Observation of the penetration of subbarrier ultracold neutrons through beryllium foils and coatings

The subbarrier passage of ultracold neutrons through beryllium foils and coatings with a probability much higher than that of tunneling is observed. This effect may be responsible for the so-called anomalous loss of ultracold neutrons. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 317–322 (10 September 1997)     

Search for anomalous transmission of ultracold neutrons through metal foils

Results are presented on the search for anomalous transmission of ultracold neutrons (UCNs) through beryllium (thickness ∼0.14 mm), stainless steel (0.05 and 0.015 mm), and copper (0.01 and 0.018 mm) foils. This anomalous transmission is considered to be a possible reason for the disappearance of UCNs from beryllium bottles, an effect which was discovered in experiments at the St. Petersburg Nuclear Physics Institute and which was recently observed in the experiment of V. E. Varlamov et al., JETP Lett. 66, 336 (1997). No transmission was found in our measurements at the 10⁻⁷ level except in the case of copper foils, which we attribute to the presence in the UCN flux of an admixture of neutrons with energies higher than the boundary energy for copper. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 440–444 (10 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

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     

Experimental check of the dispersion law for ultracold neutrons

An experiment to check the standard dispersion law for ultracold neutrons is described. The experiment is based on searching for a shift of the resonance line of a neutron interference filter as the neutron velocity component parallel to the filter surface is varied. The first results attest to a statistically significant effect. No mimicking effects were found in a control experiment, but their possible existence cannot be completely ruled out at present. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 746–751 (25 May 1998)     

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.     

Features of nuclear reactions induced by ultracold neutrons passing through thin films

The probabilities of elastic scattering and capture of ultracold neutrons in thin films are calculated in terms of the time quantum theory. It is shown that at a neutron wavelength exceeding considerably the film thickness, the macroscopic probability of the (n, γ)-reaction can decrease as compared to the macroscopic probability of elastic scattering.     

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.     

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)     

Size resonances in penetration of intense femtosecond laser pulses through ultra-thin foils

An analytic expression for the transmission coefficient as a function of the foil thickness d describing penetration of intense femtosecond laser pulses through ultra-thin foils with a thickness of the order of 30–100 nm is derived using the Vlasov-Boltzmann equation. It is found that the transmission of laser radiation stops at the skin depth c/ω _p , but sharp and narrow resonances occur for the foil thickness d > c/ω _p with the transmission coefficient T = 1. The article is published in the original.     

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.     

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.     

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.     

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}.     

The interaction of foils activated by thermal neutrons

The interaction of foils lying in a plane during their activation by thermal neutrons has been calculated. Numerical values of functions are listed, which can be used in calculations, especially for graphite. The interaction was measured in the graphite column of the VVR-S reactor.

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, , . , , . VVR-S.

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We thank J. Frytacký, T. Kania and V. ervenka for their help in the numerical calculations and measurements.     

Experimental investigation of a low-cost solid state detector with high spatial resolution for ultracold neutrons

An ultracold-neutron detector based on a commercial CMOS webcam was investigated for the first time at the instrument PF2 of the Institut Laue-Langevin. In this feasibility study two different neutron converters, ¹⁰B and ⁶Li, were compared. For a standard high-definition videochip with an active area of 3.2 mm × 4.8 mm, a spatial resolution of ≈ 0.3 μm is possible to obtain. For a 430 nm thin ⁶Li converter, positioned in front of the webcam, a detection efficiency of (47.1 ± 0.6)% was found.     

Bulk effects in the coherent inelastic scattering of ultracold neutrons

With the use of a theory developed earlier, bulk effects in ultracold neutron coherent inelastic scattering are considered both for solid and liquid target samples related to energy and momentum exchange with phonon and diffusion-like modes. For the neutron in a material trap, differential and integral probabilities for the energy transfer per bounce are presented in a simple analytic form which exhibits parameter dependence. As an example, the theoretical values for the ultracold-neutron loss rate from a storage bottle with Fomblin-coated walls and stainless-steel walls are evaluated. A possible contribution from incoherent inelastic scattering is discussed.     

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.     

Superthermal source of ultracold neutrons for fundamental physics experiments

Ultracold neutrons (UCNs) play an important role for precise measurements of the properties of the neutron and its interactions. During the past 25 years, a neutron turbine coupled to a liquid deuterium cold neutron source at a high-flux reactor has defined the state of the art for UCN production, despite a long history of efforts towards a new generation of UCN sources. This Letter reports a world-best UCN density available for users, achieved with a new source based on conversion of cold neutrons in superfluid helium. A conversion volume of 5 liters provides at least 274,000 UCN in a single accumulation run. Cyclically repeated operation of the source has been demonstrated, as well.