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The Crystal Growth of Cs2LiYCl6:Ce Scintillators in Microgravity (CLYC-Crystal Growth) investigation uses a new crystal that, for the first time, effectively detects both gamma-rays and neutrons. High-quality crystals are essential to a wide variety of applications and the microgravity environment produces better quality crystals. Analyzing CLYC crystals grown in microgravity helps researchers better understand exact conditions needed to produce the highest-quality, defect-free crystals. Insights from this work support commercial scale-up of CLYC production on Earth.
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The following content was provided by Alexei Churilov, Ph.D., and is maintained in a database by the ISS Program Science Office.
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Alexei Churilov, Ph.D., Radiation Monitoring Devices, Inc, Watertown, MA, United States
Radiation Monitoring Devices, Inc., Watertown, MA, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
National Laboratory (NL)
Earth Benefits, Scientific Discovery
ISS Expedition Duration
- September 2017
ISS Flight 11A
ISS Flight LF-1.1
For the Crystal Growth of Cs2LiYCl6:Ce Scintillators in Microgravity (CLYC-Crystal Growth) investigation, a series of four CLYC growth runs are planned for both the International Space Station (ISS) Solidification Using a Baffle in Sealed Ampoules (SUBSA) and the ground-based SUBSA furnaces. In the ground-based runs, the SUBSA furnace is oriented vertically to replicate the vertical Bridgman approach used in production at Radiation Monitoring Devices, Inc. (RMD).
The CLYC-Crystal Growth experiments plan to address the following research questions:
Does reduced buoyancy-driven convection in microgravity delay the onset of secondary phase precipitation in CLYC melt?
Does reduced mixing of segregated components result in less inclusions in CLYC crystals?
If detached growth is achieved, does the reduced contact between the growing crystal and ampoule result in less stress and better crystallinity?
Can the problem of melt leaking between the growing crystal and ampoule wall be completely eliminated in absence of Earth gravity and if so, is the quality of crystal improved?
In absence of Earth gravity force, which force dominates the contact between CLYC melt and fused quartz ampoule wall: melt surface tension or wetting of fused quartz?
Is the concentration of gas bubbles increased or reduced without gravity?
Answering these questions immediately provides insight on which crystal growth methods to use on Earth, how to improve ampoule and furnace design, and which crystal growth parameters to change and in which direction.
Crystal growth of Cs2LiYCl6:Ce (CLYC) presents a number of challenges, which limit the yield of high quality crystals. The hypothesis for Crystal Growth of Cs2LiYCl6: Ce Scintillators in Microgravity (CLYC-Crystal Growth) is that microgravity conditions would allow for the elimination, reduction, or isolation of some of the process parameters related to gravity, in order to determine the magnitude of their effect on grown crystals. This would allow focusing on optimization of parameters with the largest impact on quality, for production on Earth, which is the ultimate goal.
The main features of crystal growth in microgravity are absence of buoyancy-driven convection and weightlessness of the melt. Without convection in the melt, it is expected to achieve nearly diffusion-controlled transport of CLYC components in the melt (CsCl, LiCl, and YCl3), which should produce more axially homogeneous crystals. In addition, the weightlessness of the melt should eliminate the gravity-related problem of melt leaking into the gap between crystal and ampoule wall, which is typically observed during CLYC crystal growth on Earth.
The proposed microgravity research project focuses on developing a better understanding of the mechanisms that govern defect formation during CLYC crystal growth. The main defects include secondary-phase inclusions, cracks, and bubbles, which can degrade scintillation performance and production yield. While some understanding of the mechanisms involved in the defect formation through empirical observations on Earth has been developed, it is believed that growth in microgravity will have profound effects and further elucidate the processes.
The specific microgravity crystal growth experiments aim to answer the basic questions of how the defects originate and how they can be controlled. For the issue of secondary-phase inclusions, it is believed that there are two main causes, both of which should be greatly affected by gravity. CLYC does not grow from a stoichiometric melt and thus, secondary-phase compounds materialize along with CLYC at the tip and tail of the boules. Convective mixing of the melt is thought to be a driver for the precipitation of secondary-phase inclusions in the CLYC. Reduced convection in microgravity should thus result in fewer such inclusions and suggest approaches for ground-based solutions. Another cause of secondary-phase inclusions is thought to be diffusion of excess lithium into the crystal from the periphery after solidification. Lithium-rich melt that collects at the tail end of CLYC crystal growth has a much lower solidification temperature than CLYC itself. Therefore, in vertical Bridgman the lithium-rich melt can drip down the sides under the force of gravity when the crystal shrinks away from the ampoule wall. Greater concentrations of the secondary phase inclusions have been observed near the periphery of CLYC crystals where the dripping of Li-rich melt is more apparent, suggesting the possibility that lithium could diffuse into the crystal and produce inclusions. Since the Li-rich melt is relatively reactive, it also degrades the outside of the crystal and can lead to the crystal sticking to the ampoule. This sticking is a leading driver of crack formation because the crystal shrinks substantially as it cools. Therefore, the microgravity environment should prevent dripping of the Li-rich melt and reduce both inclusions and cracking.
Another common defect in CLYC crystals is gas bubbles, although it is a lesser problem, as compared to cracking and secondary-phase inclusions. Nevertheless, it is important to better control them for optimal crystal quality. It is expected that the bubble distribution depends on melt mixing, as well as the flotation effect of a gas bubble in liquid. Both of these effects should be greatly affected in microgravity, and thus a significant difference in bubble distribution is expected.
To address the research questions described above, the Solidification Using a Baffle in Sealed Ampoules (SUBSA) furnace on the International Space Station (ISS) for a series of four CLYC crystal growth runs is utilized. The project greatly benefits from previous experience of the investigators on the team with these facilities and equipment, who in the summer of 2002, grew eight single crystals of indium antimonide (InSb), doped with tellurium (Te) or zinc (Zn), using directional solidification in microgravity. The main goal of the SUBSA project was to achieve reproducible diffusion-controlled dopant segregation with initial transient and steady state segregation. In a similar approach, the CLYC-Crystal Growth investigation performs four equivalent crystal growth runs in the SUBSA furnace on Earth to obtain direct comparison of the gravitational effects. The four crystal growth runs include variations in crucial growth parameters to evaluate their effects in conjunction with microgravity:
Seeded, stoichiometric composition;
Seeded, optimized composition;
Unseeded, stoichiometric composition;
Unseeded, optimized composition.
While the ground-based SUBSA growth runs provide a direct comparison to the microgravity results, the results are also compared to the substantial experience of CLYC growth using the vertical Bridgman method at Radiation Monitoring Devices, Inc. (RMD). It is expected that the knowledge gained in the microgravity work will lead to innovative new approaches to CLYC growth on the ground and thus provide a significant commercial benefit in the scale up of the production process.
After receiving the CLYC crystals grown in space, RMD performs a comprehensive evaluation of all crystals that are grown within this program, including the ground-based ones. This includes crystal characteristics, as well as the scintillation performance. The crystal properties to be evaluated include visible and microscopic defects that can affect the light output, such as cracks, inclusions, and bubbles. For each crystal, the density and distribution of all defects are measured to enable an effective comparison between crystals. The scintillation performance is also measured at RMD using various gamma-ray and neutron radiation sources. The important scintillation properties include: light output, energy resolution and linearity, emission spectrum, and pulse rise/decay characteristics. The scintillation performance provides an important quality test to gauge the overall effects of all defects contained in the crystals.
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These experiments address important questions about crystal growth in microgravity versus normal gravity. Results contribute to improvement in the methods used to produce crystals in space and provide insight on the best methods for crystal growth on Earth. In general, this investigation furthers human space exploration efforts by contributing to technological and biological advancements.
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A compact, inexpensive and more sensitive single instrument for gamma ray and neutron detection has tremendous potential in nuclear and radiological applications. Such detectors have use in homeland security and nuclear non-proliferation applications, oil and gas exploration, particle and space physics, non-destructive testing, and scientific instruments. Commercial availability of CLYC detectors supports development of next-generation instruments for these uses.
Operational Requirements and Protocols
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Four sealed ampoules with CLYC charge materials are processed in microgravity. The spacing between the experiments is not critical. There are 4 runs of the CLYC Melt Growth (10 days), 3 runs of the InI Melt Growth (2.5 days) and 3 runs of the InI Vapor Growth (7.5 days). The downlink data consists of live video and 5 thermocouple readings. Ampoules are returned for processing.
Decadal Survey Recommendations
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Radiation Monitoring Devices, Inc.
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Crystal Growth of Cs2LiYCl6: Ce Scintillators in Microgravity (CLYC-Crystal Growth) defects in CLYC crystals. Image courtesy of Radiation Monitoring Devices, Inc.
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