Vertex detector geometries of the two design options. Left: 5 single ladders (VTX-SL). Right: 3 double ladders (VTX-DL).
Unfortunately the aluminium is only transparent for a very short moment, so there is still a long way until the material scientists can compete with Scotty. And 50nm is also not really a thickness one can handle, and of course not build a basin to transport a pot whale through time….
But why am I interested in thin material? Something really thin but very stable is what we need to build the vertex detector at the Linear Collider. The vertex detector is the innermost detector directly around the beam pipe measuring the first few cm of the created particles’ tracks (vertex). We need to build this really thin to allow high precision tracking also with low momentum tracks. If we chose the material to thick, this for our studies very important particles loose too much energy before they can reveal their secrets to us.
And by “thin” we do not mean only the thickness measured in mm, but also the material properties are counted. We usually speak of radiation length X0, a characteristic of a material, related to the energy loss of high energy particles when passing through the material. X0 is both, the mean distance over which a high-energy electron loses all but 1/e* of its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. We usually do not give the length itself, but we rather say that a certain thickness of a material is x % of a X0. This helps us to compare material directly. The radiation length X0 for Silicon is 93.7 mm so we say 100micrometer of Silicon is 0.106% of an X0. But for example 100 micrometer of lead are 1.9% of X0, so it is way less “transparent” to particles than silicon and we of course would not build the vertex detector out of lead.
Typically the sensor material for a vertex detector is Silicon due to the very nice possibilities to measure particle tracks with it, as for example described by Frank or me earlier on this blog.
The vertex detector for the Linear Collider should be in the order of 0.8% of material. Looking at the numbers above one can see that this is a real challenge as we would like to have 5 layers of silicon. The silicon needs to be thinned to about 50 micrometer, so half the thickness of a standard piece of paper.
But a detector is not only made out of silicon. The silicon has to be mounted around the beam pipe by some mechanical structure. Carbon fiber composite is a nice material, but still too “thick” to build the vertex detector. New ideas are carbon foam making a real light-weight detector.
The biggest challenges in the material business are the so-called services. We need to get power and cooling to and from the detector, as well as digital signals to steer the electronics. And of course we want to get the data out of the detector. All of this means cables, hoses, and pipes, and this of course means material.
So you can see, building such a detector is a real challenge and we need some time to handle all this problems. Some of the problems can’t be solved; then we have to look into the impact on what we want to measure and maybe have to adjust elsewhere. And this is also an answer to Paul’s comment some time ago “It *still* seems odd to be upgrading something that isn’t running yet?”
Maybe we should try to find Scotty, he always had a solution for this kind of problems, and I would love to own a tricorder!
A tricorder
*e = 2.71828…, the Base of Natural Logarithms
