the nns operates under the same principle as a bonner sphere neutron spectrometer. the sensitive part of the spectrometer is the helium-3 proportional counter, which detects neutrons through the reaction he(n,p)h. this particular reaction has a significant cross-section only at thermal neutron energies. in order for a higher energy neutrons to be detected their energies must be decreased, or "moderated". to slow down neutrons at higher energies the nns uses different amounts of high-density polyethylene (hdpe). the he-3 proportional counter is surrounded by varying amounts of hdpe to sample various energy regions of the neutron field. the standard nns has seven hdpe cylinders which are nested together, similar to russian nesting dolls, to measure neutrons of higher and higher energies.