On the occasion of the 100th anniversary of PTR/PTB, the 1987 Helmholtz Prize was awarded in three different subject areas. In the field of “Precision Measurement of Physical Quantities”, Dr. Fritz Riehle and Prof. Dr. Burkhard Wende of PTB Berlin were awarded the prize for their work titled “Ein Elektronenspeicherring als primäres Strahlungsnormal zur Realisierung strahlungsoptischer Einheiten” (An electron storage ring as primary radiation standard for the realization of radiation-optical units).
When Max Planck drew up his famous radiation formula in 1900, he could refer to the precise measurements of the cavity radiation that had been carried out at PTR in Berlin. Based on Planck's radiation formula, high-temperature cavity radiators were used first as primary radiation standards for the realization of spectral radiant powers. However, for short wavelengths below 250 nanometres, the emission of these radiators is too weak to be used for a radiation standard. In contrast to this, electron storage rings emit intensive synchrotron radiation from the visible spectral range to the X-ray range. The spectral distribution of this radiation only depends on a few parameters and is given by a formula which was derived by Julian Schwinger in 1949 for the radiation emission of radially accelerated relativistic electrons from the laws of classical electrodynamics.
As Riehle and Wende had proven in 1984, the storage ring BESSY I in Berlin represented a new radiation standard which was clearly superior to the cavity radiator. For this purpose, the parameters of the storage ring had to be measured very accurately, and the storage ring had to be optimized in such a way that it was possible to describe its spectral radiant power with the aid of the Schwinger formula. While this formula is valid for a single electron moving on an orbit, up to 1012 electrons moved on different orbits in BESSY I. Due to this, the radiation emission of the storage ring deviated from the predictions of the Schwinger formula. By determining the spatial and the angle distribution of the electrons in the storage ring with the aid of the measured angular dependence of the radiation, Riehle and Wende were able to calculate the spectral radiant power of the storage ring with high precision with the help of the Schwinger formula.
By reducing the number of electrons stored in BESSY I to one electron, the radiant intensity could be modified by up to 12 orders of magnitude. Until its decommissioning in 1999, the storage ring standard had been used for the calibration of detector and source working standards from the near IR to the range of soft X-rays. Since January 1999, PTB has been using the electron storage ring BESSY II as a primary radiation standard, especially from the vacuum UV to the X-ray range. With the Metrology Light Source (MLS), a primary radiation standard for spectral ranges from IR to the extreme UV has been available to PTB in addition to BESSY II since spring 2008.