Dieter Renker, who made some key contributions to the design and construction of the CMS experiment at the LHC, passed away on 16 March after a short illness. Dieter was born in Bavaria and studied physics in Munich and Berlin. He obtained his PhD from the Ludwig Maximilian University in Munich, based on experiments performed at SIN, now the Paul Scherrer Institute (PSI), in Villigen, Switzerland. In 1982 he joined SIN as a staff physicist, where he remained until his retirement at the end of 2009.
At SIN/PSI he participated in many experiments, providing excellent technical support, as well as designing new beamlines at the accelerator there. His technical aptitude in due course turned to detector development, which led to his greatest achievement. In the early days of CMS there were various ideas for the design of the electromagnetic calorimeter. Among these was the use of lead tungstate crystals, which although having many suitable properties for operation at the LHC, have a relatively small scintillation-light yield. Dieter contributed the key measurements which showed that avalanche photodiodes (APDs), with their key properties of internal gain and insensitivity to shower leakage, could be used to read out the crystals. This led to lead-tungstate crystals being adopted by CMS for the design of the calorimeter. Not only did they provide superb energy resolution for electrons and photons, enabling key discoveries such as the Higgs boson in 2012, but they also enabled a more compact detector with significantly reduced overall cost.
The development of the final APD was carried out over a period of many years by Hamamatsu Photonics (Japan), but under the close guidance of Dieter. Nearly 100 different APD prototypes were tested before the technology was deemed fit to be used in CMS. The size, capacitance, speed and, above all, radiation tolerance were the key parameters that needed to be improved, and the final choice was made very close to the deadline for commencing construction of the calorimeter. A complex multi- step screening process involving gamma irradiation and annealing also needed to be developed to ensure that the APDs installed met the demanding reliability requirements of CMS. Until now there has been no recorded failure of any of the 122,000 APDs installed in CMS.
Later, Dieter turned his attention to Geiger-mode APDs, which are now widely used in particle and astroparticle physics, as well as in PET scanners. Together with researchers at ETH Zurich, he started the development of the first camera based on these novel photo sensors for Cherenkov telescopes to measure very high-energy gamma rays from astrophysical sources. This camera was installed at the FACT telescope, located in La Palma, Spain, where the HEGRA experiment had also been operated with Dieter’s active participation. The FACT telescope has now been operating successfully for more than seven years, without any sensor-related problems.
After his retirement Dieter returned to his spiritual home, Munich, where he continued his work at the Technical University.
Dieter was a curious physicist with an exceptional talent for novel detector concepts. He pursued new ideas with a strong focus on achieving his goals. He had a very open mind, and was willing to advise and assist colleagues with great patience and good humour. In his free time his interests included classical music and cooking as well as searching the woods for unusual edible mushrooms. Many colleagues and visitors have fond memories of invitations to his home, embellished with fine cooking.
His sudden illness was a shock to many. Dieter leaves behind his partner, Ulrike.