## 4.4 DECAY REGION AND HADRON ABSORBER (WBS 1.1.4)

2.  **Introduction**

The neutrino beam requires a region where secondary particles (produced
by the interaction of the incoming protons on the production target) can
decay to produce neutrinos. This region is usually under vacuum (see
section 4.7) to avoid particle loss through interaction with air
molecules. In addition, the beam needs a hadron absorber at the end of
the decay volume. At this absorber, non-interacting primary protons and
remaining secondary hadrons interact and deposit their energy in the
form of heat. The particles that are not stopped by the absorber consist
mainly of muons, neutrinos and a small fraction of neutrons.

### System Description: Decay Region

###  The parameters of the decay region are summarized in Table 4.4--1. 

  -------------------------------- --------------------------------------
  length                           thickness

  (meters)                         (inches)

  50                               72

  150                              84

  150                              66

  100                              66

  100                              63

  350                              66

  125                              56
  -------------------------------- --------------------------------------

Decay Pipe Shielding Concrete

Specification from Mars runs

  --------------------------------- ---------------- --------------------
  **diameter**                      1.98             meters

  **length**                        677.1            meters

  **minimum wall thickness**        0.375            inch

  **vacuum**                        \<1              torr
  --------------------------------- ---------------- --------------------

**Table 4.4--1** Decay Region General Specifications

A significant element of this region is the decay pipe, under vacuum as
described in section 4.7, which will be made up of steel pipe sections
of inner radius 39 in and with wall thickness 3/8 in. Pipe sections will
be fabricated in 40-ft lengths; each will have five stiffener-rings 3/8"
thick, 5" in the radial dimension. Special end sections will be shorter
in length; these will have two stiffener rings and one thrust ring. The
upstream and downstream end caps (or windows) of the decay pipe are
quite different from each other; the characteristics of each are
summarized in **Table 4.4--2.** Note that neither of the End Caps is
designed to be removable.

  ------------------------- --------------------- -----------------------
                            Upstream End Cap      Downstream End Cap

  Nominal Window Thickness  0.062"                .25"
  at r=0"                                         

  Nominal Window Thickness  0.375"                .25"
  at r=38"                                        

  Material at r=0"          Aluminum              Carbon steel

  Material at r=38"         Carbon steel          Carbon steel
  ------------------------- --------------------- -----------------------

**Table 4.4-2:** Properties of Decay Pipe End Caps

To reduce the number of particles interacting in the upstream window
(thereby increasing the flux of neutrinos) the thickness has been
minimized in the central region. This serves to minimize the effects of
both absorption and multiple scattering. A sketch of the upstream window
is shown in **Figure 4.4-1a**. **Figure 4.4-1b** is taken from an
assembly drawing for the upstream window.

![](media/image1.png){width="4.5in"
height="3.263888888888889in"}**Figure 4.4-1a**: Sketch of Upstream
Window -- Inner Window is 1/16" aluminum, Outer Window is carbon steel,
Transition Piece employs an Explosion bonded pair of rings. Transition
piece lengths are not to scale.

**Figure 4.4-1b**: Drawing of Upstream Window -- To Scale, Shown Mounted

The inner portion of the window is aluminum 1/16" in thickness. It is
joined to a steel outer portion by a transition section utilizing an
explosion bonded bi-metal pair of rings. An [ANSYS
analysis](http://www-numi.fnal.gov/numwork/tdh/Thin_Head_Final_Report.pdf)
of dynamic thermal stress due to an accident condition with full
intensity beam missing the target and impinging on the window with a
small size (resulting from missing the target) indicated that aluminum
6061-T6 was an acceptable material for the inner region; this was not
the case for stainless steel. The diameter of the aluminum inner portion
is determined by the largest diameter commercially available for the
explosion bonded bi-metal rings. Shown in Figure 4.4-1a & b are bi-metal
rings that have an outside diameter of 42" and an inside diameter of
36". The radius of 18" where there is only 1/16" of aluminum in the path
of the particles exiting horn #2 compares favorably with the distance of
14" from the beam centerline to the steel forming the top edge of the
target chase.

The design of the downstream decay pipe window has a full-diameter
semi-ellipsoidal window made of carbon steel 1/4 inches thick. A layout
drawing is shown in **Figure 4.4-1c**.

![](media/image3.wmf){width="6.013888888888889in"
height="6.305555555555555in"}**Figure 4.4-1c**: Layout Drawing for
downstream window. The diameter is 78", length is 26"

The downstream End Cap is where the pump-out port is located. Also shown
in Figure 4.4-1c is a drain stub---which would serve to allow drainage
of water or oil that manages to collect at the downstream end of the
decay pipe.

The decay pipe is surrounded in radius by \~5 feet of concrete (see
Table 4.4-1). This concrete absorbs radiation escaping the decay pipe
and constitutes the groundwater shield. The concrete shielding
surrounding the pipe is poured in place. The energy deposited in the
steel walls of the decay pipe and in the surrounding concrete is
summarized in **Table 4.4--3** **(values** **from** **table** **3,**
**[NuMI-B-610](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=718)).**
and shown in **Figure 4.4-2 (adapted from Figures 15 and 17 of
[NuMI-B-610](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=718))**

  ---------- ------- ------- ---------- ------- -------- ------- -------- -------
             Steel           Concrete                                     

                             (0-30 cm)          (30-60           (\>60    
                                                cm)              cm)      

             value   error   value      error   value    error   value    error

  PH2L       62.7    0.5     42.9       0.2     6.14     0.01    2.89     0

  PH2M       65.3    0.5     46.2       0.3     6.99     0.01    3.26     0
  ---------- ------- ------- ---------- ------- -------- ------- -------- -------

**Table 4.4-3** Total energy deposition in kW in the decay pipe steel
and the concrete surrounding the decay pipe. Results are shown for
simulations of: (1) the baseline low energy beam (PH2L), (2) baseline
medium-energy beam (PH2M),

.

![](media/image4.wmf){width="6.497222222222222in"
height="3.060416666666667in"}

**Fig. 4.4-2a**: Energy deposition distribution for low-energy
configuration of target and horns

![](media/image5.wmf){width="6.496527777777778in" height="3.0375in"}

**Fig. 4.4-2b:** Energy deposition distribution for medium-energy
configuration of target and horns

Twelve cooling pipes are placed at 30° intervals of azimuth around the
decay pipe\--in order to remove the heat generated in that region. The
effectiveness of the concrete shielding in protecting the groundwater is
discussed in Chapter 5**.** At its downstream end the decay pipe will be
followed by instrumentation (Downstream Hadron Monitors -- see section
4.5) whose primary purpose is to measure the proton beam position during
low intensity commissioning and later special runs, with the target
removed from the beam's path. Following this instrumentation will be the
Hadron absorber.

**4.4.3 System Description:** **Hadron Absorber**

As mentioned, the function of the hadron absorber is to absorb the
energy of all remaining hadrons in the beam at the end of the decay
pipe. The absorber design has a water-cooled core surrounded on five
sides by steel shielding. On the beam east side, and at the downstream
end, there is an additional layer of shielding concrete, three feet in
thickness. The specifications of the steel in the hadron absorber are
summarized in **Table 4.4--5.**

  ------------------- ---------------- -----------------------------------
  Position            \# steel blocks  steel dimensions

                      52\" x 52\" x    W x H x L (inches)
                      26\"             

  bottom center       16               52 by (4 x 26+ 2) by 4 x 52

  bottom BE           16               52 by (2 x 26 + 9.125 + 2 x 26) by
                                       4 x 52

  bottom BW inner     16               52 by (2 x 26 + 9.125 + 2 x 26) by
                                       4 x 52

  bottom BW outer     12               26 by (52 + 9.125 + 2 x 52) x 4 x
                                       52

  BE side             8                52 by 2 x 26 by 4 x 52

  BW side             8                52 by 2 x 26 by 4 x 52

  top                 24               3 x 52 by (1 + 2 x 26) by 4 x 52

                                       

  total               100              
  ------------------- ---------------- -----------------------------------

**Table 4.4-5** Location and Size of Steel shielding in absorber

The entire absorber resides in the absorber cavern as shown in **Figure
4.4-4**.

![](media/image6.png){width="6.030555555555556in"
height="4.863888888888889in"}

**Fig. 4.4-4:** absorber in cavern (some elements are outdated)

Another view, showing partial assembly, is given in Figure 4.4-5.

![](media/image7.png){width="6.49375in" height="3.415277777777778in"}\
\
**Fig. 4.4-5**: View of absorber in cavern (several items shown are out
of date)

Based on the expected beam energy deposition shown in **Table 4.4--6**,
a water-cooled Al core has been designed as shown in **Figure 4.4-6**.

  ----------------------------------------------------- -----------------
  Total energy in the beam                              121.2 kJ

  Energy of primary protons                             99.7 kJ

  Energy of secondaries: pi; p                          16.1 kJ

  n; e; gamma                                           5.4 kJ

  Average beam power                                    64 kW
  ----------------------------------------------------- -----------------

**Table 4.4-6a** Beam parameters in front of the absorber for the ME
beam in normal operation mode (from Table 2.1 of
[NuMI-B-652](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=673)).

  ----------------------------------- --------------- -------------------
  Part of the absorber                0\<Z \<2.4 m    2.4 \< Z \<3.7 m

  Core and subsequent steel           41.0 (Al)       5.7 (Fe)

  Surrounding steel shielding         10.2            0.14
  ----------------------------------- --------------- -------------------

**Table 4.4-6b** The average power (kW) deposited in the absorber for
the ME beam. The total deposited power is equal to 57 kW. (from Table
2.2 of
[NuMI-B-652](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=673))

![](media/image8.png){width="4.447222222222222in"
height="2.0729166666666665in"}

**Fig. 4.4-6:** Absorber Core arrangement (some items shown are
outdated)

The design incorporates a Radioactive Water (RAW) system for cooling the
core of the absorber that is described in section 4.7. The current
design has a core with transverse dimensions 51" x 51", with eight
water-cooled aluminum modules each 12" thick, followed by 10 layers of
CCS steel each 9.1 inches thick. Each aluminum module has two water
circuits.

**Table 4.4--7** shows peak temperatures in different locations in the
absorber (taken from
[NuMI-B-652](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=673)).

  ----------------- -------------------- --------------- -----------------
  Location          reference            temperature     reference
                    temperature (°C)     (°C)            

  Al module #3      37                   60              RAW water

  Al module #4      37                   60              RAW water

  US face of steel  20                   85              ambient

  US steel in core  20                   270             ambient
  ----------------- -------------------- --------------- -----------------

**Table 4.4-7** Peak Temperatures in absorber (normal operation)

**Figure 4.4-7** shows the temperature rise in module #4 if 4x10^13^
protons / 1.9 sec miss the target and reach the absorber with cooling
water flowing. This is taken from a [study by Bob
Wands](http://www-numi.fnal.gov/numwork/tdh/gundrilled_absorber_analysis_MSG-EAR-01-289.pdf).
The first part of the curve is less sensitive to the assumptions made
about the rate of cooling water flow & is therefore more reliable.

![](media/image9.wmf){width="5.972222222222222in"
height="3.4583333333333335in"}\
\
**Fig. 4.4-7:** Temperature Rise in Aluminum Module #3 or 4 versus time,
beam missing target

In the unlikely event of a water leak in one of the two circuits in a
module, that particular circuit could be shut off since the remaining
circuit would provide sufficient cooling. Even in the more unlikely case
of a failure also in the second circuit of a module, there is sufficient
heat transfer to the two adjacent, water-cooled modules such that the
temperature in the aluminum module with no water-cooling will not become
high enough for structural damage. This is shown in **Figure 4.4-8.**
Since modules #3 and #4 both receive 8 KW of power from energy
deposition during normal operation, this figure indicates that the
temperature would rise to 450 °F if there were no water flow in that one
module.

![](media/image10.wmf){width="6.263888888888889in"
height="6.111111111111111in"}

**Fig. 4.4-8:** Module cooling by convection to adjacent modules (no
water flow)

The temperature of the core will be monitored by strategically mounted
thermocouples. The full design of the thermocouple system has not
started.