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An Explanation of some Scientific Terms:


Materials for Fission

Atom Bomb

Solving the problem to produce the atombomb

Schroedinger's cat

The Manhattan Project

Principle of Uncertainty

Theory of Complementarity

The Copenhagen Interpretation

Excerpts from Heisenberg's paper on the Theory of Copenhagen - not difficult to read.

World War II Timeline

Photos of Hiroshima


The spontaneous or induced disintegration of a heavy atomic nucleus into two or more lighter fragments. The energy released in the process is referred to as nuclear energy.

Materials for fission:

Fissile material Although sometimes used as a synonym for fissionable material, this term has acquired a more restricted meaning. Namely, any material fissionable by thermal (slow) neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.


A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons and is fertile. Natural uranium also includes a minute amount of uranium-234.

U235 and fission

The uranium atom was actually split into two atoms of approximately the same size -- and fission was accomplished. This released significant amounts of energy. It was found that an isotope of uranium, uranium-235, easily fissioned with slow neutrons to yield krypton and barium. (Taylor, 353) Unfortunately, uranium-235 is found in naturally occurring uranium only about 1 part in 137. Extracting it is not an easy process. (Segre, "From X-rays...",

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The element with the most protons known at the time was uranium. It has 92 protons and 146 neutrons. (It is usually known as uranium-238.)


One of two or more atoms with the same number of protons, but different numbers of neutrons in their nuclei. Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element carbon, the numbers denoting the approximate atomic weights. Isotopes have very nearly the same chemical properties, but often different physical properties (for example, carbon-12 and -13 are stable, carbon-14 is radioactive).


Plutonium has assumed the position of dominant importance among the trasuranium elements because of its successful use as an explosive ingredient in nuclear weapons and the place which it holds as a key material in the development of industrial use of nuclear power. One kilogram is equivalent to about 22 million kilowatt hours of heat energy. The complete detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical explosive. Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world's nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. The various nuclear applications of plutonium are well known. 238Pu has been used in the Apollo lunar missions to power seismic and other equipment on the lunar surface. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals.

Source: Los Alamos National Laboratory, US Department of Energy.

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Fast Reactors

A reactor in which the fission chain reactionis sustained primarily by fast neutrons rather than by slow moving neutrons. Fast reactors contain little or no moderator to slow down the neutrons from the speeds at which they are ejected from fissioning nuclei.


A material, such as ordinary water, heavy water, or graphite, that is used in a reactor to slow down high-velocity neutrons, thus increasing the likelihood
of fission.

Heavy Water

Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.

Uncertainty Principle

Definition: The principle that it is not possible to know with unlimited precision both the position and momentum of a particle. This principle was discovered in 1927 by Werner Heisenberg.

Theory of complementarity

In quantum mechanics, the wave and particle models are complementary. A measurement showing the wave character of an object cannot prove the particle character in the same measurement and vice versa.

Bohr's Copenhagen Interpretation:

According to the Copenhagen interpretation it is neither possible nor reasonable to search for properties of a quantum system as such. Since we can only communicate what we have found by using our classical language, questions concerning properties of systems only make sense, strictly speaking, as questions about classical properties of a classical apparatus.
See On the Interpretation and Philosophical Foundation of Quantum Mechanics
Anton Zeilinger:

Schrödinger's cat (referred to in the play)

 Definition: A thought experiment introduced by Erwin Schrödinger in 1935 to illustrate the paradox in quantum mechanics regarding the probability of finding, say, a subatomic particle at a specific point in space. According to Niels Bohr, the position of such a particle remains indeterminate until it has been observed. Schrödinger postulated a sealed vessel containing a live cat and a device triggered by a quantum event such as the radioactive decay of the nucleus. If the quantum event occurs, cyanide is released and the cat dies; if the event does not occur the cat lives. Schrödinger argued that Bohr's interpretation of events in quantum mechanics means that the cat could only be said to be alive or dead when the vessel has been opened and the situation inside it has been observed. This paradox has been extensively discussed since its introduction with many proposals made to resolve it.

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The above definitions found at:
and the U.S Regulatory Nuclear Commission site:

Solving the problem so that fission could be used for the Atomic bomb:

"Exactly what would happen, they asked themselves, if you could cull from natural uranium a mass composed purely of the rare uranium-235? Bohr and others had told the public that there could be enough energy there to blow up a city, but nobody had worked it out as a serious technical possibility. Now Frisch and Peierls realized that with fissionable uranium-235 atoms all crammed together, there would be no need for a moderator to slow the neutrons down, since even the fast neutrons emitted in each fission would have a good chance to provoke another fission. The whole chain reaction would go so swiftly that, before the mass had a chance to blow itself apart, a run away [reaction would allow] many of the uranium-235 atoms [to] split and release energy." (Weart, 84)

This question may have been left academic for years had it not been for World War II. As the awesome power of an atomic bomb was realized by leaders of several countries, a race began to be the first to make a working bomb. As a result, a simpler method was discovered than separating uranium-235 from uranium-238.

This simpler method starts when uranium-238 absorbs a single neutron a new element, called neptunium-239, is created. (Neptunium-239 has 93 protons and 146 neutrons.) This element decays into plutonium-239 (94 protons and 145 neutrons). Plutonium is stable and also has the property of undergoing fission with slow neutrons. Hence, the atom bomb was conceivable. Plutonium was produced in a reactor. (Weart, 87)

ATOM BOMB is a weapon with great explosive power that results from the sudden release of energy upon the splitting, or fission, of the nuclei of such heavy elements as plutonium or uranium.

When a neutron strikes the nucleus of an atom of the isotopes uranium 235 or plutonium-239, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In the process of splitting, a great amount of thermal energy, as well as gamma rays and two or more neutrons, is released. Under certain conditions, the escaping neutrons strike and thus fission more of the surrounding uranium nuclei, which then emit more neutrons that split still more nuclei. This series of rapidly multiplying fissions culminates in a chain reaction in which nearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb.

Many isotopes of uranium can undergo fission, but uranium-235, which is found naturally at a ratio of about one part per every 139 parts of the isotope uranium-238, undergoes fission more readily and emits more neutrons per fission than other such isotopes. Plutonium-239 has these same qualities. These are the primary fissionable materials used in atomic bombs. A small amount of uranium-235, say 0.45 kg (1 pound), cannot undergo a chain reaction and is thus termed a subcritical mass; this is because, on average, the neutrons released by a fission are likely to leave the assembly without striking another nucleus and causing it to fission. If more uranium-235 is added to the assemblage, the chances that one of the released neutrons will cause another fission are increased, since the escaping neutrons must traverse more uranium nuclei and the chances are greater that one of them will bump into another nucleus and split it. At the point at which one of the neutrons produced by a fission will on average create another fission, critical mass has been achieved, and a chain reaction and thus an atomic explosion will result.

In practice, an assembly of fissionable material must be brought from a subcritical to a critical state extremely suddenly. One way this can be done is to bring two subcritical masses together, at which point their combined mass becomes a critical one. This can be practically achieved by using high explosives to shoot two subcritical slugs of fissionable material together in a hollow tube. A second method used is that of implosion, in which a core of fissionable material is suddenly compressed into a smaller size and thus a greater density; because it is denser, the nuclei are more tightly packed and the chances of an emitted neutron's striking a nucleus are increased. The core of an implosion-type atomic bomb consists of a sphere or a series of concentric shells of fissionable material surrounded by a jacket of high explosives, which, being simultaneously detonated, implode the fissionable material under enormous pressures into a denser mass that immediately achieves criticality. An important aid in achieving criticality is the use of a tamper; this is a jacket of beryllium oxide or some other substance surrounding the fissionable material and reflecting some of the escaping neutrons back into the fissionable material, where they can thus cause more fissions. In addition, "boosted fission" devices incorporate such fusionable materials as deuterium or tritium into the fission core. The fusionable material boosts the fission explosion by supplying a superabundance of neutrons.

Fission releases an enormous amount of energy relative to the material involved. When completely fissioned, 1 kg (2.2 pounds) of uranium-235 releases the energy equivalently produced by 17,000 tons, or 17 kilotons, of TNT. The detonation of an atomic bomb releases enormous amounts of thermal energy, or heat, achieving temperatures of several million degrees in the exploding bomb itself. This thermal energy creates a large fireball, the heat of which can ignite ground fires that can incinerate an entire small city. Convection currents created by the explosion suck dust and other ground materials up into the fireball, creating the characteristic mushroom-shaped cloud of an atomic explosion. The detonation also immediately produces a strong shock wave that propagates outward from the blast to distances of several miles, gradually losing its force along the way. Such a blast wave can destroy buildings for several miles from the location of the burst. Large quantities of neutrons and gamma rays are also emitted; this lethal radiation decreases rapidly over 1.5 to 3 km (1 to 2 miles) from the burst. Materials vaporized in the fireball condense to fine particles, and this radioactive debris, referred to as fallout, is carried by the winds in the troposphere or stratosphere. Since the radioactive contaminants include such long-lived radioisotopes as strontium-90 and plutonium-239, they can have lethal effects for weeks after the explosion.

The first atomic bombs were built in the United States during World War II under a program called the Manhattan Project. One bomb, using plutonium, was successfully tested on July 16, 1945, at a site 193 km (120 miles) south of Albuquerque, N.M. (see photograph). The first atomic bomb to be used in warfare used uranium. It was dropped by the United States on Hiroshima, Japan, on Aug. 6, 1945. The explosion, which had the force of more than 15,000 tons of TNT, instantly and completely devastated 10 square km (4 square miles) of the heart of this city of 343,000 inhabitants. Of this number, 66,000 were killed immediately and 69,000 were injured; more than 67 percent of the city's structures were destroyed or damaged. The next atomic bomb to be exploded was of the plutonium type; it was dropped on Nagasaki on Aug. 9, 1945, producing a blast equal to 21,000 tons of TNT. The terrain and smaller size of Nagasaki reduced destruction of life and property, but 39,000 persons were killed and 25,000 injured; about 40 percent of the city's structures were destroyed or seriously damaged. The Japanese initiated surrender negotiations the next day.

After the war, the United States conducted dozens of test explosions of atomic bombs in the Pacific at Enewetak (Eniwetok) atoll and in Nevada. In subsequent years, the Soviet Union (1949), Great Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998) tested fission weapons of their own. The great temperatures and pressures created by a fission explosion are also used to initiate fusion and thus detonate a thermonuclear bomb..

From the encyclopedia Britannica,5716,10254+1,00.html

The Manhattan Project

On the 2nd August 1939 some scientists wrote to President Roosevelt of efforts in Nazi Germany to purify Uranium-23 5 with which might in turn be used to build an atomic bomb. It was shortly thereafter that the United StatesGovernment began the serious undertaking known only then as the Manhattan Project. The Manhattan Project was designed to research and production that would produce a usable atomic bomb. The Project was named after the Manhattan Engineer District of the US Army Corps of Engineers, because a lot of the early research was done in New York.

In 1942 General Leslie Grove was chosen to lead the project. He brought a site at Oak Ridge, Tenn. For facilities to separate the necessary uranium-235 from the much more common uranium-238. Robert Oppenheimer was appointed to lead the day to day running of the project. The team of scientists who worked on the atom bomb worked 6 days a week and often 18 hours a day.

By 1945 the project has nearly 40 laboratories and factories which employed 200,00 people. That was more than the total amount of people employed in the US automobile industry in 1945. The total cost of the Manhattan project was $2-billion which is about the equivalent of $26 billion today.

The Copenhagen Theory: An explanation

The Copenhagen Interpretation: Jacques Mallah

Historically, the Copenhagen interpretation was the first to recieve wide recognition (1927); it basically asserts that experimental predictions are the reality, and that attempts to model underlying mechanisms are futile. There is virtually no agreement on the details, even among advocates. Today, it is widely seen .... as only one among the possible interpretations. Most other interpretations are ontological, which means they assume some underlying model for reality which, if not exactly right, is at least supposed to capture whatever features are relevant of whatever model would be exactly right.

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The Copenhagen Interpretation of quantum theory by Heisenberg (1958)

Heisemberg's explanation. Not difficult to read.

Excerpts from Heisenberg's explanation:

(1) A real difficulty in the understanding of this interpretation [of quantum theory] arises, however, when one asks the famous question: But what happens 'really' in an atomic event? It has been said before that the mechanism and the results of an observation can always be stated in terms of the classical concepts. But what one deduces from an observation is a probability function, a mathematical expression that combines statements about possibilities or tendencies with statements about our knowledge of facts So we cannot completely objectify the result of an observation, we cannot describe what 'happens' between this observation and the next. This looks as if we had introduced an element of subjectivism into the theory, as if we meant to say: what happens depends on our way of observing it or on the fast that we observe it. Before discussing this problem of subjectivism it is necessary to explain quite clearly why one would get into hopeless difficulties if one tried to describe what happens between two consecutive observations.

(2) To make this point clearer we have to analyse the process of observation more closely.

To begin with, it is important to remember that in natural science we are not interested in the universe as a whole, including ourselves, but we direct our attention to some part of the universe and make that the object of our studies. In atomic physics this part is usually a very small object, an atomic particle or a group of such particles, sometimes much larger - the size does not matter; but it is important that a large part of the universe, including ourselves, does not belong to the object.

Now, the theoretical interpretation of an experiment starts with the two steps that have been discussed. In the first step we have to describe the arrangement of the experiment, eventually combined with a first observation, in terms of classical physics and translate this description into a probability function. This probability function follows the laws of quantum theory, and its change in the course of time, which is continuous, can be calculated from the initial conditions; this is the second step. The probability function combines objective and subjective elements. It contains statements about possibilities or better tendencies ('potentia' in Aristotelian philosophy), and these statements are completely objective, they do not depend on any observer; and it contains statements about our knowledge of the system, which of course are subjective in so far as they may be different for different observers. In ideal cases the subjective element in the probability function may be practically negligible as compared with the objective one. The physicists then speak of a 'pure case'.

When we now come to 'the next observation. the result of which should be predicted from the theory, it is very important to realize that our object has to be in contact with the other part of-the world, namely, the experimental arrangement, the measuring rod, etc., before or at least at the moment of observation. This means that the equation of motion for the probability function does now contain the influence of the interaction with the measuring device. This influence introduces a new element of uncertainty, since the measuring device is necessarily described in the terms of classical physics; such a description contains all the uncertainties concerning the microscopic structure of the device which we know from thermodynamics, and since the device is connected with the rest of the world, it contains in fact the uncertainties of the microscopic structure of the whole world. These uncertainties may be called objective in so far as they are simply a consequence of the description in the terms of classical physics and do not depend on any observer. They may be called subjective in so far as they refer to our incomplete knowledge of the world.

After this interaction has taken place, the probability function contains the objective element of tendency and the subjective element of incomplete knowledge, even if it has been a 'pure case' before. It is for this reason that the result of the observation cannot generally be predicted with certainty; what can be predicted is the probability of a certain result of the observation, and this statement about the probability can be checked by repeating the experiment many times. The probability function does - unlike the common procedure in Newtonian mechanics - not describe a certain event but, at least during the process of observation, a whole ensemble of possible events.

(3) With regard to this situation Bohr has emphasised that it is more realistic to state that the division into the object and the lrest of the world is not arbitrary. Our actual situation in research work in atomic physics is usually this: we wish to understand a l certain phenomenon, we wish to recognise how this phenomenon follows from the general laws of nature. Therefore that part of matter or radiation which takes part in the phenomenon is the natural 'object' in the theoretical treatment and should be separated in this respect from the tools used to study the phenomenon. This again emphasises a subjective element in the description of atomic events, since the measuring device has been constructed by the observer, and we have to remember that what we observe is not nature in itself but nature exposed to our method of questioning. Our scientific work in physics consists in asking questions about nature in the language that we possess and trying to get an answer from experiment by the means that are at our disposal. In this way quantum theory reminds us, as Bohr has put it, of the old wisdom that when searching for harmony in life one must never forget that in the drama of existence we are ourselves both players and spectators. It is understandable that in our scientific relation to nature our own activity becomes very important when we have to deal with parts of nature into which we can penetrate only by using the most elaborate tools.

Source: Physics and Philosophy (1958) publ. George Allen and Unwin Edition, 1959. Chapters 2 (history), 3 (Copenhagen interpretation) and 5 (HPS).
This can be found at the following site

World War II Timeline :

Photos of Hiroshima after the bombing :

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