http://www.ck12.org Chapter 25. Nuclear Physics
First, we find the difference between the mass of the polonium atom and sum of the masses of the lead atom and the
alpha particle.
∆m=mpo−mpb−mα= 209. 982848 u− 205. 974440 u− 4. 002602 u= 0. 005806 u→( 0. 005806 u) 1. 6605 × 10 −^27
kg
u= 9. 640863 × 10 −^30 kg→E=∆mc^2 = 9. 640863 × 10 −^30 kg(
2. 997 × 108
m
s) 2
= 8. 65943 × 10 −^13 → 8. 659 × 10 −^13 J
b. Express the result of part (a) in MeV.
Solution:
E=^8.^659 ×^10
− (^13) J
- 60 × 10 −^19 eVJ =^5.^41 ×^10
(^6) eV→ 5. 41 MeV
Learn more about alpha decay by following the link below.
http://phet.colorado.edu/en/simulation/alpha-decay
Beta decay
Beta decay occurs when a neutron within the nucleus changes into a proton by emitting an electron. This is not
so remarkable since a neutron, removed from a nucleus, has a mean life-time of about 15 minutes. Free neutrons
decay into a proton, an electron, and a subatomic particle called an antineutrino(ve). Neutrino (or antineutrino)
means “small neutral one,” in Italian. It is an elementary particle with a mass much smaller than that of the electron.
Neutrinos hardly interact with matter. In fact, as you sit and read this, trillions of neutrinos are passing through you
without interacting with the atoms that make your body.
Note: The electron emitted during beta decay is not an orbital electron but an electron emitted from the nucleus of
the atom. We often call these electrons, beta(β)particles.
An example of beta decay is the transformation of sodium
( 24
11 Na)
into magnesium( 24
12 Mg)
.
24
11 Na→
24
12 Mg+β+ve
Notice that the mass number(A)during the transmutation process has not changed. It remains 24. What has
changed is the atomic numberZ, since there is now one more proton in the nucleus.
Learn more about beta decay by following the link below.
http://phet.colorado.edu/en/simulation/beta-decay
Gamma Decay
We discussed earlier that the electrons of an atom occupy different stationary states or energy levels. It turns out
that the nucleons within an atomic nucleus also occupy different energy levels. However, nuclear energy states are
separated by much larger energy gaps. When a nucleon falls to a lower energy level in the nucleus, a high-energy
electromagnetic pulse, called a gamma ray(γ)is emitted, just like a photon is emitted when an electron falls to a
lower energy level in the atom. It is good to keep in mind that the only difference between a “photon” and a “gamma
ray” is that the photons ejected or absorbed by an electron in an atom have much lower energy than gamma rays.
Gamma rays have energies millions of times more than visible light photons.
Note that ejection of a gamma ray from a nucleus does not alter either the atomic mass or atomic number of the
atom.
Here is an example of gamma decay involving an excited cobalt 60 nucleus^6027 Co∗. The asterisk signifies the nucleus
in an excited state.