Low-energy heating of ultracold neutrons during their storage in material bottles

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1 Physics Letters A 309 (2003) Low-energy heating of ultracold neutrons during their storage in material bottles A.P. Serebrov a,, J. Butterworth b,m.daum c, A.K. Fomin a, P. Geltenbort b,k.kirch c, I.A. Krasnoschekova a,m.s.lasakov a, Yu.P. Rudnev a,v.e.varlamov a, A.V. Vassiljev a a St. Petersburg Nuclear Physics Institute, Gatchina, Russia b Institut Max von Laue Paul Langevin, Grenoble, France c PSI, Paul-Scherrer-Institut, Villigen-PSI, Switzerland Received 1 March 2002; received in revised form 29 October 2002; accepted 30 January 2003 Communicated by P.R. Holland Abstract We have studied the low-energy heating of ultracold neutrons during their storage. On fomblin, it is greatly suppressed for the low temperature liquid and at solidification. For non-magnetic solid materials, upper limits of (Be) and (Cu) per collision were found. Therefore, low-energy heating is not the reason for anomalous losses of (2 3) 10 5 per collision on beryllium surfaces Elsevier Science B.V. All rights reserved. PACS: v Keywords: Ultracold neutrons; Anomalous losses; Low-energy heating In recent years, the origin of anomalous losses of ultracold neutrons (UCN) during their storage [1] has attracted considerable attention. In several experimental investigations [2,3], low-energy heating of UCN was considered as the possible origin of these anomalous losses by increasing the UCN energy above the critical energy of the storage vessel material. In this experiment, we investigate in detail low-energy heating by: (1) temperature dependence of low-energy heating in fomblin oil; (2) low-energy heating due * Corresponding author. address: serebrov@pnpi.spb.ru (A.P. Serebrov). to magnetic interaction; (3) low energy heating of non-magnetic solid materials. The experiment was performed at ILL (Grenoble) with a new apparatus emphasizing on the suppression of possible systematic effects. The experimental setup (see Fig. 1) consists of a gravitational spectrometer for shaping a well defined UCN energy spectrum and a UCN trap in which samples were placed. The three detectors A,B and C were typical proportional chamber UCN detectors. Detectors A and C counted upscattered neutrons. For this purpose, 17 µm thin aluminium windows with a diameter of 78 mm were placed behind the shutters 2 and 5, in front of these detectors. The maximal energy /03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 A.P. Serebrov et al. / Physics Letters A 309 (2003) Fig. 1. Experimental setup. 1 6: UCN shutters; A, B, C: UCN proportional chamber detectors. of the UCN spectrum was chosen to be below the critical energy of the window material (53 nev for Al). UCN, upscattered into an energy range above the selected energy spectrum, can penetrate the windows and be registered in the detectors. For the measurements with the samples placed in the UCN trap, upscattered UCN were registered in detector C, while for the investigation of the upscattering from the spectrometer surfaces, detector A was used. In addition to the window foils, the detectors can be closed by shutters for background investigations. Detector B served for monitoring: (1) the density in the gravitational spectrometer during UCN filling; (2) the density in the UCN trap; (3) for normalization of the registered data. For the investigation of temperature dependences, a cryo-insert can be placed inside the UCN trap; for the studies of magnetic effects, a superconducting solenoid was used for polarizing neutrons and generation of a Oe strong magnetic field inside the UCN trap at the position of the samples. 1 During the measurements, the vacuum in the experimental setup was 10 4 mbar. We did not use any special procedures like sample heating for surface treatment. For the investigation of low-energy heating from fomblin oil, we used a cryo-insert in the trap for temperature variation. Its surface of 0.45 m 2 was coated with fomblin. For the other samples, surface 1 For the investigations with fomblin and the other non-magnetic samples, the solenoid in the setup (Fig. 1) was switched off. sizes similar to that of the trap were used ( 1m 2 ), and for the investigation of beryllium, surface sizes of up to 2 m 2 were chosen in order to increase the sensitivity of the experiment. The gravitational spectrometer has an inner diameter of 49 cm and a height of 2.2 m. Its inner surface was coated with beryllium. As a result, a storage time of 200 s at room temperature was achieved for UCN with energies up to about 50 nev. Since UCN are affected by gravity, this energy value is defined throughout the Letter for the central part of the trap, i.e., at the center of the aluminium foils. The second storage vessel (trap, Fig. 1) for the investigation of different material samples and the temperature dependence of the low-energy upscattering is made from copper (diameter 49 cm, height 49 cm). The inner surface was coated with sputtered graphite. The storage time was 230 s at room temperature for UCN energies 50 nev. However, because of the wide dispersion of the graphite density and the resulting dispersion of its critical energy between 100 and 170 nev, the storage time was only 70 s for UCN in the energy region nev. With different samples in the trap the storage time for UCN energies 50 nev decreased depending on the sample s material and size: for example, it was 180 s for a Be sample with a surface area of 0.6m 2 and in the worst case it was 110 s for weak-magnetic stainless steel with a surface area of 1.4m 2. The experimental procedure and the data taking is displayed in Fig. 2, here for the case of a fomblin sample at room temperature. For times 0 <t<100 s, the gravity spectrometer was filled with UCN from the ILL turbine [4]. This process is monitored by the detector B: UCN leak through unavoidable slits between the closed shutter 3 and the spectrometer wall; the total area of these slits is about 0.1 cm 2. The shutters 1, 4, 6 are open, and detector C is recording background events. After about 100 s, detector B indicates a count rate close to saturation, i.e., further filling will not increase appreciably the number of UCN ( )in the gravitational spectrometer. For times 100 <t<310 s, the UCN energy spectrum was shaped in the spectrometer. Neutrons with energies higher than the critical energy of the aluminium foil in front of detector C in the trap are completely removed by placing the polyethylene absorber in the gravitational spectrometer at a height

3 220 A.P. Serebrov et al. / Physics Letters A 309 (2003) of 65 cm above the spectrometer bottom, i.e., 44.5 cm from the lower edge of Al foil with its critical energy of 53 nev (52 cm). Only shutters 4 and 6 are open, i.e., detector C measures the background level. The energy shaping is monitored by detector B, as above. Fig. 2. Time dependent count rates in the detectors B and C during one experimental cycle (see text); s: filling of the spectrometer from the UCN source; s: UCN energy spectrum formation in the spectrometer; s: UCN transfer from the spectrometer to the trap; s: low-energy heating measurement; s: emptying, background measurement and calibration. For the time interval 310 <t<390 s, the UCN trap is filled from the gravitational spectrometer only with neutrons of a well defined energy distribution. The filling occurs through Be coated UCN guides with an inner diameter of 70 mm. The filling process of the trap is monitored by detector B counting UCN leaking through slits between shutter 6 and the trap bottom. UCN from quasi-elastic scattering on fomblin are registered in detector C (the shutters 3, 4, 5 are open). For 390 <t<790 s, UCN are stored in the trap which is monitored by detector B (slits at shutter 6). Heated neutrons passing through the aluminium foil (the shutter 5 is open) are registered in detector C. For 790 <t<950 s, the neutrons which did neither decay nor leave the trap are registered in detector B with open shutter 6. With this procedure, the trap is emptied and the integral count rate with open shutter is used for normalization (see below). Detector C is measuring background (shutter 5 is closed). The temperature dependence of the low-energy heating of UCN from fomblin was obtained by varying the temperature of the fomblin coated cryo-insert by cold nitrogen gas and performing the experimental procedure described above. The results are displayed in Fig. 3. From the recorded data, the probability β of Fig. 3. Temperature dependence of UCN low-energy heating by fomblin. On the left vertical scale, the integral counts in detector C from 380 < t < 780 s before background subtraction are plotted. On the right vertical scale, the probability β of the quasi-elastic heating is indicated (see text). The solid curve is taken from Ref. [6]; it was calculated for capillary waves on the surface of fomblin with initial UCN energies between 0 and 52 nev and final energies between 52 and 106 nev. The data and the calculation are qualitatively in good agreement.

4 A.P. Serebrov et al. / Physics Letters A 309 (2003) the quasi-elastic heating (right vertical scale of Fig. 3) was obtained from the ratio ε 1 t 2 t β = 1 n C (t) dt (1) N ν Σ n B (t 2 ) t 2 t 1 n B (t) dt. Here, n C (t) is the count rate in detector C after background subtraction, t 1 and t 2 mark the start of the filling and the end of the storage in the trap, t 1 = 310 s, t 2 = 790 s; ε is the registration efficiency for UCN with an energy E above the critical energy of the aluminium foil; ν the average collision frequency for one UCN in the trap; n B (t) the count rate of UCN in detector B which pass through small slits around the closed shutter 6; N Σ the total number of neutrons registered in B after opening shutter 6 at the end of the cycle, and n B (t 2 ) the detector B count rate in the monitoring mode, i.e., with closed shutter 6, at the end of the storage at t 2. With this ratio β we can determine the probability for low-energy heating from the measured events and the calculated number of collisions during storage in the trap. The nominator in Eq. (1) is the number of counted upscattered UCN, corrected for the registration efficiency ε. The denominator represents the total number of collisions in the trap, which is the product of the collision frequency ν, the counting rate of UCN in the trap integrated over the time (310 <t<790 s) and the ratio. This ratio allows to determine the N Σ n B (t 2 ) real number of UCN in the trap and thus serves as a calibration. The collision frequency ν was calculated using a Monte Carlo simulation of UCN motion in a gravitational field. Using the results from this calculation, we can estimate the frequency of collisions for samples at different heights in the trap and for different spectra. The registration efficiency ε was determined experimentally as follows. With the gravitational spectrometer, we selected a well defined energy spectrum by varying the absorber height in five equidistant steps between 72 and 197 cm. After filling with UCN and energy shaping, shutter 4 was closed and either shutter 5 or shutter 6 was opened; the UCN were registered in either detector B or C (with Al foil in front of C). Then, the differences of the total counts N B and N C from each detector between step number i and i 1 were calculated and ε was determined from the ratio ε i = N C(i) N C (i 1) N B (i) N B (i 1),whereε i is a function of the UCN Fig. 4. Dependence of the registration efficiency on the difference between the energy of the upscattered UCN and the critical energy of the aluminium foil. The black squares correspond to the detection efficiency when a Be foil (0.6 m 2 ) is placed inside the trap. The efficiency of the setup with an empty trap is practically the same. The white triangles correspond to the detection efficiency when a stainless steel foil (1.4 m 2 ) is placed inside the trap. The thick solid and dashed lines present the results of the Monte Carlo simulations of the detection efficiency for the trap with Be and stainless steel samples, respectively. In these calculations, the effect of albedo reflection of UCN from the Al foil was taken into account. The thin solid and dashed lines show results from calculations without taking into account the effect of albedo reflection. Note: the kinetic energy of the UCN is defined to be as at the center of a foil with a diameter 8 cm (this gives the offset of 4cm). energy interval between step i 1andi. A correction was applied for possible different detection efficiencies in the two detectors by repeating this procedure without the Al foil in front of detector C. Results for such experimental determinations, here for a trap with a beryllium and a stainless steel sample, respectively, are shown in Fig. 4. To understand better this experimental result, we used Monte Carlo calculations of the registration efficiency. The agreement with the experimental data was only obtained when we took into account the effect of UCN scattering on material inhomogeneities (albedo reflection) in the Al foil. The scattering cross section can be described in the following form [5]: σ(q) = σ 0 exp[ (qr G ) 2 /3], where σ 0 = σ(q 0), q = 4π λ sin θ 2, R G is the Guinier radius. The fit was obtained for R G = 150 Å and the macroscopic cross section Nσ 0 = 700 cm 1.Theresults of the two calculations of the registration efficiency, taking albedo effects into account and ignoring them, are included in Fig. 4.

5 222 A.P. Serebrov et al. / Physics Letters A 309 (2003) Table 1 Measured probability β per collision of quasi-elastic scattering of UCN from the energy interval 0 < E < 50 nev to an energy interval 80 <E<150 nev for various materials. The accuracy of measurements in the table is defined by the statistical accuracy of measurements only. The accuracy in calculations of the number of collisions does not exceed 20 30%, and thus contributes correspondingly to the absolute experimental uncertainties Material β (per collision) Upper limit 90% C.L. Be coating of the gravitational spectrometer ( 4.7 ± 2.3) 10 8 < Be foil (2.2 ± 1.6) 10 8 < Be coating of copper rings + Be foil (1.1 ± 1.3) 10 8 < Be weighted average (0.5 ± 0.9) 10 8 < Stainless steel (non-magnetic) ( 1.6 ± 3.9) 10 8 < Cu (99.9%) ( 1.9 ± 2.2) 10 8 < Graphite coating (1.7 ± 0.8) 10 8 < Stainless steel (weak-magnetic) (0.1 ± 1.9) 10 8 < Fomblin (5.0 ± 0.1) 10 6 Stainless steel [3] in the Earth magnetic field Stainless steel [3] in a magnetic field of Oe Stainless steel [3] (after etching with HC1) (4.1 ± 0.4) 10 7 (1.2 ± 0.6) 10 7 (1.2 ± 0.5) 10 7 The registration efficiency for heated neutrons is mainly determined by their transmission through the aluminium foil, however, it also depends on the neutron storage time for heated neutrons in the trap. Therefore, the registration efficiency was measured each time, a new sample was placed inside the trap. This efficiency amounts to 10% at E 20 nev, and does not exceed 40% for E nev, see Fig. 4. Since the final energy of neutrons was not measured in this experiment, the probability of UCN heating can be calculated only on a conditional basis. For example, if the energy of the heated neutrons is nev higher than the critical energy of aluminium (53 nev), then the probability of heating by fomblin at room temperature amounts to per collision. However, if the energy is only 20 nev higher, the probability is approximately per collision. For further calculations, we assumed that the average heating energy was about nev higher than the critical aluminium energy and an average registration efficiency of 30% was used. Thus, the experimental data are analysed to give the probability β for quasielastic scattering of UCN from an energy interval 0 < E<50 nev to an energy interval 80 <E<150 nev. Besides fomblin, we also investigated the low energy heating of beryllium, copper, graphite, and stainless steel; for beryllium and stainless steel, the same samples were used as in the experiment [2]. The experiments showed no effects of UCN heating for the materials presented in Table 1, except for fomblin and a special kind of stainless steel. For most materials, only upper limits for the probability of quasi-elastic scattering from the energy interval 0 <E<50 nev to the energy interval 80 <E< 150 nev were established. In these cases, the background measurements with detector C (time intervals 0 <t<310 s and 800 <t<950 s) and measurements of effects (310 <t<790 s) did not give any statistically significant differences. The same holds for a comparison between measurements with and without samples inside the trap. Our sensitivity δβ in case of zero effect depends on the accuracy of the background measurement in detector C (δn C = N C ) as well as on the total number of UCN collisions with the sample during the measurement (ν N 0 τ with t 2 t 1 τ ). We thus obtain δn C δβ (2) ε ν N 0 τ, where N 0 is the number of UCN in the trap after filling, τ the storage time in the trap with the sample, and N C the integral background counts during the cycle. In a cycle time of 950 s, we typically have an integral background of 5 events, i.e., δn C = 5. With ε = 0.3, ν = 15 s 1, N 0 = ,andτ 150 s, the sensitivity of our setup is for one single

6 A.P. Serebrov et al. / Physics Letters A 309 (2003) measurement as shown in Fig. 2. Long measurements, e.g., over two days, allow us to reach a sensitivity of for a null effect, as is presented in Table 1 for the Be foil. With these sensivities, we obtain an upper limit of the heating probability for all measurements with beryllium β per collision, and β per collision in copper, both at a confidence level of 90% for the low-energy heating of UCN from an energy interval 0 <E<50 nev to 80 <E< 150 nev. These values are in severe disagreement with the discoveries reported in Ref. [2]. The distinct features of the present experiment are: (1) Thorough formation of UCN spectra prior to the measurements, i.e., during 210 s after filling. This is possible because of a long storage time (τ stor 200 s) in the Be-coated gravitational spectrometer. (2) No absorber movements during a measurement. (3) No fomblin contamination of the spectrometer and samples and no oxidation of surfaces after sample heating. Besides the measurements with samples inside the trap, direct measurements with the gravitational spectrometer were carried out. In this case, the same method as in Ref. [2] was used. Here, the difference in counting rate of detector A, behind the Al foil was measured for two cases: with lifting the absorber after the shaping of the UCN spectrum and without lifting the absorber. The results are shown in the first lines of Table 1. In the most recent measurement, this experiment was repeated with an aluminium foil coated on both sides with copper and installed in front of detector A of the gravitational spectrometer (Fig. 1). In this case, quasi-elastic scattering from the beryllium coated spectrometer walls from a UCN energy interval 80 <E<165 nev to an energy interval 200 < E<240 nev was measured and the probability for upscattering was less than per collision at a confidence level of 90%. This result was obtained in a high statistic run for the higher energy range of the copper coated foil (critical energy 167 nev) in comparison with the aluminium foil. Thus, for solid substances, no low-energy heating was observed in this experiment. The only exception was a special polished stainless steel sample from which the previous gravitational spectrometer was made. In the present work, this material was studied thoroughly with the conclusion, that most probably the upscattering observed in Ref. [3] originates from magnetic heating in small ferromagnetic clusters in the stainless steel surface. This process can be described as follows: if a neutron approaches a ferromagnetic cluster, it can be accelerated in the magnetic field gradient. If a spin flip occurs during reflection, the neutron can be accelerated again in the same magnetic field gradient. Thus, the key element of the upscattering process is the spin flip without which the UCN interaction with the ferromagnetic cluster remains elastic. It is possible to affect the spin flip by creating a sufficiently strong magnetic guiding field of Oe at the location of the stainless steel foils by means of a superconducting solenoid (see Fig. 1). The foils were arranged so that the magnetic field was oriented mainly along their surface. With the magnetic field on, the effect of low-energy UCN heating was reduced by a factor 3.4. This fact clearly indicates that the low-energy heating of UCN from this specific stainless steel is of magnetic nature. In order to verify the hypothesis that ferromagnetic clusters in the stainless steel with a significant magnetization are responsible for the heating, we tried to remove the ferromagnetic clusters by treating the samples with a 7% hydrochloric acid solution using the fact, that etching with the acid proceeds faster in iron than in stainless steel. After this treatment, the UCN heating effect was reduced by a factor of 3.3, thus confirming our hypothesis. In conclusion, our experimental investigations of UCN low-energy heating during their storage resulted in: (1) The UCN low-energy heating exists in case of reflections from fomblin and has a strong temperature dependence. The probability of the UCN low-energy heating by reflections from fomblin is inversely proportional to the viscosity of fomblin and proportional to its temperature as calculated for capillary waves on the surface of fomblin [6]. Thus, the UCN low-energy heating from fomblin can be explained by quasi-elastic scattering. (2) The low-energy heating on stainless steel in a gravitational spectrometer [3] is apparently connected with ferromagnetic effects in stainless steel and can be explained by scattering from ferromagnetic material cells causing a UCN spin-flip.

7 224 A.P. Serebrov et al. / Physics Letters A 309 (2003) (3) UCN low-energy heating with an energy transfer 30 <E<100 nev has not been observed for solid substances (Be, Cu, stainless steel). The low-energy heating probability in Be with such an energy transfer is less than per collision. (4) The UCN low-energy heating cannot explain the phenomenon of UCN anomalous losses, the probability of which is [1] for Be. Acknowledgements References [1] V.P. Alfimenkov, et al., JETP Lett. 55 (1992) 84. [2] V.V. Nesvizhevsky, et al., Pis ma Zh. Eksp. Teor. Fiz. 70 (1999) 175. [3] A. Strelkov, et al., Nucl. Instrum. Methods A 440 (2000) 695. [4] A. Steyerl, et al., Phys. Lett. A 116 (1986) 347. [5] D.I. Svergun, L.A. Feigin, X-ray and Neutron Small-Angle Scattering, Nauka, Moscow, [6] S.K. Lamoreaux, R. Golub, nucl-ex/ This work was supported by PNPI (Russia), ILL (France), PSI (Switzerland), and INTAS grants No , RFBR No