Dry Air

Gases & Other

Density: 0.001205 g/cm³

Atom density: 0.00005 atoms/b·cm

Dry air near sea level, for streaming paths, room modelling, and atmospheric transport.

Where these numbers come from

PNNL-15870 Rev. 2, "Air (dry, near sea level)" (id air-dry-near-sea-level): density and full isotopic composition are taken from the compendium unchanged.

Open the compendium entry for the isotope-by-isotope table and the compendium's own notes.

Material compositions from PNNL-15870 Rev. 2 (April 2021), R.S. Detwiler, R.J. McConn Jr., T.F. Grimes, S.A. Upton, and E.J. Engel, Compendium of Material Composition Data for Radiation Transport Modeling, Pacific Northwest National Laboratory. Full report (DOI 10.2172/1782721)

Composition

Nuclide (ZAID)	Weight fraction	Atom fraction	Atom density (atoms/b·cm)
C-12 (6012)	1.2256e-4	1.4859e-4	7.4118e-9
C-13 (6013)	1.4365e-6	1.6071e-6	8.0164e-11
N-14 (7014)	7.5232e-1	7.8157e-1	3.8986e-5
N-15 (7015)	2.9441e-3	2.8553e-3	1.4243e-7
O-16 (8016)	2.3115e-1	2.1024e-1	1.0487e-5
O-17 (8017)	9.3580e-5	8.0085e-5	3.9948e-9
O-18 (8018)	5.3454e-4	4.3204e-4	2.1551e-8
Ar-36 (18036)	3.8527e-5	1.5583e-5	7.7731e-10
Ar-38 (18038)	7.6672e-6	2.9381e-6	1.4656e-10
Ar-40 (18040)	1.2781e-2	4.6526e-3	2.3208e-7

This is the material as formulated. The cards below carry weight fractions, written negative, which is what MCNP and Serpent read as a mass basis.

What ENDF/B-VII.1 forced

O18 folded into O16: ENDF/B-VII.1 has no O18 evaluation (5.3454e-4 by weight, so the spectrum does not care).

The cards keep the material's mass and atom count; a nuclide the library cannot name hands its share to one it can. That is why a card row can differ slightly from the table above.

Generated material cards

MCNP

c Dry Air
c PNNL-15870 Rev. 2, "Air (dry, near sea level)" -
c reactormc.net/nrdp/material/air; rho = 0.001205 g/cm3 (use -0.001205 on the
c cell card)
c O18 folded into O16: ENDF/B-VII.1 has no O18 evaluation (5.3454e-4 by weight,
c so the spectrum does not care).
m38    6000.80c  -1.2400e-4  $ C
       7014.80c  -0.752316  $ N14
       7015.80c  -0.002944  $ N15
       8016.80c  -0.231688  $ O16
       8017.80c  -9.3580e-5  $ O17
       18036.80c  -3.8527e-5  $ Ar36
       18038.80c  -7.6672e-6  $ Ar38
       18040.80c  -0.012781  $ Ar40

MCNP and Serpent cards use .80c (ENDF/B-VII.1 at 293.6 K) — renumber m1 and change the suffix to match your deck. SCONE compositions are atom densities in atoms/barn-cm with the .03 suffix matching temp 300. See the library reference before changing a suffix: the mapping is not portable between vendors.

Notes

The compendium formulation carries the 124 ppm of carbon that atmospheric CO₂ contributes, and expands nitrogen, oxygen and argon over their isotopes. Ar-40 capture makes Ar-41 (1.83 h), which is the reason containment air is monitored.

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