Quantum effects in a one-dimensional magnetic gravitational trap for ultracold neutrons

JETP Letters - The problem of storing ultracold neutrons over a plane magnetic mirror in the presence of gravity is considered. For neutrons with a definite polarization, the sum of the magnetic...     

Neutron radiography with ultracold neutrons

The neutron transmission factor of very thin films may be low if the neutron energy is comparable to the pseudo-potential of the film material. Surprisingly, perhaps, it is relatively easy to obtain neutrons with such low energies in sufficient numbers to produce neutron radiographs.     

Unconventional trapping of ultracold neutrons

In unconventional storage experiments we filled ultracold neutrons (UCN) into a Fomblin-grease coated trap and then immediately removed the UCN from the storage volume by an absorber, until their residual density in the trap was measured to be negligible. When subsequently the absorber was withdrawn a significant number of UCN of higher energies emerged from the trap. Their appearance cannot be attributed to heating or cooling of residual UCN. Further experiments were performed to investigate the origin of these UCN which we call `late UCN'. We noticed that application of a magnetic field gradient at the trap wall as well as a replacement of Fomblin grease on the surface by Fomblin oil gave rise to small but measurable alterations of storage behavior. These phenomena are consistent with the hypothesis of temporary adhesion of a few UCN to a rough wall.     

Replica neutron guides for experiments with ultracold neutrons

Technical Physics - The method for producing neutron guides for ultracold neutrons based on the replica method has been described. A comparative analysis of the quality of replica neutron guides,...     

A Michelson Interferometer for ultracold neutrons

We propose a neutron Michelson Interferometer installed within a focussing gravity diffractometer for ultracold neutrons. In this arrangement the expected interference pattern depends only on the well-defined vertical component of neutron wavevector. Possible applications of such an interferometer are discussed.Work supported in part by the German Bundesministerium für Forschung und Technologie     

A quantum time lens for ultracold neutrons

The possibility of creating a time lens, an analogue of the zone plate in X-ray optics, for ultracold neutrons is experimentally demonstrated. The neutron energy was changed by means of a purely quantum effect: the phase modulation of a neutron wave at a variable modulation frequency. The modulator was a phase grating with variable spatial period moving across the neutron beam.     

Heating of ultracold neutrons on beryllium surfaces

The temperature dependence of the loss factor for ultracold neutrons owing to heating at thermal energies on the surface of a beryllium sample is studied. The probability of heating ultracold neutrons is anomalously high throughout the entire measured temperature interval, but especially at low temperatures. Zh. éksp. Teor. Fiz. 114, 786–797 (September 1998)     

Determination of the axial-vector weak coupling constant with ultracold neutrons

A precise measurement of the neutron decay β asymmetry A? has been carried out using polarized ultracold neutrons from the pulsed spallation ultracold neutron source at the Los Alamos Neutron Science Center. Combining data obtained in 2008 and 2009, we report A? = -0.119?66±0.000?89{-0.001?40}{+0.001?23}, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon g{A}/g{V}=-1.275?90{-0.004?45}{+0.004?09}.     

Experimental study of quasi-elastic scattering of ultracold neutrons

Ultracold neutrons (UCN) are lost from traps if they are quasi-elastically scattered from the wall with an energy gain sufficient to exceed the Fermi potential for the wall. Possible mechanisms of a quasi-elastic energy transfer are, for instance, scattering from hydrogen diffusing in an impurity surface layer or on surface waves at a liquid wall. Using two different experimental methods at the UCN source of the Institut Laue-Langevin we have investigated both the energy-gain and the energy-loss side of quasi-elastic UCN scattering on Fomblin grease coated walls. For Fomblin oil and similar new types of oil we report up-scattering data as a function of temperature and energy transfer. These low-temperature oils may be used in an improved measurement of the neutron lifetime, which requires extremely low wall reflection losses. Received 13 March 2002 Published online 31 July 2002     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.     

On the depolarization of ultracold neutrons in traps

The method of pulse duration control is proposed for intense molecular beams. The method is based on the shortening of a primary molecular-beam pulse through the formation of a pressure shock ahead of a solid surface through which the beam is passed. The method was used to obtain intense SF₆, H₂, He, SF₆/H₂ (1/10), and SF₆/He (1/10) molecular beams with a pulse duration of ≤10?15 μs and a spatial length of ≤1?2 cm.     

Cooling and heating of ultracold neutrons during storage

The rare processes of weak heating and cooling of ultracold neutrons reflected from the surface of fluorosubstituted oil are studied. The probability of these processes is estimated at 10^?6 per single reflection at energy transfer commensurate with the primary neutron energy. Weak heating and cooling are shown to be a manifestation of a more general phenomenon—quasielastic neutron reflection whose probability is dependent on temperature.     

Diffuse reflection of ultracold neutrons from low-roughness surfaces

We report a measurement of the reflection of ultracold neutrons from flat, large-area plates of different Fermi potential materials with low surface roughness. The results were used to test two diffuse reflection models, the well-known Lambert model and the micro-roughness model which is based on wave scattering. The Lambert model fails to reproduce the diffuse reflection data. The surface roughness b and correlation length w , obtained by fitting the micro-roughness model to the data are in the range 1 $ \le$ b $ \le$ 3 nm and 10 $ \le$ w $ \le$ 120 nm, in qualitative agreement with independent measurements using atomic force microscopy.     

Neutron-antineutron oscillation of ultracold neutrons in storage vessels

If neutron-antineutron oscillations exist in nature, UC neutrons, confined in a storage box, may develop a tiny component of antineutrons that eventually annihilate inside the confining walls. The annihilation rate for this process is evaluated taking into account the effect due to the Earth's gravitational field. The result is employed to discuss how UC neutron experiments would compare with experiments with neutron beams.     

A new escape channel for ultracold neutrons in traps

A surprising new escape channel for ultracold neutrons (UCNs) in traps was reported recently. It could be relevant to the long-standing puzzle of the “too high” loss rate of UCNs from traps, which has yet to be completely understood and eliminated. In the present work we positively identify the new phenomenon and investigate it in detail. The escape of UCNs from traps is due to rare events in which their energy undergoes a small increase (∼10⁻⁷ eV). The reason for such an energy gain and its impact on the physics of UCN storage is still to be investigated. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 175–180 (10 August 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Total cross-sections of various homogeneous substances for ultracold neutrons

By transmission measurements at the time-of-flight spectrometer for ultracold neutrons at the FRM Munich, the total cross-sections of gold, aluminium and copper were determined as a function of neutron energy in the range 10^?7<E<5× 10^?4eV and the temperature range 33–303 °K. No significant deviations from the expected 1/v dependence of the cross-sections were found if the neutron velocityv is corrected for the index of refraction. From the measured total cross-sections at low temperature the following values for the thermal capture cross-sections could be calculated: $$Au:99.3 \pm 0.5b;Al:227 \pm 6mb;Cu:3.77 \pm 0.12b.$$ For aluminium in addition a value of 389±50 °K for the Debye temperature at 298 °K could be obtained from the temperature dependence of the total cross-sections. Additional measurements on glass, mica and air also showed proportionality of the total cross-sections to 1/v.     

Superfluid-helium converter for accumulation and extraction of ultracold neutrons

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.