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Units for material properties [message #1273] Wed, 07 November 2007 16:10 Go to previous message
partridge
Messages: 11
Registered: October 2005
There are several methods defined in the IMaterial interface for retrieving material properties. I was particularly interested in getting the radiation length to calculate multiple scattering errors (more on this below).

Since there are no units listed in the Javadoc, I printed out the properties of some materials in the tracking volume from a simulation using the sid01 detector. My empirical results are below:

getName: Matches material name in the compact.xml file

getDensity: gm/cm^3 of simulated material (i.e., correctly accounts for cases where nominal density is over-ridden in the compact.xml file)

getZ: Looks reasonable for both elemental and mixture materials

getA: Looks reasonable for both elemental and mixture materials

getMeltingPoint: Always 0

getBoilingPoint: Always 0

getTemperature: Units are Kelvin, nominal value looks like 0 C

getPressure: Units are atmospheres?

getMolecularWeight: Always 0

getEffectiveNumberOfNucleons: Always 0

getNuclearInteractionLength: Units are gm/cm^2

getNuclearInteractionLengthWithDensity: Units are cm

getRadiationLength: gm/cm^2

getRadiationLengthWithDensity: NAN is always returned

getMoliereRadius: NAN is always returned

getState: "Unknown" is always returned

Comments:

1. Javadoc comments are essential for people to know what is being returned here.

2. Those items returning NAN should be fixed.

3. The use of "cm" units is dangerous since it requires people to do unit conversion. At a minimum, the "WithDensity" methods for radiation and nuclear interaction lengths should return units of mm. I also worry that people will either assume the "non density" values are in mm, rather than gm/cm^2, or divide these values by the density and assume the result is in mm whereas the current units will give a result in cm. My suggestion would be to have just one value for radiation length that returns mm, and the same for nuclear interaction lengths. I don't feel strongly about whether the density should be gm/cm^3 or gm/mm^3, as long as there is some Javadoc documentation of the units.

4. Are boiling point and melting point useful? Will they ever be filled in? If the answer is no to either question, consider removing these properties. Similarly for molecular weight, effective number of nucleons, and state.
 
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