Physical Quantity
Symbol
Dimensions
Units
Remarks
Atomic mass unit
[M]
u
Unit of mass for
expressing atomic or
nuclear masses One
atomic mass unit equals
1/12th of the mass of 12C
atom Disintegration or
l
[T –1]
s–1
decay constant
Half-life
T1/2
[T]
s
Time taken for the decay
of one-half of the initial
number of nuclei present
in a radioactive sample Mean life
t
[T]
s
Time at which number of
nuclei has been reduced to
e–1 of its initial value
Activity of a radio-
R
[ T–1]
Bq
Measure of the activity
active sample
of a radioactive source