1. Radioactivity
GEOG/PHYS 182

2. Radioactive Decay – The process by which the nucleus of
an unstable atom loses energy by the emission of radiation
Radioactive decay was initially
discovered by Henri Becquerel (1896)
Nobel Prize (1903) -Becquerel, M. Curie, and P. Curie
SI unit of radioactivity: Bq (becquerel) -Defined as one transformation per second

3. Proton: p+
Mass = x kg
Charge = x C
Neutron: n0
Mass = x kg
Charge = 0 C
Electron: e-
Mass = 9.11 x kg
Charge = x C

4. Isotope – Same atomic number but different atomic mass
-Each isotope has the same atomic number (# of protons).
-Each isotope of an element has a different atomic mass.
-Atoms of these isotopes have a net electric charge of zero.

5. All elements heavier than lead are energetically unstable

6. Most Common Radioactive Decay Types:
1) Alpha Decay -Emission of alpha particle (helium nucleus) 2) Beta Decay – Two types: i) p+ changes to n0, plus ν (neutrino) and e+ (positron) emitted ii) n0 changes to p+, plus antineutrino and e- (electron) emitted
3) Gamma Decay
-Emission of a high energy photon (more energetic than X-rays)

7. Atomic mass
Number of protons

8. Neutron decays into a neutron + antineutrino + electron
Proton decays into a neutron + neutrino + positron

9. -Still stronger than gravity -Acts only over VERY short distances
Fundamental Forces:
Gravity
Electromagnetism
Weak
Strong (Nuclear)
Weak Force
-Still stronger than gravity
-Acts only over VERY short distances
-Mediated by W+ and Z bosons
-Responsible for beta decay
The daughter nucleus is usually left in an “excited state” and emits a gamma ray.

10. The half-life lab used an
isomer of Ba-137, which
was extracted from a
container of Cs-137.

11. Nuclear Fission – A heavy nucleus splits into a lighter one with other byproducts
such as neutrons which can
cause further fissions.
Nuclear Fusion – Lighter nuclei fusing together to create heavier nucleus with other byproducts

12. Stopping Power Alpha particles (He nuclei) can be stopped by a sheet
of paper or by your skin.
Beta particles (e- or e+) can be stopped by a thin metal shield
or by 1 cm of polyethylene.
Gamma particles (photons)
can be attenuated by THICK
slabs of lead.

13. Geiger-Mueller Counter
-Used to detect radiation -Ionizing radiation ionizes the detector gas -Each ionization event creates a pulse of current
which is then counted

14. Example Beta- decay (t1/2 = 30.1 years) Ba-137m to Ba-137
(t1/2 = 2.6 minutes)
Ba-137m is a metastable nucleus because it has too much energy. It decays into a lower energy state to Ba-137 (the same nucleus) and emits a photon γ (a “gamma ray”).
We can use the Geiger-Muller tube to detect this photon.

15. N0 = N(t = 0) = Number of radioactive nuclei present at t = 0
Characteristic time = τ -a measure of the decay rate
Half-life = t1/2 = τ*ln(2) = τ*(0.693)
-Time for half of the starting nuclei to decay

16. Practice Problem
Cesium has a half-life of years. What is its characteristic time?
t = t½ / = years
If a sample of Cesium has No = 100,000 radioactive nuclei
at t = 0 s, how many nuclei remain after 5 years?
N = No e –t/t = 89,149
Plot the number of nuclei remaining at 5 year intervals from 0 to 120 years.

17. Practice Problem
Suppose that a sample of radioactive material contains
No = 50,000 radioactive nuclei at t = 0 second. If the
half-life for the radioactive decay in this sample is
measured to be t½ = 12 minutes, how many nuclei remain
after t = 15 minutes?

18. Practice Problem A sample of radioactive material has half-life
t1/2 = 90 minutes. How long will it take 30,000 radioactive nuclei to decay to 20,000?