IBDP Physics- E.4 Fission- IB Style Questions For SL Paper 1A -FA 2025
▶️ Answer/Explanation
1. Function of Moderator:
A moderator slows down fast neutrons to thermal energies to facilitate fission.
2. Evaluate Materials:
- (A) Cadmium: Used for control rods (absorbs neutrons).
- (B) Concrete: Used for shielding.
- (C) Uranium-238: Fuel.
- (D) Water: Often graphite or water (heavy or light) is used as a moderator because hydrogen atoms have similar mass to neutrons, maximizing energy transfer during collisions.
✅ Answer: (D)
▶️ Answer/Explanation
In nuclear fission, a very heavy nucleus splits into two medium-mass nuclei. Medium-mass nuclei lie higher on the binding-energy-per-nucleon curve than very heavy nuclei. Hence the binding energy per nucleon increases in the products.
Therefore, \(\dfrac{\text{BE per nucleon of }\mathrm{Y}}{\text{BE per nucleon of }\mathrm{X}} > 1\).
Because energy is released in fission, the total binding energy of the products (\(\mathrm{Y}\) and \(\mathrm{Z}\)) is greater than the total binding energy of the original nucleus \(\mathrm{X}\). Hence, \(\dfrac{\text{Total BE of }\mathrm{Y}\text{ and }\mathrm{Z}}{\text{Total BE of }\mathrm{X}} > 1\).
✅ Answer: (A)
▶️ Answer/Explanation
In a thermal reactor, fast neutrons produced by fission are less likely to cause further fission of \(\,^{235}\text{U}\) than slow (thermal) neutrons.
A moderator slows neutrons down by repeated collisions, so it reduces the kinetic energy of neutrons.
Control rods (made of neutron absorbers such as boron, cadmium, or hafnium) absorb neutrons.
By inserting or withdrawing the rods, the reactor can be kept critical so the chain reaction proceeds at a constant rate.
✅ Answer: (C)