Radioisotope technique and methods

A SEMINAR REPORT ON THE PRINCIPLES & APPLICATIONS OF RADIOISOTOPES.

By Abhishek .A. GiriM.Sc. (part-I)Roll no. 1326056

Paper - IV , Unit - II

A PRESENTATION ON THE PRINCIPLES & APPLICATIONS OF RADIOISOTOPES.

By Abhishek .A. Giri

Topic to cover:-

What are radioisotopes? Uses of radioisotopes Units of radioactivity Method of scintillation counting Autoradiography

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 are emitted.o They become stable isotopes by the process of

radioactive decay.

What is a Radioisotope?

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?

1. MEDICAL TRACERS 2. STERILISATION

3. STERILZING FOOD

Uses of radioisotopes

UNSTERILIZED Gamma source STERILIZED

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o In SI unit the curie has been replaced by becquerel (Bq), where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

DETECTION & MEASUREMENT OF RADIOACTIVITY

o There are three commonly used methods of detecting & quantifying radioactivity.

o These are based on the ionization of gases, on the excitation of solids or solutions, & the ability of radioactivity to expose photographic emulsions

(i.e. autoradiography)

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 

o The three types of gaseous ionization detectors are:1. Ionization chambers2. Proportional counters3. Geiger-Muller tubes

Types

Ionization chambers

Operate at a low electric field strengthThe 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.

o Good uniform response to gamma radiation and give an accurate overall dose reading

o Will measure very high radiation rateso Sustained high radiation levels do not degrade fill gas

Advantages

o Very low electronic output requiring sophisticated electrometer circuit

o Operation and accuracy easily affected by moistureo Cannot measure energy of radiation - no spectrographic

information

Disadvantages

Proportional Counter

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.

o Can measure energy of radiation and provide spectrographic information

o Can discriminate between alpha and beta particleso Large area detectors can be constructed

Advantages

o Anode wires delicate and can lose efficiency in gas flow detectors due to deposition

o Efficiency and operation affected by ingress of oxygen into fill gas

o Measurement windows easily damaged in large area detectors

Disadvantages

Geiger-Muller Tube

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.

Cheap, robust detector with a large variety of sizes and applicationsLarge output signal from tube requiring minimal electronic processing for simple countingCan measure overall gamma dose when using energy compensated tube

Advantages

Cannot measure energy of radiation - no spectrographic informationWill not measure high radiation rates due to dead timeSustained high radiation levels will degrade fill gas

Disadvantages

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 event.

Method based upon Excitation

SCINTILLATION COUNTER

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.

operation

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 radiation.

Schematic of a scintillating crystal combined with a photomultiplier.

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.Highly automated.

Advantages

Cost of the instrument & cost per sample.Potentially high background noise.QuenchingChemiluminescencephospholuminescence

disadvatages

Scintillation counters are used to measure radiation in a variety of applications.

o Hand held radiation survey meterso Personnel and environmental monitoring for Radioactive

contamination.o Medical imaging.o National and homeland security.o Border security.o Nuclear plant safety.o Oil well lodging.

Applications

AUTORADIOGRAPHYRadiography 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.

• Living cells are briefly exposed to a ‘pulse’ of a specific radioactive compound.

• Samples are taken, fixed, and processed for light or electron microscopy.

• Left in the dark for days or weeks (while the radioisotope decays).• The photographic emulsion is developed (as for conventional

photography).• 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.

Autoradiography Method

Radioisotopes by D. Billington, G. JaysonPrinciples & techniques in practical biochemistry by Keith Wilson, John walkerPrinciples & techniques of Biochemistry & molecular biology by R.J.Slaterwww.ansto.gov.au/NuclearFacts/.../Radioisotopes/index.htmwww.kentchemistry.com/links/Nuclear/radioisotopes.htmwww.lab.anhb.uwa.edu.au/hb313/main_pages/.../autoradiography.htmwww.wikipedia.org/wiki/Liquid_scintillation_counting

References

Thank you