From the U.S. Government Accountability Office, www.gao.gov

Title: Depiction of Neutron Detectors

Description: This animation depicts how neutron detector components -- including moderator, conversion material, and signal processing -- interact to detect neutrons.

Related GAO Work: GAO-11-753, Neutron Detectors: Alternatives to Using Helium-3

Issue date: September 29, 2011

[First screen]
This screen has the GAO logo and the following text: “Depiction of Neutron Detectors,” “Animation to GAO-11-753,” “Neutron detectors: Alternatives to using helium-3”.

[Second screen]
This screen contains a legend identifying fast neutrons as a heavy, bold black arrowed line; slow/thermal neutrons as a thinner black arrowed line; and background radiation as a wavy red arrowed line.  On the left of the screen, a circular neutron source has arrows depicting fast neutrons radiating out from it in all directions.  A detector is grayed out on the right side of the screen and includes a rectangular box labeled “Moderator” closest to the neutron source, a rectangular box labeled “Conversion material” further from the source, and a computer terminal labeled “Signal processing”. 

[Narrator: ] A neutron source emits fast neutrons in all directions.

[Second screen]
This screen contains the elements of the first screen, and the “Moderator” box is no longer grayed out.  Some fast neutron arrows hit the moderator and change to slow/thermal neutron arrows, which change direction – some continue to the right of the screen, while one goes back towards the neutron source.  One fast neutron is seen to miss the detector components.

[Narrator: ]  Neutrons that pass into the detector first encounter the detector’s moderator. These neutrons bounce off of atoms in the moderator, slowing down the neutrons and reflecting some of them.

[Third screen]
This screen contains the elements of the second screen, and the “Conversion material” box and “Signal processing” computer are no longer grayed out.  One slow/thermal neutron is seen to pass from the moderator to the conversion material, where it continues through the conversion material without further interaction with the detector.  Another slow/thermal neutron is seen to pass from the moderator to the conversion material, where it stops and creates a flash effect, followed by a flashing dashed line labeled “Signal collection” running from the conversion material to the signal processing computer. Background radiation is seen to come from the left of the screen, pass through the moderator, and strike the conversion material, creating a smaller flash effect.  This is followed by another flashing of the signal collection line.

[Narrator: ]  Some neutrons pass from the moderator to the detector’s conversion material. These neutrons have a chance of being absorbed by the conversion material, creating a signal. Background radiation may also create a signal in the conversion material.

[Fourth screen]
This screen contains all elements present at the end of the third screen.  The neutron source and background radiation line are flashed as the narration is read.

[Narrator: ]  Detector signals are collected and processed to determine if they are from a neutron or from a background source.