ACHRE Report


The Atomic Century

Before the Atomic Age: "Shadow Pictures," Radioisotopes, and the Beginnings of Human Radiation Experimentation

The Manhattan Project: A New and Secret World of Human Experimentation

The Atomic Energy Commission and Postwar Biomedical Radiation Research

The Transformation in Government - Sponsored Research

The Aftermath of Hiroshima and Nagasaki: The Emergence of the Cold War Radiation Research Bureaucracy

New Ethical Questions for Medical Researchers


The Basics of Radiation Science

What Is Ionizing Radiation?

What Is Radioactivity?

What Are Atomic Number and Atomic Weight?

Radioisotopes: What Are They and How Are They Made?

How Does Radiation Affect Humans?

How Do We Measure the Biological Effects of External Radiation?

How Do We Measure the Biological Effects of Internal Emitters?

How Do Scientists Determine the Long-Term Risks from Radiation?

What Is Radioactivity?

What causes radioactivity?

As its name implies, radioactivity is the act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it "wants" to give up some energy in order to shift to a more stable configuration. During the first half of the twentieth century, much of modern physics was devoted to exploring why this happens, with the result that nuclear decay was fairly well understood by 1960. Too many neutrons in a nucleus lead it to emit a negative beta particle, which changes one of the neutrons into a proton. Too many protons in a nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron. Too much energy leads a nucleus to emit a gamma ray, which discards great energy without changing any of the particles in the nucleus. Too much mass leads a nucleus to emit an alpha particle, discarding four heavy particles (two protons and two neutrons).

How is radioactivity measured?

Radioactivity is a physical, not a biological, phenomenon. Simply stated, the radioactivity of a sample can be measured by counting how many atoms are spontaneously decaying each second. This can be done with instruments designed to detect the particular type of radiation emitted with each "decay" or disintegration. The actual number of disintegrations per second may be quite large. Scientists have agreed upon common units to use as a form of shorthand. Thus, a curie (abbreviated "Ci" and named after Pierre and Marie Curie, the discoverers of radium[87]) is simply a shorthand way of writing "37,000,000,000 disintegrations per second," the rate of disintegration occurring in 1 gram of radium. The more modern International System of Measurements (SI) unit for the same type of measurement is the becquerel ( abbreviated "Bq" and named after Henri Becquerel, the discoverer of radioactivity), which is simply a shorthand for "1 disintegration per second."

What is radioactive half-life?

Being unstable does not lead an atomic nucleus to emit radiation immediately. Instead, the probability of an atom disintegrating is constant, as if unstable nuclei continuously participate in a sort of lottery, with random drawings to decide which atom will next emit radiation and disintegrate to a more stable state. The time it takes for half of the atoms in a given mass to "win the lottery"--that is, emit radiation and change to a more stable state--is called the half-life. Half-lives vary greatly among types of atoms, from less than a second to billions of years. For example, it will take about 4.5 billion years for half of the atoms in a mass of uranium 238 to spontaneously disintegrate, but only 24,000 years for half of the atoms in a mass of plutonium 239 to spontaneously disintegrate. Iodine 131, commonly used in medicine, has a half-life of only eight days.

What is a radioactive decay chain?

Stability may be achieved in a single decay, or a nucleus may decay through a series of states before it reaches a truly stable configuration, a bit like a Slinky toy stepping down a set of stairs. Each state or step will have its own unique characteristics of half-life and type of radiation to be emitted as the move is made to the next state. Much scientific effort has been devoted to unraveling these decay chains, not only to achieve a basic understanding of nature, but also to design nuclear weapons and nuclear reactors. The unusually complicated decay of uranium 238, for example--the primary source of natural radioactivity on earth--proceeds as follows:[88]

U-238 emits an alpha

Thorium 234 emits a beta

Protactinium 234 emits a beta

Uranium 234 emits an alpha

Thorium 230 emits an alpha

Radium 226 emits an alpha

Radon 222 emits an alpha

Polonium 218 emits an alpha

Lead 214 emits a beta

Bismuth 214 emits a beta

Polonium 214 emits an alpha

Lead 210 emits a beta

Bismuth 210 emits a beta

Polonium 210 emits an alpha

Lead 206, which is stable

How can radioactivity be caused artificially?

Radioactivity can occur both naturally and through human intervention. An example of artificially induced radioactivity is neutron activation. A neutron fired into a nucleus can cause nuclear fission (the splitting of atoms). This is the basic concept behind the atomic bomb. Neutron activation is also the underlying principle of boron-neutron capture therapy for certain brain cancers. A solution containing boron is injected into a patient and is absorbed more by the cancer than by other cells. Neutrons fired at the area of the brain cancer are readily absorbed (captured) by the boron nuclei. These nuclei then become unstable and emit radiation that attacks the cancer cells. Simple in its basic physics, the treatment has been complex and controversial in practice and after half a century is still regarded as highly experimental.
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