Carbon
−
14
-
is produced when neutrons from cosmic rays bombard
.
7
N
14
.
.
.
7
N
14
+
.
.
0
n
1
→
.
.
6
C
14
+
.
.
1
H
1
.
Plants use
C
O
2
C
for photosynthesis, therefore they have
C
−
14
C
also.
As long as plants are alive, the amount of
C
−
14
C
in them is constant. When plants die ,
C
−
14
C
starts decaying and by comparing the amount of
C
−
14
C
in the living plants and deat plant, age of dead plant can be determined.
