The details of the methods for preparing samples and analyzing the resulting spectral data for the two types of sampling measurements made by Radwatch, gamma measurements and neutron activation analyses, are described here.

Gamma Measurement Details


All fish samples have been purchased from local fish markets in the Bay Area. Fish farmed or caught locally on the West Coast are given preference, in order to understand the local sea environment. However, we are also measuring samples of more inland animals like chicken and pork, which people consume regularly.

Figure 1: Fish sample before drying process.



Any liquid from water content or fats would attenuate the gamma radiation and weaken the radiation signal we're trying to detect. This moisture content is removed by baking the sample for a couple hours. Samples are continually cut with a clean knife to speed up the drying process and make it easier to fit inside a Marinelli beaker.

Figure 2: Same fish sample after drying and cutting process.



Once samples are dry, they are then placed in a Marinelli beaker and weighed. The wet weight is provided by the local fish market. The dry mass is collected after sample processing and is used in radioisotope calculations.

Figure 3: Marinelli beaker with dried sample inside. The label indicates what type of food sample, the date it was measured, and its wet and dry mass.



Samples are placed in front of a high purity Germanium (HPGe) detector and measured for a day. The day long measurement is for good statistics and to see slow-building features that wouldn't be noticeable in a shorter measurement.

Figure 4: Sample placed into lead chamber. The front of the lead cave is open in order to place the sample. During measurements, this is closed up using additional lead bricks.



When the measurement is finished, an energy spectrum is collected, like the one in Figure 5. We search for peaks in the energy spectrum that correspond to radioisotopes of interest. Each radioisotope has its own set of gamma ray emissions that are listed in the table below.

Figure 5: Characteristic Energy spectrum obtained from samples measured by the HPGe.



We search for peaks in the energy spectrum that correspond to radioisotopes of interest. Each radioisotope has its own set of gamma ray emissions that are listed in Table 1.

Isotope Energy (keV) Gamma Branching Ratio (%)
Cs-134 604.72 97.62
Cs-137 661.66 85.1
K-40 1460.83 11
Bi-214 609.31 45.49
1120.29 14.91
1764.49 15.28
Tl-208 583.19 30.6
2614.51 35.9


Simply finding the peaks and summing the total counts is not sufficient for calculations. This is due to Compton scattering that can occur inside the detector material. Gamma radiation can scatter inside the detector, which can contribute counts at energies below its full energy. Since there are multiple gamma rays from various radioisotopes, a Compton background builds up in the spectra, with a noticeable 'plateau' coming from gammas associated with K-40.

This compton background must be subtracted from the total peak area in order to know the contribution purely from the radioisotope itself. This is done by looking at counts in regions around the peak and estimating the Compton background. Subtracting this from the total peak area gives us our net area contributed by the radioisotope gamma ray. An example of this procedure is illustrated in Figure 6.

Figure 6: Illustration of the side-band procedure for determining the Compton background under a peak of interest. The two side-band regions around each peak are indicated in green and the peaks of interest in red. The side-bands around each peak are averaged to estimate the background under the peak.



Subtaction of the Compton background alone is not sufficient to evaluate concentrations. We also need to account for background from the environment around the detector. Even though the detector is inside a lead chamber, small amounts of radiation can still get through. To account for this, we take a background spectrum by measuring an empty chamber. This measurement is normally taken for a longer period of time in order to measure more precisely the count-rates at all energies.

Figure 7: Characteristic Energy spectrum from background for the HPGe under the same conditions as when sample measurements are taken.



Once we subtract the background, we then can calculate the countrate from our sample by dividing the net area by the measurement time (specifically, the time the detector was active or “live”). The countrate enables us to calculate the activity of a certain radioisotope using the following relation:

Activity = countrate/(efficiency * branching ratio)


Where the efficiency is the detector efficiency at a specific energy and the branching ratio is the probability for a given radioisotope to produce a gamma ray at that energy. We know the efficiency of our detector at all energies for samples in the marinelli beaker. The above relation allows use to evaluate the activity within a given food sample.

For the case of K-40, Bi-214, and Tl-208, these are naturally occurring isotopes that can be more accurately quantified with the use of a reference sample. The reference sample has known quantities of these three isotopes and a measured countrate associated with them. Comparing the sample countrate to the reference countrate allows us to be much more accurate in calculating these concentrations.

Acknowledgements

The methodology described here was developed by Christopher Figueroa, under the supervision of Dr. Brian Plimley. This page is edited and maintained by Dr. Ali Hanks.