Harald H Rossi, the `father of microdosimetry', died on the first day of the new millennium at the age of 82 in his home near New York. Harald was one of the truly great radiation physicists and will be missed by many.

Harald Rossi (all the first-born men in his family had names ending in -ald and he did not like to be called Harold) was born in Vienna where he studied at the university before he came to the United States via England in 1939. During the war he participated in the Manhattan Project, and after the war he joined Professor Failla's group at Columbia University where he became engaged in research, hospital physics and radiation protection. When Failla retired in 1960, Harald succeeded him as Director of the Radiological Research Laboratory.

In 1959 Rossi introduced microdosimetry with an article in the journal Radiation Research . He contributed strongly to its development in a particularly successful collaboration with Albrecht Kellerer. He designed several unique radiation detectors, such as the proportional chamber known as the Rossi chamber that is used for radiation protection measurements in complicated radiation fields, for example on aircraft and around accelerators.

Harald has told how he got involved in microdosimetry when he began to measure ionisation in gas in order to determine LET. He found unexpectedly large fluctuations, and Failla warned him that his method would not work because of the great variation in the signals. But Harald, who realised that the fluctuations might carry useful information, replied that it was precisely the variation he wanted to study. It is the hallmark of a great scientist to turn problems into success.

In addition to his scientific achievements, reflected in several hundred scientific articles, Harald is remembered as a forceful member of the International Commission on Radiation Units and Measurements (ICRU) and its Committee on Quantities and Units. It has been said that a committee was never pale or boring with Harald on it. Many of us also remember Harald's powerful interventions at meetings of UNSCEAR and other international groups as he tried, and succeeded, to educate those who handled quantities and concepts with insufficient rigour.

My own best memory of Harald is when he chaired two joint ICRP/ICRU Task Groups, one on the conceptual basis for determination of dose equivalent, the other on the quality factor in radiation protection. Harald felt that the quality factor for high LET radiation was underestimated and he convinced his colleagues.

During his last years Harald was a sharp critic of imprudent handling of the assumption of a linear, non-threshold dose-response relation.

I made friends with Harald in the 1950's when I worked for a year at the UNSCEAR secretariat on Manhattan. When I returned to Sweden, I sold him my Chevrolet. He, in turn, sold his old car (which he called Bouncy) to a friend for one bottle of whiskey for each month the friend could make use of the car. That was typical of Harald's unconventional and imaginative approach to practical problems.

Bo Lindell