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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 2TARGET MULTIWIRE FOR THE FERMILAB BOOSTER
NEUTRINO BEAMLINE
R. Prokop†, Fermi National Accelerator Laboratory, Batavia, IL, USA
Abstract
The Booster Neutrino Beamline experiment requested a
new secondary electron emission multiwire profile monitor
installation. The device had to be durable in high radiation
conditions and mounted within a large 10-foot airtight steel
fixture for installation near the beam target. Previous iterations of multiwire suffered radiation damage to both theconnectors and wires. To ensure accurate horizontal andvertical beam profile measurements, an updated design
was proposed, designed, and constructed. The new BNB
multiwire utilizes 3 mil diameter gold-plated tungsten
sense wires soldered to vertical and horizontal Alumina-96
ceramic planes, 50 wires per plane. Radiation hard Kapton
insulated 30-gauge wires carry the output signals. Profiles
are readout through charge integrator scanner electronics.
This paper will detail the design and functionality of theBNB target multiwire and present relevant beam profile
data.

signal is expected to be proportional to the amount of beam
particles striking the wires [2].
Bias planes are comprised of titanium foils, and act toenhance the secondary emission effect by collecting strayelectrons. The bias operates at +200 VDC.

FUNCTION AND APPLICATIONThe Booster Neutrino Beamline Multiwire for Horn 5 isa unique secondary emission monitor design at Fermilab
utilized specifically for profile measurements of high intensity beam, up close to the experimental target [1]. This
specialization calls for high durability and several modifications from the typical multiwire. The goal is to get cleangaussian profile measurements from small diameter wire,using wire/cable that can withstand high radiation environments. Previous iterations of this design used the classic
multiwire flat Kapton ribbon cables with zero insertion
force connectors glued to the ceramic frames. This is notideal, as the intense environment of this device causes
damage to the connectors. To remedy this problem, smallgauge (30-40 AWG) Kapton insulated wire are individually cut to length and wrapped in bundles, soldered to boththe frame and readout connectors. This presents many engineering challenges, but also opportunities to improve andcraft a very durable and effective multiwire. This projecthas been a joint effort between the Fermilab Accelerator
Directorate Instrumentation, External Beams & Target
Systems groups.
The BNB multiwire operates on the principle of secondary electron emission, much the same as all other SEM profile monitors at Fermilab. The particle beam made up ofprotons strikes the sense wires and knocks off surface electrons from the metal material. This phenomenon leaves a
resultant net positive charge on the wires which can bereadout as a current through the signal cables. This current
is integrated through transimpedance amplifiers in thefront-end electronics and digitized as a voltage value to beplotted
as a final beam profile measurement. The resultant___________________________________________* This work was supported by the U.S. Department of Energy undercontract DE-AC02-07CH11359† rprokop@fnal.gov

Figure 1: Multiwire assembly drawing.

DESIGN
The BNB Target multiwire is an open air mounted secondary emission monitor designed for the high intensity
beam readout. The multiwire assembly as shown in Fig. 1,consists of two ceramic planes, one of vertical orientationand one horizontal. The ceramics are 4.5 x 4.5 inch with acenter hole of diameter 1.25 inch. They have 50 pads at
1mm spacing for signal wires, and 100 pads at 0.5 mm
spacing for sense wires across the center hole. There arethree titanium foils of .005-inch thickness, which act asbias planes to assist in maintaining an electric field forcharge deposition on the sense wires. The multiwire assembly is held together by front and back support brackets aswell as spacers between the planes. The multiwire mounting assembly is slid into a larger horn mechanical enclosurefor insertion close to the target. Discussion of the entire
mechanical mounting assembly is beyond the scope of thisdocument.
The signal carrying conductors are fashioned in a similar
way to previous the MiniBooNE target multiwire, but without the zero insertion force connectors or Kapton ribbon
cables. This new design instead utilizes 100 individual
Kapton insulated wires of 0.25 mm diameter for the signal
wires, terminated at one end at the pads of the ceramic andat two 50 pin D-Sub connectors on the other end. The useof connectors and epoxy was not ideal for this application,
as in previous designs the epoxy wore down and connectors became damaged due to heat and radiation. 0.003-inch
diameter AuW wires are soldered pad to pad for the centerbeam sense wires. AuW wires provide acceptable signal
integrity through collected charge, as well as having excel-WEP022
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03 Transverse Profile and Emittance Monitors

IBIC2023, Saskatoon, CanadaJACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 2

lent density and durability with high intensity beam [3]. A
small diameter of 3 mil is typical for applications such as
this; it limits the amount of material in the beam while also
giving ample signal.

Wire Preparation
 The Kapton insulated wires make for a challenge whenassembling the multiwire. To account for every sensewire reading, there must be 50 wires per plane, totalling to 100 signal wires. Additionally, there is one highvoltage wire which supplies the bias. The signal wires
for the multiwire must measure at minimum 10-feet inlength. When accounting for some slack, they were cut
to ~11 ft. The wires must reach from the multiwire
planes to the D-Sub connectors for readout. To achievea neat and organized bundle of wires, the decision wasmade to separate the wires into sets of 10. Each plane
has five sets of 10 wires, with each set taped together
at intervals along the 10-foot length run. The mechanical assembly, shown in Fig. 2, has a 9-foot metal pipe
which carries the wire bundles from the wireplane
mount to the connectors. There is a slight angular bendat the end of the pipe, which makes the pipe length andwireplane fixture distance approximately 9.5 ft.
 The ends of the Kapton wires are stripped to reveal a
few mm length of centre conductor for soldering. It is
important to consider the order of the wires when preparing bundles. Each set of 10 wires has a red tickmark on the electrical tape near the first wire. A multimeter was used to check continuity between ends ofthe wires and ensure that each wire is properly markedon both sides.

The central sense wires are soldered with an intricate
method by a wireplane specialist at Fermilab. She winds
the wires on a transfer plane to get tension and the correct
spacing, 0.5 mm in this case. They are aligned to the padson the ceramic thereafter. Then the wires are temporarily
epoxied to tape at the end of the substrate and soldered.
To circumvent damaging the ceramic with high heat, andallow for fast and easier reflow soldering, Alpha CVP-520
low temp solder paste [4] was initially chosen as the bonding agent. This paste has a melting point of 138 oC and an
optimum reflow temperature between 150 oC and 190 oC.
A soldering iron would work for drag soldering but runs
the risk of applying too much focused heat to the ceramic.
A heat gun causes far too much spread, with risk of affecting the sense wires or too much airflow. The ideal tool forthe job is an adjustable airflow heat wand with changeable
tips. Keeping airflow relatively low allows more heat withless risk of paste blowing across the ceramic. A temperature of 190 oC/374 oF provides a fast and effective reflow
for the solder paste and bonds the wires efficiently.
The 10-foot bundles were routed through 9-foot pipe tothe readout box at MI-8 during the full assembly procedure. The readout cable and connector soldering process
involved several days and many hours of stripping and soldering each wire end to the D-Sub connectors bolted to thereadout box. A high voltage wire was also connected to theplanes and fed to an SHV bulkhead connector.

Figure 3: Wireplane with Kapton insulated signal wires.

INSTALLATION AND RESULTS

Figure 2. Multiwire assembly completed and mounted.

Soldering Procedure
The soldering of signal wires requires a pre-made wire
containing fixture, as a procedure like this has never been
attempted before with a multiwire. The signal wires are
soldered to pads directly on the ceramic itself, much different than other modern multiwires which have connectors
and 50 conductor cables or ribbon cables. The fixture issimply a metal block, with 0.25 mm width slots to sit thewires in. A top block is fastened above the wires to clamp
down and secure them in place. This wire fixture is essential for the completion of the multiwire assembly (seeFig. 3).

The multiwire is mounted on a mechanical assembly andaligned with precision instrumentation. This process ensures that the center of the wireplane grids and the centerof the beam pipe are within ~1 mm tolerance. Kapton insulated cabling is routed through a 9 ft stainless steel pipe
for protection. At the end of the pipe is a welded cap with
D-Sub connector feedthroughs and one SHV-5 high voltage connector feedthrough. The signal wires have beencarefully soldered individually to each pin and tested forcontinuity as well as shorts.
The mounting assembly is on rails, and during installation it slides into a larger housing unit. This unit is sealed
completely airtight and placed near the beam target. Due tothe proximity to the beam target and the high radiation hazard, access to the internals of the unit for repairs or
modifications is very difficult and time consuming. Allalignment and testing are done to the highest of standards
to reduce the risk of failures.
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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 2

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

Readout Electronics
The BNB target multiwire uses standard Fermilab
readout electronics for beam profile monitors. These chassis are called scanners, shown in Fig. 4. A set of boards
housing 96 channel analog front end integrator circuits receive the signals from the current carrying cables and integrate them to voltages for digitization. Wire 1 and 50 on
the vertical and horizontal wireplanes act as ground wires,
tied to chassis ground at the scanner. The modern scanners
have ethernet capability, for fast and reliable control anddata capture. Every scanner is connected to the Fermilab
Accelerator Control Network front end for viewing dataand making changes to settings. The Fermilab Controls Department manages these devices and provides Instrumentation with reliable readout electronics.

the charge collection as a histogram of voltages on eachwire. Due to the nature of the BNB multiwire and it’s very
difficult to reach location, as well as nearby hazards, it ischallenging to repair. A complete failure or wire replacement would result in the entire assembly’s removal and adisassembly of the wireplane fixture.
Another Target multiwire for BNB Horn 4 is coming,and there are lessons learned and proposed modifications
the team has come up with. For the signal wire readout cables, there is a ceramic coated small gauge wire produced
by a vendor which may be a suitable substitute for the Kapton insulated wire. The ceramic coated wire is highly durable and highly flexible, while maintaining its form. The ceramic coating is extremely small, thin, and porous. The primary challenge when implementing this material will bestripping the coating from the central Copper/Nickel conductor. Using a molten hot solution is one method, buthighly hazardous and not ideal for 100 wires. Fermilab engineers who have done research this potential wire type,have proposed a few alternate stripping and mounting
methods which will be tested.

CONCLUSION
Figure 4: Rear view of scanner electronics.

Profile Data
The data pictured below is not from the BNB target multiwire, as this device is recently installed and has not yet
seen beam. The image below shown in Fig. 5 is a screen
capture from a similar device, a standard multiwire SEM.
This is the horizontal and vertical display showing expected results with a Gaussian fit. The histogram plot has
each wire voltage value represented as a blue bar. The center of beam is ideally the center peak of the profile.
The environment which the BNB target multiwire is installed is considered a high radiation area, due to its proximity to the beam target. Exact radiation measurements are
not known but are enhanced by secondary beam materials
such as neutrons. The beam bunch intensity is approximately 4E12 protons/bunch and the proton rate is around
3E16 protons/hr at a proton beam energy of 8 GeV.

The BNB target multiwire was a collaborative effort between multiple departments and teams at Fermilab. It overcame numerous challenges during design and assembly.
The target systems group expects full operation during the2023-24 accelerator run, with hopes for quality profile datato follow. The radiation-hard connectors and wire strain relief, as well as the Kapton insulation should provide thismultiwire with many years of safe operation within theBNB target enclosure. Lessons learned from this projectwill be carried over into future installations.

ACKNOWLEDGEMENTS
Thanks to the following past and present Fermilab teammembers who lent their knowledge, skills, and time to thisproject. Daniel Schoo, retired Fermilab Electrical Engineer. Gianni Tassotto, retired Fermilab Engineering Physicist. Thomas Kobilarcik, Fermilab Engineering Physicist.
George Lolov, Fermilab Mechanical Engineer. Hiep Le,late retired Fermilab Physicist. Stephen Rocos, Fermilab
Technician. Wanda Newby, Fermilab Technical Specialist.
Paula Lippert, Fermilab Technical Specialist. Cervando
Castro, Fermilab Technical Specialist.

REFERENCES[1] Fermilab, https://ad.fnal.gov/ebd/

Figure 5: Beam profiles, horizontal and verticalFuture
The readout electronics consist of two flat-to-round signal cables, vertical and horizontal, connected to a Controls
Scanner rack-mount chassis. The readout electronics arehoused at Main Injector Service Building 12. Testing thedevice is possible using the common flash test procedurefor multiwire profile monitors. This test uses an integratingscanner, high voltage supply, and video monitor to view

[2] G. Tassotto and J. Zagel, “Beam Profile Detectors at the newfermilab injector and associated beamlines”, in Proc. DIPAC'99, Chester, UK, May 1999, paper PS17, pp. 141-143.
[3] Hsing-yin Chang, Andrew Alvarado, Jaime Marian, “Calculation of secondary electron emission yields from low-energy
electron deposition in tungsten surfaces”, Applied Surface
Science, vol. 450, pp. 190-199, 2018.
doi:10.1016/j.apsusc.2018.04.155[4] Alpha,https://www.macdermidalpha.com/assembly-solutions/products/solder-paste/alphacvp-520-solder-paste

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