The medusa and the snail lewis thomas

Pronominal 1 possessives hers, its, theirs, yours, and ours have no apostrophe. Indefinite pronouns, however, use the apostrophe to show possession. It was previously conventional to use an apostrophe in expressions of time involving a plural reference […] The apostrophe is now often left out.

The medusa and the snail lewis thomas

To keep the crew and the computers alive you have to shield them from both gamma rays and pions. As far as the crew is concerned both reaction products come under the heading of "deadly radiation.

Given an absorbing propellant or radiation shield of a specific density you can figure the thickness that will stop all the pions. In table the columns under the yellow bar show how many centimeters the "range" of the given stopping material is required to absorb MeV of pion energy.

The two sets of orange bars is because while the range is relatively constant for all high energies, the range becomes dramatically less at the point where the pion energy drops below MeV the "last MeV". But you only need 27 centimeters of water to absorb MeV from a 75 MeV pion. Since hydrogen, helium, and nitrogen have regrettably low densities the reaction chamber will have to operate at high pressure to get the density up to useful levels.

The Space Shuttle engines operated at a pressure of atmospheres, is a bit excessive. So of the gases nitrogen might be preferrable, even though you can get better specific impulse out of propellants with lower molecular weight.

Range of charged pions from Antiproton Annihilation Propulsion Using more calculations that were not explained figure was produced. The curve is the relative intensity of a charged pion at a given kinetic energy in MeV.

The medusa and the snail lewis thomas

The MeV pions are the most intense there are more of themthe average energy is MeV. Mean Life is the lifespan not half-life of a pion at that energy in nanoseconds.

The range of a pion at that energy can be measured on the RANGE scales below, traveling through vacuum, hydrogen H2 propellant at atm, nitrogen N2 propellant at atm, and tungsten radiation shielding. Gamma Rays Sadly gamma rays cannot be used to propel the rocket well, actually there are a couple of strange designs that do use gammasall they do is kill anything living and destroy electronic equipment.

The medusa and the snail lewis thomas

So you have to shield the crew and electronics with radiation shielding. This is one of the big drawbacks to antimatter rockets. Gamma-rays would be useful if you were using antimatter as some sort of weapon instead of propulsion.

A small number of "prompt" gamma-rays are produced directly from the annihilation reaction. The prompt gammas have a whopping MeV, but they only contribute about 0. A much larger amount of "delayed" gamma-rays are produced by the neutral pions decaying 90 attoseconds after the antimatter reaction.

As mentioned abovethe antimatter reaction is basically spitting out charged pions and gamma rays. The pions can be absorbed by the propellant and their energy utilized.

The gamma rays on the other hand are just an inconvenient blast of deadly radiation traveling in all directions. Since gamma rays are rays, not particles, they have that pesky exponential attenuation with shielding.

Particle shielding on the other hand have a thickness where nothing penetrates. Gamma rays with energies higher than MeV have a "attenuation coefficient" of about 0. Since tungsten has a density of Table gives the attunation for various thickness of tungsten radiation shields.Here is your handy-dandy cheat-sheet of rocket engines.

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The Medusa and the Snail: More Notes of a Biology Watcher by Lewis Thomas