# The photofission reaction against the illicit transport of nuclear material in cargo containers: A review on recent achievements and future challenges in active photon interrogation

Adrien Sari

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Adrien Sari. The photofission reaction against the illicit transport of nuclear material in cargo containers: A review on recent achievements and future challenges in active photon interrogation. ANIMMA 2025 - 9th Advancements in Nuclear Instrumentation Measurement Methods and their Applications, Jun 2025, Valence, Spain. pp.06015, 10.1051/epjconf/202533806015. cea-05490179

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# The photofission reaction against the illicit transport of nuclear material in cargo containers: A review on recent achievements and future challenges in active photon interrogation

Adrien Sari¹,*,

¹Université Paris-Saclay, CEA, List, F-91120 Palaiseau, France

(*) adrien.sari@cea.fr

**Abstract**—X-ray scanning is widely used by customs for visual inspection of cargo containers at border controls. When Special Nuclear Material (SNM) such as uranium or plutonium is suspected, an active non-destructive nuclear measurement method serves as a second-line tool. The photofission reaction, central to the Active Photon Interrogation (API) technique, enables the direct detection of SNM. This paper reviews European advances in API over the past decade through three Horizon 2020 projects. In C-BORD (2015–2018), Europe’s first photofission system was tested at the Maasvlakte terminal of the Port of Rotterdam (Netherlands), with experiments on depleted uranium and container mock-ups demonstrating integration into a stationary 9 MeV industrial X-ray scanning facility. The ENTRANCE project (2020–2023) evaluated a mobile 7 MeV X-ray scanner at the Škrljevo terminal of the Port of Rijeka (Croatia), investigating the 5–6 MeV actinide photofission threshold and assessing operational performance. The MULTISCAN 3D project (2021–2025) explored new approaches using high-energy photons from laser–plasma sources. Collectively, these developments have strengthened nuclear security and enhanced Europe’s capacity to counter illicit SNM trafficking.

**Keywords**—Border monitoring; Cargo containers; X-ray scanning; Active Photon Interrogation (API); Photofission; Special Nuclear Material (SNM).

## I. INTRODUCTION

THE concept of photofission—the induction of fission in heavy nuclei by high-energy photons—was first predicted [1] and later demonstrated experimentally [2] in early nuclear physics studies. The reaction occurs when photons with energies above ~5–6 MeV interact with fissile nuclei ($^{235}$U, $^{239}$Pu, and others) or fertile nuclei ($^{238}$U, $^{240}$Pu, and others), producing fission fragments along with prompt neutrons and gammas, while subsequent decay of the fission products emits delayed neutrons and gammas. A schematic of the photofission reaction is shown in Fig. 1.

Building on these foundations, Active Photon Interrogation (API) has been developed as a non-destructive nuclear measurement method [3]. API employs high-energy photons, typically generated by a linear electron accelerator (linac) via a

Bremsstrahlung target, to induce photofission reactions on actinides. API overcomes the limitations of passive measurements and Active Neutron Interrogation (ANI), both constrained by matrix effects—passive methods relying on spontaneous SNM emissions, ANI limited in large volumes by an interrogative thermal neutron beam—using photofission signatures [4, 5].

API has been tested in realistic port environments with cargo containers, demonstrating the feasibility of both stationary [6] and mobile [7] systems, and confirming its potential for operational deployment at borders. Given the ongoing challenges in securing borders against SNM illicit trafficking and the rapid technological developments in photon sources and detection methods, a comprehensive review of European advances in API over the past decade is both timely and essential. This paper focuses on three Horizon 2020 projects: C-BORD, ENTRANCE, and MULTISCAN 3D. While the first two projects focused on linac-based systems, the MULTISCAN 3D project explored novel photon sources based on laser–plasma interactions, paving the way for next-generation photofission-based SNM detection.

The implementation of photofission-based inspection systems responds to the growing demand for robust nuclear security measures at European borders. These technologies not only enhance the detection of SNM but also support international trade by enabling efficient, non-intrusive inspections. Over the past decade, Europe has led in developing and testing these advanced systems, setting a benchmark for global security practices and demonstrating the feasibility of integrating API into operational port environments.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Scheme of the photofission reaction. A high-energy photon interacts with a heavy nucleus (e.g., $^{235}$U, $^{238}$U, $^{239}$Pu, etc.), inducing fission into two or three fragments and emitting prompt neutrons and gammas. Subsequent decay of the fission products produces delayed neutrons and gammas.</figcaption></figure>
## II. PIONEERING PHOTOFISSION IN EUROPE: THE FIRST INSPECTION SYSTEM

The C-BORD project (2015–2018) aimed to develop and demonstrate advanced technologies for non-intrusive inspection of cargo containers at European border controls. Within this framework, photofission (API) was deployed as a second-line, active non-destructive nuclear measurement method, complementing conventional X-ray scanning. A stationary photofission inspection system was installed in 2018 at the *Maasvlakte* terminal of the Port of Rotterdam (Netherlands) on an existing 9 MeV X-ray scanning facility operated by customs.

Several technical challenges had to be addressed during deployment. The geometry of the installation imposed a distance of more than 10 m between the linac and the container, while the photon beam was strongly collimated to a width of only a few millimeters. These constraints required careful alignment and optimization of the measurement setup.

Field experiments were conducted in 2018 using 10 ft mock-up containers loaded with depleted uranium (DU) samples and three representative cargo matrices: aluminum, fertilizer, and tobacco. In a realistic “blind test” configuration, the matrices and DU locations were prepared by customs, and their arrangement was unknown to the technical partners. The photofission subsystem was operated in a stationary “top linac” mode at 100 Hz, with active background acquisition followed by irradiation of the suspicious area. Measurements were performed at two positions, and results compared. A statistically significant increase of the neutron or gamma signal (≥ 3σ) triggered a red-light warning on the Graphical User Interface (GUI), while a green-light status indicated no anomaly. The setup included dedicated detectors for prompt neutrons, delayed neutrons, and delayed gammas (see Fig. 2).

These trials marked the first deployment of a photofission inspection system on an operational industrial site in Europe, demonstrating the feasibility of integrating active photon interrogation with existing high-energy X-ray scanning facilities.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Field trials of the C-BORD 9 MeV stationary photofission inspection system at the *Maasvlakte* terminal of the Port of Rotterdam (Netherlands, 2018) [6]. The setup included dedicated detectors for prompt neutrons, delayed neutrons, and delayed gammas, integrated with an X-ray scanner initially designed for visual inspection of cargo containers.</figcaption></figure>

## III. ON THE ROAD TO OPERATIONAL FLEXIBILITY: DEVELOPMENT OF A MOBILE SYSTEM

The ENTRANCE project (2020–2023) aimed to advance mobile photofission inspection systems for detecting Special Nuclear Material (SNM) at European ports. Photofission (API) was deployed as a second-line, non-destructive nuclear measurement method, complementing conventional X-ray scanning. The system was tested at the *Škrljevo* terminal of the Port of *Rijeka* (Croatia) in 2023, demonstrating operational capability under realistic conditions.

Deployment posed several technical challenges. Unlike the stationary C-BORD system, the ENTRANCE setup was fully mobile, installed on a truck, and relied on a 7 MeV linac, close to the 5–6 MeV photofission threshold. Accurate positioning relative to the container was critical for reliable measurements.

Field experiments were conducted using mock-up containers loaded with depleted uranium (DU) samples and various cargo matrices. The photofission subsystems provided delayed neutron and delayed gamma detection (see Fig. 3), with immediate feedback via a Graphical User Interface (GUI) showing green/red lights. Prior X-ray scanning allowed visual inspection and accurate localization of suspicious areas.

The ENTRANCE project successfully validated the feasibility of a 7 MeV mobile photofission inspection system, providing insights for flexible, rapid-response port operations.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Field trials of the ENTRANCE 7 MeV mobile photofission inspection system at the *Škrljevo* terminal of the Port of Rijeka (Croatia, 2023) [7]. The setup included dedicated detectors for delayed neutrons and delayed gammas, integrated with an X-ray scanner initially designed for visual inspection of cargo containers.</figcaption></figure>

## IV. TOWARD A TECHNOLOGICAL REVOLUTION: EXPLORING LASER/PLASMA SOURCES

The MULTISCAN 3D project (2021–2025) investigated next-generation photofission (API) inspection systems based on high-energy photons generated by laser-plasma sources. This pioneering approach aims to develop compact, high-brightness photon sources capable of inducing photofission in actinides. It enables flexible, rapid, and efficient SNM detection in cargo containers of various sizes and compositions. Laboratory-scale proof-of-concept experiments have been performed to demonstrate feasibility, leveraging the underlying laser-plasma interactions responsible for photon generation. These experiments highlight the potential for integrating advanced photon sources with existing inspection infrastructure. Detailed experimental results and a full technical characterization will be reported in a dedicated publication.
# V. CONCLUSIONS AND FUTURE CHALLENGES

## A. Conclusions

Over the past decade, the European projects C-BORD, ENTRANCE, and MULTISCAN 3D have advanced photofission-based inspection technologies for cargo container security:

### C-BORD (2015–2018)

* Stationary photofission inspection system deployed at the Port of Rotterdam (Netherlands).

* Operated with a 9 MeV linac, integrated into an industrial X-ray scanning facility.

### ENTRANCE (2020–2023)

* Mobile photofission inspection system deployed at the Port of Rijeka (Croatia).

* Operated with a 7 MeV linac, demonstrating the feasibility of a mobile system embarked in a truck.

### MULTISCAN 3D (2021–2025)

* Proof-of-concept of photofission experiment using a femtosecond laser-driven Inverse Compton Scattering (ICS) source.

* First worldwide experiment at the intersection of laser, plasma, and nuclear physics.

* Paves the way for next-generation inspection technologies.

## B. Future challenges

### Short-term outlook

* Development of a passive/active multi-threat mobile inspection system for detecting SNM, radioactive sources, narcotics, explosives.

* Integration of multi-technology approaches to enhance detection efficiency.

### Long-term outlook

* Expansion to emerging applications, including recycling and metallurgical industries.

* Cross-technology synergy combining ANI, API, NAA, and PAA methods.

* Strengthening interdisciplinary collaboration among nuclear physicists, laser/plasma experts, and border security practitioners.

### Overall impact

* Highlights the growing potential of photofission technologies.

* Sets the stage for the next generation of nuclear security tools, combining mobile deployment, high-precision photon sources, and advanced detection systems.

* Strengthens border security and facilitates trade, while complementary nuclear techniques ensure robust detection of multiple threats.

# ACKNOWLEDGMENT

The author gratefully acknowledges the active involvement and support of the project partners:

### C-BORD (2015–2018)

* Smiths Detection, France

* National Centre for Nuclear Research (NCBJ), Poland

* Symetrica, UK

* Dutch Customs Administration (DCA), Netherlands

* CEA-List technical and coordination teams, France

### ENTRANCE (2020–2023)

* Smiths Detection, France

* Ruđer Bošković Institute (RBI), Croatia

* The Ministry of Interior, the Ministry of Finance – Customs Administration, and the Port of Rijeka (Luka Rijeka d.d.), Croatia

* CEA-List technical and coordination teams, France

### MULTISCAN 3D (2021–2025)

* Ludwig-Maximilians-Universität München – Centre for Advanced Laser Applications (LMU-CALA), Germany

* University of Padova (UNIPD), Italy

* CEA-List technical and coordination teams, France

# FUNDING

The C-BORD, ENTRANCE, and MULTISCAN 3D projects have received funding from the European Union's Horizon 2020 research and innovation programme under grant agreements No. 653323, 883424, and 101020100. The work presented in this presentation reflects only the authors' views and the Commission is not liable for any use that may be made of the information contained therein.

# REFERENCES

[1] N. Bohr and J. A. Wheeler, “The Mechanism of Nuclear Fission,” Phys. Rev., vol. 56, no. 5, pp. 426–450, 1939, DOI. 10.1103/PhysRev.56.426.

[2] R. O. Haxby, W. E. Shoupp, W. E. Stephens, and W. H. Wells, “Photofission of Uranium and Thorium,” Phys. Rev., vol. 58, no. 1, p. 92, 1940, DOI. 10.1103/PhysRev.58.92.

[3] A. Lyoussi, et al., “Transuranic waste detection by photon interrogation and on-line delayed neutron counting,” Nuclear Instrum. and Methods in Phys. Res. B, vol. 160, no. 2, pp. 280–289, 2000, DOI. 10.1016/S0168-583X(99)00586-8.

[4] J. L. Jones, et al., “Photonuclear-based, nuclear material detection system for cargo containers,” Nuclear Instrum. and Methods in Phys. Res. B, vol. 241, no. 1–4, pp. 770–776, 2005, DOI. 10.1016/j.nimb.2005.07.242.

[5] J. L. Jones, et al., “Detection of shielded nuclear material in a cargo container,” Nuclear Instrum. and Methods in Phys. Res. A, vol. 562, no. 2, pp. 1085–1088, 2006, DOI. 10.1016/j.nima.2006.02.101.

[6] A. Sari, et al., “Deployment of the First Photofission Measurement System Dedicated to SNM Detection in Europe: Outcomes and Future Prospects,” in 2019 IEEE NSS/MIC/RTSD, Manchester, UK, 2019, pp. 1–2, DOI. 10.1109/NSS/MIC42101.2019.9059765.

[7] A. Sari, et al., “Deployment of a 7 MeV Mobile Photofission Inspection System at the Rijeka Seaport for the Detection of SNM in Cargo Containers,” in 2024 IEEE NSS/MIC/RTSD, Tampa, FL, USA, 2024, pp. 1–2, DOI. 10.1109/NSS/MIC/RTSD57108.2024.10655794.