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How does the energy response of a radiation survey meter impact measurement results and work-flow? We sat down with Magnus Smith, Product Manager of the RaySafe 452 Radiation Survey Meter, to discuss some common questions on the topic.
I have read that the RaySafe 452 has a flat energy response and does not require correction factors. What does “a flat energy response” mean?
A flat energy response means that the survey meter does not under‑respond or over‑respond for the measured dose at certain energies. Simply put, if the resulting dose from an exposure is 0.5 mSv, the meter should indicate 0.5 mSv, regardless of the energy of the detected photons.
Don’t all survey meters have a flat energy response?
On the contrary, many survey meters under‑respond or over‑respond in some energy ranges. For instance, pressurized ionization chambers typically under‑respond below 50 keV, due to the chamber wall thickness that causes attenuation of low energy photons; and thin‑window Geiger-Müller chambers (“pancake detector”), inherently have a strong over-response around 50 keV.
What is the impact of a varying energy response of a survey meter?
That depends on the application and what parameters you need to measure. A varying energy response is not always a drawback. For instance, the over‑response of a Geiger-Müller pancake may be beneficial for detection of radioactive spills, since it brings an increased sensitivity. However, as soon as you need to quantify your measurements, a meter with a flat energy response is required to accurately quantify the measurement.
But if I know the response of my meter, can’t I use correction factors for dose and dose rate measurements?
For monoenergetic radiation beams this is true, however, not many real-world radiation sources have a single energy. For instance, there’s always a spectrum of photon energies generated from an X-ray machine. The figure below shows the resulting photon energy spectrum from a fluoroscopy machine at 120 kV with 0.3 mm Cu filtration (calculated in SpekCalc).
Even though the voltage is set to 120 kV, the resulting photon energies have a spread from zero to about 110 keV. This complex combination of photon energies is what you measure with your survey meter. For a wide distribution of photon energies, a single correction factor cannot make up for the varying energy response of a survey meter.
The RaySafe 452 contains both a Geiger-Müller pancake and semiconductor diodes. How can it have a flat energy response?
The Geiger-Müller pancake detector and the semiconductor diodes in the RaySafe 452 do have a varying energy response, but they are energy compensated internally. The RaySafe 452 has different filters that are utilized to attenuate certain energies and boost others, resulting in a flat energy response over a wide range of energies.
What benefits do a flat energy response of a survey meter bring for me as a user?
With a flat energy response, there is no need for manual correction factors. Therefore, you decrease the risk of underestimating dose to patients and staff. From a work-flow perspective, a flat energy response means that you decrease the risk of human error and you save time during data analysis.