B. H. Fields - Understanding Neutrons

To Susan, who long ago set off a chain reaction for me.

Published in 2016 by Cavendish Square Publishing, LLC
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Copyright 2016 by Cavendish Square Publishing, LLC

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Library of Congress Cataloging-in-Publication Data

Fields, B. H., author.

Understanding neutrons / B.H. Fields and Fred Bortz.

pages cm (Exploring the subatomic world)

Includes bibliographical references and index.

ISBN 978-1-50260-542-9 (hardcover) ISBN 978-1-50260-543-6 (ebook)

1. NeutronsJuvenile literature. 2. Particles (Nuclear physics)Juvenile literature.

I. Bortz, Fred, 1944- author. II. Title. III. Series: Exploring the subatomic world.

QC793.5.N462F48 2015

539.7213dc23

2014049225

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Editor: Andrew Coddington

Copy Editor: Cynthia Roby

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Printed in the United States of America

Contents

Atoms and Molecules

The Nucleus

Nuclear Forces

Neutrons and Nuclear Fission

Readers of the entire Exploring the Subatomic World series will notice many similarities between this book and Understanding Protons. That is unavoidable. To physicists such as myself, the proton and the neutron are two different aspects of the same particle that we sometimes call the nucleon. The historical aspects of their stories are quite similar, and thus many parts of the two books are also nearly the same. But there are significant differences between the books as well. I hope you will use both the similarities and differences to understand both types of nucleon and to appreciate what that understanding means to science and technology.

All matter is made of atoms. That statement may seem obvious to you, but when English chemist John Dalton (17661844) made that statement in 1803, it transformed science.

Yet Daltons idea was far from new. The ancient Greek philosophers Leucippus and Democritus had suggested it twenty-three centuries earlier when they asked themselves how small they could cut a piece of matter without changing its nature. They described the smallest possible piece as atomos, meaning indivisible. But their ideas were only of the mind, while Dalton connected the notion of atoms to laboratory observations.

In Daltons theory, the smallest pieces of most substances turn out not to be atoms but molecules. A water molecule is made of two hydrogen atoms and one oxygen atom (H2O). Thus it is not indivisible, but it is still the smallest speck of matter that can still be called water. Dalton spoke of two kinds of substances: elements, which are made of only one kind of atom; and compounds, which are made of only one kind of molecule.

Dalton, like Leucippus and Democritus, thought that atoms were indivisible, but that turns out to be incorrect. As scientists discovered more elements, they began to wonder what distinguished one element from another. Might atoms be made of even smaller particles, just as molecules are made from atoms? By the end of the nineteenth century, physicists (scientists who study matter and energy) had begun to find evidence that the answer was yes.

Today, students like you learn that atoms contain protons, neutrons, and electrons. But that is not the full set of subatomic particles. This series, Exploring the Subatomic World, tells how those particles were discovered and how they combine to give atoms their properties. This book is the story of one of those particles. It will carry you deep inside the atom to explore the nucleus. There you will find neutrons and the forces that act on them. You will also discover how understanding neutrons enabled us to unleash great power, first in war and then for the benefit of the world.

ATOMS
and Molecules

T he ancient Greek idea of atoms was very different from the one we have today. They suggested that the atoms of each different kind of matter had a shape and texture that matched the properties of the substance itself. They expected that water atoms would be round and smooth, while the atoms in rock would be hard and sharp or gritty.

Unlike Dalton, they never considered testing their atomic theory by observing and experimenting. That was not because atoms were too small to see (although they are). It was because testing ideas through observation, a cornerstone of modern science, had not yet become part of human culture. In fact, Greek philosophy took an opposite approach, valuing pure human thought above all else. Logical thinking and reason alone were considered sufficient to deduce the truth. Great thinkers such as Socrates, Aristotle, and Plato used their powerful minds and logic to deduce what they believed to be the truth about the world around them.

Aristotle was considered so brilliant that people would rarely question his ideas, even long after his death. For nearly two thousand years, people simply accepted Aristotles explanation that all the worlds matter was made of four elements: earth, air, fire, and water. The idea of atoms all but disappeared. Today we know that both Democritus and Aristotle were right in one way but wrong in another.