Advisory Committee On Human Radiation Experiments Final Report (2024)

ACHRE Report

Introduction

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

What Is Radioactivity?

What causes radioactivity?

As its name implies, radioactivity is the act of emitting radiationspontaneously. This is done by an atomic nucleus that, for some reason, isunstable; it "wants" to give up some energy in order to shift to a more stableconfiguration. During the first half of the twentieth century, much of modernphysics was devoted to exploring why this happens, with the result that nucleardecay was fairly well understood by 1960. Too many neutrons in a nucleus leadit to emit a negative beta particle, which changes one of the neutronsinto a proton. Too many protons in a nucleus lead it to emit a positron(positively charged electron), changing a proton into a neutron. Too muchenergy leads a nucleus to emit a gamma ray, which discards great energywithout changing any of the particles in the nucleus. Too much mass leads anucleus to emit an alpha particle, discarding four heavy particles (twoprotons 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 howmany atoms are spontaneously decaying each second. This can be done withinstruments designed to detect the particular type of radiation emittedwith each "decay" or disintegration. The actual number of disintegrations persecond may be quite large. Scientists have agreed upon common units touse as a form of shorthand. Thus, a curie (abbreviated "Ci" and named afterPierre 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 disintegrationoccurring in 1 gram of radium. The more modern International System ofMeasurements (SI) unit for the same type of measurement is the becquerel( abbreviated "Bq" and named after Henri Becquerel, the discoverer ofradioactivity), which is simply a shorthand for "1 disintegration persecond."

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 unstablenuclei continuously participate in a sort of lottery, with random drawings todecide which atom will next emit radiation and disintegrate to a more stablestate. The time it takes for half of the atoms in a given mass to "win thelottery"--that is, emit radiation and change to a more stable state--is calledthe half-life. Half-lives vary greatly among types of atoms, from lessthan a second to billions of years. For example, it will take about 4.5billion years for half of the atoms in a mass of uranium 238 to spontaneouslydisintegrate, but only 24,000 years for half of the atoms in a mass ofplutonium 239 to spontaneously disintegrate. Iodine 131, commonly used inmedicine, 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 aseries of states before it reaches a truly stable configuration, a bit like aSlinky toy stepping down a set of stairs. Each state or step will have its ownunique characteristics of half-life and type of radiation to be emitted as themove is made to the next state. Much scientific effort has been devoted tounraveling these decay chains, not only to achieve a basic understanding ofnature, but also to design nuclear weapons and nuclear reactors. The unusuallycomplicated decay of uranium 238, for example--the primary source of naturalradioactivity 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. Anexample of artificially induced radioactivity is neutron activation. Aneutron fired into a nucleus can cause nuclear fission (the splitting ofatoms). This is the basic concept behind the atomic bomb. Neutron activationis also the underlying principle of boron-neutron capture therapy for certainbrain cancers. A solution containing boron is injected into a patient and isabsorbed more by the cancer than by other cells. Neutrons fired at the area ofthe brain cancer are readily absorbed (captured) by the boron nuclei. Thesenuclei then become unstable and emit radiation that attacks the cancer cells.Simple in its basic physics, the treatment has been complex and controversialin practice and after half a century is still regarded as highlyexperimental.