A question we’re often asked is “Do I need a neutron survey meter to survey Cf-252?” One would think since the primary emission from Cf-252 are neutrons that a neutron survey meter, such as a BF₃ for be essential.  The answer is it depends on what you do.  If you are the manufacturer that is transferring the source from the shielding container to the source holder to use, having an exact reading is necessary.   However, the licensee who uses Cf-252 in their plant is limited on the activities that they are able to perform.  The two tasks necessary for the licensee are:

1. Performing a Transport Index (TI) survey on the shipping container when received. As a refresher, the TI is the highest radiation level one meter from the package.  When receiving the package from the transporter, it is the licensee’s responsibility to ensure that the shipping papers are correct for officially receiving the package.  This requires conducting a TI survey.
2. The other time a survey is needed is placing the incoming package into temporary storage until the manufacturer comes on site to transfer the source from the shipping container into the source holder. Then, remove the decayed sources from the source holder and place into the shipping container to return to the manufacturer.

So, if the primary emission is neutrons shouldn’t a neutron meter be required?  The answer is, NO.  What is important is to know the ratio between the neutron emissions to gamma emissions.  That ratio is approximately 3 neutrons to 1 gamma.  So, when I use my Geiger-Mueller counter that measures gammas, I can take the TI reading a one meter from the package.

Let’s assume that the gamma reading is 2 mR/hour.  So, if the relationship between neutrons to gammas is 3 to 1, then the neutron dose rate is 2 X 3 or 6 mrem/hour.  Remember the neutron dose rate is expressed in mrem, not mR as it’s not measuring ionizations due to the gamma interaction with the orbital electrons.  So, our total TI is 2 + 6 = 8 mrem.  Since the TI is without units, the TI for the package would be 8.

The gamma survey would also be required when placing the package in the temporary storage.  When this is done, after the package is placed in the secured area, a survey of the perimeter is necessary to ensure that the radiation levels do not exceed 2 mrem/hour.

If you have a small storage area and the radiation levels exceed the 2 mrem/hour, you have two choices.  One option is to make the area larger to increase the distance; or, place shielding around the package.  One inexpensive method is to surround the package with drums of water, sheet rock or bags of cement.

When one is recording the results of the survey on documents, recommend posting the actual gamma reading in writing, then show your calculation (as above) to explain your result.