**4.2.3 Target**

The neutrino beam is produced by pion decay. Those pions are produced by
interactions of the primary proton beam with the target. The proton beam
spot size should be no smaller than 1.0 mm RMS horizontally and
vertically at full baseline beam intensity to limit beam induced stress
on the target.

The target material is graphite, type ZXF-5Q (POCO Graphite), density
1.78 g/cm^3^. The graphite segments are machined as narrow fins and
mounted to stainless steel water-cooling pipes, as shown in **Figure
4.2-12** and **Figure 4.2-13**. The airtight casing has beryllium
windows for the primary beam entrance and exit.

The main target is 47 vertical target segments, each 20.0 mm long, with
0.3 mm spacing between segments, for a total length of 95.38 cm. The
segments are 6.4 mm wide. The height is sculpted (see Figure 2). A
48^th^ segment (not shown) is mounted horizontally, in the target
canister, with the fin center 157.3 mm upstream of the upstream edge of
the main target.

**Figure 4.2-12**![](media/image1.wmf){width="5.997222222222222in"
height="3.888888888888889in"} The target and target vacuum canister.

**Figure 4.2-13** Beam\'s eye view of target. Dimensions shown in mm.

![](media/image2.png){width="3.75in" height="2.670138888888889in"}

The target fin plus cooling tube is electrically isolated from the
casing by ceramic spacers and water line breaks. A wire connected to the
cooling tube allows measurement of the delta-ray charge knocked out of
the target when it is hit by beam. This horizontal "Budal monitor"
provides a cross check of the position of the target by measuring the
response as the beam is scanned horizontally across the target.

The Budal signal on the 48^th^ target segment is separately read out,
and provides a position check as the beam is scanned vertically across
the target. The cooling for this segment is provided by conduction
through its clamping plates to the canister. The clamping plates are
anodized to provide the required electrical isolation.

The aluminum casing around the target is electrically insulated from the
base canister. This provides an electrical break from possible voltage
from the horn inner conductor. A wire connected to this casing can be
used to check if the casing is touching the horn (there is only 2.7 mm
clearance between the target casing and horn).

The target casing may be evacuated during low intensity beam scans,
which should improve the signal-to-noise of the Budal monitor. During
high intensity running, the target canister will be He filled, to
provide heat transfer from the aluminum casing to the cooling pipes.

During continuous operation, the center of the highest temperature
segment will cycle between 58 deg C and 330 deg C, with resultant
stresses, energy deposit and water cooling parameters shown in **Table
4.2-4** through **Table 4.2-6**.

  ------------------------------------ ---------------------- ------------------
                                       Center of segment      Corner of segment

  Max. equivalent stress               27.4 MPa               23.5 MPa

  Type of stress                       All-axis-compression   All-axis-stretch

  Strength limits                      210 MPa compressive    95 MPa tensile

  Derating factor high cycle fatigue   0.5 to 0.6             0.5 to 0.6
  endurance                                                   

  Resulting safety factor after many   2.2                    1.8
  cycles                                                      
  ------------------------------------ ---------------------- ------------------

**Table 4.2-4** Thermal stresses in target segment with highest energy
deposition density.

  --------------------------------------------------- -------------------
  Graphite Segments                                   2.96 kw

  Cooling pipes and water                             0.42 kw

  Aluminum casing                                     0.15 kw

  Total                                               3.52 kw
  --------------------------------------------------- -------------------

**Table 4.2-5** Energy deposition in target for baseline beam intensity
(4e13 protons per 1.87 seconds).

  ------------------------------------------- --------- --------- ---------
  **Velocity of cooling water**               **2 m/s** **3 m/s** **4 m/s**

  Heat transfer coefficient, kW/m^2^/K        10        14        18

  Pressure drop, atm                          0.32      0.68      1.2

  Water flow rate, l/min                      2.7       4.1       5.5

  Water temperature rise, K                   18        12        8.8
  ------------------------------------------- --------- --------- ---------

**Table 4.2-6** Water cooling parameters as a function of water
velocity.

The construction drawing set for the target is IHEP 7589-00-00-00. See
also "[Advanced Conceptual Design of the Low Energy Target and Beam
Plug](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=706)"
(NuMI-B-543), "[Dynamic Stress Calculations for ME and LE Targets and
Results of Prototyping for the LE
target](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=676)"
(NuMI-B-675), "[Further Studies of Target for the Low Energy NuMI
Beam](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=723)"
(March 30, 2001), "Temperature and Stresses in the LE Target with 6.4 mm
Wide Segments" (June 20, 2001), and "[NuMI LOW ENERGY TARGET
SPECIFICATION](http://www-numi.fnal.gov/numwork/tdh/spec_le.pdf)" (June
25, 2001), for more details.