Concept of radioactivity, radioactivity counting methods with principles of different types of counters

1. Submitted to:
Dr. Nageshwar Singh
Submitted by:
Amit Rana
A-2019-40-013
ASSIGNMENT TITLE:
Concept of Radioactivity, Radioactivity
counting methods with principles of different
types of counters
Course Title:
TECHNIQUES IN BIOCHEMISTRY (1+3)
BIOCHEM 551

2. RADIOACTIVITY

3. DISCOVERY OF RADIOACTIVITY
❑ In 1896, Henri Becquerel discovered that uranium containing
crystals emitted rays that could expose and fog photographic
plates. He found that these “rays” originated from changes
within the atomic nuclei of the U atoms.
❑ He proposed that the uranium atoms were unstable. They
emitted particles and/or energy to become more stable. For his
discovery of radioactivity, Becquerel was awarded Nobel
prize for Physics in 1903.
❑ Marie (1867-1934) and Pierre (1859-1906) Curie study of
radioactivity is an important factor in science and medicine.
❑ They coined the term “radioactivity” and isolated two
previously unknown radioactive materials, polonium and
radium.

4. INTRODUCTION
❑ The atom is the basic constituent of all matter and is one
of the smallest units into which matter can be divided.
Each atom is composed of a tiny central core of particles,
or nucleus, surrounded by a cloud of negatively charged
particles called electrons.
❑ Most atoms in the physical world are stable, meaning that
they are not radioactive. However, some atoms possess
excess energy, which causes them to be physically
unstable. In order to become stable, an atom rids itself of
this extra energy by casting it off in the form of charged
particles or electromagnetic waves, known as radiation.
❑ The nuclei of naturally occurring heavy elements like U,
Th, Ra and Po are unstable and keep on emitting
spontaneously invisible rays or radiations (α, β, γ -rays)
and give more stable elements. These heavy elements are
called radioactive elements. The property of emitting
these rays called radioactivity of the elements

5. RADIOACTIVITY
❑ Radioactivity is the phenomenon of the spontaneous
disintegration of unstable atomic nuclei to form more
energetically stable atomic nuclei.
❑ Radioactivity refers to the particles which are emitted
from nuclei as a result of nuclear instability. The most
common types of radiation are called alpha, beta, and
gamma radiation, but there are several other varieties
of radioactive decay.
❑ Depending on how the nucleus loses this excess
energy either a lower energy atom of the same form
will result, or a completely different nucleus and
atom can be formed.

6. UNITS OF RADIOACTIVITY
❑ The SI unit of radioactivity is becquerel (Bq) and this term has been kept after Henri Becquerel. It is
defined as the activity of a quantity of radioactive material where one decay takes place per second
1 becquerel = 1 radioactive decay per second = 2.703×10-11
Other Radioactivity Units:
❑ An older radioactivity unit is the curie (Ci) and the name has been taken from Pierre and Marie Curie.
❑ It is defined as that quantity of any radioactive substance which gives 3.7 X 1010s-1 disintegration
(dps).
❑ Sometimes millicurie (mc) and microcurie (mc) are also used.
❑ Another unit is Rutherford (rd) and it is defined as the amount of radioactive substance which gives
106 disintegrations s-1 (dps).
❑ 1 curie = 3.7×1010 radioactive decays per second
❑ 1 becquerel = 1 radioactive decay per second = 2.703×10-11 Ci.
❑ 1 rutherford = 1.106 radionuclide decays per second.

7. UNITS OF RADIOACTIVITY
❑ Counts per second [c.p.s]: The recorded rate of decay
❑ Disintegrations per second [d.p.s]: The actual rate of decay
❑ Curie [Ci]: The number of d.p.s. equivalent to 1g of radium (3.7x1010d.p.s)
 Becquerel [Bq] = 1 d.p.s
 1 Ci = 3.7x1010 Bq
❑ Rad [rad]: The dose that gives an energy absorption of 0.01 Jkg–1 [100 ergg-1]
❑ Gray [Gy]: The dose that gives an energy absorption of 1J kg–1.
1 Gy= 100 rad
❑ Rem [rem]: The amount of radiation that gives a dose in humans equivalent to 1 rad of X-rays
❑ Sievert [Sv]: The amount of radiation that gives a dose in humans equivalent to 1 Gy of X-rays
1 Sv= 100 rem
❑ Roentgen [R]: The amount of radiation that produces 1.61x1015 ion pairs kg–1
❑ 1 Electron volt [eV] = 1.6x10-19 J

8. RADIOACTIVE DECAY LAW
❑ Radioactive Decay is a nuclear transformation process in which
the radioactive rays are emitted from the nucleus of the atom.
This process cannot be accelerated and slow down by any
physical or chemical process.
❑ According to the radioactive decay law, when a radioactive
material undergoes either 𝛼 or β or ℽ decay, the number of
nuclei undergoing the decay per unit time is proportional to the
total number of nuclei in the given sample material.
❑ The radioactive decay law states that “The probability per unit
time that a nucleus will decay is a constant, independent of
time”.
❑ It is represented by λ (lambda) and is called decay constant.
❑ The mathematical representation of the law of radioactive
decay is:

9. HALF-LIFE
❑ Half-life is the interval of time required for one-half of the atomic nuclei of a radioactive
sample to decay (change spontaneously into other nuclear species by emitting particles and
energy), or, equivalently, the time interval required for the number of disintegrations per
second of a radioactive material to decrease by one-half.
 Here we consider the following,
 N0 =the initial quantity of the substance
 N(t) = the quantity that is left over
 t1⁄2 = half-life
 r = mean lifetime of the decaying quantity
 λ = decay constant(½)

10. DISCOVERY OF RADIOACTIVITY
In 1898, Ernest Rutherford began
studying the nature of the rays that
were emitted and classified into
three distinct types according to
their penetrating power.
1. Alpha decay (α) – positively
charged; can barely penetrate a
piece of paper
2. Beta Decay (β) – negatively
charged; pass through as much
as 3mm of aluminium
3. Gamma Decay (γ) – neutral;
Extremely penetrating

11. ALPHA DECAY ()
 An alpha particle is identical in makeup to the nucleus of a helium atom, consisting of
two neutrons and two protons. Alpha particles have a positive charge and have little or
no penetrating power in matter. They are easily stopped by materials such as paper and
have a range in air of only an inch or so. Naturally occurring radioactive elements such
as uranium and radon daughters emit alpha radiation.
 Radioactive nuclei eject alpha particles in order to reach the stable state. This decay
occur only in heavy nuclei (Z > 83).
Characteristics of alpha particle(α) :
❑ a helium nucleus
❑ two protons and two neutrons
❑ charge +2e
❑ can travel a few inches through air
❑ can be stopped by a sheet of paper, clothing.

12. BETA DECAY (-)
❑ Beta radiation is composed of particles that are identical to
electrons. As a result, beta particles have a negative charge.
Beta radiation is slightly more penetrating than alpha but
may be stopped by materials such as aluminum foil and
Lucite panels. They have a range in air of several feet.
Naturally occurring radioactive elements such as potassium-
40 (K-40) emit beta radiation.
❑ This type of decay occurs in neutron rich nuclei. one
neutron converts to proton and electron, then the electron
(beta particle) will be ejected from the radioactive nucleus.
Characteristics of Beta particles
❑ Beta particles have the same charge (-e) and mass as
"normal" electrons.
❑ Can be stopped by glass plate, aluminum foil or a block of
wood.

13. Gamma decay()
• Gamma radiation is a form of electromagnetic radiation,
like radio waves or visible light, but with a much shorter
wavelength. It is more penetrating than alpha or beta
radiation, capable of passing through dense materials such
as concrete. X-rays are similar to gamma radiation.
• Gamma decay is an emission of nuclear photon from
excited nucleus. The unstable nucleus has excess of energy.
It expels this energy as Gamma ray in order to be stable.
• The nuclide retains its identity.
Characteristics of gamma rays
1. Gamma rays are electromagnetic radiation with high
frequency. (have NO mass and NO charge)
2. Most Penetrating, can be stopped by 1m thick concrete or
a several cm thick sheet of lead

14. DETECTION & MEASUREMENT
OF RADIOACTIVITY

15. DETECTIONANDMEASUREMENTOFRADIOACTIVITY
Radioactive isotopes interact with matter in two ways, ionization and excitation.
There are three commonly used methods of detecting and quantifying
radioactivity.
These are based on-
❑Ionization of gases (Geiger- Muller counters)
❑On the excitation of solids or solutions (Scintillation counting)
❑The ability of radioactivity to expose photographic emulsions
(Autoradiography)

16. 1. METHODS BASED UPON GAS IONIZATION
❑ When a charged particle passes through a
gas, its electrostatic field dislodges orbital
electrons from atoms sufficiently close to
its path and causes ionization. The ability
to induce ionization decreases in the order
α > β > γ (10 000: 100: 1)
❑ Accordingly, α- and β-particles may be
detected by gas ionization methods, but
these methods are poor for detecting γ-
radiation.
❑ If ionization occurs between a pair of
electrodes enclosed in a suitable chamber, a
pulse (current) flows, the magnitude of
which is elated to the applied potential and
the number of radiation particles entering
the chamber.

17. METHODS BASED UPON GAS IONIZATION
❑ In the ionization chamber region of the curve, each radioactive particle produces only one ion-pair per
collision. Hence the currents are low, and very sensitive measuring devices are necessary. This method
is little used in quantitative work, but various types of electroscopes, which operate on this principle,
are useful in demonstrating the properties of radioactivity.
❑ At a higher voltage level, electrons resulting from ionization move towards the anode much more
rapidly; consequently they cause secondary ionization of gas in the chamber, resulting in the
production of secondary electrons, which cause further ionization and so on. Hence from the original
event a whole torrent of electron reaches the anode. This is the principle of gas amplification and is
known as the Towensend avalanche effect, after its discoverer.
❑ As in the proportional counter region the number of ion-pairs collected is directly proportional to the
applied voltage until a certain voltage is reached, when a plateau occurs. Before the plateau is reached
there is a region known as the limited proportion region, which is not often used in detection and
quantification of radioactivity.
❑ The main drawback of counters that are manufactured to operate in the proportional region is that they
require a very stable voltage supply because small fluctuations in voltage result in significant changes
in amplification.
❑ Proportional counters are particularly useful for detection and quantification of α-emitting isotopes,
but it should be noted that relatively a few such isotopes are used in biological work.

18. METHODS BASED UPON GAS IONIZATION
❑ In the Geiger-Muller region all radiated β-particles, induce complete ionization of the gas in
the chamber. Thus the size of the current is no longer dependent on the number of primary
ions produced. Since maximum gas amplification is realized in this region, the size of the
output pulse from the detector will remain the same over a considerable voltage range (the so-
called Geiger- Muller plateau). The number of times this pulse is produced is measured rather
than its size.
❑ Therefore, it is not possible to discriminate between different isotopes using this type of
counter. Since it takes a finite time for the ion-pairs to travel to their respective electrodes,
other ionizing particles entering the tube during this time fail to produce ionization and hence
are not detected, thereby reducing the counting efficiency. This is referred to as the dead time
of the tube and is normally 100 to 200µs.
❑ When the ions reach the electrode they are neutralized. Inevitably some escape and produce
their own ionization avalanche. Thus, if unchecked a Geiger- Muller tube would tend to give
a continuous discharge.
❑ To overcome this, the tube is quenched by the addition of a suitable gas, which reduces the
energy of the ions. Common quenching agents are ethanol, ethyl formate and the halogens.

19. GEIGER-MUELLER (GM) COUNTER
❑ One of the first and most sensitive devices employed to detect and measure low levels of ionizing
radiation.
❑ The underlying principle is based on the emission of radiations that remove electrons from the atoms
it strikes producing ions which passes through the oppositely charged plates resulting in a flow of
current which is read on a meter.
❑ Geiger counter apparatus consists of Argon and Neon gas filled metal tube. A thin wire acts as a metal
conductor with applied voltage of 500 volts, runs inside the tube to a connector on the tube body.
Charged particles when enters the GM counter tube, knock off electron (ionization) from gaseous
atoms upon collision. The ionized gas produces current which is further amplified to be measured.
Geiger counter also contains a quenching agent to stop current flow after few microseconds.
❑ Older GM tubes used methane gas which upon each detection break downs resulting in a limited tube
lifetime. In contrast, modern GM tubes uses gases that does not break increasing tube dependability
and reliability. Each gas discharge incident is measured in counts per minute (CPM) in GM counter
with the capability to detect even a single emitted particle. However, ionization chambers cannot
detect a single particle emission but requires high levels of radiations to be detected. GM counters are
smaller in size with nearly 100% efficiency for measuring alpha and beta particles but only 1 to 2%
efficiency is reported in case of gamma and X-ray’s detection.

20. 1. The tube contains argon gas at low pressure.
2. The end of the tube is sealed by a mica 'window' thin enough to allow alpha particles to pass into the tube as well as
beta and gamma radiation.
3. When a charged particle or gamma-radiation enters the tube, the argon gas becomes ionized. This triggers a whole
avalanche of ions between the electrodes.
4. For a brief moment, the gas conducts and a pulse of current flows in the circuit.
5. The circuit includes either a scaler or a ratemeter. A scaler counts the pulses and shows the total on a display.
6. A ratemeter indicates the number of pulses or counts per second. The complete apparatus is often called a Geiger
counter.
GEIGER COUNTER

21. 2. METHODS BASED UPON EXCITATION:
❑ As outlined above, radioactive isotopes interact in matter in two ways, causing ionization,
which forms the basis of Geiger-Muller counting, and excitation. The latter effect leads the
excited compound (known as fluor) to emit photons of light. This fluorescence can be
detected and quantified. The process is known as scintillation and when the light is detected
by a photomultiplier, forms the basis of scintillation counting.
❑ The electric pulse that results from the conversion of light energy to electrical energy in the
photomultiplier is directly proportional to the energy of the original radioactive event. This is
considerable asset of scintillation counting, since it means that two, or even more, isotopes
can be separately detected and measured in the same sample, provided they have sufficiently
different emission energy spectra.

22. 2. METHODS BASED UPON EXCITATION:
❑ The modern electronic scintillation counter was invented in 1944 by Sir Samuel Curran
when he was working on the Manhattan Project at the University of California at Berkeley
❑ Scintillation counter is an instrument for detecting and measuring ionizing radiation by
using the excitation effect of incident radiation on a scintillator material and detecting the
resultant light pulses
PRINCIPLE
❑ When high energy atomic radiations are incident on a surface coated with some fluorescent
material, then flashes of lights (called scintillations) are produced. PMTs absorb the light
emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect.
The photon from the scintillation strikes a photocathode and emits an electron which
accelerated by a pulse and produce a voltage across the external resistance
❑ The subsequent multiplication of those electrons (sometimes called photo-electrons) results
in an electrical pulse which can then be analyzed and yield meaningful information about
the particle that originally struck the scintillator.
❑ This voltage is amplified and recorded by an electronic counter
❑ Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on
the other hand, detect incoming photons by the excitation of charge carriers directly in the
silicon. Silicon photomultipliers consist of an array of photodiodes which are reverse-biased
with sufficient voltage to operate in avalanche mode, enabling each pixel of the array to be
sensitive to single photons.

23. The Scintillation counter has mainly three parts:
1. Scintillator- Scintillator is made from a single, transparent crystal that should have following features:
• High efficiency
• High resolving power
• Available in proper form
Popular Type of Crystals used as scintillator-
Cesium iodide, Zinc sulphide, Phosphor, Plastic, Organic liquid
2. Photomultiplier Tube- Photomultiplier tube (PMT) is very sensitive which converts the light to an electrical
signal and electronics to process this signal
3. Counter

25. METHODS BASED UPON EXCITATION:
Types of Scintillation Counting
1. Solid scintillation counting
❑ In solid scintillation counting, the sample is placed
adjacent to a solid flour (e.g. sodium iodide)
❑ Solid scintillation counting is particularly useful for
gamma-emitting isotopes. This is because they can
penetrate the fluor. The crystal that is normally used
is sodium iodide, whereas for α-emitters zinc
sulphide crystals are preferred and for β-emitters
organic scintillators such as anthracene are used.
❑ The counter can be small hand-held devices with
the fluor attached to the photomultiplier tube or
larger bench-top machines with a well-shaped fluor
designed to automatically count many sample.

26. METHODS BASED UPON EXCITATION:
2. Liquid scintillation counting
❑ Liquid scintillation process is the conversion of the energy of a
radioactive decay event into photons of light in a liquid.
Photomultipliers (PM-tubes) detect the emission of light and
convert the light pulse into an electrical signal. The intensity of
the light pulse (number of photons emitted) is proportional to the
energy of the radioactive decay event.
❑ Scintillation fluid (cocktails) contain solvent (such as toluene or
diisopropylnaphthalene) and fluors such as 2,5- diphenyloxazole
(PPO), 1,4-bis (5-phenyloxazol-2-yl) benzene (POPOP) or 2-(4’-t-
butylphenyl)-5-(4”-bi-phenyl)-1,3,4-oxydiazole (butyl-PBD).
❑ This method is particularly useful in quantifying weak beta-
emitters such as 3H, 14C and 35S which are frequently used in
biological work
❑ Scintillation fluids are called ‘cocktails’ because there are different
formulations made of solvent plus flours
❑ Cocktails can be designed for counting organic samples or may
contain detergent to facilitate counting of aqueous samples

27. APPLICATIONS OF SCINTILLATION COUNTING:
❑ Scintillation counters are used to measure radiation in a variety of applications
including hand held radiation survey meters, personnel and environmental monitoring
for radioactive contamination, medical imaging, radiometric assay, nuclear security
and nuclear plant safety
❑ Scintillation counters designed for border security, ports, weigh bridge applications,
scrap metal yards and contamination monitoring of nuclear waste
❑ Scintillation counting is used in thin layer chromatography as radioactivity co-
migrating with ceramide phosphate is scraped from the plate and quantified by liquid
scintillation counting
❑ Liquid scintillation counting is extensively used for in-vivo and in-vitro biomedicine
research
❑ Liquid scintillation counting is used in in-vitro studies to determine the properties such
as dissolution, partitioning between two phases and binding to other compounds
❑ Different radiolabeled molecules are used in a wide range of metabolite transport
applications

28. ADVANCES IN SCINTILLATION COUNTING:
❖Reddy P.J. , Bhade S.P.D. , Babu D.A.R. and Sharma D.N. (2019).
At Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai analysed triple-
lableled radio nuclides samples (3H, 35S, and 36Cl) by using liquid scintillation counting
Hence it is revealed that simultaneous determination of activities in multilabeled samples by using
liquid scintillation counting results in the easy sample preparation as compared to traditional method
of chemical separation method which is time-consuming and can be successfully used for more than
three combinations of radionuclides
❖Sonali P. D. Bhade, P. J. Reddy, D. A. R. Babu, D. N. Sharma (2019).
At Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai working on
simultaneous determination of gross alpha and beta activities in composite samples by using Packard
Tri- Carb 2900TR liquid scintillation counter automatic efficiency control (AEC) technique which
provides automatic counting region adjustments for single and dual label samples according to the
degree of quench present in the sample
AEC is a simple, less time consuming technique for the simultaneous measurement of gross activities in
a sample and gave accurate results

29. 3. 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.
❑ Niepse de Saint Victor first described the phenomenon of autoradiography.
❑ An autoradiograph is an image on an X-ray film or nuclear emulsion produced by the pattern
of decay emissions from a distribution of a radioactive substance.
❑ It is very sensitive technique.
❑ Used in a variety of biological experiments
❑ The emulsion or film contains silver halide crystals and as energy from the radioactive
material is dissipated the silver halide becomes negatively charged and is reduced to metallic
silver thus forming a particulate latent image.
❑ Photographic developers shows these silver grains as a blackening of the film , then fixer are
used to remove any remaining silver halide and a permanent image results.
❑ Weak β-emitter such as 3H, 14C and 35S which are suitable for autoradiography .

30. AUTORADIOGRAPHY
❑ In autoradiography a photo graphic emulsion is used to visualize molecules labeled
with a radioactive element . The emulsion consists of a large number of silver halide
crystals embedded in a solid phase such as gelatin .
❑ As energy from radioactive material dissipated in the emulsion , the silver halide
becomes negatively charged & is reduced to metallic silver.
❑ Techniques of autoradiography have become more important in molecular biology.
❑ 3H is the best radioisotope , since it’s all energy will get dissipated in the emulsion .
❑ Electron microscopy can then be used to locate the image in the developed film .
❑ For location of DNA bands in electrophoretic gel, 32 P labeled nucleic acid probes are
useful .
❑ After hybridization, hydrolysis & separation of DNA fragments by electrophoresis , a
photographic plate is applied to to the covered gel & allowed to incubate

31. AUTORADIOGRAPHY
Fluorography
❑ If low energy β-emitters are used it is possible to enhance the sensitivity several
orders of magnitude by using fluorography. A fluor (e.g., PPO or sodium silicate) can
be used to enhance the image.
❑ The β-particles emitted from the isotope will cause the fluor to become excited and
emit light, which will react with the film.
❑ This has been used for example for detecting radioactive nucleic acids in gels.The
fluor is infiltrated into the gel following electrophoresis; the gel is dried and then
placed in contact with a pre-flashed film.
Pre-flashing
❑ The response of a photographic emulsion to radiation is not linear and usually
involves a slow initial phase(lag) followed by linear phase.
❑ Sensitivity of films may be increased by pre-flashing.

32. AUTORADIOGRAPHY
❑ Pre-flashing involves a millisecond light flash prior to the sample being
brought into juxtaposition with the film. It is often used where high sensitivity
is needed or if results are to be quantified.
Intensifying screens
❑ Intensifying screens are used when obtaining fast result is more important than
high resolution.
❑ It is useful for example in gel electrophoresis or analysis of membrane filters
where high energy β-emitters (32P-labelled DNA) or γ-emitting isotopes
(125I- labelled protein) are used.
❑ The intensifying screen consists of a solid phosphor, and it is placed on the
other side of the film from the sample.
❑ When high energy radiation passes through the film, causes the phosphor to
fluoresce and emit light, which in turn superimposes its image on the film.

33. AUTORADIOGRAPHY
PRINCIPLE
❑ Autoradiography is based upon the ability of radioactive substance to expose the
photographic film by ionizing it.
❑ In this technique a radioactive substance is put in direct contact with a thick layer of a
photographic emulsion (thickness of 5-50 mm) having gelatin substances and silver
halide crystals.
❑ This emulsion differs from the standard photographic film in terms of having higher
ratio of silver halide to gelatin and small size of grain.
❑ It is then left in dark for several days for proper exposure.
❑ The silver halide crystals are exposed to the radiation which chemically converts
silver halide into metallic silver (reduced) giving a dark color band.
❑ The resulting radiography is viewed by electron microscope, preflashed screen,
intensifying screen, electrophoresis, digital scanners etc.

34. AUTORADIOGRAPHY
BASIC MECHANISM
❑ Penetration of negatively charged beta particles emitted by radioactive salts
through silver halide film emulsion causes activation of silver present in the
emulsion.
❑ Activated silver crystals are very unstable therefore quickly reduced to black
silver particles which is easily detectable.
❑ Autoradiography sensitivity is improved by carrying the detection process at
70°C and pre-flashing the film before use.
❑ Pre-flashing needs only one hit per crystal deposited to increases sensitivity

36. FACTORS AFFECTING EFFICIENCY OF
AUTORADIOGRAPHY
❑ Energy of emitter: Higher the energy longer is the track length and so it’s
difficult to localize the points in the low density region of the same track.
❑ Distance and Thickness of sample : If either the sample is very thick or the
sample is far away from the emulsion film resolution will be lost.
❑ Grain size and amount of silver halide crystals : The grain size should be
smaller so that there is more availability of AgX crystals. Also concentration of
gelatin should be less in emulsion as comapred to AgX crystals.
❑ Exposure time : An autoradiogram must be exposed for a sufficiently long
time for proper exposure to view pattern of the track length.

37. APPLICATIONS
As said earlier, the recent advances in autoradiography have brought about its importance in the world. It is broadly
used in the following fields:
❑ Histopathology: Quick detection of pancreatic Carcinoma by imaging lactose and assessment of treatment
response
❑ Immunoelectrophoresis: Immunoelectrophoresis is a method of separating and identifying a mixture of
antigens using electrophoresis to separate them and an antigen-antibody reaction to identify them.
❑ Pharmacology
❑ Radiopharmaceuticals
❑ Biochemistry
❑ In imaging and analyzing rock porosity
❑ In matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI), and secondary ion
mass spectrometric imaging (SIMS-MSI) for pharmaceutical discovery and development
❑ In whole body imaging.
❑ Tool for genetic studies.
❑ Determining gross absorption and utilization of foliar applied nutrients etc.

38. RECENT ADVANCES IN AUTORADIOGRAPHY
❑ In recent autoradiography, the major concern of the scientist is not only to view the
quantitative details of the specimen but also its qualitative clarity, amidst other advances, two
major innovations has brought autoradiography from darkness to light. These are:
1. Gas-Counter Technology in Autoradiography
2. Electronic Autoradiography
❑ CCD (charged coupled device) and CMOS (Complimentary Metal-Oxide Semiconductor)
imaging technologies can be applied to thin tissue autoradiography as potential imaging
alternatives to using Conventional Film Emulsion.

39. ASPECTS OF COUNTING RADIOACTIVITY
The difference between amount of radiations emitted and detected actually depends on number of factors
which can be classified as follows:
1. Counter Characteristic:
(a) Background count:
❑ Radiation counter of all types always register an “unwanted” back-ground count, even in the absence
of radioactive material in the apparatus. This may be due to such sources as cosmic radiation, natural
radioactivity in the vicinity, nearby X-ray generators, and/or circuit noises and may interfere in a
measurement.
❑ By use the lead shielding, this background radiation may be considerably reduced, but its values must
always be recorded and accounted for in all experiments. Some commercial experiments have
automatic background subtraction facilities.
(b) Dead time:
❑ Detector systems (like GM Counter) designed for quantitative measurement has electronic equipment
to count and record the number of pulses received from detector. All such equipments require a finite
time to record a pulse, and during this time it is “dead” to any other pulse which may arrive.
❑ Thus at very high count rates in Geiger- Muller counting, counts are lost due to the dead time of the
Geiger-Muller tube. Dead time is not a problem in scintillation counting.

40. ASPECTS OF COUNTING RADIOACTIVITY
(c) Geometry:
❑ Those factors which determine whether the radiation misses or hits the detector are collectively referred to as
counting geometry. These factors are:
i. Geometric relationship between the source of the radiation and detector.
ii. The presence of material which absorbs or scatters the emitted radiation. Therefore, when samples with an end-
window ionization counter, such as a Geiger- Muller tube, are compared, it is important to standardize the
position of the sample in relation to the tube, otherwise the fraction of the emitted radiation entering the tube
may van’ and hence will vary the observed count.
2. Sample and Isotope Characteristic:
(a) Self-absorption:
❑ Self-absorption is primarily a problem with low energy P emitters; radiation is absorbed by the sample itself.
❑ It can be a serious problem in the counting of low energy radioactivity by scintillation counting if the sample is
particulate or is, for instance, stuck to membrane filter. Care should be taken to ensure comparability of samples
because the methods of standardization outlined earlier will not correct self- absorption effects.
❑ Automated methods for calculating counting efficiency in a scintillation counter will not correct for self
absorption effects. Particulate samples should be digested or otherwise solubilised prior to counting if quench
correction is required.

41. ASPECTS OF COUNTING RADIOACTIVITY
(b) Half-life:
❑ The half-life of an isotope may be short and, if so, this must be allowed for analysis of the data.
(c) Statistics:
❑ Emission of radioactivity is a random process and the spread of results forms a normal distribution.
The standard deviation can be calculated by taking a square root of the counts.
❑ The more counts we take the smaller the standard deviation is as a proportion of the mean count rate.
❑ The more counts measured the more accurate the data.
3. Supply, Storage and Purity of Radiolabeled Compound:
❑ There are several suppliers of radiolabeled compounds. This is because several types of
decompositions can occur; for example internal decomposition resulting from radioactive and external
decomposition where emitted radiation is absorbed by other radioactive molecules, causing impurities.
❑ The extent to which decomposition occurs is dependent on many factors such as temperature, energy
of radiation, concentration and the formulation of the compound. It is therefore, imperative to store
radioisotopes by the method recommended by the supplier and to maintain sterility of the stock. If
necessary, chromatographic procedures will be required to check on the purity of the labelled
compound.

42. ASPECTS OF COUNTING RADIOACTIVITY
4. Specific Activity:
❑ Specific activity of radioisotope is defined as the amount of radioactivity or the decay
rate of a particular radionuclide per unit mass of radionuclide.
❑ Specific activity expressed by units such as Bq/mol, Ci/mol or d.p.m./mol.
❑ The higher the specific activity the more sensitive the experiment. Because the higher
the specific activity the smaller the quantities of labelled substance that can be
detected.
5. The Choice of Radionuclide:
❑ This is the complex question depending on the precise requirements of the experiment.
❑ Safety
❑ The type of detection to be used
❑ The sensitivity required
❑ Cost

43. THANK YOU