 Educators Study Questions Atomic Physics - Solutions Atomic Physics - Solutions 1. Define isotope.

Isotopes are elements with the same number of protons and different numbers of neutrons.

2. What is the relationship between the atomic mass number, the atomic number, and the number of neutrons in a nucleus.

A = N + Z
where A is the atomic mass number, N is the number of neutrons and Z is the atomic number.

3. What does E=mc2 mean?

When mass is converted into energy the amount of energy released is equal to the mass multiplied by the speed of light squared.

4. How many protons and neutrons are in Plutonium 239?

Z = 94
N = A - Z
239 - 94 = 145

5. How much energy is released in the total conversion of 10g of mass into energy? How many megatons of TNT equivalent are released?

E = mc2 (Make sure you use consistent units i.e. mks, cgs)
10 g = 0.1 kg
Energy = .01 kg (3.0 x108 m/s)2 = 9 x1014 joules = .22 Mt.

6. What is an alpha particle? What is a beta particle?

An alpha particle is a helium nucleus. A beta particle is an electron.

7. What is meant by the half-life of a radioactive substance?

The half life is the time it takes for one half of a given radioactive substance to decay.

8. What is spontaneous fission? Why is it important?

Spontaneous fission is fission that occurs without any external trigger i.e. a neutron. It is important because neutrons are released in this process. Those neutrons can then trigger other fission reactions. If spontaneous fission were to occur just as two halves of a gun type weapon were being assembled there is a chance that the weapon will fizzle.

9. Iodine 131 is a prominent fallout particle. After undergoing beta decay what element does it become?

I 131 → Xe 131 + b ; where Xe is xeonon.

10. When uranium 238 decays by alpha particle emission, what nucleus is formed?

U 238 → Th 234 + a ; where Th is thorium.

11. Plutonium is usually the radioactive material used in the primary of a hydrogen bomb. If we eliminated all of our existing nuclear weapons today, and then Stored the plutonium, how long would it take for this stockpile to decay to one-quarter of its original amount? To one-sixteenth? Is there anything we can do to speed up this process? (Pu 239 has a half-life of 24,400 years).

For plutonium to decay to one-quarter of the original amount it will take two half-lives (1/2 x 1/2 = 1/4) or 48,800 years. To decay to one-sixteenth, (1/2 x 1/2 x 1/2 x 1/2= 1/16), or 97,600 years. It is not possible to speed up nuclear reactions. However, you can artificially transmute the element into another radioactive that has a shorter half-life. This is being pursued as a clean-up option for nuclear reactors.

12. Why is an antineutrino needed in beta decay?

The antineutrino is necessary in beta decay to conserve energy. 