Lecture 26 Radiation and Radioactivity

LECTURE 26 RADIATION AND RADIOACTIVITY

Instructor: Kazumi Tolich Lecture 26

¨ 30.4 Radiation and radioactivity ¤ Alpha decay ¤ Beta decay ¤ Gamma decay ¤ Decay series ¤ Nuclear radiation is a form of ionizing radiation

¨ 30.5 Nuclear decay and half-lives ¤ Activity ¤ Radioactive dating Quiz: 30.4-1

3 ¨ A uranium nucleus U undergoes an alpha decay. What is/are its decay product(s)? Choose all that apply. A. He B. U C. U D. Th E. Pu F. Th Quiz: 30.4-1 answer

¨ In an alpha decay, a nucleus decays by emitting an alpha particle. It loses two protons and two neutrons:

X → Y + He + energy

¨ Here, X is the parent nucleus and Y is the daughter nucleus. ¨ U → Th + He + energy ¨ The energy released in an alpha decay ends up mostly as the kinetic energy of the alpha particle. Alpha decay results in more stable nuclei � ≈ ∆� = � − � − � � Quiz: 30.4-2

5 ¨ What is the daughter nucleus when a potassium nucleus K undergoes a �-decay? A. Ca B. K C. Cl D. Sc E. Ar F. K Quiz: 30.4-2 answer

¨ In a � -decay, a neutron is converted into a proton, an electron and an antineutrino: n → p + e + �̅ ¨ The charge is conserved. ¨ A nucleus that undergoes a � -decay loses a neutron and gains a proton: � → � + e + �̅ ¨ K → Ca + e + �̅

¨ Beta decay occurs only if � > �. ¨ If a nucleus undergoes a � -decay, a proton has become a neutron emitting a positron and a neutrino : � → � + e + � Quiz: 30.4-3

¨ An excited nucleus undergoes a gamma decay. How do the atomic number and neutron number change? Choose all that apply. A. The atomic number increases by 1. B. The atomic number decreases by 1. C. The atomic number remains the same. D. The neutron increases by 1. E. The neutron decreases by 1. F. The neutron number remains the same. Quiz: 30.4-3 answer

¨ The atomic and neutrons numbers both remain the same.

¨ Alpha and beta decays are often followed by a gamma decay.

¨ A gamma ray is a photon emitted by a nucleus as it returns to its ground state. Quiz: 30.4-4

31 32 ¨ Phosphorus has one stable isotope, P. The iso to p e P is a neutron- rich radioactive isotope that is used in nuclear medicine. What is the likely daughter nucleus of 32P decay? A. 32S B. 32Si C. 28Si D. 31P E. 28Al Quiz: 30.4-4 answer

¨ Cloud Chamber ¤ Tracks of alpha particles.

¨ Geiger counter ¤ Comparison between � decays and � decays. n � rays are more penetrating. ¤ Salt substitute is radioactive! (and so are many others, like a banana) 30.4 Decay series

¨ The sequence of isotopes, starting with the 235U decay series. original unstable isotope and ending with the stable isotope, is called the decay series. Quiz: 30.4-5

¨ The apple is irradiated for 2 hours by a very strong radioactive source. Is the apple now radioactive? A. Yes B. No C. It depends on what kind of radiation it is. Quiz: 30.4-5 answer

¨ No

¨ The energies of alpha and beta particles and the gamma-ray photons have high enough energy to ionize atoms and to break molecular bonds.

¨ Ionizing radiation causes damage to the body by driving chemical reactions or damaging DNA molecules, which can create a mutation or a tumor.

¨ However, the radiation does not change the nuclei in the apple. 30.5 Nuclear decay and half-lives

¨ Start with � unstable nuclei, the number of nuclei � remaining at time � is given by

1 ⁄⁄ � = � = � �⁄ 2

where half-life �⁄ is the half-life (average time required for one-half the nuclei to decay), and � = ⁄ is the time constant.

¨ No matter how many nuclei there are at any point in time, the number decays by half during the next half-life. Quiz: 30.5-1

¨ You have 400 g of a radioactive sample with a half-life of 20 years. How much in grams is left after 50 years? Quiz: 30.5-1 answer

¨ You have 400 g of a radioactive sample with a half-life of 20 years. How much is left after 50 years? ⁄⁄ ⁄ ¨ � = � = 400 g = 71 g Quiz: 30.5-2

¨ A radioactive isotope has a half-life of 10 s. You are observing a sample of this isotope. After approximately one minute of observation, there is only one nucleus of this isotope left in your sample. How many atoms of this isotope will be left in your sample 15 s later? A. Definitely 1. B. Approximately 0.35. C. Definitely 0. D. Possibly 1 but probably 0. E. Possibly 0 but probably 1. Quiz: 30.5-2 answer

¨ Possibly 1 but probably 0.

¨ As with other quantum processes, the time any particular nucleus will decay cannot be predicted. ⁄⁄ ⁄ ¨ � = � = 1 = 0.35 ¨ But, nuclei are quantized, so you can either have 1 nucleus or 0 nuclei. 30.5 Activity

¨ The activity � of a radioactive sample in terms of the number of nuclei � and the time constant �, or in terms of the initial activity � is

� 1 ⁄⁄ � = = � = � �⁄ � 2

¨ The units of activity are either curie (Ci) or becquerel (Bq). ¤ 1 Ci = 3.7 × 10 decays⁄s ¤ 1 Bq = 1 decay⁄s Quiz: 30.5-3

¨ You have a radioactive sample A with a half-life of 100 years, and another sample B with a half-life of 1000 years. Both samples have the same number of radioactive nuclei. Which sample has the higher activity? A. sample A B. sample B C. both the same D. impossible to tell Quiz: 30.5-3 answer

¨ Sample A

¨ If a sample has a shorter half-life, this means that it decays more quickly. ⁄ ¨ � = ¨ � = = = ⁄ ⁄ ¨ Activity is inversely proportional to the half-life.