# Magnetic shielding for large photoelectron multipliers for the OSIRIS
facility of the JUNO detector

O. Smirnov, 11footnotetext: corresponding author, D. Korablev  A. Sotnikov  A.
Stahl  J. Steinmann  V. Khudyakov  I. Avetissov  M. Zykova

###### Abstract

We present technical design and characteristics of the magnetic shield
developed for 20" PMTs of the low-background OSIRIS facility. A ribbon of
amorphous alloy with extreme magnetic permeability was used in its design,
providing excellent efficiency in screening the Earth’s magnetic field with a
relatively small amount of material. The mass of materials is crucial to
construction of low-background facilities because of radioactive backgrounds.
Using amorphous materials is cost-efficient compared to other methods for
screening the Earth’s magnetic field.

##  1 Introduction and brief overview of methods for screening weak magnetic
fields in scintillation detectors

Normal operation of photolelectron multipliers (PMTs) is disturbed in the weak
Earth’s magnetic field of about 50 $\mu$T. Special measures are usually taken
in order to avoid degradation of detector characteristics in large volume
scintillator detectors in the weak magnetic field. The most straightforward
method is compensation of the Earth’s magnetic field (EMF) at PMT locations by
rather large Helmholtz coils placed outside the detector. This method was used
in the Super-Kamiokande [1] and KamLAND detectors [2]. The EMF compensation
will be used in the JUNO detector [3] in the same way. In the SuperKamiokande
location, the Earth’s magnetic field with a magnitude of 45 $\mu$T is oriented
at 45 degrees to the horizon. Geomagnetic field compensation is provided by 26
Helmholtz coils. The average residual EMF in the detector does not exceed 5
$\mu$T with current in coils ON. As a matter of fact, only the local field at
the position of a PMT is important for its proper operation, so the
compensation field should be tuned correspondingly. The magnetic field at
various PMT locations was measured before the tank was filled with water. In
accordance with the technical report of KamLAND, the EMF at the underground
laboratory is reduced to below 5 $\mu$T from the original 35 $\mu$T, which
ensures normal operation of 20" PMTs.

20" PMTs of the JUNO detector, both of the central detector and of the water
Cherenkov detector, lose up to 60% of their efficiency in the non-compensated
EMF. Magnetic field compensation is provided by 32 coaxial Helmholtz coils
which surround the detector. The calculated residual field should not exceed 5
$\mu$T at the central detector and 10 $\mu$T at the location of PMTs of the
water Cherenkov detector [4]. The PMTs are insensitive to the field of this
intensity.

Should EMF compensation be for some reasons unsuitable, PMT screening with
high magnetic permeability is needed. The common solution is a cylinder or
truncated cone made of permalloy sheets placed coaxial to PMTs. This scheme
was applied to the Borexino prototype detector CTF [5] and Borexino detector
[6]. The magnetic shield of the Borexino detector is shown in Fig. 1.

Figure 1: PMTs of the Borexino detector are equipped with magnetic shields
(yellow truncated cones at the PMT base in the photo, the colour is due to
waterproof coating) and light concentrators. PMTs are not visible in the
photo. The highest point of the PMT photocathode touches the plane of the
upper edge of the magnetic shield. Courtesy of the Borexino collaboration.

The results of magnetic shield testing for CTF/Borexino were presented in [7].
"Open" screens are quite effective in reducing the EMF component in the
direction perpendicular to the axis. Along the axis, the reduction of the
field is rather moderate because of the open structure. In general, the
magnetic field penetrates into the open structure at a depth comparable to the
size of the hole, and the usual recommendation is to extend the magnetic
shield about one diameter beyond the cathode plane [8]. It is impossible to
follow the recommendation in a straight way as the shadow of such a size will
cut a significant part of the light field at the OSIRIS facility.
Nevertheless, due to the fact that PMTs with a linear focussed dynode
structure are less sensitive to the field along the axis [7], a shorter "open"
shield is acceptable for application to PMTs with a linear focussed dynode
structure.

In order to suppress the EMF more uniformly, a number of experiments used
cages made of permalloy wires: DarkSide-50 [9], Antares [10], Baikal-GVD [11].
The use of a non-transparent material in front of the photocathode leads to
partial shadowing of light, reducing the total photoelectron detection
efficiency (PDE). In practice, the loss is about 5%. Moreover, it is
impossible to achieve a significant magnetic field reduction with a cage, the
field is reduced by a factor of 2.0-2.5. A common serious disadvantage of
using permalloy screens (both types: either made of sheets or wire), is non-
stability of shielding efficiency. Permalloy screens require annealing for
achieving a high magnetic permeability. Magnetic properties of the shield can
degrade because of mechanical stress. In this respect, cages are extremely
sensitive to conditions of transportation and treatment - any mechanical
deformation can lead to a partial loss of magnetic properties.

To make the description complete, let us mention a scheme of the EMF partial
compensation for PMTs with a linear focussed dynode structure proposed in
[12]. The scheme is based on different sensitivities of PMTs to two mutually
orthogonal directions (x-axis and y-axis, perpendicular to z, the PMT axis of
symmetry). Orienting PMTs by rotation around the z-axis, it is possible to
minimize the projection of the EMF in a single, most sensitive direction. In
such a way, the EMF influence is minimized without using screens or Helmholtz
coils. Naturally, two less destructive EMF components remain uncompensated.

When being used in low-background detectors, the magnetic shield should not
significantly contribute to the total radioactive background. In practice, the
radioactive background of the shield should be lower than that of a PMT. From
this point of view, cages are less radioactive compared to full-metal shields
because of a lower amount of material used. Nevertheless, modern PMTs with a
photocathode 20" in diameter will need some kilograms of material to provide
reasonable screening. In Fig. 2, we present, as an example, one of the
prototype cages developed at our Laboratory within JUNO activities for 20"
Hamamatsu PMTs (type R12860).

Figure 2: A prototype cage for a large PMT (20"). It consists of two separate
parts. The diameter of the widest part (nominal "equator") is approximately 50
cm. Permalloy wire 1 mm in diameter was used, the size of a single cell in
front of the photocathode is 50 mm. The wiring below the equator is denser by
a factor of 3.

Permalloy wire 1 mm in diameter was used for the cage with a 50 mm cell, the
light loss on the mesh is about 5%222Because of photocathode sphericity and
dependence of quantum efficiency on the angle of incidence, the loss of light
does not coincide with geometric shadowing.. The cages provided an acceptable
reduction of the magnetic field by a factor of 2, but the annealing process
was complicated because of the cage size - specialized large-volume ovens are
quite rare, and waiting for processing reaches several months. Moreover, the
cages need special coverage with protecting lacquer because PMTs will be
placed in water. This also causes technical issues as one needs "cold"
covering in order to preserve magnetic properties after annealing. The JUNO
collaboration decided for a global compensation of the EMF within the detector
volume, but the idea of magnetic screens came back to the agenda during
discussions about the OSIRIS facility design developed for measuring ultra-low
radioactivity of JUNO liquid scintillator before filling the main JUNO
detector. It is impossible to use Helmholtz coils at the OSIRIS facility at
the Jiangmen Underground Neutrino Observatory (JUNO) because of the restricted
space.

As the experience with large-size cages was unsatisfactory, we decided to use
an open structure magnetic shield similar to the one used in the Borexino
experiment but manufactured from modern materials: amorphous alloy and
composite materials. The PMTs of the OSIRIS facility will be immersed in
water, and hence all the materials and water should remain unchanged for a
rather long period. At the same time, the materials used should not contribute
much to the total radioactivity of the setup.

In the present article, we describe the successful application of the
amorphous alloy ribbon for production of EMF shields for the PMTs of the low-
background OSIRIS facility. The shields are made of glass fibre and/or carbon
fibre composite material and are coated outside with black gelcoat providing
necessary waterproof properties to the construction. The matted black colour
of the covering excludes parasitic reflections. The shields are equipped with
an auxiliary aluminium foil layer serving as a radiofrequency screen. In order
to satisfy the demands for low radioactivity, all the materials were carefully
selected based on the results of measurements.

An important advantage of using amorphous alloy is a significant decrease in
the mass and cost of material while providing the same screening effect:
magnetic permeability of non-annealed 333Annealing could further increase
magnetic permeability by a factor of 2 [15], but it would be technically
complicated in our case. amorphous alloy achieves $10^{6}$ compared to the
typical magnetic permeability of annealed permalloy of $10^{4}$, the price is
roughly an order higher. Replacing permalloy with the amorphous tape in such a
way, we gain two orders of magnitude in mass and an order of magnitude in
price for the same screening coefficient.

In this paper, we present a report based on the experience of manufacturing 64
shields using carbon fibre composite for the PMTs of the main detector of the
OSIRIS facility with very strict requirements for material radioactivity and
12 shields for the PMTs of the muon veto detector with less strong
requirements for radioactivity.

At the time our paper was being prepared, an article about a similar approach
to magnetic shielding using amorphous alloys has been published [13]
describing magnetic screens for 20" PMTs with MCP plates for the LHAASA-WCDA
experiment. The screens surrounding the photocathode, have a cylindrical shape
and are 41 cm in height. The protection against oxidation in water is provided
by a polymer film. The main difference from our shields is the absence of any
demands on screen radioactivity. Another significant difference is that we use
the internal surface of the shield as a light concentrator.

##  2 OSIRIS facility

The OSIRIS facility is schematically shown in Fig. 3. The facility uses 64
PMTs to detect light from the central liquid scintillator detector; the other
12 PMTs are used as the muon veto (water Cherenkov detector), 8 of them are
installed in water at the bottom of the detector, and 4 are placed at the top
of the detector. As one can see from Fig. 3, the PMTs of the central detector
are oriented either vertically, below and above the cylindrical sensitive
volume, or horizontally around the central detector. 8 PMTs of the muon veto
at the bottom are oriented vertically and 4 PMTs placed at the top are
oriented horizontally.

Figure 3: A schematic view of the OSIRIS facility. The side projection of the
central cylindrical detector is represented by a rectangular shape in the
centre of the image. The active volume is viewed either by the PMTs
horizontally placed around the active volume or by ones vertically installed
at the bottom and at the top of the cylinder. Muon veto PMTs are installed on
the "floor" (8 PMTs) or on the "ceiling" (4 PMTs) of the detector. Courtesy of
the OSIRIS collaboration.

The JUNO detector and its facility, OSIRIS, are under construction at the
Jiangmen Underground Neutrino Observatory (JUNO) in Kaiping, China. The
intensity of the geomagnetic field in the laboratory location (22.1∘ North) is
45 $\mu$T: 38 $\mu$T in the horizontal direction to geographic north and 24
$\mu$T in the vertical direction. The inclination of the field is 32.0∘ to the
horizon. Because of the stronger horizontal component of the field at the
laboratory compared to the vertical one, open shields effectively suppress the
most sensitive component of the field (orthogonal to the PMT axis) for the
PMTs vertically installed. The factor of reduction is the same for all PMTs.
For the PMTs installed horizontally, the screening factor depends on
orientation: efficiency of reduction of the field component parallel to the
PMT axis will depend on the angle between the PMT axis and the horizontal
projection of the field.

The Online Scintillator Internal Radioactivity Investigation System (OSIRIS)
is a 20-ton liquid-scintillator detector for monitoring radiopurity of liquid
scintillator during the JUNO filling phase. The OSIRIS will operate 20"
Hamamatsu PMTs (type R15343). This PMT type uses an electron multiplier with a
linear focussed structure (Box and linear focussed) [14], also see datasheet
R15343. The sensitivity of characteristics of PMTs of this type to components
of the field orthogonal to the PMT axis is noticeably stronger than to those
directed along the PMT axis (see. [7]). For the simplicity of notations in the
following discussion, we will use the notation z for the axis along the PMT
axis and x and y for two other orthogonal axes. For graphic representation of
the coordinate system, see Fig. 4 .

Figure 4: The coordinate system for characterization of PMT sensitivity to the
magnetic field is linked to the linear dynode structure in a way schematically
presented in the drawing. The y-axis is directed from the observer.

Developing a magnetic shield design, we decided to use the inner part of the
shield protruding from the PMT equator as a light concentrator. The height of
the cylindrical part of the shield was chosen on the basis of Monte Carlo
studies. The main parameter under study was efficiency of collection of light
from the central detector: a cone being too high would shadow a fraction of
incident light arriving at large angles at the PMT photocathode.

##  3 Description of the magnetic shield for PMTs of the OSIRIS facility

The magnetic shield consists of two parts: the main conic part and the
auxiliary cylindrical one, see Figs. 5 and 6.

Figure 5: Photos of the magnetic shield prototype: the main (left) and
auxiliary (right) elements of the magnetic shield are shown separately. A
copper contact of the RF screen is visible at the top of the main element. The
auxiliary element is equipped with the corresponding contact, too. In the
final design, the copper contact was replaced by a food grade steel in order
to avoid corrosion when placed in high-purity water.

Figure 6: On the left: two elements of the magnetic shield are shown in an
exploded view. On the right: the magnetic shield is shown together with a PMT
mounted. The main element of the magnetic shield is placed below the "equator"
of the PMT bulb. The auxiliary element is installed above the "equator".

The magnetic shield is formed by using the AMAG-170 ribbon (manufactured by PC
MSTATOR, Borovichi, Russia). The amorphous soft magnetic alloy used in the
production of the AMAG-170 ribbon has extreme magnetic permeability of an
order of $10^{6}$. The high magnetic permeability of the ribbon allows using
relatively small amounts of material, and this is of great importance for the
OSIRIS facility sensitive to radioactivity of construction materials. Another
important feature of the amorphous soft magnetic alloy is the stability of its
properties. The material does not require mandatory annealing to restore its
magnetic properties after processing.

Both parts of the magnetic shield are made from composite materials (carbon
fibre and glass fibre), 8 layers of the AMAG-170 ribbon $\simeq 25$ $\mu$m
thick and 30 mm wide (both dimensions are maximal available) are placed
between the layers of composite material in a way to guarantee an average
thickness of the shield of 0.2 mm. The uniformity of tape placement does not
play a big role, the total screening is defined mainly by the total mass of
the material used. One shield contains about 1.5 kg of amorphous tape.

An additional layer of aluminum foil is laid above the soft magnetic tape, it
serves as a screen against electric fields (radio frequency noise). Amorphous
metal does not provide effective RF screening because of rather moderate
conductivity.

Two shield variants were produced: one for protection of the muon veto PMTs
(12 in total) and another for shielding the PMTs of the main detector (64 in
total). The main difference is the type of the composite materials used. The
muon veto shields are produced using less expensive glass fibre composite
materials. Glass fibre is not appropriate for the main detector in respect to
radioactivity because of the high content of potassium (in this case from U
and 40K), so a more expensive carbon fibre composite material on the basis of
potassium-free polyether resin was used. This material provides the matted
black colour. The choice of polyether resin results from its long-term
resistance to chemically ultrapure water. Masses of the material used for both
shield variants are presented in Tables 2 and 3.

Glass fibre shields are covered with black gelcoat to prevent soaking of
construction materials when placed in water. The black matted colour of the
protection excludes parasitic reflection of the light from surfaces. The
carbon fibre composite is originally black. It was covered with a transparent
gelcoat (topcoat). The auxiliary cylindrical part is covered from inside with
white gelcoat. Titanium dioxide was used as a filler (10% TiO2 by mass). The
outside surface is covered with black gelcoat and matted in the case of glass
fibre shields and left untouched for carbon fibre items. In accordance with
Monte Carlo simulations, we expected an about 20% light collection improvement
in the air. Measurements showed a 19% improvement in light collection
efficiency (see Section 6).

##  4 Radioactivity of the magnetic shield

The purpose of the OSIRIS facility is low-background measurements of the
extremely low radioactivity levels of the liquid scintillator before filling
in the main JUNO detector. The levels needed for the JUNO physics programme
are at a level of $10^{-17}$-$10^{-16}$ g/g in U, Th and K and are unreachable
with common methods. The flux from radioactive impurities in magnetic shield
materials should not exceed the corresponding flux from PMTs.

The analysis of chemical impurities was performed at the Mendeleev University
of Chemical Technology of Russia by inductively coupled plasma mass
spectrometry (ICP-MS) with preliminary transfer of solid samples to the liquid
phase. Extra pure water (AquaMax-Ultra 370 Series, Young Lin Instruments Co.
Ltd., Korea, Anyang) with a resistance of 18 M$\Omega$·cm was used. Aluminum
foil, steel ribbon and carbon plastics were dissolved in high-purity nitric
(HNO3) acid (7N7) purified by the Berghof BSB-939-IR surface distillation
system in the SPEEDWAVE-FOUR microwave decomposition system (BERGHOF GmbH &
Co. KG, Germany, Drolshagen) equipped with DAP-100 PTFE autoclaves (BERGHOF
GmbH & Co. KG, Germany, Drolshagen). Polymer materials were dissolved in high-
purity sulfuric (H2SO4) acid (8N Ultrapur, Sigma Aldrich) by using the same
microwave decomposition system. Analytical measurements were carried out with
the NexION 300D inductively coupled plasma mass spectrometer (PerkinElmer
Inc., USA, MA, Waltham). The quantitative analysis of Th and U was carried out
using the “additives” method, taking into account the concentration of the
main (matrix) elements in the solution analyzed. Standard solutions
(PerkinElmer Inc.) were used for calibration. The optimized operating mode of
the NexION 300D spectrometer applied to the impurity analysis of samples is
presented in Table 1.

Nebulizer type | Concentric (Meinhard), PFA
---|---
Spray chamber | Scott double-pass chamber, PFA
Argon flow rate, L/min through the nebulizer | 0.98
plasma-forming | 15
auxiliary | 1.2
Generator power, W | 1500
Number of scan cycles | 15
Table 1: The operating mode of the NexION 300D instrument for conducting the
impurity analysis of samples.

The results of measurements recalculated to the content of radioactive
impurities are shown in Tables 4 and 5. Note that only preselected materials
which satisfy radioactivity requirements are listed in the Tables. We did not
test food grade stainless steel of the electric contact because of the low
mass of the material used. Even the worst case of natural content would not
harm the total balance of radioactivity. One can see (Table 4) that the glass
fibre shield exceeds the reference values by an order of magnitude in U and by
a factor of 4 in potassium, that is why it is not appropriate for the main
detector.

Material | Manufacturer | Main | Auxiliary | Total
---|---|---|---|---
|  | element | element |
Glass fibre | ECC Interglas 92140, aero | 0.8 | 0.36 | 1.16
Vinylester resin | Derakane MomentumTM 470-300 | 1.018 | 0.458 | 1.476
Black gelcoat | Maxguard™ GN-NTRL S + carbon black | 1.4 | 0.35 | 1.75
White gelcoat | Maxguard™ GN-NTRL S + 10% TiO2 | - | 0.35 | 0.35
Amorphous alloy | MSTATOR AMAG-170 | 1.015 | 0.55 | 1.565
Aluminum foil |  | 0.06 | 0.03 | 0.09
Steel ribbon | 12х18н10т type | 0.03 | 0.02 | 0.05
Total |  | 4.343 | 2.128 | 6.471
Table 2: Weight of materials used in kg (glass fibre items with black gelcoat coating). We used steel Type 12х18н10т made in Russia, its western analogue is AISI 304. Material | Manufacturer | Main | Auxiliary | Total
---|---|---|---|---
|  | element | element |
Carbon fibre | G. Angeloni GG245T | 0.57 | 0.25 | 0.82
Vinylester resin | Derakane MomentumTM 470-300 | 0.82 | 0.37 | 1.19
Transparent topcoat | Maxguard™ GN-NTRL S | 1.26 | 0.32 | 1.58
White gelcoat | Maxguard™ GN-NTRL S + TiO2 | - | 0.35 | 0.35
Amorphous alloy | MSTATOR AMAG-170 | 1.015 | 0.55 | 1,565
Aluminum foil |  | 0.06 | 0.03 | 0.09
Steel ribbon | 12х18н10т type | 0.03 | 0.02 | 0.05
Total |  | 3.755 | 1.890 | 5.645
Table 3: Weight of materials used in kg (carbon fibre items with transparent coating). Material | U | Th | K | m(U) | m(Th) | m(40K)
---|---|---|---|---|---|---
| ppb | ppb | ppm |  $\mu$g |  $\mu$g |  $\mu$g
Glass fibre | 5833 | <3 | 1460 | 6770 | <4 | 198
Vinylester resin | <0.1 | 0.9 | 0.78 | <0.15 | 1.3 | 0.13
Black gelcoat | <3 | <0.3 | 177 | <5.3 | <0.5 | 36
White gelcoat | 7 | 7 | 4.33 | 2.5 | 2.5 | 0.2
AMAG-170 ribbon | 3 | <5 | 0.84 | 4.7 | <7.8 | 0.154
Aluminum foil | 170 | 26 | 0.96 | 15.3 | 2.4 | <0.01
Total | - | - | - | 6798 | <18 | 234
PMT glass | 400 | 400 | 60 | 3600 | 3600 | 63
Table 4: Abundances (three first columns) and the total content (in $\mu$g, three last columns) of natural long-lived radioactive isotopes in construction materials of the glass fibre shield. The last row presents corresponding reference values for glass with a total mass of 9 kg. The total radioactivity of glass is $\simeq$80 Bq. The total radioactivity of the glass fiber magnetic shield is $\simeq$150 Bq, almost twice the radioactivity of the glass. Material | U | Th | K | m(U) | m(Th) | m(40K)
---|---|---|---|---|---|---
| ppb | ppb | ppm |  $\mu$g |  $\mu$g |  $\mu$g
Carbon fibre | 3 | <5 | 0.84 | 0.25 | <4.9 | 1.4
Vinylester resin | <0.1 | 0.9 | 0.78 | <0.12 | 0.11 | 0,1
Transparent topcoat | <3 | <0.3 | <4.33 | <4.8 | <0.5 | <6.8
White gelcoat | 7 | 7 | 4.33 | 2.5 | 2.5 | 0.2
AMAG-170 ribbon | 3 | <5 | 0,84 | 4.7 | <7.8 | 0,154
Aluminum foil | 170 | 26 | 0.96 | 15.3 | 2.4 | <0.01
Total | - | - | - | <27.7 | <18 | <8.7
PMT glass | 400 | 400 | 60 | 3600 | 3600 | 63
Table 5: Abundances (three first columns) and the total content (in $\mu$g,
three last columns) of natural long-lived radioactive isotopes in construction
materials of the carbon fibre shield. The last row presents the corresponding
reference values for glass with a total mass of 9 kg. We used the best value
of those measured for black and white gelcoats as an upper limit for
transparent topcoat. Due to the absence of fillers, the transparent topcoat
should be less radioactive, but even with these values, we are still well
below the levels imposed by glass radioactivity. The total radioactivity of
the magnetic shield is below 3 Bq, which is negligible compared to the
radioactivity of 80 Bq of the glass of the PMT.

##  5 Manufacture of the magnetic shield

Elements of the magnetic shield were manufactured by Hydromania Ltd (Minsk,
Belarus). The production line was placed in a dust-free isolated room, and all
the measures were taken to prevent accumulation of any impurities on surfaces.
Immediately after manufacturing, the shields were sealed in polyethylene bags
to prevent contact with air and, correspondingly, contamination of surfaces
with products of radon decay 444About 0.1% of styrene used for polymerization
of gelcoat (20 grams of 20% solution of styrene in parafene were used) remains
unbounded and evaporates. This process caused the accumulation of gaseous
styrene in the sealed plastic bag, and a strong smell was noticed after
unpacking. With the initial content of styrene in rough materials of about 4 g
for a carbon fibre shield (by a factor of 6 more for one of glass fibre),
about 4 mg per shield of residual styrene could emanate. When working in a
large workshop, the concentration of styrene remains within safe levels (the
legal airborne permissible exposure limit is 100 ppm averaged over an 8-hour
workshift). Anyway, the work was done using special masks for respiratory
protection. Before the final washing, the items were left in climatic boxes
for removal of gaseous styrene.. The shields will be washed with pure water
before the installation in the OSIRIS facility.

The shields are produced using the technology of manual (contact) formation on
mandrels. The mandrels were made of waterproof plywood and coated with a
protective glass fibre layer and instrumental (matrix) gelcoat. After
hardening, the gelcoat surface was polished to the necessary grade of
roughness.

The sequence of technological operations for carbon fibre items was as the
following:

  * •

coating of the surface with gelcoat: transparent gelcoat was used for the main
element and white gelcoat for the auxiliary one;

  * •

inner layers of carbon fibre with resin are placed after partial
polymerisation of gelcoat (to a tack condition);

  * •

four layers of the amorphous ribbon are placed on the carbon fibre surface.
The orientation of the ribbon in layers was chosen to avoid empty spaces in
the shield;

  * •

aluminum foil and stainless steel lamellas are placed on layers of the
amorphous ribbon; the outside electric contact is formed by the protruding
stainless steel lamella part;

  * •

four last layers of the amorphous ribbon are placed above aluminum foil, the
result is shown in Fig.7;

  * •

outer layers of carbon fibre were placed after the installation of magnetic
shields and RF screens;

  * •

finally, the outer surfaces were coated with a transparent gelcoat (topcoat).

Figure 7: The view of the magnetic shield before covering with outside
protective layers. The location of the amorphous alloy in the shield is
clearly visible, there was no need in magnetic shielding of the narrow part of
the cone since this part of the PMT is insensitive to magnetic fields.

##  6 Efficiency of the magnetic shield

The suppression of the weak magnetic field inside the shield is shown in Fig.
8. The unit of measurement is 50 $\mu$T (roughly corresponds to the EMF
strength), the suppression coefficient is a fraction of the total field
measured at the given point inside the shield.

Figure 8: Suppression of the weak magnetic field for two mutually orthogonal
components: perpendicular to the PMT axis and along the PMT axis, or z-axis.
The position of the PMT glass bulb is shown for reference, no PMT was
physically present at the time of measurement. The location of the magnetic
shield in the cone is shown with a thick line.

Most critical for the normal PMT operation is the direction along one of the
axes orthogonal to the PMT symmetry axis (z-axis in the chosen coordinate
system, see Fig. 4), namely the direction along the dynodes (y-axis), see [7].
Transport of electrons inside the PMT bulb is most sensitive to the field in
this direction. The estimates of shield efficiency were performed for the
field orientation along the y-axis. The estimates obtained correspond to the
maximum possible influence of the magnetic field in the direction orthogonal
to the PMT symmetry axis.

As one can see in Fig. 8, the shield suppresses the external field in the
direction orthogonal to the symmetry axis by a factor of 3 to 12. This means
that a large part of the working space inside the PMT volume is well protected
against this component of the EMF. For the vertically installed PMTs, the
residual sensitive component will range from 3 to 12 $\mu$T depending on the
position inside the PMT bulb. The integral effect should be checked anyway,
because a noticeable reduction of PMT efficiency is observed for uniform
fields of above 5 $\mu$T.

The suppression coefficient along the PMT axis is significantly weaker, its
maximum value close to the position in the PMT bulb center is just 2.
Nevertheless, due to the much less pronounced sensitivity of PMTs to this
component, we do not need its strong suppression.

Earlier measurements with 20" dynode PMTs of the JUNO detector showed a weak
dependence on time characteristics (below $\pm$0.2 ns) in the external
magnetic field of up to 20 $\mu$T [16]. Due to the fact that the magnetic
shield suppresses the external field to this value, we focused on the study of
light detection efficiency losses in external magnetic fields, anyway, we
checked the influence of the magnetic field on the transit time spread.

The estimate of magnetic shield efficiency is obtained by comparing light
detection efficiency in the absence of the magnetic field and when the
magnetic field is applied. As reference values, we used the measurements in
the compensated magnetic field with the magnetic shield installed. The
measurements with and without a magnetic shield in the compensated field
allowed estimating the improvement of light collection due to the light
concentrator. It is $(19.0\pm 0.7)$% 555In the air. In experimental
conditions, PMTs will be placed in a medium with a higher refraction
coefficient (in water), this leads to improvement of light detection with a
spherical photocathode. Correspondingly, the improvement of light collection
efficiency could be lower in water..

Tests of the magnetic shield were performed at a special laboratory equipped
with Helmholtz coils for EMF compensation [17]. Changing the orientation of a
PMT in space and adjusting currents in compensation coils, it was possible to
apply fields from 0 to 50 $\mu$T along the PMT axis or in the orthogonal
direction. The light field was created by a short-pulse laser. The efficiency
of light detection was estimated by an average number of photoelectrons (p.e.)
registered for one trigger of the laser. In order to provide the stability of
the system, a special care was taken. The stability was estimated by taking
data in the same conditions at different periods of time. We used in total 15
datasets of 30000 events in each, the variance of central values was found to
be $\delta\mu=0.026$ p.e. while the average p.e. count was
$\overline{\mu}=1.63$ with expected statistical uncertainty of
$\sigma_{\mu}=0.012$ p.e. Using these values, the systematics of the
measurements can be estimated $\mu=0.023$ p.e. or 1.4%.

When the maximum field of 40 $\mu$T is applied along the most sensitive
y-axis, the average count of photoelectrons drops by a factor of 2 ($\mu$=0.88
p.e., with a reference value in the compensated field of $\mu$=1.85 p.e.). The
electronic gain of a multiplier estimated from charge histograms decreases by
a factor of 1.4. The decrease in light detection efficiency was not so
pronounced for the same field applied along the x-axis: light detection
efficiency decreased by a factor of 1.09, and the electronic gain decreased by
a factor of 1.14. Uncertainties of the values quoted are at a level of 2% with
dominating systematics.

Light detection efficiency of the PMT with the magnetic shield in the maximum
field of 40 $\mu$T coincides with reference values within the error of
measurements.

The transit time spread (TTS) was checked for a field of up to +/-40 $\mu$T
directed along the x-, y- and z-axes. In the worst case, the TTS for the PMT
without a magnetic shield increases by about 30% in the presence of the
magnetic field (from 0.9 to 1.2 ns for the PMT under tests), the TTS for the
PMT equipped with the magnetic shield is about 0.9 ns for all possible
directions of the magnetic field, which is consistent with the value measured
in the absence of the magnetic field within the precision of measurement.

We can state that the magnetic shield developed at our laboratory provides
practically ideal protection of PMTs in arbitrarily directed external magnetic
fields of up to 40$\mu$T.

##  7 Conclusions

There is no available space for installation of EMF compensation coils due to
the configuration of the OSIRIS facility, and the detector PMTs will operate
in the non-compensated EMF. We have developed open magnetic field shields
using soft magnetic materials on the basis of amorphous alloys. The laboratory
tests demonstrated very good efficiency of these magnetic shields for PMTs
arbitrarily oriented in the EMF. Taking into account that the detector PMTs
will not operate at extreme values of magnetic fields applied during the
tests, the expected performance will approach an ideal one in a completely
compensated field. Amorphous alloys reduce the amount of material needed for
manufacturing due to their extremely high magnetic permeability. All the
construction materials were tested for the content of radioactive long-lived
natural isotopes U, Th and K. The total radioactivity of the magnetic shields
produced on the basis of carbon fibre satisfies the low-background experiment
requirements. The upper part of the magnetic shield is used as a light
concentrator providing an additional 19% in light collection efficiency in the
air.

##  8 Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the
Russian Federation, Contract 075-15-2020-778 of the Program of Major
Scientific Projects within the National Project Science. We are grateful to N.
Mazarskaya for her valuable help in preparing the manuscript.

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