50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2143.pdf

PROGRESS ON THE DEVELOPMENT AND TESTING OF NEW SCINTILLATORS FOR PLANTARY GAMMA-RAY SPECTROSCOPY. T. H. Prettyman1, A. Burger2, N. Yamashita1, R. Hawrami2, E. Ariesanti2, E. Rowe2, V. Buliga2, N. Pearson1, M. E. Landis1, 1Planetary Science Institute, Tucson AZ ([email protected]), 2Fisk University, Nashville TN.

Introduction: Since the 1960s, planetary gamma ray emissions have been measured by sensors deployed on orbiting spacecraft and landers [1]. Gamma rays are produced by the decay of natural radioelements found in rock-forming minerals and by the steady interaction of cosmic rays with surface materials. The gamma ray leakage spectrum can be analyzed to determine the elemental composition of the regolith to depths up to a meter, providing constraints on planetary evolution and formation, and complementing information provided by other remote sensing methods. The earliest planetary spectrometers were scintilla- tors, which are transparent to ultraviolet and visible light emitted in response to excitation by ionizing radi- ation. For example, the interaction of gamma rays with the scintillator produces swift elections that ionize the medium. Scintillation light is detected by an optical Figure 1. Gamma ray pulse height spectrum acquired by the sensor, such as photomultiplier or photodiode and con- NASA Dawn mission’s Gamma Ray and Neutron Detector verted to a pulse height spectrum. Peaks in the spec- from low altitude at dwarf planet Ceres [5]. trum identify specific nuclear reactions occurring with- yield and electron energy for energies lower than about in the planetary surface, from which the target ele- 100 keV. This results in additional variability in light ment/isotope can be identified (Fig. 1). The intensity of production, further broadening gamma ray peaks. Non- the peaks is proportional to elemental concentration, proportionality is an intrinsic property of scintillating subject to correction for various systematic effects. materials and is a major factor limiting energy resolu- The energy resolution currently achieved by scintil- tion [6, 7]. For example, despite having lower light lators is inferior to cryogenically-cooled, intrinsic ger- yield, CLYC has better energy resolution than NaI(Tl), manium, the “gold standard” for gamma-ray spectros- (Table 1). For CLYC, contributions from nonpropor- copy [e.g. 2]. Nevertheless, scintillators have ample tionality are relatively low compared to NaI(Tl). Im- resolution for many applications and they fill an im- provements in the resolution of inorganic scintillators portant niche for missions that require rugged, low-cost have resulted primarily from the discovery of crystals sensors that operate at ambient temperatures, with min- with low nonproportionality. imal operational complexity and low impact on pay- Detection efficiency is another key consideration in load resources. We describe the development, testing the design of planetary gamma ray spectrometers. Effi- and evaluation of the next generation of scintillators, ciency is determined by crystal size, geometry and including europium-activated strontium iodide, SrI2:Eu gamma ray attenuation coefficient of the scintillating [e.g. 3], and self-activated Cs2HfCl6 (CHC) [e.g. 4]. material. The latter increases with the density and ef- These scintillators have attributes attractive for plane- fective atomic number (Zeff) of the scintillator (Table tary applications, including improved energy resolution 1). Bismuth germanate has the highest Zeff of any scin- compared to scintillators with flight heritage, such as tillator used for planetary applications. Large, BGO bismuth germanate (BGO) [5], and high efficiency for crystals can be manufactured, resulting in very high gamma ray detection. They are not hygroscopic and are detection efficiency. easier to grow due to their cubic structure. New scintillators combine high light yields and low Scintillator selection: Gamma ray energy resolu- nonproportionality resulting in up to a factor of four tion is fundamentally controlled by the light yield of improvement in resolution compared to BGO. Of the scintillator, the number of photons produced per these, LaBr3:Ce is not well suited for planetary applica- MeV of energy deposited by swift electrons. Statistical tions, despite having flight heritage [8]. Self-activity fluctuations in the number of photons detected results (radiolanthanum) obscures planetary gamma ray emis- in broadening of the gamma ray peaks. Moreover, all sions up to 3 MeV. Strontium iodide, which yields scintillators exhibit nonproportionality between light 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2143.pdf

higher efficiency with equivalent energy resolution, the BGO crystals in visible to infrared wavelengths. and is therefore the focus of our research (Table 1) [9]. The samples were placed in storage at room tempera- In addition, we are evaluating CHC for planetary ap- ture for a long time prior to characterization. Thus, our plications. Its cubic structure will enable the growth of preliminary experiments constrain the effects of deep large crystals that are not hygroscopic. Substitution of defects introduced by exposure to protons. Cs with Tl may yield scintillators with improved ener- Next Steps: Based on lessons learned, additional gy resolution and efficiency equivalent to BGO [10]. accelerator testing is planned to explore radiation dam- Testing and evaluation: Specific attributes of age effects and annealing of SrI2:Eu and CHC crystals scintillators evaluated by our project include suscepti- fabricated at Fisk University along with BGO crystals bility to radiation damage, sources of background inter- from commercial sources. The proton response of the ference, and the response to gamma rays and energetic scintillators will also be characterized. These studies particles. In addition, we are optimizing the mechanical will provide data needed for sensor design and to de- and optical design of scintillation detectors for space velop a models of damage effects useful for mission applications, including the evaluation of silicon photo- planning. The models will be validated against flight multipliers for compact sensors deployed on microsat- data from the Dawn mission. In addition, are develop- ellites. ing prototype instrument for planetary applications Preliminary radiation damage tests of SrI2:Eu were using large volume (2” × 2” right circular cylinder) carried out at the Loma Linda University Medical Cen- SrI2:Eu scintillator, which will be ready for testing in ter Proton Facility. Strontium iodide ingots were grown 2019. using the vertical Bridgman process by Fisk University. References: [1] Prettyman T. H., "Remote Sensing of Small crystals (10×10×10 mm3) were harvested from Chemical Elements Using Nuclear Spectroscopy," the ingots and polished, etched and encapsulated. Pre- Encyclopedia of the Solar System (Third Edition), T. Spohn, irradiation tests included measurement of pulse height T. Johnson and D. Breuer, eds., pp. 1161-1183: Elsevier, 2014. [2] Goldsten J. O. et al. (2007), Space Science resolution and gamma-ray nonproportionality. The Reviews, 131, 1, 339-391. [3] Cherepy N. J. et al. (2008), crystals were exposed to a range of doses, up to Applied Physics Letters, 92, 8, 083508. [4] Burger A. et al. 20 krad in 100- and 200-MeV proton beams. Bismuth (2015), Applied Physics Letters, 107, 14. [5] Prettyman T. H. germanate crystals of the same size were simultaneous- et al. (2019), LPS L, Abstract #1356. [6] Cherepy N. J. et al. ly exposed. These served as reference that can be con- (2009), IEEE Transactions on Nuclear Science, 56, 3. [7] nected to doses received in flight. For example, BGO Valentine J. D. et al. (1998), IEEE Transactions on Nuclear on Dawn was exposed to ~0.2 krad/yr during more than Science, 45, 3, 512-517. [8] Zhu M. H. et al. (2013), Sci 10 years in deep space [11]. A noticeable loss in gain, Rep, 3, 1611. [9] Prettyman T. H. et al. (2015), SPIE Newsroom, DOI: 10.1117/2.1201510.006162. [10] Fujimoto likely due to darkening of the BGO crystal in response Y. et al. (2018), Sensors and Materials, 30, 7, 1577–1583. to radiation damage, was observed during flight. Nev- [11] Prettyman T. H. et al. (2003), Nuclear Science, IEEE ertheless, performance of the sensor was acceptable Transactions on, 50, 4, 1190-1197. [12] Knoll G. F., through the end of the mission. Radiation detection and measurement: John Wiley & Sons, Post-irradiation characterization of SrI2:Eu crystals Inc., 1989. revealed decreased gain with exposure, likely the result Acknowledgements: This work is supported by the of damage induced darkening; although, neither differ- NASA Planetary Instrument Concepts for the Ad- ential nonlinearity nor energy resolution were signifi- vancement of Solar System Observations (PICASSO) cantly affected. Two-way transmission measurements program, grant number NNX16AK42G. using a spectrophotometer demonstrate darkening of

Table 1. Selected properties of some inorganic scintillators. Data for light output and energy resolution are from [4, 6, 12]. Quoted values for resolution are for small crystals with optimized light collection and readout.

*MFP200 is the approximate mean free path of 200 keV gamma rays.