| The measurement of radon isotopes and their daughters of significance and value in radiationprotection studies,as well as in mineral resources exploration and exploitation, earthquake science,hydrology and OceanographyCoincidence counting method is one of the basic nuclear detection technologies,accordingto time sequence decay characteristic of radon isotopes and their daughters, a kind of delayedcoincidence counting may be employed to discrimination radon isotopes219Rn and220Rn,thismethod features that it can discriminate219Rn and220Rn in environment of222Rn and low levelactivity. The technology has successfully applied to measurement of low-level220Rn in air andindirect measurement of Radium isotopes tracer in Oceanography.Distinguishing radium isotopesto an important mean and key technology of oceanographic research,is currently a research focus.The paper introduces the principle of radon isotopes delayed coincidence countingtechnique,researh into the correction and error data processing of delayed coincidence counting;discusses the theory of pseudo random number generator and first suggests software delayedcoincidence technology to discrimination radon isotopes, and investigate the better measure bysumulating the decay of each of these radon isotopes to a short-lived polonium isotope withMatlab,Several simulation experiment is conducted by altering the delay time and differentnumber of radon isotopes,and found that optimal coincidence window is3-5times radonprogency’s half-life. By comparing the experimental data with the references quoted, the resultsalso shows that the method can offer better convenience and flexibility, is also referential to thecurrent applications.Digital delayed coincidence counting(DCC) for discrimination radon isotopes is also firstproposed.a design scheme for digital coincidence was described in this paper. With sampling thedetector signals to the computer by the high speed data acquisition card, the sampling data isstored and processing by digital method instead of by hardware circuits. In order to get thecoincidene information(amplititude,pulse arriving time and interval), This dissertation researchesthe follows:anylize the signal of detectors, digital pulse shape,baseling correction.The timingcircuit is the basic unit of analog circuit of nuclear timing detector, including leading-edge timing,zero-crossing timing, constant fraction timing(CFD) on a regular timing circuit.Compared with analog timing circuits, digital timing circuit was detail investigated into CFD and leading-edgetiming, zero-crossing timing discriminator in MATLAB,an improved digital leading-edge timingis also studied in this article, and it has analyzed the status and trends of nuclear pulse timingimplemented by MATLAB software. Moreover, A DCC system was constructed for radonisotopes discrimination experiment.and initial results is descibed and analyzed.DCC method requires high speed data acquisition and mass storage devices, is fit to forlaboratory measurements.For portable use the software delayed coincidence counting(SCC) deviceis recommended. Unlike DCC The device based on embedded system. was constructed to recordtime serials information of nuclear pulse. According to the different half-life of Po short-lifedaughters of radon isotopes, a method of software delayed coincidence counting was used todiscriminate radon isotopes. Experiment system was established for measurement of low-level220Rn by comparative measurement with RAD7.The results shows that220Rn of decay rate of3dpm～4dpm can be measured by the system with error of±8%,when the background count rateof2cpm～4cpm. The study also demonstrated that the effectiveness and the validity of theproposed software delayed coincidence method.The conventional coincidence counting(CCC) usually employs traditional hardware coincidencemodules,which has complicated circuit structure and is difficult to be implimented.With thedevelopment digital technology, the CCC has many obvious disadvantages such asanti-disturbancecapability, visualization,reproducibility,information sharing,easy operation,instruments upgrading. The above proposed DCC and SCC characterize compact structure,flexible expansion, adaptable to measurement object and environment,strong liability, is promisingflexible new approaches to the absolute measurement of radioactivity.The research is of greatsignificance to the future research on digital coincidence related technologies such ascoincidence,delayed coincidence,anti-coincidence,coincidence spectry. This technology is helpfulto radiation protection and environment protection.,also serves improvement to earthquake science,hydrology,oceanography and somr other related technologies... |
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