Diagram showing the first standard dosimeter in The Netherlands. Design by Dr. Daniel den Hoed and physicist Dr. Walch, 1928. image from dissertation of Den Hoed.
Within the International Committee on Radiological Units (ICRU) in 1925, it is agreed that the r-unit (röntgen) will be used as a measure of the delivered dose. The principle of dosimetry in air using an ionisation chamber is introduced. It takes a number of years before the new unit is generally accepted and the old unreliable chemical dosimeters are no longer used.
Dr.Den Hoed recognizes the importance of the exact measure of dose for radiotherapy and in 1926 takes the initiative to develop the new dosimetry technique in the Netherlands Cancer Institute and to encourage its use nationwide .
Design sketch of the Standard Dosimeter by Den Hoed. In 1928, together with physicist Walch of the University of Amsterdam (UvA), he constructs the first standard dosimeter in The Netherlands (Archive Den Hoed, courtesy H. van der Gugten).
In 1932, Den Hoed together with physicist Mozes Kwieser, develops a portable dosimeter .With this secondary standard, Den Hoed carries out calibrations throughout The Netherlands and abroad. In 1932, during a visit to Dresden for a conference, he compares the Amsterdam instrument with that of the Physikalische Technische Reichsanstalt for the determination of the international r-unit. An example of such a portable dosimeter has been kept at the NKI, but needs restoration. This is possibly the original instrument used by Den Hoed.
Up until 1940, the national dosimetry standard remains in the NKI and from Amsterdam the dosimeters are calibrated for users in The Netherlands. Regular use is made of the calibration service set up by Den Hoed, in a cooperation with the engineer and entrepreneur Louis Koopman. The latter was managing director of Almara, the firm that was the sole distributor of medical instruments and apparatus from Siemens and other German manufacturers. After World War II, the physicist Somerwil in the Rotterdam Radiotherapy Institute (RRTI) takes over this activity. Later Somerwil joins the Dutch state institute for public health and environment (RIVM) to set up a country-wide service in order to maintain the national standard and to calibrate dosimeters.