The first part of this series explained radiometric measurement and its working principle. It also mentioned gamma rays in the process of measurement. This second part of the series will review how we use gamma detectors.
All radioactive materials decay naturally over time. A material’s half-life is the amount of time it takes for its radioactivity to drop by half: cobalt 60 has a half-life of 5.3 years, and Cesium 137 has a half-life of 30 years.
Cs-137 isotopes have a longer half-life than Co-60 because emit less radiation. This means that Cs-137 can have smaller and cheaper shielding containers than Co-60, even though lower penetration also. Therefore, Cs-137 are the most commonly used isotopes for industrial level measurement.
Gamma ray detectors
The industry uses two standard types of gamma detectors:
- Ionization detectors: These rugged devices use ionization chambers that can withstand high temperatures without water cooling.
- Scintillation detectors: These detectors, based on plastic rod scintillators, have more than 10 times the sensitivity of ionization chambers. So they can operate with very low and safe radiation levels, a minimum dose rate of about 0.1 µSv/h. Despite needing water cooling for ambient temperatures higher than 50 ºC, they’re the more popular method.
Example: discharge blockage detection
A radiometric measurement system configured for gamma limit detection can detect a reactor discharge blockage. No other system works as well for this application – despite temperatures above 800 ºC, build-up, and abrasion – because it’s a non-invasive method. It also requires no maintenance.
For this application, we can measure the build-up rate and predict the best times for reactor maintenance. And if build-up reaches the established limit, the system detects it and trips an alarm in the control system.
Is radiometric measurement dangerous?
Because of nuclear weapons and the Chernobyl disaster, some people see radiometry as complicated and dangerous, but only because they don’t understand this technology.
Radiometric measurement uses very low radiation levels, thanks to the high sensitivity of scintillation detectors, which can operate with dose rates as low as 0.1 µSv/h.
The gamma source stays shielded inside a lead-filled container to ensure that radiation emits only in the detector. Operators also must set up a control area around the source, with safe dose rates under 3 µSv/h according to German standards and 7.5 µSv/h according to American and Asian standards.