Depolarization of ultracold neutrons during their storage in material bottles

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1 Physics Letters A 313 (2003) Depolarization of ultracold neutrons during their storage in material bottles A.P. Serebrov a,,m.s.lasakov a, A.V. Vassiljev a, I.A. Krasnoschekova a, Yu.P. Rudnev a, A.K. Fomin a,v.e.varlamov a, P. Geltenbort b, J. Butterworth b, A.R. Young c,u.pesavento c a St. Petersburg Nuclear Physics Institute, Gatchina, Russia b Institut Max von Laue Paul Langevin, Grenoble, France c Princeton University, Princeton, NJ, USA Received 1 March 2002; received in revised form 16 May 2003; accepted 16 May 2003 Communicated by P.R. Holland Abstract The depolarization of ultracold neutrons (UCN) during their storage in traps has been investigated. The neutron spin-flip probability for the materials studied amounts to (1 2) 10 5 per collision and does not depend on the temperature. The possible connection between the phenomenon of UCN depolarization and that of anomalous losses is discussed Elsevier B.V. All rights reserved. PACS: v Keywords: Ultracold neutrons Depolarization of UCN during storage has been studied, and corresponding experiments have been carried out for the following reasons. Firstly, the ability to store polarized UCN provides new perspectives for the experimental study of neutron β-decay asymmetries [1 3]. Secondly, experimental studies of UCN depolarization enable us to verify the hypothesis concerning a connection between UCN anomalous losses [4] and * Corresponding author. address: serebrov@pnpi.spb.ru (A.P. Serebrov). incoherent interactions of UCN with surfaces [5,6]. The idea is that incoherent interaction on the surface causes some UCN to be scattered into vacuum and into substance. It is the reason for anomalous losses of UCN in substance and depolarization of UCN scattered into vacuum. The anomalous losses can be observed only at the temperature 4 K. They cannot be observed at the room temperature because of significant losses caused by an upscattering process on hydrogen contaminations of the surface. But the existence of incoherent interactions can be revealed by the existence of spin-flip processes. Therefore, studies of UCN depolarization allow us to determine the contribution due to incoherent scattering of UCN during reflection from /03/$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /s (03)00848-x

2 374 A.P. Serebrov et al. / Physics Letters A 313 (2003) a substance. The first experiment to study UCN depolarization during storage confirmed that this process exists [7], however, more detailed studies of this phenomenon are important. The aims of this experiment were: (1) search for low-depolarization materials which could be useful for neutron β-decay program; (2) studies of temperature dependence of UCN depolarization in order to obtain additional information about the depolarization process and the process of incoherent scattering on the surface. In particular, the probability for UCN, which were incoherently scattered into substance, to come back in vacuum can depend on the temperature of substance. The experimental setup is presented in Fig. 1. It consists of a gravitational spectrometer, a superconducting solenoid and a UCN storage volume, where the samples to be studied are placed. In order to study temperature dependent effects a cryo-insert can be placed inside the UCN trap and the samples were fixed to a cold table. The gravitational spectrometer is used to shape the UCN spectrum prior to filling the storage trap. The gravitational spectrometer consisted of a copper cylinder with inner diameter 49 cm and height 2.2 m. Its inner surface was coated with beryllium. As a result, the storage time for low energy UCN (< 50 nev) is about 200 s and about 160 s for UCN Fig. 1. The experimental setup. 1, 2, 3, 4, 5, 6 are UCN valves. A, B, C are UCN detectors. with energies less than 200 nev. The total number of UCN in the gravitational spectrometer with energies less than 200 nev can be up to The superconducting solenoid is used to fill the trap with polarized UCN and also to analyze the polarization after storage. The magnetic field inside the solenoid is 4.5 T therefore the neutrons with one spin component and with energy less than 270 nev will be reflected. The efficiency of the polarizer has to be equal to 100% because the neutron spin behavior in strong magnetic field is perfectly adiabatic. The neutron guide which goes through the solenoid is made from copper tube with diameter 70 mm. The inner surface of the guide is polished and coated with beryllium. The trap for the investigation of different material samples is made from copper (diameter 49 cm, height 49 cm). The inner surface was coated with sputtered graphite; its storage time was 230 s for UCN energy 50 nev and was about 150 s for energy 100 nev. Immediately after filling the trap the number of polarized UCN with energy less than 100 nev was equal to The storage time was decreased by a factor 1.5 when samples with surface area 1 m 2 or the cryoinsert were placed inside the trap. The cryo-insert was made from copper and coated with graphite. For lower temperature measurements the cryo-insert was filled with liquid nitrogen. The three detectors A, B and C were typical proportional chamber UCN detectors. Detector A was used for monitoring the gravitational spectrometer, detectors B and C were used for monitoring the storage trap. Before discussing the procedure used for the depolarization measurements let us consider how the process of depolarization during storage in the trap can be described. For these purposes we can use the following system of differential equations: dn + (t) = ανn (t) (α + µ)νn + (t), (1) dt dn (t) = ανn + (t) (α + µ)νn (t), (2) dt where N + (t) and N (t) are the number of neutrons in the trap with positive and negative polarization, α is the probability of a spin-flip during reflection, µ is the loss probability during a collision with the wall, ν is the average frequency of wall collisions per UCN.

3 A.P. Serebrov et al. / Physics Letters A 313 (2003) For normalized values of the spin components R + (t), R (t) and polarization P(t) we have R ± (t h ) =[1 2R 0 ] 1 e 2ανt h + R 0, (3) 2 P(t h ) =[1 2R 0 ]e 2ανt h, (4) where t h is the holding time of UCN in the trap, R ± = N ± /(N + + N ), R 0 = N + (t f )/[N + (t f ) + N (t f )], where t f is the filling time. There are the following simple relations between polarization P(t) and normalized values of spin components R + (t), R (t): P(t)= 2R + (t) 1 = 1 2R (t). Therefore the measurement of R + (t) corresponds to the measurement of UCN polarization in the process of UCN storage in the trap. If ανt h 1 we will have the following linear dependence for R + on t h : R + (t h ) =[1 2R 0 ]ανt h + R 0. (5) Therefore by measuring R + (t h ) as a function of the holding time of UCN in the trap, we can determine the probability of a spin-flip per second (αν) orthe probability of a spin-flip per collision (α), if we know the frequency of collisions with the trap walls (ν). In order to study the probability of a spin-flip upon reflection from various materials, samples of these materials were placed in the trap, and the probability of a spin-flip was measured before and after introduction of the samples. The frequency of UCN collisions for both cases was calculated from gas-kinetic ratios taking gravitation into account. The measurement of R exp + (t h) was made in two steps: measurement of N exp + and measurement (N+ + N ) exp. In order to measure the number of neutrons that flipped their spin, as a function of the holding time N exp + (t h), the following sequence of operations was carried out. Firstly, a spectrum of neutrons with a given maximum energy was prepared in the gravitational spectrometer with the help of a polyethylene absorber placed at the height which was 5 10 cm less than the height corresponding to the critical energy of the studied sample. After filling the gravitational spectrometer over 100 s and holding them in the spectrometer for 10 s to eliminate the high energy part of the UCN spectrum, neutrons filled the trap for 70 s, passing through shutters 3, 4 and through the superconducting solenoid. As only one spin component passes through a strong magnetic field (4.5 T), and the other one is reflected, the trap was filled with polarized neutrons. However, partial depolarization of UCN occurs already during the filling and the emptying processes, therefore R 0 is not zero. The filling process could be monitored by means of detector C (Fig. 2). Although of shutter 6 is closed detector C can monitor the density of UCN in the trap through a small slit of about 40 µm with total area of about 0.1 cm 2. At different stages of the measurement neutrons were held in the trap for different periods t h, between 10 s and 200 s. In Fig. 2 the holding time was 150 s (between 180 and 330 s). After UCN storage, shutters 3 and 4 were opened, and neutrons, that had not changed their spin state, returned to the spectrometer, where the polyethylene absorber was now placed at its lowest possible position, in order to absorb the returning neutrons. Neutrons, that had changed their spin state, were not able to leave the trap through the solenoid due to the magnetic potential barrier. In order to remove all neutrons of the initial spin component from the trap, the emptying time was set to 300 s (from 330 s up to 630 s in Fig. 2). Detector C in monitoring mode demonstrates the emptying process. The first exponent τ 43 s corresponds Fig. 2. The sequence of R + exp measurements for the empty trap with absorber position 80 cm. Time interval s corresponds to the measurement of N +, time interval s corresponds to the measurement of (N + + N ). are measurements with magnetic field in the superconducting solenoid, are control measurements without magnetic field. (See text for detailed explanation.)

4 376 A.P. Serebrov et al. / Physics Letters A 313 (2003) to the emptying process for the initial spin component, the second exponent τ 160 s corresponds to the storage of UCN with the opposite spin component, because for neutrons that had changed spin state the trap is closed by means of the magnetic field. Then the shutter 6 was opened and neutrons N + exp were counted ( s) with detector C. The exponential time constant for emptying the storage trap into detector C is 34 s. This is less than the emptying time for the initial spin state through the solenoid because the guide has a larger diameter (78 mm) and it is inclined downwards towards the detector. In Fig. 2 for the time interval s two curves are shown. The lower curve which is about 10 times smaller corresponds to the control measurement, when the superconducting solenoid was switched off. The measurement without magnetic field with unpolarized neutrons was normalized to the same initial UCN density in the trap. The control measurements demonstrate that about 85% of the observed effect is connected with the process of UCN depolarization and only 15% are neutrons which still remain in the trap after 300 s of emptying and produce background effect. For the measurement of (N + + N ) exp as a function of the storage time the same procedure was repeated, but the shutter 4 was not opened after storage. This process is shown in the second part of Fig. 2 ( s). By repeating the same procedure for different holding times t h the following experimental dependence R + exp (t h) was determined: N + exp R exp + = (N + + N ) exp = [1 2R 0 ]ανt h e ανt sp + R 0 + R b, (6) where t sp is the time required to separate the two spin components, R b is a background effect connected with neutrons, which still remain in the trap after the t sp. Its value was estimated by means of using the same procedure of the measurements but without magnetic field. We assume that the UCN trap was sealed equally well by both the magnetic barrier and mechanical valve 4, therefore the storage times are the same for both cases: because of this µ is excluded from Eq. (6) as well as from Eq. (5). Eq. (6) is different from Eq. (5) by coefficient e ανt sp, which is very close to 1 ( ). The presence of the coefficient e ανt sp in the formula (6) is explained by the fact that during the separation of the spin components a leakage of neutrons N + occurs due to a second spinflip. The formula (6) was used to fit the experimental data R exp + (t h) using two parameters: αν and R 0.Only the term αν is important. After calculation of the frequency of collisions, ν, we can then determine the probability of depolarization per collision, α. The parameter R 0 is also determined in the experiment but influence of R 0 for accuracy of determination of (αν)-parameter is not significant as one can see from Eq. (6), where the coefficient (1 2R 0 ) is before (αν)- parameter. The control measurements have been carried out in our previous experiment [7] and in our present experiment. They demonstrate the following regularities: (1) the effect of depolarization is proportional to the holding time [7]; (2) the effect of depolarization is proportional to the surface area of the sample [7]; (3) the effect of depolarization arises inside the trap and does not arise in the UCN guide placed in magnetic field gradient near the superconducting solenoid [7]; (4) the effect of depolarization does not depend on the reduction of magnetic field in the UCN trap by the factor of 2, which was done by means of displacement of UCN trap with respect to the superconducting solenoid; (5) the effect of depolarization does not depend on the time of spin component splitting. However, the background effect (R b ) can be increased considerably by decreasing the time of spin component splitting. At last the energy dependence of UCN depolarization in the trap is similar to the energy dependence of UCN losses in the trap (Fig. 3). The measured results for R exp + for various samples in the trap at temperature of 300 and 80 K are presented in Fig. 4. The dashed line corresponds to control measurements for the empty trap without magnetic field in the superconducting solenoid. There is a background effect (R b ) because after emptying the trap approximately % of UCN still do not leave the trap. When the superconducting solenoid was switched on the UCN depolarization was measured for the empty trap (graphite coated copper surface

5 A.P. Serebrov et al. / Physics Letters A 313 (2003) Fig. 3. Results of measurements of probability of spin-flip (αν) = τfl 1 and probability of losses (µν) = τloss 1 in the trap for the UCN spectra with different upper energy, which were selected by means of the absorber position in gravitational spectrometer (h, mm). is (µν) = τloss 1 1 (left scale), is (αν) = τfl (right scale), is the number of UCN captured in the trap at the different positions of the absorber (relative units). Fig. 4. Measured results for R + exp(t h ) for various samples in the trap at temperatures of 300 and 80 K. The dashed line shows the reference measurements for the empty trap without magnetic field in the polarizer. Curve 1 is the R + measurements for the empty trap with polarization. Curves 2 and 2 are for the trap with cryo-insert at the 300 and 80 K, respectively. Curves 3 and 3 are after putting the copper rings with BeO coating onto the cryo-insert. Curves 4 and 4 are after adding Be foils to the previous configuration. (See text for detailed explanation.) curve 1). R + is not equal to zero at zero storage time (R 0 ) because depolarization takes place already during filling and emptying of the trap. When the cryo-insert was installed, the depolarization in the trap was increased because of the new surface and the increased number of collisions (curve 2). Cooling of the cryo-insert down to liquid nitrogen temperature did not change the slope of curve 2 with respect to that of curve 2. We see only a change in the background effect (R b ), which is connected with the temperature dependence of the storage time. (The low energy part of the UCN spectrum, which gives the main contribution to the background effect, will be stored more efficiently at lower temperature than the rest of the spectrum.) In the next step of this experiment we studied BeO coatings on copper rings, which were mounted on the cryo-insert. (Curves 3 and 3 at room temperature and near liquid nitrogen temperature, respectively.) Again, no temperature dependence of depolarization was observed. The same situation was repeated when beryllium foils were additionally mounted on the cryoinsert together with BeO coated copper rings. (Curves 4and4.) From these measurements we can conclude that no temperature dependence of UCN depolarization for the materials studied (C, BeO, Be) was observed. The upper limit is 20% for the temperature interval K (C.L. 90%). Besides measurements at various temperatures, measurements at room temperature without the cryoinsert were also performed. In these cases the samples were placed into the empty trap, and the resulting change in spin-flip probability was measured in comparison with the empty trap. The repeatability of results with the empty trap was checked several times in the course of measurements. The results of these measurements are presented in the Table 1. The accuracy is better for the UCN trap itself because it is measured directly; reduced accuracy for the samples is the result of using a difference measurement (trap plus sample, trap alone). The results of previous measurements for different traps with beryllium coating are also included in Table 1. One can see that the results for different Be coatings can be distinctly different. The largest depolarization was observed for BeO. It is possible that the different result for Be can be explained by different degrees of oxidation of the beryllium surface. It should be mentioned that new measurements using the gravitational spectrometer enabled us to calculate more accurately the collision frequency. As a result, we had to correct our earlier measurements [7]. In particular, for graphite, copper and teflon the probability of a spin-flip turned out to be remarkably

6 378 A.P. Serebrov et al. / Physics Letters A 313 (2003) Table 1 The probability of a UCN spin-flip per collision for various materials at room temperature. The accuracy of the measurements in the table is defined by the statistical accuracy of the measurements only. The accuracy of calculations of the number of collisions does not exceed 20 30%, and thus contributes correspondingly to the absolute experimental uncertainties Material Trap coating (Be) I (measurement in 1998) Trap coating (Be) II Be foil (measurements of 1998) Be foil (measurements of 2000) Be coating on copper rings Be coating on Al foil Teflon Fomblin 1 Cu (99,9%) Good Fellow Cu (a trap before graphite coating) Graphite foil Graphite trap coating Graphite coating on copper rings BeO coating on copper rings 1 Liquid perfluorinated polyether. α (0.72 ± 0.07) 10 5 (2.07 ± 0.05) 10 5 (1.58 ± 0.20) 10 5 (2.17 ± 0.21) 10 5 (1.15 ± 0.09) 10 5 (1.23 ± 0.21) 10 5 (0.60 ± 0.24) 10 5 (0.61 ± 0.13) 10 5 (0.73 ± 0.14) 10 5 (1.70 ± 0.10) 10 5 (0.59 ± 0.10) 10 5 (1.06 ± 0.06) 10 5 (0.70 ± 0.10) 10 5 (3.75 ± 0.33) 10 5 surface demonstrates that spin-incoherent scattering on hydrogen does not significantly contribute to the observed depolarization. In conclusion we summarize that during experimental studies of UCN depolarization processes during storage in material traps the following results have been obtained. Fig. 5. Measured results for the UCN loss probability µ (left scale) and UCN spin-flip α (right scale) as a result of water vapor condensing onto the cryo-insert surface. higher (at a level of 10 5 ). Unfortunately, attempts to find materials with spin-flip probabilities significantly lower than 10 5 per collision met with no success. Finally, Fig. 5 presents a measurement of the probability of a UCN spin-flip as a result of water vapor condensing onto the cryo-insert surface. The left vertical axis represents the probability of losses, and the right axis the probability of a spin-flip. A conclusion can be made that in spite of the significant influence of frozen water vapor on the loss probability, there is no sign of any influence on depolarization. The experiment with water vapor freezing over the cryo-insert (1) The process of UCN depolarization during their reflection from a substance exists for all the materials studied (Be, BeO, C, Cu, fluoroplastic, fomblin). The probability of a spin-flip is at a level of several units 10 5 per collision. Thus, the process of UCN reflection from surfaces is not completely coherent. (2) No temperature dependence of the spin-flip probability was observed for Be, BeO, C in the temperature range 300 to 80 K. The absence of any temperature dependence of the spin-flip probability indicates that after an incoherent interaction UCN scattered into the substance do not return into the vacuum. The general conclusion from the studies carried out is that depolarization exists for all the studied materials. The probability of depolarization is approximately (1 2) 10 5 per collision and is rather close to the probability of anomalous losses for Be ( ) [4]

7 A.P. Serebrov et al. / Physics Letters A 313 (2003) and, apparently, for solid fomblin ( ) [8]. It is very probable, that the phenomena of UCN depolarization and UCN anomalous losses are explained by spin-incoherent scattering on paramagnetic centers (clusters) or ferromagnetic clusters on the surface of the substance. Moreover, UCN scattered into the vacuum with a spin-flip result in the observed depolarization, and UCN scattered into the substance are the cause of anomalous losses. However, the real reason behind UCN depolarization is not yet clear and it is very surprising that hydrogen in frozen water did not increase the observed depolarization. The similarity of the values for the probability of depolarization and the probability of anomalous losses, and the absence of temperature dependence for both processes does not yet prove the above assumption about the connection between these phenomena. To obtain strict indications, it is necessary to measure the ratio of the probability of a spin-flip and the probability of anomalous losses in one experimental setup, in order to eliminate the uncertainty connected with calculation of the number of collisions. Due to the fact that anomalous losses can be observed at a lower temperature, this installation has to be a cryostat with the temperature of 4 K. Within the frame of the simplest hypothesis, the ratio of the probability of spinflip and the probability of anomalous losses has to be equal to 2/3. The simplest hypothesis is that after 4πscattering on the surface the UCN scattered into substance ( 2π)never come back into vacuum, and UCN scattered into vacuum (+2π)have, in accordance with the law of spin incoherent scattering, the probability of 2/3 to flip neutron spin. The building of the new experimental installation is planned to verify this hypothesis. Acknowledgements This work has been carried out at ILL and done with support of PNPI (Russia), ILL (France), and Princeton University (USA), and also grants INTAS No , RFBR No References [1] A.P. Serebrov, et al., LNPI preprint 1391, Leningrad (1988). [2] A.P. Serebrov, et al., PNPI preprint 1835, St. Petersburg (1992). [3] T. Bowles, et al., A letter of intent, LANL, Los Alamos, USA, [4] V.P. Alfimenkov, et al., JETP Lett. 55 (1992) 84. [5] A.P. Serebrov, et al., PNPI preprint 2193, Gatchina (1997). [6] A. Serebrov, in: Proceedings of the fifth International Seminar on Interaction of Neutrons with Nuclei (ISINN-5), Dubna, 1997, p. 67. [7] A. Serebrov, et al., Nucl. Instrum. Methods A 440 (2000) 717. [8] S.S. Arzumanov, et al., in: Proceedings of the third UCN Workshop, Pushkin, 2001.