RADIOISOTOPE RH-101 AS X-RAY SOURCE FOR INSTRUMENTS ON SPACE MISSIONS

Transcription

1 167 RADIOISOTOPE RH-101 AS X-RAY SOURCE FOR INSTRUMENTS ON SPACE MISSIONS Ch. Stenzel 1, Ch. Schroer 2, B. Lengeler 3, M. Rasulbaev 4, R. Vianden 4 1 Astrium GmbH, Friedrichshafen, Germany 2 Technical University of Dresden, Germany 3 RWTH Aachen, Germany 4 HISKP, University of Bonn, Germany ABSTRACT Our work shows that XRD and XRF studies on missions to Mars and Saturn's moons could be carried out using the radioisotopes Rh-101 or Am-241 as sources of radiation. We discuss geological questions which we believe should be answered before any search for biological structures or pre-forms of life. We also suggest a method to produce Rh-101. Key words: Isotopes Rh-101, Am-241, XRD, XRF, Mars, Saturn's moons INTRODUCTION Search for early forms of life on Mars or the Saturnian's moons starts with profound knowledge on topology, geology and mineralogy. Previous missions to Mars were concentrated on the examination of the topology, mainly by optical observation with cameras operating in the visible range. Summarizing all the results of these missions in the recent decades, Mars became the best mapped planet in our universe with a highly-resolved cartography. Residuals of ancient water structures and ice formation on the poles have been detected (see Fig. 1). The Saturnian's moons Titan and Enceladus are further highly interesting candidates for the search of precursors of living structures. Enceladus surface is covered with water ice with apparent cryo-volcanic activities. Titan's atmosphere contains a significant amount of methane

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 168 and liquid methane lakes have been detected at the surface (see Fig. 1). Both features could enable the creation of quite different forms of higher organic molecules and chemistry [1]. Fig. 1: Left: Crater ice on Mars Right: Liquid methane lakes on Titan But there are still some relevant geological questions open in this context, before any search for biological structures or pre-forms of life would make sense: Where did all the water go on the Martian surface? Are there differences on sediment and other geological structures between a water and methane driven chemistry in the atmosphere? Was there volcanism and tectonic dislocations in ancient times? Was there an active magnetism? To tackle these questions, the following scientific activities could be employed: Observation of surface Study of the elemental composition Investigation on the abundance of significant tracers. Hence, upcoming international exploration missions to Mars (NEXT) and to the Saturnian's moons Titan and Enceladus (TANDEM) will concentrate first on the investigation on geology and

4 169 mineralogy rather than a direct search for life. For both missions the implementation of a combined instrument for XRD and XRF on a respective lander is discussed. X-ray techniques like diffraction (XRD), to analyse the crystallinity, and fluorescence (XRF) to study the element composition, represent vital scientific methods to achieve the mentioned goals [2-5]. Thereby these methods shall be employed on pre-selected and crushed samples of rocks. The requirements on the X-ray source are driven by the mission scenario and boundary conditions: Low mass and low power consumption (< 100 W) Autonomous operation Life time of more than 3 years Robust design (to be integrated on a space mission). Due to the severe limitations on weight and power consumption, the utilization of a microfocus tube as X-ray source is not feasible. Hence the X-ray source must consist of a radionuclide. The radioactive decay of this isotope must include a strong gamma-ray transition with energy in the X-ray region. The half-life of the isotope has to be matched with the anticipated mission duration of several years. Furthermore, in order to offer optimum conditions for powder diffractometry or fluorescence under the severe restrictions of an exploration mission, a photon flux of more than 10 6 photons/s must be provided as target value. Low fluxes would increase the measuring time which has an important impact on the consumption of valuable resources like power and data processing. The photon energy shall be in the range of 8-20 kev, the higher energies are definitely to be preferred because of the reduction of background. POTENTIAL RADIONUCLIDE CANDIDATES A review has yielded two potential candidates for the X-ray source: Am-241 and Rh-101. The characteristic features of these isotopes are given in the table below [6]. For comparison the values for Fe-55 which has been used as radionuclide in former missions are included in Table 1 as well:

5 170 Rh-101 Fe-55 Am-241 Half-life 3.3 y 2.7 y 433 y X-ray energy 19 kev 6 kev 14 kev, 60 kev Initial photon flux ph/s ph/s ph/s Photon flux after 5 y ph/s ph/s ph/s Table 1: Comparison of performance data for different radionuclides to be utilized as X-ray source for an XRD and XRF instrument The given photon flux for Rh-101 represents optimized values which can be extrapolated from the presented first results described below. The values for Am-241 are adapted to a source size of mm 3 and take the strong absorption of the Roentgen X-rays in the source itself into account. PRODUCTION OF RADIOISOTOPE RH-101 This production of Rh-101 makes use of a nuclear reaction with light ions in a cyclotron. For isotopes which are close to the "valley of stability", the cross sections for these reactions are sufficiently high [6]. The selected nuclear reactions (d, 2n) or (d, 3n) to produce Rh-101 were Ru d Rh n and Ru d Rh n Prototype radionuclides have been produced via a nuclear reaction as described at the isochronous cyclotron at the HISKP of the University of Bonn, Germany. A natural Ruthenium foil served as target (cross section of 20 mm 2 and a thickness of about 0.3 mm) which contained both isotopes, Ru-101 and Ru-102. The relevant parameters for this nuclear reaction were: Deuteron energy: 27 MeV Cross section: 1000 mbarn Deuteron beam intensity: 1 µa Irradiation time: 2 d Rh-101 activity after cool-down period of 43 d: 1 MBq

6 171 The decay scheme of the radioactive Rh-101 nucleus reveals in addition to the wanted X-ray photon with 19 kev two other strong gamma-rays at energies of 127 kev and 198 kev [7]. The strong occurrence of theses line and the K α line of Ru, as evident from Fig. 2, indicate the great selectivity of the chosen nuclear reaction and a sufficient reaction yield. However, first experiments using semiconductor devices for detection of the gamma ray radiation revealed that reactions of the gamma-rays at energies of 127 kev and 198 kev with the matter of the semi-conductor material inside the detector cause a high background. Hence, a dedicated detector, e. g., a proportional gas counter would have to be applied here. In Fig. 2 the measured spectra in the X-ray region and for higher gamma ray energies are plotted. For the production of the flight X-ray source a cyclotron facility with a high-intensity beam (up to 50 µa) is necessary to produce the Rh-101 isotope with a required activity. An isotopically enriched Ru-101 target would increase the yield furthermore. A much thicker and larger target has to be used followed by a subsequent radiochemical preparation of the final X-ray source. The latter step would require some development effort, but represents a common procedure in producing radionuclides for radio-medical applications [8]. Am-241: A very convenient route to provide X-rays with the appropriate properties is to look for "conventional" radioactive sources which are commercially available. The α-particles emitting isotope Am-241 delivers also two X-rays which could be used for diffractometry, with 14 kev (37%) and 60 kev (36%). The long half-life of 433 y would result in a constant photon flux over a mission duration of 10 years. This isotope is envisaged for utilisation in the APXS-instrument on the NASA Mars Science Laboratory in It can be procured in large quantities (grams) from Oak Ridge National Laboratory, USA. By placing this source in an appropriate housing the alpha particles can be shielded very easily while the X-rays are emitted. However, a certain drawback of this solution represents the relatively low specific activity (radioactivity/mass) of 126 GBq/g which is further reduced by a strong self-absorption of the 14 kev X-rays in the source itself. Rough estimation yield that for the specified dimensions of the source a photon flux of ph/s could be obtained.

7 172 Fig. 2: Measured gamma spectra of Rh-101 in the X-ray region (top) and at higher energies (bottom)

8 173 EVALUATION AND SUMMARY Concerning all critical criteria as mentioned above Rh-101 turns out to be a feasible solution: It provides the highest photon flux at the optimum energy and has a half-life which fits well to the mission duration. A reliable and efficient production route has been identified as well. However, a detector system which is only sensitive to the 19 kev gamma radiation from this source has to be employed for this radioisotope. The subsequently listed improvements can be employed to increase the activity to 4 GBq compared to the presented first measurements: Employment of pure Ru-101 target (activity increase by a factor of 6) Irradiation at a high-power cyclotron with 50 µa (factor 50) Elongation of irradiation time to 40 d (factor 960) Optimization of target thickness and beam energy (factor 3) A radiochemical separation of Rh-101 from the irradiated Ru material would greatly reduce the absorption of the 19 kev line in the sample and allow the generation of an optimum geometrical shape for XRD. Shielding against high energetic gamma-rays is necessary; appropriate shielding measures must be implemented into the design of the instrument. Am-241 would represent another good candidate radioisotope for a XRF experiment. However, the Fe-55 isotope is regarded as completely non-appropriate because The initial photon flux is already two orders of magnitude smaller than that of Rh-101 With 6 kev the photon energy is too low for high-performance XRD and XRF The short half-life would prohibit a mission to the Saturnian's moons.

9 174 REFERENCES [1] "The Mystery of Methane on Mars and Titan", by Sushil K. Atreya; Scientific American, May 2007 [2] "Handbook of Synchrotron Radiation", vol 3. chap. 5: D. E. Cox, "Powder Diffraction", p.155 (1991) and vol 4. chap.9: A. Iida, Y. Goshi, "Trace element analysis by x-ray fluorescence", p. 307 (1990) [3] "Forschung mit Röntgenstrahlen", F. H. W. Heuck, ed. Macherauch, Springer 1996 [4] "Microscopic X-Ray Fluorescence Analysis", K. H. A. Janssens, F. C. V. Adams, A. Rindby, J. Wiley, 1999 [5] "Fundamentals of Crystallography", C. Giacovazzo et al., Oxford Science Publications 1998 [6] A. Hermanne, M. Sonck, S. Takacs, F. Tarkanyi, Y. Shubin Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) p [7] "Table of Isotopes", CD ROM Edition, Version 1.0, March, 1996 by Richard B. Firestone, Virginia S. Shirley Editor, S.Y. Frank Chu CD-ROM Editor Coral M. Baglin and Jean Zipkin Assistant Editors, Wiley Interscience, New York [8] Zhuang Guisun, Qian Yine, Hua Zhifen, J. of Nuclear and Radiochemistry 4 (1982) 62