April 21, 1995

One Minute Papers - Questions and Answers

Can (or has) the nucleus be seen through microscopes?

Not exactly. A microscope "sees" an objects by sending a beam of particles at that object and then looking at the reflected or transmitted particles. For example, a common light microscope sends light at an object and then looks at the transmitted or reflected light. But light cannot resolve details smaller than about 1/2 its wavelength. To see smaller objects, you must use particles with shorter wavelengths. Electron microscopes can resolve details down to about 1/2 the electron wavelength (in principle) and that brings their resolution down toward the level of individual atoms. But to see nuclei, which are much smaller than atoms, you need particles with even smaller wavelengths than are available in electron microscopes. However the electrons in particle accelerators do have such small wavelengths that they can resolve features of nuclei. There are no simple images produced, but it is possible to learn much about the structure of the nuclei. In fact, people now look at features even smaller than nuclei, at the structures of the individual nucleons (protons and neutrons) that make up nuclei.

Why can't you make nuclear weapons with any old element?

Only a few elements/isotopes are fissionable, meaning that only a few elements/isotopes have nuclei that shatter when struck by a neutron. Moreover, only a few of this fissionable nuclei release more neutrons than they take to fission. Of naturally occurring isotopes, only Uranium 235 is suitable for nuclear weapons. Plutonium 239 is also suitable, but it must be made artificially in a nuclear reactor.

How do we use (nuclear energy) as a source of energy?

That is the subject of the lecture on April 24, 1995.

If we were ever to have a nuclear war, would we have to live underground?

The long term effects of nuclear war would come primarily from the release of radioactive isotopes into our environment. Large nuclei, such as that of uranium 235, have many more neutrons than protons. These neutrons "dilute" the repelling protons and made these large nuclei less unstable. But once a large nucleus shatters into fragments of medium size, these fragments acquire electrons and become "normal" atoms with medium sized nuclei. Unfortunately, these medium sized nuclei need fewer neutrons than they wind up with and they are generally unstable. While they resemble normal atoms chemical, they contain unstable cores and eventually decay. The decays release energy and this energy can do chemical damage to surrounding material. If the atom has been incorporated into a biological system (e.g. a person), it can do chemical damage to that biological system, perhaps causing cancer or genetic damage. To avoid this insidious damage, people would have to stay away from the fallout chemicals. That would be a difficult task, even underground.

When a plane drops a nuclear bomb, what sets the detonation process into effect?

The altitude at which the bomb explodes affects its results. Near or on the ground, the blast would cause incredible local damage, but less long range damage. Above the ground, the blast would cause less local damage, but more long range damage. So the bomb-makers build altitude sensing equipment into the bomb; probably a pressure sensing or radar-based altimeter. When the bomb has determined that it is at the right height, it triggers. High explosives assemble the critical mass as quickly as possible (typically by crushing the central sphere with carefully shaped high explosive charges). Once the fissionable material exceeds its critical mass, the chain reaction starts and the bomb explodes.

How do you keep the nuclear bomb stable until you're ready to use it? (For example, on the way to Hiroshima)

The nuclear material will only explode if it is assembled to the point of critical mass. If that assembly is done slowly, the material will overheat and melt, perhaps causing a minor explosion but creating more of a radiation hazard than a nuclear detonation. Only if the assembly is done rapidly to well over the critical mass will the bomb explode. To keep a nuclear weapon "safe", the bomb makers ensure that the assembly cannot occur prematurely. They probably remove the triggers for the high explosives or block the paths through which the nuclear material must move. In most cases, even an accidental triggering of the high explosives use in assembly wouldn't cause the bomb to explode because all of the high explosives must be triggered at the same time for the assembly to work properly. If only part of the explosives ignited, the bomb would fizzle (very loudly).

What is the difference between the nuclear bomb and the H bomb?

The fission bomb (uranium or plutonium bomb) derives its energy from the shattering of large nuclei; those in uranium or plutonium. The H bomb (hydrogen, thermonuclear, or fusion bomb) derives most of its energy from the fusion or coalescence of small nuclei; those in hydrogen. The H bomb releases more energy per kilogram than the fission bombs and can be made larger than the fission bombs. However, triggering a hydrogen bomb requires the enormous temperatures of a fission bomb.

By firing neutrons into a nucleus to change an atom, can you make gold using other cheaper metals?

Yes. However, trying to build gold with nuclear reactions is an expensive way to make the precious metal. Furthermore, you would probably end up with radioactive gold because at least some of the nuclei you made would have the wrong numbers of neutrons in them and would be unstable.

How do you do work on an atom so that the nuclear force can overcome the repulsive force.

Building large nuclei is a curiosity in modern science, not a sensible scheme for synthesizing elements. Most of the heavy elements in our world were made during a supernova explosion sometime before the formation of our present solar system about 5 billion years ago. During the supernove explosion, the temperatures became so high that nuclei of all sorts crashed into one another violently and many heavy nuclei were created. The work needed to build these giant nuclei came from the heat of this horrendous explosion.