Radioactivity Model Answers

Question 1

Compare the methods of measuring nuclear radii: alpha scattering and electron diffraction by referring to the principles on which each experiment is based, an explanation of what may limit the accuracy of each method, and the advantages and disadvantages of each method.

Answer 1

alpha particle scattering involves coulomb or electrostatic repulsion
electron diffraction treats the electron as a wave having a de Broglie wavelength

alpha particle scattering only measures the least distance of approach, not the radius
alpha particles have a finite size which must be taken into account
electrons need to have high speed/kinetic energy
to have a small wavelength or wavelength comparable to

Question 2

3a

Answer 2

Probability of decay per unit time/given time period
OR fraction of atoms decaying per second
OR the rate of radioactive decay is proportional to the number of unstable nuclei and nuclear decay constant is the constant of proportionality

Question 3

3e

Answer 3

the boat may have been made with the wood some time after the tree was cut down
the background activity is high compared to the observed count rates
the count rates are low or sample size/mass is small or there is statistical variation in the recorded results
possible contamination
uncertainty in the ratio of carbon-14 thousands of years ago

Question 4

m3bii

Answer 4

count for large periods OR

repeat counts and average

Question 5

1ci

Answer 5

handle with long 30cm tweezers
because the radiation intensity decreases with distance

store in a lead box (immediately) when not in use
to avoid unnecessary exposure to radiation

Question 6

1c

Answer 6

alpha particles
coulomb/electrostatic/electromagnetic repulsion
or KE converted to PE as alpha particle approaches nucleus
mass of nucleus is most of the mass of an atom

high energy electron scattering
diffraction of de Broglie waves by nucleus

Question 7

exam question c

Answer 7

alpha scattering involves coulomb or electrostatic repulsion
electron diffraction treats the electron as a wave having a de Broglie wavelength

alpha particles only measure the least distance of approach, not the radius
alpha particles have a finite size which must be taken into account
electrons need to have high speed/kinetic energy
to have a small wavelength or wavelength comparable to nuclear diameter, the wavelength determines the resolution
the wavelength needs to be of the same order as the nuclear diameter for significant diffraction