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A novel apparatus for the investigation of material properties for the storage of ultracold neutrons
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Nuclear Instruments and Methods in Physics Research A 550 (2005) 637–646 www.elsevier.com/locate/nima
A novel apparatus for the investigation of material properties for the storage of ultracold neutrons T. Brys´ a, M. Dauma, P. Fierlingera,b,, P. Geltenbortc, D. Georgea, M. Guptaa, R. Hennecka, S. Heulea,b, M. Horvata, M. Kasprzaka,d,e, K. Kircha, K. Kohlika, M. Negrazusa, A. Pichlmaiera, U. Straumannb, V. Vrankovica, C. Wermelingerf a
PSI, Paul Scherrer Institut, CH 5232 Villigen PSI, Switzerland b Physik-Institut, Universita¨t Zu¨rich, Switzerland c ILL, Institut Laue-Langevin, Grenoble, France d Jagiellonian University, Cracow, Poland e SMI, Stefan-Meyer-Institut, Vienna, Austria f ETHZ, Eidgeno¨ssische Technische Hochschule Zu¨rich, Switzerland Received 14 April 2005; accepted 11 May 2005
Abstract We have built a novel apparatus for the investigation of materials for the storage of ultracold neutrons. Neutrons are filled into a storage volume, confined at the bottom by a magnetic field, at the top by gravity and at the sides by the slitless sample surface under investigation. For different beryllium and diamond-like carbon samples, storage times up to 200 s were obtained at room temperature. The corresponding loss parameters Z for ultracold neutrons varied between 4.2 and 6:8 104 per wall collision. r 2005 Published by Elsevier B.V. PACS: 29.25.Dz; 28.20.v; 55.55.jd Keywords: Ultracold neutrons; Diamond-like carbon; Ultracold neutron storage; Ultracold neutron losses; Anomalous losses; Magnetic mirrors
1. Introduction
Corresponding author. Tel.: +41 56 310 5291;
fax: +41 56 310 3294. E-mail address: peter.fi[email protected] (P. Fierlinger). 0168-9002/$ - see front matter r 2005 Published by Elsevier B.V. doi:10.1016/j.nima.2005.05.076
Coherent interaction of very slow neutrons with material surfaces can be described by the Fermi potential. It is of the order of a few hundred nanoelectron volt and positive, i.e. repulsive, for most materials. Neutrons with kinetic energies
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lower than a Fermi potential of 300 neV, corresponding to temperatures of a few millikelvin, are called ultracold neutrons (UCNs). They are totally reflected from surfaces under all angles of incidence with losses on a level of 104 2105 per wall collision. This allows for their storage in vessels with a storage time of several hundred seconds for precision experiments in Nuclear and Particle Physics, e.g. the search for the electric dipole moment of the neutron [1] and the determination of the neutron lifetime [2,3]. Further improvements of the sensitivity of such precision experiments call for higher UCN densities and more sophisticated tools to handle systematic uncertainties. For the next generation of ultracold neutron sources, solid deuterium [4] or superfluid helium [5] will be used as UCN converters. An increase in several orders of magnitude in the neutron density is expected compared to todays leading facility at ILL, the Institute Laue Langevin in Grenoble, France [6]. This was demonstrated in reactor experiments [7] and at neutron spallation facilities [8–10]. In parallel to the construction of higher intensity UCN sources, new materials for coatings of storage vessel surfaces and UCN guides with optimal reflection performance are developed and investigated. For suitable coating materials, the highest Fermi potential was found for 58Ni (335 neV); other materials are beryllium (252 neV) and BeO (261 neV) [11]. While 58Ni is predominantly used for high transmission UCN guides, its use as a coating material for UCN storage volumes is limited by the relatively high absorption cross section (4.6 b [12]). Beryllium and BeO are frequently used in UCN applications. These materials, however, are toxic resulting in complications in the production process and handling. A very promising alternative is diamond-like carbon (DLC). First characterizations and developments have been performed successfully [13–17]. In this paper, we describe a novel apparatus for the investigation of surface properties of such new materials for the storage of ultracold neutrons in future high intensity UCN sources and experiments. The measurements of storage times with this apparatus allows for extraction of UCN losses
on material walls and, simultaneously, the precise determination of depolarization effects.
2. Experimental apparatus In order to achieve long storage times, UCN bottles need to be hermetically sealed. Therefore, the construction of mechanical neutron shutters is demanding and absorbing slits are practically unavoidable. Another issue is the maintenance of excellent vacuum conditions in a sealed container during times of up to several hundred seconds. In the experiment described here, we get around these problems by replacing the mechanical shutter at the bottom of the storage volume by a rapidly switchable magnetic field which inherently lacks all gaps.1 We use a vertical tube made from the material to be tested as storage volume, closed for UCN by gravity on the top but open for vacuum pumping. In this way, the storage vessel does not contain slits, which are a well known source of UCN losses.2 As a result, the apparatus represents an excellent tool for the investigation of material surfaces for the storage of ultracold neutrons. 2.1. Magnetic shutter The magnetic shutter is shown schematically in Fig. 1. It consists of a H-type dipole magnet, 80 cm high, 125 cm wide and 40 cm long; its weight is 3500 kg. The magnet poles have diameters of 40 cm at the top (and bottom) and 27.2 cm at the gap. A bore for the vacuum chamber to be inserted for the UCN storage sample and the neutron guides (see below) was drilled vertically through the yokes and the poles at the centre of the magnet. The bore diameters are 121 mm in the top yoke and 81 mm through the poles and the bottom yoke. For calibration purposes, the magnetic field in the centre of the magnet, i.e. in the midplane of the gap and in the centre of the bore, was measured as 1
The principle to store very slow neutrons in magnetic fields was first discussed by Vladimirski [18]. First experiments with such a system were performed by Abov et al. [19]. 2 A cylindrical sample layout is usually well suited for the production and application of coatings.
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Fig. 1. Dipole magnet for the activation of the magnetic shutter field for the storage of polarized ultracold neutrons. The magnet is 80 cm high, 125 cm wide and 40 cm long; for details, see text. The field lines in the relevant area of the vertical bore are indicated. 1: magnet yoke; 2: coils; 3: magnet poles; 4: bore for the vacuum chamber, sample, etc. for the storage of UCN.
Fig. 2. Magnetic field in the centre of the cylindrical bore and in the midplane of the gap as a function of power supply current.
a function of the magnet current, see Fig. 2. For the storage of neutrons, the magnetic field along the z-axis, i.e. in the vertical direction in the centre of the bore, is important. The field distribution along z as a function of the magnet current was measured with a Hall probe and is shown in Fig. 3. At the maximum current of 300 A, the field distribution in the midplane of the gap and along the pole radius r was calculated [20]. The result is displayed in Fig. 4. At the maximum current of
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Fig. 3. Magnetic field along the vertical (z) axis in the cylindrical bore ðr ¼ 0Þ as a function of the power supply current.
Fig. 4. Calculated vertical magnetic field component in the midplane of the gap ðz ¼ 0Þ for a magnet current of 300 A as a function of the pole radius r. For the calculation, we used the Poisson Superfish code [20].
300 A, the magnetic field in the centre of the gap between the pole faces is 2.75 T; it decreases to 1.52 T in the centre of the bore. The latter value is in agreement with the magnetic field measurements, cf. Fig. 3. The magnet coils have an ohmic resistance of 0:582 O. The switching time, t ¼ L=R, where L is the inductance and R the ohmic resistance of the magnet, for various magnetic field excitations is shown in Fig. 5.
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Fig. 5. Turn-on time for the magnetic field at point (r ¼ 0, z ¼ 0) for different magnet currents. The arrow indicates the point in time when the power supply voltage is turned on.
2.2. Experimental setup The experimental setup is shown in Fig. 6. It consists of several components which are specified below. 2.2.1. Cryo cooler The upper part of the experimental setup consists of a two-stage cryo cooler. It is a He driven compressor based on a Gifford–Mc Mahon cycle (25, Fig. 6). It has a 70 K stage (60 W) and a second stage with a cooling power of 10 W at 10 K. The cryo cooler housing is fixed to the magnet with a support frame. In order to avoid propagation of mechanical vibrations to the samples, the cryo cooler is only loosely coupled to the vacuum system through stainless-steel vacuum bellows. 2.2.2. Thermal shield The 70 K stage of the cooler is connected by flexible copper connections to the thermal copper shield (17, Fig. 6) which reaches down inside the vacuum chamber into the yoke of the magnet. 2.2.3. Sample vacuum and samples The 10 K stage of the cooler is connected to the sample by flexible copper connections (21, Fig. 6). The sample and the thermal shield are open at their upper ends in order to allow for vacuum pumping. This is done by a turbomolecular pump
Fig. 6. Experimental apparatus for the investigation of materials for the storage of UCNs: 1: Ni-coated neutron guide from the turbine; 2: beam line shutter; 3: stainless steel neutron guide; 4: neutron switch, see Fig. 7; 5: detector; 6: vacuum pumping port; 7: vacuum separation foil; 8: vertical neutron guide; 9: sample; 10: magnet yoke; 11: magnet coils with poles (horizontal); 12: insulation vacuum; 13: vacuum pumping port; 14: sensor feedthroughs; 15: holding field coils; 16: vacuum separation membrane; 17: thermal shield (copper); 18: vacuum pump; 19: clean sample vacuum; 20: sensor feedthrough; 21: flexible thermal copper connections; 22: 10 K cold stage; 23: 70 K stage; 24: vibration decoupling; 25: cryo cooler.
system (18, Fig. 6) with a drag of 500 dm3/s for air and 480 dm3/s for hydrogen. The sample vacuum is separated from the thermal isolation vacuum in the lower part of the system by a teflon ring (16, Fig. 6). The sample ends near the lower pole face.
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Pressures of 1 106 hPa for the insulation vacuum and 1 107 hPa for the sample vacuum were obtained at room temperature; at 70 K, 1 107 and 1 109 hPa were reached, respectively. The temperatures were measured with various 100 Pt and Cernox sensors distributed along the sample (outer surface) and the thermal shield. The thermal shield reached a temperature of 70 K. The temperature of the sample had a gradient: at its upper end, i.e. close to the cold stage, the temperature was 40 K; at the lower end, 100 K were measured. As samples, aluminium and quartz cylinders coated with beryllium and one aluminium cylinder coated with DLC were used. The samples with Aluminium substrate were cylindrical tubes with a length of 1080 mm. They had an inner diameter (ID) of 70 mm and an outer diameter (OD) of 90 mm in the upper part, i.e. above the upper pole of the magnet. Within the magnet pole, the outer diameter was 73 mm. The quartz sample was also 1080 mm long and had OD ¼ 70 mm and ID ¼ 65 mm. 2.2.4. Main vacuum chamber Below the above-mentioned vacuum pumping unit (18, Fig. 6), a vacuum tube (diameters 120 mm in the upper part and 80 mm in the lower part) extends into the magnet. At its upper end, the teflon ring mentioned above (16, Fig. 6), separates the sample vacuum from the thermal insulation vacuum. This thermal insulation vacuum is pumped by two turbomolecular pumping units, each with a drag of 60 dm3/s nitrogen, connected to the flanges (6 and 13, Fig. 6) of the vacuum system. 2.2.5. Neutron switch Below the cross-shaped vacuum vessel with the pumping flange (6, Fig. 6), the neutron switch is installed, see Fig. 7. It consists of a horizontally movable piston in a special vacuum chamber. This piston has two positions. (1) An electro-polished stainless steel mirror for the deflection of the incoming ultracold neutrons (1, Fig. 6) into the vertical neutron guide and the sample tube (8 and 9, Fig. 6). The
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Fermi potential of stainless steel is 190 neV [21]; the vertical distance from the mirror to the top of the sample is 1750 mm. (2) An open bore with an electro-polished stainless steel surface as a connection from the sample tube to the detector (5, Fig. 6); in this position, the switch serves also to block the incoming UCN beam. The top flange of the neutron switch and the short vertical tube to the vacuum separation foil (7, Fig. 6) is made from one piece of stainless steel. A thin (7:5 mm) Mylar foil, supported by two congruent honeycomb grids made of 0.2-mm-thin stainless steel separates the vacua of the neutron switch and the horizontal neutron guide (1 and 3, Fig. 6) from the vertical neutron guides and the sample tube (8 and 9, Fig. 6). The transparency of the honeycomb grids was 93%. 2.2.6. Vertical neutron guide The neutron guide (8, Fig. 6) reaches up to the lower pole of the magnet. The sample tube (9, Fig. 6) is sitting on the upper end of the neutron guide, as shown in Fig. 8. The connection between the two components is not vacuum tight; however, the leak rate between the two components is very small and has only negligible influence on the sample vacuum. All neutron guides are electropolished stainless steel tubes. The upper part of the equipment (sample, thermal shield etc.) could be heated to 100 1C.
3. Measurements with UCN The experiment was installed at the UCN beam line at ILL PF2 [22,23]. It was operated in the following way: ultracold neutrons from the turbine [6] enter the installation from the right through a rotatable shutter (2, Fig. 6) in open position and a feeding guide (3, Fig. 6). With the UCN switch (4, Fig. 6) in filling position, UCNs are reflected into the vertical neutron guide (8, Fig. 6) and the sample volume (9, Fig. 6). After 15 s of filling, equilibrium neutron density in the sample is reached and the magnetic field is turned on for the storage of UCNs in the sample tube. Then, the
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Fig. 7. Neutron switch for the deflection of UCNs. Left: stainless steel mirror for the deflection of the incoming UCNs into the vertical neutron guide; ‘filling’ position. Right: vertical neutron guide as a connection from the sample under investigation to the detector, ‘emptying’ position. This position blocks UCN incident from the right.
mechanical neutron switch is moved within 1 s from the filling position to the emptying position. The shutter (2, Fig. 6) is also closed for improved background suppression. 3.1. UCN storage
Fig. 8. Schematic view of the interconnection between the neutron guide and the sample tube. The horizontal dashed line represents the midplane of the 3-cm-high magnet gap, the vertical one the centre of the cylindrical bore through the magnet. 1: sample; 2: vertical neutron guide; 3: vacuum chamber; 4: magnet poles. The neutron guide from below ends near the lower pole face with a sharp edge on which the sample is quasi tightly sitting. In this way, the slit between neutron guide and sample, i.e. between the sample vacuum and the isolation vacuum, is minimal.
When a neutron in the storage volume enters the magnetic field B, it is reflected if the orientation of its magnetic moment is antiparallel to the direction of the magnetic field and if its kinetic energy is EpE B ¼ 1:91mN B ¼ 60 neV=T B, with mN the nuclear magneton. Neutrons with energies E4E B can penetrate the magnetic field barrier. Neutrons with their magnetic moments oriented parallel to B are accelerated into the field and thus fall into the detector. In our case, the latter occurs over a period of about 100 s after the magnetic field is switched on. The procedure to remove neutrons with the wrong spin orientation or too
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Fig. 9. Detector counts for sequences ‘filling’, ‘cleaning’, ‘holding’, and ‘emptying’ for different holding times ti , t1 t0 ¼ 20 s (top) and t2 t0 ¼ 220 s (bottom). The magnetic field B as a function of time is also shown.
‘high’ energies from the storage volume is called ‘spectral cleaning’, see Fig. 9 between 15 and 115 s. Simulations [24] showed that the cleaning time depends critically on the efficient removal of UCN with energies slightly above E B . These neutrons can only pass the magnetic field in the relatively rare case that they enter it on a trajectory nearly parallel to the field direction. It was found that the cleaning time could be shortened by ramping the field initially to 0:9B and increasing it to the full value only after 100 s.3 This procedure limits the maximum energy of storable UCN to 90% of the theoretically possible energy but guarantees that the numbers of UCN with ‘wrong’ spin state and of neutrons with energies above the magnetic barrier in the storage volume at the beginning of the measuring cycle are negligible. After cleaning, the neutrons are stored for the holding time ti (20 s up to a few hundred seconds). The events reaching the detector during this holding time originate from wall interactions in which the spin of the neutron was flipped. 3
From a Monte Carlo calculation [24] we found a minimum of 60 s for cleaning. With a safety margin we choose, however, 100 s.
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After the holding time, the magnet is switched off and all remaining neutrons fall into the detector where they are counted as N i . The whole procedure is repeated with different holding times. From the measured neutron counts at different holding times, one obtains the storage time tst (see below). The data taking procedure is illustrated in Fig. 9. As detector, we used a gas counter with 18 hPa 3He and 10 hPa CO2 in about 1100 hPa Ar. In order to penetrate the aluminium entrance window of the detector (critical energy of aluminium E c ðAlÞ ¼ 54 neV), the neutrons were accelerated by gravity over a vertical distance of 100 cm. The detector was shielded using polyethylene and borated rubber. Its background was (4:8 0:1Þ 103 s1 over the whole beamtime.
3.2. Monte Carlo calculations For the simulation of UCN motion in storage bottles, a Monte Carlo programme [24] based on GEANT4 [25] was used. The new programme was tested successfully by calculating the cleaning process in our storage sample, see Fig. 10. By switching on the magnetic field in order to start the storage process, neutrons that are in the magnetic field region during the magnetic field ramping experience an energy shift originating from the interaction of the neutron magnetic moment with the increasing magnetic field. This process was simulated by the same MC code; the result is displayed in Fig. 11. With a switching time of 3 s, the zero level is shifted by 35 neV; neutrons with the lowest energies, i.e. low velocities, stay in the magnetic field region for longer times and thus experience a larger energy shift during ramping. For ‘higher’ energies, the corresponding shift is smaller, see Fig. 11. The integrated energy spectrum of the stored neutrons was determined by performing measurements with the magnetic field B set to different values between 0.90 and 1.52 T. At each magnet setting B, the respective energy spectrum of stored UCN is registered; this is called the integral energy spectrum.
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Fig. 10. Measured (full dots) and calculated neutron counts in the detector during the ‘cleaning’ process, see text. The discrepancy at t ¼ 0 originates from the uncertainty in the initial conditions.
Fig. 12. Measured integral (top) and derived differential energy spectrum of stored UCN. The small triangles in the differential spectrum are Monte Carlo data [26]; the line is to guide the eyes.
Fig. 11. Calculated UCN energy spectrum in the sample before and after switching on a magnetic field of 1.52 T. After switching on, the zero level is shifted upwards by about 35 neV.
N 2 , the storage time tst of the neutrons in the storage volume is given by tst ¼
The differential UCN energy spectrum is obtained from the integral spectrum by taking the differences of the number of neutrons between neighbouring field values, see Fig. 12. The spectra were corrected for the field-dependent energy shift of each magnet setting with the MC code [24]. 3.3. Results For two different holding times (t1 t0 ¼ 20 s and t2 t0 ¼ 220 s) and detector counts N 1 and
t2 t1 . lnðN 1 =N 2 Þ
(1)
In Eq. (1), we replace N 2 by N 2 ¼ N 2 þ N sf , where N sf corresponds to the number of spin-flipped neutrons measured between t1 and t2 , see Fig. 9. The number N sf , which is about an order of magnitude larger than the background, is corrected for background and for the total storage time; for details see Ref. [26]. The spin-flip corrected storage time t st is connected to the neutron lifetime tn and the
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wall-loss lifetime tloss : ðt st Þ1 1 ¼ t1 n þ tloss Z EB Z ¼ t1 þ n 0
HðEÞ
nðh; EÞmðh; EÞgðEÞ dh dE. 0
ð2Þ The wall-loss lifetime tloss depends on the wall collision frequency nðEÞ,4 the loss probability per wall collision mðEÞ and the stored normalized UCN energy distribution gðEÞ. The integration over the vertical tube dimension h is necessary since gravity causes the UCN velocity and density distribution to vary over the tube height. Evaluation of mðEÞ from Eq. (2) thus requires solving the integral which can be done analytically for simple cases only (see, e.g., Refs. [11,27]). In our case, the distribution of the UCN energy is additionally modified by the magnetic field, cf. Fig. 11. Therefore, we used MC calculations to evaluate Eq. (2); the calculated magnetic field was used as an input parameter. The energy dependence is given by the equation (see also Ref. [11]) " 1=2 # 1=2 V V 1 E mðEÞ ¼ 2Z sin 1 . (3) E V E Here, V is the real part of the Fermi potential and E the kinetic energy of the neutron. The energy dependence (Eq. (3)) was taken from Ref. [27], which successfully describes the energy dependence of the losses on Fomblin oil and grease. The dependence of the loss parameter Z on the measured storage time as obtained from our Monte Carlo calculation of Eq. (2) is displayed in Fig. 13. This allows one to determine the loss parameter Z from the measured storage time. We investigated different samples with (i) beryllium coating on aluminium substrates (two different samples), (ii) beryllium coating on quartz, and (iii) DLC coating on aluminium. All data were taken with the full UCN spectrum (E B o83 neV) storeable in our apparatus. The results are summarized in Table 1. In all cases, the losses due to depolarization (spin flip) during storage were more than an order of magnitude smaller 4
Fig. 13. Dependence of the loss coefficient Z on the storage time tst according to Eq. (2). The upper curve is for our aluminium tubes, the lower for the quartz tube which has a slightly different geometry, see text (2.2.3).
than the losses originating from wall collisions, see also Refs. [26,28]. The uncertainties in our results include the statistical errors of the measurements and an estimate of a systematic contribution due to the spectral distribution. Due to lack of time during the first data-taking period, in all cases the sample surfaces were measured without any elaborate conditioning. It is well known, however, that the losses on beryllium decrease considerably e.g. by baking at 620 K in helium or deuterium atmosphere [29]. For comparison, the literature value of Z for beryllium at room temperature ranges around 1:1 104 24:3 104 [29]. To our knowledge, no values for the loss parameter Z have been reported for DLC coatings so far. The detailed analysis of the data including depolarization losses due to wall interactions with spin flip are in progress. Data at a sample temperature of 70 K and for a variety of substrates and coatings were taken successfully at a later date. The analysis of these data is still going on and the results will be published elsewhere [26,28].
4. Conclusions
1
We calculated average wall-collision frequencies nðEÞ30 s depending on sample geometry and the magnetic field value during ‘cleaning’.
The apparatus described above allows for the investigation of materials for the storage of
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Table 1 Measured numbers of neutrons and the corresponding loss parameter Z after storage for various samples at room temperature Sample no.
Sample
Substrate
N 1 ðavÞ
1 2 3 4
Be Be Be DLC
Al Al Quartz Al
381 292 262 321
N 2 ðavÞ
714 712 73 713
145 71 63 93
75 73 71 74
tst ½s 200 138 133 161
711 75 74 76
Z [104 ] (4.2 (6.8 (5.0 (5.8
70.3) 70.3) 70.1) 70.3)
The cleaning time was 100 s at 90% of the magnetic field. The storage times were t1 ¼ 20 s and t2 ¼ 220 s at 100% field value (1.52 T). The numbers of neutrons N i are averaged over several data-taking cycles.
ultracold neutrons (UCNs). UCNs are confined by a magnetic field of 1.52 T at the bottom and by gravity at the top. The apparatus is designed to avoid any gaps contributing to UCN losses. It allows for measuring UCN losses per wall collision. Simultaneously, one can determine the depolarization of polarized UCN from the material surface under investigation. First experimental tests at the ILL UCN source were performed very successfully, resulting in storage times of up to 200 s for different surface materials at room temperature. The corresponding loss parameters Z for UCNs varied between 4.2 and 6:8 104 per wall collision.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements [17]
The experiment was designed and built at PSI. We thank the workshops at PSI, especially M. Mu¨ller and U. Bugmann, for their outstanding work in fabrication of many apparatus components. The data acquisition took place at ILL, Grenoble. We acknowledge the great support of Th. Brenner and the ILL reactor crew. We also thank A. Strelkov, JINR, for providing us with a 3 He UCN detector. We are very grateful to I. Altarev, TU Munich, M. Lasakov, PNPI, M. Makela and B. Vogelaar from Virginia Tech, and A. Young from NCSU, who prepared the beryllium and DLC coating for the samples.
[18] [19] [20]
[21] [22] [23] [24] [25] [26] [27]
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Nuclear Instruments and Methods in Physics Research A 550 (2005) 637–646 www.elsevier.com/locate/nima
A novel apparatus for the investigation of material properties for the storage of ultracold neutrons T. Brys´ a, M. Dauma, P. Fierlingera,b,, P. Geltenbortc, D. Georgea, M. Guptaa, R. Hennecka, S. Heulea,b, M. Horvata, M. Kasprzaka,d,e, K. Kircha, K. Kohlika, M. Negrazusa, A. Pichlmaiera, U. Straumannb, V. Vrankovica, C. Wermelingerf a
PSI, Paul Scherrer Institut, CH 5232 Villigen PSI, Switzerland b Physik-Institut, Universita¨t Zu¨rich, Switzerland c ILL, Institut Laue-Langevin, Grenoble, France d Jagiellonian University, Cracow, Poland e SMI, Stefan-Meyer-Institut, Vienna, Austria f ETHZ, Eidgeno¨ssische Technische Hochschule Zu¨rich, Switzerland Received 14 April 2005; accepted 11 May 2005
Abstract We have built a novel apparatus for the investigation of materials for the storage of ultracold neutrons. Neutrons are filled into a storage volume, confined at the bottom by a magnetic field, at the top by gravity and at the sides by the slitless sample surface under investigation. For different beryllium and diamond-like carbon samples, storage times up to 200 s were obtained at room temperature. The corresponding loss parameters Z for ultracold neutrons varied between 4.2 and 6:8 104 per wall collision. r 2005 Published by Elsevier B.V. PACS: 29.25.Dz; 28.20.v; 55.55.jd Keywords: Ultracold neutrons; Diamond-like carbon; Ultracold neutron storage; Ultracold neutron losses; Anomalous losses; Magnetic mirrors
1. Introduction
Corresponding author. Tel.: +41 56 310 5291;
fax: +41 56 310 3294. E-mail address: peter.fi[email protected] (P. Fierlinger). 0168-9002/$ - see front matter r 2005 Published by Elsevier B.V. doi:10.1016/j.nima.2005.05.076
Coherent interaction of very slow neutrons with material surfaces can be described by the Fermi potential. It is of the order of a few hundred nanoelectron volt and positive, i.e. repulsive, for most materials. Neutrons with kinetic energies
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lower than a Fermi potential of 300 neV, corresponding to temperatures of a few millikelvin, are called ultracold neutrons (UCNs). They are totally reflected from surfaces under all angles of incidence with losses on a level of 104 2105 per wall collision. This allows for their storage in vessels with a storage time of several hundred seconds for precision experiments in Nuclear and Particle Physics, e.g. the search for the electric dipole moment of the neutron [1] and the determination of the neutron lifetime [2,3]. Further improvements of the sensitivity of such precision experiments call for higher UCN densities and more sophisticated tools to handle systematic uncertainties. For the next generation of ultracold neutron sources, solid deuterium [4] or superfluid helium [5] will be used as UCN converters. An increase in several orders of magnitude in the neutron density is expected compared to todays leading facility at ILL, the Institute Laue Langevin in Grenoble, France [6]. This was demonstrated in reactor experiments [7] and at neutron spallation facilities [8–10]. In parallel to the construction of higher intensity UCN sources, new materials for coatings of storage vessel surfaces and UCN guides with optimal reflection performance are developed and investigated. For suitable coating materials, the highest Fermi potential was found for 58Ni (335 neV); other materials are beryllium (252 neV) and BeO (261 neV) [11]. While 58Ni is predominantly used for high transmission UCN guides, its use as a coating material for UCN storage volumes is limited by the relatively high absorption cross section (4.6 b [12]). Beryllium and BeO are frequently used in UCN applications. These materials, however, are toxic resulting in complications in the production process and handling. A very promising alternative is diamond-like carbon (DLC). First characterizations and developments have been performed successfully [13–17]. In this paper, we describe a novel apparatus for the investigation of surface properties of such new materials for the storage of ultracold neutrons in future high intensity UCN sources and experiments. The measurements of storage times with this apparatus allows for extraction of UCN losses
on material walls and, simultaneously, the precise determination of depolarization effects.
2. Experimental apparatus In order to achieve long storage times, UCN bottles need to be hermetically sealed. Therefore, the construction of mechanical neutron shutters is demanding and absorbing slits are practically unavoidable. Another issue is the maintenance of excellent vacuum conditions in a sealed container during times of up to several hundred seconds. In the experiment described here, we get around these problems by replacing the mechanical shutter at the bottom of the storage volume by a rapidly switchable magnetic field which inherently lacks all gaps.1 We use a vertical tube made from the material to be tested as storage volume, closed for UCN by gravity on the top but open for vacuum pumping. In this way, the storage vessel does not contain slits, which are a well known source of UCN losses.2 As a result, the apparatus represents an excellent tool for the investigation of material surfaces for the storage of ultracold neutrons. 2.1. Magnetic shutter The magnetic shutter is shown schematically in Fig. 1. It consists of a H-type dipole magnet, 80 cm high, 125 cm wide and 40 cm long; its weight is 3500 kg. The magnet poles have diameters of 40 cm at the top (and bottom) and 27.2 cm at the gap. A bore for the vacuum chamber to be inserted for the UCN storage sample and the neutron guides (see below) was drilled vertically through the yokes and the poles at the centre of the magnet. The bore diameters are 121 mm in the top yoke and 81 mm through the poles and the bottom yoke. For calibration purposes, the magnetic field in the centre of the magnet, i.e. in the midplane of the gap and in the centre of the bore, was measured as 1
The principle to store very slow neutrons in magnetic fields was first discussed by Vladimirski [18]. First experiments with such a system were performed by Abov et al. [19]. 2 A cylindrical sample layout is usually well suited for the production and application of coatings.
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Fig. 1. Dipole magnet for the activation of the magnetic shutter field for the storage of polarized ultracold neutrons. The magnet is 80 cm high, 125 cm wide and 40 cm long; for details, see text. The field lines in the relevant area of the vertical bore are indicated. 1: magnet yoke; 2: coils; 3: magnet poles; 4: bore for the vacuum chamber, sample, etc. for the storage of UCN.
Fig. 2. Magnetic field in the centre of the cylindrical bore and in the midplane of the gap as a function of power supply current.
a function of the magnet current, see Fig. 2. For the storage of neutrons, the magnetic field along the z-axis, i.e. in the vertical direction in the centre of the bore, is important. The field distribution along z as a function of the magnet current was measured with a Hall probe and is shown in Fig. 3. At the maximum current of 300 A, the field distribution in the midplane of the gap and along the pole radius r was calculated [20]. The result is displayed in Fig. 4. At the maximum current of
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Fig. 3. Magnetic field along the vertical (z) axis in the cylindrical bore ðr ¼ 0Þ as a function of the power supply current.
Fig. 4. Calculated vertical magnetic field component in the midplane of the gap ðz ¼ 0Þ for a magnet current of 300 A as a function of the pole radius r. For the calculation, we used the Poisson Superfish code [20].
300 A, the magnetic field in the centre of the gap between the pole faces is 2.75 T; it decreases to 1.52 T in the centre of the bore. The latter value is in agreement with the magnetic field measurements, cf. Fig. 3. The magnet coils have an ohmic resistance of 0:582 O. The switching time, t ¼ L=R, where L is the inductance and R the ohmic resistance of the magnet, for various magnetic field excitations is shown in Fig. 5.
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Fig. 5. Turn-on time for the magnetic field at point (r ¼ 0, z ¼ 0) for different magnet currents. The arrow indicates the point in time when the power supply voltage is turned on.
2.2. Experimental setup The experimental setup is shown in Fig. 6. It consists of several components which are specified below. 2.2.1. Cryo cooler The upper part of the experimental setup consists of a two-stage cryo cooler. It is a He driven compressor based on a Gifford–Mc Mahon cycle (25, Fig. 6). It has a 70 K stage (60 W) and a second stage with a cooling power of 10 W at 10 K. The cryo cooler housing is fixed to the magnet with a support frame. In order to avoid propagation of mechanical vibrations to the samples, the cryo cooler is only loosely coupled to the vacuum system through stainless-steel vacuum bellows. 2.2.2. Thermal shield The 70 K stage of the cooler is connected by flexible copper connections to the thermal copper shield (17, Fig. 6) which reaches down inside the vacuum chamber into the yoke of the magnet. 2.2.3. Sample vacuum and samples The 10 K stage of the cooler is connected to the sample by flexible copper connections (21, Fig. 6). The sample and the thermal shield are open at their upper ends in order to allow for vacuum pumping. This is done by a turbomolecular pump
Fig. 6. Experimental apparatus for the investigation of materials for the storage of UCNs: 1: Ni-coated neutron guide from the turbine; 2: beam line shutter; 3: stainless steel neutron guide; 4: neutron switch, see Fig. 7; 5: detector; 6: vacuum pumping port; 7: vacuum separation foil; 8: vertical neutron guide; 9: sample; 10: magnet yoke; 11: magnet coils with poles (horizontal); 12: insulation vacuum; 13: vacuum pumping port; 14: sensor feedthroughs; 15: holding field coils; 16: vacuum separation membrane; 17: thermal shield (copper); 18: vacuum pump; 19: clean sample vacuum; 20: sensor feedthrough; 21: flexible thermal copper connections; 22: 10 K cold stage; 23: 70 K stage; 24: vibration decoupling; 25: cryo cooler.
system (18, Fig. 6) with a drag of 500 dm3/s for air and 480 dm3/s for hydrogen. The sample vacuum is separated from the thermal isolation vacuum in the lower part of the system by a teflon ring (16, Fig. 6). The sample ends near the lower pole face.
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Pressures of 1 106 hPa for the insulation vacuum and 1 107 hPa for the sample vacuum were obtained at room temperature; at 70 K, 1 107 and 1 109 hPa were reached, respectively. The temperatures were measured with various 100 Pt and Cernox sensors distributed along the sample (outer surface) and the thermal shield. The thermal shield reached a temperature of 70 K. The temperature of the sample had a gradient: at its upper end, i.e. close to the cold stage, the temperature was 40 K; at the lower end, 100 K were measured. As samples, aluminium and quartz cylinders coated with beryllium and one aluminium cylinder coated with DLC were used. The samples with Aluminium substrate were cylindrical tubes with a length of 1080 mm. They had an inner diameter (ID) of 70 mm and an outer diameter (OD) of 90 mm in the upper part, i.e. above the upper pole of the magnet. Within the magnet pole, the outer diameter was 73 mm. The quartz sample was also 1080 mm long and had OD ¼ 70 mm and ID ¼ 65 mm. 2.2.4. Main vacuum chamber Below the above-mentioned vacuum pumping unit (18, Fig. 6), a vacuum tube (diameters 120 mm in the upper part and 80 mm in the lower part) extends into the magnet. At its upper end, the teflon ring mentioned above (16, Fig. 6), separates the sample vacuum from the thermal insulation vacuum. This thermal insulation vacuum is pumped by two turbomolecular pumping units, each with a drag of 60 dm3/s nitrogen, connected to the flanges (6 and 13, Fig. 6) of the vacuum system. 2.2.5. Neutron switch Below the cross-shaped vacuum vessel with the pumping flange (6, Fig. 6), the neutron switch is installed, see Fig. 7. It consists of a horizontally movable piston in a special vacuum chamber. This piston has two positions. (1) An electro-polished stainless steel mirror for the deflection of the incoming ultracold neutrons (1, Fig. 6) into the vertical neutron guide and the sample tube (8 and 9, Fig. 6). The
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Fermi potential of stainless steel is 190 neV [21]; the vertical distance from the mirror to the top of the sample is 1750 mm. (2) An open bore with an electro-polished stainless steel surface as a connection from the sample tube to the detector (5, Fig. 6); in this position, the switch serves also to block the incoming UCN beam. The top flange of the neutron switch and the short vertical tube to the vacuum separation foil (7, Fig. 6) is made from one piece of stainless steel. A thin (7:5 mm) Mylar foil, supported by two congruent honeycomb grids made of 0.2-mm-thin stainless steel separates the vacua of the neutron switch and the horizontal neutron guide (1 and 3, Fig. 6) from the vertical neutron guides and the sample tube (8 and 9, Fig. 6). The transparency of the honeycomb grids was 93%. 2.2.6. Vertical neutron guide The neutron guide (8, Fig. 6) reaches up to the lower pole of the magnet. The sample tube (9, Fig. 6) is sitting on the upper end of the neutron guide, as shown in Fig. 8. The connection between the two components is not vacuum tight; however, the leak rate between the two components is very small and has only negligible influence on the sample vacuum. All neutron guides are electropolished stainless steel tubes. The upper part of the equipment (sample, thermal shield etc.) could be heated to 100 1C.
3. Measurements with UCN The experiment was installed at the UCN beam line at ILL PF2 [22,23]. It was operated in the following way: ultracold neutrons from the turbine [6] enter the installation from the right through a rotatable shutter (2, Fig. 6) in open position and a feeding guide (3, Fig. 6). With the UCN switch (4, Fig. 6) in filling position, UCNs are reflected into the vertical neutron guide (8, Fig. 6) and the sample volume (9, Fig. 6). After 15 s of filling, equilibrium neutron density in the sample is reached and the magnetic field is turned on for the storage of UCNs in the sample tube. Then, the
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Fig. 7. Neutron switch for the deflection of UCNs. Left: stainless steel mirror for the deflection of the incoming UCNs into the vertical neutron guide; ‘filling’ position. Right: vertical neutron guide as a connection from the sample under investigation to the detector, ‘emptying’ position. This position blocks UCN incident from the right.
mechanical neutron switch is moved within 1 s from the filling position to the emptying position. The shutter (2, Fig. 6) is also closed for improved background suppression. 3.1. UCN storage
Fig. 8. Schematic view of the interconnection between the neutron guide and the sample tube. The horizontal dashed line represents the midplane of the 3-cm-high magnet gap, the vertical one the centre of the cylindrical bore through the magnet. 1: sample; 2: vertical neutron guide; 3: vacuum chamber; 4: magnet poles. The neutron guide from below ends near the lower pole face with a sharp edge on which the sample is quasi tightly sitting. In this way, the slit between neutron guide and sample, i.e. between the sample vacuum and the isolation vacuum, is minimal.
When a neutron in the storage volume enters the magnetic field B, it is reflected if the orientation of its magnetic moment is antiparallel to the direction of the magnetic field and if its kinetic energy is EpE B ¼ 1:91mN B ¼ 60 neV=T B, with mN the nuclear magneton. Neutrons with energies E4E B can penetrate the magnetic field barrier. Neutrons with their magnetic moments oriented parallel to B are accelerated into the field and thus fall into the detector. In our case, the latter occurs over a period of about 100 s after the magnetic field is switched on. The procedure to remove neutrons with the wrong spin orientation or too
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Fig. 9. Detector counts for sequences ‘filling’, ‘cleaning’, ‘holding’, and ‘emptying’ for different holding times ti , t1 t0 ¼ 20 s (top) and t2 t0 ¼ 220 s (bottom). The magnetic field B as a function of time is also shown.
‘high’ energies from the storage volume is called ‘spectral cleaning’, see Fig. 9 between 15 and 115 s. Simulations [24] showed that the cleaning time depends critically on the efficient removal of UCN with energies slightly above E B . These neutrons can only pass the magnetic field in the relatively rare case that they enter it on a trajectory nearly parallel to the field direction. It was found that the cleaning time could be shortened by ramping the field initially to 0:9B and increasing it to the full value only after 100 s.3 This procedure limits the maximum energy of storable UCN to 90% of the theoretically possible energy but guarantees that the numbers of UCN with ‘wrong’ spin state and of neutrons with energies above the magnetic barrier in the storage volume at the beginning of the measuring cycle are negligible. After cleaning, the neutrons are stored for the holding time ti (20 s up to a few hundred seconds). The events reaching the detector during this holding time originate from wall interactions in which the spin of the neutron was flipped. 3
From a Monte Carlo calculation [24] we found a minimum of 60 s for cleaning. With a safety margin we choose, however, 100 s.
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After the holding time, the magnet is switched off and all remaining neutrons fall into the detector where they are counted as N i . The whole procedure is repeated with different holding times. From the measured neutron counts at different holding times, one obtains the storage time tst (see below). The data taking procedure is illustrated in Fig. 9. As detector, we used a gas counter with 18 hPa 3He and 10 hPa CO2 in about 1100 hPa Ar. In order to penetrate the aluminium entrance window of the detector (critical energy of aluminium E c ðAlÞ ¼ 54 neV), the neutrons were accelerated by gravity over a vertical distance of 100 cm. The detector was shielded using polyethylene and borated rubber. Its background was (4:8 0:1Þ 103 s1 over the whole beamtime.
3.2. Monte Carlo calculations For the simulation of UCN motion in storage bottles, a Monte Carlo programme [24] based on GEANT4 [25] was used. The new programme was tested successfully by calculating the cleaning process in our storage sample, see Fig. 10. By switching on the magnetic field in order to start the storage process, neutrons that are in the magnetic field region during the magnetic field ramping experience an energy shift originating from the interaction of the neutron magnetic moment with the increasing magnetic field. This process was simulated by the same MC code; the result is displayed in Fig. 11. With a switching time of 3 s, the zero level is shifted by 35 neV; neutrons with the lowest energies, i.e. low velocities, stay in the magnetic field region for longer times and thus experience a larger energy shift during ramping. For ‘higher’ energies, the corresponding shift is smaller, see Fig. 11. The integrated energy spectrum of the stored neutrons was determined by performing measurements with the magnetic field B set to different values between 0.90 and 1.52 T. At each magnet setting B, the respective energy spectrum of stored UCN is registered; this is called the integral energy spectrum.
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Fig. 10. Measured (full dots) and calculated neutron counts in the detector during the ‘cleaning’ process, see text. The discrepancy at t ¼ 0 originates from the uncertainty in the initial conditions.
Fig. 12. Measured integral (top) and derived differential energy spectrum of stored UCN. The small triangles in the differential spectrum are Monte Carlo data [26]; the line is to guide the eyes.
Fig. 11. Calculated UCN energy spectrum in the sample before and after switching on a magnetic field of 1.52 T. After switching on, the zero level is shifted upwards by about 35 neV.
N 2 , the storage time tst of the neutrons in the storage volume is given by tst ¼
The differential UCN energy spectrum is obtained from the integral spectrum by taking the differences of the number of neutrons between neighbouring field values, see Fig. 12. The spectra were corrected for the field-dependent energy shift of each magnet setting with the MC code [24]. 3.3. Results For two different holding times (t1 t0 ¼ 20 s and t2 t0 ¼ 220 s) and detector counts N 1 and
t2 t1 . lnðN 1 =N 2 Þ
(1)
In Eq. (1), we replace N 2 by N 2 ¼ N 2 þ N sf , where N sf corresponds to the number of spin-flipped neutrons measured between t1 and t2 , see Fig. 9. The number N sf , which is about an order of magnitude larger than the background, is corrected for background and for the total storage time; for details see Ref. [26]. The spin-flip corrected storage time t st is connected to the neutron lifetime tn and the
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wall-loss lifetime tloss : ðt st Þ1 1 ¼ t1 n þ tloss Z EB Z ¼ t1 þ n 0
HðEÞ
nðh; EÞmðh; EÞgðEÞ dh dE. 0
ð2Þ The wall-loss lifetime tloss depends on the wall collision frequency nðEÞ,4 the loss probability per wall collision mðEÞ and the stored normalized UCN energy distribution gðEÞ. The integration over the vertical tube dimension h is necessary since gravity causes the UCN velocity and density distribution to vary over the tube height. Evaluation of mðEÞ from Eq. (2) thus requires solving the integral which can be done analytically for simple cases only (see, e.g., Refs. [11,27]). In our case, the distribution of the UCN energy is additionally modified by the magnetic field, cf. Fig. 11. Therefore, we used MC calculations to evaluate Eq. (2); the calculated magnetic field was used as an input parameter. The energy dependence is given by the equation (see also Ref. [11]) " 1=2 # 1=2 V V 1 E mðEÞ ¼ 2Z sin 1 . (3) E V E Here, V is the real part of the Fermi potential and E the kinetic energy of the neutron. The energy dependence (Eq. (3)) was taken from Ref. [27], which successfully describes the energy dependence of the losses on Fomblin oil and grease. The dependence of the loss parameter Z on the measured storage time as obtained from our Monte Carlo calculation of Eq. (2) is displayed in Fig. 13. This allows one to determine the loss parameter Z from the measured storage time. We investigated different samples with (i) beryllium coating on aluminium substrates (two different samples), (ii) beryllium coating on quartz, and (iii) DLC coating on aluminium. All data were taken with the full UCN spectrum (E B o83 neV) storeable in our apparatus. The results are summarized in Table 1. In all cases, the losses due to depolarization (spin flip) during storage were more than an order of magnitude smaller 4
Fig. 13. Dependence of the loss coefficient Z on the storage time tst according to Eq. (2). The upper curve is for our aluminium tubes, the lower for the quartz tube which has a slightly different geometry, see text (2.2.3).
than the losses originating from wall collisions, see also Refs. [26,28]. The uncertainties in our results include the statistical errors of the measurements and an estimate of a systematic contribution due to the spectral distribution. Due to lack of time during the first data-taking period, in all cases the sample surfaces were measured without any elaborate conditioning. It is well known, however, that the losses on beryllium decrease considerably e.g. by baking at 620 K in helium or deuterium atmosphere [29]. For comparison, the literature value of Z for beryllium at room temperature ranges around 1:1 104 24:3 104 [29]. To our knowledge, no values for the loss parameter Z have been reported for DLC coatings so far. The detailed analysis of the data including depolarization losses due to wall interactions with spin flip are in progress. Data at a sample temperature of 70 K and for a variety of substrates and coatings were taken successfully at a later date. The analysis of these data is still going on and the results will be published elsewhere [26,28].
4. Conclusions
1
We calculated average wall-collision frequencies nðEÞ30 s depending on sample geometry and the magnetic field value during ‘cleaning’.
The apparatus described above allows for the investigation of materials for the storage of
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Table 1 Measured numbers of neutrons and the corresponding loss parameter Z after storage for various samples at room temperature Sample no.
Sample
Substrate
N 1 ðavÞ
1 2 3 4
Be Be Be DLC
Al Al Quartz Al
381 292 262 321
N 2 ðavÞ
714 712 73 713
145 71 63 93
75 73 71 74
tst ½s 200 138 133 161
711 75 74 76
Z [104 ] (4.2 (6.8 (5.0 (5.8
70.3) 70.3) 70.1) 70.3)
The cleaning time was 100 s at 90% of the magnetic field. The storage times were t1 ¼ 20 s and t2 ¼ 220 s at 100% field value (1.52 T). The numbers of neutrons N i are averaged over several data-taking cycles.
ultracold neutrons (UCNs). UCNs are confined by a magnetic field of 1.52 T at the bottom and by gravity at the top. The apparatus is designed to avoid any gaps contributing to UCN losses. It allows for measuring UCN losses per wall collision. Simultaneously, one can determine the depolarization of polarized UCN from the material surface under investigation. First experimental tests at the ILL UCN source were performed very successfully, resulting in storage times of up to 200 s for different surface materials at room temperature. The corresponding loss parameters Z for UCNs varied between 4.2 and 6:8 104 per wall collision.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements [17]
The experiment was designed and built at PSI. We thank the workshops at PSI, especially M. Mu¨ller and U. Bugmann, for their outstanding work in fabrication of many apparatus components. The data acquisition took place at ILL, Grenoble. We acknowledge the great support of Th. Brenner and the ILL reactor crew. We also thank A. Strelkov, JINR, for providing us with a 3 He UCN detector. We are very grateful to I. Altarev, TU Munich, M. Lasakov, PNPI, M. Makela and B. Vogelaar from Virginia Tech, and A. Young from NCSU, who prepared the beryllium and DLC coating for the samples.
[18] [19] [20]
[21] [22] [23] [24] [25] [26] [27]
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