The Los Alamos Primer

The First Lectures on How to Build an Atomic Bomb, Updated with a New Introduction by Richard Rhodes

Robert Serber

• Publisher: University of California Press
• Year: 2020
• ISBN:
  9780520374331
• Edition: 1
• Topic: History
• Subtopic: North American History
• Index: History

More than seventy years ago, American forces exploded the first atomic bombs over the Japanese cities of Hiroshima and Nagasaki, causing great physical and human destruction. The young scientists at Los Alamos who developed the bombs, which were nicknamed Little Boy and Fat Man, were introduced to the basic principles and goals of the project in March 1943, at a crash course in new weapons technology. The lecturer was physicist Robert Serber, J. Robert Oppenheimer's protégé, and the scientists learned that their job was to design and build the world's first atomic bombs. Notes on Serber's lectures were gathered into a mimeographed document titled The Los Alamos Primer, which was supplied to all incoming scientific staff. The Primer remained classified for decades after the war. Published for the first time in 1992, the Primer offers contemporary readers a better understanding of the origins of nuclear weapons. Serber's preface vividly conveys the mingled excitement, uncertainty, and intensity felt by the Manhattan Project scientists. This edition includes an updated introduction by Pulitzer Prize–winning historian Richard Rhodes. A seminal publication on a turning point in human history, The Los Alamos Primer reveals just how much was known and how terrifyingly much was unknown midway through the Manhattan Project. No other seminar anywhere has had greater historical consequences.

The Los Alamos Primer

The following notes are based on a set of five lectures given by R. Serber during the first two weeks of April 1943, as an “indoctrination course” in connection with the starting of the Los Alamos Project. The notes were written up by E. U. Condon.

Everybody assembled in the big library reading room on the first floor of the Technical Area, the building where the theoretical physicists had their offices. We had a little blackboard set up in front and a lot of folding chairs spread around the room. Fifty people on hand, something like that. Scientific staff, a few visiting VIPs. There was hammering off in the background, carpenters and electricians working out of sight but all over the place. At one point during the lectures a leg came bursting through the beaverboard ceiling. One of the workmen misstepped and they had to pull him out.

1. Object

The object of the project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.

I started lecturing. I started talking about the “bomb.” After a couple of minutes Oppie¹ sent John Manley up to tell me not to use that word. Too many workmen around, Manley said. They were worried about security. I should use “gadget” instead. In the Primer Condon wrote it down both ways. But around Los Alamos after that we called the bomb we were building the “gadget.”

Section 1 emphasizes that our purpose at Los Alamos was to build a practical military weapon—one small enough and light enough that an airplane could carry it.² There was no use making something that weighed one hundred tons. That was our concern.

We meant to build this weapon by utilizing the energy from nuclear fission. Fission had a history. For a long time before 1939, people were bombarding uranium with neutrons. Uranium was the heaviest element known up to 1939. People had the idea that the uranium they were bombarding was capturing neutrons and transmuting to heavier elements, elements beyond uranium on the periodic table, transuranics. I remember seminars in Berkeley in the 1930s when the chemists discussed the trouble they were having explaining the chemistry of these supposed transuranic elements. The chemistry didn’t seem to be working out right. Then Otto Hahn and Fritz Strassmann, in Germany, working with the physicist Lise Meitner, found out that making transuranics wasn’t what was usually happening at all. Instead, the uranium nucleus was actually splitting into two big pieces, and doing it with the release of a great deal of energy (and a couple of extra neutrons, as several people soon demonstrated). As soon as that was discovered, everybody realized the possibility both of making weapons and of getting power.

Finally, the reaction we were interested in was a fast neutron chain reaction, which I’ll discuss later in these notes.

2. Energy of Fission Process

The direct energy release in the fission process is of the order of 170 Mev per atom. This is considerably more than 10⁷ times the heat of reaction per atom in ordinary combustion processes.

In Section 2³ we immediately come to the heart of the matter: that the energy released in the fission of the uranium nucleus is considerably greater than 10⁷—that is, ten million times the energy released in a typical chemical combustion such as an explosion or a fire. All else follows from this fact. So we should try to understand where this large number comes from.

We can do so because the origin of the energy released in fission is exactly the same as the origin of the energy released when two atoms or molecules react chemically. It’s the electrostatic energy between two similarly charged particles. Two similarly charged particles repel each other. There’s an electrical force pushing them apart. Work has to be done to overcome that repulsion and push them together from a large distance, up to a point of separation we can call R.

To start with a simpler particle than an atom, let’s look at two electrons pushed together. If you released them, they would fly apart with an amount of energy equal to the work that went into pushing them together. That energy E is given by the formula

where e is the electron charge, e² is e multiplied by itself, and R is the distance between the particles. The electrostatic energy thus ends up as kinetic energy, the energy of motion. In chemical reactions—the burning of hydrogen and oxygen in a rocket engine, for example—electrons bound in atoms or molecules change their positions, and the change in electrostatic energy is what appears as the energy of the chemical reaction.

Now let’s consider the electrostatic energy in the uranium nucleus. The uranium nucleus contains 92 protons, each of which has the same charge as an electron, though of opposite sign—particles of opposite sign attract each other, those of the same sign repel. So the uranium nucleus has a charge 92 times as great as an electron; it’s positive rather than negative, + rather than −, but since only the square of the charge is involved, that difference doesn’t matter in equation (1). The numerator of (1) is thus 92² times bigger than for a chemical reaction. For our purposes, 92² is close enough to call 100². So the numerator for a uranium atom would be greater by a factor of 100², 100 times ...

Table of contents

• Title
• Copyright
• Contents
• Introduction by Richard Rhodes
• Preface by Robert Serber
• Illustrations
• The Los Alamos Primer
• 1 Object
• 2 Energy of Fission Process
• 3 Fast Neutron Chain Reaction
• 4 Fission Cross-sections
• 5 Neutron Spectrum
• 6 Neutron Number
• 7 Neutron Capture
• 8 Why Ordinary U Is Safe
• 9 Material 49
• 10 Simplest Estimate of Minimum Size of Bomb
• 11 Effect of Tamper
• 12 Damage
• 13 Efficiency
• 14 Effect of Tamper on Efficiency
• 15 Detonation
• 16 Probability of Predetonation
• 17 Fizzles
• 18 Detonating Source
• 19 Neutron Background
• 20 Shooting
• 21 Autocatalytic Methods
• 22 Conclusion
• Endnotes
• Appendix I: The Frisch-Peierls Memorandum
• Appendix II: Biographical Notes
• Notes
• Index