A novel detector for low-energy photon detection with fast response

Abstract

A new avalanche silicon detector concept is introduced with a low gain in the region of 5 to 10, known as a Low Gain Avalanche Detector, LGAD. The lower gain reduces noise in comparison with a standard avalanche photodiode. The LGAD can be segmented to produce hybrid pixel detectors for low energy X-ray detection. Thin LGAD′s produce the same, or larger, signals as standard PIN diode detectors produced by minimum ionising particles (mips), however the collection time is reduced. The thickness reduction factor of an LGAD sensor is equal to the gain of the LGAD. For example; a 30µm thick sensor with a gain of 10 will give the same signal from a mip as a 300µm thick PIN diode. The LGAD has a fast rise time, useful for fast silicon timing detectors with sub-ns rise times from X-ray or mip interactions. Simulation of LGADs, using Sentaurus TCAD, of a pixelated device has been performed to determine the implant structures for the required gain and high voltage characteristics and to understand the in-pixel gain uniformity for a range of pixel sizes. Devices have been fabricated at Micron Semiconductor which produce the desired gain (measured to be approximately 10) and electrical performance with a breakdown voltage > 400V. LGAD sensors compatible with the Timepix readout system with varying pixel sizes have been fabricated. X-ray measurements have been performed on 5mm pads with a minimum detetable energy of 17.5 keV. Along with the detection of low energy photons these devices have been shown to produce a very fast response. Devices with this technology will be used in both the ATLAS and CMS timing detectors using pixels of the order 1mm x 1mm. Pixels have been fabricated in 2x2, 3x3 and 5x5 arrays. The measurements of gain and gain uniformity are presented.