7th International Workshop "Ultra Cold & Cold Neutrons. Physics & Sources". (8-14 of June 2009) Project leader: Anatoli SEREBROV

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1 7th International Workshop "Ultra Cold & Cold Neutrons. Physics & Sources". (8-14 of June 2009) PROJECT OF SUPERFLUID HELIUM UCN SOURCE AT PNPI Arcady ZAKHAROV Project leader: Anatoli SEREBROV

2 Reactor WWR-M

3 Diagram for neutron calculation 1 reactor core, 2 beryllium reflector, 3 lead shield, 4 separator, 5 water, 6 iron shield, 7 бетонная shield, 8 Pb or Bi shield, 9 graphite moderator, 10 front part of graphite moderator, I, II, III, IV cell with superfluid helium Theoretical Physics Division of PNPI A.N.Erykalov, M.S.Onegin

4 Helium cell shielding Neutron flux in superfluid helium: for bismuth shield n/cm 2 smwå for lead shield n/cm 2 smwå Heat release in the cell (Al+HeII) at Pb shielding is 30% less than at Bi one.

5 Cold pre-moderator C D 2 O эв МВт D 2 Be 2 (E), н/см =9 Е E, эв Differential density of neutron flux in the helium cell depending on the pre-moderator material: 20К, liquid 20К, heavy 20К, 20К.

6 Position on the reactor Т=1.2 К Т=20 К Т=300 К Ф=10 14 n/(сm 2 s) Q=16 МВт

7 Heat release in material CUCN= n/(сm 3 s) Т=1.2 К Т=20 К Ф= n/(сm 2 s) Q He =6 W 19 W Q Al =13 W Т=300 К Q C =750 W Q Bi =16 kw Ф=10 14 n/(сm 2 s) Q=16 МW

8 Neutron spectra from helium cell He (50 l) 1K =0.10 rad free gas =0.10 rad (HeII.lib) 10 9 F, n/cm 2 s sr A MW , A Yield at solid angle Ω=2π θ

9 UCN density

10 Progress in UCN production

11 UCN storage time in He II s 20 s 30 s 70 s s detec ctor counts 93 s (T=0.49 K) ge time, s stora s T=1.16 K holding time, s 1.0 K temperature, K 1.3 K Z.Phys.B Condensed Matter 51,

12 Experimental data obtained at ILL neutron count storage time, s time, s temperature, K neutron count UCN density =С Ф( =9Å)= n/(сm 2 s А) С UCN production rate ( ) n/(сm 3 s) - storage time 10 сm -3 wavelength, A T=0.1К changes the storage time of about 15%. Phys. Lett. A 308 (2003) 67-74

13 Is it possible to use 3 Не at 20W heat release to reach a lower temperature? Helium saturated pressure Heat removal is produced due to the helium evaporation. Pressure, torr He 4 He 3 Не evaporation 0,6К - 10,5 J/g 4 Не evaporation 1,2К - 21,5 J/g 1E-3 1E T, K Employment of 3 Не To remove 20 W it is necessary to withdraw by pumping ~ 2 g/s. To cool down incoming Не from 3.2 К to 0.6 К it is necessary to withdraw by pumping ~ 2.2 g/s. Vacuum facility capacity: 63.5 m 3 /s (Т = 273К, Р = 50 Pа) Employment of 4 Не To remove 20 W it is necessary to withdraw by pumping ~ 1g/s. To cool down incoming Не from 4.2 К to 1.2 К it is necessary to withdraw by pumping ~ 0.5 g/s. Vacuum facility capacity: 17 m 3 /s (Т = 273К, Р = 50 Pа)

14 Diagram of low temperature part 1 He II cell; 2 UCN neutron guide, 3 CN neutron guide, 4 He II supply pipe, 5 lower 1.2 К, 6 intermediate 1.2 К, 7 3 Не filter, 8 level sensor, 9 upper 4.2 К, 10 helium supply valve, 11 level sensor, 12 vacuum pipe (gravitation trap for UCN), 13 vacuum pipe for lower bath, 14 vacuum pipe for intermediate bath, 15 main vacuum manifold, 16 UCN neutron guide membrane, 17 CN neutron guide membrane, 18 thermal 20 К, 19 vacuum jacket, 20 UCN outer neutron guide, 21 CN outer neutron guide, 22 helium supply at temperature of 4.2 К, 23 pipe for helium vapour removal, 24 helium supply for thermal shield 18, 25 helium removal from thermal shield 18, 26 pumping of vacuum jacket.

15 UCN storage time in the neutron guide Storage time: -1 = t n, t total cross section, n amount of atoms in unit volume, - velocity of atoms. For helium t = 0 = 0,8 ±0,2 barn. Cross section of m 2 is used for calculation. Amount of atoms in unit volume can be defined from equation: P = n k T, Р gas pressure [Pа], Т temperature [K], k Boltzmann const. : 1, [J/К]. Amount of atoms in unit volume: n = 3, Most likely velocity of atoms: = (2 k T / m) 0/5, m mass of helium atom: 6, kg. = 70 m/s at temperature of 1.2 К. UCN storage time in the neutron guide filled with helium at 55 Pa and temperature of 1.2 К is equal to = 54 s. Storage time obtained from experimental data [1] for conditions in the neutron guide: = 54 ± 4 s. 1. Ю.Ю. Косвинцев, Ю.А. Кушнир, В.И. Морозов, Г.И. Терехов. Взаимодействие ультрахолодных нейтронов с газообразной средой. Нейтронная физика. Часть 1, М.,1980, с

16 Diameter of pipe for helium vapour pumping Molecular flow: m mass flow rate; R universal gas constant, M molecular mass, Т gas temperature. At flow rate of 1 g/s and temperature of 1.2 К Q = 2.5 Pа m 3 /s. Allowable pressure drop in the pipe dp= 0.3 Pа. Q m R M T Pipe conductivity: С = Q/dP Diameter of pipe: DdP ( ) 4 Q dp 128 L gc p - dynamic viscosity, р average pressure in the pipe, L length of pipe, gc the conversion rate coeff. The necessary diameter of pipe is equal to 5.2 cm. UCN neutron guide diameter is 10 cm.

17 Surface area which is necessary for evaporation in the cell At equilibrium conditions at saturated pressure P n : Q ev Q con N a P n 2 MRT AkT Withdrawal gas flow: Q = Qev Qcon Q N a 2 MRT AkT ( P n P) Gas flow to be removed: Q m R M T Necessary surface for evaporation: A surface area, m 2 k Boltzmann const., J/К Т temperature, К R universal gas constant М molecular weight, kg/mol N a Avogadro const., 1/mol Р pressure above the surface, Па Р n saturated pressure, Па A Q 2 MRT N kt ( P P) a n AP L It is necessary to have the surface area equal to m 2 to remove a gas flow of 2.5 Pа m 3 /s at pressure above the surface of 5 Pa below the saturated pressure P L

18 Capacity of the thermo-mechanical mechanical pump (filter) H. Nakai, N. Kimura, M. Murakami *, T. Haruyama and A. Yamamoto. Superfluid helium flow through porous media. Cryogenics, 36, 1996, pp

19 General technical specification of low temperature part Pressure in the neutron guide Temperature of helium vapour UCN storage time in the guide Neutron guide diameter UCN neutron guide 55 Pа 1.2 K 54 s 10 cm UCN loss in the gas ~ 2% Intermediate bath at 1.2 К Surface are m 2 Diameter of vacuum pipe Volume of He II Diameter of vacuum pipe Volume of He II Dimensions: Lower bath at 1.2 К He II cell length diameter ~ 7 cm ~ 2,5 l ~ 5 cm ~ 2,5 l 50 cm 30 cm Surface area m 2 He II temperature Diameter of He II supply pipe Flow rate of He II Volume of He II 1.15 К ~ 1 cm 1 g/s 35 liters Total amount of He 1.2K is about 40 liters

20 WWR-M reactor neutron beam area Thermal neutron area Ultra-cold neutron area Very-cold neutron area Cold neutron area

21 Technological general diagram Helium refrigerator Helium liquefier Vacuum facility WWR-M reactor UCN source Reactor core

22 Vacuum facility general diagram Vacuum facility capacity is 17 m 3 = 273 К and Р = 50 Pа. Vacuum pipeline between UCN source and vacuum facility has diameter 50 cm and length about 20 meters. It transfers 1.5 g/s gaseous helium at 50 Pa.

23 Helium liquefier

24 Helium tank

25 Helium refrigerator

26 Gas management systems

27 General view

28 General view