Since the end of 1999 the high-current injector of the SIS deliver ion beams with up to 10¹⁰ particles /spill even for the heaviest accelerated ions like the uranium ions with Z=92. For lighter ions the beam intensity increases up to 10¹³ particles /spill. The detector devices used in heavy-ion experiments with such intense beams must be radiation hard, provide fast collection of charge, be as thin as possible and in case of strip-or pixel detectors remove heat from readout electronics. The most promising detector material for this purpose is CVD diamond.

The Polycrystalline CVD-Diamond Material

The Chemical Vapor Deposition (CVD) synthesis technique for diamond [1] is based on the decomposition of gaseoues hydrocarbon molecules in a gas mixture containing 98% hydrogen and about 1% to 2% (usually) methane. The reactants are activated by thermal or microwave-plasma assisted processes. The initial reactions are the decomposition of molecular hydrogen into atomic hydrogen and the formation of methyl radicals.

The microscope pictures show the very different surface topology of the three detector sides. The real material is of high transparency. The colours are affected here by the illumination and the photographic process. (Microscope Pictures: H. Folger GSI)

The reaction products are transported to the deposition surface (usually silicon- or molybdenium wafers) on which graphite and diamond start to grow. The deposition of diamond can only be realized because diamond has a higher stability against the influence of atomic hydrogen than graphite. However, due to a mismathing of the lattice constants in the interface layer CVD diamond grows polycrystalline.

Polycrystalline CVD-Diamond Heavy-Ion Detectors

Polycrystalline CVD-diamond detectors are used for heavy-ion measurements in which a good time resolution, resp. a very high counting-rate capability is required. Due to a wide band gap of 5.5 eV no pn-junction and no cooling is needed. After metallisation of the electrodes (evaporation or sputtering of Cr-Au or TiW-Au) the detectors are ready to use.The diamond signals are processed in a single-particle mode. They are amplified with low-noise, broadband amplifiers DBA ( Diamond Broadband Amplifier) developed ( P.Moritz@gsi.de) especially for the ultra-fast but weak CVD-diamond signals (FWHM < 1 ns possible). In order to avoid signal reflexions, every single detector channel is built as a 50 W microstrip line. An intrinsic time resolution of 29 ps and a count-rate capability per channel > 10⁸ particles/s are achieved.

Although the complicated charge collection phenomena in CVD diamond are not completely understood yet, satisfying heavy-ion results led to a vareity of applications  of CVD-diamond detectors in accelerator beam diagnostics and in heavy-ion physics experiments as well.

Single-Crystal CVD-Diamond Detectors

Striving for radiation hard detectors of superior performance in both, time and energy resolution we investigate homoepitaxial single-crystal CVD diamond materials, which are grown on single-crystal diamond substrates made by High-Pressure-High-Temperature (HPHT) processes.Single-crystal CVD-diamond detectors show complete charge collection, an excellent energy resolution which is superior to the resolution of silicon detectors in the case of relativistic heavy ions,and a comparable time resolution to the ultra-fast polycrystalline CVD-diamond detectors. The following figures illustrate the high potential of this material for the most important detector categories in heavy-ion and hadron physics research.

(Page in preparation: Figures will be published soon!)

References

[1] Synthetic Diamond, Emerging CVD Science and Technology, ed. by Karl. S. Spear and John P. Dismukes

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