Gas filled detectors

Gas Filled detectors, nuclear radiation detection, GM Counters, Proportional Counters, Ionization Chamber.

1. Radiation Detectors
Gas Filled Detectors
Ammara Usman
MPhil Physics
2nd sem.
University of Education

3. Types of detectors (cont.)
•Detectors may also be classified by the type of information
produced:
–Detectors, such as Geiger-Mueller (GM) detectors, that indicate
the number of interactions occurring in the detector are called
counters
–Detectors that yield information about the energy
distribution of the incident radiation, such as NaI scintillation
detectors, are called spectrometers
–Detectors that indicate the net amount of energy deposited in
the detector by multiple interactions are called dosimeters

4. Types of gas-filled detectors
•Three types of gas-filled detectors in common use:
–Ionization chambers (continuous current output)
–Proportional counters (pulse current output)
–Geiger Muller counters (pulse current output)
•Type determined primarily by the voltage applied between the two
electrodes
•Ionization chambers have wider range of physical shape (parallel
plates, concentric cylinders, etc.)
•Proportional counters and GM counters must have thin wire anode

5. Gas-filled detectors
•A gas-filled detector consists of a volume of gas between two
electrodes, with an electrical potential difference (voltage) applied
between the electrodes
•Ionizing radiation produces ion pairs in the gas
•Positive ions (cations) attracted to negative electrode (cathode);
electrons or anions attracted to positive electrode (anode)
•In most detectors, cathode is the wall of the container that holds the
gas and anode is a wire inside the container
•The electrons make a meter needle deflect and, if a loudspeaker is
connected, you can hear a loud click every time particles are
detected.

6. Practical Gaseous Ionisation Detection Regions

7. Dead time
•In pulse mode detectors, two interactions must be separated by a
finite amount of time if they are to produce distinct signals
•This interval is called the dead time of the system
•If a second interaction occurs in this interval, its signal will be lost; if
it occurs close enough to the first interaction, it may distort the
signal from the first interaction
•GM counters have dead times ranging from tens to hundreds of
microseconds, most other systems have dead times of less than a
few microseconds
–Detector has longest dead time in GM counter systems
–In multichannel analyzer systems, the analog-to-digital converter
often has the longest dead time

8. Ionization chambers
•ionization chamber measures the charge from the number of
ion pairs created within a gas caused by incident radiation.
•Ion-pairs move towards opposte polarity electrodes, generating an
ionization current which is measured by an electrometer circuit.
•The chamber cannot discriminate between radiation types (beta or
gamma) and cannot produce an energy spectrum of radiation.
•Straight pleateu as there is no “multiplication” and also no
“recombination”.

9. Ionization chambers
•The advantages are good uniform response to gamma radiation
and accurate overall dose reading, capable of measuring very high
radiation rates, sustained high radiation levels do not degrade the fill
gas.
•The disadvantages are 1) low output requiring sophisticated
electrometer circuit and 2) separation and accuracy easily affected
by moisture.

11. Ionization chambers
Applications
Nuclear industry
Used where a constant high dose rate is being measured as
they have a greater operating lifetime than standard GM tubes,
which suffer from gas break down
•Smoke detectors
Ionization chamber contains an alpha-emitter, producing
constant ion current. Smoke enters, disrupts this current
because ions strike smoke particles and are neutralized. This drop in
current triggers the alarm.
•Medical radiation measurement
Ionization chambers are used to ensure that the dose delivered
from a therapy unit

12. Ionization chambers
Advantages and Disadvantages:
•The advantages are good uniform response to gamma radiation and
accurate overall dose reading, capable of measuring very high
radiation rates, sustained high radiation levels do not degrade the fill
gas.
•The disadvantages are 1) low output requiring sophisticated
electrometer circuit and 2) peration and accuracy easily affected by
moisture.

13. Proportional counters
•Proportional counters operate at a slightly higher voltage, selected
such that discrete avalanches are generated. Each ion pair
produces a single avalanche so that an output current pulse is
generated which is proportional to the energy deposited by the
radiation. This is in the "proportional counting" region.

14. Proportional counters
Applications
•Commonly used in standards laboratories, health physics laboratories, and for
physics research
•Seldom used in medical centers
•The wire chamber is a multi-electrode form of proportional counter used as a
research tool.
•The proportionality between the energy of the charged particle travelling
through the chamber and the total charge created makes proportional counters
useful for charged particle spectroscopy.
•Proportional counters are also useful for high energy photon detection, such
as gamma-rays, provided these can penetrate the entrance window. They are
also used for the detection of X-rays to below 1 Kev energy levels, using thin
walled tubes operating at or around atmospheric pressure.
•These are used extensively to check for radioactive contamination on
personnel, flat surfaces, tools and items of clothing.

15. Proportional counters
•The advantages are the ability to measure energy of radiation and
provide spectrographic information, discriminate between alpha and
beta particles, and that large area detectors can be constructed.
•The disadvantages are that anode wires delicate and can lose
efficiency in gas flow detectors due to deposition, the efficiency and
operation affected by ingress of oxygen into fill gas, and
measurement windows easily damaged in large area detectors.

16. Geiger–Müller
counter
a hand-held radiation
survey instrument

17. GM counters
•consists of a GM tube, the sensing
•element, and processing electronics.
•Filled with an inert gas at Low
•pressure, and high voltage.
•each ion pair creates an avalanche,
•but by the emission of UV photons,
•multiple avalanches are created
•which spread along the anode wire.
•Ionization amplified within tube by Townsend Discharge Effect to
produce an easily measured detection pulse.
Ionization occurs mainly due to electrons emitted by the walls of
chamber by process of photoelectric effect when gamma rays enter
the chamber. In order to avoid this effect, 10% ethyl alcohol and
90% argon is added. Ethyl alcohol absorbs photons emitted by the
de-excitation of gas atoms. This is called adding Quenching gas.

18. This large pulse from tube makes
the G-M Counter relatively cheap.
Townsend Avalanche is a gas
ionization process where free
electrons are accelerated by an
electric field, collide with gas
molecules, and consequently free
additional electrons.
Those electrons are in turn
Accelerated and free additional
electrons.
The result is an avalanche
multiplication that permits electrical
conduction through the gas.
The discharge requires a source of
free electrons and a significant
electric Field.

19. GM counters

20. Readout
Count per second
•Normally used when alpha or beta
particles are being detected
Absorbed dose
•Radiation dose rate, displayed in a unit such as the ”Sievert”
•Normally used for measuring gamma or X-ray dose rates
The readout can be analog or digital, modern
instruments are offering serial communications
with a host computer or network.
An option produces audible clicks
representing the number of ionization
events detected.

21. Advantages an Disadvantages OF GM
COUNTER
The advantages are that they are a
•cheap detector with a large variety of sizes and applications,
•large output signal is produced from tube
•can measure the overall gamma dose when using an energy
compensated tube.
The disadvantages are that it
•cannot measure the energy of the radiation (no spectrographic
information)
•Can not measure high radiation rates due to dead time

22. APPLICATIONS OF GM COUNTER
Particle Detection:-
•For alpha particles and low energy beta particles the "end-window" type of
G-M tube has to be used as these particles have a limited range even in
free air, and are easily stopped by a solid material. High energy beta
particles can also be detected by a thin-walled "windowless" G-M tube,
which has no end window.
Gamma and X-Ray Detection:-
•Geiger counters are widely used to detect gamma radiation, and for this the
windowless tube is used.
Neutron Detection:-
A variation of the Geiger tube is used to
measure neutrons, where the gas used is
Boron Trifluoride or Helium-3 and a plastic
moderator is used to slow the neutrons. This
creates an alpha particle inside the detector
and thus neutrons can be counted.

23. LIMITATIONS OF GEIGER-MULLER COUNTER
•The current pulses produced by the ionising events can derive a visual
display of count rate or radiation dose, and usually in the case of hand-
held instruments, an audio device producing clicks
There are two main limitations of the Geiger counter.
Because the output pulse from a Geiger-Müller tube is always the same
magnitude regardless of the energy of the incident radiation, the tube
cannot differentiate between radiation types.
A further limitation is the inability to measure high radiation rates due to
the "dead time" of the tube. This is an insensitive period after each
ionization of the gas during which any further incident radiation will not
result in a count, and the indicated rate is therefore lower than actual.