## 4.2.6 Horn Alignment Cross-hair System

# 4.2.6.1 Alignment cross-hair function and specifications

The purpose of the NuMI horn alignment cross-hair system is to allow
locations of the NuMI magnetic horns to be determined relative to the
incident proton beam centerline. This is to be accomplished by scanning
the incident beam, at low intensity (roughly 10^12^ protons per pulse),
horizontally and vertically across the cross-hair locations. The
locations of the cross hairs are determined by observing the changes in
signal level in a beam loss monitor (BLM) located downstream of each
horn. The BLMs are mounted well away from the beam axis and detect
secondaries from beam proton interactions in the cross-hair material.
The cross hairs consist of horizontal and vertical aluminum strips
attached to the downstream end of Horn 1 and the upstream and downstream
ends of Horn 2. While it is not possible to attach a cross-hair
structure to the upstream end of Horn 1 due to the production target
location, the narrow neck of the inner conductor of Horn 1 provides
essentially the same alignment function as an upstream cross-hair
structure when it is scanned by the incident beam.

Adam Para described the initial conceptual design of the cross-hair
alignment system in
[NuMI-B-796](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=735).
Byron Lundberg used MARS to perform a series of beam-heating
calculations for different cross-hair materials
([NuMI-NOTE-BEAM-882](http://www-numi.fnal.gov/numwork/tdh/numi0882.pdf)).
It can be concluded from this study that aluminum will survive normal
full-intensity operation of the NuMI beam if the cross hairs were
centered 2.5 mm from the beam centerline and are not more than 1.0 mm
wide.

The performance of the cross-hair system has been simulated and
documented by Debbie Harris
([NuMI-NOTE-BEAM-864](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=734)).
The simulation was performed only for the low-energy neutrino beam
configuration. The conclusion of this study is that the transverse
alignments of the upstream and downstream ends of each horn can be
determined to a precision of ±0.5 mm. The study concluded that the
aluminum cross-hair material of the cross hairs should be 6 mm deep
along the beam line for the downstream cross hairs on both horns and 18
mm deep for the Horn 2 upstream cross hairs, in order to achieve an
acceptable signal to noise ratio in the BLMs. It has been decided to
double the thickness of the cross hair material (to 12 mm and 36 mm
respectively) to allow for the possibility of worse signal to noise. The
simulations indicate signal-to-noise ratios of 0.6 to 0.8 for the
cross-hair scans (with the doubled thicknesses) and \>10 for the Horn 1
neck scan. The simulations also indicate that the cross-hair material
causes changes of less than 0.2% in the neutrino flux and the near/far
detector event rates.

#### The cross hair system consists of the following components:

1.  Three aluminum cross hair structures and hardware to mount them to
    the horns. The locations must not be permanently affected (±0.25 mm)
    by temperature changes, horn pulsing and other perturbations that
    occur during NuMI operation. **Figure** **4.2-1** shows an end-on
    view of the Horn 1 cross-hair structure.

2.  Two beam loss monitors (BLMs), one located downstream of each horn,
    and associated hardware and readout electronics, including
    gun-drilled holes for cable penetrations. BLMs will remain in
    position during high intensity running but they must be replaceable
    if they are damaged.

3.  Vertical and horizontal trim magnets, located roughly 23 m upstream
    of the upstream face of Horn 1. These are used to perform scans of
    each set of cross hairs and of the neck of Horn 1.

The cross-hair system specifications include the following:

1.  The system must determine the transverse location of Horn 1 to a
    precision of ±0.5 mm and of Horn 2 to ±1.0 mm, relative to the
    proton beam centerline. These requirements limit the distortion of
    the neutrino energy spectrum by horn misalignment to an acceptable
    level.

2.  The proton beam location is assumed to be determined to a precision
    of ±0.1 mm and ±15 rad by multiwire chambers upstream of the NuMI
    target.

3.  The system requires the ability to perform horizontal and vertical
    scans of the proton beam position, at low intensity, over a range of
    ±3.5 mm at the locations of the cross hairs and ±12 mm at the
    location of the neck of Horn 1. Scans are performed with the target
    and baffle moved out of the beam.

4.  BLM response should be stable and sensitive to rate changes of \~5%
    at intensities expected during scans (10^7^ particles/cm^2^/10^12^
    protons/spill).

5.  The proton beam is assumed to have an rms size of 1 mm and
    divergence of 0.910^-4^ in both horizontal and vertical at the NuMI
    target. Primary beam instrumentation that monitors proton beam
    intensity and location must perform according to the same
    specifications at low intensity (10^12^ protons per pulse) as at
    high intensity.

6.  The signal-to-noise determined by the simulation will degrade if the
    material in the beam is increased beyond that assumed.

7.  The cross hairs are mounted permanently to the horns at locations
    close enough to the beam centerline to provide adequate signals for
    the scans and far enough away to prevent damage to the cross hairs
    during full intensity running.

8.  The cross hair material should have a negligible (1%) effect on the
    neutrino flux.

9.  The upstream and downstream cross hairs on Horn 2 should be located
    on opposite sides of the beam so that they can be scanned
    individually.

### B. Alignment cross-hair system parameters

**Table** **4.2-9** summarizes the parameters used for the locations and
dimensions of the aluminum cross hair material in the simulations study
([NuMI-NOTE-BEAM-864](http://beamdocs.fnal.gov/cgi-bin/public/DocDB/RetrieveFile?docid=734)).
The BLMs for Horns 1 and 2 are located at z = 4.01 m and 14.14 m
respectively (in the Monte Carlo coordinate system, relative to z = 0 at
MCZERO at the entrance to Horn 1). Note that the GEANT simulation used
slightly different values for these locations, z = 4.099 m and z =
14.573 m respectively. The signal-to-background ratio was found to be
independent of the distance between a BLM and the beam centerline for
distances of 15 to 35 cm. Each BLM is located at beam elevation and
displaced horizontally by 30 cm from the beam centerline. Each will
collect a charge of roughly 33 nC during a beam spill of 10^12^ protons
on target.

**Table** **4.2-9**. Characteristics of the three sets of cross hairs
![](media/image2.wmf){width="6.0in"
height="1.7083333333333333in"}according to GEANT

#### 

4.  
5.  
6.  

#### 

![](media/image3.jpeg){width="5.874305555555556in"
height="5.753472222222222in"}

**Figure** **4.2-****17**. Drawing of the Horn 1 cross-hair structure.

This is an end-on view, looking into the proton beam, showing how the
cross hair is mounted on bolts that are also used to tie the inner and
outer conductors together. The aluminum cross hair material is 12-mm
thick (along the beam direction) and 1-mm wide. The horizontal member is
made only long enough (4-mm long, measured from the center of the
vertical member) for incident beam scans, in order to reduce the amount
of material in the beam and to simplify the support structure.
Belleville washers provide a tension of about 80 pounds (at room
temperature) on the vertical member, which is adequate to maintain
positive tension at 100 C above room temperature (e.g., from beam
heating). The cross-hair material is nickel plated to protect against
corrosion.