Spectral emissions and dosimetry of metal tritide particulates

Inference of intakes and doses from inhalation of metal tritide particles has come under scrutiny because of decommissioning and decontamination of US Department of Energy facilities. Since self-absorption of radiation is very significant for larger particles, interpretation of counting results of metal tritide particles by liquid scintillation requires information about emission spectra. Similarly, inference of dose requires knowledge of charged particle and photon spectra. The PENELOPE Monte Carlo radiation transport computer code was used to compute spectral emissions and other dosimetric quantities for tritide particulates of Sc, Ti, Zr, Er, and Hf. Emission fractions, radial absorbed dose distributions, specific energy distributions and related frequency-mean specific energies and lineal energies, and the emitted spectra of electrons and bremsstrahlung photons are presented for selected particulates with diameters ranging from about 0.01 microm to 25 microm. Results characterising the effects of uncertainties associated with the composition and density of the tritides are also presented. Emission spectra are used to illustrate trends in the relationship between apparent and observed activity as a function of particle type and size. Emissions from metal tritide particles are weakly penetrating, and electron emission spectra tend to 'harden' as particle size increases. Microdosimetric considerations suggest that the radiation emitted by metal tritides can be classified as a low linear energy transfer radiation source. For cells less than about 7 microm away from the surface of a metal tritide, the primary dose component is due to electrons. However, bremsstrahlung radiation may deposit some energy tens, hundreds or even thousands of micrometres away from the surface of a tritide particle. The data and analyses presented in this report will help improve the accuracy of dose determinations for particulates of five metal tritides. Future work on the spectral emissions and dosimetry of metal tritide particulates needs to consider the contributions of so-called internal bremsstrahlung, an additional form of bremsstrahlung radiation emitted during beta decay.