Lecture 2 Basic Operation of Gasfilled Detectors Unit

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

Introduction u 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, 2 nd 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=__5 Guf. Mx. LFk. Swr. ASF_Ax 2 j. C 1 t. Sd. IM=&tbnid=PK 1 o. VAux 8 mis. IM: &tbnh=67&tbnw=130&prev=/images%3 Fq%3 Dproportional%2 Bcount er%26 start%3 D 42%26 ndsp%3 D 21%26 um%3 D 1%26 hl%3 Den%26 sa%3 DN available at http: //www. austinst. com/images/austin_1. jpg Accessed Sept. 5, 2008.

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

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

Current vs. Pulse Mode u 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 u Measured by height or signal vs. time From: http: //nsspi. tamu. edu/nsep/courses/basic-radiationdetection/gas-filled-detectors/introduction Accessed 30 Sep 2012.

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

u Next time – Dose Calibrator and Survey Meter Operation