BaF 2 :Ce POLYMER COMPOSITE GAMMA-RAY SCINTILLATORS M.B. Barta, J.H. Nadler, Z. Kang, B.K. Wagner Georgia Tech Research Institute 925 Dalney St, Atlanta, GA 30332 Keywords Barium halide, nanophosphor, nanocomposite, gamma-ray scintillator Abstract A nanocomposite gamma-ray scintillator contains nanoscale phosphor particles (nanophosphors) encapsulated in a polymer matrix. These scintillators show great promise for improving the cost and durability of scintillators for gamma-ray detector systems because single crystal scintillators currently in use have high cost and hygroscopicity, and are available in limited compositions and sizes. This research utilizes two different commercially available epoxies to encapsulate two different nanophosphors (BaF 2 :2%Ce and BaF 2 :15%Ce). The importance of three parameters are illustrated: photon efficiency of the encapsulated nanophosphor, refractive index (RI) match between the nanophosphor and polymer matrix, and the %transmission of the polymer at the wavelength at which the BaF 2 :Ce nanophosphors emit gamma-ray excited photons. Introduction Current gamma-ray scintillators are primarily large single crystals of Thallium doped NaI. While single crystal scintillators are currently the most efficient available, their drawbacks include hygroscopicity, a limited choice of composition, and a low-yield growth process that frequently introduces defects [1]. Nanoscale phosphor powders (nanophosphors) encapsulated in a polymer matrix may provide improved scintillation performance. Due to their potential for environmental insensitivity, mechanical durability, and flexibility both in size and geometry, nanophosphors encapsulated in a polymer matrix could provide improved performance for a range of applications [2]. Figure 1: Schematic of gamma-ray detector system
The gamma-ray detector discussed is a materials system comprised of polymer composite scintillator loaded with phosphor particles, and a photomultiplier tube (PMT) detector (see Figure 1). Upon interaction with gamma radiation, these phosphor particles encapsulated in the polymer matrix emit photons via energy transitions of excited electrons. The emitted photons are then channeled through the polymer matrix to the PMT and classified into a range of energies, thus generating a characteristic energy spectrum dependent on the radioactive isotope of the source [3]. The three main scintillator properties of interest to this investigation are matched refractive index (RI) between the nanophosphor and the epoxy matrix, the %transmission exhibited by the nanophosphor loaded epoxies, and the photon efficiency of the nanophosphors. These properties are crucial for ensuring that gamma-ray excited photons generated by the scintillator material propagate through the matrix and deposit their full energy onto the PMT, thus generating a high resolution energy spectrum. The importance of RI matching and high %transmission will be shown via quantitative data collected on nanophosphor loaded epoxy samples and qualitative determination of the RI of the nanophosphors and epoxies. The potential benefits provided by nanophosphors in gamma-ray detection include greater photon yields and shorter decay times by minimizing photon absorption and scattering due to impurities (a problem in large crystal growth) [4-5]. However, encapsulated nanophosphors can exhibit light scattering due to refractive index (RI) mismatch at the particle-matrix interfaces. Encapsulating the nanophosphors into an appropriate polymer matrix can provide an improved RI match, permitting more photons to propagate to the PMT, minimizing scattering, and thus signal loss [2]. In particular, BaF 2 :Ce nanophosphors (average size of approximately 40nm [8]) provide the highest luminescent efficiency of nanophosphors previously studied [8], and fluoresce in the near-uv range (350-400nm) as shown in Figure 2. Lower energy alpha-radiation will not excite electrons in BaF 2 particles, which allows the nanophosphor to generate scintillation photons solely from high energy gamma-ray excitation. This energy discrimination significantly reduces signal noise from the resulting energy spectrum [6]. The nanophosphors used in this study were synthesized using an adaptation of a chemical precipitation synthesis for LaF 3 :Ce [7,8]. Figure 2 shows photoluminescence (PL) data for the 2-30mol% doping levels synthesized, and indicates that BaF 2 :15%Ce produces the highest photon efficiency. In this work, BaF 2 :15%Ce and BaF 2 :2%Ce are encapsulated in polymer matrices to confirm the effects of two luminescent efficiency levels. High solids loadings (40mg per ml of epoxy or higher) of the organic matrix are required to compensate for the reduced volume of scintillating material as compared to bulk single crystal scintillators [2], and so two different solids loading levels of the BaF 2 :Ce nanophosphors are encapsulated in two types of epoxy matrices with different RI and transmission levels at 350-400nm.
Figure 2: PL data for BaF 2 :Ce using 288nm excitation wavelength [8] Photons generated by the nanophosphors must be able to propagate to the photomultiplier tube (PMT) and deposit their full energy in order to generate useful data. Sufficient matching of the RI between the nanophosphor and its matrix will promote light propagation rather than scattering within the scintillator [9]. However, due to dispersion, the RI that allows 589nm light to propagate uninhibited is different from the RI that allows 389nm or 400nm light to propagate. Thus, matching the RI of the nanophosphor and the matrix at the wavelength at which a nanophosphor will generate gamma-ray excited photons is desired. Gamma-ray measurements provide information regarding the scintillator photon yield (or efficiency) when exposed to a gamma radiation source, such as 241 Am. A good scintillator must be able to transform the full energy of the gamma-ray into emitted photons (excitons) that can be deposited onto the PMT and generate an energy spectrum [10]. Procedure Refractive Index Matching The RI of BaF 2 :Ce was measured by combining the 2mg of nanophosphor with 0.5mL of liquid with known RI at 589nm from Cargille Labs Inc. (Cedar Grove, NJ) on a standard glass microscope slide. These slides were placed on a clean silicon wafer and viewed with a Leica M165C optical microscope under 400nm LED illumination (see Figure 3). The extent of visual detection of the powder agglomerates submerged in each RI liquid was used as the criteria to estimate the RI of the particles. After a matching liquid was found, a manufacturer provided dispersion curve for the specific liquid was used to create a polynomial fit to the data and approximate the RI of the particles at 389nm (see Figure 4).
Figure 3: Refractive index matching apparatus Figure 4: Extrapolated RI curve for BaF 2 :Ce nanophosphors based on RI matching Nanocomposite Synthesis For this investigation, two different commercially available epoxies are used to encapsulate the BaF 2 :Ce nanophosphors: Epo-TEK 301-2FL and Epo-TEK 305 (from Epoxy Technology, Inc., Billerica, MA). Both polymers are two part epoxy mixtures and both have room or high temperature cure capabilities. The most notable difference is in their RI values: Epo-TEK 305 has a RI of 1.476 while Epo-TEK 301-2FL has a RI of 1.512 (both at 589nm). The difference in RI will affect the way generated photons will travel through the matrix, and thus affect the overall performance of the scintillator. BaF 2 :15%Ce and BaF 2 :2%Ce nanoparticles with loading levels of 40mg nanophosphor per ml epoxy and 100mg/mL were added gradually to 5mL samples of Epo-TEK 301-2FL and Epo- TEK 305. The phosphor loaded polymers were then poured into 32mm diameter cylindrical
molds to form disks approximately 3mm thick. The molds were placed in an ultrasonic bath to enhance particle dispersion for 20 minutes, placed under vacuum (<1torr) for 5 minutes to remove any entrapped air and any remaining agglomerations, and allowed to cure at room temperature. Following curing, the samples were polished prior to transmission or gamma-ray measurements to minimize surface scattering. Gamma-ray Measurements Gamma-ray measurements were conducted using a radionuclide source placed on top of the thin phosphor loaded polymer samples. Optical grease was applied between the radionuclide and the scintillator to prevent interface scattering. The sample and source were placed on a photomultiplier tube (PMT) and the entire assembly was shielded to block external light. Photons generated by the scintillator were recorded by the PMT, and their energy and intensity in a range of 1-12keV distributed over 1,040 channels with a total counting time of 5,000 seconds. Transmission Measurements Transmission measurements in the range of 300-450nm were conducted using a Cary 5000 UV- Vis-NIR Spectrophotometer (Varian, Inc., Walnut Creek, CA) to observe the sample behavior near the wavelength at which BaF 2 :Ce generates gamma-ray excited photons [8]. Unloaded samples of both polymers were scanned in addition to all of the nanophosphor-loaded samples. Results & Discussion Phosphor Nanopowder Encapsulation Following encapsulation of both of the BaF 2 :15%Ce and BaF 2 :2%Ce powders, the Epo-TEK 301-2FL appeared to produce the most transparent phosphor-loaded disk prior to curing. As expected, the 100mg/ml loadings in both epoxies after curing exhibited greater opacity than the 40mg/mL loadings. This difference was an expected effect of increased nanophosphor particle loading. Gamma-ray Measurements Figure 5(a) and (b) are plots of gamma-ray detector data ( 241 Am source) for BaF 2 :15%Ce encapsulated in the Epo-TEK 305 and Epo-TEK 301-2FL, respectively. Epo-TEK 301-2FL based components generated much lower photon intensities compared to Epo-TEK 305, a likely result of its higher index of refraction (1.512 at 589nm) relative to the Epo-TEK 305 and the nanophosphors. This large index mismatch would cause greater scattering of any photons traveling through the scintillator. The 100mg/mL phosphor loading in Epo-TEK 305 exhibited greater photon efficiency than the 40mg/mL loading due to a higher phosphor loading density, which would allow for greater material interaction with incident gamma-rays, and thus greater photon production. The absence of a discernable trend in solids loading in the BaF 2 :15%Ce loaded Epo-TEK 301-2FL samples is probably due to the low transmission of that epoxy in the 300-400nm range. Figure 6(a) and (b) are plots of gamma-ray detector data for the epoxies loaded with BaF 2 :2%Ce nanophosphors under a 241 Am source. The performance of the different solids loadings vary less in the BaF 2 :2%Ce measurements, but the efficiency of the BaF 2 :15%Ce nanophosphors is nearly five times greater that of the BaF 2 :2%Ce phosphors, indicating the necessity for a highly efficient nanophosphor as the scintillation material.
(a) (b) Figure 5: Photon intensities generated by BaF 2 :15%Ce loaded in (A) Epo-TEK 305 samples and (B) Epo- TEK 301-2FL samples, both excited by a 241 Am radionuclide source (a) (b) Figure 6: Photon intensities generated by BaF 2 :2%Ce loaded in (A) Epo-TEK 305 samples and (B) Epo- TEK 301-2FL, both excited by a 241 Am source Despite the superior detection performance of the 100mg/mL loading of BaF2:15%Ce in Epo- TEK 305 epoxy, the light intensity measured by the PMT remains far below that of single crystals. The best single crystal scintillator currently in use, NaI(Tl) generates photons with intensities nearly an order of magnitude higher than the nanophosphor composite scintillators, as shown in Figure 7. One way to mitigate this difference may be to increase the solids loading of the nanophosphor in the polymer matrix, providing a quantity of luminescent sites comparable to that of single crystals, which are composed entirely of scintillating material. Figure 7: Photon intensity generated by a NaI(Tl) single crystal
Transmission Measurements Figure 8(a) shows a transmission spectrum from 300-450nm for the polymer disks loaded with varying amounts of BaF 2 :15%Ce nanophosphors, while Figure 8(b) is a transmission spectrum for epoxies loaded with BaF 2 :2%Ce nanophosphors. The Epo-TEK 305 epoxy loaded with 100mg/mL BaF 2 :15%Ce exhibited the greatest transmission over the spectrum. This was unexpected to some degree as a disk containing a higher solids loading of nanophosphor would have a greater area of interfaces due to mismatched RI, and thus measured greater light scattering (as indicated by the lower %transmission of 100mg/mL BaF 2 :15%Ce in Epo-TEK 301-2FL). The discrepancy may be due to variations in particle agglomeration between samples or surface defects on the disk. However, it was expected that the Epo-TEK 305 epoxy would show improved transmission over Epo-TEK 301-2FL epoxy because according to the manufacturer provided data, Epo-TEK 301-2FL is not transparent to prolonged UV exposure. The results in Figure 8(b) indicate much lower transmission values for the epoxies loaded with BaF 2 :2%Ce nanophosphors. This is likely due to a combination of the much lower light yield of BaF 2 :2%Ce compared to BaF 2 :15%Ce and scattering from the epoxy matrix. These results suggest that it necessary to consider the photon efficiency of the phosphor in addition to RI matching and transmission efficiency of the matrix. Figure 9 shows a plot of the transmission from 300-450nm of the unloaded epoxies, and indicates that the addition of nanophosphor to the epoxy results in a nearly 50% decrease in the transmission values. These results suggest that matching the RI of the polymer matrix with the nanophosphors should improve transmission, and that an ideal polymer would have a RI of 1.485 at 389nm. (a) (b) Figure 8: %Transmission measurements of (A) BaF 2 :15%Ce and (B) BaF 2 :2%Ce loaded epoxies. (I) 301-2FL, 100mg/mL, (II) 301-2FL, 40mg/mL, (III) 305, 100mg/mL, and (IV) 305, 40mg/mL Figure 9: %Transmission measurements of unloaded epoxies
Conclusions & Future Work Nanophosphor composite scintillators promise to improve the durability and lower the cost of gamma-ray scintillators by improving photon yields and decreasing environmental sensitivity. Light scattering and low photon yield must be further investigated in order to elevate the practicality of nanophosphors to the level of single crystals. Future experiments will investigate the potential of Eu 2+ dopants to increase the photon yield. Photon scattering can also be reduced if a polymer or other material with a RI matching that of the nanophosphor (1.485 at 589nm) is procured. Acknowledgements The author gratefully acknowledges Dr. Jason H. Nadler and Dr. Zhitao Kang of the Georgia Tech Research Institute at the Georgia Institute of Technology for their supervision and guidance. Research was conducted as part of an initiative by the Domestic Nuclear Detection Office (DNDO) in response to the SAFE Port Act. References 1. C.W.E. van Eijk. Inorganic-scintillator Development, Nuclear Instruments & Methods in Physics Research, (A 460) (2001), 1-14 2. E.A. McKigney et al., Nanocomposite scintillators for radiation detection and nuclear spectroscopy, Nuclear Instruments and Methods in Physcis Research, A (579) (2007), 15-18 3. Brent K. Wagner, private communication with author, Georgia Tech Research Institute, 15 June 2009. 4. W.H. Tait. Radiation Detection; Butterworth: Boston, 1980 5. D.W. Cook, Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y 2 SiO 5 :Ce nanophosphors, Applied Physics Letters, 2006, vol. 88, 103 6. A.V. Golovin. Mechanism of short-wavelength luminescence of barium fluoride, Optics and Spectroscopy, 1988, vol. 65, 102 7. J. del-castillo. Wide colour gamut generated in triply lanthanide doped sol-gel nano-glassceramics, Journal of Nanoparticle Research, 2009, 789-884 8. M.B. Barta, Gamma Ray Scintillators via BaF 2 :Ce Nanopowders (Paper presented at the 96 th MS&T Annual Meeting, Pittsburgh, PA, 26 October 2009) 9. R.S. Meltzer et al., Effect of the matrix on the radiative lifetimes of rare earth doped nanoparticles embedded in matrices, Journal of Luminescence, (94-95) (2001), 217-220 10. B.D. Milbrath et al., Radiation detector materials: an overview, Journal of Materials Research, (23) (2008), 2561-2581