Neutron-Antineutron Oscillation Search with Cold Neutrons ILL beam experiment Improved experiment with horizontal beam: (1)Reactor source, (2) pulsed source Vertical experiment: DUSEL proposal M. Snow Indiana University/CEEM NANO Workshop Thanks for slides and calculations to: Yuri Kamyshkov, Geoff Greene, Hiro Shimizu

Neutron-Antineutron transition probability Contributions to V: ~100 neV, proportional to density =  B, ~60 neV/Tesla; B~10nT-> Vmag~ eV, both >>  Figure of merit= N=#neutrons, T=“quasifree” observation time

Neutron Cooling: MeV to neV

N-Nbar search at ILL (Heidelberg-ILL-Padova-Pavia) No GeV background No candidates observed. Measured limit for a year of running: Baldo-Ceolin M. et al., Z. Phys. C63,409 (1994).

 Bt<< ћ ILL achieved |B|<10 nT over 1m diameter, 80 m beam,one layer 1mm shield in SS vacuum tank, 1% reduction in oscillation efficiency (Bitter et al, NIM A309, 521 (1991). For new experiment need |B|<~1 nT Quasifree Condition: B Shielding and Vacuum If nnbar candidate signal seen, easy to “turn it off” by increasing B V opt t<< ћ: Need vacuum to eliminate neutron-antineutron optical potential difference. P<10 -5 Pa is good enough, much less stringent than LIGO

The conceptual scheme of antineutron detector

Better Cold Neutron Experiment (Horizontal beam) need cold neutrons from high flux source, access of neutron need cold neutrons from high flux source, access of neutron focusing reflector to cold source, free flight path of ~ m Improvement on ILL experiment by factor of ~1000 in transition probability is possible (but expensive) with existing n optics technology and sources L = 300 m D ~ 2-3 m

concept of neutron supermirrors: Swiss Neutronics neutron reflection at grazing incidence (< ≈2°) refractive index n < 1 total external reflection e.g. Ni  c = 0.1 smooth supermirror

Supermirror Neutron Optics: Elliptical Focusing Guides Muhlbauer et. al., Physica B 385, 1247 (2006). Under development for neutron scattering spectrometers Can be used to increase fraction of neutrons delivered from cold source (cold source at one focus, nbar detector at other focus)

“Supermirrors”:  critical  m  critical 1  c Reflectivity  c m ~ 1000 layers Commercial Supermirror Neutron Mirrors are Available With m ≈ Phase space acceptance for straight guide  m 2, more with focusing reflector “Items of commerce” Multilayer mirror

H. Shimizu, KEK/Japan Supermirror Neutron Optics: Higher m and reflectivity m=10! From H. Shimizu

Prototype supermirrors with m~6 produced Useful neutron flux scales roughly as m 2 Image Courtesy; H. Shimizu

New Experiment at Existing Research Reactor? Requests to all >20 MW research reactors Requests to all >20 MW research reactors with cold neutron sources Cutaway view HFIR reactor at ORNL need close access to cold source to fully illuminate elliptical reflector need close access to cold source to fully illuminate elliptical reflector Can a reactor be found? Not yet… Can a reactor be found? Not yet…

Advantages of a Next-Generation Pulsed Neutron Source (ESS) for A Neutron-Antineutron Oscillation Experiment Possibility to use active neutron optics to partially correct for gravitational defocusing Possibility to optimize the target/moderator system with this experiment in mind, and take advantage of a “colder” cold source if moderator research is successful. Possibility to upgrade reflector with other developments in neutron optics (higher m higher reflectivity supermirrors, …)

At a pulsed neutron source like ESS, neutrons of a given speed reach the mirror at a known time. We can therefore imagine an array of mirrors tiling an ellipse and phased to the source to condition the beam Piezodrivers This tilting can be used to counteract the defocusing of the beam from gravity, thereby reducing the beam/detector size and therefore reduce the cost of the experiment.

N  t 2 distribution vs vertical Y in the target plane m=4; | x target | < 1 m. Radius of beam is smaller by~factor of 2 ->cost of experiment is smaller (scales generically as the area)

Supermirror Neutron Optics: Future Possibilities “In the future one may consider varying the shape of the guides actively by means of piezo actuators. If used at pulsed sources, beam size and therefore the divergence for each wavelength during a neutron pulse can be optimized. This corresponds to a kind of active phase space transformation that will allow the circumvention of Liouville’s theorem. The combination of fast mechanical actuators with supermirror technology may become useful for active phase space transformation.” P.Boni, NIM A586, 1 (2008). One could design the experiment to be able to take advantage of such advances in active neutron optics technology through modification of the reflector

· ~3 MW TRIGA research reactor with vertical hole and cold neutron moderator ® vn ~ 1000 m/s · Vertical shaft ~1000 m deep with diameter ~ 4-5 m at proposed US DUSEL facility · Large vacuum tube, focusing reflector, magnetic shielding · Detector (similar to ILL N-Nbar detector) at the bottom of shaft Letter of intent to DUSEL submitted Scheme of Vertical N-Nbar experiment

Annular core TRIGA reactor (General Atomics) for N-Nbar search experiment annular core TRIGA reactor 3.4 MW with convective cooling, vertical channel, and large cold LD 2 moderator (T n ~ 35K). Courtesy of W. Whittemore (General Atomics) ~ 1 ft GA built ~ 70 TRIGA reactors 0.01¸14 MW (th) 19 TRIGA reactors presently operating in US (last commissioned in 1992) 25 TRIGA reactors operating abroad (last commissioned in 2005) some have annular core and vertical channel Well-established technology

Cold Neutron Source Example Made in PNPI, Russia Liquid hydrogen at 20K Inserted vertically into research reactor Delivered to new Australian research reactor, 18 MW power

3.4 MW annular core research TRIGA reactor with Liquid D2 cold neutron moderator TRIGA = Training Research Isotopes from General Atomics

Vertical flight path1-1.5 km Shaft diameter15-20 ft Focusing mirror reflector4  c Vacuum chamber with 10  5 Pa Active + passive magnetic shield1 nT Annular core TRIGA reactor3.4 MW LD 2 cryogenic cold moderator; neutron temperature35K Running time3-5 years Robust detection signature nC  several pions 1.8 GeV Annihilation properties are well understood LEAR physics Active magnetic shielding allows effectON/OFF Free-n sensitivity increases more than  1000 Expected background at max sensitivity  0.01 event

Talks posted at 1km Vertical Space Working Group NNbar: search for neutron to antineutron transitions (Yuri Kamyshkov/UT) Study of diurnal Earth rotation (Bill Roggenthen/SDSMT) Physics of cloud formation (John Helsdon/SDSMT) Search for transitions to mirror matter (n  n) (Anatoli Serebrov / PNPI) Cold atom interferometry for detection of gravitational waves (Mark Kasevich / Stanford U) ExperimentLengthDiaPressureMag. shield Purpose NNbar1.5 km4-5 m <10  Pa ~ 1 nT Mirror neutrons1.5 km4-5 m <10  Pa ~ 1 nTn disappearance Atom interferometry1-4 km0.3 m <.1  Pa ~ 1 nT grav. wave detection Cloud Form Physics0.5-1 km3-5 m  0.2 atm N/Aatm. physics facility Diurnal rotation0.1-1 km1m <10  Pa N/AE&O

Sources of x1000 Improvement on ILL Experiment with Cold Neutrons -increased phase space acceptance of neutrons from source (using m=3 supermirrors): x~60 -increase running time: x~3 -increase neutron free-flight time (t 2 ): x~100 (vertical), ~4-10 (horizontal) -source brightness : x~1/20 (vertical 3.4 MW TRIGA) X~1/2 (horizontal, 20-60MW research reactor) For horizontal experiment: greater source brightness ~counteracted by (dispersive) gravitational defocusing of Maxwellian neutron spectrum

Sources of x1000 Improvement on ILL Experiment with Cold Neutrons from CW Source -increased phase space acceptance of neutrons from source (using m=3 supermirrors): x~60 -increase running time: x~3 -increase neutron free-flight time (t 2 ): x~4-10 (horizontal) -source brightness : x~1/2 (horizontal, 20-60MW research reactor) For CW horizontal experiment: greater source brightness ~counteracted by (dispersive) gravitational defocusing of Maxwellian neutron spectrum