The Start detector for heavy ion experiments in HADES is used to determine the reaction time (T0) and monitor properties and quality of the beam. A scCVD (Chemical Vapour Deposition) diamond detector has been used for this purpose. CVD diamond material is mainly known for its excellent charge collection efficiency which results in very good energy resolution and high drift velocity of both electrons and holes. These properties are demonstrated in the animation below. The scope waveforms were collected with a scCVD diamond detector with thickness of 50um irradiated with Au beam ions @ 1.25 AGeV.
T0 determined by the Start detector is a part of the Time of Flight measurement used for the particle identification and therefore a precision better than 50 ps (sigma) is required. In the upgraded HADES spectrometer, prepared for high multiplicity and high rate experiments, reactions at Au-beam intensities of about 2-3*106 ions/s can be studied. Such beam intensities pose challenges on the start detector mainly in terms of the rate capability and the radiation hardness.
Excellent beam monitoring capability of the device are demonstrated in the histograms showing profile of Au beam during the HADES experiment. Archived beam focus is better than 1.3 mm (3 sigma) in Y direction and 1.0 mm (3 sigma) in X. Such a beam spot at the HADES focal point is essential as the HADES has to use very small target elements (2.2 mm in diameter) in order to reduce the background processes.
Following the requirement the Start detector delivers a time measurement with precision better than 50ps (sigma) for a single beam particle. At the same time it can stand the beam intensities requested by the physics program which are about 2-3*106/s/mm2. The spectrum below was taken with an Au beam at 1.25 AGeV with the scCVD based Start detector in HADES measured in HPTDC chip which run in 25ps/bin mode. Obtained time resolution was better than 50ps (sigma).