References- Wilson and walker, google Methods based upon excitation (Scintillation counting) have not been explained.

1. 3/18/2021
1
-Gokul Arora
-M.Sc Medical
Biochemistry,
JIPMER

2. CONTENTS
• METHODS BASED UPON GAS IONISATION
• METHODS BASED UPON EXPOSURE OF PHOTOGRAPHIC
EMULSIONS
• OTHER PRACTICAL ASPECTS OF COUNTING RADIOACTIVITY
AND ANALYSIS OF DATA
• SAFETY ASPECTS
3/18/2021
2

3. DETECTION AND MEASUREMENT OF
RADIOACTIVITY
3/18/2021
3

4. METHODS BASED UPON GAS IONISATION
• If a charged particle passes through a gas, its electrostatic field dislodges orbital
electrons from atoms sufficiently close to its path and causes ionisation
• A >> (10 000 : 100 : 1)
3/18/2021
4

5. • The three basic types of gaseous ionization detectors are:-
3/18/2021
5

6. IONIZATION CHAMBERS
• Ionization chambers operate at a low electric field strength, selected such that no gas
multiplication takes place.
• The ion current is generated by the creation of "ion pairs", consisting of an ion and
an electron.
• The ions drift to the cathode while free electrons drift to the anode under the
influence of the electric field.
• This current is independent of the applied voltage if the device is being operated in
the "ion chamber region"
3/18/2021
6

7. 3/18/2021
7

8. • ADVANTAGES
uniform response to gamma
radiation and accurate overall dose
reading
Measuring very high radiation rates
No dead time
Sustained high radiation levels do
not degrade the fill gas.
• DISADVANTAGES
low output requiring sophisticated
electrometer circuit
operation and accuracy easily
affected by moisture
3/18/2021
8

9. PROPORTIONAL COUNTERS
• The key feature is its ability to measure the energy of incident radiation, by
producing a detector output pulse that is proportional to the radiation energy
absorbed by the detector due to an ionizing event.
• A proportional counter uses a combination of the mechanisms of a Geiger–Müller
tube and an ionization chamber, and operates in an intermediate voltage region.
3/18/2021
9

10. • ADVANTAGES
Provide spectrographic
information.
discriminate between alpha
and beta particles, and that
large area detectors can be
constructed.
• DISADVANTAGES
anode wires are delicate
and can lose efficiency in
gas flow detectors due to
deposition.
the efficiency and operation
affected by ingress of
oxygen into fill gas
measurement windows
easily damaged in large area
detectors.
3/18/2021
10

11. GEIGER-MULLER COUNTER
• The Geiger–Muller counter has a cylindrical-shaped gas chamber and it operates
at a high voltage.
• This makes the instrument less dependent on a stable voltage, so the counter is
cheaper and lighter.
3/18/2021
11

12. • ADVANTAGES
cheap and robust detector with a
large variety of sizes and
applications.
large output signal is produced from
tube which requires minimal
electronic processing for simple
counting
measure the overall gamma dose
when using an energy compensated
tube.
• DISADVANTAGES
it cannot measure the energy of the
radiation (no spectrographic
information)
it will not measure high radiation
rates due to dead time
sustained high radiation levels will
degrade fill gas
3/18/2021
12

13. Applications
• Routine monitoring of the laboratory to check for contamination.
• Quick screening of radioactive gels prior to autoradiography
• Checking chromatographic fractions for labelled components.
3/18/2021
13

14. METHODS BASED UPON EXPOSURE OF
PHOTOGRAPHIC EMULSIONS
• Ionising radiation acts upon a photographic emulsion or film to produce a latent
image much as does visible light. This is called autoradiography.
• The emulsion or film contains silver halide crystals.
3/18/2021
14

15. 3/18/2021
15

16. Suitable isotopes
• Weak -emitting isotopes ( 3H, 14C and 35S) = cell and tissue localisation
experiments.
• Radiations with higher energy give faster results but poorer resolution
3/18/2021
16
The energy of the radiation is low.
The higher energy negatrons produce much
longer track lengths, exposing a greater surface
are of the film, and results in less discrete
images.

17. Choice of emulsion and film
• Autoradiography emulsions are solutions of silver halide that can be made to set
solid by the inclusion of materials such as gelatine.
• This can be used for autoradiography of microscopic slides.
• X-ray film is the alternative and is used for gels.
3/18/2021
17

18. 3/18/2021
18

19. Direct autoradiography
• In direct autoradiography, the X-ray film or emulsion is placed as close as
possible to the sample and exposed at any convenient temperature.
• Quatitative images are produced until saturation is reached.
• Isotopes with an energy of radiation equal to, or higher than, 14C (Emax = 0.156
MeV) are required.
• The higher the energy the quicker the results.
3/18/2021
19

20. Fluorography
• If low-energy -emitters are used it is possible to enhance the sensitivity several
orders of magnitude by using fluorography.
• A fluor (e.g. 2,5-diphenyloxazole or sodium silicate) can be used used to enhance the
image.
3/18/2021
20
Isotope
-particles
Fluor
becomes
excited
Light
emitted
React with
the film

21. Intensifying Screens
• Intensifying screens are used when obtaining a fast result is more important than
high resolution.
• The intensifying screen consists of a solid phosphor, and it is placed on the other
side of the film from the sample.
• It is useful for example in gel electrophoresis or analysis of membrane filters where
high-energy -emitters (e.g.32P-labelled DNA) or -emitting isotopes (e.g. 125I-
labelled protein) are used.
3/18/2021
21

22. Low-temperature exposure
• When intensifying screens or fluorography are used the exposure should be done
at low temperature. This is because the kinetics of the film’s response are
affected.
• To stop the back reaction which cancels the latent image.
• Low temp. = -700 C.
3/18/2021
22

23. Preflashing
• The response of a photographic emulsion to radiation is not linear and usually
involves a slow initial phase (lag) followed by a linear phase.
• This involves a millisecond light flash prior to the sample being brought into
juxtaposition with the film.
• Used where high sensitivity is required or if results are to be quantified.
3/18/2021
23

24. Quantification
• It is possible to obtain quantitative data directly from autoradiographs by using
digital image analysis.
• Preflashing combined with fluorography or intensifying screens create the best
conditions for quantitative working.
3/18/2021
24

25. Other practical aspects of counting radioactivity
and analysis of data
3/18/2021
25

26. Self-absorption
• Self-absorption is primarily a problem with low energy -emitters: radiation is
absorbed by the sample itself.
• Serious problem in the counting of low energy radioactivity by scintillation
counting.
• Particulate samples should be digested or otherwise solubilised prior to counting
if quench correction is required.
3/18/2021
26

27. Specific activity
• The specific activity of a radioisotope defines its radioactivity in relation to the
amount of material.
• Expressed by units such as Bq mol-1, Ci mmol-1 or d.p.m. mmol-1 .
• The highest specific activities are associated with isotopes with short half-lives,
since the rate of decay per unit mass (or mol) is higher.
3/18/2021
27
Specific activity More sensitive the experiment

28. Statistics
3/18/2021
28
The more counts measured the more accurate the data.

29. The choice of radionuclide
• The key factors in the decision are often based on
Safety
 the type of detection to be used
the sensitivity required
the cost.
3/18/2021
29

30. 3/18/2021
30

31. SAFETY ASPECTS
3/18/2021
31

32. 3/18/2021
32

33. 3/18/2021
33

34. 3/18/2021
34
The ingestion of one ALI results in a person receiving a dose limit to the whole body or to
a particular organ.

35. THANK YOU
3/18/2021
35