PHY 102 - Chapter 15 Test

Radioactive isotope technetium-99m has a half-life of 6 hours. What fraction of a sample of technetium-99m remains after 24 hours?

1/16

X-rays were discovered in

1895.

A Radioactive isotope has 1/64 of the original material left after 10 minutes. Its half life is

2 minutes

Light can't penetrate even a millimeter into plutonium, so why is a neutron able to travel centimeters into plutonium?

A neutron has no charge and only interacts with nuclei that are so small that they're hard to hit.

Lead, with 82 electrons per atom, is an excellent absorber of X-rays. Why?

Almost any X-ray matches the energy of one of lead's many electrons and thus can cause efficient photoelectron emission. (Stable elements that are very dense with heavy nuclei can absorb x-rays.)

Suppose that oxygen nuclei frequently absorbed neutrons. How would that behavior affect water's performance as a moderator?

By absorbing neutrons, the water would suppress the chain reaction. (The best moderators are nuclei that rarely or never absorb neutrons and don't fall apart during collisions with them (e.g. heavy water).)

Control rods are usually installed on top of the reactor where their weights tend to pull them into the core. Why is this arrangement much safer than putting the control rods at the bottom of the reactor?

By installing the control rods on top of the reactor, if the fission rate becomes too high, the control system can simply drop the control rods into the core to decrease the fission rate. (The attraction of the control rods with the earth can quickly slow down the fission rate.)

What is needed for a nuclear chain reaction?

Critical mass

What is not needed for a nuclear chain reaction?

Excess electrons

What two Japanese cities were attacked with nuclear bombs during World War II?

Hiroshima and Nagasaki

The most troubling radioactive isotopes in nuclear waste are those with half-lives of between a few years and a few hundred thousand years. Explain.

It's possible to wait out isotopes with short half-lives because they decay away quickly and long half-life isotopes decay so slowly that it's unlikely they'll decay during a person's lifetime.

Magnetic resonance imaging isn't good at detecting bone. Why not?

MRI detects hydrogen and bone contains little hydrogen.

Magnetic resonance imaging (MRI) differs from computed tomography imaging in that it involves no "ionizing radiation." What electromagnetic radiation is used in MRI, and why aren't the photons of this radiation able to remove electrons from atoms and convert those atoms into ions?

MRI uses radiowaves or microwaves and photons of those two types of electromagnetic radiation have far too little energy to cause chemical damage to molecules.

No magnetic metals such as iron or steel are permitted near a magnetic resonance imaging machine. In part, this rule is a safety precaution since those magnetic metals would be attracted toward the machine. However, the magnetic fields from these magnetic metals would also spoil the imaging process. Why would having additional magnetic fields inside the imaging machine spoil its ability to locate specific protons inside a patient's body?

Magnetic metals would change the local magnetic fields inside a patient and the machine needs to be able to know exactly what the fields are at each point in the patient in order to know where the protons it is detecting are located.

Some satellites have been lauched with small nuclear reactors on board to provide electric power. Why must these small reactors use highly enriched uranium or plutonium rather than natural or slightly enriched uranium?

Natural or slightly enriched uranium alone cannot use the slow-moving-neutron effect to maintain a chain reaction because the fast moving neutrons are quickly absorbed by the more abundant 238U nuclei.

The nuclear bombs developed during World War II used what principle?

Nuclear fission

What percentage of an atom's mass is in its nucleus?

Over 99.9%

Which of the following is NOT a criterion to make a nuclear bomb?

Sufficient number of electrons to propagate the chain reaction.

Why is it important to keep foods and drugs out of direct sunlight, even when they're not in danger of overheating?

The high-energy photons in sunlight can cause chemical damage. The high-frequency photons in sunlight can cause chemical damage. The short-wavelength photons in sunlight can cause chemical damage. (All of the above choices are correct.)

The stronger the magnetic field used in MRI, the larger the fraction of protons that align their spins with the field. Why does this increased alignment make it easier for the MRI to study the protons?

The more spin-aligned protons in a patient, the more radiative transitions the MRI system can produce and the more easily it can detect tissue.

Museums often display priceless antique manuscripts under dim yellow light. Why not use white light?

The photons in blue and ultraviolet light have a high energy and thus can cause chemical damage to the molecules in the manuscripts whereas yellow light cannot because its energy is less. The photons in blue and ultraviolet light have a high frequency and thus can cause chemical damage to the molecules in the manuscripts whereas yellow light cannot because its frequency is lower. The photons in blue and ultraviolet light have a short wavelength and thus can cause chemical damage to the molecules in the manuscripts whereas yellow light cannot because its wavelength is longer. (All of the above choices are correct.)

Why are many drugs packaged in amber-colored containers that block ultraviolet light?

The photons in ultraviolet light have a high energy and thus can damage the molecules in medicines.

A fission chain reaction can occur at room temperature, so why must hydrogen be heated to astronomical temperatures to get its nuclei to fuse?

The protons in hydrogen nuclei repel one another tremendously at short distances; therefore, they need to be moving very quickly with gigantic amounts of thermal energy for them to touch. (To make them approach one another closely enough to stick, the nuclei must be heated to temperatures of more than 100 million degrees Celsius.)

Once fallout from a nuclear blast distributes radioactive isotopes over a region of land, why is it virtually impossible to separate many of those radioactive isotopes from the soil?

The radioactive isotopes have unstable nuclei and are chemical indistinguishable from ordinary elemental constituents of the soil.

The uranium fuel in a thermal fission reactor must be replaced every so often. When that fuel is removed, it's still mostly uranium. Why can't that uranium be cleaned and reused until the uranium is completely consumed?

The reactor is consuming only the 235U and the uranium that is left in the fuel is mostly 238U which is not fissionable. (The uranium that is consumed is 235U because the nuclei are so good at catching slow-moving neutrons that they easily win out over the more abundant 238U nuclei.)

Why does the strength of the magnetic field used in MRI affect the frequency of the radio waves used to detect the protons?

The stronger the magnetic field, the more energy a proton needs to change from aligned to anti-aligned and thus the radio wave photons must have more energy which requires a higher frequency. (The higher the frequency, the stronger the magnetic field required in order to detect the protons.)

Sunscreen absorbs ultraviolet light while permitting visible light to pass. Why does this coating reduce the risk of chemical and genetic damage to the cells of your skin?

The sunscreen blocks the high-energy photons in ultraviolet light before they reach your skin. The sunscreen blocks the short-wavelength photons in ultraviolet light before they reach your skin. The sunscreen blocks the high-frequency photons in ultraviolet light before they reach your skin. (All of the above choices are correct.)

The three major nuclear reactor accidents are

Windscale Pile 1 (1957), Three Mile Island (1979) and Chernobyl (1986).

Fission chain reactions in uranium are sustained by neutrons, each of which is released by one fission and may induce a subsequent fission. Each fission also releases other particles, which include protons. However, proton-containing fission fragments are not as effective at causing subsequent fissions because they

are repelled as they try to approach other uranium nuclei.

Isotopes do not share

atomic mass number.

In a well-designed water-cooled thermal fission reactor, the water is acting as a moderator for the neutrons. If some of this water is lost, the chain reaction in the reactor core will slow down because neutrons in the reactor core will

be traveling too fast and will be absorbed by uranium 238 nuclei.

When an x-ray photon is absorbed in a tumor, it may cause the tumor cell to die because the photon

carries enough energy to damage molecules and cause chemical injury to the cell.

In a common X-ray tube, electrons accelerate from the

cathode to anode.

Most nuclear reactors in the United States are thermal fission reactors, reactors that slow their fission neutrons by sending those neutrons through materials known as moderators. Water is a common moderator. When a fast moving neutron enters water, it usually slows down because it

collides with the water nuclei and transfers most of its energy and momentum to those nuclei.

Which of the following does a moderator not have an effect on?

electron speed

The fallout from a nuclear explosion and the waste from a nuclear reactor contain radioactive isotopes. The atoms of these isotopes

have unstable nuclei but are chemically indistinguishable from stable atoms.

Imaging x-rays go easily through living tissue but not so easily through bone. This is primarily because the calcium atoms in bone have

many more electrons than the atoms in tissue have.

The fallout from a nuclear explosion and the waste from a nuclear reactor contain radioactive isotopes. For example, iodine-131 (a fission byproduct) is radioactive while iodine-127 (normal iodine) is not. An atom of iodine-131 differs from an atom of iodine-127 because the iodine-131 atom has more

neutrons in its nucleus.

In many circumstances it is very important to have control of a fission nuclear chain reaction with a moderator. A moderator must therefore have a profound effect on the motion of

neutrons.

When uranium undergoes fission, it releases an enormous amount of energy. That energy was stored in the uranium nuclei as

nuclear potential energy from the nuclear forces between nucleons.

The two main developments essential to the making of the atomic bomb are

relativity and quantum physics.