Multi-element neutron activation analysis of China geochemical standard samples

Zhang Yunjun, Li Xingbin, Song Linshan

Institute of Geophysics and Geochemical Exploration, Ministry of Geology and Mineral Resources, China

Yuan Ling, Chen

Institute of Atomic Energy of China Academy of Sciences

Such as Wang, Sun Jingxin

The Institute of High Energy Physics of the Chinese Academy of Sciences

The concentrations of 36 elements in geochemical standard samples of China Ministry of Geology and Mineral Resources were determined by neutron activation analysis. Three laboratories (Institute of Atomic Energy, China Academy of Sciences (ENAA INAA), Institute of High Energy Physics, Chinese Academy of Sciences (INAA) and Institute of Geophysics and Geochemical Exploration, Ministry of Geology and Mineral Resources (PNAA INAA)) systematically studied the samples, and adopted three main technologies: instrument, superheated neutron and pre-irradiation neutron activation analysis (INAA, ENAAA and PRAA). Long radiation and short radiation as well as Ge(Li) and HPGe detectors are used. A data processing auxiliary software package is developed. Among the data measured by neutron activation method, about 865438 0% is consistent with the recommended value, and the error is within 65438 05%.

The Institute of Geophysical and Geochemical Exploration of the Ministry of Geology and Mineral Resources has prepared three series of geochemical reference samples: stream sediment (GSD), soil (GSS) and rock (GSR). Eight stream sediment samples, GSD 1-8, were selected from areas considered to represent the types of sediments in China. Xie (1) gives the lithology of rocks in the province where the sampling point is located and the sampling basin.

experimental

standard

Primary solution standards are used for standardization. A set of 3-5 elements without major energy interference is combined to form a multi-element standard. Reference samples USGS GXR 1-6, USGS-AGV- 1, NES-SRM- 1362a, CCRMP SY-2 and SY-3 were also used. Suck a predetermined volume of solution standard onto a plastic film or circular filter paper, and package the film or filter paper into a high-purity aluminum foil (2).

Sample preparation and irradiation

Dry the uniform sample powder in an oven at 90 ℃- 1 10℃ for more than 3 hours, and then accurately weigh the sample part (20- 100mg) onto the aluminum foil. For INAA and PNAA, all samples and standards are placed in aluminum irradiation containers, and for ENAA, in boron nitride (BN3) containers. The irradiation time of dysprosium and manganese is 2 to 5 minutes, silver is 72 to 65 hours, and other elements are 3 to 23 hours. Swimming pools and heavy water reactors are used for irradiation. The thermal neutron flux es is (1-2.4) *101cm-2s-1and (5-7) *13n, respectively.

Pre-irradiation separation technology is used to improve the analytical sensitivity of gold and rare earth elements. For gold, the sample part (1 0-20g) was dissolved in aqua regia, and then gold was adsorbed on activated carbon, while for rare earth, the sample part (1g) was melted in sodium peroxide, and the sample was dissolved in PMBP( 1- phenyl 1-3- methyl/kl).

Equipment and measurement

Two types of Canberra data acquisition systems (SCORPIO-3000 and JUPITER) are used to measure gamma spectra. Coaxial germanium (lithium) detector covers the energy range of 60 to 2000 keV, and planar high purity germanium (HPGe) detector covers the energy range of 30 to 300 keV. All three laboratories use germanium (lithium) for measurement, but in addition, the Institute of Geophysical and Geochemical Exploration also uses high-purity germanium to measure many rare earth elements, such as hafnium, tantalum and tungsten.

After a given decay period, the irradiated sample is transferred to a counting bottle in a glove box. Adjust the distance between the irradiated sample and the detector to avoid the dead time of ADC exceeding 10%. Different cooling time is selected to determine isotopes with different half-lives. According to the intensity of sample activity, the data acquisition time is 500, 1000 or 2000 seconds.

data processing

The spectra of various samples are transferred to CD-ROM, processed by software packages SPECT-RAN-F and SPCSUP, so as to obtain the concentration of elements in the samples and print out various reports. SPECTRAN-F was developed by Canberra; SPCSUP is a software package that supplements SPECT-RAN-F, written in BASIC language and designed by the laboratory of Institute of Geophysical and Geochemical Exploration. When the standard number of elements is equal to or greater than 3, the calibration curve is calculated by least square method. U-fission interference of standard and sample is automatically corrected.

Up to four gamma rays are used to determine the concentration; The results obtained for each line are stored independently. According to the cooling time and the complexity of the sample matrix, only gamma ray results with allowable interference are included in the average value. The main parameters of NAA listed in table 1 are obtained from (3-5).

Results and discussion

Thirty-six elements in GSD 1-8 were determined in three neutron activation laboratories, which accounted for 66.7% of the total number of certified elements (54) in the recommended value (1). The results of 36 elements of GSD 1-8 obtained by NAA are listed in Table 2 together with the recommended value (RV). Two or three NAA laboratories have obtained the data of 3 1 element. The other five elements are provided by a laboratory of NAA: gallium is provided by IAE Institute of Atomic Energy, bromine and nickel are provided by IHEP Institute of High Energy Physics, and gold and holmium are provided by Institute of Geophysical and Geochemical Exploration. g)。 The standard deviation of most elements is less than or equal to 15%.

Most of our data are not only consistent among the three laboratories, but also consistent with the recommended values, which are calculated from the data by 4/kloc-0 laboratories using various analytical methods. The Institute of Geophysics and Geochemistry made a statistical analysis of the difference between the data of Li detector and the recommended value. Of the 2 14 differences, 95 (44.4%) were consistent with RV within 5%. 173, or 80.8% agree 15%; And 195 or 9 1. 1% are within 25% of the recommended value. The data of germanium (lithium) detector are generally consistent with the recommended values, which confirms the importance of NAA application in geochemistry.

The Institute of Geophysical and Geochemical Exploration has obtained the results of nine elements (cerium, gadolinium, hafnium, neodymium, samarium, tantalum, thulium, tungsten and ytterbium) with high purity germanium, and the resolution in low energy range is higher than that of germanium (lithium). For example, HPGe detector can better distinguish the following peaks: Gd- 153 of 97.4 keV, ofTa- 182 of 100.3 keV and Sm- 153 of 103.2 keV. There is no need to wait a long time for the attenuation of Sm- 153, and there is no need to correct the interference of Ta- 182 for the measurement of Gd- 15397.4 keV peak.

In the determination of gallium and tungsten, ENAA is helpful to inhibit the interference of Na-24 and sometimes the interference of Br. The interference of Na-24 in INAA of some geological samples is a serious problem, which should always be corrected.

The advantage of Au pre-radiation separation technology is that it can avoid interference (EU-15241kev and Ho- 166m 465 438+00.8keV are used for Au19841/kloc-. Using a large sample (10-20g) instead of a small sample (less than 100mg) usually used in INAA program is helpful to avoid a big difference in gold data, which may be caused by a small amount of small particles of free gold in river sediments.

The pre-irradiation separation technology of rare earth elements not only avoids similar effects, but also helps to determine other rare earth elements more accurately, such as Ho, Tm and Gd. It is difficult to determine Ho using INAA.

admit

Thanks to Li Chang Cuo, Zhao Meizhuo and Shi Jianwen from the Institute of Geophysical and Geochemical Exploration of the Ministry of Geology and Mineral Resources and Zhang Yuanji from the Institute of High Energy Physics of the Chinese Academy of Sciences for their participation in this work. We would also like to thank Cheng of the Central Laboratory of Shaanxi Geological Bureau, who helped to separate the rare earth elements. Special thanks to F.J.Flanagan of USGS for his careful revision and editing of this article.

Table 1. The main parameters used in NAA (in the gamma-ray column, A, B and C are additional subscripts used by the Institute of Geophysical and Geochemical Exploration in its data processing to identify the same isotopes with different primary gamma rays. Germanium (lithium) detector. H, HPGe detector)