The Radioactive Decay of Tritium . cont'd

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DOE-HDBK-1132-99
of energy loss per unit path length (linear energy transfer, or LET) increases as
the velocity of the beta particle slows, a distinct maximum range can be
associated with beta particles of known initial energy.
The beta decay energy spectrum for tritium is shown in Figure 1. The maximum
energy of the tritium beta is 18.591 ± 0.059 keV. The average energy is 5.685
± 0.008 keV. The maximum range of the tritium beta (i.e., the mass attenuation
coefficient) is 0.58 mg/cm2.
The adsorption of energy from beta particles that emanate from a point source
of tritium has been shown to occur nearly exponentially with distance. This is a
result of the shape of the beta spectrum as it is subdivided into ranges that
correspond with subgroups of initial kinetic energies. As a consequence, the
fraction of energy absorbed, F, can be expressed as
F = 1 - e-(µ/
)( )(x)
,
(3)
where µ/ is the mass attenuation coefficient of the surrounding material,
is
the density of the surrounding material, and x is the thickness of the
surrounding material. For incremental energy absorption calculations, Equation
(3) can be restated as
F = 1 - e-µx ,
(3a)
where µ (i.e., the linear attenuation coefficient) is the product of the mass
attenuation coefficient (µ/ ) and the density ( ), and x is the incremental thickness
of choice. In gases at 25EC, at atmospheric pressure, for example, the linear
attenuation coefficients for the gases hydrogen (H2), nitrogen (N2), and argon (Ar),
are 1.81 cm-1, 11.0 cm-1, and 12.9 cm-1, respectively. A 5-mm thickness of air will
absorb 99.6 percent of tritium betas. A comparable thickness of hydrogen (or
tritium) gas will absorb only 60 percent of the tritium betas.
Absorption coefficients for other media can be estimated by applying correction
factors to the relative stopping power (the scattering of the material
of interest. For the most part, these will be directly proportional to ratios of
I-91

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