Polyethylene

(CH₂)ₙ

Shielding

Density: 0.93 g/cm³

Atom density: 0.119785 atoms/b·cm

High-density polyethylene — the densest practical hydrogen source, and so an efficient neutron shield per centimetre.

Where these numbers come from

PNNL-15870 Rev. 2, "Polyethylene, Non-borated" (id polyethylene-non-borated): 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)
H-1 (1001)	1.4369e-1	6.6659e-1	7.9848e-2
H-2 (1002)	3.3026e-5	7.6667e-5	9.1835e-6
C-12 (6012)	8.4637e-1	3.2977e-1	3.9501e-2
C-13 (6013)	9.9195e-3	3.5667e-3	4.2723e-4

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.

Bound scattering

ENDF/B-VII.1 has a thermal scattering law for this material as formulated, so the cards attach poly.20t. Free-gas scattering would misplace the thermal spectrum here.

Generated material cards

MCNP

c Polyethylene
c PNNL-15870 Rev. 2, "Polyethylene, Non-borated" -
c reactormc.net/nrdp/material/polyethylene; rho = 0.93 g/cm3 (use -0.93 on the
c cell card)
m31    1001.80c  -0.143686  $ H1
       1002.80c  -3.3026e-5  $ H2
       6000.80c  -0.856276  $ C
mt31   poly.20t

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

Hydrogen is bound in the polymer, so the card attaches poly.20t. Every moderated neutron ends in a 2.2 MeV H(n,γ) gamma, which is why shields are often borated polyethylene backed by a gamma layer.

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