Table 4.20 Analytical results for the CI meteorite Orgueil by MPGA.
Element MPGA SPGA
Compilation datadEnergy paira'b (kev) Contents
Energya
(keV)
Contents (n=3)Contents Ne
Gd, pglg
Sm, ptglgTi, O/o Ca, O/o Ni, O/o
Co, pgtg
Fe, O/o Mg, O/o
si, o/,
Cr, pgtg Mn, O/o K, O/o Na, O/e Cl, pglg
s, o/o
1 82-780
334-439 1762-1499 2002-1943 339-2842
230-497 352-1261 585-2828 1273-2093
835-1785 105-271 1159-770
91-781 1165-786 841-238O
O.190 Å} O.082 O.133 Å} O.039 0.0562 Å} O.O060
O.913 Å} O.053 1.17 Å} O.09
563 Å} 19 18.9 Å} O.3 9.01 Å} O.24 9.98 Å} O.38 2964 Å} 211 O.195 L O.O12 O.0422 Å} O.O076
O.492 Å} O.089 781 Å} 25 5.20 Å} O.08
182 334 1382 1943 465 277 1725 2828 3539 749 212 770 472 1165 2380
O.208 Å} O.021
c
O.0504 Å} O.O059 O.976 Å} O.096
1.10 Å} O.078
536 Å} 38
18.9 Å} O.7 8.24 Å} OAI 11•8 Å} Oe4 3141 Å} 134
O.218 Å} O.O17
c O.539 Å} O.034 776 Å} 8.58 5.83 Å} O.29
O.201Å}O.Oll 8 O.145Å}O.O15 17 o.o43gÅ}.oo3o ls
O.908Å}O.066 14 1.07Å}O.06 33
507Å}24 19
18.35Å}O.79 24 9.59Å}O.44 17 10.57Å}O.28 11
2630Å}150 16
O.192Å}O.OIO 16
4.9 References
Chu, S. Y.; Nordberg, H.; Firestone, R. B.; Ekstrom, L. P. Isotope Euplorer 2.23. US Deparment of Energy, 1999. hmp:11dbserv.pnpi.spb.rulelbibltablisothoi981wvvwlisoexpYisoexpl.htm (accessed on October, 201 1).
Ember, P. P.; Belgya, T.; Molnar, G. L. 2002. improvement of the capabilities of SPGAA by
coincidence techniques. Appl. Radiat. Isot., 56, 535-541.Gardner, R. P.; Mayo, C. W.; El-sayyed, E. S.; Metwally, W. A.; Zheng, Y.; Poezart, M. 2000. A feasibility study ofa coincidence counting approach for PGNAA applications. Appl. Radiat. Isot.
53, 515-526.
lmai, N.; Terashima, S.; Itoh, S.; Ando, A, 1995. 1994 compilation of analytical data for minor and trace elements in seventeen GSJ geochemical reference samples, "Igneous rock series". Geostand.
Newsl. 19, 135-213.
Jarosewich, E.; Clarke Jr., R. S. and Barrows, J. N. 1987. The Allende meteorite reference sample.
Smiths. Contrib. Earth Sei. 27, 1-49.
Kaliemeyn, G. W. and Wasson, J. T. 1981. The compositional classification of chondrites-I. The carbonaceous chondnte greup. Geochim. Cosmochim. Acta 45, 1217-1230.
Kallemeyn, G. W.; Rubin, A. E.; Wang, D. and Wasson, J. T. 1989. 0rdinary chondrites: Bulk
compositions, classification, lithophile-element fractionations, and composition-petrographic type relationships. Geochim. Cosmochim. Acta 53, 2747-2767.Latif, Sk A.; Oura Y.; Ebihara, M.; Kallemeyen, G. W.; Nakahara, H.; Yonezawa, C.; Matsue, T.;
Sawahata, H. 1999. Prompt gamma-ray analysis of meteorite samples, with emphasis on the
determination ofSi. J. Radioanal. Nucl. Chem. 239, 577-580.Lodders, K. 2003. Solar system abundances and condensation temperatures of the elements.
Astrophys. J. 591, 1220-1247.
National Nuclear Data Center (NNDC), Brookhaven National Laboratory, Upton, NY 11973-5000, USA, last updated July 7, 2010. http:11wvvw.nndc.bnl.govlcaSPGAmlindex.hmi (accessed on
October, 201 1).Paul, R. L.; Lindstrom, R. M. and Heald, A. E. 1997. Cold neutron prompt gamma-ray activation analysis at NIST-recent developments. J. Radioanal. Nucl. Chem. 215, 63-68.
Shinotsuka, K. and Ebihara, M. 1997. Precise detemination of rare earth elements, thorium and
uranium in chondritic meteorites by inductively coupled plasma mass spectrometry- a comparative study with radiochemicalneutron activation analysis, Anal. Chimica Acta 338, 237-246.
Terashima, S.; imai, N.; Tominaga, M.; Hirata, S.; Taniguchi, M. 2000. Preperation of a new GSJ geochemioal reference material: JSO-2 soil. Bunseki Kagaku 49, 319-324.
Toh, Y.; Oshima, M.; Fumtaka, K.; Kjmura, A.; Koizumi, M.; Hatsukawa, Y.; Goto, J. 2008b.
Development of a neutron beam line and detector system for multiple prompt gamma-ray analysis.
Z Radioanal. 7Vucl. Chem. 278, 703-706.
Toh, Y.; Oshima, M.; Koizumi, M.; Kimura, A.; Hatsukawa, Y. 2008a. Development of multiple
prompt gamma-ray analysis. J Radioanal. Nucl. Chem. 276, 217-220.Wapstra, A. H. 1979. Alpha-, beta- and gamma-ray spectroscopy, Vol. 1, 5th ed., North-Holland
Publishing Company, Amsterdam, New York, Oxford, p. 539.
Yonezawa, C. 1993. Prompt gamma-ray analysis of elements using cold and thermal reactor guided neutron beams, Anal. Sci. 9, 185-193.
CHAPATER 5
Comparison of MPGA and SPGA for the analysis of geological and cosmochemical samples
5.1 IntroductionThe importance of the SPGA for elemental analysis of extraterrestrial materials was discussed in Chapter 1. ln Chapter 4, MPGA and its expected outcomes for elemental analysis were explained. ln this chapter, analytical capabilities of these two methods are compared for elemental analyses of geological and cosmochemical samples. SPGA is a sensitive, precise, multi-elemental and non-N destmctive analytical method of elements. However, SPGA has an inherently low signal to noise ratio primarily because of the large background associated with its spectmm. Therefore, difficulty of quantification arises when the gamma-ray intensity from the trace element of interest is not strong
enough in comparison with the background gamma-ray from 1arge amounts of other elements present in the sample. ln such a situation, MPGA method can be used to reduce background level (Ember et
al., 2002; Wang et al., 2011). Since in MZPGA, only the gamma-gamma coincident events from a
nucleus that simultaneously emits two or more cascade prompt gamma rays in de-excitation of the nuclei witlm a short interval of time are collected, the background caused by a single-gamma-ray-emitting nuclide can be reduced.Among meteorites, CI and some of CM are known to have high contents (up to 2 O/o in mass) ofH.
Elemental analysis of CI group of meteorites is of special importance because they have essentially the same chemical composition as that of the sun except for a few extremely volatile elements (Anders and Ebihara, 1982; Anders and Grevesse, 1989). in SPGA analyses ofhydrogenous samples, Compton-scattered gamma-ray ofH causes a considerable increase in the low energy background of the spectrum (Toh et al., 2008a). Therefore, MPGA can be used to analyze hydrogenous meteorites to
reduce low energy background caused by H in the sample. Hence, in MPGA, some trace elements
could potentially be analyzed by using only measurable lower energy prompt gamma-ray which is not detectable in SPGA.ln analyzing geological and cosmochemical samples, XRF and INAA are commonly used.
Although XRF .technique is relatively simpler than SPGA and MPGA, the application XR F is limited to the surface analysis of solid samples because of lower penetration ability ofX-ray compared with neutron and gamma-ray. Therefore, XRF is not suitable for whole rock analysis of heterogeneous solid samples. To overcome such a difficulty, compositionally homogeneous bead samples are to be prepared. Once the beads are prepared, the samples cannot be reused for other purposes. This can be a fata1 defect in analyzing a limited amount of precious matter like meteorites. INAA can detemine most maj or elements of silicate rocks but Si, one of the most important constituent elements of silicate
rock samples, is hardly deterrnined. Another defect of INAA can be caused by the residua1
radioactivity in samples once irradiated with neutron even in a short term. ln SPGA and MPGA,samples need not to be physically decomposed before assaying and no significant amount of
radioactivity remains in the samples after an appropriate cooling interval (normally few days).Although SPGA is usable for detemning most major elements simultaneously in rock samples, some elements cannot be always determined with high accuracy and precision dne to known and
unlmown interference from matrix elements and background sources (Karouji and Ebihara, 2008).Magnesium (Mg) is one representative element for such a case. As, in MPGA, coincidence events
caused by cascade prompt gamma-ray from a nuclide are collected in a two dimensional energy
matrix, the spectral interferences from the other elements can essentially be avoided. ln this study,newly developed MPGA system at the JAEA was used to analyze hydrogenous meteorites as well as rock samples, and was characteristically compared with SPGA (conventional PGA) based on their analytical capabilities.
5.2 Experimental
5.2.1 Samples and standards
The Smithsonian Allende meteorite powder sample (10S mg) (prepared by E. Jarosewich, the
Smithsonian lnstitution; splitlposition = 2216) and geoiogical rock sample JB-1 (110 mg) (a basalticstandard rock sample issued by the Geological Survey ofJapan (GSD) were analyzed repeatedly by
MPGA in gamma-gamma coincidence mode (4 MeV energy range). Analyses were repeated four
times over halfa year for JB-1 and Allende with the same samples. The powder samples were sealedinto thin FEP (fluorinated ethylene propylene) film bags. Pmalytical grade chemical reagents
(chemical form and other information are given in section 2.2.1 of Chapter 2) for all studied elementsexcept Sm and Gd were used as reference standards for comparison method. For Sm and Gd, recommended values of JB-1 from the literature Imai et al. (1995) were used. MPGA was also
applied to some hydrogenous meteorites, Orgueil, Ivuna, Alais, 9801 15, B-7904, 793321 and Y-86720, as well as to a reference rock sample GSJ-JB-2 (mass range about 25 - 192 mg each). SPGA was applied only to Orgueil, Ivuna, Allende and GSJ-JB-2.5.2.2 MPGA and SPGA irradiation and analysis
MPGA
The MPGA irradiations of the samples were done for 2.5 to 4.4 hrs at a newly installed IN,ff)GA system by using guided cold neutrons (flux: 106-7 n cm-2s"i) from JRR-3M research reactor of JAEA.
The detailed descriptions on the developments of neutron beam line and detector system for MPGA were reported elsewhere (Toh et al., 2008b; 2008c). The present detector system consists of eight
clover Ge detectors with bismuth germanium oxide (BGO) Compton suppressors. The relative
counting efficiency of each clover detector was approximately 1200/o relative to that of 3 in. x 3 in.Nal detector. MPGA system at JAEA gains high gamma-ray detection efficiency with sophisticated
clover Ge detector system in tight geometry and overcomes the wealmess of the conventional
coincidence technique with low detection efficiency (Gardner et al., 2000). The energy calibration of
the MPGA detector system was performed with the gamma-ray emitted by i52Eu, 57Fe and 66cu. For dead time correction, the counts of 81 keV and 356 keV coincident-peak of i33Ba were used. The sample to detector distance was approximately 5 cm. Neutron beams were collimated to size of 30 mm x 20 mm at the position of sample irTadiation, and the air in the beam line was replaced with heliumlcarbon dioxide gas (3L!min.) to reduce the background gamma-ray caused by neutron capture reactions with nitrogens. The neutron attenuators made up of acrylic plates were used to optimize the rate of true coincidence to the accidental coincidence by adjusting neutron beam intensity, if necessary. Blank sample (FEP film bag with sample holder) was also irradiated in every run for background correction. Events comprising a pair of coincident prompt gamma-ray were collected in the experiment. The add-back soning mode was used for offiine soning of the coincidence data. ln MPGA, each clover detector is composed of four closely packed Ge crystals. in add-back mode, all output signals are summed up to obtain the total pulse height for an incident gamma-ray that scatters from one crystal and is absorbed by one of the other three crystals, consequently increasing photo peak efficiency of a clover detecter, which was evaiuated in Chapter 4. With the sorted data, a
histogram of the gamma-gamma energy correlation, which was a two dimensional gamma-ray matrix spectmm, was constructed for elemental analysis.