1987I6
Carbon 14 is produced in the natural environment from the bombardment of nitrogen
atoms in the atmosphere by neutrons according to the following reaction :
14
7N

1
0n


14
6C

1
1H
(a) Carbon 14 is radioactive and undergoes -decay with a half-lift of 5760 years.
Write down the corresponding reaction equation representing the decay of carbon
14.
(b)
out of syllabus
(c)
Carbon 14 can be used to measure the approximate age of archaeological wood
samples. For this purpose it may be assumed that the proportion of carbon 14 in
the natural carbon of living wood is everywhere and at all times the same and
that it begins to change only after death. Suppose a piece of timber has been
recovered from an excavation, and from which 10 g of carbon is prepared.
Determine the number of carbon 14 atoms originally present in the sample,
given that the ratio of carbon 14 to natural carbon is 1.251012.
( Relative atomic mass of natural carbon
= 12
Avogadro constant
= 6  1023 mol1 )
(d)
If the activity of the sample is found to be 191012 Ci ( 1 Ci corresponds to
3.71010 disintegrations per second), estimate
(i)
the number of carbon 14 atoms in the timber, and
(ii)
the age of the sample in years,
given that the half-life of carbon 14 is 5760 years.
(e)
List TWO advantages which make carbon 14 a suitable nuclide for dating
archaeological sample.
1995I4(AS)
A geologist wants to find the age of a sample of rock containing 40K which decays to
give the stable isotope 40Ar . The activity of the sample is found to be 1.6 Bq while
the original activity of a similar rock having the same mass is 4.8 Bq. The half-life of
40
K is 1.3  109 years.
(a) (i) Find the decay constant of 40K.
(ii) Give the physical meaning of the decay constant of a radioactive isotope.
(b)
Find the age of the rock sample.
(c)
Give two factors that determine the activity of a radioactive source.
(d)
The decay of 40K to 40Ar is spontaneous. How is the magnitude of the binding
energy of 40K compared to that of 40Ar .
(e)
Mention a difficulty involved in measuring such a small decay rate of 1.6 Bq.
1987I6
Carbon 14 is produced in the natural environment from the bombardment of nitrogen
atoms in the atmosphere by neutrons according to the following reaction :
14
7N

1
0n


14
6C

1
1H
(b) Carbon 14 is radioactive and undergoes -decay with a half-lift of 5760 years.
Write down the corresponding reaction equation representing the decay of carbon
14.
(b)
out of syllabus
(c)
Carbon 14 can be used to measure the approximate age of archaeological wood
samples. For this purpose it may be assumed that the proportion of carbon 14 in
the natural carbon of living wood is everywhere and at all times the same and
that it begins to change only after death. Suppose a piece of timber has been
recovered from an excavation, and from which 10 g of carbon is prepared.
Determine the number of carbon 14 atoms originally present in the sample,
given that the ratio of carbon 14 to natural carbon is 1.251012.
( Relative atomic mass of natural carbon
= 12
Avogadro constant
= 6  1023 mol1 )
(d)
If the activity of the sample is found to be 191012 Ci ( 1 Ci corresponds to
3.71010 disintegrations per second), estimate
(i)
the number of carbon 14 atoms in the timber, and
(ii)
the age of the sample in years,
given that the half-life of carbon 14 is 5760 years.
(e)
List TWO advantages which make carbon 14 a suitable nuclide for dating
archaeological sample.
1995I4(AS)
A geologist wants to find the age of a sample of rock containing 40K which decays to
give the stable isotope 40Ar . The activity of the sample is found to be 1.6 Bq while
the original activity of a similar rock having the same mass is 4.8 Bq. The half-life of
40
K is 1.3  109 years.
(a) (i) Find the decay constant of 40K.
(ii) Give the physical meaning of the decay constant of a radioactive isotope.
(b)
Find the age of the rock sample.
(c)
Give two factors that determine the activity of a radioactive source.
(d)
The decay of 40K to 40Ar is spontaneous. How is the magnitude of the binding
energy of 40K compared to that of 40Ar .
(e)
Mention a difficulty involved in measuring such a small decay rate of 1.6 Bq.