Micro-magnetic susceptometer for the actinides

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Micro-magnetic susceptometer for the actinides

S. Nave, P. Huary

To cite this version:

S. Nave, P. Huary. Micro-magnetic susceptometer for the actinides. Journal de Physique Colloques,

1979, 40 (C4), pp.C4-114-C4-115. �10.1051/jphyscol:1979436�. �jpa-00218832�

JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-114

Micro-magnetic susceptometer for the actinides (*)

S. E . N a v e ( t ) a n d P . G . H u a r y

Department of Physics, University of Tennessee, Knoxville, TN 37916, and Transuranium Research Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37830, U.S.A.

Résumé. — Un susceptomètre magnétique à courant continu, comprenant un « S.Q.U.I.D. » comme détecteur de flux magnétique, a été construit avec une sensibilité permettant de mesurer la susceptibilité magnétique d'échantillons d'actinides à une échelle inférieure au microgramme en fonction de la température. Pour un échantillon de deux microgrammes la susceptibilité minimum est 10 CGS pour un champ de 2 000 gauss. La sensibilité a été déterminée par une série de calibrations en utilisant comme échantillons des sphères de plomb à 4,2 K. Pour réduire le bruit on a utilisé des bobines détectrices insensibles à des variations de champs spatialement uniformes et à des champs dont la dérivée première est constante. Le champ est piégé par un cylindre de niobium pour produire un champ appliqué stable et uniforme. Le flux produit par l'échantillon dans les bobines détectrices est proportionnel à la susceptibilité magnétique de l'échantillon et est fonction de sa position.

Abstract. — A d.c. magnetic susceptometer incorporating a S.Q.U.I.D. as a magnetic flux sensor has been constructed with sensitivity to measure the magnetic susceptibility of submicrogram actinide samples as a function of temperature. For a two microgram actinide sample the minimum measurable dimensionless susceptibility is ~ 10 CGS in a 2 000 gauss field. The sensitivity has been determined in a series of calibration experiments using samples of lead spheres at 4.2 K. Pick-up coils insensitive to changes in spatially uniform fields and fields with a constant first derivative were used to reduce noise. The field was trapped in a niobium cylinder to produce a uniform stable applied field. The flux produced by the sample in the pick-up coils is proportional to the magnetic susceptibility of the sample and is a function of the sample position.

1. Introduction. — With t h e d e v e l o p m e n t of t h e S u p e r c o n d u c t i n g Q u a n t u m I n t e r f e r e n c e D e v i c e ( S . Q . U . I . D . ) a high sensitivity m a g n e t i c s u s c e p t o - m e t e r h a s b e c o m e available for u s e in determining t h e magnetic p r o p e r t i e s of limited s a m p l e q u a n t i t i e s . W e h a v e c o n s t r u c t e d an a p p a r a t u s b a s e d o n t h e s e e l e c t r o n i c s w h i c h e m p h a s i z e s a large s a m p l e t o p i c k - u p coil flux transfer r a t i o . A t t h e s a m e time w e h a v e r e t a i n e d t h e versatility of variable s a m p l e t e m - p e r a t u r e b e t w e e n 4.2 K a n d r o o m t e m p e r a t u r e , a n d a n e x t e r n a l l y applied m a g n e t i c field, u p t o 2 000 g a u s s .

2. Apparatus description. — A cross-sectional dia- g r a m of t h e l o w - t e m p e r a t u r e part of this a p p a r a t u s is s h o w n in figure 1. T h e c r o s s h a t c h e d parts- of t h e a p p a r a t u s r e m a i n i m m e r s e d in a liquid helium d e w a r . T h e pick-up coils a r e l o c a t e d in t h e g e o m e t r i c c e n t r e of a s u p e r c o n d u c t i n g m a g n e t (not s h o w n ) . T r i m coils a b o v e and b e l o w this m a g n e t p r o v i d e a null field in t h e n e i g h b o u r h o o d of t h e S . Q . U . I . D . a n d maintain a b e l o w critical field for t h e shield tubing a n d lead w i r e s . A t h e r m a l l y activated niobium cylinder (Fig. 1) is rigidly a t t a c h e d a b o u t t h e pick-up coils t o

Fig. 1. — Cross-section of the low temperature part of the micro- magnetic susceptibility apparatus.

shield t h e m f r o m external e l e c t r o m a g n e t i c noise a n d t o t r a p t h e u n i f o r m applied m a g n e t i c field. T h e pick-up coils a r e w o u n d w i t h a 1.5 m m inside d i a m e - t e r in a second derivative fashion s o t h a t t h e y d o n o t (*) Research sponsored by the Division of Nuclear Sciences,

U.S. Department of Energy under contracts EY-76-S-05-4447 with The University of Tennessee (Knoxville) and W-7405-eng-26 with Union Carbide Corporation.

0) Supported by Oak Ridge Associated Universities.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979436

MICRO-MAGNETIC SUSCEPTOMETER FOR THE ACTINIDES C4-115 respond significantly to changes in a spatially inva-

riant magnetic field nor to changes in a field with constant first derivative. This geometry, shown in figure 2, reduces the effect of external noise, and ideally gives a persistent response only when a sample is displaced relative to the coils in the presence of an applied field. The design achieves a maximum fractional coupling of 0.002 9 of the sam- ple flux to the S.Q.U.I.D. element.

x

= - 1/4 .rr due to its perfect diamagnetism at 4.2 K. A plot of the theoretical magnetic flux per unit dipole moment of the sample, @ / p , versus position in our pick-up coils is illustrated in figure 2.

Points taken from an experimental measurement, GSi,,,, versus position of the sample in our pick-up coils is also given for comparison in figure 2. For a sphere the sample moment is

i

^{lso where} m is the sample mass, p is its density, and H is the applied field. Thus,

a -

A

^S f (the flux transfer ratio) is the fraction of the

o sample flux coupled to the S.Q.U.I.D. and depends on the various coil geometries through their induc- tances. The value of f obtained by using the respon- se when the sample is in the geometric centre of the

Y

^{- -100 coils is 0.002 9.}

-1000-

For the 8.3 gauss measurement shown in figure 2,

1 1 1 1 1 l 1 1 1 1 1 1 l 1 1 l 1 1

-8 -6 -4 -2 0 2 4 6 8

POSITION (MM) a maximum signal of 163 @, was obtained.

Fig. 2 . - A plot of the theoretical magnetic flux divided by the sample moment @ / p (in cm-') coupled to the persistent pick-up coils (also indicated) as a function of the position (in rnm) of a spherical sample. Points taken from an experimental measure- ment, @,,,, ,,, are also indicated for a 60 pg lead sphere in a uniform 8.3 gauss field.

The samples are held by G.E. 7031 varnish in a 0.2 mm cavity drilled in the side of,= miform 0.5 mm diameter gold wire. The wirdis enckpsed inside a quartz tube for positioning a$d insuldtion.

The gold wire is held by a copper chuck which contacts a thermal ballast (Fig. 1). The ballast contains heaters and a variety of thermometers. The chuck, gold wire, and sample may be removed through a vacuum interlock for sample exchange.

The entire chuck and ballast may also be driven up or down by a motor at a rate of 15 p,m/s. This displacement moves the sample through the pick-up coils and is detected by a linear potentiometer.

A series of experiments were conducted to cali- brate the apparatus. In one a 60-microgram sphere of lead was used to define the sample susceptibility as

Go = 2.07 x lo-' gauss

.

cm2

.

Since

xHm

it may be seen that a 0.1- microgram sample of '@Cf metal (which has

x

= lop3 at 50 K) is expected to give a signal of 0.6 @, in a field of 2 000 gauss. This measurement is accessible with our system noise level of Go. For a two microgram actinide sample in a 2 000 gauss field our minimum measurable dimensionless susceptibility is thus

AX

-

^{lo-' CGS (or}

^{AX^} -

^10- cm3

-)

_g

With these sensitivity levels measurements should be of sufficient accuracy so as to be limited by other uncertainties such as sample geometry, bulk and surface contamination, and reproducibility of weighing. A computer program designed to compen- sate for a distribution of magnetic moments has been written to deconvolute the flux vs. position data.

With this program we may to first order account for geometrical sample or coil irregularities.