1. Nuclear Radioactive Decay
Atoms: Nuclear Interactions

2. Nuclear Stability
The nucleus consists of protons and neutrons that are held together by the nuclear force – a strong force of attraction between nucleons.
Nucleus stability is determined by the ratio between the neutrons and protons.
For elements up to Z=20, the ratio between neutrons and protons is between 1:1 and 1.1:1
Above Z=20, the ratio increases to 1.5:1
There are no stable nuclides above element 83

3. Nuclear Stability
“magic numbers” – number of nuclear particles that shows stability: 2, 8, 20, 28, 50, 82 and 126
There also seems to be greater stability when the protons and neutrons both are even numbered and much less stability when both are odd numbered
157 52 50 5
Number of protons
Even
Odd
Number of neutrons

4. Nuclear Stability 39 19 20 19 Neutrons Protons K The ratio = =
Calculate the stability (possible radioactivity) of the following isotopes: 39 19 20 19
Neutrons
Protons K
The ratio = =
= 1.05:1
Stable 14 6 8 6
Neutrons
Protons C
The ratio = =
= 1.33:1
Unstable
208 84 Po
The element is greater than Z = 83
Unstable

5. Natural Radioactive Decay
There are 5 main types of radioactive decay:
1. Alpha Emission (α) – the emission of a He nucleus
This occurs for all elements Z > 83
Conservation of Matter: = 88 and = 226 4 2
226 88 4 2
222 86
Ra → He Rn
234 90 4 2
230 88
Th →
He + Ra

6. Natural Radioactive Decay-1
2. Beta Emission (β or e) – emission of a high speed electron
This occurs when the N/Z (neutron/proton) ratio is too large
Notice the conservation of matter again: 6 =
The ratio for carbon-14 should be 1:1 since it is Z < 20, but the ratio is 8:6 or 1.33:1 making it unstable – the final ratio of nitrogen-14 is 1:1, making it stable 14 6 -1 14 7
C → e N

7. Natural Radioactive Decay1
3. Positron Emission (β+ or e) – emission of positron
This occurs when the N/Z ration is too small
Notice the ratio of carbon-11 is 5:6 or 0.8:1 which is smaller than it should be, making it unstable 95 43 1 95 42
Tc → e Mo
= ? 11 6 1 11 5
C → e + B
= ?

8. Natural Radioactive Decay
4. Electron Capture (EC) – unstable nucleus captures an electron
In essence a proton is changed into a neutron
5. Gamma Emission (γ) – emission of a gamma photon from an excited nucleus (metastable) 1 -1 1
p e → n
99m 43 99 43
Tc → Tc γ

9. Natural Radioactive Decay
Decay Type
Symbol
Change in Atomic #
Change in Atomic Mass
Change in # of Neutrons
Alpha
α or He -2 -4
Beta
Β or e +1
No change -1
Positron
β+ or e
Electron Capture EC
Gamma γ 4 2 -1 1

10. Natural Radioactive Decay
The penetrating ability of
alpha (α), beta (β) and
gamma (γ) rays. Gamma rays
are the most penetrating; alpha
particles are the least.
Alpha (α) is stopped by paper
Beta (β) is stopped by aluminum
Gamma (γ) is stopped by lead
sheet or concrete block

11. Radioactive Decay Sequences
A decay sequence is one where the daughter nuclide (product) is still unstable and continues to decay
Thorium-232 decay sequence
Uranium-238 decay sequence