Answer to Question #6694 Submitted to "Ask the Experts"

Q

Calibration standards for solid and liquid geometries are available in plenty, whereas availability of gaseous standards especially for FPNG (fission product noble gases) is almost rare, and getting the same in the required geometry is also difficult. I have done some experiments with radon, which is a gaseous source and whose daughters do have plenty of gamma energies covering a range of 200 keV to 2,500 keV. I have injected the radon gas into a one-liter PVC (polyvinyl chloride) bottle and I have continuously obtained a gamma spectrum of the same for about five times over a period of one week. After that, I have sampled and determined the radon activity present in the bottle using a Lucas (alpha scintillation counting with ZnS) cell. Once radon activity is calculated, the daughter activities present in the bottle at different periods of time can be calculated. Efficiency for each energy can be determined and the same can be plotted. Using an energy versus efficiency plot, the efficiency for the energy required can be calculated. Kindly let me know whether my approach is correct or not.

A

While you can purchase some fission product noble gases in known quantities, it is true that you may have problems getting all you need and in obtaining them in the geometries desired.

The approach you have outlined would be appropriate if there was a way to ensure that the radon progeny would remain airborne in the bottle over the period of your measurements. Unfortunately, this is not likely the case, and daughter products will deposit on the inner surface of the bottle, thus changing the counting geometry compared to what is observed if all products remain airborne. The degree of impact that the change in geometry has on the observed counting efficiency depends on a number of factors, including the size and shape of the bottle and the distance and orientation of the bottle with respect to the detector.

As you know, the major radionuclides that contribute significant gamma rays are the metal ions/atoms of ²¹⁴Pb (half-life of 26.8 min) and ²¹⁴Bi (half-life of 19.9 min), and these are subject to deposition on the walls of the container. You may be able to get an idea of how much the deposited progeny are contributing to counts in the gamma spectrum at a given elapsed time past filling with radon by counting the bottle containing the radon and progeny at an elapsed time of interest, then flushing the radon and airborne progeny from the bottle with clean air and recounting it to observe the gamma pulse height distribution from the deposited progeny.

If one corrects for individual decay of the progeny, one would expect the fractional deposition of the progeny to increase with increasing elapsed time up to a half day or so and then to level off as an equilibrium situation takes place in which deposited activity is decaying as rapidly as new activity is being deposited.

If you don't correct for decay, the observed deposition will appear to increase, go through a maximum, and then decrease over several hours. (In order to observe such behavior you would have to take numerous measurements over a number of hours, each time having filled the bottle with a fixed amount of radon and allowed a specified amount of time to elapse before flushing out the radon and counting the deposited progeny.)

If you find that the deposited progeny are contributing a negligible fraction of the observed total counts, then you can neglect their effect on the counting efficiency. If the counts contributed by deposited progeny are significant, you can attempt to correct for their effect by subtracting counts observed for the deposited progeny in each respective energy region of interest from the total counts (before flushing out the radon) in the same respective regions of interest.

Naturally you must decay correct each radionuclide to the same point in time before doing the subtraction. There may be some error in this method because the process of flushing out the radon and airborne progeny from the bottle may produce some additional deposition of progeny because of the turbulence effect that might result from the flushing. I do not think the turbulent deposition would be a severe effect if the flushing is done quickly.

Good luck in your attempts to evaluate the efficiencies you require.

George Chabot, PhD, CHP