## 4.5 NEUTRINO BEAM MONITORING (WBS 1.1.5)

**4.5.1 Introduction**

The neutrino beam monitoring system enables the users to verify the
quality of the beam being delivered to the experiment(s). The monitors
measure the flux and spatial distribution of:

-   secondary hadrons (and non-interacting primary protons) measured by
    the "Hadron Monitor", located just upstream of the hadron absorber,

-   muons, which are directly associated with the production of
    neutrinos, measured by the "Muon Monitors," located downstream of
    the hadron absorber and at various locations within the dolomite
    muon shield.

The locations of the monitors are shown in **Figure 4.5-1**. The
monitors' intensity measurements are normalized to the incoming proton
intensity while the measured spatial distributions of particles are
compared to nominal spatial distributions recorded as the beam is being
commissioned.

![](media/image1.png){width="4.8125in"
height="2.5208333333333335in"}**Figure 4.5-1** Plan view of the
monitoring system: the hadron monitor is upstream of the absorber stack
and the muon monitors are in the absorber enclosure and within the
dolomite shielding

Both the Muon and Hadron Monitors are used to commission the NuMI
beamline. In addition, during normal neutrino beam running for the MINOS
experiment, the monitors serve as accurate neutrino rate monitors.
Finally, during periodic specialized runs the monitors serve to re-check
the neutrino beam direction toward Soudan.

**4.5.2 The Hadron Monitor**

**4.5.2.1 System Function**

The hadron monitor is designed primarily for commissioning the NuMI
beamline. Specifically, it will align the proton beam prior to the
installation of the horns and target in the target hall. The proton beam
alignment is set within the quantitative limits described in Section 4.6
(about 50μrad).[^1] **Figure 4.5-2** shows the signal expected in the
Hadron Monitor during commissioning.

During normal neutrino beam operations with the horns and target in
place, the hadron monitor tracks the proton spot, and can also monitor
the integrity of the NuMI target. However, these latter functions are
redundant with other instrumentation, such as the multiwires and BPM\'s
or the muon monitors, so are not essential. If the Hadron Monitor should
fail after extended running, it will have been highly radioactivated by
the beam. As a result, the hadron monitor will only be replaced if a
long shutdown permits. The likelihood or urgency of the replacement
requirement will depend on operational experience with the NuMI beam.
**Figure 4.5-2** shows the expected signal during normal beam
operations: the magnitude of particle scattering in the target serves as
a check of the full target integrity.

![](media/image2.png){width="5.90625in"
height="2.8854166666666665in"}**Figure 4.5-2** (left) Charged particle
fluxes at the hadron monitor as a function of the distance from the
center of the beamline. Fluxes for running with the target in (nominal)
and out are shown. (right) Effect on Hadron Monitor signals if 15 cm of
the 1 m target breaks off.

![](media/image3.wmf)**Figure 4.5-3** Schematic layout of the Hadron
Monitor gas vessel and 7x7 array of ion chambers.

**4.5.2.2 System Description**

The hadron monitor is a 1x1 m^2^ Aluminum gas vessel encasing an array
of parallel plate ionization chambers (see **Figure4.5-3**). The
chambers are mounted to the rear wall of the gas vessel on electrical
feedthroughs. A thin front window of 0.010\'\' Aluminum is mounted to
the box. It is welded to a flange that compresses an Aluminum wire
gasket to the box. It is not anticipated to open this lid for chamber
repair after the hadron monitor has been in the beam. The vessel is 34"
x 34". Each 4"x4" ion chamber plate is separated from its neighbor by
0.5".

**Figure4.5-4** shows the drawings for the ceramic plates of the
parallel plate ion chambers. The wafers are 1mm 96% pure Aluminum oxide,
with Pt-Ag alloy electrodes.[^2] Electrical connections to the grounded
guard ring, the sense pad, and the HV plane are made through the corner
holes to the rear of the board, where solder connections are made.
Deviations from flatness, based on a preliminary order of 20 boards, are
less than 0.003\'\' across the 4\'\' plate. The plates are separated by
1mm, the gap defined by precision ceramic washers.

![](media/image4.png){width="3.138888888888889in"
height="4.372916666666667in"}![](media/image5.png){width="3.265972222222222in"
height="4.38125in"}

**Figure 4.5-4** Design of the metalized ceramic pads that make up one
parallel plate ion chamber inside the hadron monitor: (above) HV plan
(below) signal plane with guard ring.

**Figure 4.5-5** shows how the parallel plate chambers are mounted on
the rear wall. Two commercially available HV feedthroughs (rated to 10
kV in vacuum) penetrate the rear Aluminum wall of the hadron monitor
box.[^3] The feedthroughs consist of a central stainless pin
Kovar-sealed to a ceramic insulator. The ion chamber plates are soldered
directly to the feedthrough pins at the corners, thus providing both the
mechanical support of the chamber and the electrical contacts. An
explosion-bonded Al-Stainless piece is used to make the weldments
between the feedthroughs and aluminum box.

The cables delivering HV and bringing signals out of the hadron monitor
consist of two pieces. Immediately behind the monitor, where the rad
levels are highest, the coax cables are made of aluminum central and
outer conductors with ceramic tubing as the dielectric. All the parts
are made of commercially available wire or tubing. At the edge of the
monitor, these cables are attached to kapton-insulated coax cable.[^4]
The kapton cable brings the lines out to the edge of the absorber
shielding when the hadron monitor is in place, where a transition box
will convert this to standard twist-n-flat.

![](media/image6.jpeg){width="4.2756944444444445in"
height="3.001388888888889in"}

**Figure 4.5-5:** The mounting and signal feedthrough for a single ion
chamber. Ceramic-insulated feedthroughs both support the chamber and
deliver HV/signal through the gas vessel wall. The HV and signal
feedthroughs are at opposite diagonal corners of the plates, and the
other two corners are simple Aluminum standoffs.

**4.5.2.3 Fluences, Doses**

The hadron monitor is exposed to up to 10^9^ charged
particles/cm^2^/spill during beam operation, consisting predominantly of
120 GeV protons which fail to react in the target. In addition, the
monitor sees 2 x 10^9^ neutrons/cm^2^/spill, largely as a result of
splashback from the hadron absorber located 80 cm downstream of the
monitor. The particle fluences result in 1.3GRads/year at the monitor
center and 1.0Grad/yr at its edge

It is estimated that the total mass of the Hadron Monitor, including all
cables that must be disposed of with the detector in the event of
failure, is of order 55 pounds. Of this mass, 47lbs. is Aluminum or
Aluminum oxide so will have short half life after irradiation by the
beam. The remaining 5 pounds is predominantly stainless steel used to
make the vacuum connections, and will account for 90% of the residual
activation. The residual activation is approximately 50Rem/hr on contact
after 1 year of irradiation and 1 week cooldown.

**4.5.2.4 System Repair**

Should the hadron monitor fail and require replacement, the failed
monitor will be disposed of. Given the intense activation of both the
monitor and the surrounding shielding and absorber core, special care
must be taken for personnel safety.

A conceptual idea for the sequence is indicated in **Figure 4.5-6**. The
cables and gas lines for the hadron monitor protrude through the
polyethylene blocks. Disconnections of these cables will be the first
task. Provision for disconnections will be made just outside the
absorber shielding. As the exterior of the absorber is expected to be 20
mRem/hr on contact after operations, this disconnection must be readily
done.

Second, the two polyethylene blocks (not shown in the figure) must be
extracted from the slot in the concrete shielding. They can be pulled
out using pulley (A) and lowered down to ground level using pulley (B).
Because these blocks will be activated they will be lowered into a
shielded cart. The cart is rolled out of the passageway.

Removal of the hadron monitor is also accomplished using the two pulleys
(A) and (B). Two steel cables will have been attached to the hadron
monitor when it was inserted into the absorber and left attached to the
hadron monitor (*i.e.*: they also protruded through the polyethylene
blocks). These cables will be used to extract the hadron monitor (over
pulley (A)) and lower it into a shielded coffin/cart (over pulley (B)).
Because the cords were left attached to the HadMon during installation,
the personnel can stand far away from the slot during this procedure. In
**Figure 4.5-6**, the hadron monitor is shown at three successive
locations during the procedure to extract it from the absorber
(indicated as (1) through (3)).

![](media/image7.jpeg){width="7.5in"
height="6.370833333333334in"}**Figure 4.5-6** Rail and Pulley system for
hadron monitoring replacement scheme as viewed from the downstream end
of the absorber alcove.

![](media/image8.png)Figure 4.5-7 Rail system for hadron monitor
replacement scheme.

The rail system to install and align the Hadron Monitor is shown in
**Figure 4.5-7**. The lower rail is a 6" I-beam and supports the monitor
weight. The monitor is in sliding contact with the rail. The upper rail
consists of 6" "L" bracket that is straddled by fins on the top of the
monitor so as to maintain verticality. A hard stop on the lower rail
defines the fully-inserted monitor position, as well as the reproducible
position to which replacement monitors may be inserted. Alignment and
placement of the monitor is done during initial installation, but not
after replacement.

![](media/image9.png){width="5.46875in" height="2.6666666666666665in"}

**Figure 4.5-8** Muon fluxes in the 3 muon monitoring locations: Alcoves
1, 2 and 3 . Fuxes assume 4x 10^13^ protons on target per spill, and
only include the muons, as predicted by GEANT. Shown are the fluxes in
the three alcoves for nominal low-energy beam running (left plot), for
special monitoring runs in the semi-medium energy beam (right). The
fluxes in alcoves 2 and 3 have been multiplied by 1.5 (for both plots)
so the shapes can been seen on the figure.

**4.5.3 The Muon Monitors**

**4.5.3.1 System Function**

The muon monitors are three arrays of ionization chambers located in the
four \'alcoves\' downstream of the hadron absorber in the NuMI beamline.
Each alcove will see different fluxes of charged particles due to the
attenuation expected in the absorber and dolomite rock between alcoves,
with the downstream alcove seeing the most energetic muons only (see
**Figure 4.5-8**).

The muon monitors are approximately 2 m by 2 m arrays of ion chambers
that measure an ionized charge proportional to the number of muons
passing through the array. Because each muon plane is segmented into an
array of 9 by 9 chambers, the individual signals from each chamber in
the array help locate the center of the muon (hence neutrino) beam. The
chambers will not be absolutely calibrated to provide a charge-per-muon
conversion factor, but will be relatively calibrated and this
calibration will be tracked over time during operation of the NuMI beam.
In this way, the relative signal seen from chamber to chamber indicates
the relative magnitude of (muon) beam intensity at a particular location
within each muon array, and also the comparison of rates between alcoves
can be used to infer information on the energy spectrum of muons.

![](media/image10.png)**Figure 4.5-9** One array of 9 muon chamber
tubes. Each tube is 2m in length, and separated from neighboring tubes
by 25 cm (center-to-center).

An ion chamber tube is approximately 8 feet in length, made of 6\'\' by
2\'\' extruded aluminum tubing, 1/8\'\' wall. Nine tubes will be used to
form the 9 by 9 array (see **Figure 4.5-9**). The tube has 3/4\'\' thick
endflanges welded to each end to which are bolted endplates which handle
all electrical and gas connections. The endplates are gas-sealed to the
endflanges by an Aluminum wire gasket that is compressed by the endplate
and endflanges.

The ion chambers themselves are mounted onto trays that slide into the
tubes. A drawing of one of the ion chambers mounted on the channel is
shown in **Figure 4.5-10**. The tray is rigidly bolted to one of the
endplates via a tab (shown in red in the figure). The channel is shown
in light blue, the endplate is shown in dark gray. The 6\'\' by 2\'\'
tube and its endflange is also shown (in mustard brown). Not shown in
the figure are the two kapton-insulated coax cables per ion chamber,
which bring signal and HV to the chamber from the feedthroughs at the
endplate.

The muon monitors use the same ceramic plate ion chamber electrodes as
the hadron monitor. The gas gap, however, will be 3mm (not 1mm as in the
hadron monitor). With this gas gap and Helium gas, it is anticipated to
collect 1400, 170 and 80 pC per in the central ion chamber in Alcoves 1,
2, and 3, respectively.

The endplate with all electrical (signal and HV) connections is shown in
**Figure 4.5-11**. The high voltage and signal feedthroughs are
commercially-available ceramic-insulated vacuum feedthroughs. The
signals for the 9 chambers are fed into a 9-pin D-type connector[^5]
with ceramic insulator, gold-plated stainless pins, and a stainless
jacket. It is welded directly to the endplate. The high voltage
(typically less than 600 Volts) is brought in through a straight-pin
feedthrough.[^6] One HV feedthrough delivers the HV for one chamber.

To maintain chamber-to-chamber calibrations over the course of NuMI
operations, as well as help calibrate temperature or pressure
variations, each chamber will have a 1 μCi Am^241^ alpha source (*E* =
5.4 MeV) mounted to it. During dedicated calibration runs, the
electronics will trigger between NuMI spills so as to integrate the
charge from the ionization created by the alpha's, not the NuMI beam.
Such sources in bench measurements have been shown to ionize
approximately 2.5pA of current in He gas, which is sufficient for
calibration but a small background to actual beam spill measurements.

![](media/image11.jpeg){width="4.361111111111111in"
height="3.2777777777777777in"}**Figure 4.5-10**. Tray inside a muon tube
onto which the parallel plate chambers are mounted. Each ion chamber is
connected to the electrical feedthroughs at the endplate by coax kapton
cable (not indicated in drawing).

![](media/image12.png)**Figure 4.5-11**. Endplate of the muon monitor
tube will electrical feedthroughs. A single 9-pin D-type connector
brings out the signal lines and a ceramic HV feedthrough is used for
each individual chamber HV. A Swagelok feedthrough brings in the gas
line.

![](media/image13.wmf)**Figure 4.5-12**. Elevation view of the support
structure and the muon monitor in Alcove 1. The muon detectors are
supported at beam center. The top of the detector is 7" from the drip
ceiling.

A frame has been designed to support the muon beam monitoring chambers
at beam height in any of four alcoves downstream of the hadron beam
absorber. **Figure 4.5-12** shows a side view of this frame as seen in
Alcove 1. Unobstructed access to the detectors is permitted from the
upstream side for quick mounting or service of individual muon tubes.
The chamber tubes hang by studs that fasten to the upper cross beam of
the picture frame. The upper studs serve to locate the tube centers.
Lower studs are less constrained in order to allow for welding
inaccuracies and warp in the extruded tubes. We strive for 1 mm relative
placement of the tubes on the frame. Cabling and gas supply will be
routed along the bottom of the picture frame. Gas exhaust will be routed
along the top of the picture frame. Deflection of the upper beam when
loaded with the muon chambers should be about 0.050 inches.

Components of the frame are entirely aluminum except for steel
jackscrews in the floor mounts. Because of the difficulty of
transporting a frame to its final location underground, these frames
have been made of smaller sections, which will be bolted together in the
final alcove location. The three principal pieces are a rectangular
picture frame on which the chamber tubes are mounted vertically, a
diagonal bracing section which supports the picture frame, and a
rectangular base section which has a 3-point mount on the floor.

Beam height in each alcove determines the vertical location of the
frame. A single frame design should work well for all four muon alcoves.
Jack screws in the feet allow a vertical adjustment of ±3 inches. Larger
adjustments may be accomplished via extra bolt holes which allow the
horizontal supports of the picture frame to be adjusted an additional
amount vertically. Once the frame is bolted to the floor, no horizontal
position adjustment is possible. The mounting frame is about 6 inches
below the drip ceiling and 15 inches from the downstream wall. There
exist about 52 inches of working space upstream of the muon chambers in
alcoves 2 and 3. Part of this space may later be filled with light
material (such as polyethylene) to reduce electrons in the beam flux.

**4.5.4 System Description: Electronics**

The essential features of the electronics used to acquire data are shown
in the simplified schematic of **Figure 4.5-13**. An operational
amplifier takes the signal and stores it on a precision 100pF
integration capacitor. The voltage on the capacitor is subsequently
digitized by an ADC for computer read-out. Voltage on the input to the
integrator is maintained at a virtual ground subject to the limitations
of the amplifier. This differs from pulse height read out systems that
employ a load resistor to develop a voltage at this point in the
circuit. The integration capacitors will be modified where necessary to
accommodate the range in expected signal size. An additional resistor
has been added across the common integrator output buses to eliminate
cross talk for large signals.

The complete system used for data acquisition is the same as those used
for SWIC Readout (**Figure 4.5-14)**. Each SWIC electronics box has 96
channels, each of the monitors use that same electronics (although the
hadron monitor, of course, does not fill all the channels). There is a
slight modification to the integration capacitor and an additional
resistance is needed to reduce channel cross talk for large output
signals. The integration time is to be 5msec.

![](media/image14.jpeg){width="4.614583333333333in" height="2.9375in"}

**Figure 4.5-13**: Schematic of ionization chamber and input stage of
electronics

The map of where the electronics is placed can be seen in plan view in
**Figure 4.5-1**. The electronics racks and high voltage are located in
the bypass tunnel, which leads off to the west from the absorber cavern.
Although the downstream hadron monitor is not shown in the diagram, both
the hadron and the muon monitor in Alcove 1 will be serviced by SWIC
controllers near the electronics rack. Electronics for all monitoring
locations are accessible during normal running.

![](media/image15.wmf){width="5.840972222222222in"
height="3.0729166666666665in"}**Figure 4.5-14** Layout of the 96-channel
SWIC box.

**4.5.5 System Description: Gas System**

The monitors are to be supplied with helium, which must be maintained at
a high purity (less than 10ppm impurities). Gas will be stored near the
MINOS shaft, sent through a two stage regulator followed by a line
filter to a gas distribution panel in the hallway near the electronics,
which splits the one line to four, which then go to each of the four
monitoring planes.

Exhaust gas is returned to the distribution panel where there will be
bubblers, and then sent to an external vent, part of the HVAC system.
**Figure 4.5-15** shows a schematic of the gas system.

There are two sets of "8-pack" high-pressure manifolds, one of which
will be set up to a two-stage regulator. This allows operation in the
semi-automatic mode, with one side in reserve at all times. The gas
pressure after the two-stage regulator will be approximately 30psi. The
gas will then be put through a line filter, and sent to the distribution
panel near the electronics for the monitoring system. Each line going to
a plane of monitors will be connected to two flowmeters in parallel, as
well as a pressure gauge, and finally a valve to allow one to turn off
the flow to only one alcove in case a muon tube needs to be replaced.

Gas is supplied to the main distribution rack located outside the last
radiation barrier, via a continuous length of, 3/8\" OD stainless
tubing. The main distribution rack allows for adjustment of flow to a
given alcove. Four sets of flow-meters control gas flow to each of the
four monitoring planes. One flow meter will allow for fast purging of
the monitor, while the other will be used for normal low flow running.

The chambers are to be operated at slight overpressure (\<0.5 inches of
water). Each tube has a gas volume of approximately 15 liters (three
stations having total volume of 405 liters). The hadron monitor is 19
liters. Each bottle contains about 8000 l of gas, (300 ft^3^). At the
rate of one volume change per \~2hrs for all three alcoves plus the
hadron monitor, we consume an 8-cylinder pack every week.

![](media/image16.wmf){width="6.472916666666666in"
height="4.532638888888889in"}**Figure 4.5-15** This shows a schematic
drawing of the gas system.

## 4.5.6 System Description: Online Information 

There is much information to be recorded during data taking to be able
to ensure both that the monitors themselves are performing well, and
ultimately that the beamline itself is performing as expected. Of course
the information about the beamline is contained in the signals of these
arrays, and the concurrent calibration constants that are to be
calculated online. This information is to be summarized as an overall
flux of particles (and a peak flux for the Downstream hadron monitor),
and some shape information, including the first and second moments of
the horizontal and vertical projections of the signals. The information
that arrives at the monitors spill by spill will be transferred by the
XML-RPC system which is currently also being used by MiniBooNE.

Because ionization chambers are used, we record signals from the
detectors along with calibrations that are going on during off-spill
gates. An ethernet controlled DAC sets voltages on 16 channels of
modified Droege HV supplies. Seven of these supplies serve the
Downstream Hadron Monitor. Each of the three muon alcoves is served by
three HV channels. The current and voltage of each channel is read back
over ethernet. Low voltages in the SWIC electronics and the HV NIM bins
are also remotely monitored. Pressure, humidity, and temperature are
monitored at the location of each detector. An overview of the ethernet
system can be found in **Figure 4.5-16**.

![](media/image17.wmf){width="6.49375in"
height="4.009722222222222in"}**Figure 4.5-16** This shows the list of
devices that DCS expects to get from the beamline monitoring devices.

The DCS will export its data to the rotorooter process and then on to
the DISPATCHER where it along with the RPC XML data will be put in
offline data streams, and will be available to the experimenters.
Summary information will be put into the main ORACLE database by an
offline job. In addition it is planned to store all the data acquired by
the DCS on a disk of the DCS computer where time histories and
correlations can be extracted if needed.

[^1]: Smart, W. NuMI-L-221, 31 Oct. 1996

[^2]: Cer-Tek, Incorporated, El Paso, Texas.

[^3]: Made by Insulator Seal, Incorporated, Hayward, CA.

[^4]: Insulator Seal, Inc.

[^5]: Ceramaseal, Incorporated.

[^6]: Insulator Seal, Incorporated.