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Optimization of 18O measurement using NRA for studies of isotopic content in fossil meteorites
Nuclear Instruments and Methods in Physics Research B 269 (2011) 2229–2232
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Optimization of 18O measurement using NRA for studies of isotopic content in fossil meteorites M. Borysiuk ⇑, P. Kristiansson, N. Arteaga-Marrero, M. Elfman, P. Golubev, E.J.C. Nilsson, C. Nilsson, J. Pallon, N. Salim Division of Nuclear Physics, Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
a r t i c l e
i n f o
Article history: Available online 26 February 2011 Keywords: Ion beam analysis Nuclear microprobe NRA Semiconductor d
a b s t r a c t In this work, we discuss the possibility of a new approach to measuring oxygen isotope ratios in fossil meteorite samples, specifically one based on nuclear reaction analysis (NRA). Variations of oxygen ratios within meteoritic chromite grains can help to determine the type of meteorite to which the grains originally belonged. In this work, we have evaluated the possibility to use the reaction 18O(p, a)15N just above the 846 keV resonance to estimate the relative oxygen-18 content in a number of test samples. Another technique has to be employed for oxygen-16 measurements. A large area segmented silicon detector is used to detect the produced a particles. Results of the experimental 18O measurements for a number of samples including four extraterrestrial chromite grains are presented and compared with SIMNRA simulations. The advantage of a segmented silicon detector in the form of inherent pile-up suppression can be clearly seen in the current work. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is the third most abundant element in the universe. It has three stable isotopes: 16O, 17O, and 18O and many physical and chemical processes affect isotopic fractionation of this element, which makes oxygen analysis useful in several different contexts. Geology, hydrology and climatology are areas where measurements of oxygen isotopic content have found an application [1]. Variations in 18O/16O ratios are used to determine for instance geologic temperature records using ice cores, water movement and mixing of water reservoirs. Analytical techniques most often used for isotopic content determination in those areas are various forms of mass spectrometry, which are destructive, require sample preprocessing and relatively large amounts of material for analysis. The particular application under consideration in this work, is the analysis of oxygen isotopic ratios in fossil meteorites [2]. Meteorites are remainders of meteoroids that have fallen down to earth. Meteoroids themselves are solid bodies of different sizes and compositions found throughout our solar system. Most of them broke off larger bodies (asteroids) through collisions and were transported from the asteroid belt towards the sun. Meteorites are divided into many different categories depending on their chemical composition. Most of the known meteorites fall into the group of chondrites, which are themselves a type of stony meteorite, as ⇑ Corresponding author. Tel.: +46 2227733. E-mail address: [email protected] (M. Borysiuk). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.02.035
opposed to iron or stony-iron meteorites. Today, the most common meteorite group (by number of discovered samples) is the ordinary chondrites group which can be further divided into three subgroups, H, L and LL, depending on their overall iron and metal content. Many of the chondritic meteorites that have fallen to earth can be associated with a single break up event of a large parent body roughly 470 million years ago [2]. Some samples of fossil meteorites that have fallen close in time to the original event were preserved in marine sediments and recovered from limestone quarries. Most of the original minerals in those fossil samples were severely altered by metamorphic processes making analysis and classification of the extraterrestrial material difficult. In chondritic meteorites small grains of highly resistant mineral chromite (FeCr2O4) can be found unchanged by geological processes, this material usually makes up less than 0.5% of the meteorite by weight and is found in grains of up to a few 100 lm in diameter. Variations of oxygen isotope ratios within chromite grains can help to determine the type of meteorite to which the grains originally belonged. The technique previously used in those types of isotopic studies, in situations where samples are small, rare and not readily available, is secondary ion mass spectrometry (SIMS) [3]. SIMS is a form of mass spectrometry which requires a very small amount of material to be consumed and can be used for both 2D and 3D sample imaging. Some processing of the sample is still required and the analysis method still causes localized damage to the sample. In this work, we apply a new approach to oxygen determination in fossil meteorite samples, specifically an approach based on
M. Borysiuk et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2229–2232
nuclear reaction analysis (NRA). NRA is a method within ion beam analysis that has proven to be very effective in a few well defined cases. It is a technique that can be used for measuring specific isotopes or isotopic ratios [4]. There is a number of well known and well studied nuclear reactions which can be used for oxygen analysis, such as the reactions 16O(d, p0)17O, 16O(d, a)14N and 18O(p, a)15N. The last reaction mentioned is used in this work for the evaluation of the possibility to measure 18O with high enough statistical precision. The other two reactions will be evaluated in a forthcoming experiment regarding 16O. Measurements of 18O using NRA have proven to be useful in material science [5] specifically for studying oxidation, oxygen diffusion and oxygen transport. In those cases, most often a given sample is submerged in an artificially enriched 18O atmosphere and the processes of interest can be studied with ease. The low natural content of 18O in geological samples presents its own specific challenge. This challenge will require a new experimental approach which we are hoping to provide.
Normalized Yield (Counts/(nC*sr))
2230
0.15
O-18 yield for Quartz O-18 yield for Chromite
0.1
0.05
0
130
135
140
145
150
155
160
165
Scattering Angle (deg.) Fig. 1. Reaction yield plotted as a function of the scattering angle just above the 846 keV oxygen-18 nuclear resonance peak. Results are consistent with previous measurements [6] and show that the maximum yield is reached for small reaction angles.
2. Experiment The reaction used to measure the oxygen-18 content, 18O(p, a) N, is a well-known reaction that has several low lying resonances, which are accessible also for a small accelerators [4]. The most interesting resonance for the current investigation is at proton energy of 846 keV. At this particular energy the resonance has a width of 47 keV [6] and at the angular interval of interest alpha particles with energy around 3.5 MeV are emitted. When measuring the 18O concentration, a beam with energy of 850–860 keV, which is above the resonance peak, is used in order to maximize the reaction yield. Using beam energies above the resonance means that the bulk of the reaction takes place below the sample surface and that the contributions from possible surface contaminations should be minimized. The large width of the resonance makes depth profiling difficult but instead it provides a high nuclear reaction rate. The Lund Nuclear Microprobe facility is used to produce a stable and focused beam of protons at the required energy. The facility is built around a NEC 3MV Pelletron accelerator and a detailed description of the beam line can be found in [7,8]. High vacuum conditions were kept during the entire experimental run with pressure in the experimental chamber around 6 10 6 mbar. Sub-micron resolution was not required for this project, so instead a beam spot of several microns in diameter was used to allow for a high beam current in the range of 1–3 nA. The charge was integrated on the sample for thick targets and in a post sample faraday cup for thin targets. The charge normalization during quantitative measurements can be performed using a pre-chamber faraday cup described in reference [9]. This will solve all potential problems associated with correction for secondary electron emission. A circular 96 channel Double Sided Silicon Strip Detector (DSSSD) [10] was placed in backscattering geometry and used to detect backscattered protons and alpha particles from the nuclear reaction. A thicker version of the DSSSD detector was used in this test and this device has the following specifications: 500 lm thickness, 0.6 lm dead layer and a leakage current on the order of 1 lA. As with the other DSSSD [10], this device has 64 strips in the front and 32 rings in the back giving the total of 2048 detector elements. No noticeable degradation of the DSSSD was observed after more than 100 h of beam time, as exemplified by a stable leakage current and a stable detector resolution throughout the experiment. The distance between the target and detector was chosen to be d = 32 ± 2 mm which gives a large solid angle coverage of 3.54 sr. A small distance d allows for the inclusion of lower scattering angles (see Fig. 1) which helps to achieve an as high as possible 15
oxygen-18 integrated reaction yield. The drawback of this small distance is a minor sacrifice to the energy resolution due to geometrical straggling and a small increase in pileup from backscattered protons. As previously reported [10], the readout electronics consists of 6 Mesytec 16 channel charge sensitive preamplifiers, MPR-16. Four were used for the front side of the detector and two for the back side. Also, 6 Mesytec shaping/timing filter amplifiers STM-16 were used, one for each preamplifier. Gates for the front end data acquisition system (three ADC modules, CAEN V785) were created using standard NIM modules and the leading edge discriminator built into the timing filter portion of the STM16 amplifiers. For scanning the beam over the chromite samples, the readout electronics were connected to the existing CAMAC data acquisition system [11] using standard NIM modules in such a way that the information regarding the charge deposited on the sample and the number of alpha particles produced at each point in the sample was preserved.
3. Results Since the particles expected in the experiment are protons from elastic scattering and alpha particles from the nuclear reaction, the energy calibration of the detector was performed both with alpha particles and protons. A thorium source produced alpha particles at energies of 5.42 MeV and 5.68 MeV. Protons with appropriate energies were extracted from elastic backscattering of carbon and oxygen in a thin plastic foil at an incoming beam energy of 2.55 MeV. This gives us four calibration points, two above and two below the alpha particles from the 18O reaction. A large portion of the detector functioned in a satisfying manner, 58 out of 64 strips and 26 out of 32 rings produced signal that could be calibrated. This amounts to roughly 74% of the detector’s surface being fully functional during the entire experimental run. A number of properties of the relevant oxygen reaction were measured. The angular distribution of the emitted alpha particles for the oxygen reaction around the resonance energy of interest, which is 846 keV, was investigated and the results were consistent with previous experiments [6]. The results of the current measurements are shown in Fig. 1. Integrated yields of the 18O reaction were observed for various beam energies and for a number of thick and oxygen rich test samples (all with natural oxygen isotopic content). Around the resonance peak, the number of counts from the reaction rose to several thousand after less than an hour, for a current on the order of 1 nA. The results of the energy scan, for
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Yield (counts /(nC*sr))
0.7
one sample of quartz and one of mylar plastic are summarized in Fig. 2. Several spectra for the 18O measurement were produced by simulation above, below and at the resonance energy, in order to estimate the expected yield. All the relevant experimental parameters were chosen as in the actual experiment using the simulation program SIMNRA [12], with one crucial difference, being that the detector in the simulation is not segmented. This spectrum can be seen in Fig. 3a. The actual measured spectra for quartz and for one of the chromite grains with the beam energy directly above the resonance energy summed over all of the detector elements can be seen in Fig. 3b and c. Here one directly sees the advantages
Mylar Yield Quartz Yield
0.6 0.5 0.4 0.3 0.2
720
740
760
780
800
820
Energy (keV) Fig. 2. The oxygen-18 integrated reaction yield measured in two thick samples, mylar and quartz, at different energies below the 846 keV resonance. Error bars in the figure represent statistical deviation only. Some variation between the two plots can be seen probably due to improper removal of pile up and errors in charge measurements.
a
b
c
Fig. 4. Sample of chromite, which is a highly resistant mineral and the only part of the fossil chondritic meteorite left unchanged following the metamorphic processes the meteorite undergoes after deposition on the surface of earth. The count rate detected from both elastically scattered protons and alpha particles in (a) and the number of alpha particle counts in (b). The yield is normalized to charge. No immediate structures related to oxygen concentrations can be observed within the samples. The average relative oxygen count was measured for all four samples and can be compared with average values for a quartz glass sample measured at one point. The total scan area is 800 800 lm.
Table 1 Average relative oxygen-18 content (the reaction yield normalized to charge) in the four chromite grains shown in Fig. 4 as well as in a sample of epoxy plastic and in a quartz plate. Sample # Fig. 3. Typical spectra resulting from reaction between oxygen-18 and protons at 846 keV from thick samples: a) SIMNRA simulated quartz spectrum (note the large pile-ups marked in the figure), b) measured spectrum from quartz and c) measured spectrum from chromite. In all three spectra the same structures can be seen, the edge of the elastic scattering peak at low energies, proton pile-up (marked in the figures) and alpha particles from the oxygen reaction at high energy. Note the absence of pile-up above the alpha peak in the experimental spectra.
Chromite Chromite Chromite Chromite Epoxy Quartz
1 2 3 4
O-18 relative content
Standard deviation
0.5494 0.4726 0.4797 0.4682 0.1801 0.3675
0.0136 0.0118 0.0133 0.0210 0.0050 0.0046
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of the segmented detector in the form of inherent pile-up suppression in the DSSSD. The studied samples of fossil chondritic meteorites were acquired from the Department of Geology at Lund University as test samples. Four chromite grains encased in an epoxy plastic were scanned with the nuclear microprobe (NMP) and their relative oxygen-18 content was analyzed. The images in Fig. 4 show the count rate of valid events (protons and alpha particles) in (a), as well as the number of valid alpha particles events in (b), produced at each point in the sample. Insufficient statistics were collected at each individual point on the sample to be able to draw firm conclusions about the possible spatial structure of oxygen-18 within the chromite grains. The oxygen reaction yields averaged over each grain and normalized to the charge deposited in each grain are presented in the Table 1. The data for chromite samples can be compared with a quartz sample. We see that one of the grains has a significantly higher 18O concentration and all four grains contain more 18 O then the studied quartz sample. The data was collected with 74% of the detector functioning over a period of five hours. Considering that a large portion of the scanned sample was the epoxy plastic not relevant in the analysis, the collected statistics would be enough to measure the 18O concentration with per mil sensitivity for a single chromite grain, if the measurements were focused on an individual grain and the detector fully functional. 4. Summary and discussion In this work, the use of the 18O(p,a)15N reaction around the 846 keV resonance for quantitative analysis was evaluated and, as an illustration, used to measure the relative content of oxygen-18 in chromite samples. NRA, especially when using light ions, can be a much less destructive method, and easier to implement than SIMS. The advantage of the DSSSD over a regular silicon detector comes at a price of much more complicated readout electronics. This is why this setup requires much more automation for repeated use and analysis of a large number of samples. In addition, techniques to measure 16O simultaneously, or separately but in a similar way, have to be developed to be able to extract the interesting variations in isotope ratios. There is a number of attractive methods within NRA for both 16 O and 18O measurements, but in order to compete with SIMS, a good method for 17O measurement is required as well. So far no good NMP alternative has been suggested. There is no standard reference material specific to geological studies of oxygen isotopes and NMP methods. The standard which exists is a liquid mixture which is rather difficult for NRA and was designed with mass spectrometry in mind. It fixes the relative oxygen isotopic content
to a hypothetical mix of seawater samples (Vienna Standard Mean Ocean Water VSMOW with 18O/16O fixed at (2005.2 ± 0.45) 10 6). This should, in theory, not be a problem since NRA can be done without a standard and the oxygen concentrations can be estimated directly from the experimental parameters. This is however, a rather challenging process and relies heavily on a very detailed knowledge of the reaction cross sections, i.e. a good standard would be preferred. One possibility would be to produce a standard locally in the laboratory by oxidation of silicon. In addition, the long runs required in order to reach the desired sensitivity are prohibitive. In summary, many crucial improvements are still necessary. Acknowledgment Professor Birger Schmitz, Department of Geology, Lund University, is acknowledged for supplying the chromite grains.
References [1] M.M. Ghomshei, D.M. Allen, Potential application of oxygen-18 and deuterium in mining effluent and acid rock drainage studies, Env. Geol. 39 (2000) 767– 773. [2] A. Cronholm, The flux of extraterrestrial matter to Earth as recorded in Paleogene and Middle Ordovician marine sediments, PhD thesis, Lund University 2009. [3] P.R. Heck, T. Ushikubo, B. Schmitz, N.T. Kita, M.J. Spicuzza, J.W. Valley, A single asteroid source for extraterrestrial Ordovician chromite grains from Sweden and China: high-precision oxygen three-isotope SIMS analysis, Geochim. Cosmochim. Acta 74 (2010) 497–509. [4] G. Amsel, D. Samuel, Microanalysis of the stable isotopes of oxygen by means of nuclear reactions, Anal. Chem. 39 (14) (1967) 1689–1698. [5] J.R. Liu, Y.P. Li, Q.Y. Chen, X.T. Cui, R. Christoffersen, A. Jacobson, W.K. Chu, Depth resolution and dynamic range of 18O(p, a)15N depth profiling, Nucl. Instr. and Meth. B 136–138 (1998) 1306–1311. [6] D.D. Cohen, A. Katsaros, D. Garton, Some characteristics of the 18O(p, a)15N reaction, Nucl. Instr. and Meth. B 15 (1986) 555–558. [7] A. Shariff, C. Nilsson, V. Auzelyte, M. Elfman, P. Kristiansson, K. Malmqvist, J. Pallon, M. Wegdén, The Lund nuclear microprobe sub-micron setup. Part 1: ion optics calculations, Nucl. Instr. and Meth. B 231 (2005) 1–6. [8] M. Elfman, J. Pallon, V. Auzelyte, P. Kristiansson, K. Malmqvist, C. Nilsson, A. Shariff, M. Wegdén, The Lund nuclear microprobe sub-micron setup. Part 3: sample stage, optical imaging and detector configuration in the experimental chamber, Nucl. Instr. and Meth. B 231 (2005) 14–20. [9] P. Kristiansson, M. Borysiuk, N. Arteaga-Marrero, M. Elfman, E.J. C Nilsson, C. Nilsson, J. Pallon, A pre-sample charge measurement system for quantitative NMP-analysis, Nucl. Instr. and Meth. B 268 (2010) 1727–1730. [10] P. Golubev, P. Kristiansson, N. Artega-Marrero, M. Elfman, K. Malmqvist, E.J.C. Nilsson, C. Nilsson, J. Pallon, M. Wegden, First results from the Lund NMP particle detector system, Nucl. Instr. and Meth. B 267 (2009) 2065–2068. [11] M. Elfman, P. Kristiansson, K. Malmqvist, J. Pallon, The layout and performance of the Lund nuclear microprobe trigger and data acquisition system, Nucl. Instr. and Meth. B 158 (1999) 141–145. [12] SIMNRA ver. 6.03 http://www.rzg.mpg.de/~mam/.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Optimization of 18O measurement using NRA for studies of isotopic content in fossil meteorites M. Borysiuk ⇑, P. Kristiansson, N. Arteaga-Marrero, M. Elfman, P. Golubev, E.J.C. Nilsson, C. Nilsson, J. Pallon, N. Salim Division of Nuclear Physics, Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
a r t i c l e
i n f o
Article history: Available online 26 February 2011 Keywords: Ion beam analysis Nuclear microprobe NRA Semiconductor d
a b s t r a c t In this work, we discuss the possibility of a new approach to measuring oxygen isotope ratios in fossil meteorite samples, specifically one based on nuclear reaction analysis (NRA). Variations of oxygen ratios within meteoritic chromite grains can help to determine the type of meteorite to which the grains originally belonged. In this work, we have evaluated the possibility to use the reaction 18O(p, a)15N just above the 846 keV resonance to estimate the relative oxygen-18 content in a number of test samples. Another technique has to be employed for oxygen-16 measurements. A large area segmented silicon detector is used to detect the produced a particles. Results of the experimental 18O measurements for a number of samples including four extraterrestrial chromite grains are presented and compared with SIMNRA simulations. The advantage of a segmented silicon detector in the form of inherent pile-up suppression can be clearly seen in the current work. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is the third most abundant element in the universe. It has three stable isotopes: 16O, 17O, and 18O and many physical and chemical processes affect isotopic fractionation of this element, which makes oxygen analysis useful in several different contexts. Geology, hydrology and climatology are areas where measurements of oxygen isotopic content have found an application [1]. Variations in 18O/16O ratios are used to determine for instance geologic temperature records using ice cores, water movement and mixing of water reservoirs. Analytical techniques most often used for isotopic content determination in those areas are various forms of mass spectrometry, which are destructive, require sample preprocessing and relatively large amounts of material for analysis. The particular application under consideration in this work, is the analysis of oxygen isotopic ratios in fossil meteorites [2]. Meteorites are remainders of meteoroids that have fallen down to earth. Meteoroids themselves are solid bodies of different sizes and compositions found throughout our solar system. Most of them broke off larger bodies (asteroids) through collisions and were transported from the asteroid belt towards the sun. Meteorites are divided into many different categories depending on their chemical composition. Most of the known meteorites fall into the group of chondrites, which are themselves a type of stony meteorite, as ⇑ Corresponding author. Tel.: +46 2227733. E-mail address: [email protected] (M. Borysiuk). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.02.035
opposed to iron or stony-iron meteorites. Today, the most common meteorite group (by number of discovered samples) is the ordinary chondrites group which can be further divided into three subgroups, H, L and LL, depending on their overall iron and metal content. Many of the chondritic meteorites that have fallen to earth can be associated with a single break up event of a large parent body roughly 470 million years ago [2]. Some samples of fossil meteorites that have fallen close in time to the original event were preserved in marine sediments and recovered from limestone quarries. Most of the original minerals in those fossil samples were severely altered by metamorphic processes making analysis and classification of the extraterrestrial material difficult. In chondritic meteorites small grains of highly resistant mineral chromite (FeCr2O4) can be found unchanged by geological processes, this material usually makes up less than 0.5% of the meteorite by weight and is found in grains of up to a few 100 lm in diameter. Variations of oxygen isotope ratios within chromite grains can help to determine the type of meteorite to which the grains originally belonged. The technique previously used in those types of isotopic studies, in situations where samples are small, rare and not readily available, is secondary ion mass spectrometry (SIMS) [3]. SIMS is a form of mass spectrometry which requires a very small amount of material to be consumed and can be used for both 2D and 3D sample imaging. Some processing of the sample is still required and the analysis method still causes localized damage to the sample. In this work, we apply a new approach to oxygen determination in fossil meteorite samples, specifically an approach based on
M. Borysiuk et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2229–2232
nuclear reaction analysis (NRA). NRA is a method within ion beam analysis that has proven to be very effective in a few well defined cases. It is a technique that can be used for measuring specific isotopes or isotopic ratios [4]. There is a number of well known and well studied nuclear reactions which can be used for oxygen analysis, such as the reactions 16O(d, p0)17O, 16O(d, a)14N and 18O(p, a)15N. The last reaction mentioned is used in this work for the evaluation of the possibility to measure 18O with high enough statistical precision. The other two reactions will be evaluated in a forthcoming experiment regarding 16O. Measurements of 18O using NRA have proven to be useful in material science [5] specifically for studying oxidation, oxygen diffusion and oxygen transport. In those cases, most often a given sample is submerged in an artificially enriched 18O atmosphere and the processes of interest can be studied with ease. The low natural content of 18O in geological samples presents its own specific challenge. This challenge will require a new experimental approach which we are hoping to provide.
Normalized Yield (Counts/(nC*sr))
2230
0.15
O-18 yield for Quartz O-18 yield for Chromite
0.1
0.05
0
130
135
140
145
150
155
160
165
Scattering Angle (deg.) Fig. 1. Reaction yield plotted as a function of the scattering angle just above the 846 keV oxygen-18 nuclear resonance peak. Results are consistent with previous measurements [6] and show that the maximum yield is reached for small reaction angles.
2. Experiment The reaction used to measure the oxygen-18 content, 18O(p, a) N, is a well-known reaction that has several low lying resonances, which are accessible also for a small accelerators [4]. The most interesting resonance for the current investigation is at proton energy of 846 keV. At this particular energy the resonance has a width of 47 keV [6] and at the angular interval of interest alpha particles with energy around 3.5 MeV are emitted. When measuring the 18O concentration, a beam with energy of 850–860 keV, which is above the resonance peak, is used in order to maximize the reaction yield. Using beam energies above the resonance means that the bulk of the reaction takes place below the sample surface and that the contributions from possible surface contaminations should be minimized. The large width of the resonance makes depth profiling difficult but instead it provides a high nuclear reaction rate. The Lund Nuclear Microprobe facility is used to produce a stable and focused beam of protons at the required energy. The facility is built around a NEC 3MV Pelletron accelerator and a detailed description of the beam line can be found in [7,8]. High vacuum conditions were kept during the entire experimental run with pressure in the experimental chamber around 6 10 6 mbar. Sub-micron resolution was not required for this project, so instead a beam spot of several microns in diameter was used to allow for a high beam current in the range of 1–3 nA. The charge was integrated on the sample for thick targets and in a post sample faraday cup for thin targets. The charge normalization during quantitative measurements can be performed using a pre-chamber faraday cup described in reference [9]. This will solve all potential problems associated with correction for secondary electron emission. A circular 96 channel Double Sided Silicon Strip Detector (DSSSD) [10] was placed in backscattering geometry and used to detect backscattered protons and alpha particles from the nuclear reaction. A thicker version of the DSSSD detector was used in this test and this device has the following specifications: 500 lm thickness, 0.6 lm dead layer and a leakage current on the order of 1 lA. As with the other DSSSD [10], this device has 64 strips in the front and 32 rings in the back giving the total of 2048 detector elements. No noticeable degradation of the DSSSD was observed after more than 100 h of beam time, as exemplified by a stable leakage current and a stable detector resolution throughout the experiment. The distance between the target and detector was chosen to be d = 32 ± 2 mm which gives a large solid angle coverage of 3.54 sr. A small distance d allows for the inclusion of lower scattering angles (see Fig. 1) which helps to achieve an as high as possible 15
oxygen-18 integrated reaction yield. The drawback of this small distance is a minor sacrifice to the energy resolution due to geometrical straggling and a small increase in pileup from backscattered protons. As previously reported [10], the readout electronics consists of 6 Mesytec 16 channel charge sensitive preamplifiers, MPR-16. Four were used for the front side of the detector and two for the back side. Also, 6 Mesytec shaping/timing filter amplifiers STM-16 were used, one for each preamplifier. Gates for the front end data acquisition system (three ADC modules, CAEN V785) were created using standard NIM modules and the leading edge discriminator built into the timing filter portion of the STM16 amplifiers. For scanning the beam over the chromite samples, the readout electronics were connected to the existing CAMAC data acquisition system [11] using standard NIM modules in such a way that the information regarding the charge deposited on the sample and the number of alpha particles produced at each point in the sample was preserved.
3. Results Since the particles expected in the experiment are protons from elastic scattering and alpha particles from the nuclear reaction, the energy calibration of the detector was performed both with alpha particles and protons. A thorium source produced alpha particles at energies of 5.42 MeV and 5.68 MeV. Protons with appropriate energies were extracted from elastic backscattering of carbon and oxygen in a thin plastic foil at an incoming beam energy of 2.55 MeV. This gives us four calibration points, two above and two below the alpha particles from the 18O reaction. A large portion of the detector functioned in a satisfying manner, 58 out of 64 strips and 26 out of 32 rings produced signal that could be calibrated. This amounts to roughly 74% of the detector’s surface being fully functional during the entire experimental run. A number of properties of the relevant oxygen reaction were measured. The angular distribution of the emitted alpha particles for the oxygen reaction around the resonance energy of interest, which is 846 keV, was investigated and the results were consistent with previous experiments [6]. The results of the current measurements are shown in Fig. 1. Integrated yields of the 18O reaction were observed for various beam energies and for a number of thick and oxygen rich test samples (all with natural oxygen isotopic content). Around the resonance peak, the number of counts from the reaction rose to several thousand after less than an hour, for a current on the order of 1 nA. The results of the energy scan, for
2231
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Yield (counts /(nC*sr))
0.7
one sample of quartz and one of mylar plastic are summarized in Fig. 2. Several spectra for the 18O measurement were produced by simulation above, below and at the resonance energy, in order to estimate the expected yield. All the relevant experimental parameters were chosen as in the actual experiment using the simulation program SIMNRA [12], with one crucial difference, being that the detector in the simulation is not segmented. This spectrum can be seen in Fig. 3a. The actual measured spectra for quartz and for one of the chromite grains with the beam energy directly above the resonance energy summed over all of the detector elements can be seen in Fig. 3b and c. Here one directly sees the advantages
Mylar Yield Quartz Yield
0.6 0.5 0.4 0.3 0.2
720
740
760
780
800
820
Energy (keV) Fig. 2. The oxygen-18 integrated reaction yield measured in two thick samples, mylar and quartz, at different energies below the 846 keV resonance. Error bars in the figure represent statistical deviation only. Some variation between the two plots can be seen probably due to improper removal of pile up and errors in charge measurements.
a
b
c
Fig. 4. Sample of chromite, which is a highly resistant mineral and the only part of the fossil chondritic meteorite left unchanged following the metamorphic processes the meteorite undergoes after deposition on the surface of earth. The count rate detected from both elastically scattered protons and alpha particles in (a) and the number of alpha particle counts in (b). The yield is normalized to charge. No immediate structures related to oxygen concentrations can be observed within the samples. The average relative oxygen count was measured for all four samples and can be compared with average values for a quartz glass sample measured at one point. The total scan area is 800 800 lm.
Table 1 Average relative oxygen-18 content (the reaction yield normalized to charge) in the four chromite grains shown in Fig. 4 as well as in a sample of epoxy plastic and in a quartz plate. Sample # Fig. 3. Typical spectra resulting from reaction between oxygen-18 and protons at 846 keV from thick samples: a) SIMNRA simulated quartz spectrum (note the large pile-ups marked in the figure), b) measured spectrum from quartz and c) measured spectrum from chromite. In all three spectra the same structures can be seen, the edge of the elastic scattering peak at low energies, proton pile-up (marked in the figures) and alpha particles from the oxygen reaction at high energy. Note the absence of pile-up above the alpha peak in the experimental spectra.
Chromite Chromite Chromite Chromite Epoxy Quartz
1 2 3 4
O-18 relative content
Standard deviation
0.5494 0.4726 0.4797 0.4682 0.1801 0.3675
0.0136 0.0118 0.0133 0.0210 0.0050 0.0046
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of the segmented detector in the form of inherent pile-up suppression in the DSSSD. The studied samples of fossil chondritic meteorites were acquired from the Department of Geology at Lund University as test samples. Four chromite grains encased in an epoxy plastic were scanned with the nuclear microprobe (NMP) and their relative oxygen-18 content was analyzed. The images in Fig. 4 show the count rate of valid events (protons and alpha particles) in (a), as well as the number of valid alpha particles events in (b), produced at each point in the sample. Insufficient statistics were collected at each individual point on the sample to be able to draw firm conclusions about the possible spatial structure of oxygen-18 within the chromite grains. The oxygen reaction yields averaged over each grain and normalized to the charge deposited in each grain are presented in the Table 1. The data for chromite samples can be compared with a quartz sample. We see that one of the grains has a significantly higher 18O concentration and all four grains contain more 18 O then the studied quartz sample. The data was collected with 74% of the detector functioning over a period of five hours. Considering that a large portion of the scanned sample was the epoxy plastic not relevant in the analysis, the collected statistics would be enough to measure the 18O concentration with per mil sensitivity for a single chromite grain, if the measurements were focused on an individual grain and the detector fully functional. 4. Summary and discussion In this work, the use of the 18O(p,a)15N reaction around the 846 keV resonance for quantitative analysis was evaluated and, as an illustration, used to measure the relative content of oxygen-18 in chromite samples. NRA, especially when using light ions, can be a much less destructive method, and easier to implement than SIMS. The advantage of the DSSSD over a regular silicon detector comes at a price of much more complicated readout electronics. This is why this setup requires much more automation for repeated use and analysis of a large number of samples. In addition, techniques to measure 16O simultaneously, or separately but in a similar way, have to be developed to be able to extract the interesting variations in isotope ratios. There is a number of attractive methods within NRA for both 16 O and 18O measurements, but in order to compete with SIMS, a good method for 17O measurement is required as well. So far no good NMP alternative has been suggested. There is no standard reference material specific to geological studies of oxygen isotopes and NMP methods. The standard which exists is a liquid mixture which is rather difficult for NRA and was designed with mass spectrometry in mind. It fixes the relative oxygen isotopic content
to a hypothetical mix of seawater samples (Vienna Standard Mean Ocean Water VSMOW with 18O/16O fixed at (2005.2 ± 0.45) 10 6). This should, in theory, not be a problem since NRA can be done without a standard and the oxygen concentrations can be estimated directly from the experimental parameters. This is however, a rather challenging process and relies heavily on a very detailed knowledge of the reaction cross sections, i.e. a good standard would be preferred. One possibility would be to produce a standard locally in the laboratory by oxidation of silicon. In addition, the long runs required in order to reach the desired sensitivity are prohibitive. In summary, many crucial improvements are still necessary. Acknowledgment Professor Birger Schmitz, Department of Geology, Lund University, is acknowledged for supplying the chromite grains.
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