Lesson 28: Radioactive Decay! ! Radioactive decay is a process in which an unstable nucleus undergoes change by emitting radiation. ! ! Radiation is a...
Lesson 28: Radioactive Decay! ! Radioactive decay is a process in which an unstable nucleus undergoes change by emitting radiation. ! ! Radiation is a general term for anything that can be radiated out of the nucleus. Radiation can come in the form of particle ( ) or energy ( ) emission. !
!
Radioactivity was discovered by accident by Henri Becquerel in 1896. Becquerel placed a rock containing uranium on a piece of photographic paper inside a dark drawer. He found that the paper developed an image as if it has been exposed to a strong light source. !
! Common types of radiation ! ! Antimatter:! ! ! !
Beta negative particle
!! !! !! !! !! ! !! !! !! !! !! !
Beta positive particle
alpha particle
neutrino
antineutrino
gamma photon
Nuclear Equations!
!
Nuclear equations are written like chemical equation, with the original material on one side of the equation and the resulting material on the other. The two sides are separated by an arrow. !
! ! ! ! ! ! !
Conservation laws and nuclear equations:! Conservation of nucleons
If you find the total mass of the parent isotopes for a nuclear decay and compare it to the total mass of the daughter isotopes, there will often be some missing mass. This mass has been converted into energy, which is released in the kinetic energy of the alpha and beta particles as well as gamma radiation.!
!
Example:! ! ! ! !
Calculate the energy released during the alpha decay of - Mass of thorium-230 is 230.033134 u! - Mass of radium-226 is 226.025410 u! - Mass of alpha particle is 4.002603 u!
230 90
Th
!
! ! ! ! ! ! ! ! ! ! ! ! ! ! Radiation Hazards! !
Ionizing radiation consists of particles that have sufficient energy to knock an electron out of an atom or molecule, thus forming an ion. A single alpha or beta particle has enough energy to ionize thousands of molecules.! Nuclear radiation is potentially harmful to humans because the ionization it produces can significantly alter the structure of molecules with in a living cell. !
!
Risks of radiation exposure:! -!
! -! ! -! ! -! ! -! !
Type of Radiation Alpha Beta Gamma
Mass
Speed
Penetrating Power
Ionization
Radioactive Decay rates!
!
A stable nucleus is one that is much less likely to undergo radioactive decay. ! In order for a nucleus to be stable, there must be a balance between the number of protons and the number of neutrons in the nucleus. !
Example: Polonium-210 has a half life of 138 days. After 3.8 years, how much of a 765 g sample of polonium-210 is left?!
! ! ! ! !
Example: The half-life of a radioactive isotope is 6.8 years. If the activity of the original sample of this isotope was 4.9 x 105 Bq, what would its activity be after 100 years?!
! ! ! ! ! !
Example: You have 75 g of lead at the start of an experiment. How many half lives have passed when 9.3 g remains?!
! ! ! ! ! ! !
! Practice Problems:! ! Remember 1 u = 1.660539 x 10 kg! ! 1. Write the alpha-decay process for ! -27
234 91
Pa
.!
2. Which type of beta decay transmutes carbon-14 into nitrogen? Write the process for this decay.! 3. Which type of beta decay transmutes the sodium isotope decay.!
22 11
Na
into
22 10
Ne
? Write the process for this
!
4. Explain whether the atomic number can increase during nuclear decay. Support your answer with an example. !
! 5. What isotope will β decay of thallium-202 produce? Write the process for this decay.! ! +
6.!
Following are some equations of artificial transmutations produced by particle bombardment. Using a periodic table, if necessary, determine the other product in each case.! 27 13
a.!
12 6
b.!
9 4
c!.!
1 1
e.!
!
7.!
g.!
14 7
d.!
!
! ! ! !
1 0
14 6
1 0
n → ?
! !
H
C
! !
!
2 1
Na + H → ? + 4 2
N + He → ? +
1 1 1 1
H
!
H
!
In each of the following equations, determine the emitted particle(s) in each case.!
c.!
!
2 1
H +
H
H
1 1
N+ n → ? +
f.!!
b.!
!
1 1
C+ H → ? +
23 11
a.!
8.!
2 1
1 1
Be + H → ? +
14 7
d.!
Al + 42 He → ? +
!
222 86
238 92 34 17
Rn →
U→
Cl →
218 84
Po + ?
234 90
Th + ?
!
e.!
!
f.!
35 16
S + ?! 226 222 88 Ra → 86 Rn + ? !
g.! h.!
141 57
La →
141 58
138 60
Nd →
138 59
212 82
Ce + ?
Pb →
Pr + ?
!
!
212 83
Bi + ? ! 215 211 84 Po → 82 Pb + ? !
Radium-226 decays to polonium-214 as follows:!
! 226 88
(a)!
Ra →
222 86
(b)!
Rn →
(c)! 218 84
Po →
(d)! 214 82
Pb →
(e)! 214 83
Bi →
214 84
Po !
What kind of particle is emitted in each of the transmutations labeled (a) to (e).!
9. Find the energy in MeV released when alpha decay converts thorium-228 (228.028715 u) into radium-224 (224.020186 u). The mass of an alpha particle is 4.002603 u.!
! ! ! !
! 10. Find the quantity of nuclear energy liberated in each of the following nuclear reactions: ! ! 235
!
! ! !
! !
1
U
n
92 0 ! ! ! +! (235.043925 u) (1.00867 u)
3 2
He !
3
—>!
140 54
Xe
94
+!
2 1H ! ! (1.00728 u) !
Sr
38 ! +! !+ ! (139.92161 u) (93.915367 u)!
4
2 He ! 2 He ! +! —>! (3.01603 u)! (3.01603 u)! (4.00260 u)!
1
n
20 ! (1.00867 u) !
1
(Answers: 2.96 x 10-11 J, 2.2267 x 10-12 J)!
!
11. The half-life of a radioactive isotope is 2.5 years. If the activity of the original sample of this isotope was 3.2 x 103 Bq, what would be its activity after 5.0 years? [8.0 x 102 Bq]!
!
12. What percentage of a polonium-210 sample will remain after 172 days if it has a half-life of 138 days? [42.2%]!
!
13. A 2.0 g sample of a radioactive isotope undergoes radioactive decay. If the half-life of this isotope is 45 mins, how much of this isotope remains after 5.0 hours. [0.020 g]!
!
14. In 9.0 days the number of radioactive nuclei decreases to one-eighth the number initially present. What is the half-life (in days) of the material? [3 days]!
!
15. Strontium-82 has a half-life of 25.0 d. If you begin with a sample having a mass of 140 g, in how many days will you have only 17.5 g of strontium-82 left? [75 days]!
! !
16. The half-life of a radioactive isotope is 2.5 years. If the activity of the original sample of this isotope was 3.2 x 103 Bq, what would its activity be after 5.0 years? [8.0 x 102 Bq]!
