T9 Neutrons Assignment

Q.1 Consider thermal neutron capture by 109Cd

1. Write the reaction equation.
2. Calculate the Q-value for this reaction.
3. Is the activation product stable or radioactive? (consult an appropriate reference)
4. If radioactive, what is its decay mode? And what does it decay to?

If stable, is its binding energy per nucleon (B/A) greater or less than that of the initial isotope?

Q.2 Consider the following n-gamma reaction:

¹H(n,y)²H

where an incident neutron is captured by the target proton, forming a 2H nucleus (i.e. a deuteron).

Q. When the incident neutron is captured by the struck proton, why is a gamma ray emitted?
Hint: compare the mass defects of the initial and final nuclei
(this is fundamentally the calculation of the Q value!)

Q.3 Calculate the Q-value for the spontaneous decay of a free proton into a neutron and a positron. Comment.

Q.4 Consider the process of neutron activation of a metallic sample that was previously not radioactive.

1. What is the role of the moderator in a neutron activation experiment?
2. Define the term “cross section” as it relates to nuclear reactions. What 4 parameters does cross section depend upon?
3. The reaction rate R is relevant to the production of new isotopes by neutron capture. For this problem we will irradiate a thin circular foil (5.00 cm diameter and 1.25 mm thick) made from 1007Au. The density of Gold is 19.32 g/cm3 and the molar mass of Gold is 196.96655 g/mol. The cross section for the reaction 197Au(n,?) 198Au by thermal neutrons is 98.70 barns.

Determine the reaction rate if the neutron flux from an AmBe neutron source = 3.5 x 109 neutrons cm-2 s-1, and the neutrons are being thermalised by a moderator surrounding the source and foil. [3+4+3 = 10 marks]