3. Topic to cover:-
What are radioisotopes?
Uses of radioisotopes
Units of radioactivity
Method of scintillation counting
4. o An isotope is one of two or more atoms having the same atomic
number but different mass numbers.
o Unstable isotopes are called Radioisotopes.
o When an unstable nucleus disintegrates into more stable one radiations
o They become stable isotopes by the process of radioactive decay.
What is a Radioisotope?
5. 1) It is possible to detect radioactivity with exquisite sensitivity.
2) it is possible to follow what happens in time.
3) it is possible to trace what happens to individual atoms in a pathway.
4) we can identify a part or end of a molecule , & follow it very precisely.
Why do we use Radioisotope?
7. o The original unit for measuring the amount of radioactivity was the curie
(Ci)- first defined to correspond to one gram of radium – 226
& more recently defined as : 1 Curie = 3.7x1010 radioactive decays/sec .
o In SI unit the curie has been replaced by becquerel (Bq), where
1 becquerel = 1 radioactive decay per second = 2.703x10-11 Ci .
Units of Radioactivity
8. DETECTION & MEASUREMENT OF
o There are three commonly used methods of detecting & quantifying
o These are based on the ionization of gases, on the excitation of solids or
solutions, & the ability of radioactivity to expose photographic emulsions
9. o Gaseous ionization detectors are radiation detection instruments used in
radiation protection applications to measure ionizing radiation.
o They use the ionizing effect of radiation upon a gas-filled sensor.
o Particle having enough energy to ionize a gas molecule, the resulting
electrons & ions cause a current flow which can be measured.
Method based upon Gas ionization
10. o The three types of gaseous ionization detectors are:
1. Ionization chambers
2. Proportional counters
3. Geiger-Muller tubes
12. Operate at a low electric field strength
The ion current is generated by the creation of ‘ion pairs’.
The +ve ions drift to the cathode whilst free electrons drift to the anode
under the influence of the electric field.
13. o Good uniform response to gamma radiation and give an accurate overall
o Will measure very high radiation rates
o Sustained high radiation levels do not degrade fill gas
14. o Very low electronic output requiring sophisticated electrometer circuit
o Operation and accuracy easily affected by moisture
o Cannot measure energy of radiation - no spectrographic information
16. Proportional counters operate at a slightly higher voltage.
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 “prortional counting” region.
17. o Can measure energy of radiation and provide spectrographic
o Can discriminate between alpha and beta particles
o Large area detectors can be constructed
18. o Anode wires delicate and can lose efficiency in gas flow detectors due to
o Efficiency and operation affected by ingress of oxygen into fill gas
o Measurement windows easily damaged in large area detectors
20. o They operate at a very higher voltage, selected that each ion pair creates
an avalanche, but by the emission of UV photons, multiple avalanches
are created which spread along the anode wire, and the adjacent gas
volume ionizes from as little as a single ion pair event.
o This is the "Geiger region" of operation.
o The current pulses produced by the ionising events are passed to
processing electronics which 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.
21. Cheap, robust detector with a large variety of sizes and applications
Large output signal from tube requiring minimal electronic processing for
Can measure overall gamma dose when using energy compensated tube
22. Cannot measure energy of radiation - no spectrographic information
Will not measure high radiation rates due to dead time
Sustained high radiation levels will degrade fill gas
23. Radioactive isotopes interact with matter in two ways, ionisation &
excitation.the latter effects leads an excited atom or compound (fluor) to
emit photons of light.This process is known as Scintillation.
When the light is detected by a photomultiplier converts the energy of
radiation into an electrical signal, & the strenght of the electric pulse that
results is directly proportional to the energy of the original radioactive
Method based upon Excitation
25. A scintillation counter is an instrument for detecting and
measuring ionizing radiation.
It consists of a scintillator which generates photons of light in response
to incident radiation, a sensitive photomultiplier tube which converts
the light to an electrical signal, and the necessary electronics to process
the photomultiplier tube output.
27. When a charged particle strikes the scintillator, its atoms are excited and
photons are emitted.
These are directed at the photomultiplier tube's photocathode, which emits
electrons by the photoelectric effect.
These electrons are electrostatically accelerated and focused by an
electrical potential .
The scintillator must be in complete darkness so that visible light photons
do not swamp the individual photon events caused by incident ionising
29. Fluroscence is very fast so there is no dead time.
Counting efficiences are high .
Ability to count samples of many types like liquid, solids & gels.
30. Cost of the instrument & cost per sample.
Potentially high background noise.
31. Scintillation counters are used to measure radiation in a variety of
o Hand held radiation survey meters
o Personnel and environmental monitoring for Radioactive
o Medical imaging.
o National and homeland security.
o Border security.
o Nuclear plant safety.
o Oil well lodging.
Radiography is the visualisation of the pattern of distribution of radiation.
In general, the radiation consists of X-rays, gamma (g ) or beta (b ) rays,
and the recording medium is a photographic film.
In contrast, in autoradiography the specimen itself is the source of the
radiation, which originates from radioactive material incorporated into it.
33. • Living cells are briefly exposed to a ‘pulse’ of a specific radioactive
• Samples are taken, fixed, and processed for light or electron
• Left in the dark for days or weeks (while the radioisotope decays).
• The photographic emulsion is developed (as for conventional
• Counterstaining .
• Alternatively, pre-staining of the entire block of tissue can be done.
• It is not necessary to coverslip these slides
• The position of the silver grains in the sample is observed by light or
electron microscopy .
• These autoradiographs provide a permanent record.
• Full details on the batch of emulsion used, dates, exposure time and
conditions should be kept for each experiment.
34. Radioisotopes by D. Billington,G. Jayson
Principles & techniques in practical biochemistry by KeithWilson, John
Principles & techniques of Biochemistry & molecular biology by R.J.Slater