Lecture 2: Basic Operation of Gas-filled Detectors Unit II, Gas-filled Detectors "Enrico Fermi," Microsoft® Encarta® Online Encyclopedia 2006 http://encarta.msn.com © 1997-2006 Microsoft Corporation. All Rights Reserved.

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

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

How Gas Radiation Detectors Work

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.

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.

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 (usually about a billion) of electrons 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

Next time Dose Calibrator and Survey Meter Operation