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In radiation environments such as outer space, the stratosphere, medical radiation equipment, and high-energy experimental facilities, quartz devices and optical components tend to suffer from performance degradation, making long-term stability a critical challenge.
To address these harsh conditions, NDK has a lineup of Ultra High Purity Synthetic Quartz Crystal "N-Grade EX" and "N-Grade EXs", which incorporates NDK's proprietary sweeping process to achieve even greater radiation resistance.
Challenges in Radiation Environments
１．Frequency Shift
Synthetic Quartz Crystals contain trace amounts of impurities such as aluminum (Al), lithium (Li), and sodium (Na).
These impurities exist in bonded states within the crystal lattice; however, when exposed to radiation, these bonds are broken, generating lattice defects and charge traps.
As a result, the elastic properties of the crystal change, causing a shift in the resonance frequency.
Such deviations directly affect the reference signals in communication, measurement, and control systems that use crystal units and oscillators.
In particular, for long-duration missions and high-precision equipment, such shifts can lead to fatal errors.
This phenomenon has been reported in earlier studies, including Shimoda & Uno (IEICE, 1989), which confirmed frequency shifts of quartz resonators under high-energy radiation exposure.
【Frequency Shift Mechanism of Crystal Units under Radiation Exposure】
２．Degradation of optical components
When quartz is exposed to radiation, defects known as color centers are formed within the crystal, which absorb light and cause browning (discoloration).
This phenomenon is a common type of degradation observed in many transparent materials, including optical glass, leading to reduced transmittance and changes in optical properties in the ultraviolet to visible wavelength range.
In quartz, trace amounts of aluminum (Al) contained within the crystal tend to act as initiation sites for this process, and the presence of Al promotes the formation of radiation-induced defects.
【Radiation Test of General Optical Synthetic Quartz Crystal】
"N-Grade EX" Ultra High Purity Synthetic Quartz Crystal : Achieving 99.99999% (7N) Purity Without Sweeping
NDK's Ultra High Purity Synthetic Quartz Crystal "N-Grade EX" was developed based on an approach that eliminates potential degradation factors within the material itself.
Even without swept processing, it achieves an ultra-high purity level of 99.99999% (7N) by almost completely removing Li and Na during crystal growth.
Its aluminum content is also extremely low, effectively suppressing color center formation and minimizing both frequency shift and optical browning compared to conventional materials.
Demonstration tests have also confirmed that even after 31 MGy irradiation, the Ultra High Purity Synthetic Quartz Crystal maintained its transmittance in the ultraviolet to visible wavelength range, demonstrating excellent radiation resistance for optical applications.
【Radiation Test of Ultra High Purity Synthetic Quartz Crystal :
"N-Grade EX"】
In general optical glasses, compensatory measures such as the addition of cerium are employed to suppress the formation of color centers caused by radiation exposure.
In contrast, quartz has a completely different crystal structure, which allows the elimination of the primary causes of radiation-induced degradation at the material design stage.
NDK has pursued this concept, achieving stability that maintains optical performance even under high radiation doses.
"N-Grade EXs" : Compatible with NASA Specifications
While conventional Swept Quartz often retains residual aluminum, NDK's N-Grade EXs is based on the Ultra High Purity Synthetic Quartz Crystal (N-Grade EX) and further refined through NDK's proprietary sweeping process, which reduces Li, Na, and Al concentrations to virtually zero.
As a result, it provides superior frequency stability and optical performance under radiation enviroments, enabling use under even more demanding conditions.
Moreover, N-Grade EXs is compatible with NASA specifications and provides the highest level of stability suitable for extreme environments, including long-duration missions.
【Comparison of Impurity Concentrations in Synthetic Quartz Crystals】
Note: These values are for reference only and are not guaranteed.
【NDK's Swept Process】
The swept process removes Li and Na ions from Synthetic Quartz Crystals by applying high temperature (530 to 550 °C) and high voltage (1 kV/cm).
NDK's proprietary method uses carbon electrodes, preventing electrode material diffusion and minimizing crystal damage while achieving ultra-high purity.
Application Potential
of N-Grade EX
and N-Grade EXs
N-Grade EX and N-Grade EXs are ideal for applications requiring long-term stability in harsh environments, such as those involving radiation exposure.
Space Equipment
（Satellites, Probes, Space Telescopes）
Ensures frequency stability and optical performance under continuous exposure to cosmic rays and solar flare radiation.
Stratospheric Communication
（HAPS）
A concept utilizing unmanned aerial vehicles (UAVs) as base stations flying long-term in the stratosphere.
Effective in environments with strong exposure to cosmic rays and gamma radiation, where frequency fluctuations are a concern.
Medical Eauipment
（Radiation Therapy Devices, PET, CT）
Resistant to optical degradation and offers excellent frequency stability in environments where high-dose radiation affects internal components.
NDK's N-Grade EX and N-Grade EXs are Synthetic Quartz Crystals that go beyond conventional processing technologies, achieving enhanced radiation resistance from the material level.
They maintain frequency stability and optical transparency even under severe radiation exposure, providing reliable performance for the most demanding environments.
NDK is currently evaluating the performance of products utilizing this material and ongoing verification toward future application development in line with customer needs.