Answer to Question #8353 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
We will be purchasing a multi-gamma energy solution (10 ml) in 2 M HCl. This will be made up to a 200 ml solution for an efficiency calibration of a high-purity germanium (HPGe) system. I would be interested to know the best way to prepare it in order to ensure stability over a long time, the best container material to keep it in so as to avoid losses in container walls, and the ideal conditions to keep it in the laboratory.
The undiluted standard is being supplied in acidic solution, implying that the radionuclides in the standard are cations. The acid minimizes attachment of the cations to the container inner surface. This is important to keep in mind also when you make the dilution. You can check with the provider of the standard as to the stability of the radionuclides in acidic solution to determine how acidic you should make the dilution. I would recommend no less than 0.1 M HCl, but you may need to go as high as about 1 M, depending on feedback from the supplier. The weaker acid is preferable if the radionuclides will be stable in such solution because the weaker acid is less likely to attack the inner container surface. If glass is used, the strong acid causes slight dissolution of some glass constituents; this leaves the inner surface slightly more porous, and this can enhance the adsorption of cations on the surface. You can further minimize radionuclide plateout on the walls by adding a small amount of stable carrier to the solution you are preparing. Some standards already contain stable carrier material, and the supplier can tell you whether this is the case. If you do add stable material, it should be in the same chemical form as the radionuclide and in the same oxidation state. The amount of stable carrier can be quite variable, but amounts on the order of a few tens of micrograms to about 1 mg per milliliter of solution are rather common, depending on the radionuclides and the method of production (e.g., radionuclides prepared by neutron activation often have relatively large amounts of the stable nuclide, used as the target nuclide in the activation process, in the standard solution). The intent of using the carrier is to ensure that there are many more stable atoms compared to radioactive atoms of a particular element so that the stable atoms have a great advantage over the radioactive atoms in competition for adsorption sites on the container inner surface.
Plastics have also been used in preparation of aqueous standards. High-density polyethylene and polypropylene are among the more common choices. In general, plastics do not have the physical integrity of glass and sometimes deform over extended periods. You should still maintain the solution acidity (and carrier if used) to minimize adsorption. The plastics have the obvious advantage that they are not as subject to breakage as glass, but may not maintain physical integrity over long periods. Whatever type container is used, it must be equipped with a leakproof cover/lid that will prevent leakage and make evaporative losses negligible. For some containers it may be necessary to seal the cover in place with waterproof adhesive or sealant.
When you are preparing the acid/carrier solution to be used to dilute the standard, prepare enough extra radionuclide-free solution to fill the container that will be used. Allow this solution to remain in the container for about 24 hours; this preequilibration helps in deactivating sites that might otherwise adsorb the radionuclide. Then empty the container, and drain completely before making up the radionuclide solution in the container.
Standards of this type are commonly stored at ambient laboratory temperatures (20o to 25o C). They should be stored in a secure location, properly labeled and inaccessible to unauthorized users (often in a locked cabinet). Storage procedures should be consistent with radiation-safety requirements at your institution. Some users prefer to keep glass-contained standards in a secondary container that protects the glass against breakage and may contain sufficient capacity and/or absorbent material to retain the liquid in the event of breakage of the glass.I hope your experience in preparing this standard is a good one. Good luck.
George Chabot, PhD, CHP