SrI2 scintillator for gamma ray spectroscopy

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1 SrI scintillator for gamma ray spectroscopy N.J. Cherepy*, B.W. Sturm, O.B. Drury, T.A. Hurst, S.A. Sheets, L.E. Ahle, C.K. Saw, M.A. Pearson and S.A. Payne Lawrence Livermore National Laboratory, Livermore, CA 955 A. Burger Fisk University, Nashville, TN 378 L.A. Boatner, J.O. Ramey Oak Ridge National Laboratory, Oak Ridge, TN E.V. van Loef, J. Glodo, R. Hawrami, W.M. Higgins and K.S. Shah Radiation Monitoring Devices, Watertown, MA 7 W.W. Moses Lawrence Berkeley National Laboratory, Berkeley, CA 97 ABSTRACT We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of -based gamma ray spectrometers. offers a light yield in excess of, photons/mev and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of and EuI indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with (-6%Eu) of.6% at 66 kev and 7.6% at 6 kev with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu + -doped crystals. The light yield proportionality of is found to be superior to that of other known scintillator materials, such as and NaI(Tl). Keywords: Strontium Iodide, scintillator, gamma ray spectroscopy 1. INTRODUCTION Gamma ray spectroscopy of weak sources requires detector materials capable of high energy resolution, but also growable to large volumes at low cost. The lanthanum halides, and LaCl 3 offer energy resolution of.6-3.3% at 66 kev, but their growth is challenging [1]. Our collaboration evaluated europium-doped alkaline earth halides (BaI, BaBr,, SrBr, CaI and mixtures thereof) for their scintillation properties over the past few years, and identified strontium iodide as readily growable, offering high light yield, and capable of high energy resolution gamma ray spectroscopy [-8]. A few other reports of scintillation from the family of alkaline earth halides have been made [9-1]. CaI produces ~, Photons/MeV, however it possesses a hexagonal crystal structure with pronounced basal cleavage, making it is nearly impossible to grow substantial crystals. BaI and BaBr are orthorhombic crystals, and thus much more readily growable, although their light yields are only ~35, Ph/MeV. Eu-doped strontium iodide was originally reported by Hofstadter in 1968 to produce light yields of <, Ph/MeV, and gamma ray spectroscopy inferior to his better-known scintillator material, sodium iodide doped with thallium, NaI(Tl). In our work, we have found that with improved crystal growth feedstock and growth methods, we can achieve much improved performance. In fact, due to its intrinsic excellent light proportionality, energy resolution of ~% at 66 kev may be possible with. We have measured energy resolution of.6-.8% at 66 kev with small (< 1 cm 3 ) (-6%Eu) crystals, from each of the crystal growers in our collaboration. For larger crystals, energy resolution degrades. Two possible mechanisms may contribute to the degradation of resolution, and it may be possible to mitigate each of them separately. The first mechanism is inhomogeneity of the crystal or optics, either in doping or morphology, leading to non-uniform scintillation light generation as a function of position in the crystal. Crystal material and optical non-uniformity should be addressed, for example, by improving feedstock purity and exercising care in the crystal growth to eliminate thermal gradients. The second mechanism involves the scintillation photon transmission through the crystal and reflector assembly. Due to substantial overlap between the Eu optical absorption and emission, scintillation photons traversing a long path length will have a high probability of re-absorption and re-emission from additional Eu + activators, lengthening the pulse lifetime, red-shifting the emission, increasing the losses due to bulk and reflector absorption and thus reducing the pulse height for those events that have undergone light trapping or radiation trapping, as described *cherepy1@llnl.gov; ; MS-L181, Lawrence Livermore National Laboratory, Livermore, CA 955 Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XI, edited by Ralph B. James, Larry A. Franks, Arnold Burger, Proc. of SPIE Vol. 79, 79F 9 SPIE CCC code: X/9/$18 doi: / Proc. of SPIE Vol F-1

2 in Glodo, et al. [8]. Here, we address several topics critical to the development of scintillators: (1) the thermal properties of the strontium and europium iodide feedstock, needed to optimize the crystal growth parameters, () gamma ray spectroscopy with and (3) the light yield proportionality of.. EQUIPMENT AND METHODS Single crystals of were grown with the vertical Bridgman method in quartz ampoules, using a mixture of commercially available and EuI materials at doping levels of -6%, at Fisk University, Oak Ridge National Laboratory and Radiation Monitoring Devices (RMD). Differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) under dry Argon and powder x-ray diffraction under vacuum were performed to study the dehydration of the feedstock, the phase transition, melting point and the thermal stability of and EuI. For scintillation measurements, pulse height spectra with Ba-133, Am-1, Co-57, Na-, Co-6, and Cs-137 sources were acquired using a Hamamatsu R631U PMT (QE at 35 nm of 3%) and wrapping with Teflon reflector materials. The signals from the PMT anode were shaped with a Tennelec TC spectroscopy amplifier (8 μs shaping time) and recorded with an Amptek MCA8-A multi-channel analyzer; or by interfacing with a Bridgeport Instruments emorpho digitizer/multi-channel analyzer. The design of the Scintillator Light-Yield Non-proportionality Characterization Instrument (SLYNCI) is described in detail in reference RESULTS AND DISCUSSION 3.1 Thermal and Crystallographic Characteristics. DSC measurements of typical and EuI feedstock are shown in Fig. 1, left. The DSC and TGA reveal the desorption of <.1 wt% waters of hydration, well below melting, and the melting points for and EuI were found to be 53 º C and 51 º C, respectively. No phase changes are observed upon cooldown. The x-ray diffraction spectra acquired as a function of temperature identify the hydrates present as a function of temperature under vacuum (Fig. 1, right). Lastly, precise x-ray diffraction studies as a function of temperature of the dehydrated orthorhombic phase were analyzed to deduce the expansion coefficients (CTE s, Table 1) along the three crystalline axes, indicating modest anisotropy and a possible advantage of seeded growth. -1 Hydrate desorption Heat flow (uw) EuI Melting -5 3 Temperature ( C) 5 6 Figure 1. (left) Differential scanning calorimetry measurements of and EuI revealing very similar melting points. (right) X-ray diffraction spectra as a function of temperature, showing the characteristic spectra for the different hydrates that can be present during feedstock dehydration. Table 1. Thermal expansion coefficients for Strontium Iodide. Coefficient of Thermal Expansion (ºC) Lattice a x 1-5 Lattice b-.16 x 1-5 Lattice c-.9 x 1-5 Volume.66x 1-5 Density x 1-5 Proc. of SPIE Vol F-

3 3. Gamma Ray Spectroscopy. Figure shows that energy resolution obtained with small (5%Eu) crystals, <.5 cm 3, is superior to that of and significantly better than that of NaI(Tl). At higher energies, the resolution advantage for diminishes, maybe due to non-uniformity of the larger volume sampled by higher energy gammas. As crystals get larger, the energy resolution has been typically somewhat degraded. This may be a function of material inhomogeneity and/or optical properties that are not quite uniform, and thus the statistics are not as good for larger crystals. Figure 3 shows a pulse height spectrum obtained with a 9 cm 3 crystal, acquired with a Cs-137 source. The resolution of 3.38% at 66 kev is not quite as good as typically obtained for small crystals. 6 Am kev 9.79% 1 x-rays % Co Cs % 5.% NaI(Tl) Figure. Gamma spectra of compared with and NaI(Tl). (top left) Photograph of the. cm 3 crystal of (5%Eu) crystal grown at Fisk University used to acquire this data. Spectra were acquired with several sources, including (top right) Am-1, (middle left) Co-57, (middle right) Ba-133 and (bottom left) Cs-137. (bottom right) Energy resolution as a function of gamma energy for the (5%Eu) crystal compared to a commercial Brillance 38 crystal from St. Gobain. Energy Resolution (%) Ba NaI(Tl) LaB r 3 Gamma Energy (KeV) Figure 3. (left) Pulse height spectra acquired for a 9 cm 3 crystal using a Cs-137 source. (right) Photograph of the 9 cm 3 (%Eu) crystal, grown at ORNL, illuminated with a UV lamp and emitting the characteristic Eu + blue luminescence. Proc. of SPIE Vol F-3

4 For larger crystals, energy resolution is typically degraded in a characteristic manner. The photopeak exhibits a high energy tail, as shown in the Cs-137 pulse height spectrum, acquired using analog electronics and uniform gamma irradiation, in Figure (top), for a cm 3 crystal. When a collimated Cs-137 source is used to characterize the same crystal, Figure (bottom), the nature of the inhomogeneity of the photopeak is revealed to be related to the position of gamma interaction in the crystal. Then, a digital acquisition system was used to collect scintillation traces and compare the pulse height values to the decay time for each event. The pulse height spectrum acquired with the digital system is shown in Figure 5 (top), and the relationship between the pulse height and decay time is shown in Figure 5 (bottom), revealing that the photopeak events with the longest decay have the smallest pulse height, and vice versa. Figure. (top) Pulse height spectra acquired for a cm 3 crystal, under uniform irradiation and (bottom) several spectra acquired using a collimated Cs-137 source, as shown in the diagram ADC voltage (V) 5x1-3 Figure 5. (top) Pulse height spectrum acquired using a digitizer, of the cm 3 RMD crystal under uniform irradiation with a Cs-137 source. The lineshape is inhomogeneous, with a high energy tail, in good agreement with the data acquired with the analog system, shown in Fig.. (bottom) When the pulses are analyzed comparing decay time and pulse height, the photopeak pulse height position (in the - x 1-3 range) can be observed to depend directly on the decay time, as is expected for events in which significant light trapping occurs. Proc. of SPIE Vol F-

5 3.3 Scintillation Light Yield Non-Proportionality Characterization. Figure 6 shows that the light yield proportionality obtained with the SLYNCI for multiple different crystals, 1-9 cm 3 in size, and with varying doping levels, is excellent. Fits to this data (not shown) indicate that the limit to energy resolution for is about % at 66 kev [1]. Relative Light Yield RMD (.5%Eu) ORNL (%Eu) ORNL (6%Eu) NaI(Tl) Electron Figure 6. SLYNCI data acquired for several crystals with.5-6% Eu indicates slightly better proportionality for higher Eu doping. proportionality is markedly superior to that of NaI(Tl) and even better than that of.. CONCLUSIONS Strontium iodide doped with europium possesses good thermal and crystallographic properties for successful large-size crystal growth. High Eu-doping leads to light trapping which complicates the acquisition of high resolution gamma ray spectra, however, there may be ways to correct the data via digital signal processing. Light yield proportionality of is unsurpassed among known scintillators and therefore further development to achieve its limiting resolution for gamma ray spectroscopy of ~% at 66 kev is underway. ACKNOWLEDGEMENTS This work was supported by the Domestic Nuclear Detection Office in the Department of Homeland Security (Alan Janos) and by the National Nuclear Security Administration, Office of Nonproliferation Research and Development (NA-) of the U.S. DOE under Contract No. DE-AC3-76SF98, was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC5-7NA73. Oak Ridge National Laboratory is managed for the U.S. DOE by UT-Battelle under contract DE-AC5-OR75. REFERENCES [1] E.V.D. van Loef, P. Dorenbos, C.W.E. van Eijk, K. Kramer, H.U. Güdel, High-energy-resolution scintillator: Ce 3+ activated, Appl. Phys. Lett, vol. 79, p. 1573, 1. [] N.J. Cherepy, G. Hull, T.R. Niedermayr, A. Drobshoff, S.A. Payne, U.N. Roy, Y. Cui, A. Bhattacharaya, M. Harrison, M. Guo, M. Groza, A. Burger, Barium iodide single-crystal scintillator detectors, Proc. SPIE, 676, (7). [3] N.J. Cherepy, G. Hull, A. Drobshoff, S.A. Payne, E. van Loef, C. Wilson, K. Shah, U.N. Roy, A. Burger, L.A. Boatner, W-S Choong, W.W. Moses Strontium and Barium Iodide High Light Yield Scintillators, Appl. Phys. Lett. vol. 9, p. 8358, 8. [] R. Hawrami, M. Groza, Y.Cui, A. Burger, M.D Aggarwal, N. Cherepy and S.A. Payne,, a Novel Scintillator Crystal for Nuclear Isotope Identifiers, Proc. SPIE, Vol. 779, 779 (8). [5] C.M. Wilson, E.V. Van Loef, J. Glodo, N. Cherepy, G. Hull, S.A. Payne, W.-S. Choong, W.W. Moses, K.S. Shah, Strontium iodide scintillators for high energy resolution gamma ray spectroscopy, Proc. SPIE, Vol. 779, 77917, (8). [6] N.J. Cherepy, S.A. Payne, S.J. Asztalos, G. Hull, J.D. Kuntz, T. Niedermayr, S. Pimputkar, J.J. Roberts, R.D. Sanner, T.M. Tillotson, E. van Loef, C.M. Wilson, K.S. Shah, U.N. Roy, R. Hawrami, A. Burger, L.A. Boatner, W.-S. Choong, W.W. Moses, Scintillators with Potential to Supersede Lanthanum Bromide, IEEE Trans. Nucl. Sci. vol. 56, (9). [7] E.V.D. van Loef, C.M. Wilson N.J. Cherepy, G. Hull, S.A. Payne, W- S. Choong, W.W. Moses, K.S. Shah, Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators, IEEE Trans. Nucl. Sci., vol. 56, (9). [8] J. Glodo, E.V. van Loef, N.J. Cherepy, S.A. Payne, and K.S. Shah Concentration effects in Eu-doped, IEEE Trans. Nucl. Sci., in press (9). Proc. of SPIE Vol F-5

6 [9] R. Hofstadter, E. W. O Dell, and S. T. Schmidt, IEEE Trans. Nucl. Sci. NS-11, p. 1, 196. [1] R. Hofstadter, Europium Activated Strontium Iodide Scintillators, U.S. Patent No. 3,373,79, 1 March [11] R. Hofstadter, IEEE Transactions on Nuclear Science.vol.NS-7, Feb.198, p [1] J. Selling, M. D. Birowosuto, P. Dorenbos, S. Schweizer, J. Appl. Phys., vol. 11, p.391, 7. [13] W.-S. Choong, K.M. Vetter, W.W. Moses, S.A. Payne, N.J. Cherepy, J.D. Valentine and G. Hull, Design of a Facility for Measuring Scintillator Non-Proportionality, IEEE Trans. Nucl. Sci., vol. 55, p. 1753, 8. [1] S.A. Payne, N.J. Cherepy, G. Hull, J.D. Valentine, W.W. Moses and W.-S. Choong Nonproportionality of Scintillator Detectors: Theory and Experiment, IEEE Trans. Nucl. Sci., in press (9). Proc. of SPIE Vol F-6