Space qualified glass database

Mauro Ghigo, Paolo Spanò

INAF – Astronomical Observatory of Brera

January 29th, 2009

Summary- The radiation environment in the Lagrangian point L2

- Radiation damage mechanisms in glasses

- Properties of the radiation resistant glasses

- Comparison between “normal” BK7 and “hardened” BK7

- The Abbe diagram for Schott hardened glasses

- Other optical materials “naturally” radiation hard

- Considerations about the use of “normal” optical glasses

- Conclusions

The radiation environment in the Lagrangian point L2

Low energy protons (about 95%) and electrons from solar wind plasma with fluxes up to 1012 cm2/sec that can contribute to the degradation of surface materials. The proton energies can range from few eV up to 1 keV.

Transient radiation consisting of galactic cosmic ray particles and particles from solar events (flares and coronal mass ejections) having energies of up to about 100 – 200 MeV (again typically protons).

Ionizing Gamma, X and UV radiation

This environment is considered relatively moderate respect for example to the Van Allen belts

Radiation damage mechanisms in glassesSpace optics are damaged from ionizing radiation and particle fluxes:

Color centers (solarization): ionization caused by photons or particle radiation frees electron and holes in the glass. Optical absorption of visible wavelengths increases because of these defects, causing darkening of the glass. The loss of transmittance is most pronounced at the UV-edge of the spectrum.

Dimensional stability (compaction or expansion): observed after irradiation with protons or ions (atomic displacement). Also ionizing radiation can cause the effect. Generally is negligible. Alteration of the CTE of the material: is larger for low expansion glasses. Depending upon the requirements and planned operating temperature is an effect to be monitored.

Alteration of the refractive index: upper limit to proton irradiation induced change of refractive index of 0.0001 (dose of 20 Krads)

Radiation resistant optical glasses are necessary in all cases in which the total dose to be expected exceeds 103 rad.

Radiation resistant glasses

Optical glasses can be stabilized against transmittance loss caused by radiation by adding Cerium to the composition. The added cerium changes the intrinsic color of the glass. The transmittance edge is shifted to longer wavelengths. In general the Cerium content is kept low enough to keep this effect small (1-2 % in weight).

Schott optical glasses with the addition of Cerium are designated by the affix “G”, and by a code number which corresponds to the percent Cerium oxide content multiplied by tenExample: BK7G18

The Abbe diagram for Schott hardened glasses

This modified Abbe diagram contains all currently available stabilized glass types. The diagram shows that not only the transmittance curve but also the optical position of the stabilized glass types varies slightly from the non stabilized catalog glass.

Although a wide range of glasses with radiation resistance have been produced by Schott (in 1998 they had 16 off the shelf + 15 as special order items, only a part are nowadays readily available commercially (a special melt, producing about 180 kg of glass, would cost of the order of £25000)

Summary of properties of the radiation resistant glasses from SCHOTT

Glass name nF (480 nm) d Coeff. of thermal expansion (10-6K-1) Tg (C)

SF8G07 1.711 30.7 8.4 428

BK7G25 1.527 63.2 7.0 570

Data 2007

Many optical glasses and crystals are manufactured today with much higher purity than in the past in order to increase their ultraviolet transmission properties. Consequently these glasses are much more resistant to space radiation than in the past…..

Other optical materials “naturally” radiation hard

“Normal” glasses SF6 and SF59 shows little or no spectral transmission loss after proton irradiation and only a small loss (2 to 6%) or none, respectively, after gamma irradiation (ESA)

- Calcium Fluoride (CaF2 - synthetic single crystal excimer grade, free from contaminants)

- Barium Fluoride (BaF2 – monocristalline with wide transmission range)

- Magnesium Fluoride (MgF2 – radiation resistant, wide transmission range)

- Sapphire (Al2O3 – very radiation resistant, wide transmission range, high refractive index and dispersion)

- Synthetic Fused Silica (SiO2 - some losses in the ultraviolet with exposure to radiation, but has very minimal losses in the visible portion of the spectrum)

- Crystalline Quartz (no detectable solarization after irradiation)

- Infrared glasses (Zinc Sulfide (Cleartran), Zinc Selenide, Silicon, Germanium)

Considerations about the use of “normal” optical glasses

If the low number of commercially available radiation resistantglasses is insufficient to permit a good optical design and hence non-stabilized glass types must be used, sufficient shielding by other glass elements and/or by the metallic tube structure is an option that could be pursued.

Implementation of a front lens made of less sensitive materials (e.g. high purity fused silica) can be a solution to avoid degradation effects on the inner lenses.

It worth to remember also that for precision optics the use of optical cements must be avoided completely in exposed areas

Conclusions For the design of space optics it is necessary to determine in each case the total dose to be expected during the mission together with the dominant type of radiation and to define the most appropriate design configuration based upon that.

ESA should have more detailed infos about the radiation environment that the PLATO mission could face.

It is preferable that Cerium stabilized glasses (and other insensitive materials, like highest grade fused silica, etc.) be used for space optics.

An alternative approach could allow the partial use of non-stabilized glasses

The present database of space qualified glasses need to be further improved