Self-Absorption Factor (SAF) for ITPs

DOE-HDBK-1184-2004

5.2.3

Self-Absorption Factor (SAF) for ITPs

Within the ITP, the mass of the particulate absorbs a portion of the beta particles

emitted during tritium decay. The fraction of beta particles that escapes the ITP is

called the "self-absorption factor for beta particles" (SAFβ). When analyzing air

monitoring samples for ITPs using LSC, it is necessary to consider SAFβ, since

ITPs do not dissolve appreciably and release tritium to LSC cocktail. Only beta

radiation that escapes the particles in LSC cocktail is available for detection. The

SAFβ is used to estimate the "actual" ITP activity that would be "observed" when

samples are counted via LSC. This use of SAFβ has been confirmed experimentally

(Kropf, 1998), although the supporting evidence is limited.

To assess dose to the lung from ITPs properly, the "self-absorption factor for

energy" (SAFe) is an important consideration. Absorbed dose is a measure of the

energy deposited (per unit mass) in tissue. Only the beta energy that escapes the

particles contributes to dose. SAFe is the fraction of energy that escapes the

particles. Because LuDEP does not account for SAFe, refinement of the results of

LuDEP computations is necessary.

The SAFβ and SAFe for given physical particle sizes are calculated numerically here

by the methods described by Kropf (1998). The determinations of SAFβ and SAFe

are dependent on the electron density of a given material, which is proportional to

A/( Z∆), where A is the atomic mass of the material's empirical formula, Z is the

number of protons in the empirical formula, and ∆ is the density.

The values employed to calculate the self-absorption factors (SAFs) for several

representative materials are given in Table 5-8 below. DOE has handled tritides of

several metals, considered to be insoluble, which do not appear in the table (in the

interest of brevity), including scandium, yttrium, and alloys of titanium and

lanthanum. The derivation of SAFs for a representative but broad range of tritiated

particulate materials is the prime focus of this section. The materials investigated

were selected to represent a wide range of self-absorption, itself a function of the

parameter A/( Z∆). The materials in the tables of this section have values of this

parameter which vary from 0.2 to 1.9. Parameter values for scandium, yttrium, and

lanthanum ditritides are 0.72, 0.52, and 0.47, respectively. [Values for lanthanum

alloys are slightly larger than those for lanthanum because of the inclusion of lighter

elements. Values for iron/titanium alloys are close to those for titanium as given in

the tables.] With respect to SAFs, the materials included in the tables therefore

bracket the metals and alloys not included.

Table 5-8. Constants for calculating SAFs for various monodisperse (s g = 1) ITPs

∆

A/( Z∆)

Base Material*

Organic [∼(CH2)n]

0.9

1.944

Rust [∼FeO(OH)]

0.690

Ti H2

47.9

3.9

0.558

Zr H2

91.22

6.49

0.351

Hf H2

178.49

11.68

0.212

* Variable amounts of elemental hydrogen are isotopically tritium.