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Submitted on 1 Jan 1978
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A He-3 DILUTION REFRIGERATOR FOR
NEUTRON DIFFRACTION STUDY OF NUCLEAR
ANTIFERROMAGNETIC ORDER
P. Roubeau, Y. Roinel, V. Bouffard, O. Avenel
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 8, Tome 39, aotit 1978, page ~6-1146
A
He-3DILUTION REFRIGERATOR FOR NEUTRON DIFFRACTION STUDY OF NUCLEAR
ANTIFERROMAGNETIC ORDER
P. Roubeau, Y. Roinel, V. Bouffard and 0. Avenel
CEN/SacZay, Sermice de Physique du SoZide et de RBsonance MagnBtique, B.P. 2, 91190 Gif-SUP-Yvette, France
Rdsum8.- Des ddtails sont donnds sur la construction et 110p8ration d'un cryostat utilis6 dans des expgriences de diffraction de neutrons qtii mettent en dvidence l'ordre antiferromagnLtique nucl6aire. Le &ispositif qui comprend un rbfrigdrateur h dissolution pourvu d'un accbs direct B la chambre de m8lange et une bobine supraconductrice trPs homogPne de 6,5 tesla, peut tourner librement autour d'un axe vertical.
Abstract.- Details are given on the construction and operation of a cryostat used in neutron diffrac- tion experiments which give evidence of nuclear antiferromagnetic order. This set-up which includes a He-3
-
He-4 dilution stage with an axial access to the mixing chamber and a split coil high homo- geneity 6.5 tesla superconducting magnet, can be rotated freely around a vertical axis.In a previous experiment nuclear antiferroma- gnetic order had been observed in the laboratory using a special type of dilution refrigerator/]/ the main features of which were :
a) a pumped external He-4 bath (- 2.5 K) in a conventional liquid N2 shielded dewar ; this dewar containing a 5.5 tesla high homogeneity magnet/2/ ;
b) a completely sealed enclosure, from room to the lowest temperature, for the dilution stage ;
c) a one piece dilution stage inserted through the room temperature part of the He-3 pum- ping line/4/ ;
d) vacuum insulation of the dilution stage en- closure from 2.5 K down to the mixing chamber, the lower part consisting of a tail made of nonmetallic material ;
e) an axial access to this tail for quick in- sertion of a sample which should not be warmed abo- ve 77 K.
In order to be able to study the diffraction of neutrons we had to use a slightly different ex- perimental set-up which includes :
f) a 4.2 K He-4 bath i,n a new dewar and a split coil high homogeneity 6.5 tesla superconduc- ting magnet/3/ ;
g) a continuously fed 1.3 K He-4 bath sur- rounding the He-3 condenser of the dilution stage ;
h) a means of adjusting the sample level re- lative to the neutron beam and its radial orienta- tion relative to the dewar ;
i) a means which allows one to rotate the en- tire assembly including the 4.2 K dewar, the ener-
gized magnet, the operating dilution stage, and the sample itself ; this rotation being sufficiently smooth not to warm the sample even in the 100 mK
temperature range.
The detailed description of the system would be lengthy and difficult to make clear so it seems better to provide detailed figures.
Figure 1 is a view of the assembly with the dilution stage removed.
Figure 2 is a shematic representation of the low temperature part of the cryostat.
Figure 3 represents the pumping "T" which allows free rotation with very small effort.
Figure 4 is a perspective of the top flange of the dilution stage with exploded views of the sample handling and orientation mechanism.
. - . . - , . . . . - - . 2-PUMPING " 1 " FIXED I N SPACE
WHEN ASSEMBLY ROTATES
m k
3-WAVE GUIDE WITH SOME FLEXIBILITY.4.LIOUID NITROGEN 8ATH125 LITERSI
I
,-S-- z z , l R O I A I E S.S-HE-4 UPPER LEVEL i TOTAL CHARGE 7.1.3K HE-4 BATH
-8-HE-C SIPHON WITH FIXED -9-INSULATING VACUUM ~lO.LOCArlON OF DILUT;ON SlAGE ,I$-HE-4 LOWER LEVEL
12-MICROWAVE HORN 14. NOT REPRESENTED : FIELD
SWEEPING COIL, A O R F COILS IS- FLUOROTHENE I C2 F3 Cl l TAIL 16-THIN ALUMINIUM WINDOW ON
NEUTRON B E A M PATH \ll.LARGE ANGLE ACCESS 6.5
TESLA SUPERCONDUCTING MAONET .I8-GONIOMETER TURN TdBLE
Fig. 1 : General assembly (not to scale)
1.SMALL W E FOR ELECTRICAL LEADS 2. SINTERED BRONZE 30pm FILTER 3.FIXED IMPEDANCE 14 MOLESIHOW
/ 4 . S T STtEL TUBE .d 5 1 5 5mnr
5.COPPER THERMAL SHUNT
6.1.3K HE-4 BATH
10. INSULATING VACL'UM 1I.DILUTE PHASE SURFACE 12.STlLL
,II.RIGID STRUCTURE
I7.SEALED TUBES
I9.AXlAL CHANNEL s 6 mm
0. SECUNDARY IMPEDANCE
l . SINGLE TUBE HEAT EXCHANGERS
Z.CONCENTRATED PHASE LEVEL
2S.Kcl-F SAMPLE HOLDER
26.Krl-F MACHINED TAIL TUBES
~ 6 x 8 AND 9 5 x 1 2 m m
27. L,H CRYSTAL
2B.INCOMING CONCENTRATED PHASE 0.7 x l m m TEFLON TUBE
Fig. 2 : Low temperature part (not to scale).
- I - E r
2- HE-4 OVERPRESSURE TO AVOID
AIR CONDENSATION IN TUBE -7 -
WHEN CHANGING SAMPLE
\3-DIIUUON STAGE SUPPORTING
4- 6 100mm HEJ PUMPING LINE
RIGIDLY SETTLED
5-MASSIVE INTERMEDIATE SLEEVE
6.ONE OF THREE SUSPENSION TUBES
I S ALSO USED TO CONVEY HE-3
7- AXIAL STAINLESS STEEL GUIDE TUBE FOR SAMPLE INTRODUCTION 8- 0.15 mm CLEARANCE
9-RUBBER 0-RINGS ALLOWING FREE ROTATION
10-STAINLESS STEEL RIGID " T "
11-SLIDING 0-RING
1 2 - D E W R TOP FLANGE 13. HE-3 PUMPING LINE
Fig. 3 : The pumping 'IT".
Fig. 4 : The top flange : 1 level adjustment clip; 2 radial orientation adjustment plug ; 3 rubber o- rings on the sample holder.
CONCLUSION.- We have built and operated a cryogenic system which is able to cool a 5 mm sample down to 200 millikelvin with a He-3 circulation of 6 X 10-"
mole/s/5/ and e dissipation of 200 microwatt, and which attains 50 millikelvin without power while permitting smooth rotation around a vertical axis. Three different fields must be applied to the sam- ple in order to study nuclear magnetic ordering :
a microwave field, two r.f. fields and a low fre- quency field which modulates the static 6.5 tesla
D.C. field. The latter has a homogeneity of 5 x10-~ in a cylinder h =
d
= 6 mm. This system is suffi- ciently simple to be a reliable part of an other- wise "impressively multi" -component experiment.References
/ l / Roinel,Y., Bouffard,V. and Roubeau,P., (submit-
ted to J. Physique)
/ 2 / /3/ He-4 dewars and magnets from Oxford Instru- ments Ltd
/4/ Roubeau,P., A dilution refrigerator for neutron experiments. Proceedings of the sixth Interna- tional Engineering Conference (ICEC 6), Greno- ble 1976
