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Physical Review C

Loss and spinflip probabilities for ultracold neutrons interacting with diamondlike carbon and beryllium surfaces

F. Atchison, Tomasz Bryś, M. Daum, P. Fierlinger, Peter W. Geltenbort, Reinhold Henneck, S. Heule, M. Kasprzak, K. Kirch, A. Pichlmaier, C. Plonka, U. Straumann, C. Wermelinger, Geza Zsigmond

Published Oct 4, 2007• 33 citations
Publisher

Abstract

The storage of ultracold neutrons (UCN) in a combined magnetic, gravitational, and material trap is described. Wall materials investigated were diamondlike carbon (DLC) coatings on solid and flexible foil substrates as well as beryllium coatings on solid substrates. The loss coefficient per wall collision, \ensuremath{\eta}, and the depolarization probability \ensuremath{\beta} were measured simultaneously as a function of temperature (from 70 to 400 K) and energy (from 30 to 80 neV). The results at 70 K are $\ensuremath{\eta}=(0.7\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}=(15.4\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for DLC on polyethyleneterephtalate (PET) foil and $\ensuremath{\eta}=(1.7\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}=(0.7\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for DLC on aluminum foil. At room temperature the loss coefficients are larger by a factor of about 2 whereas the depolarization probabilities are found to be independent of temperature. The corresponding values for Be at room temperature are $\ensuremath{\eta}~5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}~10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. The DLC results for \ensuremath{\beta} and for the temperature-dependent part of the loss coefficient, ${\ensuremath{\eta}}_{T}$, are interpreted in terms of incoherent scattering by hydrogen. The hydrogen admixture was measured independently by elastic recoil detection analysis to be about $1\ifmmode\times\else\texttimes\fi{}{10}^{16}$ atoms/cm${}^{2}$. The data do not support the hypothesis of hydrogen being chemically bound within the top layers of the DLC. Using two different models with a thin waterlike film on top of the substrate we obtain consistency between the temperature-dependent loss contribution and the measured hydrogen contamination.

DOI
10.1103/physrevc.76.044001

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Ultracold neutrons (UCN) with energy up to the Fermi potential were stored for the first time in a volume coated with diamondlike carbon. As a function of the UCN energy and wall temperature of the storage volume, we measured the wall loss parameter \ensuremath{\eta} and obtained $\ensuremath{\eta}=(3.1\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at $T=290$ K and $\ensuremath{\eta}=(1.8\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at $T=115$ K.

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Comment to the article "Temperature dependence of ultracold neutron loss rates" E.Korobkina et al., PRB 70,035409

Anatolii P. Serebrov

In work [1] ("Temperature dependence of ultracold neutron loss rates" E.Korobkina et al., PRB 70,035409) results of measurement of temperature dependence of losses of ultracold neutrons (UCN) in a range from 4 K to 300 K at UCN storage in a copper trap are presented. At interpretation of experimental data it was artificially considered only the difference of losses rate 1/tau(T)= 1/tau_exp(T)- 1/tau_exp(10K), (tau - UCN storage time in the trap). It has been accepted for temperature dependence of losses which changes from 0 to 3.3*10-4 per one collision of UCN with a trap surface. However, the analysis of raw experimental data shows that in a trap at temperature 10 K there is losses 1.0*10-3 per collision. They are considerably (in 3 times) more than discussed temperature dependence and almost 10 times more than losses due to capture cross section on copper. It is the most probable that these losses are connected with leakage of UCN through a slit of trap shutter. Change of the size of a slits on 25% at change of temperature from 300K to 10K can quite explain discussed temperature dependence. Certainly, hydrogen is present on a surface, but as it was shown at work [2] (V.P.Alfimenkov et al., JETP., Vol. 55, No. 2 25 Jan.1992) temperature dependence in 3 times lower, even on the undegased surfaces than on the degassed and deuterated surface in work [1]. At last, deuteration of a trap surface allows almost completely to suppress temperature dependence connected with presence of hydrogen [2]. The conclusion of work [1] is that hydrogen is localized on a surface in the form of a film, instead of distribution in the surface substance. This conclusion contradicts results of measurement of energy dependence of UCN losses in work [2]. More detailed analysis of work [1] and works [2] is presented below.

arXiv:0905.0803•May 6, 2009•0 citations

First ever storage of ultracold neutrons in a magnetic trap made of permanent magnets

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Further improvement in the accuracy of any neutron lifetime experiment by means of ultracold neutrons (UCN) in material bottles is limited due to unavoidable systematic effects when the UCN are reflected from the walls. However, such effects can be excluded in principle if magnetic trapping of UCN is used. The storage of UCN in a small magnetic trap made of permanent magnets was demonstrated for the first time ever. The measured storage time in this feasibility study was (882 ± 16) s. At this level of accuracy no depolarization was observed.

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Resonance capture by hydrogenous impurities and losses of ultracold neutrons in solid material traps

G.S. Danilov

The capture of trapped ultracold neutrons (UCNs) by closed hydrogenous impurities within a solid coating of the trap is discussed as a possible cause of observed anomalously large losses of UCNs in solid material UCN traps. Then significant losses of UCNs arise only if resonances occur in the UCN-impurity scattering amplitude. For a large size impurity, higher partial waves in the UCN-impurity interaction are important, and they are taken into account in the present paper. The method of the calculation is applicable to irregular shape impurities as well. A small distortion of an impurity shape, if it splits the resonance, can increase the UCN losses by a few times. UCN losses in the beryllium trap are calculated assuming they are due to the UCN capture by ice spherical impurities within the coating of the trap walls. Both s- and p-wave resonances contribute significantly to the UCN losses considered. As an example, observed anomalous large UCN losses are achieved if the average radius of the impurity is about 600 Angstroms and the impurity density is about 3*10^{14}/cm^3. A distortion of the spherical shape of the impurity could increase the UCN losses and therefore decrease the impurity density.

arXiv:0903.0458•Mar 3, 2009•0 citations
The storage of ultracold neutrons (UCN) in a combined magnetic, gravitational, and material trap is described. Wall materials investigated were diamondlike carbon (DLC) coatings on solid and flexible foil substrates as well as beryllium coatings on solid substrates. The loss coefficient per wall collision, \ensuremath{\eta}, and the depolarization probability \ensuremath{\beta} were measured simultaneously as a function of temperature (from 70 to 400 K) and energy (from 30 to 80 neV). The results at 70 K are \ensuremathη=(0.7\ifmmode±\else\textpm\fi0.1)\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ=(15.4\ifmmode±\else\textpm\fi1.0)\ifmmode×\else\texttimes\fi10\ensuremath−6\ensuremath{\eta}=(0.7\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}=(15.4\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremathη=(0.7\ifmmode±\else\textpm\fi0.1)\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ=(15.4\ifmmode±\else\textpm\fi1.0)\ifmmode×\else\texttimes\fi10\ensuremath−6 for DLC on polyethyleneterephtalate (PET) foil and \ensuremathη=(1.7\ifmmode±\else\textpm\fi0.1)\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ=(0.7\ifmmode±\else\textpm\fi0.3)\ifmmode×\else\texttimes\fi10\ensuremath−6\ensuremath{\eta}=(1.7\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}=(0.7\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremathη=(1.7\ifmmode±\else\textpm\fi0.1)\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ=(0.7\ifmmode±\else\textpm\fi0.3)\ifmmode×\else\texttimes\fi10\ensuremath−6 for DLC on aluminum foil. At room temperature the loss coefficients are larger by a factor of about 2 whereas the depolarization probabilities are found to be independent of temperature. The corresponding values for Be at room temperature are \ensuremathη 5\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ 10\ifmmode×\else\texttimes\fi10\ensuremath−6\ensuremath{\eta}~5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},\ensuremath{\beta}~10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremathη 5\ifmmode×\else\texttimes\fi10\ensuremath−4,\ensuremathβ 10\ifmmode×\else\texttimes\fi10\ensuremath−6. The DLC results for \ensuremath{\beta} and for the temperature-dependent part of the loss coefficient, \ensuremathηT{\ensuremath{\eta}}_{T}\ensuremathηT​, are interpreted in terms of incoherent scattering by hydrogen. The hydrogen admixture was measured independently by elastic recoil detection analysis to be about 1\ifmmode×\else\texttimes\fi10161\ifmmode\times\else\texttimes\fi{}{10}^{16}1\ifmmode×\else\texttimes\fi1016 atoms/cm2{}^{2}2. The data do not support the hypothesis of hydrogen being chemically bound within the top layers of the DLC. Using two different models with a thin waterlike film on top of the substrate we obtain consistency between the temperature-dependent loss contribution and the measured hydrogen contamination.
Ultracold neutrons (UCN) with energy up to the Fermi potential were stored for the first time in a volume coated with diamondlike carbon. As a function of the UCN energy and wall temperature of the storage volume, we measured the wall loss parameter \ensuremath{\eta} and obtained $\ensuremath{\eta}=(3.1\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at T=290T=290T=290 K and $\ensuremath{\eta}=(1.8\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at T=115T=115T=115 K.
In work [1] ("Temperature dependence of ultracold neutron loss rates" E.Korobkina et al., PRB 70,035409) results of measurement of temperature dependence of losses of ultracold neutrons (UCN) in a range from 4 K to 300 K at UCN storage in a copper trap are presented. At interpretation of experimental data it was artificially considered only the difference of losses rate 1/tau(T)= 1/tau_exp(T)- 1/tau_exp(10K), (tau - UCN storage time in the trap). It has been accepted for temperature dependence of losses which changes from 0 to 3.3*10-4 per one collision of UCN with a trap surface. However, the analysis of raw experimental data shows that in a trap at temperature 10 K there is losses 1.0*10-3 per collision. They are considerably (in 3 times) more than discussed temperature dependence and almost 10 times more than losses due to capture cross section on copper. It is the most probable that these losses are connected with leakage of UCN through a slit of trap shutter. Change of the size of a slits on 25% at change of temperature from 300K to 10K can quite explain discussed temperature dependence. Certainly, hydrogen is present on a surface, but as it was shown at work [2] (V.P.Alfimenkov et al., JETP., Vol. 55, No. 2 25 Jan.1992) temperature dependence in 3 times lower, even on the undegased surfaces than on the degassed and deuterated surface in work [1]. At last, deuteration of a trap surface allows almost completely to suppress temperature dependence connected with presence of hydrogen [2]. The conclusion of work [1] is that hydrogen is localized on a surface in the form of a film, instead of distribution in the surface substance. This conclusion contradicts results of measurement of energy dependence of UCN losses in work [2]. More detailed analysis of work [1] and works [2] is presented below.
Further improvement in the accuracy of any neutron lifetime experiment by means of ultracold neutrons (UCN) in material bottles is limited due to unavoidable systematic effects when the UCN are reflected from the walls. However, such effects can be excluded in principle if magnetic trapping of UCN is used. The storage of UCN in a small magnetic trap made of permanent magnets was demonstrated for the first time ever. The measured storage time in this feasibility study was (882 ± 16) s. At this level of accuracy no depolarization was observed.
The capture of trapped ultracold neutrons (UCNs) by closed hydrogenous impurities within a solid coating of the trap is discussed as a possible cause of observed anomalously large losses of UCNs in solid material UCN traps. Then significant losses of UCNs arise only if resonances occur in the UCN-impurity scattering amplitude. For a large size impurity, higher partial waves in the UCN-impurity interaction are important, and they are taken into account in the present paper. The method of the calculation is applicable to irregular shape impurities as well. A small distortion of an impurity shape, if it splits the resonance, can increase the UCN losses by a few times. UCN losses in the beryllium trap are calculated assuming they are due to the UCN capture by ice spherical impurities within the coating of the trap walls. Both s- and p-wave resonances contribute significantly to the UCN losses considered. As an example, observed anomalous large UCN losses are achieved if the average radius of the impurity is about 600 Angstroms and the impurity density is about 3*10^{14}/cm^3. A distortion of the spherical shape of the impurity could increase the UCN losses and therefore decrease the impurity density.