The Design Of Gamma-Ray Well Logger Based On CsI(Tl) Scintilator With Avalanche Photodiode

With the rapid consumption of fossil fuels, the increasing difficulty of petroleum exploration and the increasing complexity of underneath environment, the requirements of high performance well logger gets increasingly high. The traditional methods used for well logging, such as electromagnetic logging, acoustic logging, wireline test logging,and drilling logging, not nearly meet the needs of today’s logging project.Compared with traditional logging methods,natural gamma spectroscopy logging(NGS) with relatively higher performances,like detection efficiency, stability and energy resolution. Howerver,the current NGS mainly used NaI(Tl) matched with photomultiplier tube (PMT),which has some unignored disadvantages,especially the low detection efficiency, low stability,and low energy resolution,for example,the GST digital natural gamma logger,produced by Beijing Huanding Service Company, generally has the resolution under 12% at 20 ℃ and under 15% at 175 ℃.Therefore, to furtherly improve and enhance the performance of NGS is quite necessary.The principle of NGS logging used the method of nuclear physics,and it’s one of the main techniques to help solve the problems of earth science and environmental science.Under the research,the current NGS is mainly composed of gamma ray detector, weak signal processing circuit,and data acquisition circuit. Gamma ray detector, which is commonly designed by NaI(Tl) coupled with PMT followed by preamplifier, always gets separated from the mainframe machines in the structure of NGS.The design of NSG have serious defects as following in long term using.First of all,PMT needs to work under lkV level high voltage,which can lead to super large power comsumption and can easily cause the short cut to circuits. Second,PMT is a pretty fragile glass device which can be easily damaged, so it’s not nearly appropriate for logging environment with frequently severe vibration.At last,the volume of PMT is too large to meet the needs of small bore in advanced logging project.In consideration of these obvious shortcomings in traditional NGS the actual logging requirements, and on the basis of rapidly developed microelectronic techniques,we did a lot of research on popular NGS products and we designed a new NGS with the gamma ray detector made of APD and CsI(Tl), cesium iodide active by thillium.The detector directly detect the gamma ray and convert it into electric signal. CsI(Tl) scintillator has great advantages,such as large energy sensitivity, high absorption and light yield of gamma ray,and good mechanical performance,and CsI(Tl) scintillator also extrodinarily mathes APD among its spectral rang.Besides,APD has inside gain approximate100,so it can directely convert the light signal from CsI(Tl) into weak electrical signal scintillator and amplify the weak signal.We also designed the charge sensitive preamplifier (CSP) with much lower noise and power consumption.CSP amplifies the output signal from APD,and it can significantly improves the signal to noise rate (SNR) and the system detection effeciency.The amplified signal will be sent to the signal processing circuit which can lower the noise again and get the signal shaped. Then we used the high speed analog to digital converter (ADC) to so that the following FPGA can analysis the discrete signal with different functions,such as pulse shape discrimination, baseline estimation and reduction, count rate correction, pulse phase analysis and so on.By use of these functions set in FPGA can obviously improve the system stability and energy resolution of NGS. Finally, the measured results from FPGA will be sent to the personal computer (PC) and get stored and analized.After a period of more than a year of design and research,we achieved some results as following:(1).we designed the gamma ray detector with CsI(Tl) scintillator spectally coupled with APD, and we used the polydimethulsiloxane to bond CsI(Tl) and APD, which are produced by Hammatsu Photonics.And the active area of APD is 10×10mm2 and the volume of CsI(Tl) is 10×10×10mm3,so that the high detection efficiency can be guaranteed.(2).we designed a new type of low noise and low power CSP. And we used AC-coupled and RC feedback methods within CSP under the classic folding structure.(3).we connected the signal processing circuits,ADC, and digital multichannel analyser (DMCA) with the core FPGA module.And we put the system under various situations with different bias voltage,different temperature and different radiation sources respectively.Then,we tested the system, observed the energy spectrum,and got quite a lot data analysized below.(1). we used the 137Cs under different bias voltage, ranging from 330V to 390V.And the results showed,the energy resolution rised when the temperature got higher,and we got the best energy resolution 4.98% at 370V.(2).we still used 137Cs under different temperature, ranging from -20℃ to 40℃,and we found out the energy resolution gets higher when the temperature gets lower. Especially we got 4.77% at -20℃,under the best appropriate bias voltage 370V,within 5min.(3).we set different radiation sources,137Cs,60Co,152Eu,226Ra,and the result showed us every characteristic energy peak of each sources can be clearly observed at room temperature,within 10min.Above all,the NGS we designed showed great performances,both higher stability and energy resolution compared with traditional NGS.And the preliminary design will be improved by future research.