# Optical Calibration System for the LUX-ZEPLIN (LZ) Outer Detector

W. Turner<EMAIL_ADDRESS>A. Baxter  H. J. Birch  B. Boxer  S.
Burdin  E. Fraser  A. Greenall  S. Powell  P. Sutcliffe  University of
Liverpool, Oliver Lodge Laboratory, Oxford Street, Liverpool, L69 7ZE, UK
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
STFC Rutherford Appleton Laboratory (RAL), Didcot, OX11 0QX, UK

###### Abstract

The LUX-ZEPLIN experiment will search for dark matter particle interactions
with a detector containing a total of 10 tonnes of liquid xenon. Surrounding
the liquid xenon cryostat is an outer detector veto system with the primary
aim of vetoing neutron single-scatter events in the liquid xenon that could
mimic a weakly interacting massive particle (WIMP) dark matter signal. The
outer detector consists of approximately 17 tonnes of gadolinium-loaded liquid
scintillator confined to 10 acrylic tanks surrounding the cryostat and
$228000$ litres of water as the outermost layer. It will be monitored by 120
inward-facing 8-inch photomultiplier tubes. An optical calibration system has
been designed and built to calibrate and monitor these photomultiplier tubes
allowing the veto system to reach its required efficiency and thus ensuring
that LUX-ZEPLIN meets its target sensitivity.

###### keywords:

Low-background, Optical Calibration, Water, Neutron Veto, LED

††journal: Nuclear Instruments and Methods in Physics Research Section A

##  1 Introduction

LUX-ZEPLIN (LZ) is a dark matter direct detection experiment with liquid xenon
(LXe) as the target material. Its projected sensitivity (90% C.L.) for WIMP-
nucleon spin-independent interactions reaches $1.4\times 10^{-48}$ cm2 for
$40$ GeV/c2 WIMPs [1] [2]. The core of LZ is a dual-phase xenon time
projection chamber (TPC) with an active mass of 7 tonnes which will be the
largest detector of its kind. The TPC is surrounded by an outer detector (OD)
which is fundamental in reaching the desired sensitivity. In the TPC, signals
produced by neutrons are indistinguishable from those of WIMPs on an event-by-
event basis; the purpose of the OD is to veto these neutrons [3]. LZ is in the
final stages of commissioning ${\sim}$1.5 km (${\sim}$4.5 km water equivalent)
underground at the Sanford Underground Research Facility (SURF).

Figure 1: Cutaway drawing of the LZ detector systems. The LXe TPC in the
centre is surrounded by the scintillator tanks (green) and light collection
system (white), all housed in a large water tank (blue-grey). The positions of
some of the injection points for the OD OCS are highlighted by red circles.
The route the produced light takes to reach the water tank and the monitoring
PMT housed in the dark box is shown as red dashed line. Schematic of the LZ
detector is taken from LZ TDR [3].

The OD and a LXe ‘skin’ detector compose the LZ veto system which together
form a active shield for radiation entering or escaping from the TPC, as shown
in Fig. 1. Schematic of the LZ detector taken from LZ TDR [3]. The LXe ‘skin’
detector is located between the TPC and the inner cryostat wall and is
principally used to veto MeV-scale gamma rays. The OD forms a near hermetic
seal around the cryostat and is composed of a segmented acrylic tank housing
$17$ tonnes of linear alkylbenzene based gadolinium-loaded liquid scintillator
and is surrounded by $228000$ litres of deionised water. The segmented acrylic
tank and housed scintillator are refereed to collectively as the scintillator
tanks. The primary function of the scintillator tanks is to veto neutrons that
have interacted in the TPC, predominantly via neutron capture on gadolinium
[4]. Light generated in the OD is subsequently collected by $120$ inward
facing 8-inch Hamamatsu R5912 photomultiplier tubes (PMTs) [5]. To maximise
light collection efficiency, there is a Tyvek® curtain behind, above and below
the PMTs, and a layer of Tyvek® surrounding the cryostat [3].

The main requirement to the OD is to provide a veto efficiency to the neutrons
scattered in the TPC greater than 95% with less than 5% dead time at the
effective threshold of 200 keV (to be above the end of the 14C beta spectrum.)
[1]. This requires understanding the OD optical model and PMT stability at the
$\sim$1% level. An Optical Calibration System (OCS) has been designed and
built to validate and monitor OD optical properties and calibrate the OD PMTs
at the required level. Though the expected OD threshold is 200 keV, the OCS
was designed to calibrate the OD down to 150 keV.

Section 2 of this article describes the design choices made to ensure the OCS
meets the given requirements, while Sec. 3 outlines the hardware developed to
meet these requirements as well as the hardware selection criteria. Section 6
presents the calibration procedure and Sec. 5 shows the final system
performance. Additionally, Sec. 4 presents the radioassaying results of the
OCS components and how the observed rates compare to that of the outer
detector.

##  2 System Requirements

The OCS must be capable of executing a number of routines to monitor the OD.
The requirements underpinning these routines informed the design of the OCS.
These requirements are summarised in Tab. 1 and described below.

Verification of the OD PMTs response is prerequisite to all other monitoring
techniques. Full geometry simulations were carried out using a GEANT4 based
simulation framework developed by the LZ collaboration, BACCARAT [6] [7] [8].
This demonstrated that the response of OD PMTs can be monitored via the
repeated injection of $1000$ photons from the central row of fibres. These
injections yield a high proportion of 1-photon pulses collected by each of the
120 PMTs. The BACCARAT simulations show that the average pulse area resulting
from 1 photoelectron escaping the photocathode of a PMT can be determined with
an uncertainty of $\sim$1%, given enough statistics. Therefore, any change in
PMT response above 1% would be detected.

Table 1: Requirements for the OD OCS with the reason for each requirement. Based on LZ TDR requirements for the OD [3]. Requirement |  Reason
---|---
Ability to inject $700$ – $50000$ photons per channel. |  To be able to test the threshold energy of 150 keV ($1350$ photons) as well as being able to match the signals of the calibration sources which can go up to about 2.6 MeV ($20000$ photons).
Total number of photons: 1M photons globally. |  To mimic the average muon signal.
Variation between injected pulses: $\lesssim 10\%$. |  Should be no worse than the statistical variation of the OD.
Variation between calibrations: $\lesssim 100$ photons at 150 keV. |  Need to be able to keep the threshold of the OD stable at around 150 keV.
Light intensity precision: $100$ photon precision for $700$–$2000$ injected photons; $1000$ photon precision for $5000$–$10000$ injected photons. |  To allow scanning over the OD threshold region and simplify the match to expected background and calibration sources.
Pulse width: $<20$ ns. |  Should be shorter than the amplifier shaping time of the DAQ electronics (30 ns).
Injected light wavelength: $430$ – $450$ nm, $450$ – $460$ nm, $365$ – $390$ nm. |  Matches the peak wavelength and quantum efficiency of the OD PMTs and the scintillation light from the GdLS.
Pulsing frequency: up to $10$ kHz. |  Ensure calibrations use the full readout bandwidth.
Alignment: $<5^{\circ}$ misalignment. |  To limit variation in the number of photons in each PMT. This value is an acceptable upper limit to satisfy this requirements.
Radioactivity: $<5\%$ of the total OD rate. |  To ensure the background rates in the detector are kept to a minimum.

To sufficiently monitor the OD PMT response at the 150 keV threshold (which
corresponds to about 20 detected photons and $\sim$1300 photons) a scan must
be made over this point, therefore the minimum number of photons injected by a
single fibre needs to be about 700. Around 1 million photons are required to
study afterpulsing from energetic cosmic muons; this is the maximum
requirement for the OCS. To achieve this, each channel must be capable of
injecting at least $50000$ photons which when pulsed in synchrony with each
other will reach 1 million photons.

When monitoring the light collection efficiency at the energy scale of the
calibration source, variation in the number of photons collected from an
injection needs to be less than the variation in the calibration source to
avoid widening subsequent calibration peaks. Each pulse produced from the OCS
is monitored by the rack mounted PMT. This allows for pulse-to-pulse
calibration. BACCARAT simulations show the resolution value of the peaks
resulting from the calibration sources to be ${\sim}$12%. A target of less
than 10% variation in the number of photons detected after an injection was
hence set.

To have sufficient resolution to monitor the threshold region and the energies
deposited by the calibration sources, as given in Tab. 2, the OCS injections
must have a precision of 100 photons for injections of $700$–$2000$ photons
and a precision of $1000$ photons between $5000$–$10000$ photons. High
intensity signals are not connected to the calibration sources and therefore
the precision is less of a concern.

The requirement outlining the wavelengths of light to inject is not only to
ensure the injected wavelength matches the peak wavelength and quantum
efficiency of the OD PMTs but to also ensure scintillator degradation can be
detected. The absorption length in the scintillator decreases significantly
for wavelengths below 420 nm, this region shifts to higher wavelengths as the
scintillator degrades [9]. Injections of 435 nm and 450 nm photons from the
same injection point directly below the scintillator tanks will allow
monitoring of this degradation. A similar relationship between the absorption
of UV light and the degradation of the acrylic housing of the scintillator
exists and will be monitored via the same method using one injection point
under the acrylic tank lip with fibres connected to $390$ nm and $435$ nm
LEDs.

The last three requirements in Tab. 1 are more practical in nature. To make
use of the full 4 kHz bandwidth of the DAQ [3], the OCS should be able to
complete injection at a rate faster than this. The maximum rate depends on
occupancy and it will be determined experimentally. A target requirement of 10
kHz was chosen for the OCS electronics as this would allow collection of
suitable statistics in a short amount of time. To ensure that realistic
comparisons can be made with the simulations, the misalignment of fibres in
the OD needs to be less than 5∘.

Table 2: Sources used to calibrate the OD with the energy deposited in the OD and the average number of photons produced and detected listed. To go from the energy deposited to photons produced a constant of $9000$ photons/MeV have been used, and to calculate the approximate number of photons detected the average light collection efficiency around $7\%$ and mean PMT quantum efficiency around $25\%$ were taken into account [7] [3]. Source |  Energy Deposited |  # Photons Produced |  # Photons Detected
---|---|---|---
22Na |  511 keV |  $4600$ |  $64$
22Na |  1275 keV |  $11500$ |  $160$
DD |  2200 keV |  $19800$ |  $277$
228Th |  2615 keV |  $23500$ |  $330$
Cosmic-ray Muon |  0.1 - 1 GeV |  $1000000$+ |  $20000$+

Calculation of the neutron veto efficiency is reliant on accurate optical
modelling of the OD in BACCARAT simulations. This optical model can be tuned
and verified by comparison of light collections in LZ simulation and
experimental data using processes which produce predictable light output:
cosmic ray muons passing through the OD, calibration sources, and OCS
injections. The calibration sources, shown in Tab. 2, allow absolute
calibration of the OD connecting energy depositions in the liquid scintillator
with the number of detected photons. These signals will be used as reference
points for the OCS. Table 2 uses the energy deposited in the OD from each
source to determine the approximate number of photons emitted using $9000$
photons/MeV, as well as taking into consideration the average light collection
efficiency of approximately $7\%$ and mean PMT quantum efficiency about $25\%$
to determine the approximate photons detected [7][3]. The OCS will also be
used for injections of light at intermediate and extended intensities, and in
regions of the OD which are not reachable by the calibration sources. Given
that the OCS calibration is much faster than the calibration with radioactive
sources, the OCS calibrations will be performed more frequently.

##  3 System Overview

The OCS will use duplex optical fibres to inject controlled pulses of light
produced by LEDs into the OD at 35 locations. Of these, 30 locations are
distributed evenly around the water tank (10 azimuthal positions at 3 heights)
allowing for good coverage of the detector. Additionally, four injection
points are located beneath the four side scintillator tanks in order to probe
the scintillator quality, and one injection point beneath one of the side
scintillator tanks to probe the acrylic quality.

The OCS electronics system consists of five Optical Calibration Cards (OCC).
Each card consists of a custom-made Field Programmable Gate Array (FPGA)
motherboard, which houses eight LED pulser boards as well as two photo-diode
boards. Light from each LED is fed to two outputs on the front panel and to a
photo-diode input via a custom made 3-way optical coupler. The components of
an OCC can be seen in Figs. 2 and 3.

Light is fed from the rack housing the OCS electronics to the patch panel via
40 optical fibres 15 m in length. From the patch panel, they are coupled via
SMA-SMA connectors to 21 m long duplex fibres that lead to the injection
points inside the water tank. To enter the water tank, the fibres pass through
an air-tight and light-tight feedthrough flange located on top of the water
tank. For the injection points located around the sides of the detector, only
one of the cores of the duplex fibre will be used to inject light into the
detector. The other core will be available for potential future upgrades or in
case of damage to the first core. The upward facing fibres at the bottom of
the scintillator tanks will inject light of two different wavelengths via the
two different cores, as mentioned in Sec. 2 and described in detail in Sec.
3.1.

Figure 2: A diagram showing an overview of the Optical Calibration System,
with eight LED pulsers on one Optical Calibration Card (OCC) and five Optical
Calibration Cards in the VME crate. Lines with arrows show fibre routes with
labels representing numbers of fibres from LEDs with corresponding
wavelengths. Figure 3: A picture of an Optical Calibration Card with the key
components labelled.

Precise monitoring of the light intensity is carried out in two ways; firstly,
via the FPGA board controlled photo-diode boards and, secondly, via an 8 inch
Hamamatsu R5912 PMT installed in a rack-mounted dark box close to the OCS
electronics. This PMT is identical to those used in the OD. The stability of
this PMT is also monitored, using a YAP:Ce pulser unit made by Scionix [10]
which produces light pulses corresponding to 5k photoelectrons with a rate of
20 counts/s.

The OCS will receive the pulse configuration from the LZ Slow Control system
which will also store the required OCS calibration database. Between the Slow
Control system and the OCS crate is the OCS controller. The controller will
have two purposes: to translate the Slow Control system commands into the
required FPGA board register values and read back the board temperatures and
photo-diode values as well as the FPGA status messages to display in the Slow
Control.

###  3.1 Electronics

Each FPGA motherboard sends Low Voltage Digital Signals (LVDS) to the LED
pulser boards which are then converted into a single ended Transistor-
Transistor Logic (TTL) signal. This signal then drives the base of a common
collector transistor circuit which changes the voltage on the cathode of the
LED. When the LED is turned off, the anode is held at $-$1 V. Reverse bias
ensures that the LED does not produce an unintentional light emission which
could lead to fake signals in the OD. When the LED is fired, the transistor is
turned off and the cathode voltage goes to a nominal low voltage forward
biasing the LED. The number of photons emitted per pulse is controlled by the
time width of the LVDS.

High output violet blue LEDs, BRITE-LED Optoelectronics BL-LBVB5N15C, of
wavelength 435 nm were selected to match the scintillation light from the GdLS
[4] and the peak quantum efficiency of the OD PMTs. Quality control checks
were completed to ensure that the LEDs selected had similar intensities in
order to minimise channel to channel variation. Of the 127 LEDs tested, 35
were selected for use (30 for side injections and 5 for injections beneath the
scintillator tanks). To select the LEDs, first a subset of 60 LEDs of similar
capacitance were chosen. Then, the intensities of the subset LEDs as measured
by the test stand PMT (Hamamatsu H10721-20) were compared, and the group of 35
LEDs with the lowest intensity variations were selected.

As stated previously, 5 additional LEDs were selected to monitor the
degradation of the GdLS and acrylic of the scintillator tanks: four 450 nm and
one 390 nm LEDs. Requirements for these LEDs are not as strict as for the 435
nm LEDs because they will be used only for relative measurements: stability of
light collection efficiency from these LEDs will be compared to the stability
of light collection efficiency from the 435 nm LEDs.

###  3.2 Optical Fibres

The duplex optical fibres used to direct light to injection points in the
water tank are manufactured by Mitsubishi and consist of two polymethyl-
methacrylate resin cores, with a refractive index of 1.49, surrounded by a
polyethylene jacket [11]. The fibre cores measure approximately $980$
$\mathrm{\mu}$m in diameter and the total diameter of each fibre, including
the jacket, is 2.2 mm with the total width of the duplex cable being 4.4 mm as
shown in Fig. 4. These fibres were selected based on the results of testing by
the SNO+ collaboration as our requirements on light transmission and
radiopurity are closely aligned [12].

The fibre injection ends were ice polished at Fermi National Accelerator
Laboratory to produce smooth fibre ends, therefore reducing back-scatter and
ensuring a uniform beam profile. This process involves submerging the fibre
end in liquid nitrogen in order to freeze it in a layer of ice which, not only
lubricates the polishing process, but also prevents the cladding and jacket
from splitting from the core during polishing [13] (see Fig. 4).

Figure 4: A photo of the fibre end showing the two channels with their
polythene jackets after the ice polishing.

Light profile data was obtained from each of the water tank fibres by shining
a light from 435 nm LED down one channel and projecting the emerging light
onto a Tyvek® screen underwater in a dark box. Images such as that in Fig. 5
of the light were taken using an endoscope camera and then processed to
extract the light profiles for each of the fibres.

Figure 5:  Light from a fibre projected on a screen 5 and 5 the resulting
profile plot produced by taking a sample across the middle of the image.The
black and blue profiles correspond to the two different channels of the duplex
fibre and the slight x-axis offset between them is a result of the 2.2 mm
separation between the cores.

These datas were used as a quality assurance check to ensure that the fibres
used have no major defects and the light emitted from them is distributed
symmetrically to within an acceptable range. For the fibres to be used for the
upward facing injection points, the two channels were compared to ensure they
had similar profiles to allow fair comparisons between the two different
wavelengths that will be sent through them. In total $49$ fibres were tested,
and the 35 best ones which passed the quality assurance tests were selected
for use in the detector. The potential effects of fibre bending on the light
profiles were investigated using the same dark box set up by bending a test
fibre around discs of varying diameters, from $2$ cm to $5$ cm. When comparing
the images taken for different diameters, only the smallest disc with a
diameter of $2$ cm showed a small variation in the distribution of the light
profile. From these results it was therefore determined that bending the
fibres to a diameter of less than $2.5$ cm should be avoided when they are
installed in the detector. Also, the fibre manufacturer measured a power loss
of greater than 0.5 % with a bend radius less than $2.5$ cm.

Other quality assurance checks carried out on the fibres included transmission
measurements taken with an optical power meter, and the visual inspections of
the fibre ends using a microscope. Most of the fibres had near identical power
transmission. Any which varied from the average were discarded or kept as a
spare. The full lengths of the fibre cladding were then inspected and cleaned
before the fibres were packaged in the clean room in sealed bags for shipping.
Radon assays were conducted in the manufacture and test premises to ensure
acceptable levels of contamination from radon-daughter plateout.

###  3.3 Mechanical Structures

A mechanical feedthrough is required to allow the fibres to pass from the
electronics rack where the OCS is located into the water tank. Figure 6 shows
the machined flange with 16 ports which are fitted with rubber bungs to allow
the fibres to pass into the water tank while keeping an air-tight and light-
tight seal [14]. There is also a constant nitrogen over-pressure applied above
the water level inside the water tank to ensure air does not leak in and
introduce radioactive contaminants.

Figure 6:  6 Water-tank feedthrough flange. 6 Feedthrough bung used to allow
fibre to pass into the water tank while keeping the water tank light-tight and
air-tight.

At each of the 30 injection points on the OD PMT array, the duplex optical
fibre will be held in place using a Fibre Support Structure (FSS). The purpose
of the FSS is to ensure each fibre is aligned correctly as it points in
towards the scintillator tanks and to provide a consistent 50 mm curvature of
the duplex optical fibre. Each FSS consists of 26 parts: seven machined PTFE
structural components, seven passivated bolts, nine passivated inserts, and
two PTFE inserts. A photograph of a FSS with the lid open can be seen in Fig.
7. The support structures which hold the fibres under the acrylic tanks
consist of a custom made stainless steel bracket, which attaches to the
acrylic tank platforms, and a PTFE fibre holder which directs the light
through holes in the acrylic tank platform.

Figure 7: A Fibre Support Structure (FSS) with the lid open. The key
components of the FSS are labelled.

Correct alignment of the fibres is of vital importance to allow a realistic
comparison between simulations and calibration data. It is for this reason
that the FSS are designed to allow setting the fibre direction precisely
during installation. The inward facing fibres should point perpendicularly to
the water tank wall as well as directing the centre of the beam towards the
central axis of the TPC (equivalent to the origin of the water tank coordinate
system). To achieve this, two red self-levelling cross laser beams will be
positioned equidistantly on either side of the injection point on a custom-
made jig with each of the lasers crossing on the face of the liquid
scintillator tanks. The FSSs are then aligned to this point by attaching a
rear-mounted green laser that points through the fibre injection point and
aligning the green laser to the red crosses on the scintillator tank. Once
aligned the FSS is tightened to avoid future movement leading to misalignment
when the fibre is installed.

##  4 Radioactivity Measurements

After the manufacturing of the FSSs was completed, a random selection of six
FSSs, as well as the complete set of water tank fibres, was sent to the Boulby
Underground Laboratory for radioactivity screening. The results from the
screening are shown in Tab. 3 along with the estimated activities of the OD
components. The activity for the FSSs are calculated by averaging the
radioactivity screening results and multiplying by the number of FSSs present
in the system.

Table 3: Results from the screening of the OCS components at Boulby Underground Laboratory (shown in bold) in comparison with activities of the radionuclides present in the other OD components as described in the LZ radioactivity and cleanliness control programs article [15], where a comprehensive list of the estimated activities for the rest of the detector can be found. OD Components | Mass (kg) | Activity (mBq/kg) |  Overall Contribution (%)
---|---|---|---
|  |  ${\rm{}^{238}U}_{e}$ | ${\rm{}^{238}U}_{l}$ | ${\rm{}^{232}Th}_{e}$ | ${\rm{}^{232}Th}_{l}$ | ${\rm{}^{60}Co}$ | ${\rm{}^{40}K}$ |
OD Tanks | $3200$ | 0.16 | 0.39 | 0.02 | 0.06 | 0.04 | 5.36 |  3.77
Liquid Scintillator | $17600$ | 0.01 | 0.01 | 0.01 | 0.01 | 0.00 | 0.00 |  0.14
OD PMTs | 205 | 570 | 470 | 395 | 388 | 0.00 | 534 |  94.3
OD PMT Supports | 770 | 1.20 | 0.27 | 0.33 | 0.49 | 1.60 | 0.40 |  0.59
Fibre Support Structures | 10.5 | 31 | 3.0 | 6.0 | 3.0 | 0.5 | 330 |  0.77
Optical Fibres (inc. couplers) | 5.53 | 91.5 | 8 | 10.5 | 4.5 | 1.75 | 302.5 |  0.45

These values show that the OCS contributes 1.2% of the total activity in the
OD and therefore satisfies the requirement on radiopurity (see Tab. 1). The
OCS components are also further from the cryostat than the scintillator tanks
and therefore have much less of an impact on the number of events in the TPC.
The OCS components were thoroughly cleaned before shipping as well as before
installation to reduce background contamination further. It is also worth
noting that the optical fibres were screened with the metal SMA connector
included; these sit outside of the water tank in the experiment so they will
not contribute to the detected background events.

##  5 Full System Performance

After shipping the OCS to SURF and installing the VME crate in the electronics
racks underground, a re-calibration process was carried out to validate the
performance of the system. This on-site testing used the same test stand as
used previously at Liverpool and provided a final re-calibration for each
channel as well as a cross check with the pre-shipment testing. A full system
test consisted of going through each channel one by one, and scanning over the
range of injectable photons while taking measurements with a power meter
(Thorlab PM100USB) and the photomultiplier tube (Hamamatsu H10721)[16]. The
power meter output gives the number of photons detected and the
photomultiplier tube gives the measured pulse width. Each measurement taken
consists of recording the power meter once and recording $4000$ samples of
photomultiplier tube pulses and LED trigger widths which are digitised by a
DRS4 Evaluation Board [17]. This measurement is repeated three times at each
setting. The $4000$ samples taken at each measurement are plotted on a
histogram and a Gaussian fit is used to determine the mean value and spread of
the light intensity for each set point. One of the important parameters
determined from the measurements on each channel is the LED trigger pulse
width required to produce a certain number of detected photons. The parameters
from this fit can then be used as calibration constants for each channel. A
schematic of the test stand used for the full system test is shown in Fig. 8.
A photograph of the installed Optical Calibration System can be seen in Fig.
8.

Figure 8: OCS Test Stand: 6 An overview of the test stand used at SURF showing
the connections made from the OCC to the Thorlab PM100USB Power Meter, PMT,
DRS4, Arduino Uno and data taking computer [17] [18]. 8 A photograph of the
installed Optical Calibration System, showing the VME crate (top) housing the
Optical Calibration Cards, each connected by USB to the Optical Calibration
Controller (on the shelf).

###  5.1 LED Trigger Width

The trigger sent to the LED has $504$ different indexable widths, which
corresponds to a trigger pulse width of $20$–$60$ ns. This determines the
light output, ranging from $700$–$700000$ photons per pulse, while the ability
to pulse $1000000$ photons globally is achieved by pulsing multiple channels
simultaneously. The FPGA can change the trigger pulse width with a minimum
step of $40$ ps, which is critical to having precise control over the number
of photons produced. The trigger width (TW) required for a given number of
photons can be calculated using Eq. 1, where $A$, $B$, $C$ and $D$ are channel
specific calibration parameters and $N_{ph}$ corresponds to the number of
photons which will be injected. This equation was determined empirically.

$\begin{split}TW&=A+e^{\lambda},\\\ \lambda&=Bx+Cx^{2}+Dx^{3},\\\
x&=\text{ln}(N_{ph})\end{split}$ (1)

An example of the calibration curve is shown in Fig. 9.

Figure 9: LED trigger width versus the natural logarithm of number of photons
per pulse. Equation 1 relates these two variables. Each channel is fitted with
this function and has a set of parameters for $A$, $B$, $C$ and $D$ which are
specific to that channel.

###  5.2 PMT Pulse Width

The channel pulse rate was selected to be $4$ kHz for operation in LZ. However
the DRS4 Evaluation Board, which is used to readout the waveforms of the
electrical trigger and PMT pulses, can digitise only $300$ Hz with a time
resolution of $0.2$ ns. At each setting the collected data is normalised to
the number of photons measured by the power meter and analysed to determine
pulse width by using the full width at half the maximum peak height. The
number of photons detected by the power meter is plotted against the PMT pulse
width in Fig. 10. The system satisfies the pulse width requirement where the
width of the light pulse must be less than $20$ ns. Fulfilment of this
requirement is necessary as the width of the light pulse injected by the OCS
must be similar to the width of the light pulse produced in the GdLS or water
after being processed by the pulse shaper.

Figure 10: Dependence of the number of photons per pulse on the pulse width
measured by the test stand PMT. This plot shows that the system meets the
pulse width requirement set by the experiment.

###  5.3 Stability

Optical calibration and monitoring requires precise knowledge of the numbers
of injected photons, especially for measurements related to light collection
efficiency. These requirements are reflected in Tab. 1 as pulse-to-pulse
variations and variations between calibrations. It is known that electronic
components have a temperature dependence which lead to variations in light
output from LEDs. Though an effort has been made to use precision components
and compensate for these temperature dependencies some residual effects
remain. The OCCs have temperature sensors which will allow to study these
effects further and implement corrections. Ultimately, light output monitoring
has been implemented by integrating over many pulses (photo-diode per channel)
and pulse-by-pulse (monitoring PMT for all channels) methods. The pulse-by-
pulse resolution at the light intensities corresponding to 150 keV energy
depositions in the OD is shown in Fig. 11. The PMT contributes to the width of
this distribution; therefore the actual pulse-to-pulse variation is smaller
than 10%, and it improves at higher light intensities.

Figure 11: Distribution of numbers of injected photons per pulse at the level
corresponding to 150 keV energy deposition in the OD. The resolution 10%,
which is not corrected for the spread due to the PMT measurement, shows that
the system meets the pulse width requirement set by the experiment. The
resolution is better for higher light intensities.

The variations of average numbers of injected photons between calibrations
will be monitored and corrected for using the photo-diode board and monitoring
PMT. The YAP:Ce pulser provides continuous gain calibration for the monitoring
PMT. Without optical coupling compound, it produced a peak corresponding to
$\sim$2000 photons with a resolution of 8%. Each OCS calibration will be
preceded by a $\sim$2-minute gain calibration of the monitoring PMT allowing
to reach 2% precision (10 minutes are required for the 1% precision).

###  5.4 Validation

To validate that each channel could hit key calibration points a routine was
carried out to see how accurately the channels could hit these photon levels.
As described previously, there are certain ranges of TWs reachable for each
channel. This test shows if the key calibration points falls within an
achievable ranges on each channel. The chosen calibration points were $700$
photons, $1000$ photons, $20000$ photons and $50000$ photons. Each of the
calibration points were entered into Eq. 1 along with the parameters for that
channel’s calibration curve. The resulting trigger width was checked against
the reachable range of trigger widths for that channel, taking the closest
achievable TW if it was not in an allowable range. The resulting number of
photons from this achievable TW was plotted on a histogram. Figure 12 shows
the results of this validation step where all $40$ channels injected light
with an accuracy of at least $10\%$ at each of the set points. These results
indicate the precision of the OCS internal calibration, which is used to
determine the settings necessary to achieve the required number of photons at
each set point.

Figure 12: Distribution of how accurately each channel of the OCS can
specifically inject: 12 $700$ photons per pulse; 12 $1000$ photons per pulse;
12 $20000$ photons per pulse; and, 12 $50000$ photons per pulse. This shows
the majority of the 40 channels can achieve the desired TW to reach these
arbitrary set points (or within 10%) with the current calibration curves.

##  6 Optical Calibration Procedure

An optical calibration of the OD will be initiated by the LZ Run Control
sending the required calibration routine to the Slow Control system. The Slow
Control is a SCADA platform which utilises Ignition [3] and communicates with
LZ’s subsystems via the MODBUS protocol [3]. The Slow Control system will
store and manage the calibration constants needed for the OCS in a MySQL
database. The OCS control commands will pass from the Slow Control to the OCS
controller which manages the communication with the master OCC. The OCS
controller will also measure OCC temperatures and monitor photo-diode
response.

The monitoring PMT will be connected to the LZ data acquisition system. An
analysis of OD-PMT data against the monitoring PMT data will be used to ensure
that the OD-PMTs are correctly calibrated and identify any drift in
performance. A schematic of the OCS communication with the LZ Run and Slow
Control can be seen in Fig. 13.

Figure 13: A schematic showing the order in which an OD optical calibration
will be completed.

##  7 Conclusions

The ability to perform an accurate calibration of the OD is critical to ensure
the effectiveness of the veto system used by LZ. The OD is essential for
understanding and vetoing the signal-like neutron background as well as
numerous gammas originating inside and outside of LZ. The OCS discussed in
this paper has the capability to precisely inject a known number of photons
ranging from $700$ photons up to $50000$ photons covering the energy range of
the background signals. Light will be injected into the OD using an LED-driven
system with $30$ duplex optical fibres mounted within the array of PMTs and
five duplex optical fibres mounted beneath the tanks pointing upwards into
acrylic tanks. The injection points situated within the array will maintain
the calibration of PMTs and monitor their performance. The five bottom
injection points will monitor the light transmission of the acrylic tanks and
the degradation of the liquid scintillator over time. A correctly calibrated
OD ensures that the experiment will be able to meet its projected sensitivity.

##  8 Acknowledgements

We would like to thank our collaborators in the LZ dark matter search
experiment for their continuous support, in particular, H. Kraus from Oxford,
H. M. Araújo from Imperial College London, M.G.D. van der Grinten and R.
Preece from Rutherford Appleton Laboratory, H. Nelson from U.C. Santa Barbara
for useful discussions on the OCS design and development; our colleagues at
Brandeis University for tests of the OCS parts; A. Kaboth from Royal Holloway
University of London, R. Mannino from University of Wisconsin Madison, A.
Kamaha from University at Albany, B. Penning from University of Michigan, B.
López Paredes from Imperial College London for comments on this manuscript. We
wish to thank H. Lippincott (now at U.C. Santa Barbara) and E. Hahn from FNAL
for the fibre ice polishing. Thanks are due to M. Whitley, P. Cook and the
staff working in the University of Liverpool Detector Fabrication Facility and
Advanced Materials Laboratory for hardware manufacture. We acknowledge
contributions from University of Liverpool students T. Carter, L. Hawkins, A.
Hibbert, B. Philippou. We thank N. McCauley, J. Rose, A. Pritchard and L.
Anthony from the University of Liverpool Hyper Kamiokande group for their help
and design discussions. We acknowledge additional support from the Boulby
Underground Laboratory for organising and carrying out the radiation screening
required for the OCS components. We acknowledge the LZ collaboration for
providing the BACCARAT simulation package which allowed the simulations
mentioned in this article to be carried out. With thanks going to S. Shaw from
U.C. Santa Barbara for help writing the simulations for the OCS. The
assistance of Sanford Underground Research Facility and its personnel in
providing physical access and general logistical and technical support is
acknowledged. This work was supported by the U.K. Science & Technology
Facilities Council (STFC) under award numbers ST/S000879/1 and ST/M003639/1,
and by PhD studentships ST/N504142/1 (AB), ST/R504920/1 (AB) and ST/S505547/1
(EF).

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