Chapter 16

Which type of radiation—alpha, beta, or gamma—results in the greatest change in mass number? The greatest change in atomic number?

Alpha radiation decreases the atomic number of the emitting element by 2 and the atomic mass number by 4. Beta radiation increases the atomic number of an element by 1 and does not affect the atomic mass number. Gamma radiation does not affect the atomic number or the atomic mass number. So alpha radiation results in the greatest change in atomic number, and hence charge, and mass number as well.

Uranium-238 absorbs a neutron and then emits a beta particle. Show that the resulting nucleus is neptunium-239.

Any isotope of U is atomic number 92. U-238 + 1 neutron becomes U-239. When it emits a beta, a neutron becomes a proton, making the element atomic number 93. So it is an element with atomic number 93 and mass 239. That's Np-239.

Consider a radioactive sample with a half-life of one week. How much of the original sample will be left at the end of the second week? The third week? The fourth week?

At the end of 2 weeks, 1/4 is left; at the end of 3 weeks, 1/8 is left; at the end of 4 weeks 1/16 is left.

A radioisotope is placed near a radiation detector, which registers 80 counts per second. Eight hours later, the detector registers 5 counts per second. What is the radioactive half-life?

Count: It's decreased to 40 counts per second (one half-life), 20 counts per second (2 half-lives), 10 counts per second (3 half-lives), 5 counts per second, which takes 4 half-lives. So its half-life is 8 hours/4 = 2 hours.

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

Emission of an alpha particle decreases the atomic number of the nucleus by 2. Emission of a beta particle increases the atomic number of the nucleus by 1.

Which of the three rays—alpha, beta, or gamma—has the greatest penetrating power?

Gamma rays have the greatest penetrating power of the three rays.

Which produces more radioactivity in the atmosphere, coal-fired power plants or nuclear power plants?

Global combustion of coal produces about 13,000 tons of radioactive substances; nuclear plants produce about 10,000 tons.

What do flying in a commercial airliner and a visit to the doctor's office have in common?

In both cases you are taking on the risks of radiation exposure to enjoy certain benefits. For the commercial airliner, you are exposing yourself to gamma radiation from space, but you get the benefit of high-speed travel. For a doctor visit, you may be exposed to X-rays (a form of radiation), but you get the benefit of the X- ray's diagnostic powers.

Why is cosmic radiation more intense in high-altitude cities?

In high-altitude cities, the air is thinner and provides less of a protective blanket.

Radiation from a point source follows an inverse-square law. If a Geiger counter that is 1 meter away from a small source reads 100 counts per minute, what will be its reading 2 meters from the source? 3 meters from it?

Intensity decreases with distance by the inverse-square law, twice the distance is 1/4 the intensity and 1/4 the reading, three times as far is 1/9 the intensity and 1/9 the reading.

Why does iron not yield energy if fused or if fissioned?

Iron is at the bottom of the "energy hill" of the curves shown in Figures 16.20 and 16.21, so whether fused or fissioned, the product nuclei would be more massive than the iron. This process absorbs energy rather than releasing it.

When an element like thorium decays, does it become a completely different element or a version of thorium?

It becomes a completely different element.

Why is the killing of cells by radioactivity better than damage of cells?

Killing of cells means new ones can replace them; damaged cells can be passed on to an individual's offspring.

In what form is most of the energy released by atomic transmutation?

Most is KE of the ejected particle and some is the KE of the recoiling nucleus.

Is radioactivity on the Earth something relatively new? Defend your answer.

No. It has been around longer than the human race and has been a part of the environment as long as the sun, rain and soil have been.

Why will nuclear fission probably never be used directly for powering automobiles? How could it be used indirectly?

Nuclear fission is a poor prospect for powering automobiles primarily because of the massive shielding that would be required to protect the occupants and others from the radioactivity, and the problem of radioactive waste disposal.

What is a common source of radiation found within the basements of homes?

Radon-222, which is an inert gas that accumulates in basements after seeping through cracks in the basement floor.

Oxygen and hydrogen atoms combine to form water. At the nuclear level, if one oxygen and one hydrogen were fused, what element would be produced?

The fusion product would have atomic number 9, which would be fluorine.

What is the half-life of radium-226?

The half-life of radium-226 is 1,620 years.

Explain how radioactive decay has always warmed the Earth from the inside and how nuclear fusion has always warmed the Earth from the outside.

The radioactive decay of radioactive elements found under the Earth's surface warms the insides of the Earth and responsible for the molten lava that spews from volcanoes. The thermonuclear fusion of our Sun is responsible for warming everything on our planet's surface exposed to the Sun.

What percentage of nuclear power plants in operation today are based upon nuclear fusion?

There are no fusion-based nuclear power plants in operation today. They are all of the fission type.

If a pair of carbon nuclei were fused, and the product emitted a beta particle, what element would be produced?

Two carbon nuclei would fuse to atomic number 8, and the beta emission would increase it by 1, so the element produced would have atomic number 9, which would be fluorine.

When a pair of hydrogen isotopes are fused, is the mass of the product nucleus more or less than the total mass of the two hydrogen nuclei?

When a pair of hydrogen isotopes are fused, the mass of product is less than the total mass of the nuclei before fusion.

What is the half-life of uranium-238?

4.5 billion years.