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Absorbed Dose  
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Ampere  
 (A or amp) The SI base unit of electric current.
Definition: Two parallel conductors, infinitely long and having negligible cross section, should be placed 1 meter apart in a perfect vacuum. One ampere is the current that creates between them a force of 0.2 micronewton per meter of length.
One ampere represents a current flow of 1 coulomb of charge per second. One ampere of current results from a potential distribution of 1 volt per ohm of resistance, or from a power production rate of 1 watt per volt of potential.
The unit is known informally as the amp, but A is its official symbol and is named for the French physicist AndréMarie Ampère.

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Angstrom  
 (Å
* or A*) A unit used to measure the wavelength of Xrays.
Definition: 0.1 nanometer or 10^{10} meter. 
      

Angular Frequency  
       

Bit  
 The basic unit of information.
Definition: The smallest unit of information in the storage on a computer. Eight bits are grouped together to form one byte, additional start and stop bit.
Larger units are
kilobyte (kB) = 1 000 bytes (computer storage 1024 bytes)
megabyte (MB) = 1 000 kB (computer storage 1024 kB)

      

Boltzmann Distribution  
 When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [(E1E2)/kT ] where k is the Boltzmann constant and T is the absolute temperature.
For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon. 
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Celsius  
 A metric unit of temperature.
Definition: One degree is 1/273.16 of the difference between the triple point of water (at exactly 0.01 °C) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states.
Absolute zero is the temperature at which all molecular motion discontinues.
The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (17011744), who used a similar scale.

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Coupling Constant  
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