A liquid hydrogen source of ultra-cold neutrons

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A LIQUID HYDROGEN

PHYSICS LETTERS

22 December 1980

SOURCE OF ULTRA-COLD NEUTRONS

I.S. ALTAREV, Yu.V. BORISOV, A.B. BRANDIN, V.F. EZHOV, S.N. IVANOV, G.K. KUNSTMAN, V.M. LOBASHEV, V.A. NAZARENKO, V.L. RYABOV, A.P. SEREBROV and R.R. TALDAEV Leningrad Nuclear Physics Institute of the Academy of Sciences, Leningrad, USSR Received 21 October 1980

A liquid hydrogen source of ultra-cold neutrons (UCN) developed for an experimental search for the electric dipole moment of the neutron is described. The results of an investigation of the yield of UCN from gaseous, liquid, and solid hydro2 neutron gen as a function of temperature are presented. The UCN counting rate obtained at the output of the 6 X 7 cm guide tube is 5 X 1 ~4 n/s. This counting rate corresponds to a flux of neutrons whose velocity along the axis of the neutron guide tube is below 7 rn/s. Preliminary measurements of the UCN yield from liquid and solid deuterium have been carried out.

A liquid hydrogen source of UCN has been developed at the Leningrad Nuclear Physics Institute of the USSR Academy of Sciences, to increase the experimental accuracy of the search for the electric dipole moment of the neutron [1]. The source (80 mm in diameter, 30 mm thick) contained 150 cm3 of liquid hydrogen. The chamber for liquid hydrogen was made of zirconium alloy and helium gas was used to cool the source. The hydrogen condenser was mounted directly inside the source and coupled to a ballast tank (V = 150 ~)for hydrogen gas. The cooling system and the design of the source employed made it possible to vary the temperature of the hydrogen over a wide range and study the yield of UCN from gaseous, liquid, and solid hydrogen. The neutron guide tube for the extraction of UCN was made of mirror-polished stainless steel plates and its cross section at the point of connection to the source was 56 X 68 mm2. A liquid hydrogen source of UCN was mounted in the vertical channel of the beryllium reflector of the WWR-M reactor. The channel diameter was 110 mm. The measured thermal neutron flux at the source with a reactor power rating of 16 MW was 6 X 1013 n/cm2 sand the fast neutron flux was 8 X 1012 n/cm2 s. The specific heat release in the source material was reduced to 0.3 W/g due to a lead shield located between the beryllium reflector and the core of the reactor. For liquid hydrogen, the specific heat release

due to fast and epi-thermal neutrons was 4 W/g. Studies of the UCN yield from gaseous, liquid, and solid hydrogen have been carried out. Fig. la shows the yield of UCN as a function of temperature for normal hydrogen (75% ortho, 25% para). Since no specific measurements of the hydrogen temperature in the source were made, the mean temperature of the straight-through and reverse flows of helium cooling the source was taken into consideration. The temperature dependence was obtained for a reactor power of 1.2 MW, which made it possible to study the yield of UCN from solid hydrogen. In fig. la the yield of UCN is plotted in relative units. The counting rate of UCN at a hydrogen temperature of 280 K and a pressure of 3.2 ata is taken as the unit of yield. This hydrogen pressure at the temperature specified above provides 90% of the total saturation for the yield of UCN at 280 K. Generally speaking, the effective thickness of the UCN converter from normal gaseous hydrogen at a pressure of 3.2 ata is adequate over the entire temperature range, as shown in fig. ib, which presents the yield of UCN versus pressure of gaseous hydrogen at 280 K, 85 K and 30K. Besides, we have investigated the effect of the accelerated ortho—para conversion possible under the reactor radiation. For this purpose, the source was filled with hydrogen up to pressures of 0.2 ata or 3.2 ata, and then probable changes of the UCN intensity were observed during I h. 413

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22 December 1980

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Fig. 1. (a) Curve 1: yield of UCN against temperature for hydrogen (o, experimental points obtained during cooling of the source; ., experimental points obtained during heating of the source). Curve 2: yield of UCN against temperature for deuterium (o, experimental points obtained during cooling of the source). (b) Yield of UCN against hydrogen pressure at 280 K, 85 K, and 30 K. (c) Pressure of the ballast tank against temperature of the source. I Condensation phase; II phase attended by increase in liquid hydrogen pressure; III crystallization phase.

These studies were carried out at 85 K and 30 K, mitially with the reactor power at 2 MW and subsequently at 16 MW. In all cases, no actual variation of the UCN counting rate within the above observation period 414

was detected. Hence, the thermal dependence of the UCN yield over the range 280 to 30 K presented in fig. la conforms to normal hydrogen. The experimental results thus obtained are in satisfactory agreement

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PHYSICS LETTERS

with the temperature dependence of the UCN yield calculated for normal hydrogen [2]. No studies on parahydrogen have been made. Let us consider the results obtained for liquid and solid hydrogen. It should be noted that a 150 cm3 liquid hydrogen source is a good thermalizer for thermal neutrons. From the data found in the published literature [3,41 , the effective temperature of the neutrons escaping from the liquid hydrogen moderator, 80 mm in diameter and 30 to 40 mm thick, is approximately 80 K. It is this process of thermalization of thermal neutrons which has led to an appreciable increase in the yield of UCN (fig. la) which occurred due to liquefaction of hydrogen in the source. The process of hydrogen liquefaction and the extent to which the source was filled with liquid hydrogen were controlled using the gaseous hydrogen pressure in the ballast tank. Fig. 1 c presents the hydrogen pressure in the ballast tank versus the temperature of the helium cooling the source. The solid line shows the calculated dependence which should be obtained for the process of very slow cooling of the source whose temperature field is uniform over the entire volume. The experimental dependence differs from the ideal one, but individual phases peculiar to the cooling process can be singled out, such as: I filling of the source with liquid hydrogen, II increase of the density of liquid hydrogen with a decrease in temperature, III solidification of hydrogen. The process of hydrogen liquefaction begins at 24.5 K which conforms to the condensation temperature at 3 ata, whereas the process of solidification starts at 14 K corresponding to the crystallization temperature. The phases of the cooling process described above are also presented in fig. Ia. Hence, the gain in the yield of UCN for liquid hydrogen is a factor of 20— 30. A decrease in the temperature of the liquid hydrogen makes it possible to increase the yield of UCN by a factor of 40 to 43 and for solid hydrogen the gain in the yield of UCN is a factor of 45—47. However, as a result of the low thermal conductivity of solid hydrogen, this yield of UCN is possible only for a low power reactor. With a reactor power rating of 16 MW, the cooling conditions in the source provided by the HGU-S00/1 5 refrigerator made it possible to maintain the temperature of liquid hydrogen close to the boiling point. Under

22 December 1980

these conditions, the gain in the yield of UCN is about 25. The results of preliminary studies on deuterium are given in fig. Ia. For deuterium, the dimensions of the source used are inadequate for complete thermalization of thermal neutrons and hence curve 2 cannot be interpreted as the yield of UCN from deuterium against temperature. It is merely characteristic of a liquid deuterium source with a volume of 150 cm3. For solid deuterium, the gain in the yield of UCN from the source with the above dimensions is as large as 57, which exceeds the yield from solid hydrogen. However, the yield of UCN from a source filled with liquid deuterium turns out to be half that for liquid hydrogen due to the marked temperature dependence at the full power of the reactor. When studying the temperature dependence of the UCN yield for a reliable separation of UCN from other neutrons, the UCN are registered at the output of the spectrometer trap applied in the search for the electric dipole moment of the neutron [1]. The cut-off velocity of the trap walls is 7 rn/s (using a 50% 58Ni, 50% Cu alloy for the coating). The counting rate registered at the spectrometer output for the reactor at 16 MW is 4.5 X I ü~n/s, which exceeds the counting rate obtamed earlier in ref. [1] by a factor of 7 to 8. Measurements of the UCN flux from the source at the spectrometer input or at the neutron guide tube output have also been performed. Separation of UCN from other neutrons was made at the cut-off velocity of 7 m/s using a shutter. Besides, the neutron spectrum was measured by the time-of-flight method. As registered at the spectrometer input, the flux of UCN, whose velocity along the axis of the neutron guide tube was less than 7 m/s, amounted to 5 X lO~n/s. A rough estimate shows that the neutron flux at the full velocity of 7 m/s (for the neutrons that can be stored in the trap) should be 2.5 X l0~n/s. Considering that the calculated transmission coefficient of the spectrometer is approximately 20%, it can be inferred that the results of the UCN flux measurements at the spectrometer input and output (5 X 1 0~and 4.5 X l0~n/s, respectively) are in reasonable agreement. The authors are grateful to A.I. Egorov, N.A. Efimov, G.Ya. Vasilyev, A.G. Kharitonov, V.A. Shustov, the people of the reactor theory department, the workers 415

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of the experimental shop, the cryogenic station and the staff of the WWR-M reactor for their active aid in the work done References [1] I.S. Altarev et al., Pis’ma Zh. Eksp. Teor. Fiz. 29 (1979) 749; Nucl. Phys. A341 (1980) 269.

416

[21 E.Z. Akhrnetov et al., JINR

22 December 1980

Report P3-8470 (1974) (in Russian). [3] 3. Butterworth et aL, Phil. Mag. 2 (1957) 917. [4] P. Ageron et al., Cryogenics 2 (1969) 42.