Use of polyethylene naphthalate as a self-vetoing structural material

The discovery of scintillation in the blue regime from polyethylene naphthalate (PEN), a commonly used high-performance industrial polyester plastic, has sparked considerable interest from the physics community as a new type of plastic scintillator material. This observation in addition to its good mechanical and radiopurity properties makes PEN an attractive candidate as an active structure scintillator for low-background physics experiments. This paper reports on investigations of its potential in terms of production tests of custom made tiles and various scintillation light output measurements. These investigations substantiate the high potential of usage of PEN in low-background experiments.


Introduction
Plastic scintillator materials with special properties such as high transparency, high structural integrity, high radiopurity have been of great interest for particle physics experiments. Such a scintillator candidate, namely, polyethylene naphthalate (poly(ethylene 2,6-naphthalate) or PEN), [C 14 H 10 O 4 ] n , has been suggested as an alternative to more expensive commercial scintillators with proprietary formulas [1]. PEN is a transparent polymer which has been reported to scintillate in the visible region [1,2,3] without the addition of dopants or wavelength shifters. The measurements of the mechanical properties of PEN both at room and cryogenic temperatures [3,4] have shown that both the tensile strength and the tensile modulus of PEN is higher than the one for copper, even at cryogenic temperatures; thus, it could be used as a structural material to support detectors operating in cryogenic liquids such as liquid argon (LAr) or liquid nitrogen (LN 2 ). In this work, we investigate the properties of PEN relevant to low-background rare-event physics experiments such as Germanium Detector Array, GERDA [5] and the Large Enriched Germanium Experiment for Neutrinoless double beta decay, LEG-END [6]. A further background reduction could be obtained by the replacement of optically inactive structural components with transparent structural plastic scintillators, such as PEN. These structural scintillating components can serve as an active veto, which would aid in discrimination of internal radioactivity as well as external background sources. The low-background inactive copper and the Si plate detector holder used in the GERDA experiment are such examples of an inactive structural components which could be replaced with structural plastic scintillator. Specifically, we report on radiopurity measurements, first production tests of custom made transparent PEN forms, measurements of their wavelength shifting and scintillating properties, and pulse-shape discrimination using PEN.

Availability and radiopurity of PEN
PEN is a high-performance plastic used in many industrial applications (e.g. food packing, fibers, electronics parts, reinforced tires etc.). The polymer exhibits high chemical and hydrolysis resistance, high tensile strength, high melting point and low thermal shrinkage. In this study we investigated two types of commercial PEN acquired from Tejin-DuPont: TN-8050 SC and TN-8065 S under the brand name Teonex. These materials are provided in pellet or granulate form. Radiopurity screening was done at the Gran Sasso National Laboratory (LNGS). The results of these measurements together with earlier screening results on Teonex Q51 film [9] and a laminate produced on PEN film [13] are shown in Tab. 1. The earlier results are from two experiments where PEN has been chosen for its radiopurity and used as  [12], and used for the production of HV capacitors for the GERDA experiment [9]. According to the LNGS results, the pellet samples have a much higher 40 K contamination compared to the Teonex Q51 foil, which is suspected to be due to surface contamination of the pellets; therefore, a new screening campaign is ongoing for a new batch of TN-8065S. Apart from the increased amount of 40 K which is not a determining quantity for neutrinoless double beta decay experiments, the overall results show that TN-8065S and TN-8050SC pellets have similar radiopurity levels as PEN foil and laminate used in low-background experiments. Thus, we can conclude that PEN pellets can be used to produce high-radiopurity materials. This can potentially be improved by systematic material screening of raw ingredients prior to the synthesis process.

Molding of PEN
Production of scintillator-grade PEN tiles was undertaken using injection molding. Optimal manufacturing factors were identified in order to prevent crystallization and other optical defects, which reduce transparency and thus the amount of collected scintillation light. In the injection molding process, molten polymer is injected into a temperature-controlled mold under moder-ate to high pressure. These molds consist of an empty cavity which defines the desired geometry of the part and a channel called the runner which guides the molten polymer into the cavity. There are two variations of the runner system, a hot runner and a cold runner. Both variants were tested for the injection molding tool used here. A Hot runner guides the polymer from the machine to the mold cavity. At the end of the injection process only the tile is ejected and the hot polymer remains in the hot runner ready for the second cycle. In 1 (a) the tile with the hot runner process can be seen without a gate system or a runner part. The hot runner injects the polymer at the middle of the cavity. In Fig. 1 (a) in the area of the gate, i.e. at the tip of the hot runner nozzle, strong opacities and other optical defects can be seen. Figure   Figure 1: (a) PEN tile produced with the hot runner system. Strong opacities and other optical defects are seen in the area of the gate. (b) PEN tiles produced with the cold runner system. A slight opacity is seen at the boundary where film gate meets the tiles, and the crystallinity of the sprue is evident. 1 (b) shows an injection molded part with a cold runner system. Here, a film gate with a smaller cross-section directly in front of the cavity is used where two tiles are produced simultaneously. This type of gate is used to balance the cavity filling. A slight opacity can be seen in this area, which is due to the increased shear. At the tip of the sprue (the channel through which the plastic is poured into the mold), the plastic cools more slowly due to the hot machine nozzle, which leads to a slower cooling rate at this point and thus increased crystallinity, hence opacity.
With these preliminary tests, an optimized geometry for the mold was developed resulting in standard tiles with 30 x 30 x 3 mm 3 geometry. Then, a linear, fractional-factorial design of experiments was used in order to investi-gate the following factors for optimization: melt temperature, mold temperature, injection speed, packing pressure, packing time and cooling time. Starting parameters were determined from the manufacturer's guidelines and the previous preliminary tests. The most influential factors on the transparency were found to be the melt temperature and for the given geometry, the twoway interaction of the melt temperature and the injection speed. The highest transparency was achieved with a comparatively low melt temperature of 280 • C and a comparatively low injection speed of 10 cm 3 /s. These results are consistent with previous conclusions [19] that the cooling rate of the melt as well as the shear stress of the melt during filling is primarily responsible for the formation of crystalline structures. These findings have also been used to produce larger shapes applicable in experiments with high-purity germanium (HPGe) detectors as an ongoing work, Fig. 2 shows such a form. The dome-shaped PEN of height 65 mm, inner diameter 86 mm, and thickness 3 mm is illuminated under a UV flashlight and a non-scintillating plastic tile (Poly(methyl 2-methylpropenoate) or PMMA), [C 5 O 2 H 8 ] n , is placed next to it as a reference. The blue scintillation light from the PEN is clearly visible. In order to measure the emission spectrum of PEN, a standard tile is excited in a dark box with a 382 nm light (with approximately ± 2 nm bandwidth). This light is generated by passing the continuous light output of a deuterium/halogen lamp (Spectral Products ASBN-D1-W) through a monochromator (Spectral Products CM110). The emitted light is then analysed using an Andor Shamrock 193i spectrograph. The Stokes shifted emission spectra of PEN, BC408 1 and PMMA with identical geometries are compared in Fig. 4. The wavelength of the emission peak of the PEN tile, which is obtained by fitting a second order polynomial to the peak region, is 445 ± 5 nm and matches well to the wavelength of the peak quantum efficiency of most commercial photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs).

Vacuum-UV (VUV) response for 128 nm LAr scintillation light detection
The observed Stokes shift in PEN when exposed to 382 nm light prompted further investigation down to lower wavelengths applicable to LAr scintillation detection. LAr scintillates at 128 nm and thus requires either wavelength shifters which emit near blue regime or VUV-sensitive photosensors. The light yield from VUV excitation is investigated by comparing it to that of a commonly used dye, 1,1,4,4-tetraphenyl-1,3-butadiene (TPB). 200 µg/cm 2 TPB is evaporated on a 1.5 mm thick acrylic piece to produce a standard wavelength shifting TPB tile. The 3 mm thick PEN tile and the TPB coated acrylic were illuminated in a dark box with the light from a McPherson 302 VUV monochromator fitted with a Hamamatsu L1835 deuterium lamp. The wavelength of the excitation light is scanned between 116 nm and 300 nm where the wavelength uncertainty of the measurement system is less than 1 nm, and the scintillation light is collected with a PMT (Hamamatsu model R580-15 SEL ASSY). Note that the borosilicate glass window of the PMT has a cutoff at ∼ 300 nm therefore no excitation light enters the PMT. The PMT anode current from the PEN scintillation light normalised by that from TPB coated acrylic tile is shown Fig. 5

Light output comparison with mono-energetic electrons
The scintillation light output of a standard tile is compared against a commonly used plastic scintillator, polystyrene (PS) 2 of identical geometry, which was provided by NUVIA Ltd. 3 . A 90 Sr source in combination with an electromagnet produces tunable (0.2 to 1.8 MeV) electrons with a narrow energy spread (FWHM = 1.0 ± 0.2 % at 1 MeV) [14] which is then used to excite the samples in a dark box. In order to collect the scintillation light efficiently and consistently, the samples are wrapped with a 10 µm reflective Mylar foil and then using a 1 mm thick BC-634A optical interface pad connected to a 1 inch diameter PMT (Hamamatsu model R1924A).  The comparison of the light output measured in photoelectrons (p.e.) for various energies (see Fig. 6) shows that a standard PEN tile emits about 2.5 times less light than the PS tile of the same geometry. Note that the nonlinearities in the data set are due to punch through of the electrons. They occur at different locations due to differences in the density and effective-Z of the material. Depending on the nature of excitation, the scintillation light signal measured by a PMT shows different decay times, i.e. different pulse shapes. For example, a heavy particle such as a neutron will cause a higher ionisation or excitation density which will reduce the scintillation efficiency and cause a longer/slower decay time. Two such pulse shapes separated by gating are shown in Fig. 7. Pulse-shape discrimination (PSD) methods take advantage of this feature to distinguish the type of excitation in low-background experiments. A 252 Cf fission chamber was used at Oak Ridge National Laboratory in order to test a PSD method based on charge integration on a standard PEN tile. The 252 Cf fission chamber also provides time-tagged gamma rays and neutrons which can be used to evaluate the time of flight and hence the quality of the PSD method. The PEN tile is wrapped in PTFE tape and optically coupled to a PMT (Hamamatsu model R6231-100-01 ASSY) using BC 630 optical grease. The PMT signal is digitised at 250 MHz and at 14bit using a CAEN V1725 waveform digitizer. The PSD parameter, i.e. the charge integration ratio (or tail-to-total) is extracted from the signal where the total integral is over 850 ns and the tail integral is over 750 ns starting from 50 ns from the leading edge of the pulse. The PSD parameter is plotted against the light response in Fig. 8. A clear two band structure that can be used to discriminate neutrons and gammas is visible. This is consistent with the time of flight measurement shown in Fig. 8. In order to quantify the discrimination quality, we further calculate the figure-of-merit (FOM) [15] which is defined as: where X represents the peak position of the PSD parameter of the gammas or neutrons and W represents their respective FWHM. The result as a function of light response is shown in Fig. 9. FOM is clearly better for higher light response. Similar pulse discrimination calculations has also been reported previously in Ref. [16] where 125 µm PEN films were exposed to neutrons and gammas up to 10 MeV from 239 PuBe and 238 PuBe source and the FOM was measured as 0.69 ± 0.02. For the application in low-background experiments the PSD capability of PEN is a great advantage as it allows for achieving additional information on the type of interaction and hence helps identifying background sources.

Conclusions & Outlook
In summary, the following properties of PEN have been identified in our investigations: HPGe screening measurements have shown that commercially available PEN pellets with radiopurities well below 1mBq/kg in 228Th and 226 Ra can be used to produce structural parts. Molding tests indicate that scintillator-grade arbitrary shapes such as dome-shaped structures can be produced with ease. In addition, the newly measured peak emission wavelength of PEN scintillation light (445 ± 5 nm) matches well to that of the peak quantum efficiency of most PMTs and SiPMs. The comparison of light shifting efficiency of PEN for LAr scintillation light with that of TPB show that PEN can in principle be used to shift LAr scintillation light to the wavelength readily accessible to PMTs and SiPMs. According to the results of the study of the light output of PEN using mono-energetic electrons, PEN has a linear response as a function of energy. The sample used in the investigation emitted about 2.5 times less light than a PS scintillator. This is believed to be due to the relatively low attenuation length of the self molded PEN sample of order 5 cm. Lastly, the results of the pulse-shape discrimination studies of PEN show that PEN scintillation pulses can be used to discriminate neutrons and gammas with a good efficiency for the energy range of 300 to 1300 keVee (ee stands for electron equivalent). Therefore, the combination of all of the above mentioned properties make PEN a very promising candidate as a structural scintillator. Primarily we envision its use in low-background experiments as structural self-vetoing material. Presently, efforts are underway to self synthesize PEN from carefully selected and purified ingredients. Also, measurements are being performed to further quantify the light yield, attenuation length, and wavelength shifting behavior. Also a first setup to operate a high-purity germanium detector surrounded by a custom made PEN enclosure directly in LAr is being prepared.