Overview of Glass used for Neutron Guides
Neutron guides are typically made of glass. Glass can be polished to roughness of a few angstroms and it can be machined to a few um precision. These are two important factors for building neutron guides.
There are different kinds of glass, which can be distinguished by their manufacturing process and their chemical composition.
For neutron optics, floated and cast glass are commonly used. The floated glass is solidified on liquid tin. By this process a low surface roughness is achieved. Usually there is no polishing needed for floated glass.
Borofloat and float glass (sodium-float glass) are examples for floated glass used for neutron guides. Borofloat glass is produced by Schott. Sodium-float glass, which is the standard glass for architecture glass, is produced by many suppliers for example Saint-Gobain, Guardian and Pilkington.
When exposed to a neutron flux, the Sodium and the Silicon in the Sodium-float glass emits high energy gamma rays and it stays active for some time (storage time 5.5 days). But it still has the longest lifetime of all glasses. For these reasons it is mainly - or was mainly - used close to the neutron source. There are two problems with Sodium-floatglass: First is the big waviness, we buy large plates and select the best parts, but the waviness is still much higher than for polished glass. The second problem is the quality assurance. There are many suppliers and a lot of distributors of floatglass. Usually you don't know the content and where it comes from. There is a big variety of elements allowed for floatglass. For example, depending on the raw material, the manufacturer use different kind of materials for purification. For architecture purpose, only the optical transmission is important. The content of bulk and surface of the glass is not important, not controlled and not measured. In many cases, the use of floatglass for inpile parts went well, but sometimes not. So there is a risk, which should be taken from the customer. More and more, inpile parts are manufactured from Aluminium.
Borofloat glass does content boron, which is a neutron absorber, for this reason it can be used with more variability, since it does not emit high energy gamma rays. But it is not far as resistant to neutron radiation as Sodium-float glass.
The absorption cross section for boron is 767 barn for thermal neutrons (2.2km/s, 25meV), for Sodium it is 0.53 barn. In borofloat glass, the neutrons are absorbed from boron before they hit sodium.
The best choice for neutronguides is Borkron glass.
Borkron like N-BK7 or S-BSL7 is a optical glass and manufactured for a optical performance within narrow tolerances. Borkron glass is a cast glass with Boron content. It is usually melted and cast into blocs or bars of different sizes.
Unlike floated glass, Borkron has to go through multiple processes before it can be used for neutron guides. The blocs have to be cut in plates. These plates then have to be grinded, lapped and polished. All those three steps can be done in-house at S-DH GmbH. Because of these additional steps Borkron is more expensive, than floated glass, but there are important advantages to Borkron.
When we build the first neutron guide in 1999, it was made of borofloat glass. We learned soon that the lifetime was not that high when exposed to a high neutron flux (grazing incidence!). Borkron seemed to be more promising, but at that time the polishing of Borkron was not well developed and the roughness was not sufficient for neutron mirrors with high reflectivity. We improved the superpolishing of the Borkron and were soon able to rise the reflectivity for m=2.0 from 80% to 90%. So Borkron became again attractive for neutron guides.
Tests for many years from the ILL confirmed our assumption about the lifetime. These tests showed, that Borkron is more resistant to neutron exposition and has a lifetime more than a factor 100 longer than borofloat. However, the polishing process makes the Borkron glass more expensive than borofloat. That was the reason for some customers to install borofloat guides in the past. Even self evacuated guides were build made of borofloat.
We strongly advise against it! Usually the higher price for Borkron glass will be compensated from the fact not to change the guide after some years. Not talking about security issues because of vacuum leaks due to cracks in the glass.
There are many suppliers of Borkron. Each supplier has different cast sizes, but not each size is always available. The biggest blocs have the size of 1000x700x150 mm and a weight of 300 kg. In the past we used BorkonN from Schott (similar to N-ZK7), but the limited availability of the glass lead us to test optical standard glass. In collaboration with the ILL we identified that N-BK7 and S-BSL7 from Ohara are the best types in terms of lifetime, quality, availability and price.
Even though these glasses are sufficient for most neutron optic applications, for some applications like in-pile parts they are unfortunately not sufficient. The requirements for in-pile parts are much higher, for this reason we started to develop metallic substrates for neutron guides in 2006.
Comparison of the two types of Standard Borkron and Borofloat
Nowadays we use mainly two different glass types, N-BK7 from Schott and S-BSL7 from Ohara. Both of them are manufactured to reach the same optcial properties. So it is no wonder, that the chemical content is very similar. But how big is the difference? The ILL wanted to know it precisely.
We choose randomly two glass pieces of both types and Romain Boffy from the group of Jerome Beaucour did a carefully examination of the content and the shrink after neutron radiation.
However, glass is a natural product and the composition vary from batch to batch. So the following data is just a reference point.