Question

Americium-241 undergoes fission to produce three neutrons per fission event. A change occurred that caused the rate of fission to increase. Which will be effective at achieving a critial mass? Check all that apply.

Remove a deflective shield surrounding the sample.

Remove absorbent material mixed in with the sample.

Compress the sample of americium-241.

Answer 1

Effective:
- Remove absorbent material mixed in with the sample.
- Compress the sample of americium-241.

Explanation (short):
- Removing neutron-absorbing material increases the number of neutrons available to cause further fissions (raises reactivity).
- Compressing the sample raises its density and reduces neutron leakage, increasing the probability a neutron will induce another fission (also raises reactivity).

Not effective:
- Remove a (neutron) reflective shield surrounding the sample. A reflector returns escaping neutrons to the fissile material and therefore helps reach criticality; removing it reduces reactivity. (If by “shield” you meant an absorber that removes neutrons, then removing that absorber would be beneficial — but a reflector/shield that sends neutrons back in is helpful, so you would not remove it.)