Chapter 8

Large-Volume, Magnetically Shielded Room A New Design and Material

GARY R. SCOTT and CLIFF FROHLICH

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 197 2. General Principles of Electric and Magnetic Shielding. . . . . . . . . . . . . . . . . . . . . . 199

2.1. Types of Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 2.2. Static Magnetic Shielding by Materials with Magnetic Remanence. . . . . . . . . . . . 202

3. Practical Techniques for Building Magnetically Shielded Rooms. . . . . . . . . . . . . . . . 208 3.1. Site Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 3.2. Openings, Entrances, and Access. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 3.3. Joints, Corners, and Ends of Sheets ............................. " 211 3.4. Processing and Handling of Steel Sheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 3.5. Erection of the Shield and Placement of Sheets . . . . . . . . . . . . . . . . . . . . . . . 213

4. Three Specific Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 4.1. Woodward-Clyde, Oakland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 4.2. Sierra Geophysics, Redmond. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 4.3. Caltech, Pasadena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

1. Introduction

Technical developments of the last decade have opened up new fields of research in pa-leomagnetism and biomagnetism. In particular, the main impetus has been the commercial development of superconducting quantum interference device (SQUID) magnetometers, which utilize the Josephson effect to measure extremely small magnetic fields. SQUID magnetometers measure magnetic moments about 100 times smaller than had been possible previously. This improved resolution has allowed paleomagnetists to study remanent mag-netism in most sedimentary strata, and has extended considerably both the geographical and the chronological data base available for paleomagnetic research. Similarly, super-conducting magnetometers are opening new vistas in physiology and medicine.

For several reasons, the advent of the SQUID caused researchers to become more con-cerned with magnetic shielding. First, a sample measured with a SQUID magnetometer is not spun but is held in a fixed position, and hence it is possible for the earth's field to induce a magnetization during the measurement process. Similarly, the earth's field can

GARY R. SCOTT • Lodestar Magnetics, Inc., Oakland, California 94608. CLIFF FROHLICH

•

Institute for Geophysics, University of Texas, Austin, Texas 78712.

197 J. L. Kirschvink et al. (eds.), Magnetite Biomineralization and Magnetoreception in Organisms© Plenum Press, New York 1985

198 Chapter 8

Figure 1. Generally, the shielding provided by two thin shells of material is superior to a single very thick layer. For example, for a solid spherical shell of material of constant permeability fL of 2000, inner radius R and outer radius 2R (left illustration), the interior field at A will be reduced a factor of 390 below the exterior field B. However, where the shielding is provided by two thin shells of thickness

0.05R at radius Rand 2R (right illustration), the field in the interior will be reduced by a factor of 1732, according to the formulas reported by Schwiezer (1962).

cause samples to acquire a significant moment during preparation, storage, and laboratory analysis which may be retained for days or months. As the SQUID magnetometer must measure fields of 10- 8 of the earth's field or less for biological or paleomagnetic studies, the measuring procedure is simplified if one can reduce the ambient or background field around the sample and magnetometer. Finally, by reducing the ambient field, one can realize the advantages of using a first-order SQUID gradiometer rather than using a higher-order gradiometer which is necessary when the background magnetic field is larger.

Long before paleomagnetism and biomagnetism were recognized scientific disciplines, scientists knew that it was possible to build containers or rooms to provide shielding from low-frequency and de magnetic fields, such as the earth's field (Wills, 1899). For example, if a thick-walled sphere of inner radius Hl and outer radius Hz is built of a ferromagnetic material of high permeability fL, the field inside the sphere is reduced by a factor S

S = 2fL (1 _ m) 9 m

from the ambient magnetic field outside the sphere (e.g., Jackson, 1962). Wills (1899) and subsequent workers (Wadley, 1956; Cravath, 1957; Schwiezer, 1962; Cohen, 1967; Thomas, 1968; Cohen et 01., 1970; Wikswo, 1975) showed conclusively that several layers or stages of shielding material provided a more effective shield than a single thick layer of material (Fig. 1). Using these principles, shields have been built with as many as six layers of mu metal, which is reported to reduce the interior field by a factor of about 10,000 (Erne et 01., 1981; Mager, 1981; Kelha, 1981).

Until recently, there has been very little literature published which gives practical advice for those wishing to construct magnetically shielded rooms. A notable exception is the work of Patton (1967), who gives a detailed summary of pertinent equations of estimating minimum wall thickness, expected shielding effectiveness, etc., as well as ex-amples of shielded rooms that he himself constructed. Our experience and that of others (Scott and Frohlich, 1980; Symons and Stupavsky, 1983) corroborates many of Patton's conclusions and speculations. For example, he correctly notes that permanent or remanent magnetization is responsible for providing much of the effective shielding in most existing magnetic rooms. He also speculates that for constructing magnetostatic shields, inexpen-sive materials such as transformer steel may be as effective as expansive high-fL materials such as mu metal (Table I). Indeed, within many laboratory shields the field caused by scientific equipment provides a significant proportion of the effective field in the shield's interior. In these situations, incurring great expense to reduce the interior field is not cost effective.

However, our subsequent research has not confirmed the usefulness of all the design practices advocated by Patton and Fitch (1962), Patton (1967), and Thomas (1968). For example, Patton (1967) suggests that a magnetic shield must be completely enclosed, and

Large-Volume, Magnetically Shielded Room 199

TABLE I. Comparison of Transformer Steel and Mu Metal

Transformer steel

Moderate permeability Moderate remanence High saturation induction 1982 cost $1.30/kg Annealed during manufacture Fabrication and machining during construction of

shield Flexibility in size, shape, and design changes Brittle-requires hardened cutting tools Electrical resistivity 50 x 1O- 6 0hm/cm Composition: 96% Fe, 4% Si; silicon used as an

deoxidizer in steel melts Shielding performance by remanent alignment

High permeability Low remanence

Mu Metal

Moderate saturation induction 1982 cost $9.00/kg, plus annealing High cost of annealing, 1982 cost $125/sheet Fabrication and machining before annealing

Limits to size, shape, and design changes Ductile-easy to machine Electrical resistivity 62 x 10- 6 0hm/cm Composition: 75% Ni, 18% Fe, 5% Cu, 2% Cr;

hydrogen annealed for deoxidation Shielding performance by spontaneous magnetic

alignment

thus he takes special care in describing how to construct tight metallic doors and over-lapping joints linking adjacent sheets of shielding material. Our experience indicates that the presence of open doors or close-tolerance joints is not critical for static magnetic shield-ing; instead, it is important to orient these openings properly with respect to the direction of the magnetic field, and with respect to variations or gradients in the field. Indeed, it is possible to construct effective shields with only two parallel walls and with no floor or ceiling material.

In the present paper, Section 2 discusses the different types of shields that can be constructed to reduce electric and magnetic fields in enclosed areas. We shall outline briefly the properties and materials which are desirable for obtaining effective shielding with each type of material.

In Section 3 we discuss the practical aspects of building static room-size magnetic shields out of sheets of electrical transformer steel. The main purpose of this is to explain in some detail what we know about constructing magnetically shielded rooms. These tech-niques have been developed principally from empirical studies and not from theoretical considerations. One of us (GRS) designed and constructed seven shielded rooms between 1981 and 1983, and the construction methods discussed are distilled primarily from his experience. In Section 4 we illustrate the principles covered in Sections 2 and 3 by de-scribing particular features of several of these rooms.

2. General Principles of Electric and Magnetic Shielding

2.1. Types of Shielding

In spite of magnetic shielding'S long history, the principles that govern the design and construction of magnetic shields are sometimes poorly understood even by scientists who build magnetic shields. One reason for this confusion is that there are several different reasons for building a "shielded room," each requiring a slightly different type of material and construction. In addition, in a qualitative way, different types of shields look much alike. For example, most electrostatically and magnetically shielded rooms are just large boxes enclosed by metallic sheets. Because ferromagnetic metals are relatively good con-

200 Chapter 8

TABLE II. Physical Characteristics of Various Common Materialso

Absolute initial Resistivity Relative

Material permeability (m!llcm) Remanence cost

Mu metal 20,000 62 Low High (sheets)

Purified iron 5,000 10 High Low (sheets)

Permalloy 2,500 45 High High (sheets)

Nickel 600 7.8 ?? High Aluminum 1 2.8 Low Copper 1 1.7 Low Silver 1 1.6 Very high

a From Weast (1973).

ductors, magnetically shielded rooms generally provide fairly good electrostatic and elec-tromagnetic shielding as well, even though these features may not be a part of their design criteria.

The three physical properties of a material which influence shield design are its elec-trical resistivity p, its relative permeability fL, and its magnetic remanence. The electrical resistivity of a material is a measure of its resistance to the flow of electrical currents. The resistance R provided by a material of length L and cross section A is

R = pUA

The relative permeability fL is a measure of the ability of the material to affect the magnetic field in its immediate neighborhood. If one changes the ambient magnetic field by the field within the material changes by an amount given by

=

In general, the permeability depends on the ambient field, the frequency of the applied field, the orientation of the material, and numerous other parameters of varying mathe-matical intractability. For this reason, to calculate explicitly the fields expected for special geometries, fL is usually considered to have a single, fixed constant value.

Finally, when an: applied field Bo is removed, many materials retain some magnetic moment. This is known as remanent magnetization, and materials which exhibit nonzero remanent magnetization are said to have remanence. Resistivity, permeability, and re-manence for a material all depend on how a material is prepared and on its orientation. However, for convenience, in Table II we present approximate values for these properties for various materials which might be used to construct shielded rooms.

Shielding against dc (0 Hz) electric fields, or electrostatic shielding, can be obtained by enclosing a room or container with a thin layer of any conducting material such as copper foil or even metallic paint. Regardless of the static electric field outside the enclosed space, the inside field will be identically zero. Indeed, more than a century ago Faraday used this fact to demonstrate th&t the electric field due to charges falls off as the square of the distance. The only design criterion necessary for complete shielding from outside dc electric fields is for the space to be completely enclosed by the conducting material.

Large-Volume, Magnetically Shielded Room 201

Therefore, construction details include sealable doors and special fixtures for cables and power lines that enter the shielded space. For shielding against exceedingly high electric fields, one might have to be concerned about the breakdown of air. One would want to avoid sharp points on the outside of the room, or enclose portions of the exterior within a dielectric material.

Shielding against high-frequency (AF) electric and magnetic fields, or electromagnetic shielding, can be accomplished by enclosing a room or container with a thick layer of conducting material, usually copper or aluminum. Faraday induction within the con-ducting material causes currents that oppose the external fields. Within the shielding ma-terial, the electric and magnetic fields of frequency f (Hz) drop off exponentially (by a factor of 2.718) over a distance 1) called the skin depth, given by

where p is the resistivity, foL is the permeability, and foLo = 41T X 10- 7 kg m/C2 • For a conductor like copper, 1) = 0.85 cm at 60 Hz but 1) = 0.06 cm at 10,000 Hz. Thus, a room surrounded by copper walls a centimeter thick will reduce fields with a frequency of 104

Hz by a factor of more than a million, but at 60 Hz the reduction is about a factor of three. A much thicker copper shield or shield of ferromagnetic material with a high permeability would be needed to more completely shield against 60-Hz fields. The design principle for this type of shielding is to determine the amplitude and frequency spectrum expected for the external field, and then to make sure that at each frequency the thickness of the shield material is a high enough multiple of the skin depth to lower the field appropriately. As with electrostatic shields, the best shielding is obtained if there are no holes or gaps in the shield, thereby allowing the induced currents to flow uninterrupted in any path sur-rounding the room's interior. Clearly, a room constructed in this way will provide effective electrostatic shielding as well as electromagnetic shielding.

Shielding against lower-frequency varying magnetic fields, or dynamic magnetic shields, can be accomplished by enclosing a region with layers of very-high-permeability magnetic material, such as mu metal or permalloy. This is the type of shielding discussed by Wills (1899) and Jackson (1962). The ideal material for this type of shielding is similar to the materials used for constructing transformers. It should have a high relative perme-ability foL, but as little remanence as possible. Unlike electrostatic or electromagnetic shield-ing, the geometry of dynamic magnetic shielding is critical, and will be discussed further below. Instead of using a highly permeable material, it has been possible during the last decade to use Helmholtz coils, a sensor, and a feedback loop to cancel the magnetic field within the working volume. However, this shielded region is only a small portion of the volume enclosed by the coils. The necessity of sensors and the expense of maintaining currents in large coils limit the feasibility of using Helmholtz coils for shielding room-sized volumes.

The fourth category, shielding against constant (0 Hz) magnetic fields, or magnetostatic shielding, is the primary topic of the remainder of this chapter. Previously, there has been little distinction between dynamic magnetic shielding and magnetostatic shielding, be-cause a completely effective dynamic shield also will shield against static fields. However, as some of the design criteria and the type (and expense) of materials are different for the two types of shielding, a clear distinction is important. In general, magneto static shielding can be obtained by using Helmoltz-type coils or permanent magnetic material. For room-sized volumes, permanent magnetic material would be the preferred choice. A material for magnetostatic shielding must have high remanence, must maintain its magnetization without decay, and must be readily available. Electrical transformer steel is such a material.

202 Chapter 8

X

1

r : ( uoM f 4nB

N

X

s t--- r ---<

Figure 2, When a magnetic dipole with moment M in an external field B is in its minimum energy configuratiion, there will always be a locus of points at a distance r 0 from its equator where the dipole field exactly cancels the external field.

2.2. Static Magnetic Shielding by Materials with Magnetic Remanence

It may seem paradoxical that "permanent" magnetic materials can provide magnetic shielding. Indeed, the following results show that one can construct an effective shield from the earth's magnetic field using only one sheet of magnetic material "oriented" north-south along the earth's ambient field. Even a simple magnetic dipole, however, will di-minish the field at some points in space if oriented in its minimum potential energy po-sition in the external field.

The potential energy U associated with a dipole of magnetic moment M in a field Bo is

U = -M'Bo

and so the minimum energy configuration will occur when the dipole is oriented along the direction of Bo. If r is the position of a point in space relative to the dipole, the magnetic field at r is

(1)

Thus, for points in the equatorial plane of the dipole where M'r = 0, there is a zero magnetic field (see Fig. 2) at a distance fo from the dipole given by

For groups of dipoles, the energetically preferred orientation depends on the strength of the applied field. For example, for two dipoles of moment M1 and M2 separated by a vector distance r12, the contribution to the potential energy U12 due to the interaction between the dipoles is

U12 = fLo [ri2 M1'M2 -41T r12

If r12 is along the direction of B1 , the minimum energy will always occur when the moments line up along the field direction. However, if the field is perpendicular to the vector joining the dipoles, the minimum energy configuration will depend on the strength of the applied field. For large fields, the moments will be aligned parallel along the direction of the field, but for smaller fields both the anti parallel alignment and the alignment perpendicular to the applied field will have a lower energy (Fig. 3).

Qualitatively similar results hold for thin sheets of permanently magnetized material. If the sheet is magnetized so that the moments of all the domains lie parallel within the plane of the sheet, this provides a nearly constant magnetic field outside the sheet opposite in direction to the direction of magnetization (Fig. 4).

Large-Volume, Magnetically Shielded Room

Figure 3, Four possible configu-rations of two magnetic dipoles in the presence of an external magnetic field (upward arrow in each illustration). Configuration A always has the least potential energy; however, in the presence of an external field, dipoles con-strained as in B, C, and D may prefer to be in one of several ori-entations. In large external fields, configuration C has the least en-ergy, whereas in smaller external fields, D will have the least. If the dipoles are to lie parallel to the field direction as in Band C, con-figuration B will have the least energy for low external fields.

tA

m m em

IWl

203

t 2 lJ = - - 2MB A --211 R3

m t 0

[C]J

Although it is difficult to calculate the field explicitly for rectangular sheets, one can integrate Eq. (1) explicitly for the case of a flat thin circular disk of radius R, thickness t, and magnetic moment m per unit volume directed parallel to the plane of the sheet. At a distance r from the disk along the axis of the disk, the field is

(2)

Note that when r <if R, the field is nearly constant. Perhaps more surprising is the fact that

r-- --, I I

t I

I B 0 L __ __ J

N S N S N N

S N S N S S

N N

N N

S S -----1-N N

N N

S

N N

S S -------N N

S S

N N S S

N S N S S N S N N S N S

Figure 4. The magnetic field due to a sheet of magnetic dipoles (left illustration) is opposite to the dipole direction and is ap-proximately constant in the re-gion near the center of the sheet (area within dashed lines). In an applied field with intensity Bo ,

the domains tend to line up along the field direction, creating a nearly constant low-field region between the two plates (center il-lustration). In a box with closed ends (right illustration), the do-mains at the ends have difficulty responding to the field in a co-herent fashion, and thus the ends provide little additional shield-ing in the interior regions.

204 Chapter 8

the field vanishes as R becomes very large, and we approach the case of an infinite sheet of magnetic moment.

For our purposes, we can use the above formula to calculate approximately the shield-ing factor S produced by the disk if all the magnetization is induced by the permeability 1.1. of the material. Suppose the disk is aligned so that its surface is parallel to the earth's field Bo. From Maxwell's third equation, one can show that the magnetic moment m per unit volume induced in a material by an external field Boutside that parallels the material's surface is

1.1. - 1 m = -- Boutside

1.1.0 (3)

However, the field Boutside includes both the earth's field Bo and the field due to the induced magnetic moment. From (2), since r is zero this is

(4)

By combining (3) and (4) we can solve to find the shielding factor S, or the reduction in the earth's field caused by the presence of the disk, i.e.,

= 1 + (1.1. - l)t = S Boutside 4R

(5)

Of course, this result is only approximate as we have tacitly assumed that the induced magnetization is everywhere constant and equal to its value in the center of the disk.

These results show that one can construct an effective shield from the earth's magnetic field using only one or two flat sheets of magnetic material oriented along the ambient field direction (Fig. 4). In the case of two sheets, the lowest energy configuration is for sheets oriented so that the magnetization direction is along the ambient field direction, which produces an opposing field in the central region between the sheets. Note that the shielding does not depend on having a closed region. If end sheets do exist, they contribute little to the shielding, for the domains in the end sheets cannot align easily along the ambient field direction (Fig. 4). However, the end sheets do react to the fringing fields generated by the parallel sheets.

Of course, a limitation of calculations like the one above is that at ordinary temper-atures, all real magnetic materials have both remanence and permeability. For this reason, it is not possible to separate the effects of permanent magnetization and of permeability, as we do in the discussion above, and as do authors such as Wills (1899) and Jackson (1962). In addition, remanent dipole moments in a magnetic material do not have a com-pletely fixed magnetization m; rather, m depends on the prior history of the material. Finally, the permeability 1.1. depends on the strength and frequency of the applied field.

We can demonstrate the shielding effect possible with plane sheets of transformer steel by performing a simple experiment (Fig. 5). We aligned the surface of a 66 x 71 x 0.062-cm rectangle of transformer steel along the earth's field and "demagnetized" it by passing it through a coil consisting of about 200 turns of copper wire carrying a 60-Hz AF current, thereby creating an alternating field (Fig. 6). The field created by the current is initially strong enough to reorient domains, but it decreases to zero as the coil passes away from the sheet. Thus, this "demagnetization" causes the permanently magnetized domains in the transformer steel to realign so that they have a low potential energy in the earth's field.

Large-Volume, Magnetically Shielded Room

Figure 5. If sheets of transformer steel are demagnetized in the presence of the earth's field Do by passing them through a large coil carrying a 60-Hz current, the re-sulting magnetic field near the center of the sheets (at xl de-pends strongly on the orientation of the sheets during demagneti-zation. When demagnetization occurs while the sheet is oriented parallel to the earth's field as in the illustration on the left, the field at x is reduced to only a few percent of Do, showing that shielding is occurring. However, when demagnetization occurs with the sheet oriented perpen-dicular to the earth's field, al-most no shielding occurs at x.

205

When we performed this experiment recently at the Woodward-Clyde Laboratory in Oakland, the resulting magnetic field adjacent to the center of the sheet was 1500 nT. As the earth's magnetic field in Oakland is about 50,000 nT, this demonstrated that a shielding factor of about 35 could be obtained with only a single sheet of magnetic material.

We then took the steel sheet into a shielded room, and remeasured the magnetic field near its center to get an idea of the remanent magnetization that remained. This remanent field was about 4500 nT with a direction opposed to the earth's field during AF treatment. This suggests that about 10% of the shielding provided by the sheet was from remanent magnetization, and shows that closed boxes are not essential for effective shielding.

Now suppose we repeat the entire experiment but take care to align the plane of the sheet perpendicular to the earth's field. When the field at the center of the sheet was measured outside of the shielded room, it was 50,000 nT. Effectively, in this orientation the sheet is transparent to the magnetic field. When the sheet was "demagnetized" in the same fashion inside the shielded room, the field at the center of the sheet was about 100 nT, or about the same as the field inside the shielded room. This demonstrates that the magnetization depends on the history of the material.

In general, because any two orthogonal planes contain all components of a vector, constructing only two sets of surfaces may provide sufficient shielding. However, for any given site (or vector), the simplest room design would include only a pair of walls precisely aligned with the magnetic declination. This design is not practical, but serves to illustrate the general principle that it is the planes parallel to a magnetic vector component that provide most of the shielding from that magnetic component. The east and west walls are of primary importance for shielding a typical site at intermediate latitudes, with the north and south walls of secondary importance.

206 Chapter 8

Figure 6. Photograph of the large sweeper coil used by the authors to demagnetize individual sheets of transformer steel and to reorient domains within sheets after they are in place in the shielded room. The coil consists of approximately 200 turns of l8-gauge copper wire which can be connected to a l20-V AC source. Smaller sweeper coils are used to demagnetize parts of the room with restricted access.

A serious practical limitation of theoretical treatments of shielding (e.g., Wills, 1899) is that they ignore remanence and also assume that the material permeability IL is a constant independent of the field B. In fact, the permeability becomes markedly smaller at low fields (Fig. 7) because the proportion of magnetic domains aligned with the field is reduced. In theoretical treatments of magnetics, the fraction of domains that is aligned with the applied field is known as the Langevin function L(a) (see, e.g., Kittel, 1966). Here, a =

MB/kT is the ratio of magnetic potential energy MB to thermal energy kT for individual domains, with k being Boltzmann's constant and T the absolute temperature. For regular groupings of identical isotropic magnetic domains, it can be shown that

Large-Volume, Magnetically Shielded Room

en en ::::l l\l Cl .Q

C 0

:;::; 0 ::::l

"C .E

III

20

18

16

14

12

10

8

6

4

0

.1

24 gauge transformer steel M22 FP data from U.S.STEEL

10

H DC Magnetizing Force, oersted

207

l10,OOO

9,000

8,000

>-7,000

.-

.c l\l

6,000 Q)

E ..... Q)

5,000 a.. lii

4,000 E ..... 0 Z

3,000 :t.

2,000

1,000

0

100

Figure 7. Induced field B and permeability j.L as a function of dc magnetizing force for the transformer steel used by the first author in the construction of magnetically shielded rooms. The arrow at 0.5 De shows the approximate strength of the earth's field. The graphed data are provided by the manufacturer of the steel sheet.

L(a) = coth (a) - l/a

As B is reduced, the fraction of aligned domains L(a) approaches zero, and thus the effec-tive permeability of the bulk material also becomes small. For this reason, if one wishes to use Wills's (1899) or Patton's (1967) formulas to obtain an accurate estimate of shield-ing from several layers, one must use a different effective j.L for the permeability of each layer.

Considering these problems, it at first may seem fortuitous that shields built using Wills's principles provide shielding so effectively. In addition, because domains tend to line up along the local field direction, why don't they sometimes produce a field which more than cancels the earth's field, i.e., a net field in the opposite direction from the earth's field? Unfortunately, in this chapter we cannot provide quantitative answers to these prob-lems.

However, in a qualitative way we can explain why it is reasonable that properly pre-pared rooms will behave as Wills calculated. In a high-j.L, nonremanent material, domains are largely free to realign themselves along the local direction as the field changes. In contrast, in a real material the direction of some of the domains remains fixed, especially at lower fields. However, almost any activity such as hammering, thermal agitation, or demagnetizing with AF "sweeper" coils (Fig. 6) allows the fixed domains to realign along the local field direction, i.e., the same direction as for a nonremanent high-j.L material. Thus, the shielding occurs because both remanent and nonremanent domains which realign are aligned in the same manner as calculated for a high-permeability material, and the remaining domains are either aligned randomly or aligned along some other historically

208 Chapter 8

The general tendency for domain alignment also helps explain why properly prepared real materials provide even higher shielding than calculated from laboratory measurements and formulas such as Eq. (5). The measured permeability for a material reflects only the activity of the nonremanent domains. Because the preparation of the shield causes the domains to align in the same manner, the effective permeability will be higher than the measured permeability. In the experiment performed in Oakland with the 66 x 71 x 0.062-cm sheet, we measured a shielding factor of 35. However, according to the manufacturer's specifications (Fig. 7), fL for this electrical transformer steel in the earth's field is about 3000, and so from Eq. (5) we would calculate a shielding factor of about 2.5. This difference was also demonstrated during construction of the outer layer of the shielded room in Oakland. Patton's (1967) approximate formula for the shielding factor S provided by a single layer of material of permeability fL of thickness t enclosing a space of interior di-mension L is

S = 1 + 1.34 fLt/L

For the Oakland room, Scaled = 2.7, since t = 0.124 cm, L = 3.0 m, and for transformer steel in the earth's field of 50,000 nT, fL = 3000 (Fig. 7). When the outer layer of the Oakland room was constructed, the measured shielding factor was Smeasd = 6 (internal field = 8500 nT). After some vibrational alignment (hammering) of domains, the internal field was reduced to 2900 nT. This corresponds to an effective permeability of 28,000, far above any measured value. This further demonstrates that with transformer steel, reman-ence plays a major role in shielding.

3. Practical Techniques for Building Magnetically Shielded Rooms

Many of the features of magnetic shield design and fabrication discussed here have been used by previous workers. For example, Patton (1967) speculated that effective shields could be constructed from thin sheets of electrical steel. Our usual construction technique involves building a two-stage shield with a spacing of 25-30 cm between the stages. Each stage has two thicknesses of 0.062-cm steel, for a total thickness of 0.124 cm/stage. Gen-erally, we use the shielding factors reported by Patton (1967) and others to predict ap-proximately the performance of a projected design. Like ordinary nonshielded rooms, the framing for shielded rooms is constructed of wood; however, we always take special care that the entire room is constructed with nonmagnetic materials except for the steel sheets. For example, we use aluminum nails to put together the wooden frames and brass screws to attach adjacent sheets to one another and to the frame.

Some of the construction practices we follow routinely are not always necessary to obtain good shielding performance. For example, we generally demagnetize tools such as screwdrivers or saws before using them to construct the interior shield of a room. We also never lay tools (especially power tools) on the floor of a shield; rather, we place them on wooden tables or boxes. However, localized magnetization induced by careless construc-tion practices usually can be remedied afterwards with sweeper coils.

In this section we discuss in special detail certain features of our shielded room design such as door entrances and joints which differ from the design suggested by Patton (1967). We also discuss certain practices not emphasized by previous authors, such as conducting a site survey and preparing the shielding materials before construction begins.

Large-Volume, Magnetically Shielded Room 209

3.1. Site Survey

To ensure optimum performance of a magnetic shield, a detailed magnetic site survey should be undertaken before construction begins. Besides finding the general features of the magnetic field (direction and magnitude). a survey will reveal potential problems that are the result of spatial or temporal variations at the site. The magnetic field inside larger buildings is often distorted by structural or reinforcing steel and by service pipes. If these spatial gradients are too large or extensive, they can reduce the effectiveness of a shield. In practice, magnetic shields attenuate uniform fields more effectively than magnetic gra-dients. If remanently magnetized material is the source of the gradient, it is often possible to demagnetize the material with an AF sweeper coil. Throughgoing straight pipes or beams are of little concern if they are oriented in an E-W direction, as they can be given a nearly zero field by AF demagnetization. However, large fields are associated with ferrous ma-terials having a N-S orientation, especially at bends or terminators. The best plan is to have at least a double thickness of shielding material in and around areas having large gradients. In addition, openings in the shield should be avoided in areas of large fields or gradients. Finally, keep in mind that some kinds of laboratory equipment may produce substantial fields within the shielded region.

A site survey should also search for temporal variations in the field. Because much of the shielding produced by real materials is provided by remanent domains, the shield will be less effective for temporal variations. Moving or movable magnetic sources are a common problem. Examples are vehicles (both moving and parked). furniture, filing cab-inets, laboratory equipment, and elevators. Avoidance of these problems is the best policy; we recommend choosing a site that minimizes these features. It is helpful to request that you be informed whenever equipment or furniture is moved into spaces near the shield.

Whenever the local magnetic field does change, the magnetic shield can be given a new remanence which responds to those changes by using a sweeper coil as discussed in a later section. The need for periodic servicing of a magneto static shield requires that the shielding surfaces be accessible. In general, it is sufficient to give the inner surfaces a new remanence, but for larger variations in the ambient field, the outer shield must be accessed.

3.2. Openings, Entrances, and Access

The problem of access is associated with all shielded enclosures. The solution differs markedly for different types of shielding. For electromagnetic shields, the solution is to completely enclose the shield with sealable doors without allowing any "line of site" passages. The same solution works for magnetic shielding as well; however, while this approach is effective, it restricts the design of magnetically shielded enclosures. More flexible solutions to the problem of accessways can be found by considering the working portions of a magnetic shield, and by contrasting the operation of magneto static and elec-tromagnetic enclosures.

Two features important for electromagnetic shields are unnecessary for static magnetic shields: "line of site" restrictions, and electrically sealed doors (e.g., see Fig. BA,B). In practice, the two basic classes of shielding materials are those with high electrical con-ductivity and those that exhibit ferromagnetism (Table II). Not only do these classes operate using different physical mechanisms, but in a conceptual sense they provide shielding from "sources" at right angles to one another. Because electromagnetic waves are transverse waves, a plane sheet of eddy current shielding material such as aluminum will be most effective against electromagnetic waves directed perpendicular to the plane of the material. Alternatively, if the conducting material is parallel to the direction of wave propagations,

210

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Figure 8. Vertical north-south cross section through 7-foot mu-metal cubic room at University of Texas facility in Galveston, (A) showing magnetic vectors out-side the shield. Scale indicates intensity of field vectors, indi-cated by arrows. Stars are used for measurements greater than 0.1 mT. Ambient field, Bearth is shown to scale. Declination = 6°, inclination = 65°, Bearth = 46,500 nT. (B) magnetic field generated by shield material. Vectors showing generated field are identical to magnetic field vectors in (A), except the earth's field of 46,500 nT has been sub-tracted. Note the large variations in field direction and intensity that occur near the external edges of the shield. Stars are sites where field exceeded 50,000 nT.

charges within the material can move only a small distance, and little shielding is achieved. Thus, in practice this sheet is opaque to waves impinging on its surface, but is nearly transparent when aligned with the waves. The situation for materials with ferromagnetic properties is exactly the opposite. A sheet of this material is largely transparent when oriented perpendicular to a magnetic field. However, by aligning the sheet along the field direction, magnetic shielding occurs symmetrically on both sides of the sheet. Clearly, different access openings must be designed for the two classes of shielding material.

We have built several quite effective magnetostatic shields which have no doors at the entrance (e.g., see Fig. 9A,B). From the preceding discussion, it would appear that access should be made where the magnetic field lines intersect the ends of the shielded regions, for here the planes of shielding material are perpendicular to the field and have little impact on the shield's overall effectiveness. Surprisingly enough, this is not a realistic choice. Two serious flaws are encountered; first, to approach such an entrance, one must

Large-Volume, Magnetically Shielded Room 211

pass through a high-field region (e.g., see Fig. 8). At the Woodward-Clyde shielded room in Oakland, there are fields greater than four times the ambient (e.g., 200,000 nT) just outside the lower-north and upper-south ends of the room. This suggests that openings should face perpendicular to the ambient field, for then the field encountered decreases steadily as the shield is approached from the exterior. Our experience in constructing shielded rooms with openings which face perpendicular to the ambient field fully supports this conclusion.

A second serious problem with openings which face parallel to the ambient field direction stems from the high fields and large gradients produced by the shield itself at the edges of the enclosure (Fig. 8B). When there are openings in these regions, a portion of the ambient field, and/or the field produced by the shielding material intrudes into the enclosure. If shielding material is placed over these openings, it produces shielding by responding to these locally produced fields and gradients. Of course, if a very large shield were built elongate and parallel to the ambient field, these effects could be avoided by working only in the middle areas (e.g., Wikswo, 1975). However, in most practicallabo-ratory shields, completely enclosing the end regions provides shielding from high fields as well as from shield-produced gradients.

In summary, there are two general rules for locating openings and entrances. First, access passages should face in a direction normal to the magnetic vector, and second, they should be far removed from the magnetically extreme ends of the shield.

3.3. Joints, Corners, and Ends of Sheets

Previous discussions of shield design and construction have emphasized the impor-tance of the joints between sheets of material (Patton, 1967; Thomas, 1968). This is a valid concern for electromagnetic shields because electrical conductivity is important; however, simple overlapping joints of ferromagnetic material give excellent results for magnetostatic shields. The joint system we use incorporates approximately 5 cm of overlap, with the two plates screwed together using solid brass fasteners spaced about every 15-20 cm. When a covering is used, such as plywood or gypsum board, the spacing between screws can be 25 cm or more. These values for overlap and screw spacing are conservative, as we have constructed joints with 1-cm overlaps which are virtually unnoticeable inside the shield. Generally, overlapping joints cannot be detected magnetically. Some exceptions occur, but this is restricted to within 10 cm of the joint surface and is only for joints in regions with high magnetic fields.

Our design for corners has been simple and successful. We use a special nonmagnetic bending brake, constructed from hardwoods, plywood, stainless steel and aluminum to fold the steel sheets into a right angle. As there are two steel layers on all surfaces, a set of folded sheets can be arraJ?ged so that a corner is fully covered except for the apex. For the interior, whenever a sheet ends at a large opening such as an entranceway, the sheets are bent approximately io cm outward. This places the end of the sheet and its attendant large field further from the shielded space.

If possible, we complete the construction and demagnetization of the outer stage before beginning the inner stage. For the interior sheets, we take care to perform the bending in the low-field region produced by the outer stage of the shield. After the outer shield is in place, the bending brake is moved into the partially shielded room (B = 3000 nT) and the interior sheets are folded there. This apparently reduces the alignment of high-coercivity magnetocrystalline domains in the deformed portions of the sheets.

212 Chapter 8

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Figure 9. (A) Plan view of shielded room at Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan. Numbers are field intensity (nT) measured 5 days after construction at points indicated by the filled circles. Dashed lines show approximate contours of measured fields at intensities of 100 nT, 500 nT, 1000 nT, and 5000 nT. Arrows adjacent to "E" and "Woo show position of vertical cross section in (B). Before construction, the ambient field intensity was 35,800 nT, with a declination of 8° and an inclination of 38°. (B) Vertical cross section through doorway in Taipei room, with field intensities and contours labeled as in (A). Note that strong gradients in field intensities are restricted to doorway area.

3.4. Processing and Handling of Steel Sheets

The shielding material we use is manufactured by u.s. Steel and is 24-gauge (0.62 mm), M-22, FP (fully processed), CP-3 (core plate) electrical steel with a saturation mag-netization (Hs) of 400 Oe. It comes in rolls of up to 107 cm (42 inches) in width which can be slit to any convenient width and cut to any desirable length. Generally, we have used sheets of 66 x 132 cm (26 x 52 inches) or 91 X 132 cm (36 X 52 inches). These sizes are convenient to handle during construction and require only a moderate number of joints. The weight is 11.84 kg/m2 (0.5 Iblft2 ).

Before installing the shielding material, in many of our shields we demagnetize each sheet to remove any remanent magnetization remaining from the processes of manufac-turing, cutting, or handling. For demagnetization, we pass a large electromagnetic coil (60 Hz, 15 mT; see Fig. 6) around each sheet to modify the magnetic direction for most of the remanent domains in the material. Because we control the orientation of the sheets relative to the ambient field, when the AF electromagnet is removed, the sheets acquire a mag-netization with known magnetic direction and relative intensity. The acquired magneti-zation is refered to as anhysteretic remanent magnetization (ARM). Any orientation can be used; one can even orient the sheets perpendicular to the earth's field to reduce the magnetization acquired. However, we prefer to orient the ARM direction along the length of the sheet. For this purpose we use an inclined table, with a tilt equal to the ambient field inclination and facing toward magnetic north. The direction of the ARM is then

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marked on each sheet so that it can be oriented later when the room is erected. During this demagnetization-ARM acquisition process, the sheet is passed through the center of the coil to expose the steel to the maximum fields available so that even high-coercivity domains will be realigned. Subsequent procedures with the same coil will have insufficient coercive force to realign the high-coercivity domains, but will affect only those with lower coercivity.

3.5. Erection of the Shield and Placement of Sheets

The erection sequence we use for building rooms of electrical steel is designed to simplify construction and to allow access to all surfaces (walls, floor, and ceilings) so that they can be magnetized in place. The most convenient situation is a free-standing enclosure where only the outer floor has restricted access. However, when space is at a premium, shielded enclosures can be and have been built with four or five surfaces against existing walls and ceilings. In these situations, planning must allow for these surfaces to be mag-netized before they are closed off by later construction. If possible, some limited access (even enough room for an electromagnetic coil) should be provided to allow for magnetizing the surfaces. This may be necessary both during the construction of the shield as well as later in cases where the ambient field changes because equipment or furniture is moved.

Each sheet is placed such that any previously acquired ARM is aligned with the max-imum component of the ambient field in the plane. For example, for a shielded enclosure with N-S orientation, inclination of about 600 , the sheets on the floor have their northward-directed ARM aligned toward the north. For the north and south walls, the northward ARM

214 Chapter 8

is aligned downward. Although in principle one should install the sheets on the east and west walls with their ARM inclined at 60°, in practice it is sufficient to align the ARM downward, along the largest component of the field in the plane of the sheets.

Construction usually starts with the outer-stage floor. First, we give it an overall ARM with the same coil that processed the individual sheets. In this case, we simply pass or "sweep" over the surface in much the same manner as when cleaning with a mop or push broom. The field continuously realigns perhaps 80% of the domains in an up and down direction, perpendicular to the plane of the shielding material. As the coil moves away, the ambient dc field preferentIally realigns a portion of these coerced domains along the component of the ambient field in the plane of the material. The amount of ARM acquired is proportional to the strength of the ambient dc field. Thus, the newly acquired remanence is in the same direction and has the same relative strength as the ambient field components in the plane of the shielding surface.

In general, the entire outer stage is erected and given an overall ARM. If access is restricted, this ARM acquisition procedure is repeated after the construction of each wall and ceiling. However, in the next section we will discuss an example where the inner shield walls were built first and the enclosure still provided excellent overall shielding performance. Regardless of the erection sequence, this conditioning process can be re-peated subsequently if the interior field increases, either for the entire room or for a small part of the shield. In most situations, magnetically mopping the inner shield (Fig. 6) will yield the lowest interior fields.

4. Three Specific Examples

4.1. Woodward-Clyde, Oakland

We constructed our first two-stage electrical steel, shielded room in Oakland, Cali-fornia, for Woodward-Clyde Consultants in 1980 (Fig. 10). This room had a shielded vol-ume of 23 m3 • At this time we had not yet developed our methods for remagnetizing the sheets in place with AF coils, although before installation we gave individual sheets a relatively weak ARM. When the outer layer of shielding was in place, the internal field was about 8500 nT. To improve the shielding, we first attempted to use physical shock or

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16 X 12 X 10

not necessary to obtain high outside shielding factors.

Large-Volume, Magnetically Shielded Room 215

vibration to induce domain realignment. After experimenting with various mechanical vibrators and hammers, the most effective method was found to be a hand-held rubber mallet. By pounding the entire outer shield, we achieved an average field of 2950 nT, corresponding to a shielding factor of about 15.

After installing the inner shield, the field was 490 nT. However, thoroughly pounding the inner shield only reduced the internal field to 325 nT. Apparently, we had reached the effective limits of shock magnetization, so we had to devise another more coercive method. We found that by "sweeping" over the walls with the same electromagnetic coil that processed the sheets, we immediately reduced the internal field to 126 nT. Sweeping both the inner shield and two of the outer walls produced an average field of 103 nT. This corresponds to a shielding factor of about 420.

While measuring the residual fields, we noticed an episodic magnetic field. The source of this field was vehicular traffic on a nearby six-lane boulevard centered only 15 m to the north of the shielded area. Magnetic pulses of 150 nT lasting 4-6 sec accompanied passing cars. Buses and trucks produced fields of 800 nT inside the shield. The inability to respond effectively to changes in the external field is a limitation of highly remanent materials such as electrical steel, and underscores the importance of selecting a magnet-ically quiet site.

4.2. Sierra Geophysics, Redmond

The shielded laboratory built for Sierra Geophysics near Seattle, Washington in 1981, provided a graphic illustration of the remanence characteristics of electrical steel. This room enclosed 28 m3 in a steeply inclined field (54,000 nT, inclination 72°). The con-struction sequence was:

1. Step 1, build outer floor and ceiling and treat with sweep coils. 2. Step 2, build all interior surfaces and treat with sweep coile;. 3. Step 3, build outer walls.

We selected this sequence so that the inner walls could be aligned magnetically with the nearly unshielded vertical field. After the first two steps were complete, the interior field was surprisingly low, ranging from 290 nT to 820 nT with an average of 480 nT. The next day, the outer walls were erected and given an in situ ARM with sweep coils, in effect partially shielding the inner stage. The interior field now ranged from -12,000 nT to -14,900 nT, with an average of -13,400 nT, and with a direction which now opposed the ambient field. We allowed the shield to set in this configuration for 5 days without further treatment, remeasured and found that the field ranged from -11,700 nT to -14,100 nT, with an average of -12,900 nT. We then swept the exterior walls again with a coil, and the average field changed only slightly to -13,500 nT. These experiments clearly demonstrate that remanence domains with long relaxation times (greater than 4 x 105 sec) provide a significant portion of the shielding produced by electrical steel. It also shows that the magnetic history of the shielding material is important.

After treating both interior and exterior walls extensively with sweeper coils as de-scribed below, the shield reached an interior field of - 95 nT (range from - 43 nT to -147 nT), for a shielding factor of 570. However, the interior field remained directed opposite to the ambient field. The process by which the interior field reached this low value il-lustrates the coercivity of the remanence and the interaction of the two shielding stages as they were remagnetized repeatedly (Fig. 11).

What we intended to do was to gradually demagnetize (and therefore remagnetize) the inner stage in small steps until the interior field approached zero or reversed direction.

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This appeared possible because the inner shield had been given a relatively strong ARM. After erecting the outer stage, the field at the inner walls was much smaller than the original ambient field. By sweeping the coil over the inner shield we could impose a new weaker ARM, and therefore reduce the remanence contributed by the inner shield. This new ARM could be weakened in steps with increasingly larger AFs. This would realign effectively the lowest coercivity domains first, and gradually involve more and more of the shielding material (Fig. 11).

In addition to remagnetizing the inner shield with progressively higher AFs (Fig. 11), the outer shield was remagnetized at the full coil strength of 12.5 mT after each interior step. At alternating fields of 2 and 3 mT, we repeatedly realigned the interior and then the exterior walls. This showed that it was possible to converge upon a single interior field as a general equilibrium was reached between the shielding stages. This also demonstrates that remanent magnetization of the electrical steel has a wide coercivity spectrum. This experiment shows that shielding cannot be improved indefinitely by using domain realignment to increase the effective permeability of a material. As the ambient field be-comes weaker around the innermost shield, the alignment process becomes less effective. This apparently limits the shielding that is possible. For the case of a two-stage enclosure, the empirical shielding limit seems to be about 50 nT (Table III) regardless of whether iron or high-permeability alloys are used. This corresponds to a shielding factor of about 1000.

4.3. Caltech, Pasadena

The Caltech Biomagnetic Clean Laboratory was built in 1982, and enclosed an interior volume of 38 m3 • Electrical steel shielding and clean-lab features were combined to form

Large-Volume, Magnetically Shielded Room 219

a facility for research concerning the magnetic properties of biogenic material. For the interior, epoxy paint and fireproof gypsum board were applied over the electrical steel sheets. The lab also possesses magnetically filtered positive-pressure air flow, and a pass-through shower in the entryway to the inner shield.

In addition to the clean-room features, the design of this laboratory presented two unusual problems. First was size; a N-S length of 6.2 m was required, significantly larger than any previous steel room and approaching the maximum horizontal size of mu-metal shields. The interior length was 4.2 m, with a 1.7-m-wide spacing used for the north wall. This space incorporates the compound entryway, shower, air ducts, and service pipes. The second problem was the service pipes, which could not be eliminated as they provided water and sewer needs for the entire building. Allowing these pipes, which are mostly steel or cast iron, to pass between the shielding stages, required some planning. The re-moval of pipes that terminated near the shield eliminated the large magnetic fields as-sociated with their ends. Only E-W-directed pipes were kept, as this orientation produced the minimum induced magnetization. We rearranged all other pipes to run E-W or rerouted them around and away from the shield. Wherever convenient, pipes and ducts were re-placed by nonmagnetic materials; however, concern about expense and compatibility with building codes kept us from changing most of the pipes. Because the remaining service pipes and ducts were oriented in an E-W direction, they could be demagnetized effectively. Using the same coil as for processing the shield, we produced an ARM of nearly zero along the length of the pipes. Inside the inner shield, no significant magnetic field could be attributed to these service pipes, lying as close as 0.5 m away.

The outer shield enclosed 80 m3 , with an internal field of about 4000 nT after initial treatment with the sweeper coils. With the inner shield erected, but before ARM treatment, the field averaged about 200 nT, for a shielding factor of about 215. After treating the inner shield with sweeper coils, the field was about 80 nT. The long-term stable field in the interior is about 120-200 nT, for a shielding factor of approximately 300.

5. Summary

By taking advantage of the properties of remanent magnetization, effective and rela-tively inexpensive magnetic shielding can be obtained against static fields such as the earth's magnetic field. We here described three room-sized shields constructed from two stages of ordinary transformer steel. We specially processed the steel with large AF coils so that its remanent magnetization was aligned favorably for shielding. Our research sug-gests that any ferromagnetic material will provide some level of shielding. However, the effectiveness of magnetic shielding can be enhanced by considering the magnetic history of the material and by orienting the shield and shield access ways in certain perferred geometries.

ACKNOWLEDGMENTS We thank Wulf Gose, D. T. A. Symon, and an anonymous reviewer for their suggestions concerning an earlier version of the manuscript. The motivation for using this new shielding material sprang from a seminar at the University of Texas at Dallas lead by Bob Patton in 1974. In attendance were John Foster and one of us (G.R.S.) who later proposed to build a two-layer shield in 1979 for Woodward-Clyde Consultants. Mike Stu-pavsky and David Symons generously supplied information from their experience with a single-layer shield. Duane Packer and Jeff Johnston supplied support and technical as-sistance through Woodward-Clyde Consultants. Several craftsmen and other workers were essential to the existence of these shielded laboratories, including Michael Rosenbaum, George Clark, William Richter, Henry Salameh, Pamela Cross, Carol Van Alstine, and John Sporich. This is Contribution No. 578 from the University of Texas Institute for Geophysics.

220 Chapter 8

References

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