Unit II: Nuclear Medicine Measuring Devices Part A: Gas-filled Detectors "Enrico Fermi," Microsoft® Encarta® Online Encyclopedia 2006 http://encarta.msn.com © 1997-2006 Microsoft Corporation. All Rights Reserved.

Lecture 5 Objectives Identify the components of a gas-filled detector and explains how it detects ionizing radiation Discuss the voltage response curve and identify its regions useful to nuclear medicine Define current mode and pulse mode and describe their uses in nuclear medicine instruments 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

Introduction Gas-filled detectors in nuclear medicine include: Dose calibrators Ionization survey meters Pocket survey meters Gieger-Műller survey meters

Introduction: Ionizing Radiation + α (+2) -- β (-1) Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pp 52, 53, & 54.

Introduction: Ionizations in the presence of an electric field "Electricity," Microsoft® Encarta® Online Encyclopedia 2006 http://encarta.msn.com © 1997-2006 Microsoft Corporation. All Rights Reserved

Figure 01: Block diagram of gas-filled detector Basic Operation Figure 01: Block diagram of gas-filled detector

Basic Operation: The Voltage-Response Curve Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Recombination Region If Voltage is too low between the anode and cathode, the ion pairs simply recombine before reaching the cathode or anode. The response is minimal and not really valid for radiation detection. As voltage increases, the recombinations decrease and response increases. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Ionization Region -Also called Saturation Region. This begins at 50-300 V, depending on chamber design. Region of operation for ionization chamber gas-filled detectors Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Ionization Region Two types of common nuclear medicine instruments operate in the ionization region: ionization chamber survey meters (“Cutie Pie”) and dose calibrators. Ionization Survey meters are used to accurately measure exposure rates from usually a known source. Because they work by measuring collected charges and are not sensitive to individual radiation events. Dose Calibrators operate similarly. Instead of air, they use sealed argon gas, since it is not affected by barometric pressure changes. Ionization chamber instruments are capable of measuring high levels of radioactivity. Cutie Pie Dose Calibrator Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Another type of ionization chamber-type detector is a pocket dosimeter. An interior electrode is charged, leaving a relatively negatively charged exterior casing. A needle set against a calibrated meter between the capacitors registers the accumulation of charges and thus the total exposure over time. As the air becomes ionized, it affects the voltage difference between the electrode and the casing. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 92.

Proportional Region Increasing the voltage of the system brings the output into the proportional region. Ionized electrons cause additional ionization of the gas. However, this is proportional to the amount of the original ion pairs produced. This is termed: Gas amplification Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Gas amplification (Proportional Region) In the proportional region, the increased voltage causes the ions to move with increased kinetic energy toward the electrodes. These energized electrons collide with other gas atoms and form additional ion pairs. The increased voltage affects these ion pairs also, and the effect is amplified. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 134.

A Proportional Counter Not really used in nuclear medicine, but in some lab processes like RIA. A Proportional Counter Uses noble gases that permit the free flow of ions to enhance the avalanche effect. Advantage: unlike ionization region, is capable of detecting individual radiation events and can discriminate between energy levels. Used to detect non-penetrating radiation like α and β. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 205.

A Gas Flow Proportional Counter A gas flow proportional counter http://images.google.com/imgres?imgurl=http://www.austinst.com/images/austin_1.jpg&imgrefurl=http://www.austinst.com/&h=255&w=497&sz=23&hl=en&start=44&um=1&usg=__5GufMxLFkSwrASF_Ax2jC1tSdIM=&tbnid=PK1oVAux8misIM:&tbnh=67&tbnw=130&prev=/images%3Fq%3Dproportional%2Bcounter%26start%3D42%26ndsp%3D21%26um%3D1%26hl%3Den%26sa%3DN available at http://www.austinst.com/images/austin_1.jpg Accessed Sept. 5, 2008.

Nonproportional Region An unusable region along this scale. Here the avalanche is so severe that output no longer is proportional to the amount or energy level of the ionizing events. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Geiger-Mueller Region With increase voltages from about 1000 to 1500 V, the avalanche effect reaches a saturation point. Each incident event creates a huge electric pulse with no hint of its energy level. These properties are used to construct a sensitive detector. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Geiger-Mueller Region A Geiger-Mueller counter (GM survey meter) operates in this region and is a useful tool for detecting and searching for often unknown sources of radiation. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

This produces a large pulse for each interaction Inside the GM Meter probe, in the GM region of our scale, when radiation strikes--it’s the end of the world. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 133. This extreme avalanche, called a Townsend avalanche, includes the development of UV radiation from gas excitation, Which causes additional gas ionization through the photoelectric effect. A positive ion cloud then develops enveloping the anode. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 95. This produces a large pulse for each interaction

Counting Curve of a GM Meter Threshold: the level of voltage at which the electrodes are charged enough to detect incident radiation. At a certain voltage, a plateau is reached (the knee) and increases in voltage will not affect the count rate for the same amount of radiation. At very high voltages, there is a breakdown in the composition of the gases and there is a steep rise in spontaneous discharges. At this level there can be permanent damage to the device. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 97.

Continuous Discharge Spontaneous discharge on our GM meter brings us to the continuous discharge point on our scale. No gas-filled detectors use this region. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

For our practical purposes, we only use two areas of this scale: Ionization and Geiger-Mueller. Ionization Region Devices: Ionization chamber survey meter (“Cutie Pie”) Dose Calibrator Geiger-Mueller Region Device: Question: What do those two regions have in common? GM Survey Meter Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 132.

Current vs. Pulse Mode Current mode Gas electrons neutralizing capacitor charge cause current to flow from power source as it restores the charge Current from power source is measured Based on time-averaged ionizations/second Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 90.

Current vs. Pulse Mode Current mode Needs a good amount of radiation to accurately operate Low levels of radiation result in much statistical inaccuracy. Output is in the pA range. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 90.

Current vs. Pulse Mode Pulse mode Rather than measuring the current to recharge the capacitor, the change in voltage of the capacitor is measured Electrons generated by an interaction are treated as a group The size of the pulse represents the total charge deposited on the capacitor by the group Measured by height or signal vs. time From: http://nsspi.tamu.edu/nsep/courses/basic-radiation-detection/gas-filled-detectors/introduction/introduction Accessed 30 Sep 2012.

Current vs. Pulse Mode Pulse mode An RC circuit converts current to voltage The voltage pulse can also be “shaped” Pulse shaping will be covered in scintillation detectors Detector reports pulses per second More pulses/per second = more radiation Pulses need time to be separately detected Dead time Too many pulses in too short of time will result in measuring inaccuracies

Dose Calibrator Design Features Ionization Chamber Detector Detects primary ionized electrons (no gas amplification) Operates in “Current” Mode Sealed and pressurized (12 or more atm) argon gas in its chamber (makes it impervious to barometric pressure changes). Resistor or conversion factor buttons to adjust display readout Resistor or conversion factor buttons set to commonly used radionuclides

Figure 04: Block diagram of dose calibrator

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 = Ed2/G A = Activity E = Measured Exposure Rate d = Distance between source and detector G = Specific gamma ray constant (based on specific type of radionuclide and its energy emissions) (When your pressing that isotope button, you’re changing “G.”) From: http://www.dotmed.com/listing/dose-calibrator/capintec/radioisotope-crc-12/861774 Accessed 30 Sep 2012.

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. Dose Calibrator: Isotope Selector Buttons

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. Dose Calibrator: Operation

Next time: Cutie Pies, GM Meters and Quality Control (QC) 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