Rosenau (accelerator)

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The Physics Institute of the University of Tübingen maintained the Rosenau high-voltage laboratory from 1959 to 2018 . It is located north of the natural science institutes on Morgenstelle . The accelerator is currently (2019-2021) in the approval procedure according to radiation protection law. For this purpose, the system is dismantled piece by piece and examined for radioactivity.

In 1965, after unsuccessful attempts to put an accelerator from AEG into operation, a 2 MeV accelerator from High Voltage Engineering Company was purchased. This was replaced in 1978 by a 3 MeV Van de Graaff accelerator from the same company, which had previously been in use in Hamburg for 20 years. For the first time in Germany, a used accelerator was rebuilt elsewhere, which at the time was associated with great technical difficulties. After five years of self-directed construction and control development, it went into operation in 1983 and was used primarily as a neutron generator until January 2018. The beam could be pulsed and polarized. After years of use for basic research in nuclear physics (e.g. to investigate astrophysically relevant nuclear reactions), the accelerator was mainly used as a service facility for detector developments and for training in physics studies and radiation protection from 2004.

accelerator

Accelerating voltage

Control room of the accelerator

The Van de Graaff accelerator is located in a pressure chamber with an insulating atmosphere (30% SF 6 , 30% CO 2 , 40% N 2 at 1.9 MPa). The generated voltage (maximum 3.7 MV) can be measured using a "generating voltmeter" or directly using the energy of the ions on the analyzer magnet. In order to keep the voltage as constant as possible, a control circuit is built in, which increases the voltage if the accelerated ions have a radius that is too small in the magnet, or decreases it if the radius of the ions is too large. In order to be able to regulate the high voltage fast enough, so-called corona needles are attached opposite the high voltage electrode (“terminal”). A gas discharge current flows between the needles and the terminal , the strength of which can be regulated via the distance between the needles and the terminal. This corona current is fed to a rapidly regulating tubular triode. The beam unsharpness minimized in this way is 1–2 keV.

Particle source

Accelerator pressure tank

The ions to be accelerated are generated by a high-frequency ion source in the field-free interior of the terminal. This uses the principle of ionization through high-frequency electromagnetic fields in the GHz range. The gas (you can currently choose to ionize hydrogen , deuterium , helium or carbon dioxide ) is located in a glass cylinder that is surrounded by high-frequency electrodes . A field is generated inside the cylinder, which forces the naturally present electrons on spiral paths. In doing so, they ionize the atoms and molecules of the gas and generate new electrons. The resulting plasma is concentrated near the extraction channel with the aid of a magnetic field and extracted by a superimposed electric field (U ~ 3–10 kV) between the extraction channel and the electrode at the upper end of the source. In order to get a higher source current, one focuses the exiting ion beam in front of the actual acceleration section. For this purpose, a negative focus voltage compared to the terminal is applied between the first electrode of the acceleration tube and the extraction channel. The maximum extractable current when ionizing hydrogen is approx. 2 mA.

Beam guidance

Accelerator hall with beam guidance system

The accelerated ions are guided to the experiment stations by a beam guidance system . The beam pipes are evacuated in order to avoid scattering of the ions on the residual gas and thus to guarantee a sufficiently large free path . In addition, the beam is focused by optical elements and directed onto the target .

  1. The vacuum system consists of turbo-molecular pumps , which are preceded by rotary vane pumps . This achieves a maximum final pressure of 1 × 10 −7 mbar.
  2. The beam can be shifted parallel by paired dipole magnets in order to keep it on the optical axis.
  3. An analyzer magnet selects the type of ion and determines the momentum of the ions.
  4. Quadrupole magnets focus the beam.
  5. A switch magnet (engl. Switch = Soft) directs the beam in the leading to the desired experimental space lance.
  6. The beam diameter is limited by diaphragms and the beam is limited to one type of particle directly behind the analyzer magnet. The panels are cooled.
  7. To stop the beam, you can swivel in approx. 12 mm thick, cooled copper jaws (beam stops) at various points. This allows you to change settings on the measuring apparatus without having to turn off the beam. They are also moved into the beam path to adjust the beam so as not to endanger easily destructible components.
  8. Wedler, Y-shaped pieces of metal, swing your arm periodically through the beam in a horizontal or vertical direction. In doing so, they pick up the electrical signal from the ion beam. The charge picked up in a position is proportional to the current strength of the beam. This signal is displayed on an oscilloscope . Thus one knows the beam cross section.
  9. In front of each scattering chamber there is a collimator tube , which defines the beam conditions, and another aperture.

Experiments

The ion beam generated by the generator can be directed to six different experiment stations: Experiment stations 1, 4, 5 and 6 are used to generate neutrons, which in turn are used for further experiments. At experiment stations 2 and 3, the experiments are carried out directly with the ions accelerated in the generator. For this reason, there are evacuated litter chambers at these experimental sites. The Ortec scattering chamber at experiment station 3 is made of aluminum and is electrically insulated. To focus the ion beam, a 44 cm long collimator tube extends from the end of the beam tube into the scattering chamber. In the middle there is a so-called target ladder ; it can be rotated 360 ° and can accommodate up to six targets. This enables the target to be changed without having to ventilate the chamber. (Up to) 5 silicon semiconductor detectors sit on a swivel arm at a distance of 14.8 cm from the target. The angle between two neighboring detectors is approx. 15 °.

Experiments from 2004 to 2018

Finally, silicon detectors for astronomical X-ray satellites were tested for radiation hardness against low-energy protons. For this purpose, the Ortec scattering chamber on jet pipe 3 was temporarily dismantled and the jet pipe extended. A proton beam is slowed down and fanned out to energies in the range 100 - 1000 keV by means of thin metal foils. The proton flux at the detector to be irradiated is monitored with several surface barrier detectors.

The D-chamber on jet pipe 2 was occasionally used to measure the composition of thin layers or to determine the thickness of thin films. The method used is Rutherford Backscattering Spectrometry (RBS) , in which one or more detectors measure the energy spectrum of backscattered alpha particles or protons at large scattering angles close to 180 °. With the energies achievable by the accelerator, layers up to a few micrometers thick can be examined, depending on the material.

A deuterium gas target was developed from 2013 to 2017 to investigate the radiation resistance of Si detectors, as they are to be used in the CBM experiment at FAIR. Using the DD fusion reaction, it was possible to generate neutrons of approx. 3 MeV energy when bombarded with fast deuterons. This allowed the Si detectors to be irradiated with a source strength of approx. 10 9 neutrons per second for weeks, while the changes in their electronic properties due to radiation damage and nuclear reactions were examined at the same time. The problem was the durability of the thin window between the evacuated beam pipe and the target filled with several bars of deuterium. The neutron source strength was monitored with a collimated NE213 detector and activation foils.

internship

The so-called Rosenau internship (two-week core physics compact internship) took place annually from 1986 around Easter. The target group were students of physics in their main studies as well as diploma and doctoral students. Starting in 2018, the accelerator internship was replaced by a core physics compact internship in the rooms of the Physics Institute.

tries

  1. Preliminary tests with semiconductor detectors
  2. Rutherford - scattering cross-sections from 16 O to 13 C , 13 C to 13 C ( fermions ), 12 C to 12 C ( bosons )
  3. Rutherford backscattering on various (unknown) targets
  4. Neutron activation

Coordinates: 48 ° 32 '20.1 "  N , 9 ° 1' 48.8"  E