The NNS operates under the same principle as a Bonner sphere neutron spectrometer.
[2] The sensitive part of the spectrometer is the Helium-3 proportional counter, which detects neutrons through the reaction 3He(n,p)3H.
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.
Both Measurements were made at the NIST low scatter facility in Washington, DC.
The components of a nested neutron spectrometer. Clockwise from top left: Pre-amp (white box), moderating cylinders (white cylinders), MCA (gray box), He-3 proportional counter and stand (metal cylinder), data acquisition computer (yellow), and assorted cables (red and grey).
A He-3 Proportional Counter standing beside a metal stand.
The seven standard HDPE nesting cylinders which make up the moderating assembly of the Nested Neutron Spectrometer. (Note lids of the cylinders are on.)
The seven standard HDPE nesting cylinders which make up the moderating assembly of the nested neutron spectrometer. (Note cylinders do not have their lids on.)
Nested Neutron Spectrometer measurements of AmBe neutron source. Comparison to ISO standard spectrum.
Nested Neutron Spectrometer measurements of a bare Cf source neutron source. Comparison to ISO standard spectrum.
