R. Young, M. Makela, G. Muhrer, C. Morris, A. Saunders Spallation-Driven Ultracold Neutron Sources: Concepts for a Next Generation Source R. Young, M. Makela, G. Muhrer, C. Morris, A. Saunders

Outline Motivation Extensions of the LANL SD2 source Speculative musings – a high power/high UCN current source geometry

Motivation and Goals UCN experiments: Goal for UCN Source EDM, beta-decay (lifetime and angular correlations), limits on short range forces and gravitational state studies, limits on the electric charge of the neutron, mirror-neutron and dark matter searches, N-Nbar oscillation searches Goal for UCN Source Figure of merit for loading a particular experiment to the highest density involves some combination of total UCN production (“current”) and limiting UCN density in the source →ideally highest density with shortest required storage time All of these efforts currently limited in part by available UCN density!

Area B Source Philosophy: Arrange SD2 converter close to W target In production operation since 2007! (M. Pendlebury’s talk…) Philosophy: Arrange SD2 converter close to W target to take optimal advantage of n-flux Use compact Be “flux trap” to maintain high neutron flux in vicinity of cold moderator and D2 Use well-matched cold moderator to D2 down-scatter cross-section (poly) to optimize UCN production Use relatively fast “flapper” isolation valve to decouple produced UCN from UCN-destroying SD2 (~< 0.3 s open per pulse) Large SD2 cross-section ensures high production rate Flapper valve provides 4-5 times longer system lifetime, increasing equilibrium UCN density

SD2 VOLUME 2 liters FLAPPER VALVE COOLED POLYETHYLENE MODERATOR PROTONS Be GRAPHITE He-COOLED W SPALLATION TARGET

Cold flux: Ar activation UCN flux:V foil activation Leads to consistent interpretation of extracted UCN density in LANL production source… HPGe DETECTOR PROTON BEAMLINE CALIBRATED VOLUME SD2 UCN SOURCE COLD NEUTRON DETECTOR UCN GUIDE (C. Morris) 52±9 UCN/cm3 Cold flux: Ar activation UCN flux:V foil activation A. Saunders et al., Rev. Sci. Instr. 84, 013304 (2013)

Area B source limitations 2 liters of SD2 with “equilibrium” UCN lifetime above 25 ms and relatively thick (> 6 cm typically) Cryogenic cooling available roughly 50 l/hr (> 50 W) flowing two-phase LHe near 4K Current administrative beam limit ~5 uA or ~5 kW thermal power W target size currently limited by previous, administrative constraints on radio-isotope inventory (affected volume available for target)

Extensions/Upgrades Increase current to 20 uA (now 30 uC/pulse, 5 uA) Operational changes… Increase current to 20 uA (now 30 uC/pulse, 5 uA) No heating effects observed with 100 uC/pulse (probably OK) Optimized beam structure: up to 1.85 (measurements) Measurements at low beam power, and interpretation depends on lifetime in SD2 during measurements Result: up to 385 UCN/cm3 at shield wall .74– 1.5×106 UCN/s through 3” guides Also: beam tune and duty factor!

Improved cold moderator liquid H2 →x1.2 solid methane→x1.5 More speculative: Improved cold moderator liquid H2 →x1.2 solid methane→x1.5 closer coupling→up to x2 nano-diamond flux trap 2. Increase proton beam current until beam-heating effects limit source performance… thesis, Y.-P. Xu But Area B source design not optimized for cryogenic performance! Seems to call for…

Ideally: A Systematic Investigation of high power UCN production (1) For selected source materials (SD2, LHe) identify UCN source operating temperature and cryogenic strategy (2) Optimize UCN production and beam-current one can handle for moderator geometry, consistent with (1) Given what we’ve learned in the past 10 years and will learn in the next few years, this program could define an optimal geometry for available beam power at next generation sources… Motivated by the possibility of a neutron-antineutron oscillations experiment with UCN (see Y. Kamyshkov’s talk), what we actually did was pick a specific idea and explore…

Conceptual Neutronics Modeling of a High Current, UCN Production Geometry G. Muhrer and A. R. Young Guiding concepts: Build from existing 200 kW, Lujan center target Use existing, vetted cryogenic kernels Use strategies benchmarked in development of Mark III target Use large volume (40 l) LHe as target Inspired by a suggestion from Masuda and the source design of Serebrov Assume 100W cooling power available for LHe (e.g. CERN subcooled He system)

UCN production function Korobkina et al., Phys. Lett. A 301, 462 (2002)

Lujan Geometry

Lujan like geometry 9.42*107 UCN/s/100mA Heat load @ 100mA ≡ 80KW 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) 30cm 14

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Inverse Geometry

Inverse cylindrical geometry (1) 800 MeV p+ Bi(300K) W H2 (75% ortho, 20K) 40L-He Al(20K) 53cm 6.6*107 UCN/s/100mA Heat load @ 100mA ≡ 80KW Total heat: 27.4 W Neutron heat: 17.2 W Photon heat: 9.6 W Proton heat: 0.6 W 2.4*108 UCN/s/100W (heat in the He cryostat) Cylindrical proton target (beam rastered around circumference) 17

Inverse geometry (1): Neutron spectrum in He-4

ICG (2): Be canisters W 9.5*107 UCN/s/100mA Heat load @ 100mA ≡ 80KW 800 MeV p+ Bi(300K) W H2 (75% ortho, 20K) 40L-He Be(20K) 53cm 9.5*107 UCN/s/100mA Heat load @ 100mA ≡ 80KW Total heat: 23.6 W Neutron heat: 15.5 W Photon heat: 7.5 W Proton heat: 0.6 W 4.0*108 UCN/s/100W (heat in the He) 19

Inverse geometry (2): Neutron spectrum in He-4

ICG (3): D2O pre-moderator 1.0*108 UCN/s/100mA Heat load @ 100mA ≡ 80KW Total heat: 18.8 W Neutron heat: 9.7 W Photon heat: 8.4 W Proton heat: 0.7 W 5.36*108 UCN/s/100W (heat in the He) 800 MeV p+ 800 MeV p+ 53cm 40L-He Be(20K) W W H2 (75% ortho, 20K) D2O (5cm) Bi(300K) 21

Inverse geometry (3): Neutron spectrum in He-4

ICG (4): thick D2O pre-moderator 800 MeV p+ Bi(300K) W H2 (75% ortho, 20K) 40L-He Be(20K) 53cm D2O (9cm) 9.9*107 UCN/s/100mA Heat load @ 100mA ≡ 80KW Total heat: 17.5 W Neutron heat: 8.0 W Photon heat: 8.6 W Proton heat: 0.9 W 5.64*108 UCN/s/100W (heat in the He) 23

Inverse geometry (4): Neutron spectrum in He-4

ICG (5): D2 moderator W W 1.0*108 UCN/s/100mA Heat load @ 100mA ≡ 80KW Total heat: 13.9 W Neutron heat: 10.8 W Photon heat: 2.4 W Proton heat: 0.7 W 7.14*108 UCN/s/100W (heat in the He) 800 MeV p+ 800 MeV p+ 53cm 40L-He Be(20K) W W D2 (70% ortho, 19K) D2O (5cm) Bi(300K) 25

Inverse geometry (5): Neutron spectrum in He-4

Production Study Summary Beryllium canister increases the UCN flux by 50%. Beryllium canister decreases the heat load in the He by about 15%. Heavy water pre-moderators decrease the heat load in the He by 20-25% (depending on the thickness). No significant UCN flux increase has been observed from introducing a heavy water pre-moderator. Liquid D2 moderator lowers the gamma heating in the He significantly.

UCN extraction/densities Depends on operating temperature (CERN subcooled system runs at 1.8-1.9 K), but can make other choices For subcooled system, UCN lifetime in SD2 roughly 2 s (according to Yoshiki et al., Phys. Rev. Lett. 68, 1323, 1992.) Limiting density up to 35,000 in source Extraction through 18 cm diameter guide has time constant of approximately 1.1s (imagining downward vertical extraction with a thin, supported foil) We have not (yet) considered the details of integration of specific cryogenic cooling systems and UCN extraction! Depends delicately on UCN lifetime……

and…to explore optimization of the cold moderator but…this just serves as motivation for us to integrate specific cryogenics and transport into models and…to explore optimization of the cold moderator Especially for LHe, motivation to cool CN spectrum!

Moderator R&D G. Muhrer, T. Heugle G. Muhrer: Moderator materials in confined spaces ( H2O in silica microspheres): reduction of radiation damage manipulation of excitation spectrum (more of the “right” phonons) change of solidification temperature more predictable crystallization process (2) T. Huegle:Triphenlymethane a) M. Hartl, L. Daemen and G. Muhrer, Microporous and Mesoporous Materials, 161 (2012) 7-13 b) G. Muhrer, M. Hartl, M. Mocko, F. Toveson and L. Daemen, NIMA, 681 (2012) 91-93

Triphenylmethane Melting point 93°C, boiling point 359°C Inelastic Neutron Scattering Spectrum Melting point 93°C, boiling point 359°C Three rotational modes of phenyl rings provide low energy excitations which prevent upscattering Comparably stable in radiation field

Conclusions Upgrade of the Area B UCN source presents a reasonably conservative path to significant improvements Our conceptual design provides some motivation to perform a more detailed investigation of performance characteristics and limitations for the “inverted geometry” high power UCN source Ongoing moderator research at LENS (IUCF), LANL WNR, etc... could play an important role in defining the optimized geometry!