Organic semiconducting single crystals as low cost, room temperature electrical X-ray detectors

Ionizing radiation can be detected by directly converting it into an electrical signal. Only few and expensive inorganic semiconductors (e.g. CdTe, SiC) offer the possibility of realizing portable detectors that operate at room temperature. Organic semiconductors have been so far mainly proposed as detectors for ionizing radiation in the indirect conversion approach, i.e. as scintillators, which convert ionizing radiation into visible photons, or as photodiodes, which detect visible photons coming from a scintillator and convert them into an electrical signal. Very recently we have shown the use of organic semiconducting single crystals (OSSCs) as intrinsic direct ionizing radiation detectors, thanks to their stability, good transport properties and large interaction volume [1]. X-ray detectors, based on low-cost solution-grown OSSCs are here shown to operate at room temperature, providing a stable linear response with increasing dose rate in atmosphere and in radiation-hard environments. We report here on the characterization of 4- hydroxycyanobenzine single crystals by both monochromatic synchrotron radiation (10-25keV) and a standard Mo target X-ray tube (35kV), equipped with a series of subsequent slits to focus the beam up to few mm2 area, so limiting the contribution of X-ray interaction with the substrate and the electrodes. The photoresponse to an increasing X-ray dose is always linear in the 10-35keV energy range here investigated. The linear response is maintained also at very low doses, down to 1 mGy/s. The calculated sensitivity is about 20 nC/Gy, not so far from polymeric sensor devices[2], thus assessing how OSSCs perform very well al low operating voltages (hundreds of Volts are usually required to operate room temperature inorganic semiconductor detectors). The combination of monochromatic X-rays generated by synchrotron radiation a standard Mo target X-ray tube (35kV) allowed us to better simulate the actual doses in medical diagnostic applications (typically mammography), thus covering and assessing a wide range of doses (1 – 180 mGy/s). Further development could be achieved improving the electrodes geometry to maximize the charge collection area, and to take advantage of the transport and charge collection anisotropy of the crystals