Characterization of the Radiation-Induced Damage in a PEN (Polyethylene Naphthalate) Scintillation Detector ^†

Abstract

The radiation hardness of a Polyethylene Naphthalate (PEN) thin film scintillator has been characterized in terms of the light yield loss after irradiation with 11 MeV protons and 1 MeV electrons. The light yield distributions induced by excitation with radioactive sources have been measured on samples irradiated with different doses and the induced light loss has been computed. Results showed the good radiation hardness behaviors of PEN scintillators, with a light yield loss of ~15% at 10 Mrad and ~35% at the maximum delivered dose of 80 Mrad.

1. Introduction

Scintillators are transparent materials that emit light upon excitation by energetic charged particles. They are employed in several applications in order to detect energetic particles and measure their properties [1]. Recent works proved the possibility of using common low-cost plastic polymers such as Polyethylene Naphthalate (PEN) as a scintillator for radiation-sensing applications. PEN offers excellent scintillation properties such as high density (1.33 g/cm³), a peak emission wavelength at ~425 nm, and a light yield of roughly 10⁴ photons/MeV [2,3,4].

Because of these good properties, along with its ease of manufacture and low cost, PEN has drawn the attention of the scientific community. Its field of employment ranges from dosimetry purposes in irradiation facilities or nuclear medicine [5] to particle energy measurement in High Energy Physics (HEP) colliders and underground experiments [6,7]. However, many of these applications require the instrumentation to operate in very high-radiation environments, raising the issue of the detector’s radiation hardness to a high level of priority [8]. Several works have already probed the good characteristics of PEN irradiated up to 20 Mrad [9,10,11].

In this work, we investigate the radiation hardness of a PEN thin-film scintillator. Several samples have been irradiated in air with a 11 MeV proton beam and a 1 MeV electron beam at the maximum doses of 15 Mrad and 80 Mrad, respectively. The radiation-induced damage has been measured in terms of light yield loss as a function of the dose. This investigation revealed good radiation hardness behaviors of the PEN, with a light yield loss of ~15% at 10 Mrad and ~35% at the maximum delivered dose of 80 Mrad.

2. Materials and Methods

Several tiles of 30 × 40 mm² were obtained from a Teonex^® PEN film of 100 µm thickness, as shown in Figure 1.

The samples were irradiated in air at controlled room temperature with two particle species (protons and electrons) at several doses. The proton irradiation was performed at the Tandem accelerator of the INFN Laboratori Nazionali del Sud. The actual energy of the beam on the sample surface was 11 MeV, and the maximum achieved dose was 15 Mrad. The irradiation with 1 MeV electrons was performed at the ILU-6 accelerator at the Centre for Radiation Research and Technology of Institute of Nuclear Chemistry and Technology (INCT) in Warsaw (Poland). In this case, given the high particle flux available, doses up to 80 Mrad were achieved.

The radiation-induced damage was measured in terms of the scintillator light yield loss. The PEN scintillator light yield was characterized upon excitation with radioisotope sources by coupling the samples to a Hamamatsu R5900 Photo-Multiplier Tube (PMT) with a square and flat light input surface. A schematic representation of the experimental setup is shown in Figure 2.

The samples were optically coupled to the PMT by means of a thin layer of optical grease. The radioactive source was positioned on the other face of the scintillator on top of a thin collimator. Two sources were used: ²⁴¹Am, which decays mainly via alpha-decay with the main emission of a 5.486 MeV alpha particle, and ¹³⁷Cs, which decays via beta-decay by emitting electrons with a maximum energy of 0.51 MeV.

The output signal from the PMT was sent to an amplifier whose output was read and acquired via a spectroscopy Multichannel Analyzer (MCA) interfacing with a PC.

3. Results

Prior to the irradiation campaign, all samples were characterized in terms of light yield, where they exhibited good consistency with each other.

Fluctuations remained within 2–3%, mainly due to the non-perfect reproducibility of the optical coupling with the PMT.

After the irradiation, the samples were kept in a dark environment at a controlled temperature of 25 °C for 60 days prior to characterizing them. According to [9], PEN scintillators demonstrate a partial damage recovery mechanism which reaches a plateau, with a characteristic time of about 10 days. Figure 3 shows, as an example, the light yield spectra measured on proton-irradiated samples with the Cs and Am sources. As the intent of this paper is to study the relative degradation induced by radiation, an absolute multichannel analyzer calibration was not performed. The effect of irradiation is clearly observable as a reduction of the pulse height, here measured in terms of channels of the MCA.

The percentage light loss after irradiation was then computed as a function of the delivered dose. Similar trends were observed in measurements with Am and Cs. In Figure 4, we report the percentage light yield loss measured with the Cs source in samples irradiated with protons and electrons.

This investigation revealed a reduction of the light yield emission, with similar trends for both irradiation beams. A slowing down of the damage with increasing dose is clearly observable, suggesting that most of the chemical reactions that lead to the deterioration of performances occur early in the exposure.

4. Conclusions

Polyethylene Naphthalate (PEN) thin-film scintillators have been characterized in terms of the light yield loss after irradiation with 11 MeV protons and 1 MeV electrons. A reduction of the light yield emission, with similar trends for both irradiation beams, has been observed.

This investigation revealed the good radiation hardness behaviors of PEN scintillators. A light yield loss of ~15% at 10 Mrad and ~35% at the maximum delivered dose of 80 Mrad was observed.

The results described in this work enrich the literature on radiation hardness studies on plastic scintillators, providing useful information for the introduction of PEN scintillators in nuclear, space and HEP applications.