Experimental Study On δ¹³C Measurement Of Unipolar ¹⁴C-AMS

Accelerator mass spectrometer（AMS）,as a nuclear analysis instrument for measuring long-lived radionuclides,can measure the abundance ratio of isotope in nature with high sensitivity,and was widely used in dating and tracing research.For the measurement of carbon isotope-¹³C and ¹⁴C,AMS technology has advantages of short time measurement,small sample size,high detection sensitivity,etc.With the development in the field of nuclear analysis,AMS measurement technology has gradually become one of the indispensable tools and plays an important role in the fields of the earth environment,geology,and life sciences.Guangxi Normal University GXNU-AMS is the first domestically developed accelerator mass spectrometer technology for low energy ¹⁴C measurement.For measuring ¹⁴C,it is necessary to establish and a stable carbon isotope measurement system,to determine the of the δ¹³C analysis.The measurement of δ¹³C is one of the key technologies in the experimental process of the AMS system,which can not only correct fractional effect of the AMS system but also be an important for ¹⁴C measurement.Therefore,the measurement research of carbon isotope ¹³C is very important foe the AMS measurement process.In this work,the δ¹³C values in a series of tree samples,groundwater samples,oil and water samples,etc.Were measurement with the 200k V low-energy miniaturized,¹⁴C dedicated single-stage accelerator mass spectrometer（GXNU-AMS）established in cooperation with Guangxi Normal University and the Chinese Institute of Atomic Energy.The specific work were shown as blow:（1）A preliminary attempt was made to measure the δ¹³C by apply an Treck alternating high voltage on vacuum box.Firstly,the value of the injection magnetic field was set to inject ¹³C beam,and then periodically change the treck high-voltage source voltage（±4k V）,and obtain the beam current values of ¹³C⁺and ¹²C²⁺,with the main Faraday cup and the compensation Faraday cup at high-energy side,respectively.Then,the ¹³C⁺/¹²C²⁺isotope ratios in the samples be available.（2）The method of measuring δ¹³C by periodically changing the magnetic field value of the injection magnet was established.Through the rapid conversion of the injection magnetic field,the beam current values of ¹³C⁺and ¹²C²⁺at the image points of analyze magnet were measured with the main Faraday cup and the compensating Faraday cup,respectively.To obtain the¹³C⁺/¹²C²⁺beam ratios at the high-energy end in a short time,and compare the ¹³C⁺/¹²C²⁺value of different unknown samples with the OXII standard beam ratio(¹³C⁺/¹²C²⁺)to obtain the unknown δ¹³C value.（3）The δ¹³C values in process graphite samples,groundwater samples,and oilfield oil and water samples were systematically measured,and were used in the fractional correction in ¹⁴C measurement.In this work,hundreds of samples were measured,and the ¹³C⁺/¹²C²⁺beam current ratio of the high-energy end of each group of samples was analyzed,and the fitting ¹³C⁺/¹²C²⁺beam ratio curve with time of the standard sample was drawn.The δ¹³C value of all samples was corrected with the δ¹³C value of standard sample.The results show that the relative deviation of the sample beam measurement ratio(¹³C⁺/¹²C²⁺)are within 5‰,which reflects that the ¹²C²⁺and¹³C⁺beams measurement with GXNU-AMS are high stability and which can meet the requirements of isotope fractional correction for actual samples measurement.In this work,several technical improvements have been carried out as shown bellow:（1）The manual measurement technology of δ¹³C.The manual technical method has been successfully applied to the GXNU-AMS system as a supplement to the conventional vacuum box alternate high-voltage injection method.The system beam current value is very stable and reliable during the measurement process,and can achieve high-precision measurement of δ¹³C in various carbon-containing samples;（2）During the experiment,there is a periodic drift phenomenon in the injection magnet,in order to compensate the magnetic field drift of the injected magnet,the flat top of the injection magnet should be carefully measured during the experimental beam adjusting process,and father more the platform should be regularly measured during the measurement process,so that the magnetic field is stably located in the center of the platform to ensure the stability of ¹²C and ¹³C ion beams measurement for various samples;（3）Theχ^2 test method（chi-square test）was adopted in the data analysis.This method is effective means to test the difference of the data,and accurately obtain average beam current ratio for each samples,plays an important role in the analysis of δ¹³C.