The effect of ionizing radiation on biological tissue is of great medical and public interest. The biological radiation effect depends not only on the macroscopic amount of energy that is imparted to the tissue, but also on the microscopic details of the energy transfer to the cells and their substructures. The quantities used in radiation protection, such as "equivalent dose", merely serve as a rough estimate of the radiation effect, for which no direct physical measurement procedures exist.

Thanks to a new type of measurement device which was developed by Dipl.-Phys. Uwe Titt, Dr. Volker Dangendorf and Dr. Helmut Schumacher of PTB, it became possible to improve research on the effects of radiation on matter and for applications in radiation protection and medicine. Using this device, they were able to produce three-dimensional images of the ionization tracks of charged particles in a low-pressure gas and to analyze these tracks. The three scientists were awarded the 1999 Helmholtz Prize for their work in the field of "Metrology in Medicine and Environmental Protection".

The device was based on an optically read out time projection chamber. The interaction volume consisted of a cylindrical chamber of approx. 1 litre volume which was filled with gaseous triethylamine at a pressure of 4 to 20 mbar. An energetic charged particle crossing this gas volume produced an ionization track consisting of free electrons and ions. The electrons drifted towards a detection stage where they were multiplied and converted to UV light of sufficient magnitude to be detected. Via an optical detection system, based on an intensified CCD camera, a high-resolution image of the projected ionization track was obtained. A crude reconstruction of the three-dimensional ionization track structure was obtained by recording the temporal development of the light signal.

In this way, the researchers studied the ionization track of protons, deuterons, alpha particles and various heavy ions at different energies. The spatial resolution for individual electrons in the ionization track was limited to approximately 2.7 mm and was mainly defined by the diffusion of the ionization electrons during their transport to the detection stage. Biological tissue has an atomic composition similar to the chamber gas, but it is much denser. In micro- and nanodosimetry, the distances measured in gas roughly scale with the ratio of densities of tissue to the density of the detector gas. Here, this leads to a scaling factor of approx. 25,000. The 2.7 mm resolution in gas therefore corresponded to a resolution of approx. 110 nm in biological tissue. As the microscopic pattern of energy deposition in a living cell is the primary reason for the difference of radiation effects in different kinds of ionizing radiation, it was expected that the result of these track structure measurements would provide an improved understanding of the radiation effects in biological tissue and eventually lead to an improvement of metrology in the fields of practical radiation protection and radiation therapy.

Later, the procedure was applied – using a spatial resolution that was improved approximately by a factor of three – to examine the radiation effectiveness of carbon and heavier ions. This project was performed in collaboration with the Helmholtz Center for Heavy Ion Research (GSI) in Darmstadt. The measurements especially served for the experimental verification of GSI’s models for the radiation effectiveness of carbon ions in tumour therapy. At that time, this was a new therapy modality which was pioneered at GSI and is in clinical use at several locations throughout the world today.