# Production and validation of scintillating structural components from low-
background Poly(ethylene naphthalate)

Y. Efremenko  M. Febbraro  F. Fischer  M. Guitart Corominas  K. Gusev  B.
Hackett  C. Hayward  R. Hodák  P. Krause  B. Majorovits  L. Manzanillas  D.
Muenstermann  M. Pohl  R. Rouhana  D. Radford  E. Rukhadze  N. Rumyantseva  I.
Schilling  S. Schoenert  O. Schulz  M. Schwarz  I. Štekl  M. Stommel  J.
Weingarten  E. Hoppe  I. Arnquist  K. Hobbs  A. French  M. diVacri  M.
Laubenstein  G. Zuzel

###### Abstract

Poly Ethylene Naphthalate (PEN) is an industrial polymer plastic which is
investigated as a low background, transparent, scintillating and wavelength
shifting structural material. PEN scintillates in the blue region and has
excellent mechanical properties both at room and cryogenic temperatures. Thus,
it is an ideal candidate for active structural components in experiments for
the search of rare events like neutrinoless double-beta decay or dark matter
recoils. Such optically active structures improve the identification and
rejection efficiency of backgrounds events, like this improving the
sensitivity of experiments. This paper reports on the production of radiopure
and transparent PEN plates

These structures can be used to mount germanium detectors operating in
cryogenic liquids (LAr, LN). Thus, as first application PEN holders will be
used to mount the Ge detectors in the Legend-$200$ experiment. The whole
process from cleaning the raw material to testing the PEN active components
under final operational conditions is reported.

##  1 Introduction

The next generation of low-background experiments is presently being prepared.
These ton-scale experiments demand quasi background-free environments in their
region of interest to achieve the scientific goals. For this, new innovations
in addition to the use of ultra-pure materials are required. Active shields
have been used in experimental setups to actively identify and veto background
events. This approach has proven to be very efficient as demonstrated by the
Gerda experiment in the search for neutrinoless double beta decay
($0\nu\beta\beta$) [1]. In Gerda, one of the predecessors of Legend-$200$ [2],
the Germanium (Ge) detectors were operated in liquid argon (LAr) [3]. The LAr
not only serves to cool down the detectors and as ultra-pure passive shield,
but also acts as an active veto system against external radiation thanks to
its scintillation properties. The scintillation photons from LAr have a
wavelength of $\approx\,127$ nm (vacuum ultraviolet region) [4] which is not
detectable by most standard photo sensors. In the case of Gerda, a curtain of
wavelength-shifting (WLS) optical fibers coated with tetraphenyl butadienne
(TPB) was placed around the detector strings. These fibers collect the photons
from LAr scintillation, shift their wavelength and guide them towards an array
of silicon photomultipliers (SiPMs) [5]. This configuration allows the
placement of photon detectors and their front-end electronics to be further
away from the Ge detectors, decreasing the backgrounds induced by these
components [3].

The efficiency of detecting scintillation will suffer from absorption in
optically inactive structural materials like detectors holders and other
support structures. In Gerda, high purity silicon plates were used as holder
material. These were partially between the Ge detectors. These prevent an
efficient detection of LAr scintillation light from background events
originating between the detectors.

Research is presently ongoing in developing high purity poly(ethylene
$2,6$-naphthalate) or PEN $[$C14H10O${}_{4}]_{\text{n}}$, as optically active
structural material. This research aims to replace optically inactive
materials like copper, silicon and PTFE with PEN, improving the background
detection efficiency. PEN is known to scintillate in the deep blue region [6].
In addition, PEN also can shift the vacuum ultraviolet light to easily
detectable blue light with a high efficiency [7, 8]. Furthermore, it was
recently shown that custom-made PEN structures have the necessary stability
and resilience to be used as a holding structure for detectors [9, 10]. In
this context, the principles of Gerda have been adopted for Legend-$200$,
including the LAr veto. In addition, active holding plates made from PEN will
be used. This complements the active veto system by minimizing the absorption
of photons on optically inactive materials and increasing the general veto
efficiency due to the WLS and scintillation properties of PEN.

Section 2 describes a methodology for producing active and low background
components out of commercial available PEN granulate using compression
injection molding technology. Section 3 reports the achieved radiopurity of
this PEN production for usage in the Legend-$200$ experiment. In Section 4,
preliminary results of the optical characterization are reported, while
Section 5 describes the different tests performed in close to final operation
conditions to validate the usage of PEN in the Legend-$200$ experiment.
Finally, conclusions and future prospects are presented in Section 6.

##  2 Low-background production of PEN structures

PEN raw material is commercially available in the form of white granules from
Teijin-DuPont [11]. The material used for this Legend-$200$ production has the
designation TN-$8065$ SC. The untreated PEN granulate had been previously
screened for radio-impurities. Apart from a Bq/kg level contamination with
40K, low contamination levels at the $0.2$ mBq/kg with 238U and 232Th were
measured (see Table 1) [9]. The high level of the 40K signal was attributed to
surface contamination. To mitigate these, and other possible surface
contamination, an extensive cleaning procedure was adopted. This procedure was
applied as the first step of the production process.

After cleaning, the PEN granulate was used to produce discs by a specially
optimized and controlled injection compression molding technique. Finally, the
injection molded discs were used to machine the detector holders under a
laminar flow hood. The detector holder geometry was adapted to the detector
size, while minimizing the total weight. This section describes the details of
the whole production chain.

###  2.1 Cleaning procedure

All parts that can come in contact with the granulate during the production
process were cleaned with the following procedure: First, they were rinsed off
in a flow box located in a cleanroom with $18$ MOhm water (DI water) and then
washed off with class $100$ cleanroom cloths dipped in VLSI grade
$2$-propanol. The parts were then etched in a nitric acid solution ($2\,\%$
HNO3 optima grade, $98\,\%$ DI water) for $24$ h and rinsed off again with DI
water of the same quality afterwards. Finally, the containers and sieves were
cleaned separately for $15$ minutes in an ultrasonic bath filled with DI
water.

The PEN granulate is supplied by manufacturers in large plastic containers,
containing $25$ kg each. Cleaning of the PEN granulate was conducted in
batches of $\approx 1$ kg. Each batch underwent the following cleaning steps:

  * 1.

Fill PEN granulate into cleaned stainless sieve

  * 2.

Rinse with $\approx 2$ litres of DI water $18$ MOhm inside flow box for two
minutes

  * 3.

The sieve with PEN is inserted into a PTFE container filled with $2.5$ litres
VLSI grade $2$-propanol

  * 4.

PTFE container is placed inside ultrasonic bath containing $\approx 50$ litres
of DI water for 15 minutes

  * 5.

Rinse PEN with DI water for about one minute

  * 6.

Repeat steps $3$.-$6$. in separate ultrasound bath

  * 7.

The sieve with PEN is inserted into PTFE container filled with $18$ MOhm DI
water

  * 8.

PTFE container is placed inside ultrasonic bath containing $\approx 50$ litres
of DI water for 15 minutes

  * 9.

Rinse PEN with DI water for about one minute

Figure 1: Left: PEN pellets in stainless-steel sieve during cleaning process.
The PTFE container is placed in a basin filled with DI water for ultra sound
cleaning. Right: Injection compression molding machine used for production of
PEN discs. All open parts of the machine are located inside a clean room.

Finally, the granules were collected in a larger sieve in a separate basin.
Most of the residual DI water is removed here.

The cleaned granulate was dried in two stages. In the first, $\approx 10$ kg
PEN granulate was filled into a sieve and baked out in a vacuum tank. An oil-
free roughing pump was used. The tank was heated in order to accelerate the
evaporation of the water residues using heating bands attached to the outside
tank. The temperature was set to $60\,^{\circ}$C and after $8$ hours, the pre-
dried granulate was sealed in class $100$ plastic bags. These were then packed
for transportation into another plastic bag of the same cleanliness.

###  2.2 Production of scintillator-grade PEN discs

Injection compression molding was used to transform the cleaned PEN granulate
to discs with a diameter of up to $35$ cm and $1.5$ mm thickness. The machine
used here is an Arburg Allrounder $370$S (ARBURG GmbH, Lossburg, Germany)
where the barrel and screw were replaced beforehand with new ones. The mold
consisted of two new stainless steel plates (type $316$) with a mirror-like
surface finishing (roughness $<1$ $\mu$m) which in industry is referred to as
$1$P/$2$P or No.$8$ (ASTM A$480$ /A$480$M).

All containers and areas of the molding machine that came into direct contact
with the granulate were cleaned using a nitric acid solution as described in
Section 2.1. The machine itself was located in a clean room (Figure 1, Right)
and was cleaned with $2$-propanol (VLSI grade) and Micro Cleaning Solution
(Micro-$90^{\circledR}$) [12]. The machine’s storage tank allowed to store the
PEN pellets in nitrogen atmosphere. This container can be heated to remove the
remaining moisture on the surface and inside the PEN pellets. In order to
expel any radon from the granulate, nitrogen boiled off from a liquid nitrogen
dewar was used to flush the storage volume while heating it for at least $6$
hours to $80^{\circ}$C until the proper humidity was reached.

The optimal settings for the plasticizer unit of the molding machine were
determined empirically. In first trials, the values recommended by the
manufacturer were used and then optimized for the highest possible
transparency of the outcome. For each produced PEN disc, approximately $125$
cm3 ($170$ g) of material was used. The injection time was set to $2$ s at a
volume rate of $100$ cm3/s. The optimum temperature of the plasticizer unit of
$290^{\circ}$C with a controlled temperature of the compression discs of
$30^{\circ}$C was also established empirically. The compression takes place in
two phases. At the beginning the steel plates of the mold are $5.5$ mm apart
and are closing at a speed of $3$ mm/s until the distance between the plates
reaches $2$ mm. The speed is then reduced to $2$ mm/s until the target
compression pressure of $700$ kN is reached, corresponding to a final disc
thickness of $1.5$ mm. After a cooling time of $40$ s the discs could be
removed manually without the need to use tools. Some differences in the
quality of the discs were notable from optical defects such as local
crystallization and air inclusions. These defects occurred more frequently
when the molding parameters were not fully optimized at the beginning of a
production series.

After extracting the disc from the mold (see Figure 2, Left), it was locked in
a CNC (Computerized Numerical Control) milling machine in the clean room
placed nearby the molding machine. The support surface of this CNC machine
consists of an acid etched ($2\,\%$ HNO3 optima grade, $98\,\%$ DI water, $24$
h) Polyethylene Terephthalate (PET) plate with a vacuum pump being used to
hold the disc down, like this avoiding clamps. The inside of the CNC machine’s
chamber was illuminated with UV light to reduce the static charge of the
flakes. Some of the discs were then milled down to a uniform diameter and a
hole was cut in the middle of the plate where the sprue111The sprue refers to
the hardened PEN portion that still protruded into the nozzle. was located.
This process was performed in order to match the geometry of the chamber of
the germanium screening setups [13, 14]. After machining, the PEN discs were
no longer touched with anything else than a pair of acid etched pliers with
small contact surfaces inside the center hole of the discs.

The individual PEN discs still in the clean room tent, were each sealed in
class $100$ plastic bags for transportation to the Modane underground
laboratory (LSM, France) and to the Laboratori Nazionali del Gran Sasso (LNGS,
Italy), where screening took place. For documentation, the individual bags
were numbered and the time of sealing recorded. Transportation happened in
large plastic boxes, which were sealed with a radon-impermeable film.

###  2.3 Design and processing of the Legend-200 holders

Within Legend-200 a variety of detector geometries will be used, which
requires different types of support structures. In particular, it was found
that by using three different sizes of PEN holders, all Ge detectors used in
Legend-$200$ can be accommodated. In the following, these holder geometries
shown in Figure 3 (Left) will be referred to as small, medium and large size.
In addition, a special PEN holder was designed to mount some detectors that
required two PEN holders, one at the bottom and one at the top. The design of
this special top holder used to place the high voltage is shown in Figure 3
(Right). In Legend-200, Ge detectors are mounted in so-called strings where
they hang one above the other. Each detector is supported by a holding
structure as can be seen in Figure 4. These support structures are composed of
a base holder made of PEN (previously Si), and other elements made of copper,
ULTEM and PTFE [2].

Figure 2: Left: Picture taken while removing a molded PEN disc from the CNC
milling machine inside the clean room. Right: Finished PEN holder for the
Legend-$200$ experiment machined from a bigger disc (Left). The edges and
surfaces are without any additional treatment.

In order to minimize the mass of the PEN holders and thus to minimize
background, mechanical simulations were performed. To this end, a tension load
of up to $20$ N in each string of detectors with a lateral acceleration of the
assembly up to $5$ cm/s2 were assumed. These simulations coupled with a
topological optimization using the software Abaqus $2017$ showed the areas in
the designed holders that have minimum stress values and thus could be
removed. The results allowed an average mass reduction of $28$%. Thus, the
estimated mass of the holders is $5.3$ g, $6.3$ g, and $7.1$ g corresponding
to the small, medium, and large size, respectively.

Final machining of the optimized Legend-$200$ holders was completed once the
screening results were available. Here, the milling machine was setup in a
laminar flow box. The operators wore clean room clothing and all parts of the
machine were cleaned using $2$-propanol (VLSI grade). In addition, no other
machine was running in the room during the time span of the milling process to
avoid polluting the air with dust or flakes. MKD/mono-crystalline diamond
milling cutters for high-gloss mirror finish from Karnasch professional tools
were used for the machining. These diamond tools were cleaned following the
same procedure as described in Section 2.1. The hardness of the diamond
combined with the cleaning protocol guaranteed a minimum risk of introducing
radio-impurities during the production.

During the milling process, the PEN disc was held down on a new cleaned poly-
carbonate base plate using double-sided tape. Here, the taped parts were then
cut away and tape residuals on the base plate were removed after processing
each disc. The flakes and dust produced during machining were blown away using
a nitrogen line connected directly to a $150$ l liquid nitrogen dewar. The
same nitrogen line was used for cooling the milling cutter during the milling
process. The processing of each disc took about $30$ minutes with
$30000-33000$ rpm for the drill and a feed between $480$ and $780$ mm/min. One
disc could fit up to $4$ medium/large or $8$ small size holders for
Legend-$200$. A finished holder without any additional treatment can be seen
in Figure 2 (right). The edges are transparent and almost no burr is visible.
All holders were individually packed and then sent back for another cleaning
cycle as described in Section 2.1.

Figure 3: Left: PEN holders geometries required to mount small (Left), medium
(Center) and large (Right) Ge detectors for Legend-$200$. Right: Special PEN
holder required to place the high voltage on top of some Ge detectors

##  3 Radiopurity of produced PEN discs and finished holders

The physics goal of the Legend-200 experiment, is to achieve a half-life
sensitivity of $10^{27}$ yr for the $0\nu\beta\beta$-decay in 76Ge within 5
years of data taking. To achieve this goal a background index of less than
$10^{-4}$ cts$/(\text{keV}\cdot\text{kg}\cdot\text{yr})$ is required. Thus, an
improvement of one order of magnitude with respect to GERDA background levels
(< $10^{-3}$ cts$/(\text{keV}\cdot\text{kg}\cdot\text{yr})$) is targeted [15].
In order to accomplish this background index, a maximum radioimpurity of $1$
$\mu$Bq/holder can be tolerated [16, 17, 18]. To demonstrate that the
radiopurity achieved in this PEN production meets the requirement for usage in
the Legend-$200$ experiment, independent radiopurity measurements were carried
out: HPGe screening, ICP-MS and radon emanation measurements were performed.
All measurements made on the cleaned and processed material confirm that the
radiopurity is high enough for Legend-200. Moreover, once WLS and
scintillation is taking into account, first estimations show that the achieved
background levels could satisfy Legend-1000 requirements [2]. Legend-1000 aims
to reach a half-life sensitivity beyond $10^{28}$ yr, which needs a background
index of less than $10^{-5}$ cts$/(\text{keV}\cdot\text{kg}\cdot\text{yr})$.

###  3.1 HPGe screening

Different samples of the molded plates were measured using high-purity
germanium (HPGe) detectors. One measurement was performed for about $68$ days
at the LNGS with the GeMPI$4$ setup [13], using $14.315$ kg of PEN. Another
measurement was done with the OBELIX detector at the LSM [14], using $5.231$
kg of PEN. This screening took about $79$ days.

| Raw TN-$8065$S | Discs | Discs | L200 holders
---|---|---|---|---
GeMPI$4$ at LNGS | GeMPI$4$ at LNGS | OBELIX at LSM | GeMPI$3$ at LNGS
- |  |  $14.315$ kg
---
$68$ days
|  $5.231$ kg
---
$79$ days
|  $1.07$ kg
---
$68$ days
228Ra |  $<0.15$ mBq/kg | ($92\pm 25$) $\mu$Bq/kg | ($107\pm 38$) $\mu$Bq/kg | $<0.46$ mBq/kg
228Th | ($0.23\pm 0.05$) mBq/kg | ($32\pm 16$) $\mu$Bq/kg | ($67\pm 18$) $\mu$Bq/kg | $<0.48$ mBq/kg
226Ra | ($0.25\pm 0.05$) mBq/kg | ($60\pm 15$) $\mu$Bq/kg | ($76\pm 22$) $\mu$Bq/kg | $<0.36$ mBq/kg
234Th |  $<11$ mBq/kg |  $<1.9$ mBq/kg | - | $<5.8$ mBq/kg
234Pa |  $<3.4$ mBq/kg |  $<1.7$ mBq/kg | - | $<7.0$ mBq/kg
235U |  $<0.066$ mBq/kg |  $<56$ $\mu$Bq/kg | - | $<0.22$ mBq/kg
40K | ($1600\pm 400$) mBq/kg |  $<0.24$ mBq/kg |  $<0.567$ mBq/kg | $<4.1$ mBq/kg
137Cs |  $<0.057$ mBq/kg |  $<0.15$ $\mu$Bq/kg | - | $<0.091$ mBq/kg
Table 1: Radiopurity results as achieved for molded PEN components using
commercially available PEN granulate, subsequently cleaned. Two sets of molded
PEN samples were measured at LNGS and LSM underground laboratories.
Furthermore, measurements of uncleaned granulate [9] and the finished holders
for Legend-$200$ (L$200$) are listed. Not all listed radionuclides were
analyzed in the OBELIX setup. Limits are given with $90\%$ confidence level.

The results of all HPGe screening measurements are reported in Table 1 for raw
PEN granulate, for two independent measurements of the injection molded discs
and for the final PEN holders. Clearly, the radio-impurities measured with the
granulate delivered by DuPont was significantly reduced thanks to the cleaning
procedure. Notably, it could be confirmed that the high 40K signal was due to
surface contamination. More important, the radiopurity achieved during this
first production run satisfy Legend-$200$ requirements.

It should be noted here that the mass of the measured final holders ($1.07$
kg) is much lower compared to the original big discs which were designed to
fill the chamber. Hence, no comparable exposure could be achieved and only
limits were obtained in this final measurement. These results confirm
nevertheless that no noticeable contamination has been introduced during the
post-production.

###  3.2 ICP-MS

Two finished holder samples were also sent for inductively coupled plasma mass
spectrometry (ICP-MS) at the Pacific Northwest National Laboratory (PNNL). The
ICP-MS method allows to reach a $\mu$Bq sensitivity on 232Th and 238U. The two
holders were subsampled in triplicate using new and cleaned stainless-steel
scissors. The subsamples went through a cleaning procedure including two
subsequent leaching in $5$% optima grade HNO3 for two hours. The leachates
were transferred into validated PFA Savillex vials and directly analyzed on an
Agilent $8900$ ICP-MS, to ensure any surface contamination from transportation
and subsampling had been properly removed. An external calibration standard
was used for quantitation of 232Th and 238U. After the contamination in the
leachates was measured to be low enough, within instrumental background, the
subsamples were placed into validated microwave vessels for digestion. Table 2
reports the measured values for each leach. It was found that in general the
level of 232Th and 238U decreased from the first leach to the second leach.
These levels could be representative of the remaining contamination from the
parts fabrication, handling and/or from the subsampling. It should be noted
here, that for the ICP-MS measurement of the leachates the actual sample mass
in the leach is not known. For the results, the total mass of the sample was
used. The values given can therefore be used to understand the trend but do
not accurately quantify the amount of 232Th and 238U.

Sample |  |  232Th |  238U
---|---|---|---
Weight [g] |  | ppt |  $\pm$ inst | ppt |  $\pm$ inst
| PEN #$1$
---
$0.7907$
| leach #$1$
---
leach #$2$
| $0.071$
---
$0.004$
| $0.013$
---
$0.005$
| $0.09$
---
$0.0068$
| $0.04$
---
$0.0017$
| PEN #$2$
---
$0.7136$
| leach #$1$
---
leach #$2$
| $0.10$
---
$0.0045$
| $0.03$
---
$0.0015$
| $0.031$
---
$0.007$
| $0.011$
---
$0.002$
Table 2: Results of the ICP-MS measurement of the leachates for 232Th and
238U, normalized to the mass of the sample. Absolute detection limits were
calculated as $3$ times the standard deviation of the process blanks and the
instrumental precision for a given measurement is given as $\pm$ inst.

A known amount of non-natural 229Th and 233U was then spiked into each
replicate and process blank. Subsamples were digested at $250^{\circ}$C in
concentrated optima grade HNO3 and then transferred to validated Savillex
vials. The subsamples were dried down on a hotplate, reconstituted in $2$%
optima grade HNO3, and analyzed on an Agilent $8900$ ICP-MS. Quantitation of
232Th and 238U was performed using isotope dilution mass spectrometric
methods. Table 3 reports the limits obtained from the PEN subsamples. Reported
results are given in ppt for 232Th and 238U. Absolute detection limits were
calculated as $3$ times the standard deviation of the process blanks. All
measurements for 232Th and 238U were below relative detection limits, which
were calculated by dividing the absolute detection limits by the subsample
masses. These results combined with the gamma spectroscopy measurements
confirm that an excellent radiopurity has been achieved. In addition, these
ICP-MS results suggest that remaining surface contamination can be removed and
that values reported in Table 1 should be considered as upper limits for the
bulk contamination.

|  |  |  232Th |  238U
---|---|---|---|---
Description | Replicate | Sample wt [g] | ppt | ppt
PEN #$1$ |  | $1$
---
$2$
$3$
| $0.2745$
---
$0.2481$
$0.2655$
| $<1.13$
---
$<1.30$
$<1.22$
| $<1.98$
---
$<2.08$
$<1.94$
PEN #$2$ |  | $1$
---
$2$
$3$
| $0.2784$
---
$0.1408$
$0.2940$
| $<1.02$
---
$<2.02$
$<0.97$
| $<1.81$
---
$<3.57$
$<1.71$
Table 3: Results of the ICP-MS measurement of the the PEN subsamples for 232Th
and 238U. Absolute detection limits were calculated as 3 times the standard
deviation of the process blanks.

###  3.3 Radon emanation

Complementary radiopurity information can be obtained by means of high
sensitivity Radon emanation measurement techniques [19, 20]. Five finished PEN
holders were screened using this method at the Institute of Physics of the
Jagiellonian University in Krakow. Given the small size of the samples, only
limits of less than 12 $\mu$Bq/holder and less than 16 $\mu$Bq/holder for
222Rn and 220Rn respectively were derived. These limits were obtained from two
measurements carried out using the five holders placed into a 50 liter
chamber.

##  4 Optical properties

Besides the excellent mechanical properties of PEN at cryogenic temperatures
[9] and the very good radiopurity achieved during this first production for
Legend-200, the wavelength shifting capabilities and scintillation properties
provide major advantages. Thus, the optical properties are very important and,
among others, include the surface quality, light yield and attenuation length.
Custom-made scintillators as described in this paper, could have different
properties depending on various settings in the production. In order to
classify and compare production batches, two test stations were designed. In
addition, the surface of the molded PEN discs has been characterized using
different methods. These results showed that high quality surfaces have been
obtained.

Light output222The light output was obtained exciting PEN and reference samples using electrons from a 207Bi source. The light output was then compared to the well known reference samples of type EJ-200 from Eljen Technology of the same dimensions. The quoted light yield of EJ-200 is 10000$\pm$200 photons per MeV. Thus two stacked square samples of 30$\times$30$\times$1.7 mm3 were used for both scintillators. Two samples were necessary, in order to completely absorb the high energy electrons from the 207Bi source ($\sim$ 1 MeV). These samples were coupled to five 1 inch photomultiplier tubes (PMTs) using optical grease (EJ-550). Only one 30$\times$30 mm2 side was free to place the radioactive source. | 30$\pm$2% of EJ-200
---|---
Light yield333The light yield is estimated using Geant4 simulations since a direct comparison between PEN and EJ-200 provides only a lower limit of 3000 photons per MeV. These simulations are needed to take into account attenuation effects. The bulk absorption of the EJ-200 reference samples is of the order of 4 meters, while for PEN it is only a few centimeters. Therefore, attenuation effects are expected to be important even for small PEN samples while for EJ-200 are negligible. | 5500$\pm$550 photons/MeVe-
Time constant | 25.3 $\pm$0.2 ns
Bulk absorption |  $\sim$60 mm at 450 nm
Emission spectrum maximum | 440 $\pm$ 5 nm
Table 4: Main optical and scintillation properties of Legend-200 PEN material.
[21, 10]

A detailed description of the setups and the results of the optical properties
of this Legend-200 PEN production, as well as estimation of self-vetoing
capabilities of the PEN holders is under preparation in reference [21]. The
main optical properties are summarized in table 4.

##  5 PEN as detector holder for HPGe detectors

In order to qualify the PEN detector holders for usage in Legend-200, two
proof of concept tests have been performed. In both tests, Ge detectors were
mounted on PEN holding plates from the production run described in Section 2
and operated in LAr. The goal of both tests was to qualify the mounting
process and demonstrate the stability of the holders at low temperatures under
realistic loads and close to final operational conditions. Also the baseline
of the HPGe detectors mounted on PEN detector holders was monitored in order
to verify the required stability of the leakage current over longer time
spans. An increase of the baseline can be traced back to an induced leakage
current via the resistor in the front-end electronics.

Figure 4: Left: Bottom view of Ge detector string in the Gerda setup during
the Post-Gerda test. Detector in focus is mounted on PEN holder with front-end
electronics attached. Right: Lateral view of a string deployment test at LNGS
using PPC type detectors, requiring a top PEN holder to place the high voltage
connection.

The first test was performed in the underground SCARF setup at TU Munich,
Germany [22]. Two small detectors and the front end electronics were mounted
on PEN holders. As PEN is more flexible than previously used materials, the
parameters for the wire bonding process had to be adapted. For both detectors
the bonding worked as usual and no problems due to the flexibility or other
mechanical properties of PEN arose. Then the detectors were operated in SCARF
in LAr for 10 days. The measured fluctuations in the leakage current were of
the order of pA. The change within the measured time span is according to
expectations verifying that there is no effect of the PEN holders on the
leakage current of the Ge detectors.

The second test was started in February $2020$ during the Post-Gerda test
(PGT). During the PGT several detector strings were deployed using the Gerda
infrastructure in order to validate Legend-200 electronics and other
components. Around 40% of Ge detectors were mounted using PEN holders. This
allowed a direct comparison of detectors mounted on PEN holders to detectors
mounted with conventional Si holders as used previously in Gerda. A picture of
the lower part of a detector string during the PGT can be seen in Figure 4
(Left). The detector in focus is mounted on a PEN holder. Figure 4 (Right)
shows the lateral view of a string test deployment at LNGS using PPC
detectors. The longest run lasted about $27$ days. Figure 5 shows the mean
value of the baseline for two detectors for that time period. Here, two new
inverted-coaxial point-contact (ICPC) Ge detectors are compared, one of which
was mounted on Si (Ch$37$, left) and one on PEN (Ch$39$, right). For the
determination of the mean value of the baseline, the first $1000$ samples of
the waveforms were averaged. There was a comparable behavior and no
significant variations were observed in both data sets. There were also no
significant differences in the energy resolution. Thus, for the $2614.5$ keV
$\gamma$-line of 208Tl the achieved resolutions (full width at half maximum,
FWHM) for a detector mounted on a Si or PEN holder are $2.43$ keV and $2.61$
keV, respectively [2, 23]. These deviations are in line with expectations for
the type of detector used. Extrapolated to the $Q_{\beta\beta}$ of the
$0\nu\beta\beta$-decay in 76Ge (2039 keV), the detector mounted on a Si holder
shows an energy resolution of $2.17$ keV and the one mounted on a PEN holder
of $2.33$ keV.

Figure 5: Mean baseline over time of two ICPC Ge detectors operated during the
PGT. One detector was mounted on Si (left) and one on PEN (right).

##  6 Conclusions & Outlook

The first production series of low-background components made of PEN for
Legend-200 was successfully finished. The introduction of an intensive
cleaning cycle of the raw granulate removed most of the surface contaminants
found in previous measurements. Furthermore, it could be shown that remaining
impurities on the surface can be removed by leaching. The current components
thus have an activity of less than $1$ $\mu$Bq per holder, meeting the
Legend-200 requirements. In addition, R&D is ongoing exploring self-synthesis
of the PEN raw material, which could reduce even more the achieved
radiopurity.

Preliminary estimations suggest a light yield of about $5500$ photons per MeV
and a bulk absorption of around $60$ mm at $500$ nm. In addition, the
application of PEN as a low background component was also tested in two setups
under close to final operational conditions. In both cases, Ge detectors
mounted with PEN holders were operated in LAr for a period of time on the
order of months. No complications arose either in the installation of the
electronics or in operation. Leakage current in the Ge detectors was not
affected by PEN and it was found to be comparable with previous measurements.
Direct comparisons with Ge detectors during the post-Gerda tests showed no
differences between detectors mounted with Si or PEN holders. This period of
this time is a reasonable representation of the stability needed for a 10+
year experiment.

Legend-$200$ will be commissioned in the further course of $2021$. This will
be the first time that custom-made low-background PEN scintillating components
are used in a rare event search experiment.

## Acknowledgments

This work was supported by the Ministry of Industry and Trade of the Czech
Republic under the Contract Number FV30231. Research sponsored by the
Laboratory Directed Research and Development Program of Oak Ridge National
Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.
This material is based upon work supported by the U.S. Department of Energy,
Office of Science, Office of Nuclear Physics, under Award Number DE-
AC05-00OR22725. Felix Fischer and Luis Manzanillas are supported by the
Deutsche Forschungsgemeinschaft (SFB1258). Connor Hayward is supported by the
Excellence Cluster Universe. The authors would like to thank M. Laubenstein
(LNGS) G. Zuzel (Cracow) and E. Hoppe (PNNL) for their invaluable help by
delivering the screening results of the PEN samples.

## References

  * [1] M. Agostini et al.  Final Results of GERDA on the Search for Neutrinoless Double-$\beta$ Decay.  Phys. Rev. Lett., 125(25):252502, 2020.
  * [2] N. Abgrall et al.  The Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay: LEGEND-1000 Preconceptual Design Report.  7 2021.
  * [3] M. Agostini et al.  Upgrade for Phase II of the Gerda experiment.  Eur. Phys. J. C, 78(5):388, 2018.
  * [4] T. Heindl, T. Dandl, M. Hofmann, R. Krucken, L. Oberauer, W. Potzel, J. Wieser, and A. Ulrich.  The scintillation of liquid argon.  EPL, 91(6):62002, 2010.
  * [5] Jozsef Janicsko-Csathy, Hossein Aghaei Khozani, Xiang Liu, Bela Majorovits, and Allen Caldwell.  Development of an anti-Compton veto for HPGe detectors operated in liquid argon using Silicon Photo-Multipliers.  Nucl. Instrum. Meth. A, 654:225–232, 2011.
  * [6] H. Nakamura, Y. Shirakawa, S. Takahashi, and H. Shimizu.  Evidence of deep-blue photon emission at high efficiency by common plastic.  EPL, 95(2), 2011.
  * [7] M. Kuźniak, B. Broerman, T. Pollmann, and G. R. Araujo.  Polyethylene naphthalate film as a wavelength shifter in liquid argon detectors.  Eur. Phys. J. C, 79(4):291, 2019.
  * [8] M. G. Boulay et al.  Direct comparison of PEN and TPB wavelength shifters in a liquid argon detector.  6 2021.
  * [9] Y. Efremenko et al.  Use of poly(ethylene naphthalate) as a self-vetoing structural material.  JINST, 14(07):P07006, 2019.
  * [10] I. Abt et al.  Usage of PEN as self-vetoing structural material in low background experiments.  PoS, ICHEP2020:163, 2021.
  * [11] TEIJIN.  Polyethylene naphthalate (pen) resin teonex.  https://www.teijin.com/products/resin/teonex/index2.html, 8 2021\.
  * [12] Micro cleaning solution (micro-90).  https://www.emsdiasum.com/microscopy/technical/datasheet/71054.aspx.  Accessed: 2021-11-22.
  * [13] Matthias Laubenstein.  Screening of materials with high purity germanium detectors at the Laboratori Nazionali del Gran Sasso.  Int. J. Mod. Phys. A, 32(30):1743002, 2017.
  * [14] V. B. Brudanin et al.  Development of the ultra-low background HPGe spectrometer OBELIX at Modane underground laboratory.  JINST, 12(02):P02004, 2017.
  * [15] Frank T. Avignone and Steven R. Elliott.  The Search for Double Beta Decay With Germanium Detectors: Past, Present, and Future.  Front. in Phys., 7:6, 2019.
  * [16] F. Edzards.  The Future of Neutrinoless Double Beta Decay Searches with Germanium Detectors.  J. Phys. Conf. Ser., 1690(1):012180, 2020.
  * [17] Nina Burlac and Giuseppe Salamanna.  Present and Future of 0$\nu$2 $\beta$ Searches with Germanium.  Universe, 7(10):386, 2021.
  * [18] V. D’Andrea, N. Di Marco, M. B. Junker, M. Laubenstein, C. Macolino, M. Morella, F. Salamida, and C. Vignoli.  Neutrinoless Double Beta Decay with Germanium Detectors: 1026 yr and Beyond.  Universe, 7:341, 2021.
  * [19] Marcin Wojcik and Grzegorz Zuzel.  A novel low background cryogenic detector for radon in gas.  AIP Conf. Proc., 897(1):39–44, 2007.
  * [20] M. Wojcik, G. Zuzel, and H. Simgen.  Review of high-sensitivity radon studies.  International Journal of Modern Physics A, 32(30):1743004, 2017\.
  * [21] Manzanillas L. et al.  Optical properties of low background PEN structural components for the LEGEND-200 experiment.  Being prepared, to be submitted to JINST, 2021.
  * [22] J. Janicskó Csáthy, T. Bode, J. Kratz, S. Schönert, and Ch. Wiesinger.  Optical fiber read-out for liquid argon scintillation light.  6 2016.
  * [23] Edzards F.  Characterization of Point Contact Germanium Detectors and Developement of Signal Readout Electronics for Legend.  PhD thesis, Technische Universität München, 2021.