Mainz microtron

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The Mainz Mikrotron MAMI is a particle accelerator for electron beams operated by the Institute for Nuclear Physics at the University of Mainz and used for experiments in nuclear and high-energy physics . It is constructed as a multi-stage racetrack microtron with normal conducting linear accelerators . The accelerator has been available for experiments since 1979 and has been continuously expanded since then. In the latest MAMI-C expansion stage , the accelerator can accelerate polarized electron beams ( degree of polarization typically 80%) of more than 20 µA beam current and unpolarized electron beams of up to 100 µA to relativistic energies of up to 1.5 GeV .

The MAMI is a so-called continuous wave accelerator, i. H. The beam is not split into macropulses, as is the case with some other accelerator systems, but the particle bunches pass through the accelerator in a continuous sequence. The time structure of the beam is so fine that the detectors of the experiments can no longer resolve it and the beam thus acts like a continuous direct current . This has the great advantage that the amount of experiment data is evenly distributed and not concentrated in short pulses. The accelerator generates a sharply defined beam: the beam diameter is a few 0.1 mm and the energy uncertainty is less than 13 keV. The energy of the electrons therefore only scatters by about a hundred thousandth around the target value (MAMI-C: about 110 keV or seven hundred thousandths). The position of the beam is also kept constant at less than 200 µm using complex control mechanisms.

This device is therefore very suitable for carrying out precision investigations into the structure of matter in the subatomic area. Research at the institute focuses particularly on the investigation of subatomic structures, which are composed of many particles with strong interactions . So far (2008) four experimental working groups with cooperation partners from more than ten countries have settled at the institute to use the accelerator. A group of theoretical physicists uses the knowledge gained in this way to improve their understanding of the interaction between elementary particles , in particular quarks and gluons .

In May 2008, the institute's equipment was expanded to include a supercomputer with which complex theoretical simulations can be carried out in the context of particle and high-energy physics.


The overall concept and the first stages of MAMI were developed by Helmut Herminghaus in collaboration with Karl-Heinz Kaiser , who were appointed for this project by the director of the Hans Ehrenberg Institute .

1975 First proposal for a racetrack microtron (RTM)
1979 First electron beam of the prototype RTM MAMI A1 with 14 MeV final energy
1983 Completion of the first extension MAMI A2 with 183 MeV final energy
1990 Completion of the second extension MAMI B with 855 MeV final energy
1992 Completion of a source of polarized electrons
1993 Installation of a system for coherent X-rays in the X1 experiment
2002 Installation of a FEL for infrared radiation in the X1 experiment
2006 Completion of the third extension MAMI C with 1.5 GeV final energy
2008 Installation of a computer cluster for simulations in the context of theoretical physics

Working principle

A linear accelerator for electrons typically allows only a few MeV of energy gain per meter of length. At MAMI, the electrons pass through the same linear accelerator several times, being deflected by magnets after each pass and then returned to the beginning of the linear accelerator. (Incorporated in the purchase is that the electrons in redirecting each part of their energy as synchrotron lose.) The traces see this as racecourses an ancient arena , which is why this concept as a racetrack Mikrotron ( Racetrack microtron is called RTM) . The deflecting magnets must be large enough so that the electrons of the highest energy are still completely deflected inside. For the MAMI B accelerator level, these magnets are approx. 5 m wide and 450 t in weight. This has reached the mechanical limit of the RTM concept, making MAMI the largest microtron in the world.

The latest accelerator stage therefore no longer uses two magnets that deflect by 180 ° and a linear accelerator, but rather four magnets that each deflect by 90 ° and two linear accelerators. For this new concept of the harmonic double-sided microtron , linear accelerators with a frequency of 4.90 GHz were developed and used for the first time worldwide .

Technical specifications

Final energy 855.1 MeV 1508 MeV
Rounds 90 43
Magnetic field (deflection magnets) 1.28  T 0.95-1.53 ​​T
Mass (deflection magnets) 250 t 450 t
Microwave frequency 2.45 GHz 2.45 / 4.90 GHz
Microwave power 102 kW 117/128 kW
Length (linear accelerator) 8.9 m 8.6 / 10.1 m
Size of the arrangement (L × W) 21 m × 10 m 30 m × 15 m
  • Note: The size only refers to the area enclosed by the deflection magnets.

Research priorities

The Institute for Nuclear Physics houses four experimental working groups that use the accelerator's beam in different ways for basic physical research and applied research topics.

A1 collaboration

For the A1 collaboration experiment, the electron beam is shot at solid (e.g. carbon ), liquid (e.g. hydrogen ) and gaseous targets (e.g. He ). In particular, those reactions in which additional particles are generated are examined. These newly generated particles, the electrons scattered at the target and, if applicable, the nuclear fragments knocked out of the target are then detected and identified by means of a magnetic spectrometer . The A1 collaboration has three such spectrometers, each of which can be aimed at the target at different angles and thus only specifically detect particles that have been scattered or generated at a certain angle. The spectrometers can be operated in coincidence , whereby one can filter out the reactions relevant to the question of the experiment from the large number of reactions taking place. A fourth spectrometer, the KAOS spectrometer , is also used in the measurement setup for measurements of extremely short-lived particles, the kaons . These measurements are used to determine certain form factors of protons and neutrons . With the help of these measurements it should be determined with which structure protons and neutrons are composed of their components, the quarks and gluons . In addition, studies are carried out on the structure and cohesion of light atomic nuclei .

A2 collaboration

In the A2 collaboration experiment, the electron beam is not used directly, but rather high-energy gamma radiation with energies of 100 MeV to 1.5 GeV is generated by irradiating a bremsstrahlung target (depending on the objective, a thin metal foil or diamond ) . By using a photon marking system , it is possible to individually determine the exact energy for each of the gamma quanta generated here , so that the energy dependency of the observed phenomena can also be examined. Since 2003, the A2 experiment has been using the well-traveled Crystal Ball detector as a detector , consisting of 672 sodium iodide crystals. In addition to hydrogen and deuterium , heavier nuclei including lead have also been investigated.

A4 collaboration

In the A4 experiment, the polarized electron beam with energies between 315 MeV and 1508 MeV is shot at targets made of liquid hydrogen or deuterium . The scattered electrons are detected in a calorimeter consisting of 1022 lead fluoride crystals. Specifically, those electrons are examined that have been elastically scattered (i.e. without destroying or exciting the target nucleus). When the direction of polarization is reversed, the number of scattered electrons changes by a small fraction of around a hundred thousandth, and conclusions can be drawn from these changes about the structure of the target nucleus. The A4 collaboration is investigating how strongly quantum fluctuations contribute to the internal structure and properties of protons and neutrons, and which mechanisms are at work when electrons interact with these particles.

X1 collaboration

The X1 collaboration also does not use the electron beam itself, but uses it to generate electromagnetic radiation of different wavelengths or energies. This happens in beryllium foils through transition radiation , in single crystals through parametric X-rays or completely without a medium in magnetic undulator structures . This radiation can e.g. B. can be used for X-ray structural analysis of materials. In addition, the X1 collaboration is working on the development of a free-electron laser for generating infrared radiation in the wavelength range between 0.05 and 0.20 mm using the Smith-Purcell effect .

All experimental working groups are also active in the development of detector systems and experimental equipment. Many of the developments are manufactured by the workshops located in the institute.

Theory group

In addition to the experimental working groups, there is a theoretical working group that tries to improve the understanding of the structure and interaction of the elementary particles by using the experimental results. One focus here is the chiral perturbation theory , an effective field theory that seeks the best possible approximate solutions for the analytically unsolvable equations of QCD . On the other hand, within the framework of the lattice theory , work is being carried out on determining the properties of systems with strong interaction using numerical methods ( Monte Carlo simulation ).

For this purpose, the theory group has a powerful computer cluster , consisting of 250 computing nodes, each with two quad-core processors Intel Xeon E5462 (2.8 GHz clock frequency ), which are operated via a DDR - Infiniband network with a bidirectional data transfer rate of 2.2 GByte / s are connected. This cluster achieves a computing power of 17.3 teraflops in the Linpack benchmark and an effective computing power for the QCD simulations of 3.7 teraflops.

Two working groups deal with the operation and further development of the accelerator itself:

B1 collaboration

The B1 collaboration is responsible for the operation, maintenance and further development of the accelerator. This collaboration also planned and set up the latest accelerator stage.

B2 collaboration

The B2 collaboration is responsible for the accelerator's polarized electron source . The physicists involved are investigating the properties of the semiconductor crystals and laser systems required for this in order to further improve the beam quality.


The accelerator is operated by permanent scientists and engineers, as well as by student assistant operators. The experiments are planned, set up and operated by scientific working groups (also known as collaborations). The working groups are made up of scientists and scientists from other institutes who are permanently employed at the institute, as well as students who are preparing their diploma or doctoral theses. A large part of the planning and construction work is done by the students.

The pure useful life for experiments has been an average of 5,000 hours per year in recent years, that is 57% of the year and 81% of the annual operating time. The rest of the operating time was spent on preparation and further development. Due to technical difficulties, the accelerator was out of operation for 160 hours per year, which is 3% of the annual operating time.

In May 2008, the MAMI B expansion stage of the accelerator exceeded 100,000 operating hours.

Accelerators with a similar research focus

Individual evidence

  1. ^ H. Herminghaus: From MAMI to the Polytrons. In: Proceedings of the European Particle Accelerator Conference 1992, Berlin. Volume 1, 1992, pp. 247-251.
  2. ^ A. Jankowiak: The Mainz Microtron MAMI - Past and Future. In: European Physical Journal A. Volume 28 s01, 2006, pp. 149-160
  3. ^ University of Mainz. Institute for Nuclear Physics: Annual Report 1990/91.
  4. A. Jankowiak et al. a .: Status report on the HDSM of MAMI C. In: Proceedings of the European Particle Accelerator Conference 2006, Edinburgh . 2006, pp. 834-836.
  5. Homepage of the A1 collaboration
  6. ^ Homepage of the A2 collaboration ( Memento from February 4, 2012 in the Internet Archive )
  7. Homepage of the A4 collaboration
  8. Homepage of the X1 collaboration
  9. ^ Entry in the TOP500 Supercomputing Sites list (June 2008) , accessed June 27, 2008
  10. Request to the theory group of the institute
  11. Homepage of the theory group
  12. Homepage of the B1 collaboration ( Memento from May 1, 2007 in the Internet Archive )
  13. Homepage of the B2 collaboration
  14. ^ Operation homepage of the Institute for Nuclear Physics. Retrieved April 24, 2018.
  15. a b Request to the institute's accelerator group


Web links

Coordinates: 49 ° 59 ′ 30 ″  N , 8 ° 14 ′ 11 ″  E