Why we ask

If you are every involved in a situation where you are going to be engaged in reachback, especially rad reachback, this is what you are likely to be asked - so make sure you have these answers first.

This might have been seen from those that received the DOE Triage (National Level Reachback) document entitled “Why we ask for the things we do in Triage.” If you have seen it before, apologies, otherwise it is a well written explanation of what will happen when you put that call in.

Unknown (spectrum of the item of interest):
Gamma rays are used to identify the radioactive materials present. The gamma ray energies are indicative of the decaying material(s). The height of the peaks are used to determine the number of decays that occurred during the time the spectrum was collected, which leads us to the activity (quantity) of the source. The shape of the spectrum will also provide indications of materials between the source and detector.

Background (background spectrum):
The background spectrum is the information we can subtract from the spectrum of interest. Naturally occurring radioactive material concentrations vary depending on the surrounding materials, so it’s important to have a representative spectrum to subtract. For a background spectrum to be representative it needs to be taken in the same nearby area and surroundings (outside the influence of the item of interest) with similar distances from ground and ground coverings (asphalt, concrete, soil, etc). A representative background helps us distinguish between radiation emitted from the item of interest and radiation emitted from the surrounding environment. A background spectrum should be at least 5 minutes and as long as the unknown spectrum if the unknown has a weak signal.

Known (check or calibration source spectrum):
The calibration (or known source) spectrum is used to calibrate the instrument’s response. The energy response of some instruments shifts (or drifts) with time or temperature and some radionuclides have gamma ray energies that need to be very accurately measured in order to identify them or differentiate them from others. The known spectrum should be taken near in time to the other measurements to more accurately assess any drift. Also, if we know the activity of the source on a known date, and the distance from the instrument at the time of measurement, we can calculate the instrument’s response if needed. Materials containing thorium, such as some lantern mantles or some welding rods, are preferred as calibration sources because they provide a variety of energies over most of typical spectrum data.

Collection time:
We want as long a spectrum as practicable so that statistical measurement errors are minimized and we can see gamma rays from small amounts of other materials, or gamma rays that are not emitted very often from the material(s) of interest.

Dose rates:
Radiation dose rates at two distances from an object containing a radioactive source can enable calculation of the source’s position within the object. Using the 1/r2 thumb rule, taking one measurement, and backing up until the reading is 1/4th the first will tell you that the source is the same distance away from the first measurement as your second measurement is from the first measurement. If you provide those readings (or can’t get 1/4th the dose) we can calculate position for you, and/or check your math.

Neutrons (neutron count rate):
Neutrons are rarely seen in significant numbers in natural settings, and important to a threat determination so a verification check is important. By reporting “0” we know it’s been checked. If more than one instrument was used, please include that information (data and instrument(s) used) because some instruments are more sensitive than others.
Intervening materials: The materials between the source and detector affect the spectrum, and subsequently our analysis of the radioactive material. By knowing at least a ballpark estimate of the types and amount of material(s) we can adjust our analysis. If we know it’s an assumption we will not expend as much effort on analysis as something that is measured. (DO NOT open or disturb a container to identify intervening materials.)

Event information:
Who, What, Why, Where, When, and How are things that help us to understand the situation, and context(s). Not all of this is necessary in each event, but if it seems suspicious or interesting to you it may help us (to help you) to evaluate the situation.

Pictures are informative and authoritative, and frequently provide answers to questions we don’t even know to ask. A photo showing the instrument during measurement can be used to measure the distance to the object if it’s taken from the correct perspective. An item of known size (a fiducial) in the image speeds our analysis (tape measures or dollar bills are unambiguous). Photos of the object, labels, scene, and nearby area help with situational awareness and sometimes provide us other contextual information.

Naming information:
We prefer that filenames be descriptive so when we’re sorting through an event we can use the filenames rather than referring to a submittal. Filenames such as “5min_30cm_sideA.spc” leave no doubt about what it is, and reduces the potential for error and the time to manage information. For an event that only has three files this may seem like extra work, but some events may produce 6, 8, or 10 spectra, and if we have multiple events or submitters, having descriptive filenames helps keep things straight.

Contact information:
We need to be able to contact the submitter in case we need more information and the appropriate people when we complete our work. There can be multiple people informed of our results, so be sure to include them in the “contacts” section of the submission

International requests for assistance must be initiated by the State’s (country) IAEA-identified Competent Authority or approved point of contact

Tags: Threat, Detector, Radiation

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