Foundations for Improvements to Passive Detection Systems - Final Report

Appendix A5: Remote Muon Scanning for Nuclear Materials Inside ContainershipsPeter Barnes, George Chapline, Dan Dietrich, Mark Rowland, Neal Snyderman

AbstractIt is proposed that all containerships be stopped at a suitable distance from US ports, and that a 2 GeV muon beam be used to survey the ship for nuclear materials. It is estimated that within a few hours the presence of a nuclear device could be detected with high confidence.

IntroductionThe problem of finding hidden nuclear materials onboard containerships is extremelydifficult because of the sheer size of containerships. Passive neutron and gamma ray signals from a point source may be undetectable, particularly in the case of HEU, because of the largeattenuation that these signals may suffer going through many containers. In addition, a nucleardevice could be shielded with a moderator and neutron absorber. The one thing a nuclearsmuggler cannot do, though, is to shield the nuclear material against muons. It might be recalledthat in 1968 Louis Alvarez used cosmic ray muons to survey the interior of Chephren's pyramidfor hidden chambers. Indeed given sufficient time any large structure could be surveyed usingcosmic ray muons. In the case of looking for uranium or plutonium inside a large structure,surveying the structure with negative muons provides an additional benefit in the form of aunique spectrum of characteristic 1-6 MeV muonic “x-rays”. Detection of this characteristicradiation with a -ray detector would provide unambiguous evidence for the presence of uranium 32 or plutonium. If the -ray detector had sufficient energy resolution one could even determine the isotopic composition of the nuclear material, viz. whether the material contained Pu239 or U235.

Although neutron probes can also initiate telltale signals, we believe that the using a negativemuon beam to detect hidden nuclear material may offer significant advantages when thestructure to be searched is very large.Our operational concept for inspecting containerships for nuclear materials is to introduceoff-shore interdiction points off all three coasts where containerships will be stopped andscanned with muon beams. Our initial concept for the muon source is a ship based proton

Lawrence Livermore 2004 Doc# UCRL-TR-207129

Foundations for Improvements to Passive Detection Systems - Final Report

accelerator which is used with a low Z target to generate a muon beam. The most straightforward way to generate the needed muon beams would be to guide a multi-GeV proton beam to the targets, and use the kinematic focusing of the pions produced as a result of nucleon – nucleon collisions to focus the muons onto the container stack. The ship carrying the accelerator wouldhave to be positioned next to the containership in order to avoid having to send the muon beam through water while inspecting the containers below the waterline. Positioning the inspection ship next to the containership would have the added advantage of maximizing the solid angle presented by signal detectors onboard the inspection ship. In addition, proximate positioning of the inspection ship would allow the muon beams to be slanted upwards or downwards, thus taking advantage of the low average density of the containers.

Estimated requirements for the muon sourceAlvarez needed rather high energy cosmic ray muons (~ 50 Gev) to traverse the 100 m ofsolid limestone in Chephren’s pyramid, and these would be difficult to produce in a portablesource. Fortunately, a containership is mostly air; the weight limit of 50,000 lb per containertranslates into an average density of 0.2 gm/cc. In this situation 8 containers ( i.e. halfway across current generation containerships) represent an average lateral thickness of ~ 500 gm/cm2 . The range of high energy muons is an almost unique function of the thickness measured in gm/cm2, and 500 gm /cm2 corresponds to the range of ~ 1 GeV muon. Of course, certain paths, e.g. straight across the bottom half of a horizontal row of containers, may present a higher aerial density, but by slanting the muon beam upwards or downwards it should be possible to reach the entire cargo with muons whose energy is less than ~ 2 GeV.A crucial question is whether one can produce the up to 2 GeV muons needed to survey acontainership with a portable artificial source of muons. Muons with energies up to 300 MeVmuons could be copiously produced using a cyclotron that could easily fit on a ship. If somemeans could be invented to accelerate these muons in situ, that would solve our problem.Alternatively a proton synchrotron capable of producing 2 GeV muons might conceivably befitted onto a large ship. Assuming the proton source uses a 100 nanoamp 5 GeV proton current,approximately 1012 muons a second might be produced into a cone with an opening angle of ~0.1 rad.

In order to estimate the fluence of muons needed to detect nuclear material hidden on acontainership, we need to make some assumptions regarding the efficiency for detecting 3 to 6

Lawrence Livermore 2004 Doc# UCRL-TR-207129

Foundations for Improvements to Passive Detection Systems - Final Report

MeV rays from a point source. If we assume that we have a 20 m2 detector 20 m from thesource, then the geometric efficiency is 1/250. Let’s imagine the detector efficiency is fairly highresulting in an overall efficiency ~ 10-3. The attenuation of 3-6 MeV rays will dependsomewhat on the materials in the containers, but at an assumed average density of 0.2 gm/cc this attenuation will be on the order of 10-6 . If we assume we need 100 counts to confirm theexistence of muonic lines at ~ 3 and 6 MeV, then the number of U or Pu muonic atoms thatwould need to be formed would have to be greater than 100 x106 x 103 = 1011 . If we assume thenuclear material forms a cube with side 10 cm, then a muon fluence ~ 1013 per square meter willbe required with an energy in a range to stop in the nuclear material. Fortunately, the aerialdensity of the nuclear material is a substantial fraction of the total thickness of the containers, soonly a factor ~2 larger fluence is needed for the muon beam itself.

Let us assume that the total area of one side of the container stack is 103 square meters,and that 2 beams were used to interrogate each side of the ship. Let us further assume theaverage flux of muons inside the containership is 1012 / m2 / sec. Then approximately 10 secondswould be needed to interrogate each square meter of the container stack. The entire searchvolume could be surveyed in 3 hrs. Because muon beams can be used in parallel, the search time could be reduced be reduced in proportion to the number of beams used, e.g. using 4 beams on each side could reduce the search time to 90 minutes. It should be noted that the radiation doses from the beam are substantial from the point of view of biological hazard, so the crew would have to be evacuated or at least kept away from the beam.

Lawrence Livermore 2004 Doc# UCRL-TR-207129