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Cross section measurements at neutron energies 71 and 112 MeV and energy integrated cross section measurements (0.1 < En < 750 MeV) for the neutron induced reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al
Nuclear Instruments and Methods in Physics Research B 294 (2013) 479–483
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Cross section measurements at neutron energies 71 and 112 MeV and energy integrated cross section measurements (0.1 < En < 750 MeV) for the neutron induced reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al M.W. Caffee a,⇑, K. Nishiizumi b, J.M. Sisterson c, J. Ullmann d, K.C. Welten b a
Department of Physics, Purdue University, West Lafayette, IN 47907-1396, USA Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA Francis H. Burr Proton Therapy Center, Massachusetts General Hospital, Boston, MA 02114, USA d LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b c
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 1 April 2012 Available online 23 July 2012 Keywords: Neutron induced reaction Cross sections Cosmogenic nuclides Long-lived nuclides Accelerator mass spectrometry
a b s t r a c t The cross sections for the reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al were measured at neutron energies 71 and 112 MeV. The neutron cross sections for O(n,x)10Be are higher than the corresponding proton cross sections at 70–110 MeV and the neutron induced cross section for Si(n,x)10Be at 112 MeV is slightly higher than the corresponding proton cross section. Cross sections for the production of 26Al from Si are similar to those from protons at low energies (<40 MeV) while at higher energies the measured neutron cross sections for Si(n,x)26Al are lower than the corresponding proton cross sections. Energy integrated (average) cross sections for these reactions were measured using ‘white’ neutron beams (0.1 < En < 750 MeV). The 26Al/10Be production rate ratio from SiO2 measured using ‘white’ neutrons is considerably lower than that observed in terrestrial quartz (SiO2). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Oxygen and silicon are the major constituents of solids found on the surfaces of Earth, the moon, and stony meteorites. The shortand long-lived radionuclides (e.g., 22Na, 10Be, 26Al), as well as stable nuclides, produced in these environments by cosmic-ray interactions can be analyzed to provide clues about the exposure history of the sample and in some instances the history of the cosmic rays (e.g., [1]). Techniques used to measure these cosmogenic nuclides include gamma-ray spectroscopy, particularly for the short-lived radionuclides, accelerator mass spectrometry (AMS) for the longlived radionuclides, and noble gas mass spectrometry. For many applications, both terrestrial and extraterrestrial, interpretation of the cosmogenic nuclide measurements requires knowledge of the production rates. These production rates are relatively low, especially on Earth’s surface; the primary means of determining them is by using geologic samples having a known chemical composition that have been exposed to cosmic rays for a known period of time. For terrestrial samples the exposure durations are typically >10 kyr. When determined in this manner the production rates for a specific nuclide on Earth’s surface integrate over many 22-year solar cycles, include secular changes in Earth’s magnetic field,
⇑ Corresponding author. Tel.: +1 765 494 2586. E-mail address: [email protected] (M.W. Caffee). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.07.011
and may also reflect changes in the cosmic ray flux in the inner solar system. An alternative approach to geologic calibration is the use of physics-based production rates (e.g., [2]). Production rates determined in this fashion eliminate multiple factors that influence the production of cosmogenic nuclides in geologic samples. Physics-based production rates could extend the applicability of cosmogenic nuclides to sample geometries and compositions more complicated than those seen in the geologic calibration samples. A physics-based model for cosmogenic nuclide production rates requires accurate cross section measurements over a wide energy range for the interactions of cosmic ray particles with all the major components of the sample under study (e.g., [3,4]). In extraterrestrial materials, cosmogenic nuclides are produced by solar cosmic rays (SCR), galactic cosmic rays (GCR) and the secondary neutrons produced by GCRs. SCR interactions occur in the uppermost few centimeters, primary GCR protons penetrate further into the surface, and the secondary neutron cascade penetrates to even greater depths (e.g., [4]). The rate of production from each of these primary or secondary particles depends on parameters such as the size of the body and the depth of the sample within that body. A lunar surface sample for example, could conceivably contain cosmogenic nuclides from all three of these sources. On Earth, galactic cosmic rays interact with Earth’s atmosphere, producing a secondary cascade of particles, including a significant number of neutrons. It is this secondary neutron cascade that is the principal source of cosmogenic nuclides produced on
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Earth’s surface (e.g., [5]). A comprehensive compilation of reactions and their products must necessarily consider proton and neutron reactions. Most of the relevant cross sections for proton-induced reactions, such as those producing 10Be, 26Al, and 36Cl, have been measured over a wide energy range [6]. There are however very few cross section measurements for neutron-induced reactions at energies higher than 15 MeV (e.g., [7,8]). Measurements of relevant neutron cross sections are sparse. Some estimates for neutron cross sections have been made specifically for cosmogenic nuclide studies (e.g., [3,4]). There are several strategies that are used to derive these estimates: (1) use the cross section for the corresponding proton-induced reaction; (2) use cross sections derived from nuclear physics models and calculations; (3) estimate the cross section for neutron-induced reactions from thick target simulation experiments [3] or from experimental results of lunar cores and meteorites exposed to cosmic rays [4]. None of these strategies is ideal in all cases as has been shown previously for short-lived radionuclides produced from Cu, Ti, Fe and Ni (e.g., [9,10]). In this work, we measured cross sections of 10Be and 26Al from O and Si using quasi-monenergetic neutrons with energies of 71 and 112 MeV. We also exposed SiO2 and Si targets to ‘white’ neutrons, neutrons that not monoenergetic. These new cross section measurements at high neutron energies, although few in number, for the neutron-induced reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al, are an essential first step toward improving the input data for the theoretical models used in the analyses of the cosmogenic nuclides. For the ‘white’ neutron irradiations we cannot reconstruct excitation functions but we can determine the production rate integrated over the entire energy spectrum (energy integrated) and we can also calculate the 26Al/10Be production rate ratios. Some production rates of cosmogenic nuclides from ‘white’ neutrons have already been reported: 10Be and 26Al from SiO2 or Si [11], 10Be from Cu, Ag, and Au [12], and 36Cl from Cl, K, and Ca [13]. The energy spectrum of the ‘white’ neutrons, for example at LANSCE, is not necessarily the same as secondary neutrons in the Earth’s atmosphere so we do not expect a rigorous correspondence between terrestrial cosmogenic nuclide production rates and ‘white’ neutron production rates.
Table 1 Irradiation parameters. Target
Si SiO2 SiO2 Si SiO2 SiO2
Neutron energy (MeV)
Neutron fluence (1011 neutrons cm
112.0 ± 4.1 70.7 ± 5.8 110.8 ± 4.2 0.1–750 0.1–750 0.1–750
0.456 ± 0.038 0.935 ± 0.065 0.663 ± 0.045 3.88 ± 0.39 6.37 ± 0.64 5.07 ± 0.51
2
)
Irradiation time (h)
Polyethylene (cm)
22.3 39.0 40.7 186.0 219.2 104.1
– – – 2.5 5.1 15.2
range 0.1–750 MeV). The detailed experimental procedures are described in [14]. This method uses 800 MeV proton beams incident on a tungsten (W) target to produce neutron beams. The incident protons are pulsed so time-of-flight techniques can be used to determine the energy spectrum of the neutrons. The irradiation locations are >15 m from the W target. The energy spectrum of the neutron beam can be modified using different thicknesses of polyethylene in the beam-line. The neutron fluences in these irradiations are higher than those in the quasi-monoenergetic irradiations (Table 1), however many of the neutrons having energies below the threshold for producing 10Be and 26Al. On balance the number of neutrons above threshold does not differ much from the quasi-monoenergetic irradiations. Fig. 1 shows the spectra calculated using the Monte Carlo N Particle eXtended (MCNPX) code for 2.5, 5, and 15 cm thickness of polyethylene. As the thickness of polyethylene is increased the neutron flux is suppressed at all energies. The largest suppression is at low energies (<10 MeV), energies not important for this work. The thicknesses of polyethylene used in the irradiations were 2.5, 5.1, and 15.2 cm as shown in Table 1. Since the 10Be production rates in Si and SiO2 are smaller than those for 26Al, the irradiations were designed to produce enough 10 Be atoms for precise AMS measurements. At both iTL and LANSCE, the beam size was larger in area than the target stacks, and the beam intensity is uniform in the plane perpendicular to the
2. Experimental methods 2.1. Neutron irradiations Quasi-monoenergetic neutrons were produced at iThemba LABS (iTL) cyclotron, Somerset West, South Africa, using the (p + Be) reaction where the incident protons had energies of 80 and 120 MeV. Two identical target stacks were irradiated simultaneously in beam lines at 0° and 16° to the incident proton beam. At 0° there are two components to the neutron energy spectra. The first component is the high-energy peak, which contains a significant fraction of the neutrons. The second component is a low-energy tail, which ranges in energy from just above zero to just below the energy of the incident protons. The neutrons forming this tail can produce reactions that constitute a background level (of 10Be and 26Al) that needs to be eliminated. The neutron spectra in the 16° beam line only has the lower energy tail, which closely matches the tail in the 0° beam line. A cross section for a specific neutron energy is obtained by subtracting the yield in the 16° beam line (after suitable normalization) from that measured in the 0° beam line. This procedure effectively minimizes the effect of the neutrons having less than the peak energy, i.e., the tail. The detailed experimental procedures are described in [9]. Energy integrated cross sections were measured at the Los Alamos Neutron Science Center, Los Alamos National Laboratory (LANSCE, LANL), New Mexico using ‘white’ neutron beams (energy
Fig. 1. Neutron energy spectra for beam-line 4FP15R at LANSCE, calculated using MCNPX for different thicknesses of polyethylene in front of the target irradiation position. Energy is plotted as a function of neutrons per steradian per proton per MeV. The number of protons refers to the number of protons that hit the neutron production target. Magnets placed in front of the target position alter the trajectory of the charged particles produced in the polyethylene precluding them from impacting the targets being irradiated by neutrons.
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beam axis throughout the target stacks. The neutron beams were monitored continuously throughout the irradiations. At LANSCE, an upstream transmission chamber recorded the number of neutrons and their energies above a low energy threshold (Elow), which was dependent on the exact irradiation conditions. At iTL, a neutron beam monitor, calibrated before each irradiation was located in the 8° line. Beam alignment was checked using X-ray films at iTL and reusable storage-phosphor imaging plates at LANSCE. 2.2. Targets SiO2 targets (99.9%, 5 5 cm, Goodfellow) and Si targets (99.999%, 1.5 cm or 5 cm in diameter, Leico Industries) were used for both neutron irradiation methods to measure the cross sections O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al. The detailed irradiation settings were described earlier [9,14]. The target stacks consist of one or two targets with 15 mm diameter Al, Au, Ni and Cu monitor foils. The target stack thickness is tailored to keep the attenuation of the neutron beam through the targets 610%. The neutron fluences on the second of the two targets (Si target at iTL) and on the monitor foils were corrected for the attenuation of the neutron beam through the upstream targets. The neutron fluence and irradiation time for each irradiation are shown in Table 1. Correction factors for the 16° irradiation are also shown in the table. 2.3. Radionuclide measurements After irradiation, short-lived nuclides in the targets and monitor foils were measured by non-destructive gamma-ray spectroscopy using high purity Ge detectors; the cross sections measured for several radionuclides produced in the different monitor foils serve as secondary checks of the measured neutron fluences [9,14]. The c-ray measurements in the first few days after irradiations were performed at both iTL and LANSCE, while measurements continued for several months at either the Harvard Cyclotron Laboratory or the Francis H. Burr Proton Therapy Facility, Massachusetts General Hospital. After the c-ray measurements were finished, SiO2 targets were crushed to 1 mm. The Si and the SiO2 target materials, as well as non-irradiated blank materials, were cleaned by acetone and aqua regia. After weighing, the Si targets were dissolved with an HF and HNO3 mixture along with Be (0.14–0.53 mg) and Al (0.7–4 mg) carriers. SiO2 targets were dissolved with an HF, HNO3, and HClO4 mixture along with Be (0.14–1 mg) and Al (0.7–4 mg) carriers. Be and Al were separated using ion chromatographic techniques. They were precipitated in a hydroxide form, converted to oxides and loaded into stainless steel target holders. The 10Be and 26Al concentrations were measured at PRIME Lab, Purdue University. The analytical procedures are similar to those described in [15]. The isotopic ratios were normalized to 10Be and 26Al AMS standards [16,17]. 3. Results and discussion The concentrations of 10Be and 26Al in Si and SiO2 targets and blank materials are given in Table 2. The unirradiated target materials may contain some 10Be and 26Al. The concentrations of each nuclide in the irradiated targets are blank corrected by subtracting the concentration of the nuclide in non-irradiated blanks. The quoted errors include 1r uncertainties of the AMS measurements of samples, blanks and standards, but not the uncertainty (<2–3% [16,17]) in the absolute value of the AMS standards. The measured cross sections for the reactions O(n,x)10Be, Si(n,x)10Be and Si(n,x)26Al are given in Table 3. The 10Be production from O is determined by the difference between the SiO2 and Si
Table 2 10 Be and
26
Al concentration in Si and SiO2 targets.
Target
Irradiation
Energy (MeV)
Mass (g)
10
Be (106 atom/g)
26
Al (106 atom/g)
Si Si SiO2 SiO2 SiO2 SiO2 Si SiO2 SiO2 Si Si SiO2 SiO2
iTL 0° iTL 16° iTL 0° iTL 16° iTL 0° iTL 16° LANSCE LANSCE LANSCE iTL LANSCE iTL LANSCE
112.0 112.0 70.7 70.7 110.8 110.8 0.1–750 0.1–750 0.1–750 Blank Blank Blank Blank
8.543 8.552 50.238 50.237 50.463 50.756 18.305 33.377 22.267 1.318 4.609 5.578 15.549
0.134 ± 0.025 0.073 ± 0.027 5.59 ± 0.18 2.62 ± 0.09 5.56 ± 0.29 4.42 ± 0.12 1.71 ± 0.07 21.3 ± 0.6 16.7 ± 0.8 0.044 ± 0.022 0.014 ± 0.007 0.098 ± 0.008 0.483 ± 0.011
59.7 ± 2.1 44.1 ± 1.9 38.8 ± 1.5 24.5 ± 1.3 30.1 ± 1.2 23.1 ± 1.1 122.3 ± 4.4 96.2 ± 3.8 71.3 ± 3.3 0.08 ± 0.12 <0.01 0.046 ± 0.020 2.04 ± 0.11
Table 3 Cross section measurements. Neutron energy (MeV)
Polyethylene (cm)
Target
O(n,x)10Be (mb)
Si(n,x)10Be (mb)
Si(n,x)26Al (mb)
70.7 110.8 112.0 0.1–750 0.1–750 0.1–750
– – – 2.5 5.1 15.2
SiO2 SiO2 Si Si SiO2 SiO2
1.52 ± 0.15 1.68 ± 0.25 – – 1.57 ± 0.17 1.54 ± 0.17
– – 0.08 ± 0.03 0.21 ± 0.03 – –
14.7 ± 2.4 19.7 ± 2.6 26.8 ± 3.4 14.7 ± 1.6 15.1 ± 1.5 14.0 ± 1.4
targets; the excess 10Be in SiO2 that is not produced from Si is attributed to production from O. The contribution of Si(n,x)10Be is small (but was only measured for the 112 MeV and the white neutrons); we used an average cross section of 0.08 ± 0.03 mb for both the 70.7 MeV and the 110.8 MeV irradiations at iTL and 0.21 ± 0.03 mb for the LANSCE irradiations. The error for each cross-section was calculated by adding the following errors in quadrature: the 1r error of the 10Be and 26Al concentrations (Table 2) and the error in the number of neutrons incident on the target. This latter error was taken to be 10% for measurements made at LANSCE and 7–8% for measurements made at iTL. The new cross section results are shown in Figs. 2–4, along with corresponding cross sections for the proton-induced reactions, O(p,x)10Be, Si(p,x)10Be, and Si(p,x)26Al. This compilation of proton cross sections are taken from literature data [6]. Calculated neutron energy spectra at LANSCE varied slightly depending on the thickness of the polyethylene (Fig. 1) [14]. Although adding 15 cm of polyethylene results in a harder neutron energy spectrum, the main changes in the neutron spectrum are at energies below the threshold energy (Eth) of the O(n,x)10Be (Eth 27 MeV), Si(n,x)10Be (Eth 51 MeV), and Si(n,x)26Al (Eth 16 MeV) reactions [14]. As shown in earlier studies (e.g., [18]), the cross sections for the neutron-induced reactions differ from those of the corresponding proton-induced reactions. Our neutron cross sections of 1.5–1.7 mb for O(n,x)10Be are about a factor of 3–5 higher than the corresponding proton cross section at 70–110 MeV (Fig. 2). The higher cross section for the production of 10Be from O by neutrons is consistent with the expected trend, but the neutron cross sections are not as high as estimates of 2.5–3 mb used in some Monte Carlo model calculations (e.g., [3]). The difference between measured and estimated neutron cross sections indicates that the Monte Carlo models do not entirely capture all the physics needed to correctly model these cross sections. The neutron induced cross section for Si(n,x)10Be at 112 MeV is shown in Fig. 3 along with the compiled proton-induced reaction Si(p,x)10Be [6]. Although the neutron-induced cross section is
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Fig. 2. Cross section for the reactions O(n,x)10Be (solid symbols) [this work]. The dashed line shows the excitation function of O(p,x)10Be compiled by [6]. Only an upper limit (<0.1 mb) for the O(n,x)10Be cross section at 14.6 MeV was published [21].
Fig. 4. Cross section for the production of 26Al from Si. Solid circles are measured neutron induced cross sections for Si (n,x)26Al by [this work] and open circles are measured by [7]. Both experiments used the same 26Al AMS standard. The dashed line shows the excitation function of Si(p,x)26Al compiled by [6].
Table 4 26 Al/10Be production rate ratio from SiO2.
LANSCE LANSCE LAMPF Terrestrial quartz Calculation
Fig. 3. Cross section for the reaction Si(n,x)10Be (solid symbols) [this work]. The dashed line shows the excitation function of Si(p,x)10Be compiled by [6].
slightly higher than the corresponding proton cross section, the difference is smaller than that was found for 10Be from O. This finding is not surprising given that the mass difference between Si and 10 Be is larger than that between O and 10Be. New cross sections for Si(n,x)26Al are shown in Fig. 4. Also shown are previously reported measurements for these cross sections at energies <38 MeV [7] and the compiled proton induced excitation function for Si(p,x)26Al [6]. Unlike the cross sections for the production of 10Be from O, the cross sections for the production 26Al from Si by neutrons are similar to those from protons at low energies (<40 MeV) [7]. At higher neutron energies the measured neutron cross sections for Si (n,x)26Al from this experiment are lower than the corresponding proton cross sections, especially at 71 MeV (15 mb for neutron vs 40 mb for proton). There are
Energy range (MeV)
26
Al/10Be
References
0.1–750 0.1–750 <800 Cosmic rays Cosmic rays
4.51 ± 0.22 4.24 ± 0.27 7.85 ± 0.73 6.68 ± 0.60 6.05
This This [11] [19] [2]
work work after [17] after [17]
numerous reaction pathways at these energies for either neutron or proton induced reactions from Si to 26Al. We cannot explain at this time the low neutron cross sections observed in this work. While the energy integrated production measurements reported here cannot be converted to real cross sections at specific energies the 26Al/10Be production rate ratios in targets exposed to ‘white’ neutrons can be compared to those of targets exposed to the cosmic-ray secondary neutrons on Earth. The 26Al/10Be ratio depends on the energy spectrum of the neutrons, but is independent of the neutron flux. The production rate ratio, 26Al/10Be, is an important parameter for understanding cosmic ray exposure conditions and histories of geologic samples. The 26Al/10Be ratios measured in SiO2 from the LANSCE irradiations are shown in Table 4. The ratio obtained from a previous ‘white’ neutron experiment at the Los Alamos Meson Physics Facility (LAMPF) [11] as well as the ratio found in terrestrial quartz [19] and in theoretical calculations [2] are also shown in Table 4. Note, the published ratios in [11] and [19] must be increased by a factor of 1.106 due to the renormalized 10Be AMS standard [17]. Although the neutron energy spectra for two LANSCE irradiations (with 5.1 and 15.2 cm polyethylene shielding) are slightly different, the 26Al/10Be production rate ratios in SiO2 from these two experiments are very similar, but consistently lower than those obtained by other methods. The 26Al/10Be ratio measured in a previous ‘white’ neutron experiment at LAMPF [11] is 75% higher than the ratio obtained in this work (Table 4), while the ratio in terrestrial quartz [19] is 50% higher than the ratio obtained in this work. The difference between the 26Al/10Be ratios obtained in the white neutron experiments can be explained by differences in the target positions. In the earlier
M.W. Caffee et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 479–483
LAMPF irradiations the target position was located 90° off the beam axis. The neutron spectrum was moderated by the presence in the beam stop of a large copper block filled with recirculating water. This additional shielding moderated the neutron spectrum, producing a softer spectrum. Nonetheless, the neutron spectra for these different irradiations differed enough to generate different 26 Al/10Be ratios. In particular, the calculated neutron energy spectrum at sea-level [20] shows relatively more neutrons at 10–100 MeV and less neutrons at 100–1000 MeV than the neutron spectra in Fig. 1, that is a softer spectrum. This observation is relevant to the production of cosmogenic nuclides. It is generally assumed that neutrons in the terrestrial atmosphere have reached equilibrium, i.e., the ratio of low-energy neutrons to high-energy neutrons does not change substantially with altitude. If this assumption does not hold this ratio could vary in terrestrial samples as a function of altitude.
4. Conclusions The cross sections for the reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al were measured at neutron energies of 71 and 112 MeV. The neutron cross sections for O(n,x)10Be are about a factor of 3–5 higher than the corresponding proton cross sections at 70–110 MeV, but 30–50% lower than neutron cross sections estimated by some Monte Carlo models. The neutron induced cross section for Si(n,x)10Be at 112 MeV is slightly higher than the corresponding proton cross section. Cross sections for the production of 26 Al from Si by neutrons are similar to those from protons at low energies (<40 MeV) [7] while at higher neutron energies the measured neutron cross sections for Si(n,x)26Al from this experiment are lower than the corresponding proton cross sections. The 26 Al/10Be ratio measured using ‘white’ neutrons is considerably lower than that observed in terrestrial materials, indicating a dependence of this ratio on the hardness of the neutron spectrum.
Acknowledgements This work was supported by the NASA Grants NAG5-10538, NNG06GF22G, and CO503-0018-0005 and NSF Grants EAR0345255, EAR-0345817, EAR-0851981, and EAR-0345820. This work also benefited from the use of the LANSCE funded under the auspices of the US DOE by LANL under contract W-7405ENG-36 and iThemba LABS, South Africa. We thank D.T.L. Jones, P. Binns and K. Langen for help with the irradiations at iTL and A. Buffler, F.D. Brooks, M.S. Allie and their students at the University of Cape Town for their expertise in developing the cross section measurement technique and assistance in making these measurements.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Cross section measurements at neutron energies 71 and 112 MeV and energy integrated cross section measurements (0.1 < En < 750 MeV) for the neutron induced reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al M.W. Caffee a,⇑, K. Nishiizumi b, J.M. Sisterson c, J. Ullmann d, K.C. Welten b a
Department of Physics, Purdue University, West Lafayette, IN 47907-1396, USA Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA Francis H. Burr Proton Therapy Center, Massachusetts General Hospital, Boston, MA 02114, USA d LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b c
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 1 April 2012 Available online 23 July 2012 Keywords: Neutron induced reaction Cross sections Cosmogenic nuclides Long-lived nuclides Accelerator mass spectrometry
a b s t r a c t The cross sections for the reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al were measured at neutron energies 71 and 112 MeV. The neutron cross sections for O(n,x)10Be are higher than the corresponding proton cross sections at 70–110 MeV and the neutron induced cross section for Si(n,x)10Be at 112 MeV is slightly higher than the corresponding proton cross section. Cross sections for the production of 26Al from Si are similar to those from protons at low energies (<40 MeV) while at higher energies the measured neutron cross sections for Si(n,x)26Al are lower than the corresponding proton cross sections. Energy integrated (average) cross sections for these reactions were measured using ‘white’ neutron beams (0.1 < En < 750 MeV). The 26Al/10Be production rate ratio from SiO2 measured using ‘white’ neutrons is considerably lower than that observed in terrestrial quartz (SiO2). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Oxygen and silicon are the major constituents of solids found on the surfaces of Earth, the moon, and stony meteorites. The shortand long-lived radionuclides (e.g., 22Na, 10Be, 26Al), as well as stable nuclides, produced in these environments by cosmic-ray interactions can be analyzed to provide clues about the exposure history of the sample and in some instances the history of the cosmic rays (e.g., [1]). Techniques used to measure these cosmogenic nuclides include gamma-ray spectroscopy, particularly for the short-lived radionuclides, accelerator mass spectrometry (AMS) for the longlived radionuclides, and noble gas mass spectrometry. For many applications, both terrestrial and extraterrestrial, interpretation of the cosmogenic nuclide measurements requires knowledge of the production rates. These production rates are relatively low, especially on Earth’s surface; the primary means of determining them is by using geologic samples having a known chemical composition that have been exposed to cosmic rays for a known period of time. For terrestrial samples the exposure durations are typically >10 kyr. When determined in this manner the production rates for a specific nuclide on Earth’s surface integrate over many 22-year solar cycles, include secular changes in Earth’s magnetic field,
⇑ Corresponding author. Tel.: +1 765 494 2586. E-mail address: [email protected] (M.W. Caffee). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.07.011
and may also reflect changes in the cosmic ray flux in the inner solar system. An alternative approach to geologic calibration is the use of physics-based production rates (e.g., [2]). Production rates determined in this fashion eliminate multiple factors that influence the production of cosmogenic nuclides in geologic samples. Physics-based production rates could extend the applicability of cosmogenic nuclides to sample geometries and compositions more complicated than those seen in the geologic calibration samples. A physics-based model for cosmogenic nuclide production rates requires accurate cross section measurements over a wide energy range for the interactions of cosmic ray particles with all the major components of the sample under study (e.g., [3,4]). In extraterrestrial materials, cosmogenic nuclides are produced by solar cosmic rays (SCR), galactic cosmic rays (GCR) and the secondary neutrons produced by GCRs. SCR interactions occur in the uppermost few centimeters, primary GCR protons penetrate further into the surface, and the secondary neutron cascade penetrates to even greater depths (e.g., [4]). The rate of production from each of these primary or secondary particles depends on parameters such as the size of the body and the depth of the sample within that body. A lunar surface sample for example, could conceivably contain cosmogenic nuclides from all three of these sources. On Earth, galactic cosmic rays interact with Earth’s atmosphere, producing a secondary cascade of particles, including a significant number of neutrons. It is this secondary neutron cascade that is the principal source of cosmogenic nuclides produced on
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Earth’s surface (e.g., [5]). A comprehensive compilation of reactions and their products must necessarily consider proton and neutron reactions. Most of the relevant cross sections for proton-induced reactions, such as those producing 10Be, 26Al, and 36Cl, have been measured over a wide energy range [6]. There are however very few cross section measurements for neutron-induced reactions at energies higher than 15 MeV (e.g., [7,8]). Measurements of relevant neutron cross sections are sparse. Some estimates for neutron cross sections have been made specifically for cosmogenic nuclide studies (e.g., [3,4]). There are several strategies that are used to derive these estimates: (1) use the cross section for the corresponding proton-induced reaction; (2) use cross sections derived from nuclear physics models and calculations; (3) estimate the cross section for neutron-induced reactions from thick target simulation experiments [3] or from experimental results of lunar cores and meteorites exposed to cosmic rays [4]. None of these strategies is ideal in all cases as has been shown previously for short-lived radionuclides produced from Cu, Ti, Fe and Ni (e.g., [9,10]). In this work, we measured cross sections of 10Be and 26Al from O and Si using quasi-monenergetic neutrons with energies of 71 and 112 MeV. We also exposed SiO2 and Si targets to ‘white’ neutrons, neutrons that not monoenergetic. These new cross section measurements at high neutron energies, although few in number, for the neutron-induced reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al, are an essential first step toward improving the input data for the theoretical models used in the analyses of the cosmogenic nuclides. For the ‘white’ neutron irradiations we cannot reconstruct excitation functions but we can determine the production rate integrated over the entire energy spectrum (energy integrated) and we can also calculate the 26Al/10Be production rate ratios. Some production rates of cosmogenic nuclides from ‘white’ neutrons have already been reported: 10Be and 26Al from SiO2 or Si [11], 10Be from Cu, Ag, and Au [12], and 36Cl from Cl, K, and Ca [13]. The energy spectrum of the ‘white’ neutrons, for example at LANSCE, is not necessarily the same as secondary neutrons in the Earth’s atmosphere so we do not expect a rigorous correspondence between terrestrial cosmogenic nuclide production rates and ‘white’ neutron production rates.
Table 1 Irradiation parameters. Target
Si SiO2 SiO2 Si SiO2 SiO2
Neutron energy (MeV)
Neutron fluence (1011 neutrons cm
112.0 ± 4.1 70.7 ± 5.8 110.8 ± 4.2 0.1–750 0.1–750 0.1–750
0.456 ± 0.038 0.935 ± 0.065 0.663 ± 0.045 3.88 ± 0.39 6.37 ± 0.64 5.07 ± 0.51
2
)
Irradiation time (h)
Polyethylene (cm)
22.3 39.0 40.7 186.0 219.2 104.1
– – – 2.5 5.1 15.2
range 0.1–750 MeV). The detailed experimental procedures are described in [14]. This method uses 800 MeV proton beams incident on a tungsten (W) target to produce neutron beams. The incident protons are pulsed so time-of-flight techniques can be used to determine the energy spectrum of the neutrons. The irradiation locations are >15 m from the W target. The energy spectrum of the neutron beam can be modified using different thicknesses of polyethylene in the beam-line. The neutron fluences in these irradiations are higher than those in the quasi-monoenergetic irradiations (Table 1), however many of the neutrons having energies below the threshold for producing 10Be and 26Al. On balance the number of neutrons above threshold does not differ much from the quasi-monoenergetic irradiations. Fig. 1 shows the spectra calculated using the Monte Carlo N Particle eXtended (MCNPX) code for 2.5, 5, and 15 cm thickness of polyethylene. As the thickness of polyethylene is increased the neutron flux is suppressed at all energies. The largest suppression is at low energies (<10 MeV), energies not important for this work. The thicknesses of polyethylene used in the irradiations were 2.5, 5.1, and 15.2 cm as shown in Table 1. Since the 10Be production rates in Si and SiO2 are smaller than those for 26Al, the irradiations were designed to produce enough 10 Be atoms for precise AMS measurements. At both iTL and LANSCE, the beam size was larger in area than the target stacks, and the beam intensity is uniform in the plane perpendicular to the
2. Experimental methods 2.1. Neutron irradiations Quasi-monoenergetic neutrons were produced at iThemba LABS (iTL) cyclotron, Somerset West, South Africa, using the (p + Be) reaction where the incident protons had energies of 80 and 120 MeV. Two identical target stacks were irradiated simultaneously in beam lines at 0° and 16° to the incident proton beam. At 0° there are two components to the neutron energy spectra. The first component is the high-energy peak, which contains a significant fraction of the neutrons. The second component is a low-energy tail, which ranges in energy from just above zero to just below the energy of the incident protons. The neutrons forming this tail can produce reactions that constitute a background level (of 10Be and 26Al) that needs to be eliminated. The neutron spectra in the 16° beam line only has the lower energy tail, which closely matches the tail in the 0° beam line. A cross section for a specific neutron energy is obtained by subtracting the yield in the 16° beam line (after suitable normalization) from that measured in the 0° beam line. This procedure effectively minimizes the effect of the neutrons having less than the peak energy, i.e., the tail. The detailed experimental procedures are described in [9]. Energy integrated cross sections were measured at the Los Alamos Neutron Science Center, Los Alamos National Laboratory (LANSCE, LANL), New Mexico using ‘white’ neutron beams (energy
Fig. 1. Neutron energy spectra for beam-line 4FP15R at LANSCE, calculated using MCNPX for different thicknesses of polyethylene in front of the target irradiation position. Energy is plotted as a function of neutrons per steradian per proton per MeV. The number of protons refers to the number of protons that hit the neutron production target. Magnets placed in front of the target position alter the trajectory of the charged particles produced in the polyethylene precluding them from impacting the targets being irradiated by neutrons.
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beam axis throughout the target stacks. The neutron beams were monitored continuously throughout the irradiations. At LANSCE, an upstream transmission chamber recorded the number of neutrons and their energies above a low energy threshold (Elow), which was dependent on the exact irradiation conditions. At iTL, a neutron beam monitor, calibrated before each irradiation was located in the 8° line. Beam alignment was checked using X-ray films at iTL and reusable storage-phosphor imaging plates at LANSCE. 2.2. Targets SiO2 targets (99.9%, 5 5 cm, Goodfellow) and Si targets (99.999%, 1.5 cm or 5 cm in diameter, Leico Industries) were used for both neutron irradiation methods to measure the cross sections O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al. The detailed irradiation settings were described earlier [9,14]. The target stacks consist of one or two targets with 15 mm diameter Al, Au, Ni and Cu monitor foils. The target stack thickness is tailored to keep the attenuation of the neutron beam through the targets 610%. The neutron fluences on the second of the two targets (Si target at iTL) and on the monitor foils were corrected for the attenuation of the neutron beam through the upstream targets. The neutron fluence and irradiation time for each irradiation are shown in Table 1. Correction factors for the 16° irradiation are also shown in the table. 2.3. Radionuclide measurements After irradiation, short-lived nuclides in the targets and monitor foils were measured by non-destructive gamma-ray spectroscopy using high purity Ge detectors; the cross sections measured for several radionuclides produced in the different monitor foils serve as secondary checks of the measured neutron fluences [9,14]. The c-ray measurements in the first few days after irradiations were performed at both iTL and LANSCE, while measurements continued for several months at either the Harvard Cyclotron Laboratory or the Francis H. Burr Proton Therapy Facility, Massachusetts General Hospital. After the c-ray measurements were finished, SiO2 targets were crushed to 1 mm. The Si and the SiO2 target materials, as well as non-irradiated blank materials, were cleaned by acetone and aqua regia. After weighing, the Si targets were dissolved with an HF and HNO3 mixture along with Be (0.14–0.53 mg) and Al (0.7–4 mg) carriers. SiO2 targets were dissolved with an HF, HNO3, and HClO4 mixture along with Be (0.14–1 mg) and Al (0.7–4 mg) carriers. Be and Al were separated using ion chromatographic techniques. They were precipitated in a hydroxide form, converted to oxides and loaded into stainless steel target holders. The 10Be and 26Al concentrations were measured at PRIME Lab, Purdue University. The analytical procedures are similar to those described in [15]. The isotopic ratios were normalized to 10Be and 26Al AMS standards [16,17]. 3. Results and discussion The concentrations of 10Be and 26Al in Si and SiO2 targets and blank materials are given in Table 2. The unirradiated target materials may contain some 10Be and 26Al. The concentrations of each nuclide in the irradiated targets are blank corrected by subtracting the concentration of the nuclide in non-irradiated blanks. The quoted errors include 1r uncertainties of the AMS measurements of samples, blanks and standards, but not the uncertainty (<2–3% [16,17]) in the absolute value of the AMS standards. The measured cross sections for the reactions O(n,x)10Be, Si(n,x)10Be and Si(n,x)26Al are given in Table 3. The 10Be production from O is determined by the difference between the SiO2 and Si
Table 2 10 Be and
26
Al concentration in Si and SiO2 targets.
Target
Irradiation
Energy (MeV)
Mass (g)
10
Be (106 atom/g)
26
Al (106 atom/g)
Si Si SiO2 SiO2 SiO2 SiO2 Si SiO2 SiO2 Si Si SiO2 SiO2
iTL 0° iTL 16° iTL 0° iTL 16° iTL 0° iTL 16° LANSCE LANSCE LANSCE iTL LANSCE iTL LANSCE
112.0 112.0 70.7 70.7 110.8 110.8 0.1–750 0.1–750 0.1–750 Blank Blank Blank Blank
8.543 8.552 50.238 50.237 50.463 50.756 18.305 33.377 22.267 1.318 4.609 5.578 15.549
0.134 ± 0.025 0.073 ± 0.027 5.59 ± 0.18 2.62 ± 0.09 5.56 ± 0.29 4.42 ± 0.12 1.71 ± 0.07 21.3 ± 0.6 16.7 ± 0.8 0.044 ± 0.022 0.014 ± 0.007 0.098 ± 0.008 0.483 ± 0.011
59.7 ± 2.1 44.1 ± 1.9 38.8 ± 1.5 24.5 ± 1.3 30.1 ± 1.2 23.1 ± 1.1 122.3 ± 4.4 96.2 ± 3.8 71.3 ± 3.3 0.08 ± 0.12 <0.01 0.046 ± 0.020 2.04 ± 0.11
Table 3 Cross section measurements. Neutron energy (MeV)
Polyethylene (cm)
Target
O(n,x)10Be (mb)
Si(n,x)10Be (mb)
Si(n,x)26Al (mb)
70.7 110.8 112.0 0.1–750 0.1–750 0.1–750
– – – 2.5 5.1 15.2
SiO2 SiO2 Si Si SiO2 SiO2
1.52 ± 0.15 1.68 ± 0.25 – – 1.57 ± 0.17 1.54 ± 0.17
– – 0.08 ± 0.03 0.21 ± 0.03 – –
14.7 ± 2.4 19.7 ± 2.6 26.8 ± 3.4 14.7 ± 1.6 15.1 ± 1.5 14.0 ± 1.4
targets; the excess 10Be in SiO2 that is not produced from Si is attributed to production from O. The contribution of Si(n,x)10Be is small (but was only measured for the 112 MeV and the white neutrons); we used an average cross section of 0.08 ± 0.03 mb for both the 70.7 MeV and the 110.8 MeV irradiations at iTL and 0.21 ± 0.03 mb for the LANSCE irradiations. The error for each cross-section was calculated by adding the following errors in quadrature: the 1r error of the 10Be and 26Al concentrations (Table 2) and the error in the number of neutrons incident on the target. This latter error was taken to be 10% for measurements made at LANSCE and 7–8% for measurements made at iTL. The new cross section results are shown in Figs. 2–4, along with corresponding cross sections for the proton-induced reactions, O(p,x)10Be, Si(p,x)10Be, and Si(p,x)26Al. This compilation of proton cross sections are taken from literature data [6]. Calculated neutron energy spectra at LANSCE varied slightly depending on the thickness of the polyethylene (Fig. 1) [14]. Although adding 15 cm of polyethylene results in a harder neutron energy spectrum, the main changes in the neutron spectrum are at energies below the threshold energy (Eth) of the O(n,x)10Be (Eth 27 MeV), Si(n,x)10Be (Eth 51 MeV), and Si(n,x)26Al (Eth 16 MeV) reactions [14]. As shown in earlier studies (e.g., [18]), the cross sections for the neutron-induced reactions differ from those of the corresponding proton-induced reactions. Our neutron cross sections of 1.5–1.7 mb for O(n,x)10Be are about a factor of 3–5 higher than the corresponding proton cross section at 70–110 MeV (Fig. 2). The higher cross section for the production of 10Be from O by neutrons is consistent with the expected trend, but the neutron cross sections are not as high as estimates of 2.5–3 mb used in some Monte Carlo model calculations (e.g., [3]). The difference between measured and estimated neutron cross sections indicates that the Monte Carlo models do not entirely capture all the physics needed to correctly model these cross sections. The neutron induced cross section for Si(n,x)10Be at 112 MeV is shown in Fig. 3 along with the compiled proton-induced reaction Si(p,x)10Be [6]. Although the neutron-induced cross section is
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Fig. 2. Cross section for the reactions O(n,x)10Be (solid symbols) [this work]. The dashed line shows the excitation function of O(p,x)10Be compiled by [6]. Only an upper limit (<0.1 mb) for the O(n,x)10Be cross section at 14.6 MeV was published [21].
Fig. 4. Cross section for the production of 26Al from Si. Solid circles are measured neutron induced cross sections for Si (n,x)26Al by [this work] and open circles are measured by [7]. Both experiments used the same 26Al AMS standard. The dashed line shows the excitation function of Si(p,x)26Al compiled by [6].
Table 4 26 Al/10Be production rate ratio from SiO2.
LANSCE LANSCE LAMPF Terrestrial quartz Calculation
Fig. 3. Cross section for the reaction Si(n,x)10Be (solid symbols) [this work]. The dashed line shows the excitation function of Si(p,x)10Be compiled by [6].
slightly higher than the corresponding proton cross section, the difference is smaller than that was found for 10Be from O. This finding is not surprising given that the mass difference between Si and 10 Be is larger than that between O and 10Be. New cross sections for Si(n,x)26Al are shown in Fig. 4. Also shown are previously reported measurements for these cross sections at energies <38 MeV [7] and the compiled proton induced excitation function for Si(p,x)26Al [6]. Unlike the cross sections for the production of 10Be from O, the cross sections for the production 26Al from Si by neutrons are similar to those from protons at low energies (<40 MeV) [7]. At higher neutron energies the measured neutron cross sections for Si (n,x)26Al from this experiment are lower than the corresponding proton cross sections, especially at 71 MeV (15 mb for neutron vs 40 mb for proton). There are
Energy range (MeV)
26
Al/10Be
References
0.1–750 0.1–750 <800 Cosmic rays Cosmic rays
4.51 ± 0.22 4.24 ± 0.27 7.85 ± 0.73 6.68 ± 0.60 6.05
This This [11] [19] [2]
work work after [17] after [17]
numerous reaction pathways at these energies for either neutron or proton induced reactions from Si to 26Al. We cannot explain at this time the low neutron cross sections observed in this work. While the energy integrated production measurements reported here cannot be converted to real cross sections at specific energies the 26Al/10Be production rate ratios in targets exposed to ‘white’ neutrons can be compared to those of targets exposed to the cosmic-ray secondary neutrons on Earth. The 26Al/10Be ratio depends on the energy spectrum of the neutrons, but is independent of the neutron flux. The production rate ratio, 26Al/10Be, is an important parameter for understanding cosmic ray exposure conditions and histories of geologic samples. The 26Al/10Be ratios measured in SiO2 from the LANSCE irradiations are shown in Table 4. The ratio obtained from a previous ‘white’ neutron experiment at the Los Alamos Meson Physics Facility (LAMPF) [11] as well as the ratio found in terrestrial quartz [19] and in theoretical calculations [2] are also shown in Table 4. Note, the published ratios in [11] and [19] must be increased by a factor of 1.106 due to the renormalized 10Be AMS standard [17]. Although the neutron energy spectra for two LANSCE irradiations (with 5.1 and 15.2 cm polyethylene shielding) are slightly different, the 26Al/10Be production rate ratios in SiO2 from these two experiments are very similar, but consistently lower than those obtained by other methods. The 26Al/10Be ratio measured in a previous ‘white’ neutron experiment at LAMPF [11] is 75% higher than the ratio obtained in this work (Table 4), while the ratio in terrestrial quartz [19] is 50% higher than the ratio obtained in this work. The difference between the 26Al/10Be ratios obtained in the white neutron experiments can be explained by differences in the target positions. In the earlier
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LAMPF irradiations the target position was located 90° off the beam axis. The neutron spectrum was moderated by the presence in the beam stop of a large copper block filled with recirculating water. This additional shielding moderated the neutron spectrum, producing a softer spectrum. Nonetheless, the neutron spectra for these different irradiations differed enough to generate different 26 Al/10Be ratios. In particular, the calculated neutron energy spectrum at sea-level [20] shows relatively more neutrons at 10–100 MeV and less neutrons at 100–1000 MeV than the neutron spectra in Fig. 1, that is a softer spectrum. This observation is relevant to the production of cosmogenic nuclides. It is generally assumed that neutrons in the terrestrial atmosphere have reached equilibrium, i.e., the ratio of low-energy neutrons to high-energy neutrons does not change substantially with altitude. If this assumption does not hold this ratio could vary in terrestrial samples as a function of altitude.
4. Conclusions The cross sections for the reactions O(n,x)10Be, Si(n,x)10Be, and Si(n,x)26Al were measured at neutron energies of 71 and 112 MeV. The neutron cross sections for O(n,x)10Be are about a factor of 3–5 higher than the corresponding proton cross sections at 70–110 MeV, but 30–50% lower than neutron cross sections estimated by some Monte Carlo models. The neutron induced cross section for Si(n,x)10Be at 112 MeV is slightly higher than the corresponding proton cross section. Cross sections for the production of 26 Al from Si by neutrons are similar to those from protons at low energies (<40 MeV) [7] while at higher neutron energies the measured neutron cross sections for Si(n,x)26Al from this experiment are lower than the corresponding proton cross sections. The 26 Al/10Be ratio measured using ‘white’ neutrons is considerably lower than that observed in terrestrial materials, indicating a dependence of this ratio on the hardness of the neutron spectrum.
Acknowledgements This work was supported by the NASA Grants NAG5-10538, NNG06GF22G, and CO503-0018-0005 and NSF Grants EAR0345255, EAR-0345817, EAR-0851981, and EAR-0345820. This work also benefited from the use of the LANSCE funded under the auspices of the US DOE by LANL under contract W-7405ENG-36 and iThemba LABS, South Africa. We thank D.T.L. Jones, P. Binns and K. Langen for help with the irradiations at iTL and A. Buffler, F.D. Brooks, M.S. Allie and their students at the University of Cape Town for their expertise in developing the cross section measurement technique and assistance in making these measurements.
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