  Recently, cosmic ray muons have been considered as a potential high energy
radiation probe for monitoring and interrogation of dense, well-shielded
special nuclear materials (SNM). Due to their high-penetrative nature, cosmic
ray muons can easily penetrate shielded nuclear materials with minimal
absorption and with leaving the target objects intact. However, despite the
potential benefits from using cosmic ray muons for SNM monitoring, their
widespread application has been limited for various reasons, including
relatively low cosmic ray muon flux at sea level and the difficulty of
measuring muon momentum in the field which can increase resolution and reduce
measurement time. In this work, we explore in detail the effect of cosmic ray
muon momentum measurement, focusing specifically on SNM monitoring
applications. Three different types of SNMs (HEU, LEU, and Pu) surrounded by
lead shielding with five different thicknesses (0, 5, 10, 20, and 30 cm), are
analyzed using Monte Carlo simulation for three momentum measurement resolution
levels (perfect, limited, and absent). 1000 muons were generated in the
simulation which translates to 4 minutes of measurement time for a standard
cargo container. We found that it is possible to identify and separate HEU,
LEU, and Pu with high accuracy (> 3 s.d.) when using muon momentum measurement
(perfect and limited) even when 30 cm-thick lead shielding was used. Currently,
it is not possible to identify or separate the SNMs with 30 cm thick lead
shielding without muon momentum knowledge. Our results show that the effect of
measuring muon momentum can be significant and can result in reduced
measurement times by a factor of 3 to 4 and/or improved monitoring and imaging
resolution.
