Experiments with ultracold neutrons

Ultracold neutrons (UCN) form a tiny low-energy fraction in Maxwelian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely small (~10⁻⁷ eV), their velocity is a few meters per second, and their effective temperature is as low as ~1 mK. Spec...

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Автор: Nesvizhevsky, V.V.
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Опубліковано: Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2011
Назва видання:Физика низких температур
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Цитувати:Experiments with ultracold neutrons / V.V. Nesvizhevsky // Физика низких температур. — 2011. — Т. 37, № 5. — С. 471–476. — Бібліогр.: 107 назв. — англ.

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spelling irk-123456789-1185432017-05-31T03:07:41Z Experiments with ultracold neutrons Nesvizhevsky, V.V. 8th International Conference on Cryocrystals and Quantum Crystals Ultracold neutrons (UCN) form a tiny low-energy fraction in Maxwelian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely small (~10⁻⁷ eV), their velocity is a few meters per second, and their effective temperature is as low as ~1 mK. Specific feature of UCN consists of their nearly total elastic reflection from nuclear-optical potential of many materials at any incidence angle; therefore they could be stored in closed traps for many minutes, thus they could be used for extremely sensitive measurements. A fraction of UCN in the thermal neutron flux is as low as 10⁻¹¹–10⁻¹², and serious efforts are undertaken all over the world to produce UCN in larger amounts. UCN are widely used in precision particle physics experiments. Applications of UCN are emerging in surface and nanoparticle physics. Here we will focus on recent advances in the field. 2011 Article Experiments with ultracold neutrons / V.V. Nesvizhevsky // Физика низких температур. — 2011. — Т. 37, № 5. — С. 471–476. — Бібліогр.: 107 назв. — англ. 0132-6414 PACS: 14.20.Dh, 03.75.Be, 29.25.Dz http://dspace.nbuv.gov.ua/handle/123456789/118543 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
topic 8th International Conference on Cryocrystals and Quantum Crystals
8th International Conference on Cryocrystals and Quantum Crystals
spellingShingle 8th International Conference on Cryocrystals and Quantum Crystals
8th International Conference on Cryocrystals and Quantum Crystals
Nesvizhevsky, V.V.
Experiments with ultracold neutrons
Физика низких температур
description Ultracold neutrons (UCN) form a tiny low-energy fraction in Maxwelian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely small (~10⁻⁷ eV), their velocity is a few meters per second, and their effective temperature is as low as ~1 mK. Specific feature of UCN consists of their nearly total elastic reflection from nuclear-optical potential of many materials at any incidence angle; therefore they could be stored in closed traps for many minutes, thus they could be used for extremely sensitive measurements. A fraction of UCN in the thermal neutron flux is as low as 10⁻¹¹–10⁻¹², and serious efforts are undertaken all over the world to produce UCN in larger amounts. UCN are widely used in precision particle physics experiments. Applications of UCN are emerging in surface and nanoparticle physics. Here we will focus on recent advances in the field.
format Article
author Nesvizhevsky, V.V.
author_facet Nesvizhevsky, V.V.
author_sort Nesvizhevsky, V.V.
title Experiments with ultracold neutrons
title_short Experiments with ultracold neutrons
title_full Experiments with ultracold neutrons
title_fullStr Experiments with ultracold neutrons
title_full_unstemmed Experiments with ultracold neutrons
title_sort experiments with ultracold neutrons
publisher Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate 2011
topic_facet 8th International Conference on Cryocrystals and Quantum Crystals
url http://dspace.nbuv.gov.ua/handle/123456789/118543
citation_txt Experiments with ultracold neutrons / V.V. Nesvizhevsky // Физика низких температур. — 2011. — Т. 37, № 5. — С. 471–476. — Бібліогр.: 107 назв. — англ.
series Физика низких температур
work_keys_str_mv AT nesvizhevskyvv experimentswithultracoldneutrons
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fulltext © V.V. Nesvizhevsky, 2011 Fizika Nizkikh Temperatur, 2011, v. 37, No. 5, p. 471–476 Experiments with ultracold neutrons V.V. Nesvizhevsky Institut Laue-Langevin, 6 rue Jules Horowitz, Grenoble F-38046, France E-mail: nesvizh@ill.fr Received December 12, 2010 Ultracold neutrons (UCN) form a tiny low-energy fraction in Maxwelian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely small (~10–7 eV), their velocity is a few meters per second, and their effective temperature is as low as ~1 mK. Specific feature of UCN consists of their nearly total elastic reflection from nuclear-optical potential of many materials at any incidence angle; therefore they could be stored in closed traps for many minutes, thus they could be used for extremely sensitive measurements. A fraction of UCN in the thermal neutron flux is as low as 10–11–10–12, and serious efforts are undertaken all over the world to produce UCN in larger amounts. UCN are widely used in precision particle physics experiments. Applications of UCN are emerging in surface and nanoparticle physics. Here we will focus on recent advances in the field. PACS: 14.20.Dh Protons and neutrons; 03.75.Be Atom and neutron optics; 29.25.Dz Neutron sources. Keywords: ultracold neutrons, fundamental physics, quantum phenomena. Introduction The present contribution is based on a talk given at the School of Young Scientists carried out during 8th Confe- rence on Cryocrystals and Quantum Crystals in Chernogo- lovka, Russia, on 26–31 July 2010. This contribution presents the physics with ultracold neutrons; particular attention is paid to recent advances related to the domain of interest of the author of present contribution. Ultracold neutrons (UCN) [1–4] form a tiny low-energy fraction in Maxwelian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely small (~10–7 eV), their velocity equals a few meters per second only, and their effective temperature is as low as ~1 mK. Specific feature of UCN consists of their nearly total elastic reflection from the nuc- lear-optical potential of many materials at any incidence angle; therefore they could be stored in closed traps for extended period of time, thus they could be used for ex- tremely sensitive measurements. UCN characteristic pene- tration depth is close to its wavelength and equal to a few tens nanometers. Very cold neutrons (VCN) with typical energy of 10–7–10–4 eV are totally reflected from flat sur- face only if the incidence angle is sufficiently small; so that the neutron longitudinal velocity component is lower than the material critical velocity. As a fraction of UCN in the thermal neutron flux is as low as 10–11–10–12, serious effort are undertaken all over the world to produce UCN in larger amounts, using super-thermal UCN sources or even equilibrium cooling of neutrons [5–20]. UCN are widely used in precision particle physics experiments [21], such as, for instance, searches for additional fundamental short- range forces [22–28], searches for non-zero neutron elec- tric dipole moment [29,30], precision neutron lifetime measurements [31–36] and constrains for the neutron elec- tric charge [37,38]. Applications of UCN are emerging in surface and nanoparticle physics [39,40]. We focus on re- cent advances in the field including observation of the cen- trifugal quantum states of neutrons. Combined with obser- vation of the gravitationally bound quantum states of neu- trons, this phenomenon provides the first demonstration of the weak equivalence principle for an object in a quantum state [41,42]. Also we will present a new spectrometer GRANIT constructed for precision studies of the gravita- tionally bound quantum states of neutrons and for other ap- plications in particle physics, quantum optics, and in sur- face studies [43]. A promising methodical development in the field consists of building neutron reflectors based on nanostructured materials [44–46]. Finally, unique proper- ties of UCN allow using them for experimental studies of motions of weakly bound nanoparticles [13,47]. V.V. Nesvizhevsky 472 Fizika Nizkikh Temperatur, 2011, v. 37, No. 5 Centrifugal and gravitational quantum states of neutrons Lift a ping-pong ball to a height 0H above table and let it gently falling down. The ball will accelerate in the Earth’s gravity field to the velocity 0 02V gH= , g is the gravitational acceleration; then it will reflect from the table surface. In case of perfectly elastic reflection in vacuum, the ball would return back to surface due to gravity after the period of 0 0 0( ) 8 / ;H H gΔτ = then it would continue bouncing with the frequency 0 0 0 0( ) 1/ ( )H Hν = τ = 0g / 8 H= . The smaller 0 ,H the larger 0ν : 0 (1 m)ν ≈ 010≈ Hz, 1 0 (1 cm) 10ν ≈ Hz, 2 0 (100 μm) 10ν ≈ Hz. The frequency 0ν does not depend on the ball mass M. Imagine another experiment (Fig. 1). A table is moving together with a spaceship with the acceleration = −a g far from large gravitating masses; the table surface is perpen- dicular to g. An observer in the spaceship will see a ball bouncing on the table with the same frequency 0ν as it does in the previous experiment (an observer in the rest frame will see the table accelerating towards the ball). This is a consequence of the weak equivalent principle verified with amazing accuracy of 12~ 10− for macroscopic objects [48], and with the accuracy of 4~ 3·10− for a classical ele- mentary particle [49]. What would happen in the two experiments at very small heights 0 ?H Would be these two problems still equivalent? Would the frequency seek to infinity? No, the frequency would increase only if 0H exceeds the quan- tum-mechanical limit 0 QMH that could be estimated using Heisenberg coordinate-momentum uncertainty relation: 00 2 2QM QMM gHH ≈ π , where is the Planck constant. For a ball in the Earth’s gravitational field the value 0 QMH is too small. However, quantum effects for an elementary particle, for instance, for a neutron, could be observed at relatively large height of 0 10QMH m≈ μ [41,50,51]. The condition separating quantum and classical behavior of UCN above mirror is defined by a ratio of a neutron quan- tum state width nEδ (reciprocal neutron lifetime 1 n −τ in nth quantum state) and an energy difference between neighbor quantum states 1, 1n n n nE E E+ +Δ = − (i.e. the energy-time uncertainty relation , 1 2 )n n nE +τ Δ ≈ π . The transition from a classical case to a quantum one is considered, for in- stance, in Refs. 52,53. In a quantum limit, we do not consider trajectories, heights, velocities; a frequency is defined by an energy as 00 /(2 )QM Eν ≈ π , and a characteristic height 0 QMH de- pends on mass. In accordance with the weak equivalence principle, an effective centrifugal potential [42,54] is local- ly equivalent to gravity. Thus objects do not fall in gravita- tional field and they do not move in an accelerated refer- ence system universally: massive objects could behave classically while light objects exhibit quantum properties at equal distances to mirror. Nevertheless, the weak equiva- lence principle holds: it means in our case that neutron quantum states in gravitational and centrifugal potentials are equivalent if accelerations are equal. A general solution of Schrödinger equation for a par- ticle above mirror attracted by linear potential was found in 1920th [55]. Nevertheless, it was regarded for a long time just as a beautiful quantum-mechanical text-book problem [56–62]. However, conditions corresponding to this idea- lized problem have been realized recently in experiments with slow neutrons in gravitational [41] and centrifugal [42] potentials in Institut Laue-Langevin, Grenoble, France. Due to limited space, we do not present these experiments in detail here, but refer our readers to detailed overviews [63,64], or/and to many other relevant publications [50,51,65–78]. GRANIT spectrometer Further, more precise studies of gravitational and cen- trifugal quantum states of neutrons will continue in an ad- vanced GRANIT spectrometer that is currently under com- missioning in Institut Laue-Langevin in Grenoble, France [43,79]. GRANIT is a follow-up project based on a second- generation UCN gravitational spectrometer with ultra-high energy resolution. The studies will focus on applications of these phenomena in fundamental particle physics, surface studies, methodical applications, and reflectometry with UCN as well as in quantum optics. Basic advantages of the new GRANIT spectrometer in- clude 1) much longer observation time of neutrons in a closed specular trap [72,80–82] thus much better precision in energy measurements; 2) the method of resonance tran- sitions between the gravitationally bound quantum states of neutrons [72,79]; 3) increase in UCN density using dedi- cated 4He UCN source [20] delivering UCN to the spec- trometer with no significant dilution of phase-space density Fig. 1. The quantum behavior of an object above mirror in the gravity field and that in the accelerating reference system is illu- strated schematically. The ball heights correspond to its most probable positions; the scale corresponds to the neutron mass, we consider 5th quantum state. Height over mirror 40 m� 30 m� 20 m� 10 m� Experiments with ultracold neutrons Fizika Nizkikh Temperatur, 2011, v. 37, No. 5 473 [83,84]; 4) profiting from a permanent installation of the spectrometer in a more comfortable experimental envi- ronment; 5) using polarized neutrons and polarization analysis. Nanoparticle reflectors for slow neutrons and studies of weakly bound nanoparticles Significant advance in using nano-structures in UCN physics on one hand and in studying nanoparticles and nano-structures using UCN on the other hand is due to fortunate coincidence of some their characteristic parame- ters [13]. Thus, under certain conditions, the neutron wave- length is close to nanoparticles size, simultaneously the neutron velocity is about equal to the thermal velocity of nanoparticles. Recently, powders of diamond nanoparticles have been used efficiently as the first VCN reflectors in the complete velocity range from UCN to up to ~160 m/s, thus bridging the energy gap between efficient reactor reflectors for thermal and cold neutrons, and optical neutron-matter po- tential for UCN (see Fig. 2) [13,44,85]. Moreover, VCN could be stored in traps with nano-structured walls in some analogy to storage of UCN in traps [45]. Diamond, with its exceptionally high optical nuclear potential and low ab- sorption cross-section, is a particularly suitable material for this application. Formation of diamond nanoparticles by explosive shock was first observed 50 years ago [86]. These particles measure a few nanometers. They consist of a diamond nucleus (with a typical diamond density and optical nuclear potential) within an onion-like shell of a complex chemical composition [87] (with significantly lower optical potential). The use of nanoparticles with the characteristic size of a few nanometers is needed to pro- vide a sufficiently large cross-section of coherent interac- tion and inhomogeneity of the reflector density on a spatial scale of about the neutron wavelength. A large number of diffusive large-angle neutron-nanoparticle scattering events needed to reflect VCN from powder constrains the choice of materials: only low absorbing materials with high opti- cal potential are appropriate. Studying so-called anomalous losses [88] of UCN from traps (providing an obstacle for precision neutron lifetime experiments) we observed a surprising phenomenon: the energy of stored UCN increased by ~10–7 eV with the probability of ~10–8–10–5 per collision [89]; this value exceeded any theoretical expectations by many orders of magnitude. If the neutron energy after such inelastic scat- tering exceeds some critical value it would escape from the trap. This small heating of UCN has been studied over the last years both on solid surfaces (stainless steel, copper, beryllium etc) and on liquid surfaces (different kinds of hydrogen-free oils) [47,89–98]. Only the scattering of UCN at weakly bound nanoparticles on surface with a size of ~10 μm can explain the experimental data obtained [13]. To our knowledge, such quasi-elastic scattering of UCN provides unique opportunity to measure slow motions of nano-objects as well as to study their interaction with sur- faces and with each other. Impurity gels [99–105] provide an interesting object to study using neutron techniques, as well as a tool to reflect and even to slow down neutrons [13,106,107] using the observed earlier quasi-elastic ref- lection of slow neutrons. Summary We presented recent experiments with UCN and dis- cussed further prospects in the field. These studies as well as many other applications of slow neutrons are rapidly progressing. The author is sincerely grateful to all colleagues contri- buted to the studies overviewed here, in particular to GRANIT collaborators. These experiments are supported in part by GRANIT collaboration, by ANR (Agence Na- tionale de la Recherche, France), and the Federal program “Scientific and pedagogical cadres of innovative Russia”. 1. V.I. Luschikov, Y.N. Pokotilovsky, A.V. Strelkov, and V.F. Shapiro, JETP Lett. 9, 23 (1969). 2. V.K. Ignatovich, The Physics of Ultracold Neutrons, Claren- don Press, Oxford (1990). 3. R. Golub, D.J. Richardson, and S.K. Lamoreux, Ultracold Neutrons, Higler, Bristol (1991). 4. A.V. Strelkov, Usp. Fiz. Nauk 174, 565 (2004). 5. R. Golub and J.M. Pendlebury, Phys. Lett. A53, 133 (1975). 6. A. Steyerl, H. Nagel, F.X. Schreiber, K.A. Steinhauser, R. Gähler, W. Gläser, P. Ageron, J.M. Astruc, W. Drexel, G. Gervais, and W. Mampe, Phys. Lett. A116, 347 (1986). Fig. 2. 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