Curie was originally based on the rate of decay of a gram of radium
There are $3.7 \times 10^{10}$ disintegrations per sec per gram of radium .This no is taken as a standard
=> One curie=$3.7 \times 10^{10}$ disintegrations per sec
One curie of activity is very strong source of radiation
=> 1 milli curie=1mCi=$10^{-3}$Ci
1 microcurie=1μCi=$10^{-6}$ Ci
Another unit of activity is Rutherford
$1rd=10^6$ dis/sec
Activity |dN/dt|=λN=.693N/T
=> A very short lived substance gives rise to large activity ,even it is present in minute quantities
The SI unit of radioactivity recently proposed is Becquerel (Bq) which is defined as activity done to one disintegration per sec hence
1ci=$3.7 \times 10^{10}$ bq
=37G bq
Nucleus before the decay is called parent nucleus and after the decay is called daughter nucleus
In Alpha decay, the parent nucleus AXZ emits an α particle (=4He2) leaving behind a daughter nucleus of four mass unit less and two charge units less i.e. A-4XZ-2
α decay shift the element two places to the left in the periodic tables of elements ex
All nuclides of A >= 210 and Z > 83 tends to decay by α emission
209Bi is the heaviest stable nuclide in nature
α decay in heavy nucleus occur because a too heavy nucleus becomes unstable due to coulomb repulsion and by emitting an α particle the nucleus decrease its A and Z to moves towards stability
Now the rest mass energy of parent nucleus AXZ is greater then the sum of rest mass energies of A-4XZ-2 and
4He2
The difference between the rest mass energies of initial constituents and final products is called Q-value of the process
For α decay process ,Q value is
Q=[mp -(md+mα)]c2
where mp -> Mass of parent nucleus ZAX
md -> Mass of parent nucleus Z-2A-4X
mα -> Mass of parent nucleus 24He
