Optics

Triply positive charged ions of the isotope Th-229 (Th³⁺) are of particular interest as the basis for a high-precision optical clock. As the isotope is radioactive, it is only available in very small quantities and conventional methods for realising an ion source (for example by vaporisation) are not applicable in this case. PTB has now developed an apparatus for generating, cooling and storing Th³⁺ as recoil ions from the radioactive decay of U-233. A thin film of U-233 emits the desired ions into the vacuum, but with a high initial energy of more than 80 keV. By decelerating in high-purity helium gas, it is possible to collect the ions and transfer them to an ion trap. There they are stored together with Sr⁺ ions, which are cooled to a temperature in the millikelvin range using laser light. The coupling with the Sr ions also reduces the energy of the Th³⁺ to a value that is about 11 orders of magnitude below the starting energy after...

Optical properties of atmospheric aerosols, such as aerosol optical depth (AOD), are determined from solar radiation measurements using filter radiometers, known in such measurement applications as sun photometers. Here we present calibration of filter radiometers from the European branches of aerosol monitoring networks AERONET, GAW-PFR, and SKYNET with respect to their spectral irradiance responsivities. The calibrations were carried out at PTB within the framework of the EMPIR project 19ENV04 MAPP “Metrology for aerosol optical properties” using the tunable laser-based setup, TUable Lasers In Photometry (TULIP). The realization of the spectral responsivity measurements of the filter radiometers at the TULIP setup, results, uncertainty contributions and implications are discussed in this conference paper.

A new EURAMET Traceability Project “Traceability of the GAW-PFR reference Precision Filter Radiometers to the SI” was started in collaboration between PTB, Working Group 4.11 “Spectroradiometry” and PMOD/WRC. PMOD/WRC is designated by the World Meteorological Organisation (WMO) to operate the World aerosol Optical depth Research and Calibration Center (WORCC). WORCC is responsible for hosting the world reference for Aerosol Optical Depth (AOD), formed as a set of Precision Filter Radiometers (PFR). Furthermore, it operates the AOD monitoring network established within the frame of the Global Atmospheric Watch (GAW) program (GAW-PFR network). The WORCC mandate also encompasses to provide AOD traceability from its GAWPFR reference to other global and regional AOD monitoring networks. This is achieved through the Filter Radiometer comparison campaign held a PMOD/WRC every five years, and by collocating PFR travel standards at other network...