ISSN 1063-7850, Technical Physics Letters, 2015, Vol. 41, No. 10, pp. 1016–1018. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.P. Serebrov, V.A. Lyamkin, V.V. Runov, S.A. Ivanov, M.S. Onegin, A.K. Fomin, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 41, No. 20, pp. 96–102.
Using Polycrystalline Bismuth Filter in an Ultracold Neutron Source with Superfluid Helium A. P. Serebrov*, V. A. Lyamkin, V. V. Runov, S. A. Ivanov, M. S. Onegin, and A. K. Fomin Petersburg Nuclear Physics Institute, National Research Centre “Kurchatov Institute,” Gatchina, Leningrad oblast, 188300 Russia *e-mail:
[email protected] Received June 2, 2015
Abstract—Placing polycrystalline bismuth filter in front of an ultracold neutron (UCN) source with superfluid helium at 1 K is shown to be effective. The use of this filter ensures a 30-fold decrease (down to 0.5 W) in the level of heat load in the UCN source, while reducing by 30% the flux of neutrons with 9-Å wavelength (which are converted into UCNs). The phenomenon of small-angle scattering on polycrystalline bismuth has been studied and shown to be insignificant. Cooling of the filter to liquid nitrogen temperature increases the transmission of 9-Å neutrons by only 8%; hence, creation of this cooling system is inexpedient. A project of a technological complex designed for the UCN source at the PIK reactor is presented, which ensures the removal of 1-W heat load from the UCN source with superfluid helium at a 1-K temperature level. DOI: 10.1134/S1063785015100284
At present, the creation of the PIK scientific research reactor complex at the Petersburg Nuclear Physics Institute (PNPI) is at the stage of completion. This is one of the six large projects on the territory of Russian Federation, which are included into the governmental program for creation of domestic worldclass megafacilities. Horizontal channels (GEK-3 and GEK-4) of the PIK reactor will be equipped with liquid-deuterium-based sources of cold neutrons for carrying out fundamental research and studying nanostructures. In addition, it is planned to equip these beamlines with sources producing ultracold neutrons (UCNs) [1]. The UCN sources are chambers with superfluid helium that are located on output beams of GEK-3 and GEK-4 channels and play the role of converters of cold neutrons into UCNs [2]. For increased UCN yield, the cambers must be elongated in the beam direction. Liquid helium converts neutrons with a wavelength of 9 Å into UCNs along the beam, while weakly attenuating it. The acceptable chamber length is 2–3 m. The input beam of cold neutrons is accompanied by γ quanta and fast neutrons. The thermal power released upon the absorption of this beam amounts approximately to 100 W. About 15–20% of this power is released in the UCN source chamber and in superfluid helium, which makes the creation of UCN source with superfluid helium temperature 1 К a quite difficult technological task.
We have calculated the parameters of the UCN source with a chamber volume of 152 L, comprising a cylindrical part with a length of 2 m, a diameter of 300 mm, a wall thickness of 1.5 mm, and hemispherical terminals on both ends. The average flux density of 9-Å neutrons for a chamber with this volume turned out to be 1.1 × 109 cm–2 s–1 Å–1. The total productivity of this source amounted to 1.3 × 107 UCN/s for UCN velocity spectrum from 0 to 8 m/s. The boundary velocity of 8 m/s for the given source is determined by the use of 58Ni coating on chamber walls and neutron guide walls. Calculations of energy release in the helium chamber were performed with allowance for the contribution due to fast neutrons and γ quanta. In addition to prompt γ quanta, we also took into account delayed γ quanta arriving from the active zone of reactor and those from the decay of 28Al nuclei accumulated due to neutron capture by aluminum parts. The heat load with γ quanta from various structural elements of the source in a steady-state regime amounted to 3.6 W in the shell and 11.3 W in liquid helium. This amount of heat is too large for a 1 K temperature level. The level of heat release in the UCN source can be significantly reduced by placing a polycrystalline bismuth filter in front of the source, since it is known that bismuth is a good shield from γ radiation [3]. Calculations of energy release in the UCN source with a 100-mm-thick filter showed a 30-fold decrease in the heat load as compared to that in the chamber without
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Heat load in elements of the helium chamber of the UCN source with a bismuth filter and without this filter (data in parentheses) Parameter
Chamber shell
m, g
Helium
8730 22040 0.14 (3.6) 0.26 (11.3) 0.12 (0.95) − 0.26 (4.55) 0.26 (11.3) 0.52 (15.85)
n+γ β
ΔE, W Total, W Overall, W
The bismuth filter was cooled by heat exchange with liquid nitrogen via a copper frame, which ensured a gradual decrease in the bismuth temperature. The intensity of neutron flux transmitted via bismuth was measured with 10 min intervals over the entire period of cooling. The increase in neutron transmission due to cooling of the filter amounted to 8%. Upon warming of bismuth up to room temperature, the transmission returned to the initial level.
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1.0
Initial neutron flux Neutron flux after the filter
(a) Neutron flux, a.u.
0.8 0.6 0.4
Tr = 0.84 0.2
0
0.4
0.8 1.2 1.6 Neutron wavelength, nm
2.0
350 000 300 000
(b)
250 000 Intensity, a.u.
this filter (see table). Thus, use of the filter will considerably simplify the maintenance technology of the 1K temperature level and significantly reduce the cost of necessary equipment. It is a natural question how the use of a bismuth filter will attenuate the input cold neutron flux. Investigations with crystalline bismuth filters have also been performed previously. Figure 1a shows the results reported by M. Adib et al. in 2003 [4, 5]. As can be seen, the transmission of cold neutrons through a bismuth filter cooled to 77 K is 12% higher than that at room temperature. A 20-cm-thick layer of polycrystalline bismuth transmits ~80% of neutron spectrum with λ > 0.65 nm, while the transmission of neutrons with wavelengths near 0.47 nm is below 6%. Thus, a neutron flux can be filtered nor only from γ quanta, but also from high-energy neutrons, which almost do not participate in UCN production. Another important question is related to the possible small-angle scattering of cold neutrons on a polycrystalline bismuth filter, which would increase the beam divergence so that the beam would not match the UCN source chamber size. In order to study this issue, we have performed experiments on a Vektor small-angle diffractometer of a WWR-M reactor at the PNPI. The Vektor diffractometer is a multidetector setup for measuring small-angle scattering of polarized neutrons with polarization analysis of scattered neutrons, the energy spectrum of which covers the wavelength interval of 8–10 Å [6, 7] and, hence, is well suited to study the problem under consideration. The experiments with a bismuth filter cooled to liquid nitrogen temperatures were performed using a specially designed cryostat. The filter was cooled by heat exchange with liquid nitrogen via a copper plate. The space between the liquid nitrogen vessel and the case was pumped to a residual pressure of 10–5 Torr in order to avoid moisture condensation on bismuth. The aluminum case wall thickness on the neutron beam path was reduced to 1 mm in order to decrease the amount of absorbed neutrons. Preliminary measurements of neutron transmission through the cryostat without a filter gave 96.88% for the neutron spectrum within λ = 8–12 Å. This level of neutron transmission for the cryostat with a wall thickness of 1 mm is quite acceptable. Prior to determining neutron beam divergence past the bismuth filter, it was necessary to measure the initial beam divergence. For this purpose, the initial intensity of neutrons with λ > 7 Å was measured within –0.02 to 0.02 rad with and without the filter. A neutron beam with λ = 8–12 Å exhibited no small-angle scattering on bismuth. Prior to cooling the bismuth filter, the neutron transmission was determined for “warm” bismuth at room temperature. The measurements were performed with a 9-cm-thick bismuth layer and without it. The calculated room-temperature transmission coefficient was 66%.
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200 000 150 000 100 000 50 000 0 −0.02
−0.01
0 θ, rad
0.01
0.02
Fig. 1. Examination of the polycrystalline bismuth filter of cold neutrons: (a) spectra of neutrons before and after passage through the filter; (b) neutron beam divergence without Bi filter (open circles), with Bi filter at T = 300 K (black circles), and with Bi filter cooled to 80 K (triangles).
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Adsorbers
High pressure compressors
Pumping station
Gas bags
Helium gas cylinders Operator room
Compressor system Helium liquefier GEK-3 UCN source Cryostat Bismuth filter UCN magneto UCN gravitational trap trap Cryostat Neutron EDN measurment facility Compressor system GEK-4 UCN source Helium liquefier Bismuth filter Pumping station Fig. 2. General view of a scientific station with the UCN source at the PIK reactor.
Figure 1b summarizes the results of these investigations. The value of transmission through “warm” bismuth was 66%, while the transmission though the same filter cooled to 80 K amounted to 73%. Taking into account that the cryostat attenuated the beam intensity by about 3%, there is not much sense in using a cooled filter. Thus, the use of a polycrystalline bismuth filter solves the task of reducing the heat load in the UCN source down to about 0.5 W. In this context, a project of a technological complex capable of 1-W heat load removal from the UCN source with superfluid helium on a 1-K temperature level has been developed for the PIK reactor. This complex includes the UCN source, cryogenic unit, helium vapor pumping system, compressor system, and helium storage and distribution system. The parameters of all component systems are mutually consistent. The general outlay of a scientific station with this facility is presented in Fig. 2. Acknowledgments. This study was performed at the Petersburg Nuclear Physics Institute of the National Research Centre “Kurchatov Institute” with support
from the Russian Science Foundation, project no. 1422-00105. REFERENCES 1. A. P. Serebrov, A. K. Fomin, M. S. Onegin, A. G. Kharitonov, D. V. Prudnikov, V. A. Lyamkin, and S. A. Ivanov, Tech. Phys. Lett. 40 (1), 10 (2014). 2. R. Golub and J. M. Pendlebury, Phys. Lett. A 62 (5), 337 (1977). 3. V. P. Mashkovich and A. V. Kudryavtseva, Protection from Ionizing Radiations: A Handbook (Energoatomizdat, Moscow, 1995) [in Russian]. 4. M. Adib and M. Kilany, Radiat. Phys. Chem. 66, 81 (2003). 5. M. Adib, K. Naguib, A. Ashry, et al., Ann. Nucl. Energy 29, 1119 (2002). 6. V. V. Runov, D. S. Ilyn, M. K. Runova, and A. K. Radzhabov, JETP Lett. 95 (9), 467 (2012). 7. V. V. Runov, V. N. Skorobogatykh, M. K. Runova, and V. V. Sumin, Phys. Solid State 56 (1), 62 (2014).
TECHNICAL PHYSICS LETTERS
Translated by P. Pozdeev
Vol. 41
No. 10
2015
