LECTURE 26 RADIATION AND RADIOACTIVITY

Transcription

1 LECTURE 26 RADIATION AND RADIOACTIVITY 30.4 Radiation and radioactivity Decay series Nuclear radiation is a form of ionizing radiation 30.5 Nuclear decay and half-lives Activity Radioactive dating Radiocarbon dating (it s romantic) can date a previously living organisms.

2 Quiz: ! Phosphorus has one stable isotope, 31 P. The isotope 32 P is a neutronrich radioactive isotope that is used in nuclear medicine. What is the likely daughter nucleus of 32 P decay? A. B. C. D. E. 32 S 32 Si 28 Si 31 P 28 Al

3 Quiz: answer! Phosphorus has one stable isotope, 31 P. The isotope 32 P is a neutronrich radioactive isotope that is used in nuclear medicine. What is the likely daughter nucleus of 32 P decay? A. 32 S

4 30.4 Alpha, beta, and gamma decays demo 4! Cloud Chamber! Tracks of alpha particles.! Geiger counter! Comparison between! decays and " decays. " " rays are more penetrating.! Salt substitute is radioactive! (and so are many others, like a banana)

5 30.4 Decay series! The sequence of isotopes, starting with the original unstable isotope and ending with the stable isotope, is called the decay series. 235 U decay series.

6 Quiz: ! 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.

7 Quiz: 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.! Food irradiation is used to preserve food, reduce the risk of food borne illness, and prevent the spread of pests.! Irradiation is also used for non-food applications, such as medical devices.

8 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 0 = ( ) * is the time constant. 12 /! No matter how many nuclei there are at any point in time, the number decays by half during the next half-life.

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

10 Quiz: answer 10! 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

11 Quiz: ! 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 C. Definitely 0. D. Possibly 1 but probably 0. E. Possibly 0 but probably 1.

12 Quiz: answer 12! 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?! 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.

13 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 = $ decays s! 1 Bq = 1 decay s

14 Quiz: ! 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

15 Quiz: answer 15! 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.

16 30.5 Radioactive dating 16! If you know how much of a radioactive material has decayed, you can read the elapsed time from the decay curve.! The half-lives of various nuclei can vary widely.

17 30.5 Radioactive dating - radiocarbon dating 17!!!!! "#!C % -decays with a half-life of 5730 years. Radiocarbon dating uses "#!C produced in the upper atmosphere during nuclear reactions caused by cosmic rays. "# "& "# "&!C reacts just like!c chemically, and the ratio of!c to!c in a living organism is the same as the equilibrium ratio in the atmosphere, "#!C "&!C = "&. After an organism dies, it no longer absorbs carbon, so the ratio of "#!C to "&!C continually decreases since "#!C decays. The decay rate per amount of carbon in a sample therefore yields the age of the previously living organism.

18 Quiz: ! Suppose you have a sample of old dead tree that is believed to be 100 thousand years old. Can you use carbon-14 to accurately date this sample? A. Yes. B. No.

19 Quiz: answer 19! No! The range of dates that can be determined depends on the half-life of the material.! If the time is much shorter than the half-life, not enough of the material will have decayed.! If the time is much longer than about ten half-lives, too much of the material will have decayed, and it will no longer be detectable.! Radiocarbon dating is generally limited to dating samples no more than 50,000 years old, as samples older than that have insufficient 14 C to be measurable.! # $ = 5730 y

20 For fun: Refining radioactive dating 20! There are also many nonradioactive methods of dating materials, such as tree ring analysis.! The radioactive and nonradioactive methods may be used to check each other and refine the procedure, increasing accuracy of dating.! The "#! C method gives lifetimes that are too short after about 10,000 years, probably due to fluctuations in the "#! C to "%! C ratio.

21 For fun: Age of Earth 21! The age of Earth is measured to be around 4.54 billion years using radioactivity of rocks and meteorites.! The type of the rocks used is known to reject lead during its formation.! The method uses two series of α and β decays:! "#$!" U (& ( " = 4.47 billion years ) to $"! "#-!" U (& ( " = 704 million years ) to $" ")* Pb. "). Pb.! The age of Earth is determined from the Pb to U ratios.