Upgraded ultracold neutron facility with a supermirror turbine coupled to a very cold neutron source

PHYSICA[ ELSEVIER

Physica B 213&214 (19951 869 871

Upgraded ultracold neutron facility with a supermirror turbine coupled to a very cold neutron source Y. Kawabata*, M. Utsuro, K. Okumura Research Reactor Institute, Kyoto Universi~,, Kumatori-cho, Osaka 590-04, Japan

Abstract An ultracold neutron (UCN) facility with the combination of a very cold neutron (VCN) source and a supermirror neutron turbine has been constructed in the Kyoto University Reactor. The present supermirror turbine was upgraded by changing the reflecting blades from the three-mirror sets to the five-mirror sets. The upgraded version enhances the UCN flux by a factor of about 3 compared to the original three-mirror blades, by using faster VCNs which are decelerated to become UCNs. A computer simulation of the neutron deceleration in the blade system shows the several instructive characteristics of the supermirror turbine.

1. Introduction

2. Upgrade of the supermirror turbine

Intense UCN sources previously reported have been developed in three ways: (1) direct extraction of UCNs from a cold neutron source (CNS) near the reactor core [1]; (2) very cold neutron (VCN) extraction from a CNS and conversion to UCNs by a neutron turbine I-2]; (3) a superthermal method of downscattering cold neutrons by superfluid helium [3, 4]. A VCN UCN source was developed at the 5MW Kyoto University Reactor (KURt. A preliminary VCN guide tube [5] was installed in the graphite thermal column (TC) and UCNs were generated by a supermirror neutron turbine [6]. Recently, a CNS with liquid deuterium moderator was completed in the TC [7]. VCNs are now extracted by a revised VCN guide tube in a horizontal arrangement [8] and bent by about 17' vertically by a VCN bender [9] for the turbine entrance. Our upgraded neutron turbine converts VCNs (5(~100m/s) to UCNs with an axial velocity below I0 m/s using supermirror blades.

The whole layout of the VCN UCN facilities at KUR is shown in Fig. l, and the structure of the upgraded supermirror turbine is shown in Fig. 2. The blade for the neutron deceleration is made of five fiat supermirrors. Ninety-six sets of the blades are prepared on the turbine rotor of 1 m diameter. The blade pitch is about 31 mm and the revolution speeds used are 6.(~8.5 rps. The neutron deceleration principle is essentially the same as that of Steyerl's axial turbines installed first in Munich [10] and duplicated later at the ILL I-2]. Their blades are semi-cylindrical metallic mirrors. The reasons why we adopted supermirrors are as follows: (1) The characteristic neutron velocity of the channels in the blades can be made faster with a given blade geometry. It makes use of faster feed neutrons without increasing the neutron loss. (2) A wider blade pitch, on the other hand, used at a given characteristic velocity makes the neutron loss from scattering at the edges of the blades smaller. (3) The smaller number of blades makes it easier to construct the neutron turbine.

* Corresponding author.

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Y. Kawabata et al. / Physica B 213&214 (1995) 869 871

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Fig. 3. Measured UCN intensities of the five-mirror and threemirror blade sets. The revolution speed is 7.5 rps. In this way, the design of the turbine blade was changed to the five-mirror sets from the three-mirror sets. The increase of the number of mirrors allows us to decrease the blade pitch, and therefore gives a higher characteristic velocity of neutron transmission in the blades, i.e. this modification enables the faster source VCNs to be specularly reflected with small glancing angle. The comparison of the measured UCN intensities for the two blade systems in Fig. 3 indicates that the change of the blade system increases the UCN intensity by a factor of about 3 in the UCN region below about 10m/s. Semi-cylindrical supermirror blades would enhance the UCN intensity, but curved supermirrors with high reflectivity have not yet been developed. "/

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Fig. 2. Structure of the supermirror neutron turbine. 1: VCN bender, 2: aluminum window, 3: VCN feed guide, 4: turbine rotor, 5: supermirror blade, 6: lower reflectingmirrors, 7A, 7B, 7C: UCN ports for neutron experiments, 8: turbine shaft, 9: rotation pickup, 10: evacuation port, 11: UCN chopper. In the principle of the axial neutron turbine, the velocity of incident neutrons to be decelerated to UCNs is about twice the turbine blade velocity. Thus, the UCN intensity depends essentially on the incident VCN spectrum and the revolution speed of the turbine, in the working region of the supermirror blades. The UCN intensity of the three-mirror turbine before the CNS installation gave an optimum turbine speed of about 6 rps. After the CNS installation, the source VCNs have to penetrate the aluminum walls to be extracted from the liquid deuterium moderator, which cause larger losses for slower neutrons. Thus, the information on the deformed source VCN spectrum suggested that a higher UCN flux would be expected when the velocity of the source VCNs is greater.

3. Numerical simulations on the five-mirror blade system The UCN generation in the supermirror turbine was numerically simulated by the Monte-Carlo method. The time evolutions of the positions of neutrons and the blades are calculated in the laboratory coordinate system, and the neutron position and the velocity are transformed into the coordinate system in the blade for the operation of the reflection condition on the blade mirrors. This method includes the centrifugal and the Coriolis forces rigorously. Velocity diagrams for neutron deceleration in the blade have been calculated. The velocity groups of several reflection patterns at the blade exit are shown in Fig. 4. The areas surrounded by solid and broken lines in the figure are the analytical results for the neutron deceleration in a blade which moves in a straight line 1-11]. The components fully decelerated by five mirrors indicated with 'o' in the present accurate calculation, with the rotational motion of the blade considered, show

Y. Kawabata et al.

Physica B 213&214 (1995) 869 871

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4. Concluding remarks

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a significant difference from the result of the simplified analysis, since the centrifugal a n d Coriolis forces change the neutror[ trajectories in a complicated way in the blade. This numerical simulation shows that this upgraded blade system still has n e u t r o n groups remaining to be fully decelerated such as the four-times reflected g r o u p s h o w n by 'E]' which have failed to be reflected on the fifth mirror. The a d o p t i o n of a n asymmetric blade system c o m p e n s a t i n g such trajectory changes might bring a further i m p r o v e d U C N intensity.

An U C N facility coupled to a V C N source has been constructed using a s u p e r m i r r o r t u r b i n e with u p g r a d e d five-mirror blades. The U C N intensity is e n h a n c e d by a factor of a b o u t 3 c o m p a r e d to the former t h r e e - m i r r o r blade. F u r t h e r i m p r o v e m e n t s of the turbine are in progress, for example, in the p r e p a r a t i o n of the lower reflection mirrors in the blade channel. T h e resulting U C N intensity expected is a b o u t 4 0 n c m - Z s l p_eV i at the U C N energy of a b o u t 0.2 laeV c o r r e s p o n d i n g to the axial velocity c o m p o n e n t . The three U C N ports are being used for bottle experiments, time-of-flight m e a s u r e m e n t s a n d n e u t r o n optical or magnetic experiments, respectively.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

l.S. Altarev et al., Soy. Phys. JETP Lett. 44 (1986) 344. A. Steyerl et aL Phys. Lett. lI6A (1986) 347. R. Golub et al., Z. Phys. B 51 (1983) 187. H. Yoshiki et al., Phys. Rev. Lett. 68 (1992) 1323. M. Utsuro et al., Ann. Rep. Res. Reactor Inst. Kyoto Univ. 13 (1980) 161. M. Utsuro et al., Nucl. Instr. and Meth. A 270 (1988) 456. M. Utsuro et al., Physica B 156&157 (1989) 540. Y. Kawabata et al., Proc. 5th Int. Symp. on Adv. Nucl. Energy Res., Mito, Vol. 2 (1993) 729. Y. Kawabata et al., Proc. Int. Conf. on Neutron Optical Devices and Application, SPIE Vol. 1738 (1992) 454. A. Steyerl, Nucl. Instr. and Meth. 125 (1975) 416. M. Utsuro et al., Ann. Rep. Res. Reactor Inst. Kyoto Univ. (1994), in print.