Answer to Question #2041 Submitted to "Ask the Experts"

Q

What would be the neutron shielding requirements to shield hundreds of grams of organic thorium fluorides?

A

Shielding will not be necessary. The fact that ²³⁸U has a higher specific alpha activity and higher alpha energies than ²³²Th, yet large quantities of depleted uranium fluorides are handled in uranium processing facilities without shielding, would imply that the need for shielding for hundreds of grams of thorium fluorides would not be indicated. Neutron emission rates for thorium fluoride sources were not found, but using data found in the Handbook of Health Physics and Radiological Health (1998), p. 7-5, the neutron emission rate of a ²¹⁰Po + F source with the same curie content as 200 g of thorium yields about 9 neutrons per second. This neutron emission rate yields a trivial dose equivalent rate as shown below. The ²¹⁰Po also has alphas that are much more energetic than those of ²³²Th, meaning that the neutron emission rate from a ²³²Th + F source with the same number of curies would be significantly less. But quantitative calculations of the neutron emission rate can be carried out using the thick target yields of neutrons from alphas on ¹⁹F that have been experimentally measured (Bair and del Campo 1979) and give 0.879 neutrons per 10⁶ alphas at 4.0 MeV incident alpha particle energy. Thorium has two alpha particle groups, 4.01 MeV (77%) and 3.95 MeV (33%), thus using 4.0 MeV for all ²³²Th alphas will be a good approximation. Let's assume a quantity of thorium of 200 g, which as ThF₄ would translate to 265 g, with 22 µCi of ²³²Th, or 8.15×10⁵ alphas per second. This alpha intensity gives a neutron production rate of 0.7 neutrons per second assuming the target was pure ¹⁹F. In a ThF₄ sample, of course, fluorine is only a fraction of the target material, so the neutron production rate would be much less than one neutron per second.

Also, as stated in the original question, there will be other organic constituents in the samples to further dilute the fluorine target by an additional unknown factor. The maximum neutron energy available from the reaction of 4.0 MeV alphas on ¹⁹F is about 1.6 MeV. Using a fluence per unit dose equivalent from the Handbook of Health Physics and Radiological Health, p. 13-5, the dose equivalent rate for a one-neutron-per-second source of 1.6 MeV neutrons at a distance of 30 cm is on the order of 10×10^-8 rem/h. This is definitely not a source that requires shielding for personnel protection purposes. If the thorium samples were to stay in storage for many years, there will be a buildup of ²³²Th daughters that will result eventually in a sixfold increase (assuming the ²²⁰Rn does not escape) in the total alpha emission rate with daughter alphas that are much more energetic than the alphas from the parent ²³²Th. Calculations using data in Bair and delCampo (1979) show that, when the daughters come into equilibrium (~40 y), there would be about a 100-fold increase in the neutron production rate, but the dose equivalent rate would still be below concern. (Higher-energy neutrons can be produced with the higher-energy daughter alphas, but the fluence per unit dose equivalent is very neutron-energy insensitive in the range of neutron energies under consideration here). The buildup of ²³²Th daughters with time also gives increasing beta and gamma emissions, but shielding for the gamma radiation would also not be indicated since a gamma dose rate of less than 1 mrem/h at 30 cm is calculated for 200 g of thorium in equilibrium with its daughters.

Ron Mlekodaj, CHP
Oak Ridge National Lab

References

• Shleien B, Birky B, Slaback L. Handbook of health physics and radiological health. 3rd ed. Philadelphia: Lippincot Williams & Wilkins; 1998.
   
• Bair JK, Gomez del Campo J. Nuclear Science and Engineering. 71:18-28; 1979.