Study On CARR Cold Neutron Prompt ? Activation Analysis Technology

Cold Neutron Prompt Gamma Activation Analysis is a nuclear analysis technique in the field of chemical analysis.It uses a neutron beam to bombard a nuclear?n,??nuclear reaction in a target sample.It measures the instantaneous?released by the excited state,the lifetime is less than 10^-14s,and it has high sensitivity,high accuracy,non-destructive and multi-element simultaneous measurement methods for light elements or elements with large reaction cross sections.Compared with the traditional neutron activation analysis,it has fast analysis speed and high sensitivity to light elements such as elements H?B?O?C?N which cannot be determined by conventional activation analysis,and is a supplement to the conventional activation analysis technology.To date,more than 30 laboratories around the world have established cold neutron prompt gamma activation analysis techniques.In recent years,the application of this method in high-tech fields has been increasing,such as the study of chemical and physical reaction processes of hydrogen fuel cells;the study of infiltration process of weapon materials?³H?;hydrogen storage materials/process research;hydrogen catalytic mechanism in solid catalysts;research on hydrogen embrittlement mechanism of materials such as nuclear fuel?uranium-niobium alloy?;hydrogen modification of semiconductor materials?SiO₂?Si?GaAs etc.?;determination of short-life?second or shorter?nuclear fission parameters;determination of toxic elements?such as B?Cd?in nuclear materials;determination of N content in biological materials and analysis of light elements and multi-element in standard materials.It plays an important role in the fields of high-tech materials,clinical medicine,life sciences,environmental sciences,earth and astrophysics,archaeology,industry,and“hydrogen economy”energy.And China has just started in this field.With the operation of China's advanced research reactors and the support of the National Defense Science and Technology Bureau Infrastructure Project and the Natural Science Foundation Project,the China Institute of Atomic Energy has carried out research on the cold neutron prompt gamma activation analysis and measurement system.In this context,this paper carried out some research work related to the project,using CARR cold neutron tube to provide stack cold neutron beam,according to the prompt gamma activation analysis theory and data simulation design,establish a set of cold neutrons prompt gamma activation analysis system.The paper includes the following three main tasks:1.Design of CNPGAA measuring device.Through the investigation,a measurement room with simultaneous measurement of multiple detectors was established.The measuring chamber adopts a cube design,and the interior part is composed of a sandwich carbonized boron material.Externally made of 5cm thick uncovered integrated lead cubes.The sample holder is composed of PTFE,and the sample to be tested forms a 45°with the incident neutron.This design also minimizes the transmitter rate of gamma and scattering neutrons to the detector direction.Two detectors are disposed outside the measuring chamber perpendicular to the incident neutron and in line with the sample to be tested,one on each side.The measuring chamber wall perpendicular to the detector is opened,and the inner wall of the hole is covered with 3 mm thick lithium carbonate-6 to absorb the scattered neutrons.⁶Li does not produce gamma rays when neutrons are absorbed,which does not affect the gamma ray measurement of the elements to be tested,and also avoids the influence of scattering neutrons and environmental gamma on the background.A 20mm×20mm square hole is opened in the measuring chamber wall of the incident neutron and the outgoing neutron for the passage of the cold neutron beam.The neutron beam blocker is placed 50mm away from the cold neutron exit to avoid the impact of the outgoing neutron on the environment.The design is characterized by the first step of the analysis process,which minimizes the spillover of scattering neutrons and environmental gamma generated by the interaction between the sample and the incident neutron,and reduces the background of the measurement system.At the same time,the damage of the detector by the scattering neutron is greatly reduced.2.Design of CNPGAA low background shielding system.The main content of this part is to reduce the environmental radioactivity background outside the detection system.The main considerations are three aspects of the design:On the one hand is the incident neutron.This work requires only 20mm×20mm cold neutron incident beam spot on the prompt gamma activation analysis according to the experimental requirements.Therefore,a beam arrester is set at the outlet of the CARR cold neutron conduit,such as boron carbide,lithium carbonate-6,lead block,etc.,and the activation analysis of the incident neutron beam control automatic switch ensures the safety of the non-experimental time period.The second aspect is the development of a soft body shield.The shield body is composed of colloid,boron carbide and lithium carbonate-6.It has the characteristics of heat resistance,radiation resistance,acid and alkali resistance,friction resistance,wrinkle resistance and good neutron shielding effect.At the same time,it can be made into different shapes and thicknesses as needed.The soft neutron shield has been used for neutron shields such as high-purity helium probes to prevent damage to the neutron detectors.The third aspect is gamma ray shielding around the sample measurement chamber to further reduce the environmental background.Considering the neutron blocker from the outlet of the cold neutron beam conduit to the outlet of the measuring chamber,all of which are accompanied by neutrons and gamma rays,the overall shielding of the measuring system is very important.In this project,the measuring chamber bracket is designed,and the neutron soft body shield is used for neutron shielding around the neutron beam,and then a 10cm thick lead block is installed on the bracket and the beam pipe and the measuring chamber to shield gamma rays.Thereby effectively reducing the environmental background of the measurement system.3,CNPGAA system parameter test.After the cold neutron prompt gamma neutron activation analysis measurement system is built,the system must be tested to verify it effect or further improvement.This section includes the establishment of a prompt gamma acquisition system,detector performance testing and performance comparison of different types of detectors,as well as preliminary analysis of different types of samples.Prompt gamma measurement system:Three different types of gamma detector systems were selected for this work,including a high purity germanium?Ge?gamma measurement system,a BGO detector measurement system,and a LaBr₃ detection system.These three detectors are commonly used in different locations.We first performed gamma ray measurement debugging on three detection systems.Test parameters and measurement conditions such as high voltage,amplifier,gamma energy test range of each detection system are fixed.Detector performance parameter test and performance comparison:Includes three detector energy scales,efficiency scales,and gamma ray resolution?Full Width at Half Maximum?.The role of the scale is to test for line prompt gamma activation analysis.Since Cl?chlorine?has a wide range of prompt gamma ray energies?400keV-6000 keV?and nitrogen?N?with the instantaneous gamma ray energy of 11MeV,it is used as a detector source.By testing and comparing the gamma performance of the three detectors,the results show:Compared with the energy scale,all three detectors have a good energy linear relationship;Resolution scale comparison:HPGe is better than LaBr3 than BGO;Comparison of detection efficiency:the detection efficiency of the three detectors can be well fitted,and the detection efficiency is that BGO is higher than LaBr3 than HPGe.Cold neutron prompt gamma activation analysis of test samples:In this work,the neutron fluence rate of the CARR cold neutron beam with and without cold source was tested by the gold plate activation method.The test results were 4.8×10⁸n cm^-2 s^-1and 4.1×10⁷n cm^-2 s^-1,and the neutron fluence ratio?with cold source/no cold source?was 11.7.Secondly,for the first time in China,the N element is used to measure the energy and efficiency of the wide-energy region of the detector of the system,and some important parameters such as sensitivity measurement.Then,the measurement system is used for Eu?Au?In?Gd?Al?Fe?Cu?S?Sr and other simple substances and NH₄CL?CaCO₃?NaCl?MgO?SiO₂?B₄C and the like,and a mixture thereof and GBW07401?GBW07103 and other rocks,biological and other standard materials for measurement to test the performance of the measurement system and find potential deficiencies.In addition,the measurement results are simply analyzed and fitted to build a single-mass and multi-element analysis database.This paper uses the pipeline beam cold neutron provided by CARR to establish the first cold neutron prompt gamma activation analysis system in China through theoretical and simulation design;developed an efficient cold neutron shielding soft material;the system produces a low background and can be measured simultaneously using two detectors;The performance differences,advantages and disadvantages of HPGe?LaBr₃?BGO and other detectors for prompt gamma activation analysis were studied.This result can be used as a reference for what detection system to use in different analysis environments.At the same time,the detection system was used to analyze the elements of elemental elements,compounds and unknown samples.Project characteristics and innovation:This work established China's first cold neutron prompt gamma activation analysis system,which has a low environmental background and can be analyzed and measured simultaneously using two detectors,thus improving the analysis speed.