Answer to Question #8371 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

Q

Can you calculate dose rate using a spectrum from a sodium iodide (NaI) or lanthanum bromide (LaBr3) detector?

Do you need to assess the response of these crystals with various energies and apply a weighting curve/factor to a time-integrated spectrum?

A

The respective answers to your questions are “Yes, but not easily” and “Yes.” The pulse height distribution that appears as output from the gamma spectrometer displays the photopeaks associated with the gamma rays that interact and deposit all of their energy in the detector as well as counts associated with the Compton continuum, sum peaks, escape peaks, etc. As you are likely aware, the gamma ray photopeak detection efficiency has a rather strong dependence on photon energy. Additionally, the photopeak count (rate) per unit soft tissue dose (rate) varies significantly with photon energy. This is partly a consequence of the fact that the detectors you note have effective atomic numbers very different from soft tissue, and the types and relative extent of photon interactions that occur in the detectors may be different from those that occur in tissue.

Through a proper physical calibration using photons of the energies of interest (or even through theoretical calculations, such as Monte Carlo simulations) it is possible to correlate a count in a particular photopeak with tissue dose at that energy. For a given detector such a calibration will yield somewhat variable results depending on the orientation of the detector in the radiation field. Thus, the count (rate) per unit dose (rate) at a given energy will be somewhat different in an isotropic field compared to a monodirectional field. Once you have defined the detector/field geometry, the calibration factor at a given energy may be determined. This might have dimensions of tissue dose per photopeak count (the dose quantity could represent effective dose, which can be obtained from appropriate conversion factors such as the fluence-to-effective dose factors in ICRP Publication 74). 

If an actual physical calibration is carried out with real sources, there are a few ways to proceed. Because the NaI and LaBr3 detectors are very sensitive to gamma radiation, depending on physical sizes of the detectors, the permissible gamma dose rates at the detector locations may have to be rather low in order to avoid overloading the detectors and associated electronics. If dose rate is measured directly, from a given source, a high-quality energy-independent detector is preferred. If dose rates are too low for a good-quality ionization chamber, it may be possible to make acceptable dose-rate measurements with a tissue-equivalent microrem meter. Once the dose rate has been determined at a specific location, the scintillation detector would be oriented in the desired geometry at the same location and the spectrum accumulated and the photopeak count rate determined. An alternative method is to use a calibrated source. One could then calculate the expected fluence rate of photons of a particular energy at a particular location. Dose conversion factors from ICRP 74 could then be used to determine dose rate from those particular photons. The scintillation detector would then be located in the proper geometry at the same location and the spectrum collected and the photopeak count rate determined.

Once the dose conversion factors, FE(i), have been determined for a number of energies that cover the range of interest, it may be possible and practical to fit the photopeak conversion factors to a mathematical function with sufficient accuracy that the proper conversion factor may then be quickly determined for any observed energy that falls within the range for which the function has been fit.

Once you have determined the photopeak count to dose conversion factors, FE(i) (units of dose per photopeak count), the dose, H, would be estimated from spectral data accumulated over a fixed time interval: 
   n
         H = åE(i)FE(i),
  i=1

where the index i defines the ith photon energy, n is the number of different photon energies being considered, and CE(i) is the count in the photopeak for the ith energy photon.

The dose rate, assuming no significant radionuclide decay during the counting interval, would naturally be simply the calculated dose divided by the live counting time.

Depending on the complexity of the gamma pulse-height distribution, you may have to have available techniques for dealing with overlapping photopeaks. You and/or the software must also be able to identify real photopeaks and to reject from consideration peaks that are not to be included in the dose calculations—e.g., sum peaks and escape peaks.
 
In summary, your initial inferences are generally correct. The process for making the correlation between pulse height distribution and dose is not simple, but may be well worthwhile, depending on the extent to which the dose conversions will be used. As you are likely aware, many users make energy spectral measurements for purposes of identifying radionuclides and associated gamma radiation at various locations. They may not use the gamma spectral information to calculate external dose (rate) but rather use appropriate energy-independent dose measuring instruments for the dose assessment. Thanks for the question.
 
George Chabot, PhD, CHP



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