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Experimental and theoretical total neutron scattering cross-section of water confined in silica microspheres
Nuclear Instruments and Methods in Physics Research A 681 (2012) 91–93
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Experimental and theoretical total neutron scattering cross-section of water confined in silica microspheres G. Muhrer n, M. Hartl, M. Mocko, F. Tovesson, L. Daemen Los Alamos National Laboratory, Los Alamos, 87545 NM, USA
a r t i c l e i n f o
abstract
Article history: Received 30 January 2012 Accepted 5 April 2012 Available online 13 April 2012
In the search for moderator materials encapsulated materials have been discussed, but very little is known regarding the effect of encapsulation on neutron moderation properties. As a first step toward a better understanding, we present the measured total neutron cross-section of water confined in silica microspheres and compare the measured data to the predicted theoretical cross-section. & 2012 Elsevier B.V. All rights reserved.
Keywords: Neutron scattering Neutron moderator Scattering kernel
1. Introduction Over the last 40 years, the design of moderators for neutron scattering facilities was restricted to a handful of materials, such as light water and liquid hydrogen, that have great radiation resistance and can be easily kept at a constant temperature. While these materials have served very well, after decades of optimization study opportunities for further improvement are rather limited. Hence, there is an increasing effort in the neutron source design community to develop advanced concepts (e.g. [1,2]). While these approaches use traditional moderator materials in advanced geometries, efforts are also underway to investigate none traditional moderator materials. In particular, hydrogenous materials in confined spaces have become of interest. Dore et al. [3,4] have shown that water confined in mesoporous silica can be supercooled and forms cubic ice reproducibly. These are two very interesting facts that have the potential of addressing two long standing limitations in water moderator design: temperature range and reproducibility of the ice structure. While light water is the most commonly used neutron moderator, its operating temperature is limited to its liquid phase. Ice has been used for neutron moderation, but the formation of gases (H2 and O2) over time in a radiation field forces one to melt the moderator periodically, which is not practical at high power. It has been hypothesized that the confinement of water will significantly hinder the gas production by reducing the mobility of H and O radicals that are formed in a radiation field and therefore preventing recombination of the radicals to H2 and O2. Naturally
n
Corresponding author. E-mail address: [email protected] (G. Muhrer).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.04.015
there will be limitations with regard to the supercooling process and the formation of ice in silica microspheres. While supercooling may only occur with very slow cooling and with very pure water, hexagonal ice (Ih), which is the modification that water will freeze in at 1 atm, will eventually form if the temperature is low enough or if the system is disturbed. The classical types of mesoporous silica allow the confined water a too high volatility. In order to remedy this shortcoming, Hartl et al. [5] have developed a technique that encloses the water within silica microspheres, with the spheres aligning themselves in the form of Gaussian chains. In this paper, we will present the measured total cross-section of the confined water, compare it with regular water, and with a theoretical model for the confined water.
2. Experimental total neutron scattering cross-section The water-containing silica microspheres were synthesized from tetraethoxysilane (TEOS) in a water/ethanol solution as described in Ref. [5]. The water content was measured thermogravimetrically to be 40 wt.%. The measurement of the total neutron scattering cross-section was performed at the Manuel Lujan, Jr. Neutron Scattering Center [6] on flight path 5. The experimental setup for total cross-section measurements on this beam line has been discussed previously in Ref. [7]. For the measurement presented in this paper, we only modified the setup slightly. The fission chamber was replaced by a BF3 detector, which was placed about 4 m downstream of the sample location. The modified geometry can be seen in Fig. 1. We measured five samples: (a) an aluminum sample holder filled with water confined in silica microspheres (see Ref. [5] for
92
G. Muhrer et al. / Nuclear Instruments and Methods in Physics Research A 681 (2012) 91–93
Fig. 1. Layout of Flight Path 5 at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory: (1) indicates the position of the sample which is placed in front of the collimation, (2) points to the polyethylene wall which shields the hutch from neutrons scattering off the sample and the collimation, and (3) indicates the detector position. (Sightly adjusted geometry as compared to that shown in Ref. [7].)
chemical properties) with a thickness of 20 mm in the neutron beam direction, (b) an aluminum sample holder filled with dry silica microspheres (see Ref. [5] for the drying process) with a thickness of 20 mm in the beam direction, (c) an empty aluminum sample holder (reference), (d) a quartz cell filled with water confined in silica microspheres with a thickness of 2 mm in the beam direction and (e) an empty quartz cell (reference). Fig. 1 shows that there is about 0.7 m of collimation between the sample and the detector. The collimation system prevents a very large degree scattered neutrons from reaching the detector, and thereby ensures that the collected data very closely resemble the transmitted beam through the sample. To confirm this assumption, two different sample thicknesses were used to measure water in silica microspheres (samples (a) and (d) described above). Additionally, two different thicknesses also ensure that within reasonable counting time, the statistical uncertainty of the measured data is adequate throughout the entire energy range of interest. The transmitted spectra for all the samples were collected in time-offlight mode. The measured spectra were then normalized by the integrated proton beam current on target for the duration of the measurement. The macroscopic total neutron cross-section was calculated using the attenuation equation NðxÞ ¼ Nð0ÞeSx
ð1Þ
with S being the total macroscopic cross-section and x being the thickness of the sample. N(x) and Nð0Þ are the neutron per unit current on target that were detected by the detector for a x-cm thick sample and for the reference run (empty sample holder), respectively. The total microscopic cross-section s is defined as
S ðcm1 Þ ¼ r ðatom=ðb, cmÞÞ s ðb=atomÞ
Fig. 2. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water confined in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to regular light water as measured by Russell [8] (blue squares) and Heinloth [9] (black triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
where r is the atomic density of the sample material. Based on these equations, the total microscopic cross-sections of the light watercontaining silica microspheres and the dry silica microspheres were determined. In the second step, the cross-section of the dry silica microspheres was subtracted from the water-containing silica microsphere cross-section to determine the cross-section of the water that is confined in the silica matrix. The results can be seen in Fig. 2. This figure shows that the total cross-section of regular light water and light water confined in silica microspheres starts to differ at around 500 meV, which is approximately the energy of the OH stretching mode in regular water. The cross-section of the confined water in this range is higher than that of bulk water. At lower energies ð o1 meVÞ the two cross-sections seem to converge again, however, there are only a few data points for water in this range and the uncertainty of the presented measurement in this energy range is too large to come a definitive conclusion.
Fig. 3. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), the regular light water as measured by Russell [8] (blue squares) and Heinloth [9] (black triangles) compared to the theoretical cross-section for regular light water using the LEAPR model (black filled squares). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Theoretical total neutron scattering cross-section 3.1. LEAPR model In the first step, the theoretical total neutron scattering crosssection was calculated using the LEAPR model [10]. The input deck of test problem #9 supplied with the general release of NJOY99 [10] was used. It is based on the model developed by Koppel and Houston [11]. Fig. 3 shows the result of this calculation. This figure shows that the theoretical cross-section for water and the measured data are in very good agreement above 1 meV. Below 1 meV the theoretical data and the measured data diverge. This has been already pointed out by Mattes and Keinert [12].
G. Muhrer et al. / Nuclear Instruments and Methods in Physics Research A 681 (2012) 91–93
93
data. However, as shown in Ref. [5], water confined in silica microspheres generates small angle neutron scattering, which is not taken into account in the LEAPR model. 3.2. Small angle scattering correction
Fig. 4. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to the theoretical cross-section for regular light water (black squares) and water confined in silica using the LEAPR model (blue triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
As mentioned in the previous section, Hartl et al. [5] have shown that water confined in silica microspheres generates small angle scattering, which is not included in the LEAPR model. To estimate the small angle scattering contribution to the total neutron cross-section of water, we first estimated its contribution to the total neutron cross-section for the dry silica microspheres. This was done by subtracting the free gas cross-section for silica from the measured data. This result was then corrected, using the scattering length density for water, to estimate the small angle contribution to the total neutron cross-section for water confined in silica microspheres. The results are shown in Fig. 5. This figure shows that the small-angle scattering corrected theoretical total cross-section is in good agreement with the measured data.
4. Conclusion We have shown that the encapsulation of water in silica microspheres changes its total cross-section significantly. We have also shown that the total cross-section can be predicted accurately, when the theoretical data is corrected for the small-angle scattering contribution, which is fairly substantial at longer wavelengths. If porous materials become of interest for neutron moderation, the theoretical models will have to be developed correspondingly in NJOY [10] to include small-angle scattering. In closing we would like to alert the reader to the fact that we have performed a measurement of a moderator consisting of water confined in silica microspheres. The results will be presented in a later publication.
Acknowledgment
Fig. 5. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to the theoretical cross-section for water confined in silica using the LEAPR model corrected for the small angle scattering contribution (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
However, since the measured data for water confined in silica microspheres does only extent to about 1 meV, this issue shall be ignored in the present discussion. Hartl et al. [5] have shown that confinement of water in the pores of silica microspheres has a significant impact on the excitation spectrum of water. In particular, it can be seen from their work that the density of state of the OH-bending mode was significantly reduced. If this is taken into account the theoretical total crosssection based on the LEAPR model changes as shown in Fig. 4. It can be observed in this figure that the LEAPR model, based on the excitation spectrum for water confined in silica, agrees with the measured cross-section very well above 20 meV. Below 20 meV the theoretical data diverge from the measured
This work was supported by Readiness in Technical Base and Facilities (RTBF) which is funded by the Department of Energy’s Office of National Nuclear Security Administration. It has benefited from the use of the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396. References [1] D.V. Baxter, J. Leung, H. Kaiser, S. Ansell, G. Muhrer E.B. Iverson, P.D. Ferguson, Nuclear Instrument and Methods A, in press, doi:10.106/j.nima.2010.12.027. [2] G. Muhrer, M.A. Hartl, L.L. Daemen, J. Ryu, Nuclear Instrument and Methods 578 (2007) 463. [3] J.C. Dore, Journal of Chemical Physics 258 (2000) 327. [4] D.C. Steytler, J.C. Dore, C.J. Wright, Journal of Physical Chemistry 87 (1983) 2458. [5] M. Hartl, L.L. Daemen, G. Muhrer, Journal of Microporous and Mesoporous Materials, http://dx.doi.org/10.1016/j.micromeso.2012.04.025. [6] P.W. Lisowski, K.F. Schoenberg, Nuclear Instrument and Methods A 562 (2006) 910. [7] G. Muhrer, M. Hartl, L. Daemen, F. Tovesson, A. Schnegg, M. Russian, E. Schachinger, Nuclear Instrument and Methods A 629 (2011) 251. [8] J.L. Russell Jr., J.M. Neill, J.R. Brown, General Atomic 7581 (1966) 6612. ¨ Physik 163 (1961) 218. [9] K. Heinloth, Zeitschrift fur [10] R.E. MacFarlane, D.W. Muir, The NJOY Nuclear Data Processing system, Los Alamos National Laboratory, LA-12740M, Los Alamos, 1994. [11] J.U. Koppel, D.H. Houston, General Atomic Report GA-8774, 1968. [12] M. Mattes, J. Keinert, Nuclear Data Section Report 0470, IAEA, Vienna, Austria, 2005.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Experimental and theoretical total neutron scattering cross-section of water confined in silica microspheres G. Muhrer n, M. Hartl, M. Mocko, F. Tovesson, L. Daemen Los Alamos National Laboratory, Los Alamos, 87545 NM, USA
a r t i c l e i n f o
abstract
Article history: Received 30 January 2012 Accepted 5 April 2012 Available online 13 April 2012
In the search for moderator materials encapsulated materials have been discussed, but very little is known regarding the effect of encapsulation on neutron moderation properties. As a first step toward a better understanding, we present the measured total neutron cross-section of water confined in silica microspheres and compare the measured data to the predicted theoretical cross-section. & 2012 Elsevier B.V. All rights reserved.
Keywords: Neutron scattering Neutron moderator Scattering kernel
1. Introduction Over the last 40 years, the design of moderators for neutron scattering facilities was restricted to a handful of materials, such as light water and liquid hydrogen, that have great radiation resistance and can be easily kept at a constant temperature. While these materials have served very well, after decades of optimization study opportunities for further improvement are rather limited. Hence, there is an increasing effort in the neutron source design community to develop advanced concepts (e.g. [1,2]). While these approaches use traditional moderator materials in advanced geometries, efforts are also underway to investigate none traditional moderator materials. In particular, hydrogenous materials in confined spaces have become of interest. Dore et al. [3,4] have shown that water confined in mesoporous silica can be supercooled and forms cubic ice reproducibly. These are two very interesting facts that have the potential of addressing two long standing limitations in water moderator design: temperature range and reproducibility of the ice structure. While light water is the most commonly used neutron moderator, its operating temperature is limited to its liquid phase. Ice has been used for neutron moderation, but the formation of gases (H2 and O2) over time in a radiation field forces one to melt the moderator periodically, which is not practical at high power. It has been hypothesized that the confinement of water will significantly hinder the gas production by reducing the mobility of H and O radicals that are formed in a radiation field and therefore preventing recombination of the radicals to H2 and O2. Naturally
n
Corresponding author. E-mail address: [email protected] (G. Muhrer).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.04.015
there will be limitations with regard to the supercooling process and the formation of ice in silica microspheres. While supercooling may only occur with very slow cooling and with very pure water, hexagonal ice (Ih), which is the modification that water will freeze in at 1 atm, will eventually form if the temperature is low enough or if the system is disturbed. The classical types of mesoporous silica allow the confined water a too high volatility. In order to remedy this shortcoming, Hartl et al. [5] have developed a technique that encloses the water within silica microspheres, with the spheres aligning themselves in the form of Gaussian chains. In this paper, we will present the measured total cross-section of the confined water, compare it with regular water, and with a theoretical model for the confined water.
2. Experimental total neutron scattering cross-section The water-containing silica microspheres were synthesized from tetraethoxysilane (TEOS) in a water/ethanol solution as described in Ref. [5]. The water content was measured thermogravimetrically to be 40 wt.%. The measurement of the total neutron scattering cross-section was performed at the Manuel Lujan, Jr. Neutron Scattering Center [6] on flight path 5. The experimental setup for total cross-section measurements on this beam line has been discussed previously in Ref. [7]. For the measurement presented in this paper, we only modified the setup slightly. The fission chamber was replaced by a BF3 detector, which was placed about 4 m downstream of the sample location. The modified geometry can be seen in Fig. 1. We measured five samples: (a) an aluminum sample holder filled with water confined in silica microspheres (see Ref. [5] for
92
G. Muhrer et al. / Nuclear Instruments and Methods in Physics Research A 681 (2012) 91–93
Fig. 1. Layout of Flight Path 5 at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory: (1) indicates the position of the sample which is placed in front of the collimation, (2) points to the polyethylene wall which shields the hutch from neutrons scattering off the sample and the collimation, and (3) indicates the detector position. (Sightly adjusted geometry as compared to that shown in Ref. [7].)
chemical properties) with a thickness of 20 mm in the neutron beam direction, (b) an aluminum sample holder filled with dry silica microspheres (see Ref. [5] for the drying process) with a thickness of 20 mm in the beam direction, (c) an empty aluminum sample holder (reference), (d) a quartz cell filled with water confined in silica microspheres with a thickness of 2 mm in the beam direction and (e) an empty quartz cell (reference). Fig. 1 shows that there is about 0.7 m of collimation between the sample and the detector. The collimation system prevents a very large degree scattered neutrons from reaching the detector, and thereby ensures that the collected data very closely resemble the transmitted beam through the sample. To confirm this assumption, two different sample thicknesses were used to measure water in silica microspheres (samples (a) and (d) described above). Additionally, two different thicknesses also ensure that within reasonable counting time, the statistical uncertainty of the measured data is adequate throughout the entire energy range of interest. The transmitted spectra for all the samples were collected in time-offlight mode. The measured spectra were then normalized by the integrated proton beam current on target for the duration of the measurement. The macroscopic total neutron cross-section was calculated using the attenuation equation NðxÞ ¼ Nð0ÞeSx
ð1Þ
with S being the total macroscopic cross-section and x being the thickness of the sample. N(x) and Nð0Þ are the neutron per unit current on target that were detected by the detector for a x-cm thick sample and for the reference run (empty sample holder), respectively. The total microscopic cross-section s is defined as
S ðcm1 Þ ¼ r ðatom=ðb, cmÞÞ s ðb=atomÞ
Fig. 2. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water confined in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to regular light water as measured by Russell [8] (blue squares) and Heinloth [9] (black triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
where r is the atomic density of the sample material. Based on these equations, the total microscopic cross-sections of the light watercontaining silica microspheres and the dry silica microspheres were determined. In the second step, the cross-section of the dry silica microspheres was subtracted from the water-containing silica microsphere cross-section to determine the cross-section of the water that is confined in the silica matrix. The results can be seen in Fig. 2. This figure shows that the total cross-section of regular light water and light water confined in silica microspheres starts to differ at around 500 meV, which is approximately the energy of the OH stretching mode in regular water. The cross-section of the confined water in this range is higher than that of bulk water. At lower energies ð o1 meVÞ the two cross-sections seem to converge again, however, there are only a few data points for water in this range and the uncertainty of the presented measurement in this energy range is too large to come a definitive conclusion.
Fig. 3. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), the regular light water as measured by Russell [8] (blue squares) and Heinloth [9] (black triangles) compared to the theoretical cross-section for regular light water using the LEAPR model (black filled squares). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Theoretical total neutron scattering cross-section 3.1. LEAPR model In the first step, the theoretical total neutron scattering crosssection was calculated using the LEAPR model [10]. The input deck of test problem #9 supplied with the general release of NJOY99 [10] was used. It is based on the model developed by Koppel and Houston [11]. Fig. 3 shows the result of this calculation. This figure shows that the theoretical cross-section for water and the measured data are in very good agreement above 1 meV. Below 1 meV the theoretical data and the measured data diverge. This has been already pointed out by Mattes and Keinert [12].
G. Muhrer et al. / Nuclear Instruments and Methods in Physics Research A 681 (2012) 91–93
93
data. However, as shown in Ref. [5], water confined in silica microspheres generates small angle neutron scattering, which is not taken into account in the LEAPR model. 3.2. Small angle scattering correction
Fig. 4. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to the theoretical cross-section for regular light water (black squares) and water confined in silica using the LEAPR model (blue triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
As mentioned in the previous section, Hartl et al. [5] have shown that water confined in silica microspheres generates small angle scattering, which is not included in the LEAPR model. To estimate the small angle scattering contribution to the total neutron cross-section of water, we first estimated its contribution to the total neutron cross-section for the dry silica microspheres. This was done by subtracting the free gas cross-section for silica from the measured data. This result was then corrected, using the scattering length density for water, to estimate the small angle contribution to the total neutron cross-section for water confined in silica microspheres. The results are shown in Fig. 5. This figure shows that the small-angle scattering corrected theoretical total cross-section is in good agreement with the measured data.
4. Conclusion We have shown that the encapsulation of water in silica microspheres changes its total cross-section significantly. We have also shown that the total cross-section can be predicted accurately, when the theoretical data is corrected for the small-angle scattering contribution, which is fairly substantial at longer wavelengths. If porous materials become of interest for neutron moderation, the theoretical models will have to be developed correspondingly in NJOY [10] to include small-angle scattering. In closing we would like to alert the reader to the fact that we have performed a measurement of a moderator consisting of water confined in silica microspheres. The results will be presented in a later publication.
Acknowledgment
Fig. 5. Experimental data of the total microscopic neutron cross-section (per H2O molecule) of light water in silica microspheres (black circles: 20 mm pathlength, red squares: 2 mm pathlength), compared to the theoretical cross-section for water confined in silica using the LEAPR model corrected for the small angle scattering contribution (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
However, since the measured data for water confined in silica microspheres does only extent to about 1 meV, this issue shall be ignored in the present discussion. Hartl et al. [5] have shown that confinement of water in the pores of silica microspheres has a significant impact on the excitation spectrum of water. In particular, it can be seen from their work that the density of state of the OH-bending mode was significantly reduced. If this is taken into account the theoretical total crosssection based on the LEAPR model changes as shown in Fig. 4. It can be observed in this figure that the LEAPR model, based on the excitation spectrum for water confined in silica, agrees with the measured cross-section very well above 20 meV. Below 20 meV the theoretical data diverge from the measured
This work was supported by Readiness in Technical Base and Facilities (RTBF) which is funded by the Department of Energy’s Office of National Nuclear Security Administration. It has benefited from the use of the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396. References [1] D.V. Baxter, J. Leung, H. Kaiser, S. Ansell, G. Muhrer E.B. Iverson, P.D. Ferguson, Nuclear Instrument and Methods A, in press, doi:10.106/j.nima.2010.12.027. [2] G. Muhrer, M.A. Hartl, L.L. Daemen, J. Ryu, Nuclear Instrument and Methods 578 (2007) 463. [3] J.C. Dore, Journal of Chemical Physics 258 (2000) 327. [4] D.C. Steytler, J.C. Dore, C.J. Wright, Journal of Physical Chemistry 87 (1983) 2458. [5] M. Hartl, L.L. Daemen, G. Muhrer, Journal of Microporous and Mesoporous Materials, http://dx.doi.org/10.1016/j.micromeso.2012.04.025. [6] P.W. Lisowski, K.F. Schoenberg, Nuclear Instrument and Methods A 562 (2006) 910. [7] G. Muhrer, M. Hartl, L. Daemen, F. Tovesson, A. Schnegg, M. Russian, E. Schachinger, Nuclear Instrument and Methods A 629 (2011) 251. [8] J.L. Russell Jr., J.M. Neill, J.R. Brown, General Atomic 7581 (1966) 6612. ¨ Physik 163 (1961) 218. [9] K. Heinloth, Zeitschrift fur [10] R.E. MacFarlane, D.W. Muir, The NJOY Nuclear Data Processing system, Los Alamos National Laboratory, LA-12740M, Los Alamos, 1994. [11] J.U. Koppel, D.H. Houston, General Atomic Report GA-8774, 1968. [12] M. Mattes, J. Keinert, Nuclear Data Section Report 0470, IAEA, Vienna, Austria, 2005.