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2018-07-06 13:58:51

  Lanthanum bromide (LaBr3) scintillator is a new type of inorganic crystals developed in recent years, since its excellent performance by the favor of the field of nuclear detection. Activation of neutron activation analysis of online multi elemental analyzer using prompt gamma neutron (PGNAA) technology, has a high demand for nuclear radiation detector, to have superior energy resolution, linearity, detection efficiency, by counting the rate and stability. The lanthanum bromide scintillator in the above several aspects of the performance parameters superior to other scintillation detector, have the application prospect and market value of [1] big. Due to the commercial application of lanthanum bromide crystal relatively late, on the other hand, crystal cost is higher, the current industrial nuclear instrument in the application of less. This paper will be designed by lanthanum bromide detector used in neutron activation multi-element analyzer products, so that the neutron activation multi-element analyzer made new breakthroughs in the measurement precision and repeatability on elements.
  1 Design of lanthanum bromide detector
  Lanthanum bromide scintillation detector by lanthanum bromide crystal, a photomultiplier tube, electronics parts, machinery parts. Basic lanthanum crystal itself superior performance in bromine, with electronics parts and structure of lanthanum bromide crystal applications, will eventually lanthanum bromide detector used in neutron activation multi-element analyzer products.
  Design of detector has the following 2 aspects:①The lanthanum bromide crystal and the universal application of the sodium iodide crystal properties were compared and analyzed, the choice of Saint Gobain 76cm * 76cm lanthanum bromide crystal;②Through strict theoretical calculation and practical test design electronics hardware and structure, including the selection of suitable lanthanum bromide crystal photomultiplier tube, design of high count rate, high linear voltage divider, low noise charge sensitive preamplifier, temperature controller, considering detector design for mechanical shape structure parts, reduced due to scattering of the mechanical part of caused by radiation and attenuation, detector design after the completion of testing [2] applications.
  2 The application of lanthanum bromide detector
  2.1 Analysis of self count test
  Lanthanum bromide (LaBr3) containing the La element in the La crystal, 2 natural isotopes 139La and 138la, 138la is radioactive, abundance was 0.09%, its decay is shown in Figure 1, lanthanum bromide detector from counting the bottom spectrum is shown in figure 2.

Figure 1 138la decay scheme

  138la has 66.4% branch ratio occurred in the k layer of orbital electron capture (EC) generation of excited state 138ba, 138ba back excited by 1436kev ray, at the same time as the filled orbital electron capture the formation of the k layer electron shell vacancy, outer nuclear barium electronic transition issue 35kev x 138la ray, the remaining 33.6% branching ratio generating beta decay of 138Ce formation, release 789kev gamma ray and beta endpoint energy 255kev.

Figure 2 lanthanum bromide detector from counting the bottom spectrum

  Fig. 2 shows, from left to right first see beta continuous spectrum from zero to 255kev, produced by the decay of 138la to 138Ce, from 255 to 750keV between the spectral shape for Compton scattering count 789 and 1436kev gamma rays, high energy region to be analyzed is after 789kev. Because the 789kev gamma and beta end point energy occur at the same time, enables the 789kev to about 1MeV as a continuous spectrum. The most obvious is the 1436kev gamma, we see the offset to the high-energy part, mainly by the L layer to fill the 32kev X ray produced electronic k layer and fill the 5kev X ray produced electronic l layer simultaneous contribution.
  Above 1750kev energy count mainly due to 227ac generated α ray contribution, this is due to the growth process of crystal, the self counting spectral shape 138la has the characteristics of obvious.
  2.2 The energy resolution test
  The energy resolution is an important parameter to measure the performance of the detector, said detector resolution two similar energy ray ability. Measurement of lanthanum bromide detector energy resolution is 2.8%, while the sodium iodide detector energy resolution of 7%. Lanthanum bromide test 137Cs spectrum is shown in figure 3.

Figure 3 lanthanum bromide test of 137Cs spectrum

  2.3 Lanthanum bromide detector used in neutron activation multi-element analyzer calibration test
  Neutron activation analysis of elements calibration using lanthanum bromide and sodium iodide detector calibration. Analyze the elements of calibration test, the measurement principle is shown in figure 4. Measurement and analysis of elements are mainly Ca, Si, Fe, the main energy [3], see table 2.

Fig. 4 measurement principle diagram of multi element analyzer

  Table 2 the main element analysis of neutron activation energy

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