16th Crystal Ball Meeting October, 11-13, 2010, Dubrovnik Grigory Gurevich, Yuri Usov Dubna-Mainz Dilution Cryostat of the New Frozen-Spin Target

Main technical requirements Horizontal Dilution Refrigerator to fit in the CB detector geometry Beam along cryostat axis φ symmetric, θ>160 0 Thin internal holding coil 0.4 T on 1.5 K radiation shield (longitudinal, transverse) Target material loading along the cryostat axis Base temperature in the frozen spin mode ≈ 30 mK

1.5 K radiation shield 25 K radiation shield Separator (3K) and Evaporator (1.2K) precooling stages ( 4 He) 80 K radiation shield 3 He/ 4 He Dilution stage T min ≈ 23 mK Polarization ≈ 94% Relaxation ≈ 1500 hours ≈≈

High-temperature heat exchanger (7 stages: from room to 10 K)

Intermediate tube-in-tube heat exchanger: (seven 3-mm capillaries inside a 10-mm tube: from 10 K to 4 K)

Low-temperature heat exchanger (4 stages: from 4 to ~2 K)

4 He Evaporator bath (surrounds and thermally isolates the still bath)

Still bath of the dilution unit

Preliminary heat exchanger of 3 He/ 4 He dilution stage (geometric impedance ≈ 2×10 8 cm -3 )

Sintered copper heat exchanger of the dilution unit (10 stages, total area 5 m 2 on each side)

Vacuum C 4 H 10 O – 60% 30mm 3He/4He – 6%

Loading of the target material into the cryostat

First variant of the insert Although T~30 mK was reached with this insert, its installation required using of critically large loads, while the thermal contacts turned out to be ineffective and unreliable. In spite of high power of the heater it was impossible to reach sufficiently high temperature of the indium gasket for reliable sealing, which resulted in appearing superleaks. The re-cooling line inside the insert was found to be insufficiently effective. The waveguide of a stainless steel tube had too large attenuation. NMR cables were not completely vacuum-tight.

Second variant of the insert Two-part insert has important advantages: i) a difficult procedure of introducing and vacuum-sealing of the outer insert can be performed at room temperature in usual atmosphere; ii) mounting of the inner insert with the cooled butanol sample inside the cold cryostat can be accomplished very easy, without effort, and do not require the hermetic seal (an indium gasket placed into a slot was used).

Internal longitudinal Holding coil ( solenoid coil manufactured of μm multifilamental NbTi cable and consisting of four layers, each having 600 turns wound around a 0.3-mm thick copper holder, T ≈ 1.5 K)

Internal transverse Holding coil

SUMMARY Working parameters of the dilution cryostat are in agreement with the technical requirements: - T min ≈ 23 mK; - polarization relaxation time ≈ 1500 hours (at T=30 mK); - time to cool from room temperature ≈ 5 hours; - LHe consumption in the frozen spin mode ≈ 2 l/hour Internal holding coils provide longitudinal/ transverse field 0.4 Tesla at 30 A Any combinations of beam and target polarizations are possible Two-part insert makes the sample loading operation easy and convenient Future development: New insert, containing light-guides, for active polarized target

Participants JINR (Dubna): N.Borisov, A.Fedorov, V.Kolomiets, L.Kutuzova, A.Lazarev, S.Mironov, A.Neganov, V.Pavlov, Yu.Usov INR RAS (Moscow): G.Gurevich, R.Kondratiev, Yu.Vorobiev IKPh (Mainz): A.Thomas, M.Martinez