1. Lecture 2: Basic Operation of Gas-filled Detectors Unit II, Gas-filled Detectors
"Enrico Fermi," Microsoft® Encarta® Online Encyclopedia © Microsoft Corporation. All Rights Reserved.

2. Lecture 2 Objectives
Identify the components of a gas-filled detector and explain 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

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

4. How Gas Radiation Detectors Work

5. 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.

6. A Gas Flow Proportional Counter
A gas flow proportional counter available at Accessed Sept. 5, 2008.

7. 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.

8. 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.

9. 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 (usually about a billion) of electrons
Measured by height or signal vs. time
From: Accessed 30 Sep 2012.

10. 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

11. Next time
Dose Calibrator and Survey Meter Operation