Lecture 3 (Unit II): Gas-Filled Detector Operation and Use

1. Lecture 3 (Unit II): Gas-Filled Detector Operation and Use

2. Lecture 3 Objectives Describe the operation of a dose calibrator Explain how the radionuclide buttons work for both analog and digital dose calibrator systems Determine from dose calibrator current output the appropriate activity of various radionuclides Describe the function and use of a ionization survey meter Describe the function and use of a Geiger-Műller (GM) survey meter Explain the process of radiation detection in a GM meter including the Townsend avalanche and probe recovery Discuss limitations of gas-filled detectors

3. Dose Calibrator Design Features Ionization Chamber Detector (~150 V on the voltage-response curve) Detects primary ionized electrons (no gas amplification) Operates in “Current” Mode Sealed and pressurized (12 or more atm) argon gas in its chamber Impervious to barometric pressure changes) Increases likelihood of gamma interactions Resistor or conversion factor buttons to adjust display readout Resistor or conversion factor buttons set to commonly used radionuclides

4. Figure 04: Block diagram of dose calibrator

5. Dose Calibrator:Isotope Selector Buttons Analog In older analog models, it works by measuring the total amount of ionizations produced by gamma radiation from a sample, and thus establishes an exposure rate. A = Ẋd2 Γ From: http://www.dotmed.com/listing/dose-calibrator/capintec/radioisotope-crc-12/861774 Accessed 30 Sep 2012.

6. Dose Calibrator:Isotope Selector Buttons Digital Models Digital models have microprocessors that apply conversion factors to the current for each radionuclide as its button is pushed. For example, these are a couple of conversion factors: Current flows in from the dose calibrator as pico-Amperes and is proportional to the ionizations in the chamber. The current is divided by the appropriate conversion factor to get the correct reading.

7. Dose Calibrator:Operation Cannot discriminate different levels of energy except with a shielded insert (Mo-99 breakthrough test). Watch your buttons!! It will spit out a measurement for anything that is ionizing its gas. It doesn’t care what radionuclide it is and will give you a reading on any radionuclide setting. As any ionization chamber, can measure high levels of radioactivity. Exposure rates are affected by changes in the samples size and volume (geometry). Watch for signs of contamination and scatter from outside sources. Can measure pure beta emitters from Bremsstrahlung radiation—but must be calibrated for doing so. See Table 1-1 (p. 8) showing an example on how one can determine the drawn activity from a vial of a pure beta emitter.

8. Cutie Pies & GM Meters http://www.recycledgoods.com/product_images/j/320/s_p_9543_1__33441_zoom.jpg https://www.stresslabs.com/catalog/images/CAPINTEC%20CRC-12.jpg

9. Cutie-Pie (QDπ) Survey Meters Ionization Chamber Survey Meter (a.k.a. “Cutie-Pie” ). Ionization Chamber-type Detector Response is based on the total energy deposited on the detector and is proportional to that amount of energy. Can measure in an averaged rate (mR/hr) mode or in an accumulated exposure mode (mR accumulate until measurement stopped). Roentgen = amount of radiation that produces 1 unit of charge (about 2 billion ion pairs) in 1 cubic cm of air—used in describing radiation field strength.

10. Ionization Meter Operates in current (vs. pulse) mode. Requires batteries transformed to produce 50-500V Chamber usually filled with air Measures high level of activity Good for fairly accurately measuring exposure rates from known sources, such as areas near radioactive storage or when determining clearance levels for a radiotherapy patient. Ionization Survey Meters

11. Geiger Counter Survey Meters Geiger-Műller (GM) Survey Meter Operates in the Geiger-Műller region of gas-filled detectors (400-1000V) Helium or argon gas at less than atmospheric pressure Response is based on huge electric pulses and not necessarily on the energy level of the source radiation. Uses gas amplification (Townsend avalanche) with UV emissions Its rate is only accurate for photon energies that are the same as those used to calibrate it (energy-dependent). Because of its strong response—is good for detecting unknown radioactive sources, such as when a spill is suspected or searching for contamination.

12. G-M Meter • Operates in “pulse” mode • Size of pulse represents total charge deposited by ionized electrons • RC circuit converts current to voltage and restores charge • Not accurate for measuring exposure from different gamma ray energies (off by a factor of 2 to 3) • Slow moving positive ions form an envelope around the cathode • This attracts electrons trying to reach anode • Reaction stops but pulse is created • GM meters come equipped with “quenching gas” (organic or halogen) to stop the UV rays from resulting in continuous dicharge

13. G-M meter + ion cloud Formation of positive ion cloud

14. Dead Time G-M Meter • Time constant (τ) • Short-allows quick change but also bouncing • Long-slower change but may miss high spikes • Dead time • Problem because of pulse mode • Pulse is created in about 2 microseconds and dissipates in 50-100 microseconds • Pulse has to diminish with time before a second can appear • The time required for this is called dead time.

15. G-M Meter Survey Meters Geiger-Mueller (GM) Survey Meter GM meters primarily detect gamma radiation, but many come with a slide on the probe that exposes an area of very thin aluminum to allow the detection of high energy beta radiation. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 151. Some GM Meters come with a “pancake” probe that has a very thin mica cover that allows penetration of beta and even alpha radiation.

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