Gas Detectors and GM Meters

Nuclear Medicine Instrumentation

More Radiation =

More Ion Pairs

Less Radiation =

Less Ion Pairs

Incident Ionizing Radiation

Air or Other Gas

Voltage Source

Electrical Current Measuring Device

Anode +

Cathode -

Ion Pairs

• Created when radiation interacts with atoms and molecules

• Need a small amount of radiation

# of Electrons created is

related to the amount of radiation/radioactivity present

Gas Detectors

Simplest radiation detectors
❖ Ion Pairs are created in the chamber filled with gas
❖ Have 2 basic parts:
❖ Chamber Filled with Gas
❖ Basic Electric Current

Anode

Has a + charged central wire that attracts electrons produced by ion pairs during radiation detection.

Cathode

Has a - charged metal tube that helps to collect electrons produced during ionization in gas detectors.

Power Source

Keeps the cathode and anode charged with
positive or negative charge from the power source

Electrical Current Measuring Device

Meter that is attached to the power source measures
the flow of electricity that occurs as a result of the
neutralization of charge at the anode and cathode

Gas Detector Operation

Remove the radiation source
❖ The gas between cathode and anode act as an insulator
ADD Radiation
❖ Energy over 10 electron volts (eV) passes through gas
and ionizes gas molecules, producing free electrons and
positive ions
❖ Average energy required to cause ionization is between
20-45 eV
Electrons move faster + ions
❖ Electrons collect at anode
❖ These electrons can neutralize some of the + charge
❖ Causing electricity to flow through the connecting circuit

# of electrons measured depends on

❖ # of charged particles/photons being measured
❖ Energy of the radiation
❖ Geometric Configuration of the detector
❖ Composition of gas in the chamber
❖ Applied voltage between cathode and anode

Voltage Response Curve

The curve representing the applied voltage between the cathode and anode.

There are 6 regions

Recombination Region

❖ Applied Voltage very LOW
❖ Electrons moving very slowly and do NOT reach the anode and cathode
❖ Incomplete collection of ions
NO useable detectors that operate in this region!!

Ionization Region

❖ AKA Saturation Region
❖ Applied voltage increased
❖ All electrons are being collected
❖ Saturation voltage is voltage sufficient enough to reach the saturation point
❖ Wide flat range, where even if voltage fluctuates, the same # of electrons will be collected

Useful for Radiation Detection

Proportional Region

❖ Electrons from radiation interactions are easily accelerated by higher voltages to create additional ionizations with collisions of gas molecules
❖ Gas Application
❖ Applied voltage creates signal that is larger than BUT still proportional to the # of ion pairs
NOT COMMONLY USED IN NUCLEAR MEDICINE

Limited Proportionality Region

❖ Ions are larger than electrons
❖ Drift slowly toward cathode
❖ Voltage response curve no longer changes linearly with increasing voltage
NOT USEFUL FOR RADIATION DETECTION

Geiger-Muller Region

❖ AKA-GM Region
❖ At high voltages, gas amplification effect is maximized
❖ Each electron creates many ionizations
❖ Each radiation event produces an avalanche of ions through the chamber
❖ Pulse size is essentially the same for ALL radiation events
❖ No matter the type or amount of energy transferred

GM Meters work here!!
GM Meters are a workhorse in the NM Department

Continuous Discharge Region

❖ Increase voltage above GM region
❖ Causes spontaneous ionizations in the detector without radiation present
NO USEFUL RADIATION DETECTORS HERE

Current Mode

❖ Measures the number of electrons per second required to keep the anode and cathode charged
❖ Detectors that operate in current mode are connected to a power supply that strives keep the anode and cathode fully charged
❖ Measurement is based on the time-average # of ionizations occurring per second

Ion Chambers and Dose Calibrators Operate Here

Pulse Mode

❖ Electrons created by each radiation interactions are treated as a group
❖ Pulse created as a single entity
❖ Restoring electrical potential between anode and cathode measured as a single entity
❖ Size of pulses represents total change deposited by a single radiation interaction
❖ Dead time affects the counting ability in pulse mode

GM Meters Operate Here

Ion Chambers/ Exposure Rate Meters

• Gas Filled Detectors

• Used to measure exposure rate and to determine the relative amount of penetrating and non penetrating radiation

• Uses Current Mode

• Works in Ionization Region

• Read in mR/hr, reads accurately down to 1mR/hr

• NOT appropriate to use for detecting radiation contamination

Gases Used In Ion Chambers/ Exposure Rate Meters

• Helium

• Neon

• Argon

Geiger-Mueller (GM) Detector

• One of the workhorse’s of the NM department

• Operates in the Geiger Mueller Region

• Operates in Pulse Mode

• Used as a qualitative indicator of radiation

• Excellent for finding contamination

• Highly reliable and stable

• Can read in mR/hr of cpm (counts per minute)

Types of Geiger-Mueller Detector Probes and Meters

• End Window

• Side Window (energy compensated)

• Pancake

GM Meter Quality Control Checks

• Accuracy Check or Calibration

• Daily

Accuracy Check or Calibration QC

• Performed Annually

• Sent out for Calibration and Accuracy

Daily QC

• Battery Check

• Constancy

• Voltage

When To Use A Geiger-Mueller Detector

• Monitoring radioactive packages coming and leaving the department

• Check for contamination on yourself or in a room

• End of the day room checks

Geiger-Mueller Detector Pitfalls in Inaccurate Readings

• Math Error

• User Error

  • Reading Wrong Scale

  • Multiplication Error

Factors Affecting Observed Exposure or
Count Rate – Dead Time

Instrument design to handle dead time

Paralyzable vs. Non-paralyzable

Dead Time

The detector has a limited time it takes to process and record an event

Paralyzable

• Event happening during dead time will be missed, but the dead time restarts

• Instrument reaches saturation point and unable to record and further events

Non-Paralyzable

• Event happening during dead time is lost

• With increasing event rate the detector will reach a saturation rate equal to the inverse of the dead time

What Exposure Rate is Displayed

Meter is set to x10 and the arrow is pointing to 1.0

1.0 × 10 = 10 mR/hr

What Exposure Rate is Displayed

Meter is set to x0.1 and the arrow is pointing to 0.3

0.3 × 0.1 = 0.03 mR/hr

What Exposure Rate is Displayed

Meter is set to x100 and the arrow is pointing to 0.6

0.6 × 100 = 60 mR/hr