Phys 33 Questions

Distinguish between a rad and a rem.

A rad measures energy absorbed, whereas a rem measures biological damage done.

What is a radioactive tracer?

A radioactive isotope of an element that by itself or in a molecule is used to trace biochemical pathways in plants and organisms

What kind of trail is left when an energetic particle shoots through matter?

A trail of free electrons and positive ions. Ordinary thermal motions of atoms bumping one another in a gas/liquid are not energetic enough to dislodge them.

What is the change in the atomic mass number for each of the reactions in the preceding two questions?

Alpha emission -4 (loses 2 protons and 2 neutrons), beta emission 0 (same total sum of protons and neutrons)

The alpha particle has twice the electric charge of the beta particle but, for the same kinetic energy, deflects less than the beta in a magnetic field. Why is this so?

An alpha particle has about 8000 times the mass of a beta particle, so it is (relatively) very hard to accelerate the alpha particle and it does not deflect as much as a beta particle.

What kind of rays are X-rays?

Electromagnetic

When, and by whom, did the first successful intentional transmutation of an element occur?

Ernest Rutherford, 1919. Bombarded nitrogen gas with alpha particles from a piece of radioactive ore.

Which has the higher frequency: X-rays or gamma rays?

Gamma rays

Why aren't gamma rays deflected in a magnetic field?

Gamma rays are electromagnetic radiation and so have no charge. Electric and magnetic fields deflect charges.

Which type of detector senses radiation by the ionization of gas in a tube?

Geiger counter.

What did Wilhelm Roentgen discover about a cathode-ray beam striking a glass surface?

It produced X-rays.

What is the long-range fate of all the uranium that exists in the world?

It will turn into lead. The naturally occurring isotopes U-238 and U-235 decay very slowly and ultimately become isotopes of lead.

Which contains the higher percentage of neutrons: large nuclei or small nuclei?

Large nuclei (more than 50%). The more protons there are in a nucleus, the more neutrons are needed to help balance electrical forces. For light elements, it is sufficient to have as many neutrons as protons.For large nuclei, more neutrons than protons are needed, since strong force diminishes rapidly over distance (but the electrical force does not diminish significantly over distance.) To compensate for the weakening strong force, large nuclei have more neutrons than protons.

Do humans receive more radiation from artificial or natural sources of radiation? What fraction comes from this source?

Natural, about 80%

What is the role of neutrons in the atomic nucleus?

Neutrons contribute strong nuclear force attraction without adding in electric force repulsion, so they help hold the nucleus together.

What occurs when a nitrogen nucleus captures an extra neutron?

None of the above.

What two elements did Pierre and Marie Curie discover?

Polonium and radium

Name the two different nucleons.

Proton and neutron

Which type of detector senses flashes of light produced by charged particles or gamma rays?

Scintillation counter

Which interaction tends to hold the particles in an atomic nucleus together and which interaction tends to push them apart?

The "strong force" acts between all nucleons, very strong within extremely short distances. Over distance, it loses out to the electrical force, which is relatively long-ranged, does not weaken over distance, and REPELS particles.

Why doesn't the repulsive electrical force of protons in the atomic nucleus cause the protons to fly apart?

The attractive strong nuclear force holds the nucleus together.

What is transmutation? Give one example.

The change of one chemical element into another, such as uranium to lead. This is a succession of radioactive decays of 238/92 Uranium to 206/82 lead. Series of alpha and beta decays.

What two quantities are always conserved in all nuclear equations?

The conservation of charge (the number of elementary positive and negative charges in the reactants must be the same as in the products), and the conservation of the number of nucleons .

Why is a larger nucleus generally less stable than a smaller nucleus?

The electric force of repulsion between protons is a long-range force, while the attractive strong nuclear force is short-range. Repulsion wins for larger nuclei.

How does the rate of decay of a long-half-life material normally compare to the rate of decay of a short-half-life material?

The rate of decay is lower for a long-half-life material. Half life ~ the decay rate. Longer the half life, the slower the rate.

Which is more prevalent in the food we eat: carbon-12 or carbon-14?

There is about one carbon-14 atom for every 100 billion carbon-12 atoms.

What did Henri Becquerel discover about uranium?

Uranium emitted radiation that would expose film and could be deflected by electric and magnetic fields.

Why is lead found in all deposits of uranium ore?

Uranium undergoes radioactive decay, transmuting into lead.

Is the human body radioactive? Explain.

Yes. The potassium in our bodies has a radioactive isotope that is the main internal source of radiation for a human body.

What change in atomic number occurs when a nucleus emits an alpha particle? A beta particle? A gamma ray?

-2 (loss of 2 protons), +1 (gain one proton charge), 0 (same)

What is the half-life of Ra-226?

1620 years pg. 625

When thorium (atomic number 90) decays by emitting an alpha particle, what is the atomic number of the resulting nucleus?

88 protons. Emitting an alpha particle leads to the loss of two protons and two neutrons from the nucleus: atomic number (# of protons) diminishes by two, mass number (p + n) diminishes by 4 (2 p and 2 n).

When thorium decays by emitting a beta particle, what is the atomic number of the resulting nucleus?

91 protons. A beta particle is an electron, so the atomic number of the resulting nucleus is increased by 1 proton. The resulting element has 91 protons, Protactinium. Although the atomic number has increased by 1 in this process, the mass number (p + n) remains the same. 234/90 Th >> 234/91 Pa + 0/-1 e.