Lecture 3 (Unit II): Gas-Filled Detector Operation and Use - PowerPoint Presentation

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Lecture 3 (Unit II): Gas-Filled Detector Operation and Use

Lecture 3 Objectives

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

Describe the function and use of a ionization survey meter

Describe the function and use of a Geiger-M

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ller

(GM) survey meter

Explain the process of radiation detection in a GM meter including the Townsend avalanche and probe recovery

Discuss limitations of gas-filled detectors

Dose Calibrator

Design Features

Ionization Chamber Detector (~150 V on the voltage-response curve)

Detects primary ionized electrons (no gas amplification)

Operates in “Current” Mode

Sealed and pressurized (12 or more atm) argon gas in its chamber

Impervious to barometric pressure changes)

Increases likelihood of gamma interactions

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

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.

From:

http://www.dotmed.com/listing/dose-calibrator/capintec/radioisotope-crc-12/861774

Accessed 30 Sep 2012.

Dose Calibrator:

Isotope Selector Buttons

A =

Ẋd

2

Γ

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

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

Cutie Pies & GM Meters

https://www.stresslabs.com/catalog/images/CAPINTEC%20CRC-12.jpg

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Cutie-Pie (QDπ)

Ionization Chamber Survey Meter

(a.k.a. “Cutie-Pie” ).

Ionization Chamber-type Detector

Response is based on the total energy deposited on the detector and is

proportional

to that amount of energy.

Can measure in an averaged rate (mR/hr) mode or in an accumulated exposure mode (mR accumulate until measurement stopped).

Survey Meters

Roentgen = amount of radiation that produces 1 unit of charge (about 2 billion ion pairs) in 1 cubic cm of air—used in describing radiation field strength.

Ionization Meter

Operates in

current

(vs. pulse) mode.

Requires batteries transformed to produce 50-500V

Chamber usually filled with air

Measures high level of activity

Good for fairly accurately measuring exposure rates from known sources, such as areas near radioactive storage or when determining clearance levels for a radiotherapy patient.

Ionization Survey Meters

Geiger Counter

Geiger-M

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ller (GM) Survey Meter

Operates in the Geiger-M

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ller region of gas-filled detectors (400-1000V)

Helium or argon gas at less than atmospheric pressure

Response is based on huge electric pulses and not necessarily on the energy level of the source radiation.

Uses gas amplification (Townsend avalanche) with UV emissions

Its rate is only accurate for photon energies that are the same as those used to calibrate it (

energy-dependent).Because of its strong response—is good for detecting unknown radioactive sources, such as when a spill is suspected or searching for contamination.

Survey Meters

G-M MeterOperates in “pulse” mode

Size of pulse represents total charge deposited by ionized electrons

RC circuit converts current to voltage and restores charge

Not accurate for measuring exposure from different gamma ray energies (off by a factor of 2 to 3)

Slow moving positive ions form an envelope around the

cathode

This

attracts

electrons trying to reach anode

Reaction stops but pulse is createdGM meters come equipped with “quenching gas” (organic or halogen) to stop the UV rays from resulting in continuous dicharge

G-M meterFormation of positive ion cloud

+ ion cloud

G-M MeterTime constant (

τ

)

Short-allows quick change but also bouncing

Long-slower change but may miss high spikes

Dead time

Problem because of pulse mode

Pulse is created in about 2 microseconds and dissipates in 50-100 microseconds

Pulse has to diminish with time before a second can appear

The time required for this is called dead time.

Dead Time

G-M Meter

Geiger-Mueller (GM) Survey Meter

Survey Meters

Some GM Meters come with a “pancake” probe that has a very thin mica cover that allows penetration of beta and even alpha radiation.

GM meters primarily detect gamma radiation, but many come with a slide on the probe that exposes an area of very thin aluminum to allow the detection of high energy beta radiation.

Paul Early, D. Bruce Sodee,

Principles and Practice of Nuclear Medicine

, 2nd Ed., (St. Louis: Mosby 1995), pg. 151.

http://www.alexross.com/FF8.html