Li X

A novel analog power supply for gain control of the Multi-Pixel Photon Counter (MPPC)

Abstract Silicon Photo-Multipliers (SiPM) are regarded as novel photo-detectors to replace conventional Photo-Multiplier Tubes (PMTs). However, the breakdown voltage dependence on the ambient temperature results in a gain variation of ∼3%/°C. This severely limits the application of this device in experiments with wide range of operating temperature, especially in space missions. An experimental setup was established to investigate the temperature and bias voltage dependence of gain for the Multi-Pixel Photon Counter (MPPC). The gain and breakdown voltage dependence on operating temperature of an MPPC can be approximated by a linear function, which is similar to the behavior of a zener diode. The measured temperature coefficient of the breakdown voltage is (59.4±0.4xa0mV)/°C. According to this fact, an analog power supply based on two zener diodes and an operational amplifier was designed with a positive temperature coefficient. The measured temperature dependence for the designed power supply is between 63.65–64.61xa0mV/°C at different output voltages. The designed power supply can bias the MPPC at an over-voltage with a temperature variation of ∼5xa0mV/°C. The gain variation of the MPPC biased at over-voltage of 2xa0V was reduced from 2.8 % / ° C to 0.3 % / ° C when biased the MPPC with the designed power supply for gain control. Detailed design and performance of the analog power supply in the temperature range from − 42.7 ° C to 20.9 ° C will be discussed in this paper.

Grain-Based Discrete Element Method (GB-DEM) Modelling of Multi-scale Fracturing in Rocks Under Dynamic Loading

This study aims to explore dynamic behaviours of fracturing and damage evolution of rock materials at the grain scale. A grain-based discrete element method (GB-DEM) is proposed to reveal microscale characterisation and mineral grain compositions of rock materials realistically. Micro-parameters of GB-DEM are obtained by calibrating quasi-static strengths, elastic modulus, stress–strain curves, and fracture characteristics of igneous rocks. Comprehensive numerical simulations are conducted to compare with dynamic experimental results obtained by the split Hopkinson pressure bar (SHPB). The reasonability of using the GB-DEM is presented to validate fundamental pre-requisites of the SHPB technique. Combined with crack strain and acoustic emissions, the rate dependency of crack initiation stress threshold and crack damage stress threshold is investigated. The dynamic damage evolution in the form of Weibull distribution is distinctively different from that in static tests and the shape/scale parameters are presented as functions of strain rate. Moreover, microcharacteristics of crack fracturing transition and fracturing patterns formation are discussed in detail. It is found that there exist two classes of mechanical behaviour (i.e., Class I and Class II) observed from stress–strain responses of dynamic tests. Main fracturing surfaces induced by intergranular fractures split the specimen along the direction of stress wave propagation in the type of Class I behaviour. Branching cracks derive the cracks’ nucleation and in turn increases the fragment degree. A shearing band formed near the fracture surface is caused by grain pulverisations, which eventually enhances the sustainability of rocks under dynamic loading. At last, we propose a generalised equation of dynamic increase factor in the range from 10−u20095 to 500/s, and also discuss the characteristic strain rate.

Research on Transient Wave Propagation Across Nonlinear Joints Filled with Granular Materials

A nonlinear thin layer interface model overcoming the long-wavelength assumption of displacement discontinuity model is developed to analyze the full-wavelength wave propagation across nonlinear parallel joints. In this study, the filling material is treated as a thin layer with reduced mechanical properties to reveal multiple reflections and time shifting within the filled joint. The nonlinearity of the filling material is considered, and a recursive matrix is derived in time domain. An experimental study on wave propagation across a filled joint was carried out by SHPB test. The quartz sand layer of different filling thicknesses sandwiched between Hopkinson bars was pressured by compressional waves to investigate the wave attenuation. Comparisons of the joint thickness, the wave frequency and the incident angle are carried out between the present model and the existing displacement discontinuity model. The results indicate that the thin layer interface model considering the thickness of the joint is capable of extending the long-wavelength assumption to full-wavelength research and it is more appropriate for filled joints with thick thickness that is comparable to wavelength. Then, this model is extended to parallel joints, and the properties of the filling material (i.e., initial elastic modulus and maximum closure), impact velocity, incident angle and wave frequency on wave attenuation are discussed for a joint set. The spacing dependency of the transmission coefficient for parallel joints is compared with displacement discontinuity model.

Study on temperature coefficient of CdTe detector used for x-rays detection

The temperature of the working environment is a key factor in determining the properties of semiconductor detectors, and it affects the absolute accuracy and stability of the standard detector. In order to determine the temperature coefficient of CdTe detector used for X-rays detection, a precise temperature control system was designed. In this experiment, detectors and radiographic source were set inside the thermostat with temperature of 0-40 Celsius degree, so that the temperature can be regulated for the test of the temperature coefficient of CdTe detector. Studies had shown that, with the increase of the temperature, the energy resolution and detection efficiency of the CdTe detector would deteriorate, and under 10 Celsius degree the detectors have better performance with the 8 keV X-rays.