Why is Carbon and other elements (not available) in StoppingPowerData?

I am trying to look up StoppingPowerData for a number of elements and while this snippet works for several I have tried (Aluminum, Lead, Iron for example), I get a result of (not available), but no error, when I try with others, such as Carbon. I’ve tried specifying an isotope of Carbon as well as different incoming particles and different energies.

StoppingPowerData[Entity["Element", "Carbon"], 
{"Particle" -> "Proton", "Energy" -> Quantity[500, "Kiloelectronvolts"]}, "LinearStoppingPower"]

Does anyone know what the issue might be?

Thanks.

Unfortunately StoppingPowerData does not currently include data for proton impacts on carbon. As you’ve notice data for that element is rather limited at the moment. We are always expanding our capabilities and we will work to add more data to cover this and other substances in the future.

I was afraid that was the case. Is there a way to extract a list of what properties are available for use for each entity?

This is what the result Missing[“NotAvailable”] result indicates. It means we do not have the data for those inputs. I suppose you could programmatically check substance, particle, property combinations to find the gaps. There may be some difficulties with ensuring you remain within the valid energy range but StoppingPowerData will alert you via a StoppingPowerData::orng message in those cases.