Different Types of Radioactive Counters or detectors used in analyzing low or high penetrating power radiation or particles are explained briefly with their advantages and disadvantages. Medha Thakur (M.Sc Chemistry)

1. RADIOACTIVE
COUNTERS
Presented By: Medha Thakur
MSc (Chemistry)
S.N.D.T. University

2. Contents  Introduction
 Classification Of Detectors
 Gas Ionisation System (GI)
 Types of (GI) counters
 (a) Gas Flow Proportional Counters
 b) Geiger – Muller Counters
 Types of Geiger Counters
 Solid Scintillators (SS)
 Liquid Scintillators (LS)
 Solid State (Semi-Conductor) Counters

3. Introduction
• various radiation have different
penetrating power & particles are
least penetrative.
• β particles are 10 times as penetrative
as α and γ rays are very penetrative.
• Beacuse of the wide variation in
penetrating power of the various
radiation, detectors employed for
counting the radiation will depend on
the type and energy of the radiation
emitted.
Radiation
Low
Pose a major
Problem for
detection
Medium
Detected by using
low density
materials (Gases)
Very High
Detected by using
denser material
(Solids)
With Penetrating
Power

4. Classification Of Detectors
Detectors
Gas ionisation
systems
Solid state
scintillation
counters
Liquid
Scintillators
Solid state (semi
– conductors)
counters

5. Gas Ionisation System (GI)
• Principle:
• When radiation passes through a gas, energy is
passed on to some of the gas molecules. These gas
molecules may ionise to a positive ion.
• e.g. If a suitable electric field is applied, the ions
can be collected and the magnitude of the resulting
currents can be realated to the amount of the
radiation initially responsible for the ionsation.
• The applied strength not only enables collection of
electrones or ions but also allows multiplication.

6. Types of (GI) counters
GI COUNTERS
(a) GAS FLOW
PROPORTIONAL COUNTERS
(b) GEIGER –
MULLER COUNTERS

7. (a) GAS FLOW PROPORTIONAL
COUNTERS
• In this type of detectors the applied field is such that the number of
ion pairs collected is proportional to the energy of the incident
radiation.
• This is achieved by placing the source, inside the counter and by
carefully controlling such experimental parameters as the geometry
of the counter, the flow rate of the gas, and the applied voltage.
• The output pulse is proportional to the energy of incident radiation.
Therefore by using a pulse- height analyser, it is possible to
differentiate between different isotopes.

8. GAS FLOW PROPORTIONAL COUNTER

9. GAS FLOW PROPORTIONAL COUNTERS
Disadvantages:-
• 1. A very stable high
voltage supply is
needed.
• 2. A very stable, high
gain amplifier is
needed.
• 3. The gas is used for
the counter is very
expensive.
Advantages:-
•1. gives a high and
reproducible detection
efficiency.
•2. used to investigate low
levels of radio-isotopes in
environmental sample.
•3. The sample to be counted
is normally placed inside
the counter, so that the
absorption problems of α
emitters are minimised.

10. (b) GEIGER – MULLER COUNTERS
Principle:
• In this type of detector, the applied field is increased to such
an extent that the ionisation induced by the acceleration of
initially produced ions and electrons results in virtually all of
the gas in the viscinity of the anode being ionised This is
known as ‘ avalanche effect’.
GEIGER – MULLER COUNTERS
Advantage:
They are simple and
effective.
Disadvantage:
To be compared with a standard.
They work only at high voltages (1000-
1500 V).

11. Types of Geiger
Counters
GEIGER
COUNTERS
1) END-WINDOW
used for counting solid samples.
It is cylindrical with a thin mica land
window. cylindrical case cathode and
central metal wire anode.
sample is placed directly under the End-
window. absorption of the radiation by the
window, and scattering of the radiation.
2) GEIGER COUNTER for LIQUID
SAMPLES
used for counting liquid samples
made of glass and consist of a central
gas filled tube containing the electrodes
surrounded by cylindrical ‘sheath’ into
which the liquid to be counted is placed.

12. 1) END-WINDOW GEIGER COUNTER
2) GEIGER COUNTER for LIQUID
SAMPLES

13. Solid Scintillators (SS)
Principle:
• The most significant penetration into solid materials
is achieved by γ rays.
• They cause very little ionisation in gases and hence
are poorly teasured by gas ionsiation methods.
• rays are most efficiently counted using a solid
absorber. which acts as a scintillator.
• This is usually called a ‘Crystal or phosphor’. The
most popular crystal is N2- iodide containing traces of
thallium (t) iodide. The symbol used is Na2|Th|.

14. CONSTRUCTION
• crystals are cylindrical and a light-tight
aluminium can.
• One end is kept in contact with the PM
tube at which the photocathode is located.
• reflection losses minimised by a films of
transparent silicon oil.
• Counters Used:
• Plastic scintillators are used for β
counting.
• Some specialised counters use NaI(TL) for
Low energy γ and X-ray counting.
The main Criteria for a scintillators:
• 1. Good absorption of incident radiation (i.e.
high density).
• 2. High efficiency of photon production.
• 3. Little/ no reabsorption of the photons by
crystal.
• 4. Emitted photons just have a wavelength
compatible with the PM tube (320-720 nm for
glass window).
• anthracene and stilbene meet these criteria.

15. Solid Scintillator (SS)

16. Liquid Scintillators (LS)
Principle:
• GI counters and SS counters enable us to count isotopes which
emmit gamma rays and or medium to high energy β particles.
• But, there are several low energy β emitters for which these
technologies are inappropriate because of low penetration of the
β- particles,
• it is essential that the, isotope to be counted should be internally
mixed with the scintillator and not external to it as for solid
scintillators.
• This is done by dissolving sample wholly or partly in a solvent
containing the scintillator.

17. Construction and Working
1) Solvents :- Incident radiation should readily for electronically excited
solvent molecules. These readily transfer their energy to the scintillator.
Solvents are usually aromatic hydrocarbons such as xylene, toluene,
dioxane, etc.
2) Scintillator (primary soulte) :- accepts energy from solvent and is
raised to excited state and then de-excited by emitting light photon ( λ
range 300-400 nm) which is counted by PM tube. e.g. Large organic
molecules.
3) Secondary Solute :- Whenever maximum efficiency is required a
secondary solute is used.
They emit light photons of λ range 400-500 nm, where the PM tube is
more sensitive. The excitation energy of the 1° solute molecule is
transferred to the larger 2° solute molecules.

18. Scintillation
Counters
Advantages
simple to operate.
sensitivity to all types of
ionising radiations.
Distinguishes between photons of
different energies.
Short resolving time & more
efficient, fast and accurate
counting (especially γ rays).
Dis
advantages
A massive PM tube is necessary.
At low energies, the counter
response is not 100 % .

19. Solid State (Semi-Conductor) Counters
Principle:
• They are exemplified by the Ge (Li) system.
• Detectors of this type consist of an n-type
semiconductor and p-type semiconductor; separated by
an ultra-pure radiation sensitive region.
• The n-type semiconductor contains negative sites (e-s)
for conduction. The P-type S.C. has a deficiency of e-s or
‘Positive hole’.
• The SS ionisation detector requires an element whose
atoms are not only small enough to fit into the
interstitial sites of Ge; but also readily ionise e.g. Li.

20. Construction and Working
• In Ge (Li) detector, the Li atoms are allowed to migrate or ‘drift’ to
some of the p-sites in a cylinder of p-n Ge.
• Each p-site is deficient in 1e- and each Li is readily ionised and gives
up an e-.
• Hence a p-type S.C. and an n-type S.C. are separated by an ultra-pure
and electrically neutral region (intrinsic region).
• If radiation enters the neutral region (intrinsic), it will cause
ionisation.
• By applying a suitable potential, the e-s can be collected and amplified
to a measurable pulse.

21. Advantages 1. used for γ ray counting.
2. excellent resolution – 100 times better than NaI (TL)
complexes.
3. Pulse height is directly proportional to the energy of the
incident (gamma) radiation.
Dis-
advantages
1. Li atoms are very mobile therefore the Ge(Li) detector has to
be maintained at low temp
2. poorer detection efficiency because of smaller size [as
compared to NaI (TL)]
3. not as effective as NaI (TL) crystals for the samples with
high countrate.
4. must be kept in vacuum.