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Usage of PEN as self-vetoing structural material in low
background experiments
I. Abt,𝑎B. Dial,𝑏Y. Efremenko,𝑐M. Febbraro,𝑏F. Fischer,𝑎M. Guitart,𝑎K. Gusev,ℎ
B. Hackett,𝑐C. Hayward,𝑑M. Kidder,𝑏R. Hodák,𝑒P. Krause,ℎB. Majorovits,𝑎
L. Manzanillas,𝑎,∗D. Muenstermann,𝑑R. Pjatkan,𝑖M. Pohl, 𝑓R. Rouhana, 𝑓
D. Radford,𝑏E. Rukhadze,𝑒N. Rumyantseva,ℎI. Schilling, 𝑓S. Schoenert,ℎ
O. Schulz,𝑎M. Schwarz,ℎM. Stommel𝑔and J. Weingarten 𝑓
𝑎Max-Planck-Institut für Physik, 80805 Munich, Germany
𝑏Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
𝑐Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37916
𝑑Department of Physics, Lancaster University, Lancaster
𝑒Czech Technical University, Institute of Experimental and Applied Physics, CZ-12800 Prague
𝑓Technische Universität Dortmund, Dortmund
𝑔Leibniz-Institut für Polymerforschung Dresden e.V., 01069 Dresden, Germany
ℎPhysik Department, Technische Universität, München
𝑖Nuvia a.s., 67401 Třebíč, Czech Republic
E-mail: manzanil@mpp.mpg.de
PEN is an industrial polyester plastic which has become interesting for the physics community
as a new type of plastic scintillator. PEN scintillates in the blue regime, which is ideal for
most photosensor devices. In addition, PEN has excellent mechanical properties and very good
radiopurity has been achieved. Thus, it is an ideal candidate for active structural components
in low-background experiments. One possible application are holders for germanium detectors
operating in cryogenic liquids (LAr, LN2). Such structures can help to reject surface and external
backgrounds, boosting the sensitivity of experiments. In this contribution, the R&D on PEN
is outlined and an evaluation of the first production of PEN structures for the LEGEND-200
experiment is reported.
40th International Conference on High Energy physics - ICHEP2020
July 28 - August 6, 2020
Prague, Czech Republic (virtual meeting)
∗Speaker
©Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/
arXiv:2011.08983v1 [physics.ins-det] 16 Nov 2020
Usage of PEN as self-vetoing structural material in low background experiments L. Manzanillas
1. Introduction
Rare event physics experiments such as dark matter or neutrinoless double beta decay (0𝜈𝛽 𝛽)
searches demand ultra low backgrounds. Hence, ultra pure materials are required for the structural
materials and for the detectors itself. An enormous progress has been achieved in the last years
on the production of clean materials. However, despite all this progress one major source of
background of the current and the next generation of 0𝜈𝛽𝛽 experiments are the support structural
materials (see Figure 1). Some strategies have been adopted to mitigate these external backgrounds.
Thus, in some experiments operating at cryogenic temperatures, liquid Argon (LAr) is used to cool
down the detectors and at the same time it also serves as an active shielding. Interactions of charged
particles in LAr produce VUV light (∼127 nm) [1] that can be used to veto external backgrounds.
Unfortunately, the support structures used to mount the detectors consist of inactive materials.
Besides of being a source of backgrounds, inactive materials can also absorb light produced by
the active shielding, decreasing the capabilities for background identification in the vicinity of the
detectors. In this context, a new approach is under development, and consist in using clean active
materials to build the support structures used to mount the detectors. Such materials improve the
light collection in the vicinity of the detectors and at the same time have self-vetoing capabilities.
This new approach could help to improve the discrimination of surface and external backgrounds
increasing the sensitivity of the experiments.
Figure 1: Left: Example of inactive copper structures used as support for the Ge detectors in the Majorana
experiment [2]. Center: Background components in the region of interest in the CUORE experiment, once
cuts are applied the remaining background is dominated by events originating in the support structures [3].
Right: Expected background composition in the region of interest of the LEGEND-200 experiment [2]. An
important fraction of background is expected to be produced by the support structures.
Poly(Ethylene Naphthalate) known as PEN has been identified as a potential active structural
material. PEN is a commercially available polyester, which has a yield strength higher than copper
at cryogenic temperatures. In addition, it scintillates in the blue regime, which is ideal for most of
photo-sensors. All these properties make PEN a good candidate to be used as an active structural
self-vetoing material [4].
2. PEN mechanical and optical properties
PEN [C14H10 O4] is primarily composed of only three elements. PEN can be procured as
crystalline resin or pellets, crystallized slowly, making it easier to shape transparent moulded
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Usage of PEN as self-vetoing structural material in low background experiments L. Manzanillas
products. In order to achieve high quality transparent products, PEN must be cooled from 300
°C to 220 °C in less than 10 seconds during the moulding process. The injection moulding
technology allows to produce any arbitrary shape, for example, containers, holding plates, capsules,
fibers, among others. PEN moulded shapes have a high structural stability at room and cryogenic
temperatures, with a Tensile Strength (209 MPa) higher than copper (100 MPa) at liquid Nitrogen
(LN2) temperature (77 K). In addition, PEN has a high chemical resistance to most acids and organic
solvents and therefore can be aggressively cleaned in order to remove surface contamination.
Figure 2: Left: Emission spectrum of a PEN sample excited with UV light (∼370 nm). Right: Light output
of PEN samples irradiated with a 207Bi source.
Transparent PEN moulded shapes scintillate in the blue regime, making it ideal components
to be used as active structural materials. The light emission spectrum of a PEN moulded sample
is presented in Figure 2(Left); it peaks around 445 nm, which is ideal for light detection using
standard photo-sensors such as PMTs or SiPMs. Figure 2(Right) shows the light output of PEN
samples excited with a 207 Bi source and measured using PMTs. These first measurements suggest
that the scintillation yield of PEN is about 1/3 of standard plastic scintillators. However, PEN
has the advantage of providing Pulse Shape Discrimination (PSD), which allows for alpha decay
identification. Moreover, PEN can shift the VUV light to visible light. Thus, the VUV light
produced by LAr (127 nm) or LXe (175 nm) can be shifted to about 445 nm with PEN structures
with an absolute re-emission probability of about 50% [5]. On the other hand, the light attenuation
length of PEN is of the order of few cm, which has a small impact on the light output for structures
with a thickness of the order of a few mm.
3. PEN application
One possible application of PEN as an active structural material are holders for germanium
detectors, which can be operated in LAr. In this context, using commercially available PEN
raw material in form of pellets, PEN tiles of about 1.5 mm thickness have been produced using
the injection moulding technology. Before the moulding process, the PEN pellets underwent
a systematic cleaning process with the aim of improving the radiopurity of the final moulded
products. Finally, the moulding process was realized under clean room conditions. Samples of
this production were screened at the LNGS and the LSM underground laboratories. The final
radiopurity results are presented in Table 1. These radiopurity results meet the strong requirements
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Usage of PEN as self-vetoing structural material in low background experiments L. Manzanillas
for materials to be used as support structures in the LEGEND-200 experiment. These measured
PEN tiles will be used to produce holders for the LEGEND-200 experiment.
Isotope
Radiopurity Radiopurity Expected
GeMPI4 at LNGS OBELIX at LSM radiopurity
14.315 kg 5.231 kg PEN holder
t = 5851612 s t = 6825600 s 5.3 g
228Ra 92 ±25 𝜇Bq/kg 107 ±38 𝜇Bq/kg ∼0.5 𝜇Bq/piece
228Th 32 ±16 𝜇Bq/kg 67 ±18 𝜇Bq/kg ∼0.2 𝜇Bq/piece
226Ra 60 ±15 𝜇Bq/kg 76 ±22 𝜇Bq/kg ∼0.3 𝜇Bq/piece
Table 1: Radiopurity achieved for moulded samples using commercially available PEN pellets. The pellets
underwent a cleaning process before the injection moulding process.
In order to minimize the mass of the PEN holders that will be used to mount the Ge detectors
in the LEGEND-200 experiment, mechanical simulations were used to optimize its design. An
example of a holder with optimized geometry is shown in Figure 3(Left). These PEN holders were
tested and validated using the LAr test stand facility of the Technische Universität München (TUM),
where a first deployment with HPGe detectors was performed. This study demonstrated that no
increase of leakage current is produced due to the usage of PEN holders. Later, additional PEN
holders were CNC (Computer Numerical Control) machined at the Physics machine shop of the
University of Tennessee under almost clean conditions. These holders were deployed in detector
mount prototypes during the LEGEND-200 prototyping tests at LNGS. This experience was used to
develop a protocol that will be followed during the production of PEN holders that will be mounted
in the LEGEND-200 experiment.
Figure 3: Left: Low mass PEN holder with optimized geometry being mounted. Right: Ge detectors
mounted with structures consisting of copper, Si and PEN base holders.
PEN samples from the LEGEND-200 production are being optically characterized at the Max-
Planck-Institut für Physik in Munich. To this end, different setups consisting of a spectrometer
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Usage of PEN as self-vetoing structural material in low background experiments L. Manzanillas
and several PMTs have been mounted. These setups allows to place PEN samples of different
dimensions as well as the LEGEND-200 holders in order to investigate its optical properties. Using
these measurements, first estimations of light yield and attenuation of the LEGEND-200 PEN
samples have been realized. Thus, a direct comparison of the light output of PEN samples with
polystyrene (SP32) and PVT (EJ-200) samples of the same dimensions demonstrated that the light
yield of PEN is at least 3500 photons per MeV. A complete Geant4 optical model of the setups is
being developed and will be used in conjunction with more measurements to assess the absolute
optical parameters such as light yield, absorption as function of wavelength, emission spectrum,
and surface effects. These results will be used as input in the Geant4 LEGEND-200 framework in
order to estimate the background rejection efficiency of PEN in the LEGEND-200 experiment.
4. Conclusions and perspectives
PEN is an attractive scintillator to be used as active structural material in low background
experiments. A successful production of low background PEN tiles has been achieved. These
tiles are being used to machine low background PEN holders for application in the LEGEND-200
experiment. The preliminary optical results lead to a light yield larger than 3500 photons/MeV,
which will allows for self-vetoing capabilities. Detailed Geant4 simulations of the setups used for
the optical characterization of PEN are under development. These studies will provide more precise
results of all optical parameters. Finally, further R&D is being carried out for a potential application
of PEN in the LEGEND-1000 experiment.
References
[1] T. Heindl, et al., The scintillation of liquid argon, EPL 91 (2010) no.6, 62002
doi:10.1209/0295-5075/91/62002 [arXiv:1511.07718].
[2] Yoann Kermaidic, (2020, July), [GERDA, Majorana and LEGEND collaboration], GERDA,
Majorana and LEGEND- towards a background-free ton-scale Ge76 experiment, Presented at
the Neutrino 2020 (neutrino2020), Zenodo doi: 10.5281/zenodo.4140757
[3] Thomas O’Donnell, (2020, July) [CUORE collaboration], CUORE Results and the CUPID
Project, Presented at the Neutrino 2020 (neutrino2020), Zenodo doi: 10.5281/zenodo.4134025
[4] Y. Efremenko, et al., Use of poly(ethylene naphthalate) as a self-vetoing structural material,
JINST 14 (2019) no.07, P07006, doi:10.1088/1748-0221/14/07/P07006 [arXiv:1901.03579].
[5] M. Kuźniak, et al., Polyethylene naphthalate film as a wavelength shifter in liquid ar-
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