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Physics

Electrons at high speed

Particle accelerator designed and built at the USP Physics Institute

Eduardo CesarMicrotron: nuclear physics and medicine experimentsEduardo Cesar

There are many particle accelerator models, of different sizes and with specific features. They are machines that carry particle beams inside a tube to a specific target to break an atom, crash into subatomic particles or help one understand how an organic or inorganic material is formed. The most famous particle accelerator in the world is Europe’s gigantic Large Hadron Collider (LHC). In Brazil, the largest is the Synchrotron in Campinas, inner state São Paulo. The newest machine of this kind in the country is a microtron that accelerates electrons to a speed close to that of light and was almost entirely designed and built by researchers from the Physics Institute at the University of São Paulo (USP) with funding provided by FAPESP [geared toward the purchase of equipment and grants], CNPq (the National Council for Scientific and Technological Development) and Capes (the Coordinating Office of Training for Personnel with Higher Education). The total funding amounted to R$1.5 million.

The equipment’s initial validation trials to produce electron beams were conducted in 2008 and the initial experiments with the new accelerator took place in August of 2010. This initial work dealt with analyses and diagnoses for detecting the beam and the emission of radiation generated by the accelerator, these studies being connected to the construction of the equipment’s complement. Today, the accelerator is operating with a beam of energy of 1.9 million electron-volts (MeV). The objective for the next five years is to reach 38 MeV, which will transform it into a unique machine worldwide, as there is no other microtron with this particular configuration. The largest one in the world is at Johannes Gutenberg University in Mainz, Germany, an institution that has an accelerator that goes up to 1.5 billion electron-volts (GeV) and that collaborated with the Physics Institute team throughout the development of the USP microtron.

The Brazilian machine is expected to reach an intermediary stage (6 MeV), with a good quality beam, in 2012 or 2013. “With 6 MeV, it will be possible to conduct studies with medical purposes, because this is the same energy as the accelerators used in radiotherapy to treat cancer”, says professor Vito Vanin, the microtron’s coordinator and head of the institute’s Department of Experimental Physics. “We will be able to study the interaction of radiation and the body using this. In these cases, to apply radiotherapy, what one does today is prepare a mask, so that the radiation reaches only the tumor’s location. What happens, however, is that the edges of this area are also affected and we would like to help to minimize this problem. The experimental data on the subject are scarce”, says Vanin. Researchers from the University of Barcelona and the Polytechnic of Catalonia, both in Spain, are to collaborate with the Brazilians, along with researchers from Duisburg-Essen University in Germany, all of which have been responsible for theoretical literature on the subject and plan to enter an experimental phase with the equipment at USP’s Physics Institute.

In research studies into basic physics, the new accelerator might help researchers to attain a better understanding of fission reactions in heavy nuclei, such as the those from the atoms of uranium, thorium and other elements, enabling the resumption of lines of research suspended as a result of the permanent retirement of the Physics Institute’s linear electron accelerator back in 1993, a machine that had been donated by Stanford University (USA) in 1967, thanks to the intermediation of José Goldemberg, from USP. “We had hoped to build a new accelerator with the old one still running”, says Vanin. Despite its higher energy level (60 MeV), the old accelerator was of the pulsed type, whereas the ideal and most advanced kind for the research field is an accelerator with an ongoing electron beam at extremely high speed and free of pulses. “This is an important feature, because it is far more suitable for experimental purposes, even though it is harder work to implant a continuous beam machine, which is far more complex than a pulsed one.”

Eduardo CesarComplexity to ensure a fast and continuous beamEduardo Cesar

The beam of electrons, interacting with a radiation target, normally a metallic material placed inside the tube before the material that is to be analyzed, produces photons, i.e., elementary light particles, with enough energy to investigate nuclear structures independently from the interaction processes that occur between protons and neutrons, which ensures a new tool for the study of atoms’ nuclei. The collisions of the electrons with this radiating target also generate X and gamma rays, the penetrating forms of radiation used in several types of analysis, including nuclear ones. “The interaction of the electron beam with a sample draws the electrons from the internal layer of this material; when another one of the electron’s atom fills in this hole, the bremsstrahlung effect may materialize. This is the radiation that arises when the atom’s nucleus suddenly puts a brake on the electrons. The production of X-rays in medical equipment is based on this phenomenon. These processes, plus the optical radiation of transition (i.e., the light generated by the electron when it leaves the vacuum that it travels through to enter a material environment), are being studied in our initial experiments with the accelerator.”

The new accelerator project started to take shape in the form of an agreement between USP’s Physics Institute and the National Laboratory of Los Alamos, in the United States, which provided the blueprints for constructing the accelerating structures of the microtron in the early 1990’s. The American institution was building an accelerator of this type with higher energy that actually went into operation, but turned out to be unstable and was decommissioned. “We wanted to work with lower energies and professor Jiro Takahashi [from USP’s Physics Institute] redesigned the project and built the accelerating structures”, says Vanin. When the project and accelerator construction first started, the work was coordinated by professor Marcos Martins, currently the R&D director of CNEN, the National Commission of Nuclear Energy. “All the microtron’s components were built with domestic technology, acquired from Brazilian industries, except for the Klystron valve, which amplifies micro-waves, and some accessories. By building the machine, we acquired in-depth knowhow of the experimental conditions and we learnt the limits and possibilities of all the components. Additionally, we can do the maintenance ourselves and we know whether changes will be easy or difficult, expensive or cheap to accomplish.”

Partners
Certain components, such as the vacuum chambers of a device called a booster, along the microtron, were machined by the Technological Center of the Navy, in São Paulo. It is inside these chambers, placed within electromagnets, that the electron beam goes around in order to pass through an accelerating structure and acquire speed. Another contribution came from IEAv (the Advanced Studies Institute) of the DCTA (the Department of Aerospace Technology and Science), which used the channels through which the refrigerating water of the accelerating structure runs. The machine, at this initial stage, has one segment that is six meters long, for the conditioning of the electrons, plus a few square meters for the booster.

The French Klystron valve was financed by the IDB (Interamerican Development Bank) in 1989, which provided a total of some US$200 thousand, a sum that also covered testing equipment. The valve is a microwave amplifier that supplies electromagnetic waves as a means of accelerating the electrons along the equipment until they reach the sample that is to be analyzed. This involves tens of kilowatts of power inside the tube, equivalent to about one hundred domestic microwave ovens. The electrons are generated in a gun that can produce 100 kilovolts, and that removes these particles from an electronic part called a cathode. The electron beam has an electric current of 50 microamperes, which may seem small relative to the consumption of a home electrical appliance, but that corresponds to the flow of hundreds of billions of electrons a second. The electron gun was designed and built at the Physics Institute, with  improved welding, conducted in a vacuum oven, to bond metal and ceramic parts. The ceramic tube of the electron gun was donated by the firm NGK do Brasil, which makes spark plugs for automotive engines.

The beam’s journey
After the beam is produced in the electron gun, it travels in a sort of tunnel that is one and a half centimeters wide. Along the way, when the tunnel goes through chambers called cavities, the microwaves are injected and form an electric field in the direction of the beam. At the ends of the accelerating structure of the booster, there are two large magnets that make the beam return, giving the beam a new impulse. For everything to work with no external interference, part of the equipment is shielded against magnetism, which even blocks, among other things, the Earth’s magnetic field. Along the equipment, there is a series of microcontrollers that check various parameters. One of the systems required for the microtron to work well is the personal protection one. “There is an interlock system that switches off the accelerator if anybody enters the building where the machine is located, as a precaution against possible X-ray or gamma-ray radiation – nobody stays next to the microtron when it is running”. The control of the equipment is conducted from another room in the institute, with a system that has exclusive software developed by the microtron team.

The project and construction of the microtron illustrate the search for independence by a group of researchers, providing Brazil with a tool that is not only highly important for basic science, but for industry as well. “Increasingly, technological progress will dictate the need for industrial accelerators to analyze parts with high energy beams, for example, and we have proven that we have the scientific and technological capacity to build an electron accelerator. This means we can transfer knowledge about accelerators to anyone that might want to build one”, says professor Vanin. He also states that the microtron group is interested in exchanges with researchers from other institutions interested in using the accelerator.

Acelerador

The projects
1. Assembling the microtron control room  (nº 98/15389-9); Type Regular Research Awards; Coordinator Marcos Nogueira Martins – USP; Investment R$ 40,835.45 and US$ 31,425.00 (FAPESP)
2. Data acquisition at the linear accelerator laboratory (nº 97/04084-0); Type Regular Research Awards; Coordinator Vito Roberto Vanin – USP; Investment R$ 44,047.73 and US$ 55,659.50 (FAPESP)
3. Transport system for the microtron booster beam (nº 03/07008-5); Type Regular Research Awards; Coordinator Marcos Nogueira Martins – USP; Investment R$ 166,665.00 (FAPESP)
4. Installation and characterization of the high-power microwaves of the IFUSP microtron accelerator (nº 06/01017-0); Type Regular Research Awards; Coordinator Vito Roberto Vanin – USP; Investment R$ 124,812.50 and US$ 25,700.00 (FAPESP)

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