Answer to Question #9724 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Shielding

The following question was answered by an expert in the appropriate field:


I am trying to build a thrifty epithermal neutron shield and I am concerned about secondary containment. I am using a boric acid mix now but my question is do you know of any boric acid buffer, preferably one with the highest possible boron content with a pH 7-9?


You do not specify what the physical requirements of the shield are or what are the expected magnitudes of neutron fluences or fluence rates to which the material will be exposed, and those factors may affect choices you make. There are buffer systems that would meet your pH specifications, and you would have to make some judgments about what might be appropriate.

The typical boron-based weak acid and its salt that would probably be most common as a buffer for this pH range would be a solution of boric acid, H3BO3, and the strong electrolyte (salt) sodium tetraborate, Na2B4O7, the latter usually being available as a hydrate. The hydrogen ion concentration expected from such a solution would be approximated closely by

[H+] = KaCa/Cs ,

where Ka is the acid dissociation constant, Ca is the acid molar concentration, and Cs is the salt molar concentration. Boric acid is a very weak acid for which Ka = 5.5 × 10-10. For equal concentrations of the acid and the salt, the expected hydrogen ion concentration is then 5.5 × 10-10 M, and the associated pH (negative base 10 logarithm of concentration) is then 9.26. If Ca = 3 Cs (e.g., 240 mM acid and 80 mM salt) the expected pH would be 8.78. The possible disadvantage of the sodium borate system is that the stable 23Na is subject to possible thermal neutron activation to 24Na, which is radioactive with a 15-hour half-life and which emits high-energy gamma rays. An alternative would be to use the potassium salt, K2B4O7, in place of the sodium salt. Stable 39K may activate to 40K, but the half-life of 40K is so long (1.27 × 109 years) that much less activity will be produced compared to irradiating sodium.

Others have used simple borate pH control systems prepared by dissolving only sodium tetraborate in water. The pH of a typical 10 to 20 mM solution would expectedly be about 9. Sodium hydroxide is sometimes added if a higher pH is desired. Again potassium tetraborate could be used in place of the sodium salt.

I hope this is helpful. Good luck in your project.

George Chabot, PhD, CHP

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