## 4.4 DECAY TUNNEL AND HADRON ABSORBER (WBS 1.1.4) {#decay-tunnel-and-hadron-absorber-wbs-1.1.4 .My-Title .unnumbered}

### 4.4.1 Introduction {#introduction .MyHeading}

Producing a neutrino beam requires a region where the secondary beam
pions, produced by primary beam protons hitting the target in the Target
Hall, can decay to muons and neutrinos. After some distance, when a
sufficient quantity of the pions have decayed, the pions need to be
stopped so they do not contaminate the neutrino beam. So any general
neutrino beam design needs a decay region followed by a beam absorber.

In the NuMI facility, the Target Hall is followed by the 675m long Decay
Tunnel which holds this decay region. The Decay Tunnel holds the Decay
Pipe which the particle beam passes through. To avoid a reduction of
pions due to interactions in air, the Decay Pipe is a vacuum vessel. The
Decay Pipe begins at the downstream end of the Target Hall, runs the
entire length of the Decay Tunnel, and terminates in the Absorber
Enclosure.

![](media/image1.jpeg){width="6.5in" height="3.466666666666667in"}

The Absorber Enclosure holds the Hadron Absorber, a beam dump made from
a pile of aluminum, steel and concrete. All particles remaining in the
beam at the downstream end of the Decay Pipe pass into the Absorber:
primary protons which did not interact in the target, secondary pions
which have not yet decayed, other secondary hadrons, and the muons and
neutrinos which result from pion decay. All the protons, pions and other
hadrons interact in the material of the Absorber and their energy is
deposited there as heat. The muons and neutrinos pass through the
Absorber and continue on into rock. The muons range out in the rock and
stop. To measure the decreasing numbers of muons, detectors are placed
in muon alcoves -- passages cut into the rock along the beamline. After
some distance of rock, only the neutrinos remain, and these pass into
the MINOS Hall at the very end of the tunnel excavation.

Figure 4.4-01 shows a layout of the Absorber area. The Absorber
Enclosure a \~20-ft wide \~50-ft long \~40-ft high room; it is accessed
from the MINOS Shaft elevator via the Absorber Access Tunnel. The
tunneling machine which dug the Decay Tunnel was turned off the beam
centerline upstream of the Absorber Enclosure, so as to allow the beam
to pass through rock downstream of the Absorber Enclosure. This bypass
tunnel was sealed with concrete when the Decay Tunnel shielding was
poured, and is now a dead-end. The floor space in the bypass is used as
utility area for the Decay and Absorber water pump and vacuum systems. A
Labyrinth Passage connects the Access Tunnel to the Absorber Enclosure.
The Labyrinth Passage is named for the concrete blocks placed there,
which shield the Access Tunnel from slow neutrons which are emitted from
the Absorber as a result of the beam deposited in it. A Controlled
Access Interlock door is located at the fire door in the Absorber Access
Tunnel, just downstream of the Absorber Enclosure.

### 4.4.2 System Description: Decay Tunnel and Decay Pipe {#system-description-decay-tunnel-and-decay-pipe .MyHeading}

The decay tunnel is an approximately 22ft diameter excavation created by
the tunnel-boring machine during the facility construction. The only
beamline component within the tunnel is the Decay Pipe, a vacuum region
where the produced pions decay to muons and neutrinos. The Decay Pipe is
a 6-ft diameter steel pipe which runs the entire length of the tunnel,
centered on the neutrino beam centerline; its basic parameters are given
in Table 4.4-01. The pipe was made from sections 40-ft long, each with
five stiffener-rings 3/8" wide and 5" tall in the radial dimension. Each
pipe section was surveyed into place, held up by a supporting structure
since it was positioned above the tunnel floor, and welded to its
neighbors.

  ------------------ ------------------- ---------------- -----------------
  Inner Diameter     Wall Thickness      Length           Vacuum

  1.98 meters        0.375 inch          677.1 meters     \<1 torr
  ------------------ ------------------- ---------------- -----------------

**Table 4.4-01**: Parameters describing the Decay Pipe

Once the pipe sections were in place, the Decay Tunnel was backfilled
with concrete to shield groundwater from the particles which impact the
walls of the pipe. Figure 4.4-02 shows a sample cross-section diagram of
the Decay Tunnel. As shown, the tunnel was excavated and the concrete
was formed so that a \~50-in wide egress passage remained along the east
side of the original excavation; this passage provides emergency access
between the upstream Target Hall areas and the downstream MINOS areas.

![](media/image2.jpeg){width="2.6041666666666665in"
height="2.9166666666666665in"}The concrete shielding is not of a uniform
radial thickness; it is thicker at the upstream end than at the
downstream end, because more particles impact the sides of the Decay
Pipe along the upstream end and more shielding was required there. See
Table 4.4-02 for the shield thickness versus length down the tunnel. The
effectiveness of the concrete shielding in protecting the groundwater is
discussed in Chapter 5.

The particles which impact the sides of the pipe interact and create
particle showers in the steel and the surrounding concrete. Heat is
deposited as those particles are absorbed by these materials. An
excessive amount of heat in the steel pipe might cause it to expand
enough to crack the concrete shielding, and so copper cooling lines were
installed along the length of the Decay Pipe, before the concrete was
poured. Twelve lines were placed at 30° intervals of azimuth around the
decay pipe. To prevent any chemistry between the steel pipe and copper
lines, a thin insulator was placed between them. The cooling lines exit
at each end of the Decay Pipe and are connected to a RAW circulating and
heat exchanger system.

  --------------------------- ---------- ---------- ----------- ----------
  Length in meters along the  50         150        350         425
  Decay Pipe                                                    

  Thickness of Shielding in   72         84         66          55
  inches                                                        
  --------------------------- ---------- ---------- ----------- ----------

**Table 4.4-02**: Thickness of Decay Region Shielding versus location
along the tunnel

The total energy deposited in the steel walls of the decay pipe and in
the surrounding concrete was modeled using the MARS program, which
simulates particle interactions in matter and is used to evaluate
shielding designs. The MARS results are summarized in Table 4.4-03**.**
Figure 4.4-03 shows the distribution of the energy deposition along the
length of the decay pipe.

  -------------------- ------------- ------------ ------------ -----------
                       Steel Decay   Concrete                  
                       Pipe          shielding                 

                                     0-30 cm      30-60 cm     \>60 cm
                                     radius       radius       radius

  Low Energy Beam      kW            kW           kW           kW
  Configuration                                                

                       62.7          42.9         6.14         2.89
  -------------------- ------------- ------------ ------------ -----------

**Table 4.4-03** Total energy deposition in kW in the decay pipe steel
and the concrete surrounding the decay pipe, from MARS simulations of
the NuMI beamline in low energy neutrino beam configuration.

![](media/image3.jpeg){width="6.5in" height="3.75in"}All of the decay
pipe installation described above was part of the underground
construction, and additional details may be found in the NuMI Facility
TDR. Figure 4.4-04 shows photos of the decay pipe installation in
progress. The only items remaining after the construction period were to
attach end caps onto the Decay Pipe and then install the water and
vacuum utilities; these tasks were performed as part of the NuMI
Technical Components installation.

The decay pipe ends were sealed by end caps, concave in shape to
minimize stress due to the vacuum inside the decay pipe. At the upstream
end, one wishes to preserve as many of the secondary pions as possible
in order to maximize the final flux of neutrinos. In general this means
making a thin vacuum window, although in this case thin was a relative
term. At the downstream end all pions of interest have decayed and there
are no concerns about preserving particle flux; the Downstream End Cap
can be made as thick as necessary to achieve
straight-![](media/image4.jpeg){width="6.5in" height="3.75in"}forward
structural integrity.

Vacuum windows tens of mils in thickness and 6-ft diameter have been
engineered and used. However, due to damage caused by a large particle
flux, any such window will have a limited lifetime. This might not be an
issue in some applications, but in the NuMI beamline, the Upstream End
Cap is located at the downstream end of the Target Hall, under many feet
of shielding blocks, and is quite inaccessible for repair or
replacement. So the window thickness had to be sufficient to withstand
the normal beam flux indefinitely and any possible accident conditions,
yet be as thin as possible so as to minimize the window's effect on that
same beam flux. These criteria were tempered by the fact that the window
did not have to be the full 6-ft diameter. The particles exiting the
Target Hall do not extend out to a 6-ft radius, but instead fill the
space defined by the Target Hall cave, which is approximately 3-ft
diameter. The resulting design is a thinner window embedded within a
thicker end cap. Figure 4.4-05 shows the Upstream End Cap. The inner
window material is specifically aluminum 6061-T6, selected as having the
best performance among various types of aluminum or steel. The worst
condition the material must withstand is the dynamic thermal stress due
to an accident condition where the full intensity primary proton beam
misses the target and impacts the window with a small spot size. An
ANSYS analysis of the Upstream End Cap under these conditions is
reported in MSG-EAR-02317, *Thermal and Stress Analysis of Numi Decay
Pipe Thin Head*, by Bob Wands, July 16, 2002.

The Downstream End Cap is a full 6-ft diameter semi-ellipsoidal concave
sheet made of carbon steel 1/4-in thick attached to a short section of
pipe, and is shown in Figure 4.4-06. The vacuum pump-out port is located
on the upper part of the End Cap pipe, and at the base is a line to
allow ![](media/image5.jpeg){width="6.5in" height="4.75in"}for drainage
of water that may collect in the Decay Pipe before the End Caps were
installed.

Both End Caps were constructed with an integrated carbon steel rim the
same diameter as the Decay Pipe, which allowed them to be welded
directly to the Pipe. The root weld is GTAW (a.k.a. TIG), the remaining
passes are SMAW (a.k.a stick) using 7018 filler metal. The final fillet
weld is 5/16" wide.

The Decay Pipe and End Caps have been designed to withstand the
pressures of normal operation in the NuMI beam environment,
indefinitely. The design meets the ASME pressure vessel code for
allowable stress (both primary and secondary), and the details are
described in the Decay Pipe Engineering Notes. End Cap failure, while
remote, is a hazard which must be examined, and a hazard analysis can be
found in NuMI Note 0977.

### ![](media/image6.jpeg){width="6.5in" height="5.0in"}4.4.3 System Description: Hadron Absorber {#system-description-hadron-absorber .MyHeading}

The function of the Hadron Absorber is to stop most beam particles which
remain at the end of the Decay Pipe and absorb their energy. The vast
majority of these particles are hadrons, protons, pions and kaons, and
essentially all of them are stopped by the Absorber. A relatively low
rate of thermal neutrons are emitted from the Absorber as a by-product
of the stopped hadrons. The muons in the beam are ranged out in rock
downstream of the Absorber Enclosure. The neutrinos of course pass
through all of this material and are the resulting product of the NuMI
beam.

The Absorber must be able to easily handle the energy deposited by the
beam, under all expected operating conditions and under all accident
conditions which have any reasonable probability of occurring. The basic
design of any such beam dump is a central core of metal surrounded by
additional steel shielding and by an outer layer of concrete. The metal
serves to both create and contain the particle showers. The outer steel
and concrete layers provide additional containment; the concrete also
acts to absorb thermal neutrons, those with energies of 847 keV and
below, as these are not stopped by iron.

### 4.4.3.1 Absorber Core {#absorber-core .MyHeading}

The detailed design for the Absorber core depends upon the size, energy
and intensity of the entering hadron beam. A simulation of the NuMI beam
impacting on a beam dump was used to define the basic design parameters
of the Hadron Absorber; this work is reported in NuMI-B-652, *Advanced
Conceptual Design of the NuMI Hadron Beam Absorber Core*, by A. Abramov,
et. al. ![](media/image7.jpeg){width="2.7083333333333335in"
height="4.791666666666667in"}In summary, the Absorber consists of a core
made from both aluminum and steel, surrounded by steel shielding blocks
and an outer layer of concrete shielding blocks. The core is 51-in
square transverse to the beam; this dimension was determined both by the
beam size and by the availability of specific steel blocks for the
surrounding shielding -- a core made of a size similar to these blocks
resulted in an easily stacked pile of material. The aluminum portion of
the core is 8-ft deep, longitudinal to the beam, followed by a \~7.5-ft
steel portion of the core. The aluminum portion is actively cooled by a
circulating water system, described in more detail in Chapter 4.7. The
temperature of the metal is monitored using thermocouples. An analysis
of the water-cooled aluminum blocks subjected to the expected beam
energy deposition is summarized in MSG-EAR-01-289, *Thermal Stress
Analysis of Gun-drilled NuMI Absorber Core*, by Bob Wands.

The core is built up from smaller sections. The aluminum core is made
from 8 blocks each 51" x 51" x 12". Round channels were drilled within
each block so that each of the 8 blocks has two independent water
cooling circuits. The entrances and exits to these channels are set in a
vertically staggered pattern along the sides, so that the water supply
lines to each block do not interfere with each other in the final
assembly. For ease of installation, the aluminum blocks and their piping
were assembled into two sections of 4 blocks each; this minimized the
number of delivery trips of materials to the Absorber Enclosure, and
minimized the amount of welding performed underground in the Enclosure,
where working space was tight. Figure 4.4-07 shows the three upstream
aluminum core blocks on the bench, being assembled into one of the
sections. The pipes for each Al core block are routed through the core
blocks from back to front; thus the number of pipes increases from
upstream to downstream.

The steel part of the core is made from 10 blocks each 51" x 51" x 9.1".
The steel core blocks were likewise assembled on the bench into 4
sections, of 3, 3, 2 & 2 blocks. The cooling water pipes for the
aluminum core blocks pass through the steel sections, and so the steel
blocks are machined with holes for the pipes. During installation the
pipes were carefully "threaded" through the steel core sections.

Each cooling pipe is a single continuous length through the core blocks,
not from welded sections, so as to minimize the probability of leaks.
Each pipe runs from where it is attached to its core block, through
holes in the aluminum and steel core blocks downstream, ending several
inches past the last steel core block which sits at the back face of the
Absorber. At the rear of the Absorber, additional piping was installed
to connect the core cooling pipes to a Radioactive Water (RAW) system.
Each core block has two water circuits. The two circuits are each joined
at two manifolds, so that the entire core can be cooled by one or both
of these water circuits. The report MSG-EAR-01-289 calculates that one
water circuit provides sufficient cooling. In the case of a water leak
in one circuit in any given module, the entire circuit can be removed
from the system, manually, at the manifold location outside of the
Absorber. In the more unlikely case of a failure in the second circuit,
then the piping to the particular leaking core block (on either circuit)
can be physically disconnected from the manifolds, halting water flow in
that particular block but no others; again, report MSG-EAR-01-289 shows
there is sufficient heat transfer to the adjacent blocks under this
condition that the temperature in the disconnected block would not
become high enough for structural damage. The Absorber RAW system is
described in more detail in Section 4.7.

### 4.4.3.2 Absorber Layout & Installation {#absorber-layout-installation .MyHeading}

The Hadron Absorber was constructed by a stacking of steel and concrete
shielding blocks, and this Section describes the stacking in the order
in which the parts were installed. Please refer to the diagrams in
Figure 4.4-08 throughout the stacking description; a list at the end of
this section gives the FNAL PPD Mechanical Dept. drawing numbers for the
main parts and overall assembly. Also refer to the photos in Figures
4.4-09 through 4.4-17 for views of the Absorber

![](media/image8.jpeg){width="6.579861111111111in"
height="9.110416666666667in"}

during installation. Installation started with stacked concrete shield
blocks around the Decay Pipe and the Hadron Monitor. The

Monitor is located just a few inches downstream of the decay pipe
Endcap, and both are labeled (A) in Figure 4.4-08. The concrete blocks,
labeled (B) in Figure 4.4-08, can be considered an extension of the
concrete shielding poured around the Pipe in the Decay Tunnel. The
blocks sit flush against the base of the south wall of the Absorber
enclosure, and extend 6-ft into the enclosure. The stacked blocks
provide 3-ft of shielding around the top and sides of the decay pipe
where the beam exits. The block base is built up about 7-ft high because
of the elevation of the beam centerline and Decay Pipe in the Enclosure.
On the upper east (beam-right) side, smaller blocks were stacked in a
way to create an access slot, labeled (C) in the diagrams, for the
installation of the Hadron Monitor. A sliding steel door covers the
slot, and is shown in Figure 4.4-09. Also in that figure is a view of
the Monitor support rails, showing it's location in relation to the
decay pipe and shielding. Figure 4.4-10 shows the completed Decay Pipe
concrete block ![](media/image9.jpeg){width="6.5in"
height="4.0in"}shielding, before any further Absorber installation
occurred.

As mentioned, specific steel blocks were obtained for use as the main
stacking element for the Absorber shielding; these steel blocks were
52\" x 52\" x 26\", and are the blue items labeled (D),(H) and (J) in
Figure 4.4-09. The Absorber base was stacked up in four layers of 12
steel blocks each, laid out 3 blocks wide transverse to the beam and 4
blocks deep longitudinal to the beam, labeled (D).

Between base layers 2 and 3 is a shallow steel pan which acts as a
catch-basin for any cooling water leaking from the Absorber core,
labeled (E). The pan drains to a tank, which has sufficient
![](media/image10.jpeg){width="3.75in" height="3.75in"}capacity for the
entire volume of the Absorber primary RAW system. This tank can be seen
at the back of the completed Absorber in Figure 4.4-16. Figure 4.4-11
shows two views of the base layers: the first two layers, and all four
base layers. Each layer was shimmed to be level. Cracks between blocks
were filled with grout. Note the edge of the water containment pan
between layers 2 and 3. Note also that the middle row blocks in layers 3
and 4 sits lower than the outer rows. This is because the Absorber core
carrier plate, a 1-in thick steel plate, sits on the middle row of
blocks. The core carrier plate is machined level and provides a level
base on which to set the core block sections. To compensate for the core
carrier's extra inch of elevation, 1-in thick steel plates lie on the
steel containment pan beneath the outer rows of layer 3 blocks, raising
them relative to the middle row. In the end, the Absorber base, with the
core carrier plate, sits at a level elevation.Installation of the
Absorber Core came next. The upstream face of the core is inset from the
upstream faces of the blue steel shield blocks which surround it, by
12-in. Combined with space downstream of the Hadron Monitor and inside
the concrete blocks, this makes for a volume of approximately 130-ft^3^
of air space which the beam travels through. Concerns about the release
of excess activated air, discussed in Section 4.4.3.3, led to the
filling of this volume by a box. The box is made from sheet aluminum,
welded along all edges, and is filled with air at atmospheric pressure.
The purpose of the box is simply to isolate the most intensely activated
air within the box from any air in the nooks and crannies within the
shield blocks which will eventually leak and mix with outside air. The
box was set in position just before the first half-section of the
aluminum core. Figure 4.4-12 shows the box, and then the first
half-section of the core, with its long pipes supported behind it.

![](media/image11.jpeg){width="6.5in" height="4.0in"}

The second half-section of the aluminum core came next. The steel core
sections were installed after the aluminum, and the steel sections were
carefully threaded through the pipes from the aluminum core section.
Figure 4.4-13 shows this installation process. A few inches of pipe
extend beyond the last steel core block for attaching connections to the
water system; this piping work was done later in the installation
process after most of the heavy rigging was completed.

![](media/image13.jpeg){width="6.5in" height="5.375in"}The side steel
shield blocks were installed next, labeled (H) in the diagrams in Fig.
4.4-08. These are the same blocks as used for the base layers, but
rotated so their 52-in length sits alongside the core. Two rows of steel
blocks were placed on each side of the core. These blocks did not sit
flush to the sides of the core but left an inch or so of space on each
side. The side blocks were slightly higher than the core, so that when
steel plates set on the side blocks bridged over the core about an inch
of gap existed also above the core. These gaps were measured and steel
plates fabricated to fill these small volumes, so as to exclude air
which would otherwise add to the air activation issues. The readout
cables for the core thermocouples were carefully run through the steel
filler plates out to the rear of the core.

![](media/image14.jpeg){width="6.5in" height="4.0in"}Steel bridge plates
spanning the entire width of the Absorber steel shield blocks were laid
on the side blocks so that no weight from the top layers of shielding
rest upon the core. Two more layers of steel blocks, laid in the same
way as the base layers, were set on the bridge plates, and are labeled
(J) in the diagrams. At this time, the side concrete blocks, labeled (K)
were set in place, one row down each side, 3-ft x 3-ft x 7-ft tall,
stacked two blocks high. A small air gap exists between the side
concrete blocks and the steel shield blocks. A similar wall of concrete
blocks were set across the back of the Absorber steel pile, labeled (L)
in the diagrams, but only the bottom level at this point. Figure 4.4-14
shows the core with side shield blocks and steel bridge plate, and the
first layer of top shield blocks.

At this point, the space remaining between the ceiling of the Enclosure
and the top of the blue shield blocks did not allow for the installation
of more blocks of that size, yet more shielding was required to satisfy
groundwater activation levels. Nor was there space left for the usual
3-ft of outer concrete shielding on the top of the pile, which meant
more thermal neutrons would escape along the top than on all other
sides. It was determined that 18-in more steel would satisfy the
shielding needs, and the absence of concrete would be tolerated -- if
too many thermal neutrons escaped through the top and into the
enclosure, then more concrete could be added to the Absorber Labyrinth
to block them there.

![](media/image15.jpeg){width="6.5in" height="5.0in"}In some areas on
the top of the Absorber there was barely sufficient space for this 18-in
of steel, and because of the lack of overhead space the gantry crane
could no longer be used to position blocks. This meant using more basic
rigging techniques and these techniques would be made easier if each
block were relatively small. A total of 88 blue shield blocks had been
used to this point. The final 18-in layer was made of 78 steel blocks of
various sizes. This top layer of steel blocks was set on the side
concrete blocks and on the main pile, and bridged across all gaps along
the sides and along the boundary between the front concrete shielding
and the main pile. A large gap exists at the back, between the main pile
and the outer concrete, where the core cooling pipes are routed. A
fixture was designed to support steel blocks across the top of this gap.
The top steel layer is not shown in the diagrams. Fig. 4.4-15 shows the
top steel layer during installation, from different viewpoints.

The core piping was routed along the back of the Absorber pile toward
the west wall of the enclosure; the path is labeled (N) in the diagrams.
Before the final piping connections were made, a plywood box was
constructed which filled the gap in the outer concrete shielding where
the pipes were routed. Holes drilled in the box allowed the pipes to
pass through. Once in place, the box was filled with polyethelene beads,
which are similar in shielding value to the concrete.

![](media/image16.jpeg){width="6.5in" height="3.75in"}The completed
Absorber is shown in Fig. 4.4-16. All of the outer concrete is in place.
The piping emerges from the (painted) white bead-filled box. All the top
steel is in place. The drain line from the metal containment pan, set in
the base steel layers, is routed to a holding tank at the back of the
Absorber. A heculite tarp is laid across the top, so that water from any
drips in the enclosure roof runs off down the sides, and not down into
cracks between the steel blocks. The worst (and only significant)
ceiling drip in the enclosure is caught by a pan and drained into the
gutters; however over time other drips may develop.

Once the Absorber was completed, the Absorber Labyrinth was installed in
the passage between the Absorber enclosure and the Absorber utility
area. The Labyrinth, labeled (M) in the diagrams, is simple, made from
two rows of shield blocks, each row being 6-ft thick and running between
floor and ceiling. Figure 4.4-17 shows the Labyrinth from each side.
Also in that Figure is a view of the Absorber water and vacuum utility
skids.

List of PPD Mechanical Dept drawing numbers for the main parts and
overall assembly

427776 Overall Assembly, listing all sub-drawings

427334 Concrete block stacking, around DK pipe and along sides of
Absorber

433146 Air containment box

431534 Hadron Monitor access slot shield door assembly

427127 Al core modules assembly -- 427130:135+427620+427622 individual
modules

427159 Core carrier plate assembly -- 427160, 427162, 427164 individual
plates

427154 Steel core blocks

406519, 433158, 431468, 431467 Core cooling pipes assembly

427337, 427376 Steel Blu block base assembly

427383 Containment pan assembly

427379 Steel Blu block side and top assembly

### 4.4.3.3 Control of Activated Air {#control-of-activated-air .MyHeading}

The Absorber shielding was primarily designed to protect the groundwater
in the surrounding rock from activation by particle showers, a
requirement which the Absorber satisfies. The
![](media/image17.jpeg){width="6.5in" height="3.5in"}shielding is also
sufficiently thick that residual activation rates on the outer surface
of the surrounding concrete are reasonable for an enclosure locked off
during beam and otherwise infrequently accessed. A third radiological
concern is the release of activated air, and here some steps had to be
taken.

Any stacked construction of slightly uneven blocks, such as used for the
Absorber, will create variously sized gaps and cracks. The air within
these gaps and cracks becomes activated by the particle showers created
in the Absorber material; the closer the gap or crack is to the beam
centerline the more activated the air will become. If no effort is made
to seal up the Absorber, the activated air will leak out of these gaps
and cracks, and mix with un-activated air which is flowing through the
Enclosure. The underground air flow pulls air from the Absorber
Enclosure, up the decay egress passage, and out a ventilation stack
halfway along the length of the decay tunnel. The time it takes for air
to flow along this route is too short for all activated elements to
decay, and some activated air will be released. There is an overall
budget for released activated air, and so efforts were made to predict
the contribution coming from the Absorber, which meant understanding all
the volumes of air within the Absorber pile.

And there turned out to be significant volumes, not mere cracks, located
in or near the beam center, where activation would be highest. The
largest volume was the space between the Decay Pipe Endcap and the front
face of the Absorber. The Hadron Monitor is located here, and because
"backsplash" from beam hitting the front face of the core could affect
the efficiency of the detector, by design there was some distance of
open space between the two. The full beam intensity at the end of the
decay pipe passes through this large volume of air. The next largest
volume is the gaps between the core modules and the surrounding blue
steel shield blocks. While only about an inch in thickness, the gaps
exist on the top and two sides, and extend for the entire length of the
core. Since these gaps surround the core, the air sees a large particle
flux and becomes quite activated. The third main volume is around the
upper portion of the south wall of the Enclosure, where the Decay Pipe
exits. The shielding stacking plan assumed this wall was flat, but in
fact it is not. The lower 5-ft of the wall extends 15-in further into
the enclosure than the rest of the wall, above 5-ft. The shield blocks
were placed where the wall met the floor, but that left a 15-in wide gap
above the 5-ft level, all around the Decay Pipe. The actual particle
flux passing through this volume is lower, because it is "behind" the
poured Decay Pipe shielding and off the beam axis. However, this large
gap is contiguous with the two other, more activated, air volumes
mentioned above, inside the stacked shielding, giving a clear and easy
path for the activated air inside the shielding to be pulled out and mix
with the air in the Enclosure.

The amount and activation level of air from these volumes was
calculated, and if the large gap around the Decay Pipe in the south wall
was left as is, then all the activated air would mix freely with the air
in the enclosure. From this, the predicted quantity of activated air
released to the surface could be calculated, and that quantity was
deemed to be too large a portion of the allowed fraction. But it was not
necessary to make the Absorber air-tight -- a small amount of activated
air could be allowed to escape on a continuous basis. So, during
Absorber installation, attention was paid to the particular large
volumes mentioned, to minimize or eliminate them, and to cracks between
stacked blocks, with the goal of an overall "leakage" of a few percent
of the total activated air through such cracks.

As blocks were set and leveled, cracks along the floor and between
blocks were filled with grout. Typically only the cracks along the outer
sides were filled. Certain layers were covered with grout to create a
level surface, sealing all cracks on that layer's surface: the layer on
which the core sits, and the top layer of blue shield blocks, were
treated in this way. Figure 4.4-11 shows this crack filling, and it can
be seen in other photos as well.

The first large volume, between the Decay Pipe Endcap and the front face
of the core, was mostly filled by the aluminum box shown in Figure
4.4-12. The box simply prevents the air inside of it from freely mixing
with air outside of it, removing the volume within the box from the
equation for total activated air. The box fills the space between the
Hadron Monitor and the front face of the core, which accounts for 55% of
the total volume in this area. The remainder, between the Decay Endcap
and the Hadron Monitor, cannot be similarly filled, as a box in this

![](media/image18.jpeg){width="5.079861111111111in"
height="4.688888888888889in"}

location would interfere with the installation of the Monitor. As long
as the remaining free volume of air can be held within the Absorber and
have a low leak rate into the Enclosure, then it can be tolerated.

The second main activated volume is the one around the core. The
strategy again was to mostly eliminate the volume of air from the
equation by filling the space with something else. In this case thin
steel plates were fabricated to fit as closely as possible between the
core and the steel blocks. One of the plates can be seen in Figure
4.4-14, extending beyond the end of the core block. These plates reduced
the volume of air in this space to 10% of the former amount.

The third area of attention was the 15-in gap around the Decay Pipe on
the south wall. This gap provided a large escape route for all the
activated air within the Absorber. Figure 4.4-18 shows two views of the
south wall. The first view before any shielding was installed; the 15-in
change in wall position can be seen at the level of the person's white
hard hat. The second view is a close-up of the bricked-in gap after
installation was completed. A protective foam wrap was placed around the
piping before the mortar was applied. The brick wall is not air-tight,
and was not required to be, as long as the leak rate was reduced to a
few percent of what it would be if the wall was absent.