First experiments starting

Proton microscope for FAIR

The scientists radiographed objects such as this wristwatch. Photo: Antje Stohl for FAIR

The scientists radiographed objects such as this wristwatch. Photo: Antje Stohl for FAIR

In April 2014 a facility for microscopy with protons was commissioned for the first time at the GSI ring accelerator, Darmstadt, and is to be used at the accelerator facility FAIR in the future. Protons with even more energy will be available there, thus expanding the possibilities for experiments with the proton microscope.

Protons are – alongside neutrons – the building blocks from which atomic nuclei are constructed. Similarly to X-rays, protons can also be used to radiograph objects and thus create images. They can even penetrate hot, dense materials which cannot be examined with light or X-rays.

An international collaboration encompassing researchers from GSI, Darmstadt Technical University, Los Alamos National Laboratory (LANL), Los Alamos, USA, and the Institute for Theoretical and Experimental Physics (ITEP), Moscow, Russia, has jointly constructed the proton microscope PRIOR (Proton Microscope for FAIR). In the first experiments the researchers used a proton beam, which, using the existing accelerator facility, was accelerated to an energy of 4.5 gigaelectronvolts (this corresponds to approx. 98 percent of the speed of light). A special array of four quadrupole magnets was used for focusing, similar to the glass lens in a conventional microscope. This focused the proton beam so as to provide an enlarged image of the object in the beam. Thus the scientists were able to radiograph various objects such as wires of varying thicknesses and a wristwatch comprising complex individual parts.

Extremely high resolution attained
With this measuring setup it was possible to resolve objects and structures down to a size of 40 micrometers. Thus, as early as the commissioning stage, the PRIOR facility had already attained comparable resolutions to those of the leading existing facilities in the USA and Russia. The researchers intend to improve the resolution in further experiments this year to as little as ten micrometers. Another objective is the recording of sequences of images of moving objects. In July of this year, for example, thin wires are to be vaporized using a strong electrical discharge, and this so-called plasma expansion is to be examined with the proton beam.

Tumor diagnostics and therapy
Protons penetrate hot, dense material, which is also described as plasma. This is the actual focus of the researchers’ interest for it is found in stars and gaseous planets such as Jupiter. In the laboratory such material states may be created for a short period of time with lasers or strong electrical discharges. As protons, in contrast to types of beams such as light and X-ray beams, can penetrate this matter, the proton microscope PRIOR offers unique examination possibilities: “Alongside research into processes in outer space, the technology also has very practical applications,” explains Dr. Dmitry Varentsov, from the “Plasma Physics Detectors” department at GSI. “It would be possible to radiograph running engines, for example, or even perform tumor diagnostics and therapy. We would like to pursue all these approaches.” In the precursor experiments on cancer diagnostics and therapy with protons on the setup in Los Alamos, the researchers succeeded in creating a proton image of a mouse in 2013.

The proton microscope will also play an important role at the accelerator facility FAIR (Facility for Antiproton and Ion Research), which is currently being built in the framework of an international cooperation and which will become part of the existing GSI accelerator facility. The proton microscope is to be moved there after the completion of FAIR.

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