**4. Muon scattering and tomography**

The first investigation concerned with the use of the muon scattering process to obtain a radiography of the hidden content in a volume dates back to the work by Borozdin et al. [30], who employed a set of drift chambers (60 × 60 cm) to get radiographic images of tungsten blocks. This technique proved to be very promising for several reasons: it does not introduce any additional radiation, as it is for instance for X-rays; moreover, most of the scattered muons contribute to build the image, contrary to absorption, where a large fraction of muons is absorbed by the material itself. In the scattering mode, the muon tomography technique makes use of this process, which strongly depends on the properties of the material, especially its atomic number Z, thus allowing to discriminate between low- and medium-Z elements with respect to high-Z elements.

The projected scattering angle distribution follows in a first approximation a Gaussian shape, with a width given by:

$$\Theta\_{o} = \frac{13.6 \text{MeV}}{\beta \text{cp}} \cdot Z \cdot \sqrt{\frac{\chi}{X\_{o}}} \cdot \left[1 + 0.038 \ln\left(\frac{\chi}{X\_{o}}\right)\right] \tag{7}$$

where *p* is the muon momentum, *β* is its speed, *X* the traversed thickness and *X*<sup>0</sup> the radiation length of the material, which in turn depends on the properties (Z, A) of the material roughly as

$$X\_{\alpha} = \frac{\frac{716.4 \text{ g}}{cm^2}}{\rho} \frac{A}{Z(Z+1)\log\left(\frac{287}{\sqrt{Z}}\right)}\tag{8}$$

Resistive Plate Chambers and GEM. Several contributions in this field have been reported by various groups [31–39], who have designed small scale as well as large and full-size scale

**Figure 6.** An artist view of the experimental setup being employed for muon tomography applications of cargo containers. Two muon tracking detectors, one placed above and the other placed below the container, are used to reconstruct muon

Cosmic Ray Muons as Penetrating Probes to Explore the World around Us

http://dx.doi.org/10.5772/intechopen.75426

51

For many decades, an impressive amount of used fuel has been produced, and most of this material is stored either in spent fuel pools or in dry storage casks. Such containers are sealed after filling them with the spent fuel, with no possibility to open and visually check their content. Monitoring nuclear waste is then an important aspect of the safety control in nuclear sites. Muon radiography, either in muon absorption or scattering mode, has proven to be a useful tool even for the exploration of nuclear waste storage silos, and many investigations have been already reported in this respect [40–45]. The integrity of nuclear reactors, especially after possible failures, is also a very demanding application for muon tomography [46–48] and following the Fukushima accident, it has been demonstrated that the muon scattering technique may be an answer to the problem of controlling the amount of material inside the

Also the problem of detecting the presence of orphan sources in metal scraps container has received attention by muon tomography techniques [49, 50], since it is a potential source of large industrial accidents. Blast furnaces have been also explored by cosmic muons [51].

In principle, dense structures may be explored by muon tomography, either by using the muon absorption or the scattering process, even if they are placed on the Earth surface. As an example, water towers were imaged by muon telescopes [52, 53], calibrating the response

detectors for muon tomography.

reactor core [48].

**5. Other applications**

**4.2. Nuclear reactors and waste imaging**

tracks before and after traversing the container content.

#### **4.1. Homeland security**

Due to the possibility that illicit fissile elements (Uranium or Plutonium) could be transported inside containers, this technique was suggested as a viable alternative to other traditional methods to inspect and scan a large volume. A muontomograph employing the scattering process basically requires two good muon tracking detectors, one placed above and the other placed below the volume to be inspected. Reconstruction of the muon track above and below the volume allows to evaluate the amount of scattering suffered by the muon, and in the simplest approach (where a single scattering centre is assumed), the so-called POCA (Point of Closest Approach) algorithm determines the 3D coordinates of the scattering centre (**Figure 6**). A sufficiently large number of individual tracks, traversing from any direction of the volume, allow to build 2D and 3D tomographic images. The performance of a muontomograph may be evaluated in terms of its spatial and angular resolution (depending on the tracking detectors), of the overall detection efficiency (which is the result of the detection efficiency of each tracking plane), which in turn determines the required scan time, of the capability to identify high-Z elements and discriminate them with respect to lighter elements, of the sensitivity to false-positive events, which would require an alarm and the opening of the container for a detailed control.

Several applications were oriented to the problem of scanning the content of a cargo container, searching for hidden high-Z materials, which is an important aspect of homeland security. Due to the large amount of containers travelling over the world, which is estimated to be larger than 200 M container/year, controlling the content of any of these volumes is a challenging task. At present, only a small fraction of them are checked, while laws under discussion might require more detailed procedures to be followed over the world. Following this approach, several small-scale prototypes have been built over the last years, employing a variety of detection technologies, from gas chambers to segmented strip scintillators,

**Figure 6.** An artist view of the experimental setup being employed for muon tomography applications of cargo containers. Two muon tracking detectors, one placed above and the other placed below the container, are used to reconstruct muon tracks before and after traversing the container content.

Resistive Plate Chambers and GEM. Several contributions in this field have been reported by various groups [31–39], who have designed small scale as well as large and full-size scale detectors for muon tomography.

### **4.2. Nuclear reactors and waste imaging**

makes use of this process, which strongly depends on the properties of the material, especially its atomic number Z, thus allowing to discriminate between low- and medium-Z ele-

The projected scattering angle distribution follows in a first approximation a Gaussian shape,

tion length of the material, which in turn depends on the properties (Z, A) of the material

Due to the possibility that illicit fissile elements (Uranium or Plutonium) could be transported inside containers, this technique was suggested as a viable alternative to other traditional methods to inspect and scan a large volume. A muontomograph employing the scattering process basically requires two good muon tracking detectors, one placed above and the other placed below the volume to be inspected. Reconstruction of the muon track above and below the volume allows to evaluate the amount of scattering suffered by the muon, and in the simplest approach (where a single scattering centre is assumed), the so-called POCA (Point of Closest Approach) algorithm determines the 3D coordinates of the scattering centre (**Figure 6**). A sufficiently large number of individual tracks, traversing from any direction of the volume, allow to build 2D and 3D tomographic images. The performance of a muontomograph may be evaluated in terms of its spatial and angular resolution (depending on the tracking detectors), of the overall detection efficiency (which is the result of the detection efficiency of each tracking plane), which in turn determines the required scan time, of the capability to identify high-Z elements and discriminate them with respect to lighter elements, of the sensitivity to false-positive events, which would require an alarm and the opening of the container for a

Several applications were oriented to the problem of scanning the content of a cargo container, searching for hidden high-Z materials, which is an important aspect of homeland security. Due to the large amount of containers travelling over the world, which is estimated to be larger than 200 M container/year, controlling the content of any of these volumes is a challenging task. At present, only a small fraction of them are checked, while laws under discussion might require more detailed procedures to be followed over the world. Following this approach, several small-scale prototypes have been built over the last years, employing a variety of detection technologies, from gas chambers to segmented strip scintillators,

*cm*<sup>2</sup> \_\_\_\_\_\_\_\_\_ *<sup>ρ</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>A</sup> <sup>Z</sup>*(*<sup>Z</sup>* <sup>+</sup> 1) log(

<sup>1</sup> <sup>+</sup> 0.038 *ln*(

\_\_\_ 287 √ \_\_ *Z* ) \_\_*x*

*<sup>X</sup>*0)] (7)

the radia-

(8)

√ \_\_\_ \_\_*x X*0 ∙ [

*cp* <sup>∙</sup> *<sup>Z</sup>* <sup>∙</sup>

where *p* is the muon momentum, *β* is its speed, *X* the traversed thickness and *X*<sup>0</sup>

716.4 *g* \_\_\_\_\_\_\_\_\_\_\_

ments with respect to high-Z elements.

*<sup>θ</sup>*<sup>0</sup> <sup>=</sup> \_\_\_\_\_\_\_ 13.6*MeV*

*X*<sup>0</sup> ≈

with a width given by:

**4.1. Homeland security**

detailed control.

roughly as

50 Cosmic Rays

For many decades, an impressive amount of used fuel has been produced, and most of this material is stored either in spent fuel pools or in dry storage casks. Such containers are sealed after filling them with the spent fuel, with no possibility to open and visually check their content. Monitoring nuclear waste is then an important aspect of the safety control in nuclear sites. Muon radiography, either in muon absorption or scattering mode, has proven to be a useful tool even for the exploration of nuclear waste storage silos, and many investigations have been already reported in this respect [40–45]. The integrity of nuclear reactors, especially after possible failures, is also a very demanding application for muon tomography [46–48] and following the Fukushima accident, it has been demonstrated that the muon scattering technique may be an answer to the problem of controlling the amount of material inside the reactor core [48].

Also the problem of detecting the presence of orphan sources in metal scraps container has received attention by muon tomography techniques [49, 50], since it is a potential source of large industrial accidents. Blast furnaces have been also explored by cosmic muons [51].
