Instrumentation (Gas-filled detectors)

Radiation detectors

Paramount importance in nuclear medicine. Sensors that produce signals upon interaction with radiation.

1. Photoelectric effect
2. Compton scattering
3. Pair production

X-ray and gamma ray interaction mechanism

1. Radiation energy
2. Interaction medium

Relative importance depends on:

Detector requirements

1. Sensitivity
2. Energy resolution
3. Time and position resolution
4. Counting rate performance

Sensitivity

Depends on:
1. Subtended solid angle
2. Detector efficiency for radiation interaction
3. Relevant energy range is 30-511 Kev governed by photoelectric effect

Sensitivity

Attenuation length is pzeff3-4

Ionization and Excitation

Charged particles transfer energy through these

Energy resolution

Strongly coupled to number of information carriers. Largest number produced in semiconductors.

N=E/W

Formula of energy resolution

Time resolution

Depends on 2 factors: Rest time of signal pulse and height of signal pulse

1. Fast response
2. Fast rise time
3. Light sensor's fast response

Inorganic scintillators detectors preferred because they have:

Counting rate(Dead time)

Minimum time between true events at which these are counted separately.

Gas filled-detectors

Involves the radiation entering the gas-filled chamber and ionizing gas molecule

Anode

Positive electrode

Cathode

Negative electrode

Electrode system

A central anode and cathode are placed inside the detector chamber.

Gas

Ionizing radiation interacts with gas molecule producing ion par

Quenching gas

Radiation -> Gas = Ion pairs

High voltage supply

Electric field guides the ions and electrons toward their respective electrode

Electrode system

Energetic electrons -> Produce secondary electron travelling through gas -> Secondary electrons drift to anode and ions to cathodes.

Recombinant

At very low voltage, ion pairs may recombine before reaching the electrodes leading to incomplete detection.

Ionization

All ion pairs are collected, and the current generated is directly proportional to the radiation intensity. Operational region for ionization chambers

Proportional

Electric field is strong enough to cause gas amplification.

Continuous discharge

High voltage = continuous ionization -> constant current

Geiger-Muller Region

Single ionizing event can cause a large avalanche of ionization leading to a saturated pulse output

Non-proportional

High voltage = Proportionality between the initial ionization and signal starts to break down, and non-linearities are introduced in response.

Nuclear medicine

Primarily uses counting modes, Energy information, Arrival time information

Current mode

The detector operates continuously and measures the average current produced by ionization event.

True

The current is directly proportional to the intensity of the radiation (True or False)

Pulse mode

Each ionizing event is treated as a discrete pulse of current

Paralyzable

Each new event resets the dead time countdown

Non-paralyzable

Means that while some events may be missed, the dead time does not extend due to the missed events. Detectors can handle high radiation rates.

500-1000V

Typical operating voltage of ionization chambers

Gas filled (electron ion)

30

Semi-conductor (electron hole)

3

Inorganic scintillator

25

Inorganic scintillator + PMT

100

Inorganic scintillator + Si dioxide

35

Ionization chamber

-All rays can be detected
-Simplest type of gaseous ionizing chamber
-Dosimetry devices used to measure the output of x-ray tubes

X-rays, Gamma rays, Alpha particle, Beta particle

Ionization chamber can detect these:

Dose calibrators

-Used to ensure that the dose is what intended
-Calibrated in units of Curie and Becquerel
-The outside of the chamber is shielded by lead cylinder

150 volts

Voltage supply of dose calibrators

Dipper

A gravy-ladle-shaped device fabricated out of plexiglas. Used to lower the source into cylindrical space.

Isotope selector buttons

Multiplies the measured current by a current > Activity conversion factor that is specific for each radionuclide.

Tc-99M

0.5 pA/mCi

I-131

0.2 Pa/mCi

Moly shield

A lead-vial shield or "Pig" of about 0.5 cm thickness

Cesium 137, Cobalt 37, Barium 133

Long lived

NIST

National Institute of Standards and Tracer

Linearity

-MAximum activity is dispensed down to 30UCI (1.1 MBq)
-Refers to the ability of an instant to measure variable quantities in linear fashion.

Pocket dosimeter

Used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays

False

The amount of movement is inversely proportional to the amount of ionization which occurs (True or False)

1. Temperature below -20C or above +50C
2. Greater than 50000 foot altitude in one month
3. Humidity greater than 95%. Highly resistant to shock and vibrations

Maximum change in sensitivity may occur when these conditions are met.

Audible alarm rate meters

Devices that emit a short "beep" or "chirp" when a predetermined exposure has been received

Film badges

Special film is used which is coated with two different emulsions.

Thermoluminescent dosimeters

Can measure as low as 1 millirem. Have a precision of approximately 15% for low doses.

They measure exposure by detecting liberated electron charge when x-ray photons ionize gas within the chamber

How do ionizing chambers measure?

1. Radioisotope calibrators
2. Radionuclide calibrators
3. Curie meter
4. Activity meter

Dose calibrators are also known as:

Accuracy

-ANNUALLY
-Requires 3 separate measurements of reference standard

Constancy

-DAILY before any use
-Uses Cs-137
-Verifies the proper operation of a dose calibrator on a given day.

Linearity

-QUARTERLY
-Refers to the ability of an instrument to measure unviable quantities in a linear fashion

Activity linearity

Ability of a dose calibrator to measure, in a proportional manner

Geometry

-Done UPON INSTALLATION and REPAIR
-Involves in the measurement of a variety of radioactive sources in different geometric and volumetric configurations.

1. Direct read pocket dosimeters
2. Digital electronic dosimeters

2 types of pocket dosimeters:

Digital electronic dosimeters

These records information and dose rates. Often used Geiger-muller counters

1. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure

2. Other side is coated with fine grain, slow emulsion that is less sensitive to exposure.

Film badges' 2 side

Ionization survey meters

-Primarily used to measure exposure rates from radiation shields
-Also known as "Cutie Pie"

Work function

Number that estimate the energy required to0 create one ion pair in the fill gas of meter.

Geiger-Muller survey meter

One of the workhouse for health physics and one of the oldest detectors

Hans Geiger

Invented Geiger-muller survey meter

Walther Muller

Improved Geiger-muller survey meter