Dose Calibrators

Done by: Fatma AL-Gafri

Sr. Medical Physicist

Royal Hospital

Sultanate of Oman

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Definition of Radiation

“Radiation is an energy in the form of electro-magnetic waves or particulate matter, traveling in the air.”

or X-ray

neutron

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Types of radiation detectors

Gas field detectors.• Radionuclide dose calibrator

Scintillation detector• Gamma well counter• The thyroid uptake probe• Liquid scintillation detector

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Gas Filled Detector Used every where because : Simple to make and portable . Operate by utilizing the ionization produced by radiation as it passes

through a gas. All gas-filled detectors detect radiation with different characteristics

based on the relative applied voltage between the anode and the cathode.

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PU

LSE

HE

IGH

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VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

RecombinationRegion

6 Regions of a Gas Filled Chamber

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HE

IGH

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VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

RecombinationRegion

As voltage is increased slightly above zero, the recombination region is entered. In this region, the negative ions generated are slowly attracted to the anode. Negative ions recombine with a positive ion before reaching the anode. If the voltage between cathode and the anode is high , the recombination will stop.

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PU

LSE

HEIG

HT

VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

Ion chamberRegion

• Increasing the voltage further, the ionization region (or saturation region) is entered.• In this area, all of the negative ions generated reach the anode.• characteristic of this region : low efficiency and exceedingly small pulse height.

The most widely used radiation detectors are devices that respond to ionizing radiation by producing electrical pulses.Ion chambers are the simplest of all gas filled detectors

An electric field is used to collect all the ionizations (positive and negative charged particles) produced by the incident radiation in the gas volume.

Electrical collection of ions

Used to measure dose rate

Sensitive to environmental changes.

IONIZATION CHAMBER

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IONIZATION CHAMBERS: to measure exposure rates

detector used for the measurement of exposure rate differ greatly from all other types of gas-filled detectors (e.g., G-M

and proportional detectors). radiation causes a current to flow in an ion chamber detector.

Rather than detecting a series of pulses or counts .

1010

PU

LS

EH

EIG

HT

VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

Proportional Region

• As the detector operating voltage continues to be increased, the detector enters the proportional region.

• Why???? pulse height is proportional to the energy of the photon or particle which initiated the pulse.

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Proportional Counters

Alpha-Beta discrimination

Low dead time: time after each event during which the system is not able to record another event.

Used in labs and neutron detectors

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PULSE

HEIG

HT

VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

• As the detector operating voltage continues to increase, the limited proportional region is passed through and enters the Geiger-Mueller region.

• In this region all pulse heights are equal and efficiency is relatively high, although photon efficiency will vary based on energy.

• Finally, the continuous discharge region is entered where the voltage is so high that arcing occurs.

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Geiger Mueller CounterMost common type of detector Gas amplification : multiplication of electrons.Multiplication factor 108-1010

Long dead timeUsed for: – Count rate– Dose/Dose rate– Surface activity

Battery or High Voltage

Resistor(-) Cathode

+ -

(+) Anodee-

+

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Types of radiation detectors

Gas field detectors.• Radionuclide dose calibrator

Scintillation detector• Gamma well counter• The thyroid uptake probe• Liquid scintillation detector

Dose calibratorA gas-filled ionization chamber.

Why do we need dose calibrators?– Used to measure the ionizing radiation exposure of a given

radioisotope.– Used in NM to measure the amount of radioactivity of a

radionuclide before injection into a patient

Calibrators are generally gas-filled cylinders with a well in the center of the ionization chamber into which the radioactivity is placed.

Known as:

radioisotope calibrators.

radionuclide calibrators.

curie meters.

activity meters.

Co-57, Ba-133, Cs-137, Co-60,…….

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Dose calibrator

These ionization chamber radiation detectors are typically filled with highly pressurized Argon [18-Ar] gas, compressed to around 20 atmospheres.

able to measure activities anywhere from 1μCi-20Ci (3.7kBq – 740MBq).

The highly compressed gas creates an ionic environment that favors the possibility of ionizing events.

There is a direct relation between the increased gas pressure and detector efficiency.

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Dose Calibrators designed to verify clinicallyadministered radioactivity are just one type of

radiation detector.

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Structure of Dose calibrator

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Sample holder (geometry)

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This dose calibrator dipper is specially designed to hold syringes and vials.

Why do we need to know theactivity?

We can estimate radiation absorbed doses to the organs and whole body.

Doses depend on the :activity

patient size,

Bio distribution of the specific radio labeled drug.

For a specific radio labeled drugActivity (Bq) → Radiation dose (Gy) to organs.

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How It Works HEP interact in 3 direct ways:– photoelectric effect, – Compton scattering – pair production.

In all of these interactions between photons and matter (argon gas), energy is transferred, leaving ion pairs behind.

To collect the ion pairs created from these interactions, the detector has an applied voltage with the negative cathode as the chamber wall and a positive anode within the ionization chamber.

After ionization, positively charged ions drift toward the cathode and the negatively charged ions drift toward the anode.

The positive and negative charge is supplied by a high-voltage supply, or a battery acting as a capacitor, within the dose calibrator.

This battery keeps the voltage on the cathode and anode constant and functions as a backup if there is ever a power outage to keep the calibration factors stored in memory.

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DC operates in the “ionization chamber” region of the voltage response curve where the ion pairs created by the radiation are collected.

An increase in the voltage does not significantly increase the number of ion pairs collected.

number of collected ion pairs is constant

current that is generated and measured is also constant.

ionization chamber plateau:

there is no increase in measured current because all ion pairs are collected and there is no recombination of pairs taking place .

If a volume of gas is irradiated at a constant rate, a constant amount of ion pairs are formed, and a constant current is generated.

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PU

LSE

HE

IGH

TVOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITED

PROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGE

REGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

Dose calibrator

Advantage measuring the charge

generated within the volume is that the current generated can be directly correlated to the radiation exposure.

The activity of the source correlates with the characteristic radiation exposure of each radionuclide.

Disadvantage

a lack of information about individual ionization events or the energy(s) generating the current.

This makes it impossible for the dose calibrator to identify or distinguish radionuclide's in mixed or contaminated samples.

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Dose calibrator Verses solid-crystal well counter

Dose calibrator

Gas-filled detector measures ionizations created by photons (radiation).

The ionization plateau collects all ionization events avoiding dead time effects seen with well counters.

Able to measure high levels of activity because it operates in current mode, which avoids dead-time effects.

well counter

A solid sodium iodide NaI(Tl) crystal directly detects discrete decay events (radioactivity).

Used for counting swipes from around the work area to detect small traces of contamination.

Allows for energy discrimination and presentation of energy spectra of samples.

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Dead time effectsoccur when operating in pulse mode.

NaI(Tl) crystal, used in well counters, operates in pulse mode to capture ionization events.

Each energy event creates a voltage pulse.

If the pulses are too frequent as from a high activity source, the detector is unable to count the pulses quickly enough and they begin to blend into one another.

This is problematic because information is lost and the resultant measurement isn’t a true representation of the activity of the source.

The dose calibrator does not directly measure the energy of a radionuclide only the current generated from ionization events, and the measurement is not affected by this problem.

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Dose calibrators The activity of the material is measured in term of the ionization current produced by the emitted radiations which interact in the gas.

The chamber is sealed, usually under pressure, and has two co-axial cylindrical electrodes maintained at a voltage difference derived from a suitable supply, the axial space constituting the well.

In the electrometer, the ionization current is converted to a voltage signal, which is amplified, processed and finally displayed, commonly in digital form in units of activity – Becquerel's (Bq) or curies (Ci).

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Dose calibrator

The response of the detector will depend on:

• Radionuclide ( energy of photons).• Geometry of the detector. • Geometry of the source.• The condition of instrument (QC).

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Geometric efficiency=

number of photons reaching the detector / the number of photons emitted from the sample

Increasing geometric efficiency

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Dose Calibrator

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Gamma counter

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Probe system

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Liquid scintillation counter

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