The time needed for 50 percent of the nuclei in a radioactive sample to decay is the half-life of the isotope.
Missing Word
half-life
The half-life is a constant for each radioisotope, ranging from fractions of a second to billions of years. After each half-life, half of the remaining nuclei have decayed.
Category: Radioactive Decay
Question 2
The particle emitted when a proton in a nucleus changes into a neutron, which has the mass of a beta particle but a positive charge, is a positron.
Missing Word
positron
A positron is the antimatter counterpart of the electron, with the same mass but a 1+ charge. Positron emission lowers the atomic number by one.
Category: Radioactive Decay
Question 3
A beta particle is a high-speed electron emitted from a nucleus when a neutron changes into a proton.
Missing Word
electron
A beta particle is an electron created and ejected by the nucleus, not one of the electrons orbiting the atom. Beta emission raises the atomic number by one.
Category: Radioactive Decay
Question 4
The Geiger counter detects radiation by measuring how much it ionizes the gas inside a sealed tube.
Missing Word
Geiger
In a Geiger counter, radiation ionizes gas in a Geiger-Muller tube and the resulting current pulses are counted. The rate of ionization is proportional to the amount of radiation.
Category: Radiation Effects
Question 5
An older unit of radioactivity often used in medicine, equal to 3.7 x 10^10 decays per second, is the curie.
Missing Word
curie
The curie (Ci) was originally based on the activity of 1 g of radium. It is a very large unit compared with the becquerel, which is one decay per second.
Category: Radiation Effects
Question 6
In a nuclear reactor, the fluid that carries heat away from the core so that it can be used to make steam is called the coolant.
Missing Word
coolant
The coolant transfers the heat released by fission to a steam generator, and the steam drives turbines that generate electricity. In U.S. power reactors the coolant is water.
Category: Fission and Fusion
Question 7
A substance such as graphite or heavy water that slows neutrons in a reactor to speeds at which they readily cause fission is the moderator.
Missing Word
moderator
A moderator slows fast neutrons through collisions without absorbing many of them. Slow neutrons are far more likely to be captured by uranium-235 and cause fission.
Category: Fission and Fusion
Question 8
A gamma ray is a high-energy photon given off by a nucleus.
Missing Word
photon
A photon is a packet of electromagnetic energy with no mass and no charge. Gamma photons carry much more energy than visible light.
Category: Nuclear Equations
Question 9
An alpha particle is identical to the nucleus of an atom of helium.
Missing Word
helium
An alpha particle has two protons and two neutrons, the same composition as a helium-4 nucleus. Alpha decay lowers the mass number by 4 and the atomic number by 2.
Category: Nuclear Equations
Question 10
The dense element commonly used in aprons that shield patients from X-rays during imaging is lead.
Missing Word
lead
Lead has a high density and a high atomic number, so a thin layer absorbs X-rays and gamma rays effectively.
Category: Radiation Effects
Question 11
A uranium-235 nucleus, with atomic number 92, contains 143 neutrons.
Missing Word
143
The number of neutrons equals the mass number minus the atomic number: 235 - 92 = 143.
Category: Nuclear Stability
Question 12
Increasing the proportion of uranium-235 in uranium fuel to about 5 percent, using methods such as gas centrifuges, is called enrichment.
Missing Word
enrichment
Natural uranium is mostly uranium-238, which does not sustain a chain reaction. Enrichment raises the share of fissile uranium-235 so the fuel can be used in power reactors.
Category: Fission and Fusion
Question 13
At the extreme temperatures needed for nuclear fusion, atoms are completely ionized and form the state of matter called plasma.
Missing Word
plasma
Plasma is a gas of free nuclei and electrons. Containing a plasma hot enough for fusion is one of the main challenges in building a fusion power reactor.
Category: Fission and Fusion
Question 14
A device worn by workers to record their total radiation exposure over time is a dosimeter.
Missing Word
dosimeter
A dosimeter, such as a film badge or an electronic badge, measures the cumulative dose of ionizing radiation a person receives so that exposure stays within safe limits.
Category: Radiation Effects
Question 15
A radioisotope added in small amounts so that its path through a living organism can be followed by detecting its radiation is called a tracer.
Missing Word
tracer
A tracer behaves chemically just like the stable isotope of the same element, so it goes wherever that element goes, while its radiation reveals its location.
Category: Uses of Radioisotopes