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Measurement of atomic interaction cross sections with synchrotron radiation


Representation of the double ionization chamber for the measurement of atomic photoionization cross sections with synchrotron radiation at the Metrology Light Source (MLS)

Fundamental material parameters such as the interaction cross sections used to describe the interaction of matter with radiation are very important, both for fundamental research and for diagnostic and analytical applications. In collaboration with Fraunhofer IPM in Freiburg im Breisgau, interaction cross sections for the photoionization of noble gases in the VUV spectral range have recently been measured at the Metrology Light Source. The results have allowed the existing datasets to be extended, and have smaller uncertainties and reliable metrological traceability to the International System of Units (SI). They can be used for applications in solar and atmospheric research, for the characterization of X-ray lasers and for the analysis of combustion gases.

The photoelectric effect (i.e., the fact that electrons are detached by photons that have sufficient energy) is one of the elementary processes in the interaction between light and matter. Albert Einstein was awarded the 1921 Nobel Prize in physics for describing this effect in 1905. For single, free atoms, the probability of such a process occurring is described by an effective surface – the so-called interaction cross section. For noble gases, corresponding data have long been available, though without dependable metrological traceability to the SI. In order to attain such traceability, a double ionization chamber was developed at Fraunhofer IPM and used with synchrotron radiation in a wavelength range from 25 nm to 90 nm at PTB's Metrology Light Source (MLS) in Berlin-Adlershof.

The double ionization chamber consists of a gas volume at pressures ranging up to a few 10 Pa, where photoabsorption causes the incident radiant power to be attenuated and photoelectrons to be emitted – specifically, positively charged photoions are generated. Due to their electrostatic extraction and their detection at two consecutively arranged anodes, it is possible to measure the light attenuation and thus to determine the interaction cross section for photoabsorption and photoionization. Furthermore, the two ion signals provide evidence of the incident radiant power.

During the experiments carried out at the MLS, the double ionization chamber was thoroughly characterized within the scope of a doctoral thesis at the University of Freiburg, with a specific focus on influence of pressure gradients on the signals between the moment the synchrotron radiation enters the chamber and the moment it exits the chamber. Based on this information, the photoionization cross sections of He, Ne, Ar, Kr and Xe were determined. The relative standard measurement uncertainties in the range of a few parts per 103 obtained during the determination are up to one order of magnitude lower than those from previously available data and are in excellent agreement with them within the combined uncertainties.

The results have already been used to validate the measurement data obtained with a double ionization chamber of the same type; this chamber was used in the SolACES (Solar Auto-Calibrating EUV Spectral Photometer) module of the International Space Station (ISS) from 2008 to 2017 to investigate solar radiation. Moreover, the new interaction cross sections represent an improved basis for the quantitative detection of free-electron laser radiation in the spectral range from VUV to X-ray with gas monitor detectors optimized specifically for this application. It is also planned for PTB to use these datasets to develop a gas analysis method that is traceable to the SI.



A. Gottwald, 7.13, E-Mail: Opens window for sending emailAlexander.Gottwald(at)ptb.de

R. Schaefer, A. Gottwald, M. Richter, Opens external link in new windowTraceable measurements of He, Ne, Ar, Kr, and Xe photoionization cross sections in the EUV spectral range, J. Phys. B: At. Mol. Opt. Phys. (2018)