When we receive a sample, it is promptly examined, assigned a sample number and corresponding intake documents are created to save and note any information the collector may have obtained at collection time, as well as for the purpose of documenting future information. The sample is then carefully transferred to our standard sample jar, 2.25" x 2.25" certified virgin PET, from which all subsequent measurements are conducted. Despite certification, ELA has extensively tested these jars and have found no contamination and test each time we receive a new shipment. The jar is then double-labeled, both lid and jar, with matching labels affixed to the corresponding paperwork to prevent cross-contamination by possibly inadvertently switching lids to other sample jars and/or erroneously applying the wrong data to any particular sample.

ELA then weighs the sample, subtracting the weight of the jar, documents it, and then performs three (3) - ten (10) minute average tests with the Inspector Geiger counter. The sample is then sealed and ready for testing in the scintillator chamber. Depending on the source of the sample, ELA will sometimes test this sample right away if conditions exist for interest in looking for radionuclides with short-half lives; however, if looking for radionuclides with medium to long half-lives, the sample will be set aside in a temperature controlled environment for approximately 3-4 weeks and then tested. This is to allow at least 7-8 half-lives of radon to expire, enough time for it to "build up" in the jarred sample and also allow other radionuclides with short half-lifes and stable daughters to decay as they interfere with the display of the results of other energies which may be present. This is particularly important regarding radon as, being abundant in our environment, not only does it interfere with the ratios that exist between uranium isotopes used to infer source (in fact, it interferes with the entire spectrum), but also we have found it skews the perception of a potential peak detection at 661.67 kilo-electron volts, one of the main radionuclides of interest of the analyst using sodium-iodide crystals for detection of Cesium-137. Therefore, if we are interested in looking for the presence of any given radionuclide, we must do so with respect to time and its decay rate. This principle holds valid with respect to Geiger counter testing as well so we encourage you to pay attention to all of the dates when reviewing our data. Much information can be gleaned from the passage of time if properly applied to the appropriate subject matter. Especially when dealing with expected low-count values as one hopes in all environmental samples, it is quite possible a peak can be found to be statistically significant when in fact it is due to erroneously attributing a consequence of one matter as being related to the source or result of another.

Most of our scintillation tests run for eight (8) hours, a length of time tested to afford us an expected quantity of counts in the spectrum sufficient to achieve the number needed to apply analysis using equations applicable to a normal distribution. When performing analysis and subsequently calculate activity of any given peak area we visually find significant, we subtract the background continuum from that within the sample using the "after Currie" method as described here. This is not to say an "empty chamber" background isn't performed nor necessary - quite the contrary, we perform background tests before, during and after any test. Additionally, we continually monitor our background during scintillation tests with the Inspector Geiger counter positioned within 3 feet of the castle. Should something go awry with our test and/or our background then, we will better be able to find out this way, again hopefully preventing us from erroneously attributing one consequence within a sequence of one matter to being related to the source or result of another.

When the tests have completed, analysis begins. (Please see our sample here.) We visually examine the region in the resulting spectrum where the peak we are looking for is expected to be.

If found to be present:

1. We then calculate the net peak area, its uncertainty and the uncertainty of the peak-background correction.

2. Calculate the critical limit, Lc, and compare with the net peak area.

3. If the peak area > Lc, quote a result with appropriate confidence limit.

4. If the peak area < Lc, we calculate the upper count limit (Lu), and pass that value through calculation to produce an eventual upper activity limit, or "less than" value.

Before continuing... a word on limits:

Critical limit (Lc) is a decision limit. It answers the question: Is the net count significant? Since a peak becomes non-significant by being lost in the background, uncertainties of the background must be taken into account. Taking into consideration a certain number of standard uncertainties of the distribution of counts above the background will achieve that level which we can be confident, to a degree, that a net count is valid. If our count is above this, we call it a "DETECTION" and carry the net count on through calculation to determine activity and post the result.

Upper limit (Lu) is defined as an area exceeding the actual peak area. It answers the question: Given that the peak count is NOT statistically significant, what is the maximum statistically reasonable count? It too is based on distribution if we were to count the particular sample a large number of times. If our net count then, is below the critical limit, we call it a "NON-DETECTION", calculate the upper limit and carry it on through to determine an activity - one in which we can be sure that there is a 1 in 20 chance (as in our case we use a 95% confidence level) that the true activity is greater. It is a "less than" answer.

It is the critical limit we use to determine whether our visually identified peak carries statistical significance with respect to our set confidence level we choose. We choose 95% confidence as we'd hope that should we test again that 19 out of 20 times our results would fall within the same values we measured the first time. Measuring a can of worms you think? Yes, radiation is much like a can of worms. Once opened, we know worms will eventually crawl out; however, what we can't be certain of is: how many will come out at any given time until the end when they all have (at which point we wouldn't care anymore...) so we must measure within the sequence of events to make any meaningful quantifications and allot for uncertainties. Every time we'd test it, we would come up with a different value. This would naturally produce a better estimation if those varying results were averaged, but the result would still be an estimation so the best we can hope for is the "best estimation" we are able to make with our equipment and its limitations, uncertainty and time in our day.

Fortunately, we have math to help us make those best estimations.

Further, there are other inherent uncertainties which are taken into consideration and represented by the number after an activity is quoted as a DETECTION. This number is derived from our uncertainty budget calculated specifically using factors related to our lab. While some of these values are static, others are particular to energy level. For Cesium-137 for example, ELA's calculated uncertainty at 661.67 keV is 6.1%.

So, for example, if you see in our results:

DETECTION - This means a peak was visually present for the listed radionuclide indicating its presence in the sample; and further, correcting for background and other uncertainties, if the resulting net count is determined to be above the critical limit (Lc), we count it as significant and post the activity and then its uncertainty expressed in quantity and percentage. See our Efficiency page to learn how we calculate activity.

NON DETECTION - This means a peak was determined to be below the critical limit (Lc), NOT statistically significant, and thus the upper limit (Lu) is then calculated to represent the "less than" value.

We strongly encourage you to thoroughly read Dr. Gilmore's excellent explanation of these limits in order to better understand and interpret our data as well as make determinations and better interpretations of quality and meaning of data you may encounter elsewhere.

Currently, ELA looks for the presence of Cesium-137 in environmental samples and quantifies that estimated measurement only. Because time is of the essence with respect to its presence, as noted in the comments section for each test, we indicate we and will expend further time in analysis of those samples. This includes checking for uranium ratios, uranium and radium ratios, natural radiation chains present in all samples to varying degrees, and other products of fission. Radiationfinder.com has an excellent explanation of how and why we look for uranium ratios under their "Uranium" category.