Performance Evaluation of Pulse Shape Discrimination Capable Organic Scintillators for Space Applications

The next generation of organic scintillators for fast neutron detection with pulse-shape discrimination (PSD) capability have recently been under development [1, 2, 3, 4]. These scintillators are of interest to a wide range of applications that benefit from fast neutron detection, including space-based applications such as planetary science and space science measurements. The PSD capability of these new organic scintillators provides a measure to cleanly reject gamma-ray background that organic scintillators are also sensitive to.

Some organic scintillators with PSD capability already exist. Stilbene is well known and has excellent PSD ability, however, until recently the availability of Stilbene has been limited and the cost of manufacturing large volumes high. A new growth method for Stilbene was recently developed at Lawrence Livermore National Laboratory (LLNL) [1], which opens the door to easier scalability. In addition, Stilbene produced with the new growth method showed 50$\%$ more light output than Stilbene produced using the traditional growth method [1]. Liquid organic scintillators have also been used for decades and provide good PSD, however, the challenge of flying liquid scintillators and their required size on space missions makes them an unfavorable option.

Recently, several new options for PSD capable organic scintillators have become available. In addition to the new Stilbene mentioned above, plastic scintillators with PSD capability, both unloaded [2] and loaded with ⁶Li [3] to provide thermal neutron sensitivity, and PSD glass [4] have become available.

To our knowledge, none of these new organic scintillators with PSD capability have space heritage or have undergone all of the relevant environmental testing required to assess their ultimate performance for space-based applications. The required testing includes irradiation of the scintillators to assess their tolerance to damage from total ionizing dose (TID) and displacement damage effects, and characterizing their performance after radiation exposure to typical doses and fluences of particles experienced on orbit.

Instruments in inter-planetary space or in Earth orbit are subject to high fluences of energetic charged particles. In inter-planetary space and Earth orbits outside the radiation belts, instruments are subject to $\sim 10^{9}$ protons/cm² over a 10 year mission lifetime from high-energy galactic cosmic rays (predominantly protons with an average energy of 100s of MeV). Solar energetic proton events can also result in an additional $\sim 6\times 10^{10}$ protons/cm² ($>$10 MeV protons) over the same duration. In low Earth orbit, instruments may additionally be subject to trapped protons in the radiation belts leading to higher proton flux, but these missions typically have a shorter duration.

This research focuses on the effects of radiation damage on the performance of newly developed organic scintillators with PSD capability via neutron and gamma-ray irradiation, providing critical information for assessing the future use of these scintillators for space missions.

Seven 1” right cylinder samples were obtained of four different types of scintillators: unloaded PSD plastic (EJ-276, Eljen), ⁶Li-loaded PSD plastic (EJ-270, Eljen), Stilbene (InRad Optics), and organic glass scintillator (OGS, provided by Sandia National Laboratory). The scintillator samples for each material numbered 1-7. Figure 1 shows one sample of each scintillator type. Sample 1 was measured five times over the duration of experimental measurements to establish a performance baseline and determine experimental uncertainty between measurements. Samples 2, 3, and 4 were irradiated with neutrons at the Los Alamos Neutron Science Center (LANSCE), while samples 5, 6, and 7 were irradiated with gamma rays at LANL’s Radiation Instrument and Calibration Facility (RICF) Mark2b gamma cell irradiator.

All scintillator samples were wrapped in four layers of polytetrafluoroethylene (PTFE) tape as uniformly as possible. They were then wrapped in electrical tape to maintain the integrity of the PTFE tape during handling and between measurements. The samples were coupled to 23 mm $\times$ 23 mm active area R11265U Hamamatsu photomultiplier tubes (PMTs) using optical grease. Waveforms were collected using a CAEN v1761 digitizer with a sample rate of 4 GSamples/s. The EJ-276 and EJ-270 samples were biased to $-700$V, the Stilbene to $-650$V, and the OGS to $-675$V, to limit input pulse amplitude to $<$1V as required by the digitizer.

Each full set of characterization measurements consisted of 50,000 waveforms collected from ¹³⁷Cs and ²²Na check sources to obtain Compton Edge locations for energy calibration and 100,000 waveforms collected from a neutron source (²⁵²Cf or PuBe). Particle discrimination is achieved by using PSD, which is enabled by different scintillation light decay times for neutrons and gamma rays. By integrating two regions of the scintillation light pulse, “head” (H) and “total” (T) regions, a PSD ratio is formed by $1-H/T$. The figure of merit (FOM) describes the quality of PSD, and is defined in Eq. 1 [5]:

where $\mu$ is the centroid of the neutron and gamma-ray peaks in PSD and FWHM their full-width and half-maximum.

The goal of this research was to characterize four organic scintillation detectors with PSD capability (EJ-270, EJ-276, OGS, and Stilbene), expose different samples of each material to varying doses of ionizing and non-ionizing radiation, and measure performance of the scintillators after radiation. Seven samples of each scintillation material were acquired. Samples 2, 3, and 4, were irradiated using neutrons to fluences of $1.10\times 10^{10}$, $4.96\times 10^{10}$, and $2.96\times 10^{11}$ n/cm². Samples 5, 6, and 7 were exposed to gamma rays in a Mark2b Gamma Cell Irradiator to doses equivalent to 1, 10, 50, and 100 kRad.

Samples 2, 3, and 4 were characterized after the neutron irradiation. No significant change was observed in light output reduction, average waveforms, or FOM for all the samples except for Stilbene, which showed marginal light output reduction and FOM degradation (5-7$\%$) at the highest neutron fluence. Post neutron irradiation, none of the samples showed any differences in coloration or visible damage.

After the gamma-ray irradiation, samples 5, 6, and 7 were characterized to assess damage with total ionizing dose. Stilbene presented with yellowing of the material, highest light output degradation, and least recovery over time. EJ-270 showed yellowing of the material, second highest light output degradation, yet quick recovery. EJ-276 showed little yellowing, no additional damage $>$50 kRad, and quick recovery. OGS showed no yellowing, no damage $<$50 kRad, similar light output reduction to EJ-276 at 100 kRad, and similar recovery rate as EJ-276. The average waveforms and FOM were not significantly affected, with the exception of Stilbene which showed a slight decrease in FOM after 10 kRad.

The decrease in light output observed in the experiments described is caused by radiation-induced damage in the scintillating materials. Ionizing radiation, such as gamma rays, give rise to color centers by displacing electrons which allow for new chemical bonds to form; Lima and Lameiras discuss this effect in the context of gemstones [7]. Color center formation also gives rise to absorption bands which reduce the light output of the scintillating material [8]. The susceptibility of Stilbene to higher radiation damage after exposure to gamma rays is likely due to a combination of effects, including its crystalline structure, induced phosphorescence, and optical inhomogeneity [9]. In plastic scintillators, exposure to radiation can cause breaks and cross-linking of the polymer chains that make up the material, also giving rise to color centers which absorb scintillation light and ultimately reduce the light output of the material [10]. The effect of radiation damage for different dose rates on plastic scintillators without PSD capability is also discussed in [11]. Our experimental findings highly correlate previous literature, although dose-dependent radiation damage has not been previously compared between Stilbene, PSD capable plastic, and PSD capable glass scintillators.

This work was supported by the U.S. Department of Energy through the Los Alamos National Laboratory and performed, in part, at the Los Alamos Neutron Science Center (LANSCE). The authors would also like to acknowledge Patrick Feng and Lucas Nguyen of Sandia National Laboratory for providing the organic glass scintillator samples, Steve Wender, Kranti Gunthoti, and Jeff George for supporting the LANSCE measurements, and Charity Roybal, Nick Wehmann, and Dave Seagraves for supporting the gamma cell irradiation measurements.