PFNA Technical Information

An explosive and contraband detection technique with enormous potential is Pulsed Fast Neutron Analysis (PFNA).  In this technique, short bursts (few nanoseconds) of monoenergetic, fast neutrons are scanned across the volume to be interrogated giving rise to characteristic gamma rays, which identify the isotopic content at the location of the neutrons.  Computer techniques can organize these data into three-dimensional pictures of the isotopic content of the interrogated volume.  The only practical source of pulsed, monoenergetic, fast neutrons is accelerator-based.
 
Consider the accelerator-based, pulsed neutron source as a machine gun firing slugs of neutron, at a 10-MHz rate, through a cargo container to interrogate its contents.  The machine gun is scanned up and down as the cargo container moves through the field of fire.  The physical size of the neutron slug determines the resolution of the process – the smaller the slug, the higher the resolution.  The more neutrons in each slug, the faster the scan rate and cargo movement – resulting in higher throughput.
 
The neutron slugs travel at 1/10 the speed of light.  At the 10-MHz rate, the slugs are 3 meters apart.  Hence, for a 3-m-thick cargo container, there is only one slug of neutrons in the cargo at any one time – and the system knows where it is at all times!
 
The size of the slug determines the volume of the cargo that is interrogated at any one time.  This volume, referred to as a “voxel”, is analogous to the “pixel” of digital cameras.  The transverse size of the voxel is determined by collimation of the neutron beam emerging from the neutron production target.  The length of the voxel is determined by the pulse length of the neutron production – a one nanosecond pulse yields a 3-cm long voxel of neutrons.  In order for a short pulse of neutrons to remain short as it progresses through the cargo container, the neutrons must be quite monoenergetic so that they all travel at the same speed.  The current state of the art is for voxels about the size of a soup can. 

The neutrons in the slug interact with the nuclei of the cargo, resulting in the emission of characteristic gamma rays that reveal the elemental content of the cargo.  These gamma rays, traveling at the speed of light, reveal the elemental content of the cargo within the voxel, whose location, at the time of the gamma ray detection, is known.  Each vertical scan of the neutron beam results in a two-dimensional picture of the elemental content of the cargo in a narrow vertical slice (or tome, as in tomography).  Thousands of vertical scans results in a three-dimensional picture of the elemental content of the cargo.
 
Because of the rapid-fire nature of the neutron bursts (10 MHz), corresponding to a very short duration between pulses (~100 nanoseconds), a linac-based source must remain on between pulses, implying CW (continuous wave) excitation of the linac fields.  There are very few CW linacs in the world.  Our recent development of the RFI linac structure, which is 4 times more efficient than the linac structures that were available a few years ago, reduces the rf power requirement by a large factor and makes it capable of cw operation.  The RFI linac approach to PFNA would increase its power and throughput by orders of magnitude.