M. EBIHARA, Tokyo Metropolitan University, Tokyo, Japan
In this century, our activities in science and technology will be greatly expanding. Following the Apollo missions of the USA, not a few space mission programmes aiming to collect and return extraterrestrial matter from space have been planned and will continue to be planned, not only in the USA but also in Japan and other countries. Once a sample is returned to the Earth, very serious limitations exist on its analysis for element composition, especially if the sample amount is small. Our task is to determine the exact chemical composition of such a small amount of sample non-destructively.
Why nuclear analytical techniques?
There are several requirements for analytical techniques to be applied to a limited amount of sample recovered from space.
First of all, non-destructive analysis is desirable; the same specimen used for chemical analysis can be reused for other purposes, including detailed chemical analysis by destructive methods. Secondly, the method of chemical analysis used must have high analytical sensitivity for as many elements as possible. Thirdly, the analytical data thus obtained need to be highly accurate. In consideration of these requirements, it can undoubtedly be concluded that activation analysis is the most suitable method. Here activation analysis covers neutron induced prompt g ray analysis, instrumental neutron activation analysis and instrumental photon activation analysis. These three are all methods of non-destructive analysis. In order to satisfy the second and third requirements, these methods are successively performed, as explained below in detail.
How to solve the problem?
It is well known that activation analysis has high sensitivity in determining the chemical composition of solid samples.
Recently, inductively coupled plasma mass spectrometry (ICP-MS) has been increasingly used for determining chemi-cal composition, mainly because it has high analytichemi-cal
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sensitivity for many elements and does not involve chemical procedures for assaying. Thus ICP-MS can compete with activation analysis and is even believed by some to be supe-rior to it in analytical sensitivity, and especially in cost per-formance, which must be generally admitted. One of the advantages of activation analysis over ICP-MS is that activa-tion analysis can analyse solid samples without destrucactiva-tion.
This means that the same specimen can be used for duplicate analyses using the same or different analytical methods and even for other observations such as physical and petrological analyses. This advantage becomes very significant when the sample size usable for analysis is very limited, as is the case of samples returned from space.
We used two types of activation analysis — neutron acti-vation analysis (NAA) and photon actiacti-vation analysis (PAA).
In NAA, instrumental NAA (INAA) is commonly used. In addition to INAA, we also used neutron induced prompt g ray analysis (PGA). PGA is a relatively new methodology in NAA and has several merits as a non-destructive analytical tool: PGA can be applied to large samples such as pottery and meteorites [1], most major elements constituting geological and cosmochemical samples can be non-destructively deter-mined [2], residual radioactivities are as low as natural back-ground levels [3], and so on. The last named feature enables us to reuse the same sample used in PGA for INAA and instrumental photon activation analysis (IPAA). IPAA, which can be performed by using a linear electron accelerator, is as effective as INAA in analysing solid samples and can work somewhat as a complementary analytical tool to INAA [4]. As both INAA and IPAA are non-destructive methods, the same specimen can be used for both methods if a sufficient time interval for the decay of induced radioactivities is allowed between INAA and IPAA.
The analytical procedure we designed was applied to real meteorite samples weighing 100 mg as follows. Samples were first analysed by PGA. For this analysis, we used a cold neutron beam to enhance the analytical sensitivity, because the sample amount usable for PGA was about 50 mg for each sample. We could determine a total of 16 elements by using a comparison method in which several reference standards were adopted. The remaining samples were divided into two
portions, which were subsequently subjected to INAA and IPAA. In INAA, samples were successively irradiated three times with different irradiation times (10 s, 1 min and 20 min) using two different reactors at the Japan Atomic Energy Research Institute. As a result, a total of 26 elements were determined. For determining element abundances, we used a comparison method, which will be replaced with a k0 method in the near future. In IPAA, samples were activated with photons of 30 MeV end point energy. In determining element concentrations, we adopted a comparison method using chemical reagents and geological and cosmochemical refer-ence samples. The samples were irradiated twice successively for 30 min and 6 h.
On the basis of the data obtained by three non-destructive nuclear analytical methods, we could characterize the samples cosmochemically. For characterizing extraterres-trial material in terms of chemical composition, several groups of elements play important roles. Volatile elements are highly sensitive to the thermal activity the sample experi-enced. By PGA, H and S can be non-destructively analysed.
Refractory lithophile elements also are an important group of elements. For instance, Ca/Si and Mg/Si ratios are characteris-tic of individual groups of meteorite samples. Refractory siderophile elements are another group of elements that are recognized to be as informative as refractory lithophiles.
After the initial analysis using non-destructive activa-tion methods, we plan to analyse the same sample using destructive analytical methods such as radiochemical activa-tion analysis and ICP-MS for detailed studies. Needless to say, the applicability of nuclear analytical methods described here can be extended to small sized solid samples other than geo-logical and cosmochemical samples, for example soil and sediment samples.
References
[1] NAKAHARA, H., et al., Some basic studies on non-destructive elemental analysis of bulky samples by PGA, J. Radioanal. Nucl.
Chem. 244 (2000) 405–411.
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[2] LATIF, S.K., et al., Prompt gamma-ray analysis (PGA) of meteorite samples, with emphasis on the determination of Si, J. Radioanal.
Nucl. Chem. 239 (1999) 577–580.
[3] EBIHARA, M., OURA, Y., Applicability of prompt gamma-ray analysis to the initial analysis of the extraterrestrial materials for chemical composition, Earth Planet. Sci. 53 (2001) 1039–1045.
[4] EBIHARA, M., et al., How effectively is the photon activation analysis applied to meteorite samples, J. Radioanal. Nucl. Chem. 244 (2000) 491–496.