The oldest particle accelerator in Mexico. The brief story of a 5.5 MV Van de Graaff

by David Godos Valencia

I would like to share the story of the principal equipment of the nuclear laboratory at the Institute of Physics of the Autonomous National University of Mexico (IF-UNAM, by its original name in Spanish). I am referring to the Van de Graaff (VDG) 5.5 MV particle accelerator, which celebrated its 70th anniversary in January 2024  with the ‘70th anniversary CN-Van de Graaff accelerator Workshop’ held at IF-UNAM facilities, just a day after the traditional annual Symposium on Nuclear Physics in Cocoyoc, Mexico. The accelerator has been a central piece in Mexican nuclear research, however, it was not until the 1980s that the accelerator became part of the IF-UNAM [1, 2].

This story starts in the 1950s at Rice University (RU) in Houston, Texas. The T. W. Bonner1 Nuclear Laboratory (TWBNL) at RU was the home of two particle accelerators of 5.5 MV and 6 MV, both designed by the High Voltage Engineering Company (HVE). The oldest and smallest accelerator was a CN2 vertical model installed in 1953, while the newest and most powerful one was a horizontal EN2 tandem model installed in 1961. The TWBNL building, designed by George and Abel B. Pierce, was one of the few buildings in the country to be specially designed for these kinds of accelerators, making it one of the best facilities at the time. The 5.5 MV accelerator was installed in the distinctive tower of the TWBNL, while two additional halls - one at the side of the building to house the machine, and another at the back for the detectors - were needed for the 6 MV accelerator. The building was also home to the university’s Nuclear and Energy Physics group since its opening in 1953 until its eventual demolition in 1994, when the construction of the Computational Engineering Building started. The group was moved then to the second floor of the Herman Brown building, where the researchers kept working on designing and building experimental equipment to be used at other accelerators [3, 4, 5]. 
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Nevertheless, before the demolition of the TWBNL building, by the 1970s, both accelerators were not being used because the government contracts, which represented the majority of the funding to operate the laboratory, came to an end. There was no success in finding more resources to pay the personnel needed to operate the accelerators, nor to sell them to any other US laboratory. The perception was that they had accomplished their mission and outlived their usefulness [3, 14]. The researchers at RU had moved their attention to medium and high-energy physics.

Figure 1.- Photography of the VDG 5.5 MV building at the TWBNL laboratory. Picture courtesy of Dr. Andrade.

​In 1984, both accelerators were finally moved. The 5.5 MV accelerator was donated to the IF-UNAM in Mexico, while the 6 MV accelerator was sent to the Ruder Boskovic Institute (RBI) in Croatia. Around that time, new applications for low-energy accelerators were being discovered, which were based on ion-matter interactions. The effort of giving the VDG 5.5 MV accelerator a second life in Mexico began in 1984, when Dr. G.C. Phillips3 visited the UNAM and offered the accelerator,  along with its associated equipment, as a donation to the university. Therefore, UNAM only had to cover the expenses associated with the donation, including transportation and installation of the equipment.  Dr. Eduardo Andrade Ibarra4 was in charge of this monumental project. His leadership alongside his distinguished career made him the second member of the IF-UNAM to receive the“Premio Universidad Nacional 2024”, the highest recognition of the university [5, 7].

Figure 2.- Photography collage of the VDG 5.5 MV laboratory building at the IF-UNAM. Picture courtesy of Dr. Andrade.

Similar to the RBI, only a reduced group of UNAM’s staff was in charge of the complete logistics of transporting the accelerator. It was not an easy task, as it was needed to disassemble, package, transport, and reassemble around 70 tons of the accelerator components without any external assistance from accelerator experts. The transportation from Houston to Mexico City was done using five large trailer platforms, which finally arrived at their destination in December 1984. The construction of the accelerator building started in June 1985 and finished in August 1986. The building has a 90-foot tower and a surface of about 10,000 square feet for offices and experimental areas. It still remains one of the characteristic buildings of the IF-UNAM. The accelerator’s re-assembly was done during 1987-1988, with the first beam delivered in 1988, while the installation was fully completed in 1989. From that point on, the accelerator began its second life [5, 7]. 
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In July of 1989, the mural ‘La Universidad en el umbral del siglo XXI’ of the famous painter Arturo García Bustos5 was completed. The 55 square meters mural is exhibited in the metro station Universidad, just at the entry of the Ciudad Universitaria campus of the UNAM in Mexico City. This is the closest metro station to the IF-UNAM. The mural depicts the evolution of Mexican education: from the prehispanic schools like the Calmecac6 at the left, passing to the education in colonial times, and finally to the modern technological advances at the right. On the right canvas there, we can see the VDG 5.5 MV tank and the acceleration line alongside a radiotelescope, an electronic microscope, and a TV broadcast of the moon landing. Noticeably, we can see a mushroom cloud in the background of the scene in contrast with two students up the accelerator’s tank holding a white dove, emphasizing the peaceful applications of nuclear science. Nowadays, this depiction of the VDG 5.5 MV is part of the daily life of hundreds of students and people who use the metro system [9, 10].

Figure 3.- Photograph of the right part of the “La universidad en el umbral del siglo XXI” mural in the metro station Universidad, Mexico City, Mexico. Courtesy of Daniel Caleb Robles R.

At the end of 2017, the accelerator’s ion source was severely damaged, and several power supplies were destroyed. This followed a series of malfunctions of the radio frequency ion source in the prior year. Thankfully, in 2019, some restoration work was performed, and the accelerator was once again operational. By the end of 2020, more than 100 papers had been published, and several students had been trained using the accelerator. Ion Beam Analysis (IBA) techniques have been implemented and used in many interdisciplinary research projects. In addition, international research collaboration projects between Mexico and other countries have been established using the UNAM IBA facilities. Among the analytical techniques offered by the laboratory, we can find the Energy Recoil Detection Analysis (ERDA), Rutherford Back Scattering (RBS), Nuclear Reaction Analysis (NRA), and the Particle Induced X-ray Emission (PIXE). Nowadays, the VDG 5.5 MV laboratory also utilizes two modern devices coupled to the end of the acceleration line: the SUpersonic GAs jet taRget (SUGAR) [11], and the Momentum Neutron DEtector (MONDE) [12]. These devices open new possibilities for research made with the accelerator [2, 7, 13].

It is fascinating to discover the history of this lively device and the links it has established between the USA and Mexico, specifically between the RU and the IF-UNAM. In both universities, the VDG 5.5 MV deserved the construction of a specialized new building to house it. Not only has it made a lot of contributions to Nuclear Physics, but it is still relevant in the fields of Nuclear Astrophysics and Neutron Physics. The efforts of the nuclear physics group and staff of IF-UNAM have kept the accelerator alive and working.
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Have you found the story of this particle accelerator interesting? What about the story of your laboratory? Are there any similarities with any of the devices you work with? I would love to know about their story!

Notes: 

1. The laboratory is named after Tom Wilkerson Bonner (1910 - 1961). Student, physics instructor, and chairman of the physics department of RU [6]. 
2. The CN, and EN abbreviations refer to HVE models’ nomenclature. However, the meaning of the abbreviation has been lost in time, so we are not sure what they stand for. We suspect it might be linked to the model type and type of voltage generator.
3. Dr. G.C. Phillips, retired professor and director of the TWBNL. After the sudden death of Tom Wilkerson Bonner, he was chairman of the physics department from 1961 - 1966, and then he became the first director of the TWBNL until 1991 [4, 8].
4. Dr. Edaurdo Andrade Ibarra is an Emeritus Professor in the IF-UNAM. He pursued his Physics degree at UNAM from 1961-1964, and his PhD at RU from 1969-1971. He was in charge of the VDG 5.5 MV laboratory from its beginning until 2017, when Dr. Efraín Chávez was appointed as the new director.  A video of his talk about the VDG 5.5 MV accelerator (in Spanish) can be found on YouTube at the IF-UNAM official channel.
5. Arturo Bustos Garcia (1926-2017) was a student of Frida Kahlo in the Escuela Nacional de Pintura, Escultura y Grabado. He, Guillermo Monroy, Arturo Estrada, and Fanny Rabel, were members of the group Los Fridos [9].
6. Calmecac was the school for the sons of nobles and priests in Aztec culture.

Acknowledgments:
Special thanks to Dr Andrade for the complimentary information and the pictures related to the VDG 5.5 MV accelerator transportation from RU to the IF-UNAM.

References:

1. 70th Anniversary CN-Van de Graaff Accelerator Workshop, official webpage  
2. Van de Graff 5.5 MV Laboratory, official webpage 
3. C. Spieler, The Rice Thresher 82 No 3, Ed. 1, page 5 Friday, September 9, (1994). 
4. E. Andrade, The Cornerstone 3 No 1, page 6 (1997).  
5. D. Miljanic, The Cornerstone 3 No 1, page 6 (1997).  
6. T.W. Bonner Laboratory, official webpage
7. E. Andrade, Nucl. Instrum. Methods Phys. Res. B56/57, pages 802-805 (1991).
8. Department of Physics and Astronomy of Rice University, official webpage 
9. UNAM Foundation, official webpage 
10. Metro CDMX, official webpage 
11. J. Favela, et. al, Phys. Rev. S. T. 18, 123502. 1-4 (2015).  
12. P. Santa Rita, et al, J. Phys.: Conf. Ser. 730 012019 (2016).  
13. C. G. Puigvert-Angulo, R. Espejel, et. al., JINST 16 T08013 (2021). 
14. B. León-Garcilazo, El acelerador Van de Graaff de 5.5 MeV del Instituto de Física de la UNAM (1984-1990) : movimientos y resignificaciones, UNAM, pages 67 (2025).