khayatt, PHY464 Spring 2014

Ionizing radiation By general definition ionizing radiation is characterized by its ability to excite and ionize atoms of matter Lowest atomic ionization energy is ~ eV, with very little penetration Energies relevant to radiological physics and radiation therapy are in keV – MeV range A. M. El-Khayatt, PHY464 Spring 2014

Chapter 4:Radioactive Decay Activity Activity The rate of decay → the number of atoms that decay per unit time A. M. El-Khayatt, PHY464 Spring 2014 The traditional unit of activity is the curie (Ci), which was originally the activity ascribed to 1 g of 226 Ra.

Exponential Decay The activity of a pure radionuclide decreases exponentially with time. A. M. El-Khayatt, PHY464 Spring 2014

30-MBq2.5 d Example: Calculate the activity of a 30-MBq source of after 2.5 d. What is the decay constant of this radionuclide? Solution A. M. El-Khayatt, PHY464 Spring 2014

Specific Activity SA(Bqg -1 ) A. M. El-Khayatt, PHY464 Spring 2014 The specific activity of a nuclide of half-life T (year) and atomic mass number A is therefore given by Example: Calculate the specific activity of 226 Ra in Bq g -1. This, by definition, is an activity of 1 Ci.

Example What is the specific activity of I4 C(T=5730Y)? A. M. El-Khayatt, PHY464 Spring 2014 Example: A sample contains 1 mCi of 191 Os at time t = 0. How many grams of 191 Os are present at t = 0? The mass of the sample, therefore, is

4.4 Serial Radioactive Decay The activity of a sample in which one radionuclide produces one or more radioactive offspring in a chain. A. M. El-Khayatt, PHY464 Spring 2014 Secular Equilibrium (T 1 T 2 ) At secular equilibrium ≥7 T 2

A. M. El-Khayatt, PHY464 Spring 2014 A chain of n short-lived radionuclides can all be in secular equilibrium with a long-lived parent. Then the activity of each member of the chain is equal to that of the parent and the total activity is n + 1 times the activity of the original parent. Transient Equilibrium (T 1 T 2 ) At Transient equilibrium ≥7 T 2

A. M. El-Khayatt, PHY464 Spring 2014 Example: Starting with a 10.0-GBq (= Bq) sample of pure 90 Sr at time t = 0, how long will it take for the total activity ( 90 Sr + 90 Y) to build up to 17.5 GBq? Solution (T 1 T 2 ) → Secular Equilibrium

Example How many grams of 90 Y are in secular equilibrium with 1 mg of 90 Sr? A. M. El-Khayatt, PHY464 Spring 2014 The amount of 90 Y will be that having the same activity as 1 mg of 90 Sr. The specific activity, SA, of 90 Sr (T1 = y) is

A. M. El-Khayatt, PHY464 Spring 2014 Example page 111 A sample contains 1 mCi of 191 Os at time t = 0. The isotope decays by β– emission into metastable 191m Ir, which then decays by γ emission into 191 Ir. The decay and half-lives can be represented by writing (a) How many grams of 191 Os are present at t = 0? (b) How many millicuries of 191m Ir are present at t = 25 d? (c) How many atoms of 191m Ir decay between t = 100 s and t = 102 s? (d) How many atoms of 191m Ir decay between t = 30 d and t = 40 d?

Natural Radioactivity All of the heavy elements (Z > 83) found in nature are radioactive and decay by alpha or beta emission. The heaviest elements decay into successive radioactive daughters, forming series of radionuclides that end when a stable species is produced. (n+2)The uranium series, for example, begins with 238 U-92 and ends with stable 206 Pb-82. (n+0)The thorium series, starting with Th and ending with 208 Pb-82. (n+3) the actinium series, which begins with 235 U-92 and ends with 207 Pb-82. (n+1)neptunium series 237 Np-93, has a half-life of 2.2 × 10 6 years, which is short on a geological time scale. Neptunium is not found in nature, but has been produced artificially, starting with 241 Pu-93 and ending with 209 Pb-82. A. M. El-Khayatt, PHY464 Spring 2014

Example How many alpha and beta particles are emitted by a nucleus of an atom of the uranium series, which starts as U and ends as stable Pb? Solution Nuclides of the four heavy-element radioactive series decay either by alpha or beta emission. A single disintegration, therefore, either (1) reduces the atomic number by 2 and the mass number by 4 or (2) increases the atomic number by 1 and leaves the mass number unchanged. Since the atomic mass numbers of U and Pb differ by 32, it follows that 8 alpha particles are emitted in the series. Since this alone would reduce the atomic number by 16, as compared with the actual reduction of 10, a total of 6 beta particles must also be emitted. A. M. El-Khayatt, PHY464 Spring 2014