Have the students go over the pre-lab questions and make sure they understand the underlying concepts.

Get them thinking about the logic puzzle without giving them the explicit answers. It is less important that they have figured out all of the steps than it is that they understand that the most important aspect of the experiment is showing (scientifically) that the peak they are measuring is in fact due to neutrons interacting with protons to form positronium, and not some other unrelated source of radiation.

Isolate and identify the neutron spectrum.

Characterize the response of both neutrons and gammas to different thicknesses of lead shielding.

Understand detector calibration.

Gather data with different permutations of paraffin and carbon with and without neutrons.

Check that logic is complete. If it is not get them to realize what they left out.

Any time they claim that a peak appeared, disappeared, was enhanced or reduced then need to quantify the effect by establishing the statistical significance.

They should have quantified the attenuation of gammas vs neutrons for their chosen lead shielding.

Filling in holes in logic chain.

Better quantifying the attenuation of lead on neutrons and gammas.

Quantifying peak appearance/disappearance effects.