Atomic and Nuclear Physics Flashcards

An atom of Uranium-238 (²³⁸U) and an atom of Uranium-235 (²³⁵U) both have 92 protons. Which of the following best describes their relationship?

Answer: They are isotopes of the same element.

Isotopes are variants of a particular chemical element which differ in neutron number but have the same number of protons. Both U-238 and U-235 have 92 protons, defining them as uranium, but they have different numbers of neutrons (146 and 143, respectively), making them isotopes of each other.

A radioactive source is placed in front of a Geiger counter. A single sheet of paper placed between the source and detector has no significant effect on the count rate. However, when a 5 mm thick sheet of aluminum is added, the count rate drops to nearly zero. What is the primary type of radiation being emitted?

Answer: Beta particles

Beta particles have moderate penetrating power. They can easily pass through a sheet of paper but are effectively stopped by a few millimeters of aluminum. Alpha particles would be stopped by the paper, and gamma rays would pass through both the paper and the aluminum with little reduction in intensity.

A sample of a radioactive isotope has a half-life of 20 days. If the initial sample contains 400 grams of the isotope, how much of the original isotope will remain after 60 days?

Answer: 50 grams

The time elapsed (60 days) is equal to three half-lives (60 days / 20 days/half-life = 3). After each half-life, the amount of the radioactive isotope is reduced by half. Starting with 400g: After 1st half-life (20 days) -> 200g. After 2nd half-life (40 days) -> 100g. After 3rd half-life (60 days) -> 50g.

Which statement accurately describes the process of nuclear fission as it is utilized in a nuclear power reactor?

Answer: A heavy nucleus, such as uranium-235, splits into two or more lighter nuclei after absorbing a neutron, releasing a large amount of energy and more neutrons.

Nuclear fission is the process where the nucleus of a heavy atom splits into smaller parts. In a reactor, this is typically initiated by the nucleus absorbing a neutron, which makes it unstable. This splitting releases a significant amount of energy and additional neutrons, which can then cause a chain reaction. Option A describes fusion, C describes gamma decay, and D describes beta decay.

The mass of a stable atomic nucleus is always slightly less than the sum of the masses of its individual, separate protons and neutrons. What is this difference in mass, known as the 'mass defect,' converted into?

Answer: Nuclear binding energy holding the nucleons together.

According to Einstein's mass-energy equivalence principle (E=mc²), mass and energy are interchangeable. The mass defect is the mass that is converted into the binding energy required to hold the protons and neutrons together in the nucleus. This energy release is what makes the nucleus stable.

When an atom of Carbon-14 (¹⁴C), which has 6 protons and 8 neutrons, undergoes beta-minus decay, what is the resulting nucleus?

Answer: Nitrogen-14 (¹⁴N)

In beta-minus decay, a neutron inside the nucleus is converted into a proton, and an electron (the beta particle) is emitted. This process increases the atomic number (number of protons) by one (from 6 to 7) but leaves the total mass number (protons + neutrons) unchanged at 14. The element with an atomic number of 7 is Nitrogen.