Imaging with Muon TomographyPresented by Meghan Mulryan, Gretchen Schober, Candace Spore, Jake Vautier

Background BackgroundWhat is a Muon?A muon is an elementary subatomic particle with negative charge. An elementary subatomic particle is a particle that is smaller than an atom, and is not known to be composed of smaller substructures. Another example of a subatomic particle is an electron, and muons are similar to electrons in that they both have a negative charge. The main difference between a muon and an electron is that muons are much heavier. A muon has about 200 times the mass of an electron. Furthermore, a muon is considered a lepton. Leptons are one of the families of elementary particles that are considered the building blocks of matter.

Imaging with Muons Muons Imaging withMini Muon Tomographer - MMTThe term tomography refers to imaging by sections or sectioning by the use of any kind of penetrating waves. Muon Tomography is a tomography technology using high energy muons from cosmic rays. High energy subatomic particles from distant sources in the cosmos collide with the atmospheric gases every second. Nevertheless, the major production from these collisions observed at sea level is muons at about 95% of the speed of light. Muons at such high speeds interact with matter weakly. Thus many of these muons can penetrate from long distances into dozens of meters of material. When muons pass through material, their path of travel exhibits a scattering angle. The track of high energy muons will have a larger scattering angle when passing through high density material. Conversely, the path of high energy muons will have a very small scattering angle when passing through low density material. By monitoring scattering angles and the energy loss of the muons, the identity of the composition of the material can be determined in unopened shipping containers, as demonstrated in figure 2.

Application

Application

Fukushima Daiichi Reactor Project

Where do Muons come from?Muons are generated by cosmic rays in the atmosphere. Cosmic rays come from outer space, and are composed primarily of protons (another kind of subatomic particle with positive charge). When a cosmic ray hits the earths atmosphere, the high-energy protons collide and interact with the atmospheric gas molecules. These high energy collisions result in the creation of many types of high energy particles. Some of the particles decay rapidly to form muons. Figure 1, below, provides an illustration of this phenomenon. Muons have special properties that allow them to reach the earths surface. The first property is that muons have a relatively slow rate of decay, compared to the other types of particles. The second reason is that muons have the ability to penetrate material without interacting with it.

Figure 3. Cosmic ray exposure simulation

Above is an image reconstruction of computer-simulated muon scattering data. The image is of a passenger van containing a 20 kg uranium sphere. The sphere is shown in red representing the highest level of the scattering in the image.

Figure 2. Demonstrates muon scattering angles and MMT detection

Transmission imaging with cosmic rays is somewhat different from point source x-ray imaging because both the intensity and the direction of the cosmic rays can be measured. Both intensity and direction of the cosmic ray are measured with detectors like the MMT. The trajectory information can be used to generate a projected transmission image at any distance from the detector. Conceptually the stopping length of cosmic rays in material is inversely proportional to the stopping rate and can be related to the energy spectrum, dN (E)/dE, as : 1/ = dN/Ndx = 1/N* dN/dE *dE/dx The energy loss dE/dx can be calculated using the Bethe Bloch formula. Over a wide range of momentum, the energy loss for cosmic ray muons varies only logarithmically with momentum and is approximately proportional to the electron density, Z/A.

Figure 4. Proposed detector setup for Fukushima Daiichi with two muon trackers

Figure 1. The yellow streak represents the cosmic ray as is enters the atmosphere. The green streaks represent the particles produced from high-energy proton interaction with atmospheric gas molecules. The red streaks represent the muons produced from the decayed particles in the green streaks.

The Los Alamos Muon Team proposes using muon tomography to investigate the nature of the damage at the Fukushima Daiichi reactor. Muon tomography has high sensitivity for high Z material. Cosmic ray muons are the results of hadronic showers in the upper atmosphere. Due to the mean energy of several GeV muons reach the Earths surface and are highly penetrating. The Los Alamos Team proposes that with the Muon technology approximately 10 years can be removed from the clean-up time in Japan.