Single crystal diamond as an X-ray detector

John Morse, SLAC National Accelerator Laboratory and European Synchrotron Radiation Facility

Pure crystalline diamond is a wide bandgap (5.5eV) semiconductor material. With appropriate homoepitaxial CVD growth, single crystal diamonds can now be produced with crystallographic quality and purity sufficient to provide detectors suitable for the detection of ionizing radiation ranging from deep UV photons to relativistic heavy ions. Complete collection of charge carriers over millimeter drift distances at applied fields >1V/µm are possible, and the material demonstrates sub-picoampere/cm2 leakage current at room temperature. The low atomic weight of carbon is particularly attractive in X-ray detection applications where only weak absorption of the incident radiation is a necessity, e.g. for transmissive X-ray beam monitoring at synchrotron and free electron laser sources. 3rd generation synchrotron X-ray beams present continuous ionizing dose rates to the detector >10kGy(Si)/sec, which effectively rules out the use of silicon detectors due to surface oxide charging and interface radiation damage mechanisms, while simple diamond 'ionization chambers' with metal contacts have proven radiation hard.

Beam intensity and position monitoring at synchrotrons has become more challenging as the technology of X-ray optics has been steadily improved: focusing down to the micron level is now routine, and the goal for the next few years is to operate at the 10nm scale. Beam movements result from vibrations and thermal drifts in the X-ray optical elements which are typically spread over a hundred meters length of the beamline. These movements must be monitored, and ideally compensated by feedback mechanisms with bandwidths of several kHz. We are developing thin (<100µm), position sensitive diamond devices which absorb <10% of the transmitted X-ray beam, generating electrical charge from the photoelectric absorption and Compton scattering processes. The induced signal currents are measured either with electrometers or using both wide-and narrow-band RF readout up to ~GHz frequencies. Difference-sum measurements of signals from segmented electrodes allow the determination of both beam intensity and position. Results already achieved with resolutions <20nm measured at the ESRF and DESY synchrotrons are presented. The nanosecond charge transit time in the diamond detectors and their excellent current source characteristics also allow for measurements of the X-ray parameters on a pulse by pulse basis, as required by the '4th generation' free electron laser sources such as the Linac Coherent Light Source at SLAC.