ATOMIC PHYSICS PROBLEMS
(Write equations for all the nuclear reactions mentioned in the questions)
1. Tritium 13 H has a half-life of 12.5 years against  decay. What
fraction of a sample of pure Tritium will remain undecayed after 25
years?
2. The half-life of 131
53 I against  decay is 8.07 days. This isotope is
used in thyroid gland radiotherapy. If 100mg of 131
53 I is
concentrated in the thyroid gland of a patient,
a. how much 131
53 I will be left after 48.4 days?
b. How many particles will have been produced at that time if
23
100mg of 131
atoms?
53 I contains 4.60x10
3. In Rutherford’s scattering experiment the gold foil was found to be
about 400 atoms thick. It was measured as 1x10-7m thickness.
a. Find the thickness (diameter) of 1 gold atom
b. Find how many sheets of gold foil stacked together
correspond to the thickness of a piece of paper. 80gsm
(photocopier) paper has an approximate thickness of
0.11mm.
4. One gram of Radium has (by definition) an activity of 1 curie (Ci).
1 Ci = 3.70x1010 disintegrations per second. Determine the halflife of Radium, if one gram of Radium contains 2.66x1021 atoms.
238
U against  decay is 4.51x109 years. How
5. The half-life of 923
238
U , if 1g contains
many disintegrations per second occur in 1g of 923
21
2.53x10 atoms?
238
U is found to contain 49.4%
6. An isolated sample, originally pure 923
234
(by mass) of Thorium 90Th . Use the data from question 4 above
to calculate the age of the Earth.
ANSWERS
1.
2.
3.
4.
5.
6.
¼
(a) 1.57mg (b) 4.53x1023
(a) 2.5x10-10m (b) 1,100
1,580 years
1.40x1011 dps
4.49x109 years