Radioisotopes

1. Radioisotopes The nuclei of some atoms are unstable and undergo spontaneous changes called radioactive decay. One such change is called beta decay. During beta decay a neutron changes into a proton and an electron transforming the atom to an element with an atomic number which is one higher while the atomic mass barely changes.

2. + Tritium atoms, H-3, undergo spontaneous beta decay. Shown below is a tritium nucleus. 1 proton 2 neutrons

3. The highly energetic electron is ejected from the nucleus as radiation. It travels at a speed of 1.3 x 105 km/s. The equation is: 3 3 0 1H 2He 1-e + + 2 proton 1 neutron 1 electron + v v v v v

4. Two other forms of radiation from radioactive decay are: alpha particle emission and gamma rays. An alpha particle contains 2 protons and 2 neutrons while gamma rays do not result in the release of particles. The rate of release of radiation is expressed as a half-life. A half-life is the length of time required for half of the original material to decay.

5. Tritium has a half-life of 12.26 years. 12.26 a (annum is latin for years) If 10 g of tritium were left for 12.26 a there would be 5 g left. After 24.52 a there would be 2.5 g left. Here is a table showing the quantity of tritium remaining after different time periods.

8. Here is an example of Alpha decay. Alpha decay involves the emission of a helium-4 nucleus. Write an equation which shows how uranium-235 undergoes alpha decay. 2 3 1 T h 9 0 +

9. Different radioactive isotopes decay at different rates. If 100 g of a radioactive material decays for 10 years and 50 g remains this substance is said to have a half life of 10 years. If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g 5 y 5 y After 10 y only 50 g remain

10. 200 g If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g ------->

11. 100 g left after 5 years If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g

12. 50 g left after 10 years If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g

13. 25 g left after 15 years If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g ----> 25g

14. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g

15. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g 25 da 256 g

16. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g 25 da 25 da Total - 50 da 128 g

17. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g 25 da 25 da 25 da Total - 75 da 64 g

18. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da Total - 100 da 32 g

19. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da 25 da Total - 125 da 16 g 16 g

20. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da 25 da Total - 150 da 16 g 25 da 8 g 8 g

21. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da 25 da Total - 175 da 16 g 25 da 8 g 25 da 4 g 4 g

22. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da 25 da Total - 200 da 16 g 25 da 8 g 25 da 25 da 2 g 2 g 4 g

23. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da 25 da 25 da 25 da 25 da Total - 225 da 16 g 25 da 8 g 25 da 25 da 25 da 1 g 1 g 2 g 4 g

24. If U-235 has a half-life of 7.1 x 108 y. How many years would it take 32 g to decay to 2 g? 32 g --> 16 g --> 8 g --> 4 g --> 2 g 4 half lifes 2.84 x 109 y. Cs-136 has a half-life of 13 da. If 1024 g was left to decay for 65 da how much of the original material would be left? 65/13 = 5 hl 1024 g -> 512 g -> 256 g -> 128 g -> 64 g -> 32 g or 1024 g x (1/2)5 = 32 g

25. To find the quantity of material remaining use this formula # of Half-lives # of Half-lives 1 1 2 2 Original Mass Mass remaining x = Original Mass Mass remaining x = Pb-212 has a half-life of 10.6 h. If 12.5 g of Pb-212 is left for 84.8 h how much of the original material is left? 12.5 g x (0.5)84.8/10.6 0.0488 g =