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On neutron bottles
Volume 72A, number 6
PHYSICS LETTERS
6 August 1979
ON NEUTRON BOTTLES D.J.C. YATES Corporate Research Laboratories, Exxon Research and Engineering
Co., Linden, NJ, USA
Received 19 April 1979
Aspects of the surface physics and chemistry of neutron bottles are discussed in relation to the storage time anomaly.
Very low energy neutrons should be totally reflected from solid surfaces at any angle of incidence [ 11. This prediction led to the development [2-51 of neutron bottles (NB) for the storage of ultra-cold neutrons (UCN). Unfortunately, the measured storage times of such neutrons have been much shorter than predicted [6]. Golub and Pendlebury [7] have commented that there have been several proposals to explain this discrepancy, ranging from the “sacred” to the “profane”. The sacred proposal would involve adding a new term to the Schrodinger equation, while the profane explanation is that the surfaces of the bottles are contaminated. Other explanations of the storage time anomaly have been proposed. For example, the effects of surface roughness have been discussed in detail by Steyerl [8]. Quasielastic scattering of the neutrons with the walls could also lead to higher than expected leakage of neutrons from the bottles [6]. This letter is concerned with the surface impurity problems of NB, both as regards their contamination with inherently present elements, and with avoidable contaminants. From a knowledge of the physical and chemical processes occurring at the solid-gas interface, several new procedures can be suggested to increase the storage time of UCN. As we are solely concerned with the neutron-solid interface, only those surface impurities (within, say, 200 A of the surface) which have large inelastic scattering cross sections for UCN are significant. As regards impurities inherently present in the solid surface, e.g. B in certain glasses, it is interesting that B-free glasses have much longer storage times [6] than do glasses containing boron. Evidently, this leads one to the suggestion that high purity fused silica should be
used as the material of construction for NB rather than any type of borosilicate glass or high silica glasses such as “Vycor” [9]. In confirmation of this, it is interesting that boron-containing glass gave the only experimental absorption probability that agreed with the calculated values of all the materials tested (table 1, ref. [6]), although this exception was not discussed in ref. [6]. It is to be expected that neutron absorption by the boron in the glass itself would dominate any surface contamination effects, in this particular solid. The calculated neutron absorption probability for boron glass is 320 times that for boron-free glass [6]. Methods of reducing the neutron absorption for pure silica surfaces will be discussed later. The use of metals to construct NB has been very common. However, in only one instance [ 101 has it been suspected that the oxide layer on such metals may be partially hydrated, and relatively thick (100 A). In fact it is very likely that the oxide layer on most metals is quite strongly hydrated, and that these layers on metals such as Cu will grow to thicknesses much greater than 100 A on storage in the atmosphere at ambient temperatures and humidities [ 111. Similar effects will occur with all other metals except Al and, possibly, Au. The protons in this hydroxide layer are strongly bound, probably as hydroxyl groups, and cannot be removed by the use of ultra-high vacua, unless these are combined with quite severe heating of the surface. In addition, if the metal hydroxide layer is non-porous (i.e. a normal three-dimensional coherent compound) then evidently no vacuum, however high, will remove these protons in the surface layer. The above considerations, coupled with the low vacua used, 479
Volume 72A, number 6
PHYSICS LETTERS
6 August 1979
niay well explain why heating the NB had no effect on
studies of porous glass surfaces [14—17] should lead to
the UCN storage time [2,3,5,6]. Moveover, in the one instance [4] where the degassing conditions were described in detail, for a graphite NB (400°C, 10~6Torr, one week), it was stated that the bottle was exposed to air for 6 months before use. If this was undried air, it is well known from studies in surface chemistry that but a few minutes’ exposure of a surface to ambient air will entirely negate the most severe high-temperature outgassing. We agree entirely with the earlier comments of Golub and Pendlebury [7] that the experiments done at elevated temperatures have generally used vacuurn conditions and temperatures which were not realjy sufficient to rule out surface contamination. Recent experiments by Stoika et al. [12] show that about 75% of the neutrons escape from copper neutron bottles by an inelastic upscattering process. The pro-
improved NB technology. However, if this is difficult or if the degree of water
cesses were attributed to the UCN reacting with the inner walls of the bottle, the energy probably being transferred in one single impact. Proof of the presence of hydrogen as a surface contaminant of materials used in the construction of neutron bottles has 15N been + H presented 12C + by Lanford and Golub [131, using the 4He + 4.43 MeV ‘y-ray resonance reaction. The measured surface H concentrations were shown [13] to be sufficiently high to account for the bulk of the anomalous shortening of the UCN storage times. Turning to silica and alumina surfaces, the problems which have to be overcome in the case of NB are entirely analogous to those which had to be overcome before significant infrared spectra could be obtained of molecules adsorbed on materials which transmitted infrared energy [14,151. Basically, these were that the surface to be studied had to be initially cleaned from adsorbed
alcohols.
-~
hydrocarbons by heating in air to some 450°C [9,16], then put in a vacuum IR cell and heated to 450°Cin vacuo [14,15,17]. On cooling down, one had a waterfree surface, that still contained some hydroxyl groups, but had sufficient infrared transmission to study, e.g., the infrared spectra of adsorbed NH 3 [14]. As the size of the tubes used to construct neutron bottles is much larger than the samples used in the above spectroscopic work, and in view of the rather poor vacua apparently endemic in neutron beam facilities, other methods will probably have to be used to obtain relatively protonfree surfaces. However, if effective vacuum seals can be developed then application of the techniques used in the
480
admission to the degassed NB is too high when the neu-
trons are admitted, then I suggest that the interior of the NB be made hydrophobic. The simplest way to do this is to heat the surface in methyl alcohol vapors at about 360°C[18]. Such methylated surfaces will absorb much less water from the gases admitted with the neutron beam, as their water adsorption isotherms would be convex to the pressure axis rather than concave [19]. Even if the vacua of the neutron injection system can be improved, we are still left with the fact that we have niethyl groups on the surface. As deuterons have a smaller absorption cross section for neutrons than protons, this problem can easily be overcome by depositing -CD3 groups on the surface by the use of deuterated
References [1] Zel’dovlch, Phys. JETP (1959) 293. 1389. [21 Ya. L. V.B.Groshev eta!.,Soy. Phys. Lett. 34B9 (1971)
[31 F.L. Conf.1972) on Nuclear with Shapiro, neutronsIntern. (Budapest, eds. J. structure Ero and J.study Szucs
(Plenum, New York, 1974). [4] A. Steyerl and W.D. Trustedt, Z. Phys. 267 (1974) 379. [5] A.!. Egorov, V.M. Lobashev, V.A. Nazarenko, G.D. Porsev and A.P. Serebrov, Soy. J. Nuci. Phys. 19 (1974) 147. [6] VI. Luschikov, Phys. Today 30 (1977) 42. [7] R. Golub and J.M. Pendlebury, Phys. Lett. 50A (1974) 177. [8] A. Steyerl, Z. Phys. 254 (1969) 1972. [9] M.E. Nordberg, J. Am. Ceram. Soc. 27 (1944) 299. [10] R. Golub and J.M. Pendlebury, Contemp. Phys. 13 (1972) 519. [11] N.F. and2nd R.W. properties ionic Mott crystals, ed.Gurney, (OxfordElectronic U.P., 1948) Ch. 8. in [12] A.D. Stoika, A.V. Strelkov and M. Hetzelt, Z. Phys. B29 (1978) 349. [13] W.A. Lanford and R. Golub, Phys. Rev. Lett. 39 (1977) 1509. [14] D.J.C. Yates, N. Sheppard and C.L. Angel!, J. Chem. Phys. 23 (1955) 1980. [151 N. Sheppard and D.J.C. Yates, Proc. Roy. Soc. A238 (1956) 69. [161 D.J.C. Yates, Proc. Roy. Soc. A224 (1954) 526. [17] D.J.C. Yates, Adv. Cata!. 12 (1960) 265. [18] M. 32. Folman and D.J.C. Yates, Proc. Roy. Soc. A246 (1958) [191 S. Brunauer, Physical adsorption of gases and vapors (Oxford U.P., 1943) p. 150.
PHYSICS LETTERS
6 August 1979
ON NEUTRON BOTTLES D.J.C. YATES Corporate Research Laboratories, Exxon Research and Engineering
Co., Linden, NJ, USA
Received 19 April 1979
Aspects of the surface physics and chemistry of neutron bottles are discussed in relation to the storage time anomaly.
Very low energy neutrons should be totally reflected from solid surfaces at any angle of incidence [ 11. This prediction led to the development [2-51 of neutron bottles (NB) for the storage of ultra-cold neutrons (UCN). Unfortunately, the measured storage times of such neutrons have been much shorter than predicted [6]. Golub and Pendlebury [7] have commented that there have been several proposals to explain this discrepancy, ranging from the “sacred” to the “profane”. The sacred proposal would involve adding a new term to the Schrodinger equation, while the profane explanation is that the surfaces of the bottles are contaminated. Other explanations of the storage time anomaly have been proposed. For example, the effects of surface roughness have been discussed in detail by Steyerl [8]. Quasielastic scattering of the neutrons with the walls could also lead to higher than expected leakage of neutrons from the bottles [6]. This letter is concerned with the surface impurity problems of NB, both as regards their contamination with inherently present elements, and with avoidable contaminants. From a knowledge of the physical and chemical processes occurring at the solid-gas interface, several new procedures can be suggested to increase the storage time of UCN. As we are solely concerned with the neutron-solid interface, only those surface impurities (within, say, 200 A of the surface) which have large inelastic scattering cross sections for UCN are significant. As regards impurities inherently present in the solid surface, e.g. B in certain glasses, it is interesting that B-free glasses have much longer storage times [6] than do glasses containing boron. Evidently, this leads one to the suggestion that high purity fused silica should be
used as the material of construction for NB rather than any type of borosilicate glass or high silica glasses such as “Vycor” [9]. In confirmation of this, it is interesting that boron-containing glass gave the only experimental absorption probability that agreed with the calculated values of all the materials tested (table 1, ref. [6]), although this exception was not discussed in ref. [6]. It is to be expected that neutron absorption by the boron in the glass itself would dominate any surface contamination effects, in this particular solid. The calculated neutron absorption probability for boron glass is 320 times that for boron-free glass [6]. Methods of reducing the neutron absorption for pure silica surfaces will be discussed later. The use of metals to construct NB has been very common. However, in only one instance [ 101 has it been suspected that the oxide layer on such metals may be partially hydrated, and relatively thick (100 A). In fact it is very likely that the oxide layer on most metals is quite strongly hydrated, and that these layers on metals such as Cu will grow to thicknesses much greater than 100 A on storage in the atmosphere at ambient temperatures and humidities [ 111. Similar effects will occur with all other metals except Al and, possibly, Au. The protons in this hydroxide layer are strongly bound, probably as hydroxyl groups, and cannot be removed by the use of ultra-high vacua, unless these are combined with quite severe heating of the surface. In addition, if the metal hydroxide layer is non-porous (i.e. a normal three-dimensional coherent compound) then evidently no vacuum, however high, will remove these protons in the surface layer. The above considerations, coupled with the low vacua used, 479
Volume 72A, number 6
PHYSICS LETTERS
6 August 1979
niay well explain why heating the NB had no effect on
studies of porous glass surfaces [14—17] should lead to
the UCN storage time [2,3,5,6]. Moveover, in the one instance [4] where the degassing conditions were described in detail, for a graphite NB (400°C, 10~6Torr, one week), it was stated that the bottle was exposed to air for 6 months before use. If this was undried air, it is well known from studies in surface chemistry that but a few minutes’ exposure of a surface to ambient air will entirely negate the most severe high-temperature outgassing. We agree entirely with the earlier comments of Golub and Pendlebury [7] that the experiments done at elevated temperatures have generally used vacuurn conditions and temperatures which were not realjy sufficient to rule out surface contamination. Recent experiments by Stoika et al. [12] show that about 75% of the neutrons escape from copper neutron bottles by an inelastic upscattering process. The pro-
improved NB technology. However, if this is difficult or if the degree of water
cesses were attributed to the UCN reacting with the inner walls of the bottle, the energy probably being transferred in one single impact. Proof of the presence of hydrogen as a surface contaminant of materials used in the construction of neutron bottles has 15N been + H presented 12C + by Lanford and Golub [131, using the 4He + 4.43 MeV ‘y-ray resonance reaction. The measured surface H concentrations were shown [13] to be sufficiently high to account for the bulk of the anomalous shortening of the UCN storage times. Turning to silica and alumina surfaces, the problems which have to be overcome in the case of NB are entirely analogous to those which had to be overcome before significant infrared spectra could be obtained of molecules adsorbed on materials which transmitted infrared energy [14,151. Basically, these were that the surface to be studied had to be initially cleaned from adsorbed
alcohols.
-~
hydrocarbons by heating in air to some 450°C [9,16], then put in a vacuum IR cell and heated to 450°Cin vacuo [14,15,17]. On cooling down, one had a waterfree surface, that still contained some hydroxyl groups, but had sufficient infrared transmission to study, e.g., the infrared spectra of adsorbed NH 3 [14]. As the size of the tubes used to construct neutron bottles is much larger than the samples used in the above spectroscopic work, and in view of the rather poor vacua apparently endemic in neutron beam facilities, other methods will probably have to be used to obtain relatively protonfree surfaces. However, if effective vacuum seals can be developed then application of the techniques used in the
480
admission to the degassed NB is too high when the neu-
trons are admitted, then I suggest that the interior of the NB be made hydrophobic. The simplest way to do this is to heat the surface in methyl alcohol vapors at about 360°C[18]. Such methylated surfaces will absorb much less water from the gases admitted with the neutron beam, as their water adsorption isotherms would be convex to the pressure axis rather than concave [19]. Even if the vacua of the neutron injection system can be improved, we are still left with the fact that we have niethyl groups on the surface. As deuterons have a smaller absorption cross section for neutrons than protons, this problem can easily be overcome by depositing -CD3 groups on the surface by the use of deuterated
References [1] Zel’dovlch, Phys. JETP (1959) 293. 1389. [21 Ya. L. V.B.Groshev eta!.,Soy. Phys. Lett. 34B9 (1971)
[31 F.L. Conf.1972) on Nuclear with Shapiro, neutronsIntern. (Budapest, eds. J. structure Ero and J.study Szucs
(Plenum, New York, 1974). [4] A. Steyerl and W.D. Trustedt, Z. Phys. 267 (1974) 379. [5] A.!. Egorov, V.M. Lobashev, V.A. Nazarenko, G.D. Porsev and A.P. Serebrov, Soy. J. Nuci. Phys. 19 (1974) 147. [6] VI. Luschikov, Phys. Today 30 (1977) 42. [7] R. Golub and J.M. Pendlebury, Phys. Lett. 50A (1974) 177. [8] A. Steyerl, Z. Phys. 254 (1969) 1972. [9] M.E. Nordberg, J. Am. Ceram. Soc. 27 (1944) 299. [10] R. Golub and J.M. Pendlebury, Contemp. Phys. 13 (1972) 519. [11] N.F. and2nd R.W. properties ionic Mott crystals, ed.Gurney, (OxfordElectronic U.P., 1948) Ch. 8. in [12] A.D. Stoika, A.V. Strelkov and M. Hetzelt, Z. Phys. B29 (1978) 349. [13] W.A. Lanford and R. Golub, Phys. Rev. Lett. 39 (1977) 1509. [14] D.J.C. Yates, N. Sheppard and C.L. Angel!, J. Chem. Phys. 23 (1955) 1980. [151 N. Sheppard and D.J.C. Yates, Proc. Roy. Soc. A238 (1956) 69. [161 D.J.C. Yates, Proc. Roy. Soc. A224 (1954) 526. [17] D.J.C. Yates, Adv. Cata!. 12 (1960) 265. [18] M. 32. Folman and D.J.C. Yates, Proc. Roy. Soc. A246 (1958) [191 S. Brunauer, Physical adsorption of gases and vapors (Oxford U.P., 1943) p. 150.