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Spallation-driven Ultracold Neutron Sources: Concepts for a Next Generation Source
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 51 (2014) 93 – 97
ESS Science Symposium on Neutron Particle Physics at Long Pulse Spallation Sources, NPPatLPS 2013
Spallation-driven ultracold neutron sources: concepts for a next generation source A. R. Younga,*, T. Huegleb, M. Makelab, C. Morrisb, G. Muhrerb, A. Saundersb a
North Carolina State University/Triangle Universities NuclearLaboratory, Raliegh 27695, USA b Los Alamos National Laboratory, Los Alamos 87545, USA
Abstract We present concepts for a next-generation, spallation-driven ultracold neutron source. Our source is based on a 40 liter volume of liquid He held at 1.6 K, with heat removed by “sub-cooled” He technology developed for the Large Hadron Collider’s magnet systems. We report on neutronics modeling for two geometries which utilize well-vetted scattering and absorption data developed for the Lujan Center Mark-3 target, as well as promising moderator materials for the cold neutron “pre-moderator” for this source. © 2013 2013The TheAuthors. Authors. Published by Elsevier © Published by Elsevier B.V.B.V. Selectionand andpeer-review peer-review under responsibility of scientific committee of NPPatLPS Selection under responsibility of scientific committee of NPPatLPS 2013. 2013. Keywords: ultracold neutron, spallation, source, moderator.
1. Introduction Recent years have seen the emergence of the use of ultracold neutrons (UCN) for high precision measurements of the static electric dipole moment, short range forces, and beta-decay as high priority targets for the nuclear and particle physics community, and a recognition of the potential impact of a next generation neutron-antineutron oscillations experiment [1,2,3]. An intense solid deuterium UCN source has been developed at Los Alamos to address some of the needs for UCN-based research [4]. Although this source is in the process of an upgrade,
* Corresponding author. Tel.: +1-919-513-4596; fax: +1-919-515-6438. E-mail address: [email protected]
1875-3892 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of scientific committee of NPPatLPS 2013. doi:10.1016/j.phpro.2013.12.021
94
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
motivated by the need for higher UCN densities and higher integrated currents for UCN experiments we explored the design of a next-generation high power, spallation-driven UCN source. Inspired by a suggestion from Masuda [5] and the source design of Serebrov [6], we will use a 40 l spherical volume of liquid helium (LHe) as a UCN converter. Our design permits us to exploit direct cooling of the converter material using the subcooled (pressurized) He technology developed for the Large Hadron Collider at CERN [7], providing nominally 100 W of cooling power to a superfluid He bath at temperatures of 1.6 K. Although somewhat lower operating temperatures with comparable cooling power may be achievable with this cryogenic technology, we assume our UCN source operates at 1.6 K. These features are the boundary conditions of our study. In what follows we present neutronic modeling of the existing Mark-3 Lujan Center target for this application, and then a less conventional (but effective) geometry with improved performance. We then present some notes concerning a promising cold "pre-moderator" material which might have several useful properties for a next generation UCN source.
Lujan-like Geometry
9.42*107 UCN/s/100μA 30cm
Total heat: 66.7 W Neutron heat: 44.7 W Photon heat: 18.7 W Proton heat: 3.3 W 1.41*108 UCN/s/100W( in the He)
Fig. 1. A neutronic model of a UCN source based on the existing Lujan Center Mark-3 target (side view through cylindrical geometry).
2. Conceptual Neutronic Modeling of a High Current, UCN Production Geometry The Lujan Center [8] Mark-3 production target [9] has been optimized for cold neutrons using the cold beryllium reflector filter concept [10]. While these cold neutrons are intended to be used by small angle scattering and reflectometry we first address the question how efficient this kind of target configuration would be for the production of UCN. The production of UCN in superfluid He is strongly peaked for very cold neutrons with wavelengths near 8.9 Å, so a successful UCN source design must cool spallation neutrons (produced with typical energies MeV) between 1 and 20 MeV to roughly 1 meV and shield the LHe converter material from gamma-ray and particle heating from the spallation target. Based on the design boundary conditions presented in the introduction, a detailed model of the Lujan Center production target was modified to provide cold neutrons to a LHe UCN converter. Fig. 1 shows the modified version of the Lujan target (For more detail, please see [11] and [12]). Based on Monte Carlo
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
transport calculations using MCNPX [13] the neutron flux in the He converter was calculated. The calculated flux was then convoluted with a UCN production function for LHe determined from the work by Korobkina [14]. This resulted in an estimated integrated flux of 9.4×107 UCN/s operating at nominal Lujan Center production beam conditions, 100 µA proton current and 800 MeV proton energy, which is equivalent to 80 KW proton power.
Fig. 2. (a) The “inverse” geomery (side view through a cylindrical geometry). (b) Representative calculated cold neutron fluxes for both the “Lujan like” geometry and the “inverse” geometry.
Under these conditions we have estimated that the energy deposited in the He converter and vessel is approximately 67 W. About 67% of this heat is generated by neutrons, about 28% by photons and 5% by protons. Based on the assumption that one can remove 100 W of heat from the He converter and vessel, this calculation predicts that this target could be operated at power levels as high as 120 KW, which will generate 1.4×108 UCN/s. If one further considers that the production target will need to accommodate more engineering details, the 1.4×108 UCN/s must be seen as an upper limit. We have therefore investigated a new inverse geometry, as shown in Fig. 2. This geometry is based on the backscattering concept first implemented by G. Russell [15] in the Lujan Center target system more than 10 years ago. As shown in Fig. 2a, that tungsten target is a cylinder that surrounds the moderators and the helium bath. One advantage of the backscattering geometry is evident in Fig. 2b, where the neutron spectrum in the He bath for the Lujan Center-like target has a peak equivalent to a 32 K Maxwell spectrum. The Lujan center spectra were measured and reported in [16,17]. It can be seen from this work that the spectra of the liquid hydrogen moderators at Lujan are close to a Maxwell temperature of 32 K and that the measurements agree well with the MCNPX simulations. In the case of the inverse target geometry the peak is equivalent to the peak of a 23 K Maxwell spectrum, increasing the flux of useful neutrons for UCN production. If this target were to be operated as the Lujan target with 100 μA of 800 MeV protons, the number of produced UCNs would be 108 UCN/s/100 μA. While this number is not significantly higher than the one for the Lujan-like target, the heat load is significantly lower, and equals about 14 W/100 μA. Assuming 100 W is the limit for the heat removal from the helium converter volume, the theoretical number for the UCN production rate is 7.1×108 UCN/s. So, even considering that engineering reality will drop the production rate by a factor of two, this design should still provide greater than 3×108 UCN/s, significantly greater than all current or planned UCN sources.
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We have not addressed in detail an optimized strategy to extract UCN from the source volume. From Yoshiki [18], we expect a UCN lifetime of about 4-5 s in the converter, setting the timescale for the maximum achievable densities in the source. Although UCN have been extracted with reasonably high efficiency from LHe sources [5], we note that, in order to extract UCN from our pressurized He source, we will probably need a vacuum isolation foil between the UCN source volume and the guide system to an experimental hall. The issue of UCN transport through cryogenic foils and the design of an appropriate UCN guide system are issues we focus on at present, as well as continuing our refinement of the neutronic design. 3. Triphenylmethane: a candidate cold moderator material for a next generation ultracold neutron source Carbohydrates have long been seen as impractical as moderators for high power spallation sources because of their instability in radiation fields. However T. Huegle has recently shown that triphenylmethane could be a candidate to overcome this problem [19]. It is composed of three aromatic phenyl groups that surround a central carbon atom and forms relatively stable radicals (see Fig. 3). In addition, the density of states of triphenylmethane is very rich in the lower energy regime, due to the motions of the phenyl groups, which makes it a very interesting candidate for a cold neutron moderator and for UCN production. As shown by Muhrer [20], the temperature of the moderator spectrum depends on the effective temperature of the moderator material, which can be derived from the excitation spectrum of the moderator material. In order to lower the spectrum temperature, a rich density of states below the thermal temperature of the moderator is needed. Fig. 3 shows that triphenylmethane fulfills this requirement. As a part of the design optimization, we plan to evaluate and minimize (if possible), the heat loads to the cold moderator system. This process will help define a “radiation hardness” specification for target moderators, and permit a more definitive evaluation of the appropriateness of triphenylmethane for this geometry.
Fig. 3. The chemical structure and neutron excitation function of triphenylmethane.
4. Conclusions The inverse geometry and candidate cold moderators are a work in progress. We expect that further improvements will be identified as we optimize source performance and choice of moderator. Our work suggests that the technology exists to significantly increase available UCN densities at a dedicated, spallation-driven UCN source. Acknowledgements This work supported by NSF 1005233 and DOE grant numbers DE-FG02-97ER41042 and DE-AC52-06 NA25396. This work was partially supported by Readiness in Technical Base and Facilities (RTBF) which is
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
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. References [1] [2] [3] [4] [5] [6] [7]
J. L. Hewett and H. Weerts, “Fundamental Physics at the Intensity Frontier”, SLAC-R-991 (2011). A. S. Kronfield and R. S. Tschirhart, “Project X: Physics Opportunities”, arXiv:1306.5009v2 (2013). Nuclear Science Advisory Committee’s Long Range Plan, “The Frontiers of Nuclear Science”, (2007). A. Saunders et al., Rev. Sci. Instr. 84, 013304 (2013). Y. P. Masuda, private communication and Y. P. Masuda et al., Phys. Rev. Let. 108, 134801 (2012). A. P. Serebrov et al., Physics of the Solid State 52, 1034-1039 (2010). Ph. LeBrun, private communication and Ph. LeBrun and L. Tavian, “The technology of superfluid helium”, in Proceedings of the CERN Accelerator School “Superconductivity and cryogenics for accelerators and detectors”, CERN-2004-008 375 (2004). [8] P. W. Lisowski and K. F. Schoenberg, Nucl. Instr. and Meth. in Phys. Res. A 562, 910 (2006). [9] M. Mocko and G. Muhrer, Nucl. Instr. and Meth. in Phys. Res. A 704, 27 (2013). [10] J.M. Carpenter et al., Nucl. Instr. and Meth. in Phys. Res. 189, 485 (1981). [11] J. A O’Toole et al., 2009 IEEE Nuclear Science Symposium Conference Record, p. 261 (2009). [12] M. Mocko and G. Muhrer, Nucl. Instr. and Meth. In Phys. Res. A 704, 27 (2013). [13] D. B. Pelowitz et al., MCNPX user’s manual version 2.6.0, LA-CP-07-1473 (2008). [14] E. Korobkina et al., Phys. Lett. A 301, 462 (2002). [15] G. J. Russell et al., The 14th international conference on the application of accelerators in research and industry, Denton, Texas, USA, AIP Conf. Proc. 392, 361 (1997). [16] T. Ino et al., Nucl. Instr. and Meth. in Phys. Res. A 525, 496 (2004). [17] G. Muhrer et al., Nucl. Instr. and Meth. in Phys. Res. A 527, 531 (2004). [18] H. Yoshiki et al., Phys. Rev. Lett. 68, 1323 (1992). [19] Th. Huegle et al., “Triphenylmethane, a Possible Moderator Material”, LA-UR-13-26745 (2013). [20] G. Muhrer, Nucl. Instr. and Meth. in Phys. Res. A 664, 38 (2012).
97
ScienceDirect Physics Procedia 51 (2014) 93 – 97
ESS Science Symposium on Neutron Particle Physics at Long Pulse Spallation Sources, NPPatLPS 2013
Spallation-driven ultracold neutron sources: concepts for a next generation source A. R. Younga,*, T. Huegleb, M. Makelab, C. Morrisb, G. Muhrerb, A. Saundersb a
North Carolina State University/Triangle Universities NuclearLaboratory, Raliegh 27695, USA b Los Alamos National Laboratory, Los Alamos 87545, USA
Abstract We present concepts for a next-generation, spallation-driven ultracold neutron source. Our source is based on a 40 liter volume of liquid He held at 1.6 K, with heat removed by “sub-cooled” He technology developed for the Large Hadron Collider’s magnet systems. We report on neutronics modeling for two geometries which utilize well-vetted scattering and absorption data developed for the Lujan Center Mark-3 target, as well as promising moderator materials for the cold neutron “pre-moderator” for this source. © 2013 2013The TheAuthors. Authors. Published by Elsevier © Published by Elsevier B.V.B.V. Selectionand andpeer-review peer-review under responsibility of scientific committee of NPPatLPS Selection under responsibility of scientific committee of NPPatLPS 2013. 2013. Keywords: ultracold neutron, spallation, source, moderator.
1. Introduction Recent years have seen the emergence of the use of ultracold neutrons (UCN) for high precision measurements of the static electric dipole moment, short range forces, and beta-decay as high priority targets for the nuclear and particle physics community, and a recognition of the potential impact of a next generation neutron-antineutron oscillations experiment [1,2,3]. An intense solid deuterium UCN source has been developed at Los Alamos to address some of the needs for UCN-based research [4]. Although this source is in the process of an upgrade,
* Corresponding author. Tel.: +1-919-513-4596; fax: +1-919-515-6438. E-mail address: [email protected]
1875-3892 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of scientific committee of NPPatLPS 2013. doi:10.1016/j.phpro.2013.12.021
94
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
motivated by the need for higher UCN densities and higher integrated currents for UCN experiments we explored the design of a next-generation high power, spallation-driven UCN source. Inspired by a suggestion from Masuda [5] and the source design of Serebrov [6], we will use a 40 l spherical volume of liquid helium (LHe) as a UCN converter. Our design permits us to exploit direct cooling of the converter material using the subcooled (pressurized) He technology developed for the Large Hadron Collider at CERN [7], providing nominally 100 W of cooling power to a superfluid He bath at temperatures of 1.6 K. Although somewhat lower operating temperatures with comparable cooling power may be achievable with this cryogenic technology, we assume our UCN source operates at 1.6 K. These features are the boundary conditions of our study. In what follows we present neutronic modeling of the existing Mark-3 Lujan Center target for this application, and then a less conventional (but effective) geometry with improved performance. We then present some notes concerning a promising cold "pre-moderator" material which might have several useful properties for a next generation UCN source.
Lujan-like Geometry
9.42*107 UCN/s/100μA 30cm
Total heat: 66.7 W Neutron heat: 44.7 W Photon heat: 18.7 W Proton heat: 3.3 W 1.41*108 UCN/s/100W( in the He)
Fig. 1. A neutronic model of a UCN source based on the existing Lujan Center Mark-3 target (side view through cylindrical geometry).
2. Conceptual Neutronic Modeling of a High Current, UCN Production Geometry The Lujan Center [8] Mark-3 production target [9] has been optimized for cold neutrons using the cold beryllium reflector filter concept [10]. While these cold neutrons are intended to be used by small angle scattering and reflectometry we first address the question how efficient this kind of target configuration would be for the production of UCN. The production of UCN in superfluid He is strongly peaked for very cold neutrons with wavelengths near 8.9 Å, so a successful UCN source design must cool spallation neutrons (produced with typical energies MeV) between 1 and 20 MeV to roughly 1 meV and shield the LHe converter material from gamma-ray and particle heating from the spallation target. Based on the design boundary conditions presented in the introduction, a detailed model of the Lujan Center production target was modified to provide cold neutrons to a LHe UCN converter. Fig. 1 shows the modified version of the Lujan target (For more detail, please see [11] and [12]). Based on Monte Carlo
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
transport calculations using MCNPX [13] the neutron flux in the He converter was calculated. The calculated flux was then convoluted with a UCN production function for LHe determined from the work by Korobkina [14]. This resulted in an estimated integrated flux of 9.4×107 UCN/s operating at nominal Lujan Center production beam conditions, 100 µA proton current and 800 MeV proton energy, which is equivalent to 80 KW proton power.
Fig. 2. (a) The “inverse” geomery (side view through a cylindrical geometry). (b) Representative calculated cold neutron fluxes for both the “Lujan like” geometry and the “inverse” geometry.
Under these conditions we have estimated that the energy deposited in the He converter and vessel is approximately 67 W. About 67% of this heat is generated by neutrons, about 28% by photons and 5% by protons. Based on the assumption that one can remove 100 W of heat from the He converter and vessel, this calculation predicts that this target could be operated at power levels as high as 120 KW, which will generate 1.4×108 UCN/s. If one further considers that the production target will need to accommodate more engineering details, the 1.4×108 UCN/s must be seen as an upper limit. We have therefore investigated a new inverse geometry, as shown in Fig. 2. This geometry is based on the backscattering concept first implemented by G. Russell [15] in the Lujan Center target system more than 10 years ago. As shown in Fig. 2a, that tungsten target is a cylinder that surrounds the moderators and the helium bath. One advantage of the backscattering geometry is evident in Fig. 2b, where the neutron spectrum in the He bath for the Lujan Center-like target has a peak equivalent to a 32 K Maxwell spectrum. The Lujan center spectra were measured and reported in [16,17]. It can be seen from this work that the spectra of the liquid hydrogen moderators at Lujan are close to a Maxwell temperature of 32 K and that the measurements agree well with the MCNPX simulations. In the case of the inverse target geometry the peak is equivalent to the peak of a 23 K Maxwell spectrum, increasing the flux of useful neutrons for UCN production. If this target were to be operated as the Lujan target with 100 μA of 800 MeV protons, the number of produced UCNs would be 108 UCN/s/100 μA. While this number is not significantly higher than the one for the Lujan-like target, the heat load is significantly lower, and equals about 14 W/100 μA. Assuming 100 W is the limit for the heat removal from the helium converter volume, the theoretical number for the UCN production rate is 7.1×108 UCN/s. So, even considering that engineering reality will drop the production rate by a factor of two, this design should still provide greater than 3×108 UCN/s, significantly greater than all current or planned UCN sources.
95
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A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
We have not addressed in detail an optimized strategy to extract UCN from the source volume. From Yoshiki [18], we expect a UCN lifetime of about 4-5 s in the converter, setting the timescale for the maximum achievable densities in the source. Although UCN have been extracted with reasonably high efficiency from LHe sources [5], we note that, in order to extract UCN from our pressurized He source, we will probably need a vacuum isolation foil between the UCN source volume and the guide system to an experimental hall. The issue of UCN transport through cryogenic foils and the design of an appropriate UCN guide system are issues we focus on at present, as well as continuing our refinement of the neutronic design. 3. Triphenylmethane: a candidate cold moderator material for a next generation ultracold neutron source Carbohydrates have long been seen as impractical as moderators for high power spallation sources because of their instability in radiation fields. However T. Huegle has recently shown that triphenylmethane could be a candidate to overcome this problem [19]. It is composed of three aromatic phenyl groups that surround a central carbon atom and forms relatively stable radicals (see Fig. 3). In addition, the density of states of triphenylmethane is very rich in the lower energy regime, due to the motions of the phenyl groups, which makes it a very interesting candidate for a cold neutron moderator and for UCN production. As shown by Muhrer [20], the temperature of the moderator spectrum depends on the effective temperature of the moderator material, which can be derived from the excitation spectrum of the moderator material. In order to lower the spectrum temperature, a rich density of states below the thermal temperature of the moderator is needed. Fig. 3 shows that triphenylmethane fulfills this requirement. As a part of the design optimization, we plan to evaluate and minimize (if possible), the heat loads to the cold moderator system. This process will help define a “radiation hardness” specification for target moderators, and permit a more definitive evaluation of the appropriateness of triphenylmethane for this geometry.
Fig. 3. The chemical structure and neutron excitation function of triphenylmethane.
4. Conclusions The inverse geometry and candidate cold moderators are a work in progress. We expect that further improvements will be identified as we optimize source performance and choice of moderator. Our work suggests that the technology exists to significantly increase available UCN densities at a dedicated, spallation-driven UCN source. Acknowledgements This work supported by NSF 1005233 and DOE grant numbers DE-FG02-97ER41042 and DE-AC52-06 NA25396. This work was partially supported by Readiness in Technical Base and Facilities (RTBF) which is
A.R. Young et al. / Physics Procedia 51 (2014) 93 – 97
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. References [1] [2] [3] [4] [5] [6] [7]
J. L. Hewett and H. Weerts, “Fundamental Physics at the Intensity Frontier”, SLAC-R-991 (2011). A. S. Kronfield and R. S. Tschirhart, “Project X: Physics Opportunities”, arXiv:1306.5009v2 (2013). Nuclear Science Advisory Committee’s Long Range Plan, “The Frontiers of Nuclear Science”, (2007). A. Saunders et al., Rev. Sci. Instr. 84, 013304 (2013). Y. P. Masuda, private communication and Y. P. Masuda et al., Phys. Rev. Let. 108, 134801 (2012). A. P. Serebrov et al., Physics of the Solid State 52, 1034-1039 (2010). Ph. LeBrun, private communication and Ph. LeBrun and L. Tavian, “The technology of superfluid helium”, in Proceedings of the CERN Accelerator School “Superconductivity and cryogenics for accelerators and detectors”, CERN-2004-008 375 (2004). [8] P. W. Lisowski and K. F. Schoenberg, Nucl. Instr. and Meth. in Phys. Res. A 562, 910 (2006). [9] M. Mocko and G. Muhrer, Nucl. Instr. and Meth. in Phys. Res. A 704, 27 (2013). [10] J.M. Carpenter et al., Nucl. Instr. and Meth. in Phys. Res. 189, 485 (1981). [11] J. A O’Toole et al., 2009 IEEE Nuclear Science Symposium Conference Record, p. 261 (2009). [12] M. Mocko and G. Muhrer, Nucl. Instr. and Meth. In Phys. Res. A 704, 27 (2013). [13] D. B. Pelowitz et al., MCNPX user’s manual version 2.6.0, LA-CP-07-1473 (2008). [14] E. Korobkina et al., Phys. Lett. A 301, 462 (2002). [15] G. J. Russell et al., The 14th international conference on the application of accelerators in research and industry, Denton, Texas, USA, AIP Conf. Proc. 392, 361 (1997). [16] T. Ino et al., Nucl. Instr. and Meth. in Phys. Res. A 525, 496 (2004). [17] G. Muhrer et al., Nucl. Instr. and Meth. in Phys. Res. A 527, 531 (2004). [18] H. Yoshiki et al., Phys. Rev. Lett. 68, 1323 (1992). [19] Th. Huegle et al., “Triphenylmethane, a Possible Moderator Material”, LA-UR-13-26745 (2013). [20] G. Muhrer, Nucl. Instr. and Meth. in Phys. Res. A 664, 38 (2012).
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