Dose Calibrator

1. Dose
Calibrators
Done by: Fatma AL-Gafri
Sr. Medical Physicist
Royal Hospital
Sultanate of Oman

2. 2
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

3. 3
Types of radiation detectors
Gas field detectors.
• Radionuclide dose calibrator
Scintillation detector
• Gamma well counter
• The thyroid uptake probe
• Liquid scintillation detector

4. 4
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.

5. 5
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
REGION
(100 X B)
a
(100 X Y)
a
B
Y
VS VP VL VG VD
Recombination
Region
6 Regions of a Gas Filled Chamber

6. 66
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
REGION
(100 X B)
a
(100 X Y)
a
B
Y
VS VP VL VG VD
Recombination
Region
 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.

7. 77
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
REGION
(100 X B)
a
(100 X Y)
a
B
Y
VS VP VL VG VD
Ion chamber
Region
• 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.

8. 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

9. 9
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 .

10. 1010
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
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.

11. 11
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

12. 1212
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
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.

13. 13
Geiger Mueller Counter
Most common type of detector
Gas amplification : multiplication of electrons.
Multiplication factor 108-1010
Long dead time
Used for:
– Count rate
– Dose/Dose rate
– Surface activity
Battery or High
Voltage
Resistor
(-) Cathode
+ -
(+) Anode
e
-
+

14. 1414
Types of radiation detectors
Gas field detectors.
• Radionuclide dose calibrator
Scintillation detector
• Gamma well counter
• The thyroid uptake probe
• Liquid scintillation detector

15. Dose calibrator
A 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,…….
15

16. 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.
16

17. Dose Calibrators designed to verify clinically
administered radioactivity are just one type of
radiation detector.
17

18. Structure of Dose calibrator
18

19. Sample holder
(geometry)
19
This dose calibrator dipper is specially designed to hold
syringes and vials.

20. Why do we need to know the
activity?
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 drug
Activity (Bq) → Radiation dose
(Gy) to organs.
20

21. 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.
21

22. 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.
22
PULSEHEIGHT
VOLTAGE APPLIED
I
ION CHAMBER
REGION
II
PROPORTIONAL
REGION
III
LIMITED
PROPOR-
TIONAL
REGION
IV
G-M
REGION
V
CONTINUOUS
DISCHARGE
REGION
(100 X B)
a
(100 X Y)
a
B
Y
VS VP VL VG VD

23. 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.
23

24. 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.
24

25. Dead time effects
occur 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.
25

26. 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).
26

27. 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).
27

28. Geometric efficiency=
number of photons reaching the detector / the number of
photons emitted from the sample
Increasing geometric efficiency
28

29. 29

30. Dose Calibrator
30

31. Gamma counter
31

32. Probe system
32

33. Liquid scintillation counter
33