Test

Uranium-238 atoms decay according to a decay series as noted in the chart at the end of the test. If you find a sample of uranium ore, it will also contain all of the other elements in the series. Each decay in the chain results in the emission of a significant amount of energy - in fact, this energy is the source of much of the heat in the earth’s interior.

1) Suppose you have pure, 1 gram samples of the following elements. Which sample would be give you the highest count rate on a Geiger counter placed near the sample?
²³⁸₉₂U
²³⁴₉₂U
²²⁶₈₈Ra
²²²₈₆Rn

2) The first decay in the series is the alpha decay of 238-U:
²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He
A lot of energy is released in this reaction. Thinking about the masses of the particles, we can say:
the mass of the uranium atom is greater than the mass of the thorium atom plus the mass of the helium atom
the mass of the uranium atom is less than the mass of the thorium atom plus the mass of the helium atom
the mass of the uranium atom is the same as the mass of the thorium atom plus the mass of the helium atom
all of the particles gained mass over Thanksgiving, as we all did

3) A sample of pure ²³⁸₉₂U is sealed in a box, and all of the air is pumped out, so that there is a very good vacuum in the box. Some time later, it is observed that there is some gas in the box. Most of the gas present is:
Hydrogen
Helium
Radon
Radium

4) Suppose you have a 1 gram sample of Radium-226. How long - approximately - will it be until you have only 0.1 grams of radium left?
161 years
1648 years
3214 years
5320 years

5) If you bombard uranium with neutrons, you can make other elements. Suppose 238-U absorbs a neutron, and then beta decays - as it would. This makes an atom of:
²³⁹₉₁Pa
²³⁹₉₂U
²³⁹₉₃Np
²³⁹₉₄Pu

6) A few billion years ago, when U-235 was more plentiful, there was a natural ore deposit in Africa that was rich enough to sustain a fission chain reaction. The ore deposit developed cracks, and water seeped in. Then, a fission reaction as you would find in a nuclear reactor began, with the U-235 fissioning. As the reaction proceeded, the ore heated up - and the water boiled off. At this point, the reaction stopped. After some time, the ore cooled, water seeped in, and the reaction began again. This cycle repeated itself several times. In this situation, the water was playing the role of a ______ in this natural nuclear reactor.
absorber
coolant
moderator
source of hydrogen