Fix LowDensityCarbonSteel density?

[jdkio]

Someone pointed out (Callum?) LowDensityCarbonSteel had a density ~1/10th the density of regular steel

atomic number: 39.95,	density: 1.40,	material name: LAr
atomic number: 55.06,	density: 7.90,	material name: SSteel304
atomic number: 55.61,	density: 0.75,	material name: LowDensityCarbonSteel
atomic number: 55.70,	density: 7.85,	material name: SteelTMS
atomic number: 55.84,	density: 7.87,	material name: Iron
atomic number: 63.55,	density: 8.96,	material name: Copper

Looks like it was added in this commit on purpose.

added a "LowDensityCarbonSteel" with ~1/10 the normal density to approximate the total mass of the ND-LAr cryostat (based on the 24 metric ton upstream face).

So it sounds like maybe the thickness of the LAr cryostat is 10x too high? Not 100% sure but we should fix this. We might fix the thickness but not density if we're not careful

[ast0815]

I thought this was a hack to approximate internal structure of the cryostat without having to model it.

[jdkio]

Maybe, I didn't know about that. Maybe it's a sign this wasn't documented well enough. I remember it was brought up and then I saw it again when I was finalizing the drawing script. I thought we lost track of it

[ast0815]

@mastbaum You did the commit in question. Can you shed some more light on this?

[mastbaum]

Yes, @ast0815 is correct — rather than modeling all the details of the warm steel structure (which had a lot of thickness variations due to support ribs, etc.), the idea agreed at the time was to use a simplified uniform rectangular volume and adjust the density to get the correct total mass of steel (for the right number of interactions and amount of energy loss on average). Hence the LowDensityCarbonSteel, with a density based on the total thickness and total mass of the upstream cryostat wall (as of March 2021). The support truss structure underneath was also set to be a box of the same material, again approximating structural details.