Reliability of Prompt Gamma-ray Analysis for the Determination of Na and Mg in Rock Samples

Abstract

Neutron-induced prompt gamma-ray analysis (PGA) was first introduced in the 1960's 1 as a practical analytical method. Since then, its application as a multi-element analysis tool has expanded into many fields of science and technology. Because the neutron flux used for PGA is much lower than that for conventional instrumental neutron activation analysis (INAA), the residual radioactivity in irradiated samples becomes decreased down to the natural background level after an appropriate cooling interval. Therefore, samples once used in PGA can be reused for other studies after PGA. Nondestructivity and high sensitivity for several elements are additional significant features of PGA. In fact, PGA can nondestructively determine Si in geological and cosmochemical samples, 2 which is often difficult or impossible to determine by INAA. 3 Major element compositions of these rock samples can be determined by a single measurement in PGA. These features suggest that PGA is the most suitable analytical technique for the initial analysis of geochemical and, especially, cosmochemical samples that have a generally severe limitation in the quantity usable for destructive chemical analyses.Although PGA is usable for determining most major elements simultaneously in rock samples, high accuracy and precision are not always guaranteed for all of them. According to our previous experience, PGA data of some major element contents tend to have been questioned, and not to have been listed in the final report. Sodium and Mg are two representative elements for such a case. In this study, we focused on Na and Mg, and examined the PGA procedure to get reasonably accurate and precise values of their concentrations in geological and cosmochemical rock samples. Our final goal is to establish a routine procedure of activation analyses including PGA suitable for the initial analysis of these rock samples. This paper is a part of a study to approach this goal. ExperimentalThe Smithsonian Allende meteorite powder sample (256.6 mg) (prepared by E. Jarosewich, the Smithsonian Institution; split/ position = 11/11) and JB-1 (250.9 mg) (a basaltic standard rock sample issued by the Geological Survey of Japan) were analyzed repeatedly by PGA. Analyses were repeated ten times for JB-1 and eleven times for Allende over a period of four years with the same samples. The powder samples were sealed into thin FEP (fluorinated ethylene propylene) film bags. Irradiations were done for 1 to 2 h and 2 to 3 h by using thermal neutrons (flux: 2.4 ¥ 10 7 n cm -2 s -1 ) and cold neutrons (flux: 1.4 ¥ 10 8 n cm -2 s -1 ), respectively, which were guided out of the research reactor JRR-3M at the Japan Atomic Energy Agency (JAEA). Allende was repeatedly analyzed five and six times by using thermal and cold neutrons, respectively, while JB-1 was analyzed seven and three times by thermal and cold neutrons, respectively. Neutron beams were collimated to a size of 20 ¥ 20 mm at the entrance of a sample box, which was filled with He gas. Prompt gammarays were detected by a G...

Comparison of Multiple Prompt γ-Ray Analysis and Prompt γ-Ray Analysis for the Elemental Analysis of Geological and Cosmochemical Samples

“…Although PGA is usable for determining most major elements simultaneously in rock samples, some elements cannot be always determined with high accuracy and precision due to known and unknown interferences from matrix elements and background sources . Magnesium (Mg) is one representative element for such a case.…”

“…Although PGA is usable for determining most major elements simultaneously in rock samples, some elements cannot be always determined with high accuracy and precision due to known and unknown interferences from matrix elements and background sources. 6 Magnesium (Mg) is one representative element for such a case. As, in MPGA, coincidence events caused by cascade prompt γ-rays from a nuclide are collected in a two-dimensional energy matrix, the spectral interferences from the other elements can essentially be avoided.…”