| Semiconductor detectors are used to measure particle tracks and spectrums of X-rays and γ-rays.CdZnTe is a kind of excellent semiconductor material with characteristics of high resistivity,large atomic number and wide band gap.CdZnTe are applied to physics experiments and radiation imaging because of a high energy resolution and good spatial resolution.Topmetal-Ⅰ and Topmetal-Ⅱ-are both direct charge sensors fabricated by industrial standard 0.35 μm CMOS Integrated Circuit process.The pixel array of the first design Topmetal-Ⅰ is 64x64 with a pixel size of 80×80 μm2.A naked metal node of 15×15 μm2 is placed on the top of each pixel while the total size of which is 25×25 μm2.When alpha particles pass through the region between a Topmetal-I sensor and a parallel-plate electrode,ionized charges are drifted to the surface Topmetal sensor by electric field and collected by metal nodes of pixels to form two dimensional images of charge clouds.Under the precise condition,a single alpha particle can be measured by a Topmetal-I sensor.The equivalent noise charge is calculated to be about 327 e-with a average capacitance of 207 fF in the known of the energy of incident alpha particles.The second sensor,named Topmetal-Ⅱ-,has a pixel array of 72×72 with a pixel size of 83×83 μm2,which is a little larger than Topmetal-I.However,the metal node is the same size as the Topmetal-I’s.A much lower electronic noise of 40 e-is achieved by a Topmetal-Ⅱ-sensor.The node capacitance used to collect charges is simulated to be about 5.5fF by IC design software.Charges can be continuously injected to the sensor and collected if the pixels are not saturated because the sensor does not need to be mandatorily reset in working.The equivalent noise charge(ENC)is under 15 e-when a digital trapezoidal filter is applied to shape signals induced by square waves.Topmetal-Ⅱ-is shown easily to catch tracks left by alpha particles passing through the top of the sensor when an external electric field is applied.Hence,Topmetal-Ⅱ-is sensitive to weak signals and a good choice to be utilized in beam monitoring.The sensors are designed to be applied in radiation imaging and physics experiments,such as alpha counting,beam monitor system using a gas as medium and neutrinoless double beta decay.Besides,the sensors can be coupled with semiconductor materials(such as cadmium zinc telluride,CdZnTe)to build high resolution detectors.In this paper,the three main works are as follows:Firstly,study on the performance of the Topmetal-Ⅱ-sensor at low temperature(liquid nitrogen);To measure its performance at cryogenic temperature,a Topmetal-Ⅱ-sensor is embedded into a liquid nitrogen dewar.The decay time constant of a pixel is more sensitive to the reset voltage of Vreset in liquid nitrogen,which is the reason that decay time constants and ENC of the sensor show a much wider distribution since ENC is affected by the decay time constant when trapezoidal filter is applied to shape the signal.Most probable values(MPV)of ENC are 12 and 13 e-in liquid nitrogen and at room temperature respectively,while the mean(s)value is 19.4(10.4)and 13.5(2.5)e-.The result is that Topmetal-Ⅱ-can also operate well at this low temperature with a lower ENC than that at room temperature for a single pixel.From the noise perspective,Topmetal-Ⅱ-is a promising candidate for the next generation readout of liquid argon and xenon time projection chamber(TPC)used in experiments searching for neutrinoless double beta decay and dark matter.Secondly,study on design and performance of the CdZnTe detectors based on Topmetal-I sensors;According to the characteristics of the Topmetal-I sensor to collect charge signals,the connection between the cadmium telluride crystal and the sensor adopts the method of AC coupling,the charge signal is then induced on the Topmetal-I sensor.The procedure is:a square wave voltage is applied to the metal electrode on the surface of a CdZnTe crystal;when the square wave voltage is changed to negative voltage from positive voltage,a 405 nm laser signal is injected into CdZnTe through the little square naked area without electrode;charge signal is then coupled to each pixel of the Topmetal-I sensor;taking the data before the Topmetal-I sensor reset to scan and the bias voltage is changed from the positive to the negative voltage;two dimensional signal distribution on the sensor can be obtained from the response of all the pixels on the Topmetal-I sensor;two dimensional imaging on the sensor reflects the two-dimensional distribution of the intensity of the incident particles.The surface of CdZnTe coated with a metal is used to apply square wave voltage,an area of 2×2 mm2 is naked without an electrode.The other side of the CdZnTe without an electrode is coupled to Topmetal-I sensor.Using the method described,a two dimensional image of a 405 nm laser signal is achieved by this detector.At the bias voltage of-60 V,the spatial resolution is less than 200 pm for the CdZnTe detector based on Topmetal-I.Thirdly,study on design and performance of the CdZnTe detectors based on Topmetal-Ⅱ-sensors;The Topmetal-Ⅱ-sensor does not need to be mandatorily reset in working and charges can be continuously injected to the sensor and collected.Hence,a liquid medium can be adopted to connect the sensor and a CdZnTe crystal to build a detector keeping both the uniformity and weak conductivity between them.We need to choose a medium with relatively lower resistivity than CdZnTe crystal and higher than that of a conductor,through which charges in the crystal can pass and diffuse not much while maintaining locality and then collected by metal nodes of the sensor.Because the thickness of the liquid medium will affect the charge transfer between the crystal and the chip,so the sensor and the crystal fit closely.A 650 nm laser diode is exploited to test the entire detector system,the result shows that the detector operate very well with a spatial resolution of 420 μm at a bias voltage of-2 V.A single particle of alpha is also directly measured by the detector.The performance of Topmetal-II chip in liquid nitrogen is studied in detail in this thesis.From the noise perspective,Topmetal-Ⅱ-is a promising candidate for the next generation readout of liquid argon and xenon time projection chamber(TPC)used in experiments searching for neutrinoless double beta decay and dark matter.Highly pixelated CdZnTe detectors that combine Topmetal-I,Topmetal-Ⅱ-pixel sensors and CdZnTe crystals have been successfully developed in the lab.The two detectors both show good spatial resolution.The CdZnTe detector based on Topmetal-I achieves a 200μm spatial resolution,while the CdZnTe detector based on Topmetal-Ⅱ-achieves 420 μm.At the bias voltage of-2 V,the CdZnTe detector based on Topmetal-Ⅱ-can form the two dimensional images of charge clouds ionized by single alpha particles.For now,the pixel size of CdZnTe detectors used in medical CT is around 2.5 mm,there is much room to improve considering the spatial resolution.Hence,CdZnTe detectors based on Topmetal series sensors have great potential to be applied in this application. |
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