1. Radioactivity

2. Radiation
Radiation: The process of emitting energy in the form of waves or particles.
Where does radiation come from?
Radiation is generally produced when particles interact or decay. A large amount comes
from the sun (solar) or from radioactive isotopes of the elements on earth. (terrestrial).
Radiation is going through you at this very moment!

3. Radioactivity
By the end of the 1800s, it was known that certain isotopes spontaneously emit penetrating rays. Three types of radiation were known:
Alpha particles (a)
Beta particles (b)
Gamma-rays (g)

4. How are chemical reactions different than nuclear reactions?
Radioactive atoms change into atoms of another element when they emit radiation!
In a chemical reaction, ATOMS DO NOT CHANGE INTO ATOMS OF OTHER ELEMENTS, the atoms of the elements they just rearrange and form new substances.

5. The decay chain of Uranium produces all three of these forms of radiation changing into many different types of elements!

6. So what is an Isotope and do they all spontaneously emit radiation?
What’s an isotope?
An element that has the same number of protons but different number of neutrons.
ONLY about 17% of all isotopes decay. Certain isotopes are “unstable” and decay to lighter isotopes or elements.
Let’s discuss how we can tell which isotopes are unstable and emit radiation next.
To be more clear, deuterium contains 1 proton and 1 neutron in the nucleus, and tritium contains 1 proton and 2 neutrons in its nucleus. Both isotopes behave similarly to ordinary hydrogen, as this chemical behavior is mostly driven by the atomic electrons.

7. Where do these particles come from ?
From the nuclei of atomic isotopes which are not stable.
Why are they unstable?
The nucleus contains protons + neutrons. The protons are positively charged and repel each other due to ELECTROSTATIC REPULSION FORCES.
However, the neutral neutrons can shield the protons from each repelling one another.
Also, there is something called STRONG NUCLEAR FORCES that act on p+ that are close together and overcomes the electrostatic repulsion. It even effects neutrons and holds them together!

8. So how can you predict which isotopes are unstable?
Look at their neutron to proton ratio (n/p).
Stable n/p ratios are 1:1 to about 1.5/1 ratios. Anything more than 1.5/1 ratio is unstable.
Ex. Helium has 2 neutrons & 2 protons. The n/p ratio is 2/2 = 1/1 ratio therefore it is stable
Ex. Pb-206 has 124 neutrons & 82 protons. The
n/p ratio is 124/82 = 1.51/1 ratio so it is unstable

9. Note: This is the atomic weight, which is the number of protons plus neutrons
Alpha Particles (a)
Radium
22688R
Radon
22286Rn + p n n p
a (42He)
88 Protons =
138 neutrons
86 protons
136 neutrons
2 protons
2 neutrons
Note: The 226 refers to the atomic weight, which is the equal to the number of protons plus neutrons
The alpha-particle (a) is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !

10. Beta Particles (b) Carbon 0-1B Nitrogen + 6 Protons = 8 neutrons
electron
(beta-particle)
We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus.
WAS THE EQUATION DONE RIGHT? CHECK OUT YOUR MATH ON
BOTH SIDES OF THE EQUATION: THE MASS NUMBERS SHOULD BE
EQUAL = ON BOTH SIDES OF THE EQUATION, AND THE ATOMIC
NUMBERS SHOULD BE EQUAL TOO.
Note that in beta decay, the atomic mass not change, since the neutron and proton have nearly the same mass…

11. Gamma particles (g)
In much the same way that electrons in atoms can be in an excited state, so can a nucleus.
Neon
Ne20
Neon
Ne20 +
10 protons
10 neutrons (in excited state)
10 protons
10 neutrons (lowest energy state)
gamma
A gamma is a high energy light particle.
It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

12. Gamma Rays
Neon
Ne20
Neon
Ne20 +
The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!)

13. How do these particles differ ?
Mass* (MeV/c2)
Charge
Gamma (g)
Beta (b)
~0.5 -1
Alpha (a)
~3752 +2
* m = E / c2

14. Rate of Decay
Beyond knowing the types of particles which are emitted when an isotope decays, we also are interested in how frequently one of the atoms emits this radiation.
A very important point here is that we cannot predict when a particular entity will decay.
We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time.
Some radioactive substances have a very high “rate of decay”, while others have a very low decay rate.
To differentiate different radioactive substances, we look to quantify this idea of “decay rate”

15. Half-Life
The “half-life” (h) is the time it takes for half the atoms of a radioactive substance to decay.
For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after:
Time
#atoms remaining
% of atoms remaining
1 hour (one lifetime) ?
10, (50%)
2 hours (two lifetimes) ?
5, (25%)
3 hours (three lifetimes) ?
2, (12.5%)