Comparative_X_Ray_and_Gamma_Ray_Study

A comparative study of X-ray and γ-ray spectroscopy on the planetary surface: A review Group: P (Simhachala Rao, Proloy Taran Das, Premanshu Jana, Maitreya Maity) Abstract: X- and γ- radiation are commonly observable in the vicinity of individual planets. These spectra contain different elemental lines. Spectroscopic analysis of multi channel elemental lines containing spectral data from high resolution X-ray (HRXRD) and γ-ray detectors operating in remote environments, gives us information about geo chemistries, surface morphology, mineralogy etc. Elemental compositions can be carried out from observations of these line emissions in energy range domain 0.2 keV TO 10 MeV. HRXRD studied determine the intensity of a spectral line at a particular energy level. To determine the intensity of a specific line at a particular energy level as well as new line, weak peak problems arise in high resolution γ-ray spectroscopic studies. It has been noticed clearly. Determining the specific spectral line from continuum is a very complex process due to variable irregular shapes of continuum and overlapping energy spaces of the large number of spectral lines. Key words: X- and γ- ray spectroscopy, Mineralogy, XFS, Spectroscopic analysis I. Introduction Chemical composition and its distribution related to geological process on the planetary surface helps to understand the formation and dynamics of any physical body and planetary evolution. Remote sensing measurements and passive observations from orbital spacecraft is the best way to study on chemical composition and mineralogy of the planetary surface. X- and γ-ray spectroscopic study plays a significant role to understand the surface chemical composition of atmospheric body’s such as Mercury, Moon and asteroids [1]. Electromagnetic spectrum which consists of X-ray and γ-ray radiation that are observable in the vicinity of individual planets are originated from the flux of natural phenomena. Fluorescent soft X-rays [XRF] from surfaces of the planetary surface have been studied by the instruments on several space crafts [2]. XRF or particle induced X-ray emission [PIXE] are the two ways for chemical analysis of the planetary surface. Solar coronal X-ray flux is the origin of XRF. The X-ray fluxes are sufficient to produce fluoresces measurable and detectable from the instruments on the spacecraft. On the other hand PIXE plays a vital role during particle related events only. For X-ray spectroscopy it is required to produce an excitation on emission lines characteristic of the elements in the material and measurement of their intensity along with conversion of X-ray intensity to concentration by a calibration procedure which may include correction for matrix effects. Freely propagation of cosmic rays through the atmosphere and inelastic collision with nuclei of planetary surface could produce a large number of secondary neutrons at the sub surface layer of it. Two types of nuclear reactions take place due to this interaction between outgoing neutrons and nuclei of sub layer (Fig. 1).γ-ray originates from this nuclei radiation of sub layer of the planetary surface .A series of γ-ray spectral lines produced from different nuclei of chemical elements presence in the planetary surface. A unique γ-ray comes from each individual nucleus .It is very easier to study the mineralogy of planetary surface from this γ-ray spectroscopic study [Scientific tusks off GSFa ]. The γ-ray spectrometry consists of acquisition of data pertaining to the actual energies of individual γ rays.It can be detectable of natural γ-ray emission from the transuranic series, K, Th, U. Cosmic ray induced γ-rays from H, O, Si, Fe by remote γ-ray spectrometry.X-ray spectroscopy is effective to study the chemical composition very near to the planetary surface within a few micrometer range where as γ-ray measurements gives information about the chemical composition up to 30 cm below of the planetary surface [3]. Magnesium abundance is determined from the X-ray measurement, while the potassium abundance is best obtained from the γ-measurement. Geochemical analysis of asteroids provides direct compositional data which is useful for comparable study between asteroids and meteorites, originates from asteroids. Nature of geochemical variations and whole geochemistry of surface can be used to provide information about asteroids formation [1]. Simultaneous measurements of X-ray and γ-ray are carried out a great significants from two points of view. Firstly, it is possible to comparable observational study on two sampling depths of common elements on the basis of presence or absence of a change in abundance between two measurements (X-ray and γ-ray spectrometry).This effect has a great implication on the view point of turbulence surface by the process of erosion and sedimentation. Secondly, for chemical variation also it plays a vital role even if there is no change i.e. if changes belongs to within few range, indicates availability of elements [4]. II. X-ray detectors Requirement of High energy resolution is the primary point for measurement of remote sensing X-ray spectrometer. On this view, Energy dispersive systems and imaging systems can be used. Room temperature or cryo-cool semiconductor detectors like mercury iodide (HgI2), CdZnTe (CZT) and Si PIN and APD, different proportional counters with balancing filters have a great importance correlating with remote X-ray measurement. Energy resolution of proportional counters is very poor and can’t resolve for Mg, Al and Si lines. These lines can be resolved using balancing filters with simultaneous observation. Energy resolution is much more required small area detectors for which solid state detectors are much more effective than proportional counter can be used for X-ray fluoroscopy. Better energy resolution reduces the various noisy back grounds of photo peaks besides that it improves signal-to-noise response also. Depending upon band gap operation temperatures varies. In cryo cool regime band gap (≥1eV) and in room temperature regime band gap (≥1.4eV) can be used. K-shell X-ray fluoroscopy can be used for detection of Z-elements. For lower Z-elements, K-shell X-ray lines of energy 6 keV and for higher Z-elements, K-shell X-ray lines of energy 10 keV are required. Tailing effect, reduces the energy resolution of the system is one of the most common problem in the detectors. Improving detector mechanism likes reducing weight and volume of detectors; increasing signal-to-noise response time etc. this effect could be minimized. Not only that, development of space flight analog and digital systems are also required for high Z–detectors. To obtain Mapping of planetary surface and reasonable spatial resolution, detection area range for remote sensing X-ray spectroscopy on the planetary bodies is required about of 25 cm2 has been determined by theoretical calculated results and experimental results(Apollo X-ray spectrometer Results). From outer planetary bodies it is very difficult to excite sufficient measurable amount of fluorescent X-rays on the planetary surface due to insufficiency of Solar X-ray. Fluorescent X-ray produces from inner shell ionization due to electron or protons impact. Inner-shell ionization cross sectional area due to proton impact can be comparable with electrons only when energy of protons is ~1000 times than energy of electrons. It is determined that 5 keV electrons and 4 MeV protons have the same cross sectional area of about 2 ×10 -20 cm 2 in the K- shell ionization of an aluminum target [5,6]. Based on light weight and power function of the corresponding instruments aboard outer planet missions, it is said that for imaging and spectroscopy room temperature solid state detectors has a great aspect relating to this application. In recent studies low energy fluorescent emission (ultra soft X-rays) plays a significant role rather than higher energy X-rays for remote X-ray spectrometric elemental analysis. To produce low energy fluorescent emission, low energy solar X-rays or highly energetic elements with excitation are much more required. Imaging of photons of this energy range is also much easier rather high energy X-rays. L- Or M –shell fluorescent emission for elements from Na to Ca resides in the energy range 25 to 2000 eV within which up to 600 eV is considered here. For elements from Na to Cu fluorescent emission is much more intense foe L- or M- shell than K-shell emission and used as an important tool in remote sensing of all elements .More information’s related to our solar system can be gathered from these elements which are mainly fairly large fraction’s of the air-less body. Associated with it, intensity of reflected X-rays can be compared with the intensity of the fluorescent emission, originates from bombardment of highly energetic particles. Resent observations is studied thoroughly based on fluorescent emission produced from X-rays on the dark sides of Moon by the Roentgen satellite (ROSAT) [7].Due to coherence scatter from the planetary surface and mosaic degradation arises for nature of surface and reduce the intensity of emission signal on the planetary surface, fluorescent soft energy faces an important problem. It is required to much more study on this problem theoretically and experimentally. Room temperature light solid state detectors are much more promising than others even this low energy domain also [8]. III. γ -ray detectors In remote sensing and γ –ray spectroscopy also Room temperature solid state detectors plays a significant role. Minimum energy requirement to generate an electron-hole pair and no requirement of photo multiplier and high- voltage power supplier is the key features of this type of detectors and a large number of carriers can be produced. Among CdZnTe, HgI2, Si PIN, PbI2, CdTe and GA As materials, CdZnTe is mostly studied in modern times due to its interesting features. In this time for remote sensing and γ –ray spectroscopy energy range extends from 100 to 10 MeV. Generally large volume detectors are required. It can be grown by using a larger volume or by stacking the smaller systems which has an importance because it can be used as segmented detectors along with a lot of applications in different areas like HgI2 detectors. Problem arising of unavailability of large volume solid state detectors can be replaced by using segmented detectors or combined scintillation detectors [fig.2]. Energy resolution of CsI can be enhanced by HgI2 or Si diodes photo detectors. It has been noticed resolution of ~ 5.6% can be obtained using HgI2 and 1״-2״ CsI scintillator coupled system [9].Theoretically, energy resolution is good and significant because it is limited by statistical fluctuations in number of carriers. IV. Mineralogical and sedimentary analysis on the planetary surface Surface mineralogy of a planetary surface provides useful information regarding history of climatic conditions, sedimentary weathering process or hydrothermal activity. Climate conditions of any asteroid used to determine the complete analysis of paragenetic and diagenetic histories. Sedimentology relating to stability fields, common for all rocks and also has a connection with non-equilibrium in the analysis of mineralogical data because of relationship in physical sublevel of the minerals. These phenomena can also be determined by analyzing the relict of unstable phases even when the minerals decay out. Even though mineralogical data can be used as a descriptive as well as diagnostic tool to find out the history of a planetary surface, X-ray diffraction technique is the most promising tool to determine the crystal structure of minerals without any ambiguity [10]. Radioactive heat producing elements such as K, Th, and U can be easily detected by γ – ray instruments due to its high sensitivity. Some elements produce strong spectral lines with reactions of neutrons, which are observed for O, Si, Al, Ca, Ti, Fe element. Spectra from these elements with less noise reductions after theoretical and experimental calculations [11] [Fig.3].Light elements like H, C used to represent the presence of frozen volatile materials. A detailed study of rocks mainly based upon the elemental abundances and their ratio. It has been studied that high radioactivity and lower ratio between K and Th can differentiates basal and lunar KREEP materials. Depending upon Ti, Fe contents or Fe/Ca and Mg/Ca ration a detailed study on basalts can be characterized. Well differentiation of lunar surfaces (Highland and Maria) through the ratio of abundance of Mg and Al and also relaying upon superior areal resolution. X-ray can easily detect the ration of Al and Mg from the contrast of their spectral lines. Large volume of Mg in a surface indicates the presence of olivine, pyroxene and trace amount of plagioclase in the Maria. Presence of large amount of minerals like feldspars in the highland rocks confirms existence of abundance of Al. X-ray and γ – ray geochemical instruments can provide new information’s for further study through any planetary orbiter [12-15]. V. Review and comparative study of X-ray and γ – ray spectroscopy V.1. Review The main functions of the X – ray spectroscopy includes the following measurements • The mineralogy of soil and rock samples can be determined by CHEMIN (has an ability to direct findinding of CHEmistry and MINeralogy) employing a soft X-ray. • In 1-10 keV energy domains, X-ray detectors have a capability of imaging and to analyze the spectroscopic study is studying now days. • Using 2-D charge coupled device (CCD) detector, low flux X-ray can be analyzed of all elements (Z>5). For γ – ray spectrometry, • The concentration of major rock-forming elements on a planetary surface; • The concentration of elements that are considered essential for living conditions • The concentration of trace elements on planetary surface, elements that can’t be detected using X-ray spectroscopy • The hydrogen content of a planetary surface as evidence for the presence of water or ice; • The concentration of radioactive elements on the surface. V.2. Comparative study of X- ray and γ – ray spectroscopy Both X-ray and γ –ray spectroscopy has been in use for the surface mapping of various planetary missions. The following points will give rise to the some comparison between the X-ray and γ – ray spectroscopy. The X-ray spectroscopy gives the information which will be lasts for a range of few seconds to few hours because signal to background ratio is very high. But for the γ –ray spectroscopy the composition information of surface will lasts for larger period like few hundred hours because the signal to background ratio is very low. The X-ray spectroscopy will give the limited information about the planetary surface composition about few microns thickness. The γ –ray spectroscopy will give the in depth information (about few meters) about the elements of the surface as they can penetrate more into it and also lighter elements. The γ –ray spectroscopy has higher efficiency as the number of the detected gamma rays contributing to the full energy compared to the X-ray spectroscopy [16].The γ–ray spectroscopy has higher energy resolution compared to the x-ray spectroscopy. Resolution [17] can be found out by Resolution (%) = [(FWHM) / peak energy] ×100 The X-ray spectroscopy is advantageous for getting the information just outline of the rock elements of the planetary surface as compared to γ –rays because flexibility and lower cost compared to γ – ray spectroscopy. Conclusion Room temperature solid state detectors have a great importance for spectroscopic analysis of X-ray and γ –ray on in-situ and remote sensing studies. Both spectroscopies can be used as an important tool to find out the compositional information, information about living conditions, sedimentary information etc. Chemical mapping on planetary surface can be determined by distribution model of elements. It is difficult to find out the mineralogical composition of most abundant elements on the planetary surface at different sublevel if the planet is covered by a dense atmospheric level. γ –ray spectroscopy can be provided in-depth information of all elements on the planetary surface compared to the x-ray spectroscopy, but needs a sophisticated set up of detectors. Future work should be focused to design suitable solid state device to detect the elemental spectrum even though thick atmosphere covered the planetary surface. This device will enable us to the mapping of entire range of elements using a single detector. Continuous work has been to carry out on the reduction of weight and volume of large sophisticated detectors for space mission. Suitable software is also need to be developed to give accurate in situ mapping and classification of materials from the detected spectrums. Acknowledgement We wish to thanks Dr. Aradhna Malik, IIT Kharagpur for her constant support and encouragement to write this manuscript during the ״English for Technical Writing״ course of study. References [1] Larry G. Evans, Pamela E. Clerk, J. Trombka ,Acta Astronautica,Vol. 35.Suppl. pp.79- 88 (1995). [2] Jyri Näränen, James Carpenter, Hannu Parviainen, Karri Muinonen, George Fraser, Marko Peura and Aki Kallonen, Adv. Space Res. 44), pp. 313-322 (2009). [3] Alan Owens, Burkhard Beckhoff, George Fraser, Michael Kolbe, Michael Krumrey, Alfonso Mantero, Michael Mantler, Anthony Peacock, Maria-Grazia Pia, Derek Pullan, Anal. Chem, 80, pp. 8398 8405(2008). [4] Paul Gorenstein and Herbert Gursky, Characteristic and X-radiation in the Planetary System, American Science and Engineering, Cambridge, Mass., U.S.A., (1969). [5] W. Brandt, Proc. International Conference on Inner Shell Ionization Phenomena, USAEC Conference 720404, vol. 2, pp. 948(1972). [6] L.H. Toburen, Phys. Rev. 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Acta, Suppl.6, Vol. 3, pp. 2721, Pergamon Press, (1975). [16] M.S. Skidmore, R.M. Ambrosi, H. Simon, Nuclear Instruments and Methods in Physics Research A 604 pp. 592–603(2009). [17] T. Belgya, Z. Revay, in :G.L. Molnar(Ed.),Handbook of Prompt Gamma Activation Analysis with Neutron Beams ,Kluwar Academic Publishers,Dordrecht,pp.71 (2004). [pic] Fig.1 Nuclear reactions take place in the subsurface layer of Mars under bombardment of Cosmic Rays (Picture has taken from “Scientific tasks of GRS and HEND”) [pic] Fig.2 A stacked HgI2 γ –ray detector configuration (Gerrish, 1998) [pic] Fig.3. Spectra from the experiments after noise reduction and energy calibration.The spectrum in dot- dash line in mesured with set up A and the spectrum in solid line with set up B.Counters are given in arbitary units and the spectra are normalized to unity at the Ca-Kα Peak value.Notable lines in the spectra are Si- Kα at 1.74 keV,K- Kα at 3.31 keV,Ca- Kα at 3.69 keV,Ca- Kβ at 4.01 keV,Ti- Kα at 4.51 keV,Ti- Kβ at 4.93 keV,Mn- Kα 5.89 keV,Fe- Kα at 6.40 keV,Mn- Kβ at 6.49 keV and Fe- Kβ at 7.06 keV.[Jyari Nӓrӓnen et al, Adv. In space Research ,Vol.44 (2009)] ----------------------- a: http://www.iki.rssi.ru/hend/e_page2_main.htm ,Date :19.09.2010,19:17