The base unit is the ampere (A).
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2.10-7 newton per metre of length.
The International System of units (SI) does not include a base unit for the ionizing radiation field. All the measurement units for quantities used in this field come from derived units.
The base unit is the metre (m).
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The base unit is the kilogram (kg).
The kilogram is equal to the mass of the international prototype of the kilogram.
The international prototype is an artefact made of platinum-iridium kept by the BIPM (Bureau international des poids et mesures) under the conditions specified by the 1st CGPM (Conférence générale des poids et mesures) held in Paris in 1889.
The base unit is the candela (cd).
The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540.1012 hertz and that has a radiant intensity in that direction is of 1/683 watt per steradian.
The base unit is the kelvin (K).
The kelvin is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water.
The base unit is the second (s).
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.