THREE TYPES OF RADIOACTIVE DECAY
There
are three main types of radiation:
Alpha Decay
The
reason alpha decay occurs is because the nucleus has too many protons which
cause excessive repulsion. In an attempt to reduce the repulsion, a Helium
nucleus is emitted. The way it works is that the Helium nuclei are in constant
collision with the walls of the nucleus and because of its energy and mass,
there exists a non zero probability of transmission. That is, an alpha particle
(Helium nucleus) will tunnel out of the nucleus.
Properties of Alpha Radiation
·
α-particles
are no particularly dangerous unless they are swallowed or breathed in.
·
since α radiations cannot pass through clothing.
·
But
cells they are very damaging.
Alpha
Decay of Americium-241 to Neptunium-237
Beta Decay
Beta
decay occurs
when the neutron to proton ratio is too great in the nucleus and causes
instability. In basic beta decay, a neutron is turned into a proton and an
electron. The electron is then emitted. Here's a diagram of beta decay with
hydrogen-3:
Alpha Decay of
Hydrogen-3 to Helium-3.
There
is also positron emission when the neutron to proton ratio is
too small. A proton turns into a neutron and a positron and the postiron is
emitted. A positron is basically a positively charged electron. Here's a
diagram of positron emission with carbon-11:
Positron Decay
of Carbon-11 to Boron-11.
The
final type of beta decay is known as electron capture and also
occurs when the neutron to proton ratio in the nucleus is too small. The
nucleus captures an electron which basically turns a proton into a neutron.
Here's a diagram of electron capture with beryllium-7:
Electron Capture
of Beryllium-7.
Properties of Beta Radiations
·
Beta
radiations have much greater range.
·
Hard
beta could pass through a few centimeter of lead and have considerable range in
air.
·
So
beta emissions can produce dangerous radiation even at distances from the
source.
Gamma Decay
Gamma
decay occurs
because the nucleus is at too high an energy. The nucleus falls down to a lower
energy state and, in the process, emits a high energy photon known as a gamma
particle. Here's a diagram of gamma decay with helium-3:
Properties of Gamma Radiations
·
Gamma
radiations are the most penetrating of all and can pass through many metres of
even concrete.
·
Since
their ionizing power is the least of all and many will go through the body
without effect.
HALF LIFE AND RADIOACTIVITY
Atoms
An
atom is smallest particle of an element capable of entering into a chemical
reaction. It consists of a nucleus and orbital electrons. The Nucleus which
contains neutrons and protons, small, central, positively charged region of an
atom that carries essentially all the mass. Except for the nucleus of ordinary
(light) hydrogen, which has a single proton, all atomic nuclei contain both
protons and neutrons.
The
number of protons determines the total positive charge, or atomic number; this
is the same for all the atomic nuclei of a given chemical element. The total
number of neutrons and protons is called the mass number.
A
nuclide is a species of atom characterized by its mass number, atomic number,
and energy state of its nucleus, provided that the atom is capable of existing
for a measurable time.
Isotopes
Isotopes
are nuclides having the same number of protons (same atomic number) in their
nuclei, but differing in the number of neutrons (different mass number).
Isotopes of a particular element exhibit almost identical chemical properties.
Radioisotopes
A radioisotope is an atom with an unstable
ratio of neutrons to protons in its nucleus. In an attempt to reorganize to a
more stable state, it may undergo various types of rearrangement that involve the
release of radiation. A radioisotope is a radioactive material. Radioactive
material is any material that emits radiation spontaneously. Radioisotopes
provide an easily traced “label” in various chemical compounds. Radioactive
decay is the disintegration or transformation of the nucleus of an unstable
nuclide by the spontaneous emission of charged particles and photons.
Units of Radioactivity
The
curie is the quantity of any radioactive material in which the number of
nuclear transformations is 3.7 x 1010 per second. The unit is
abbreviated Ci. The becquerel is the international (SI) unit for radioactivity.
One becquerel is equal to one nuclear transformation per second. A becquerel is
about 2.7 x 10-11 curies. Common metric prefixes are used to denote multiples
of the curie and the becquerel. A thousandth of a curie is known as a
millicurie (mCi). A millionth of a curie is known as a microcurie (µCi). A
microcurie is 3.7 x 104 or 37 kilobecquerels.
Radioactivity and Half life
Radioactive
materials have an associated half-life, or decay time characteristic of that
isotope. As radiation is emitted, the material becomes less radioactive over
time, decaying exponentially. Since it is impossible or impractical to measure
how long one atom takes to decay, the amount of time it takes for half of the
total amount of radioactive material to decay is used to calculate half-life.
Some radioisotopes have long half-lives; for example, Carbon-14 takes 5,730
years for any given quantity to decay to half of the original amount of
radioactivity. Other radioactive materials have short half-lives; Phosphorus-32
has an approximately two-week half-life, and Technetium-99m (used in human and
animal nuclear medicine diagnostic procedures) has a half-life of 6 hours.
Half-life is important for many reasons. When deposited in the human body, the
half-life of the radioactive material present in the body affects the amount of
the exposure. Materials with shorter half-lives typically cause less dose for a
given activity than materials with longer half-lives. If the radioactive
material contaminates a workbench or equipment, and is not removable, the
amount of time before the contaminated items may be used again is determined by
the radioactive half-life. Radioisotope decay (using shorter half-life
isotopes) helps to minimize costs in radioactive waste management.
The equation
used to calculate radioactive decay is shown below.
A = A0 e-λt
Where:
A = Current
amount of radioactivity
A0 = Original
amount of radioactivity
e = base natural
log (approximately 2.718)
λ = decay
constant = 0.693/t1/2 (where t1/2 = half-life)
t = the amount
of time elapsed from A0 to A
Half life
The
radioctive isotopes are characterized by their half life. The decay of
radioisotopes is irreversible and is independent of any physical parameters
such as temperature and pressure. The rate of decay always follows the
exponential law. That is, the number of atoms disintegrating at any given time
is directly proportional to the total number of atoms present at that time.
Half life (T1/2)
is the time required for the radioactivity to be reduced to one half of its
original value. This is also known as physical half life (Tp)
(T1/2) =
0.693/λ
Where λ is
radioactive decay constant
Biological half life (Tb)
Biological
half life is defined as the normal amount of time required for the turnover of
one half of the body content of a given radioactive or non radioactive element.
Effective half life (Te)
When
radioactive isotopes are used in in vivo experiments,
the turnover rate of the element in the body must also be considered. Therefore
the rate of decay of radioactivity will be a function of both radioactive decay
and metabolic turnover which is known as effective half life and denoted as Te.
Effective half
life (Te) = (Tb) x (Tp) / (Tb) + (Tp)
GEIGER-MULLER COUNTER (GM COUNTER)
The
radioactivity of isotopes can be measured either by using counting devices or
by photographic method (autoradiography). The quantity determination of
radioactivity is based on the ionization or excitation of matter induced by
radiations emanated from radioactive elements. GM counter is an important
instruments used to determine radioactivity of substances.
This
instrument works on the principle of ionization of gases & is suitable for
all kinds of radiations. The basic advantage of GM counter is derived from the
amplification (each primary ion pair yields close to 100 million secondary ion
pairs). The amplification is so great that an extremely simple external electronic
amplifier is sufficient. This simplicity imparts stability to GM counter.
It
consists of a simple & very compact ionization chamber made of glass with
two electrodes. It is filled with a gas usually argon, neon or air.The outer
electrode (cathode) is a metal tube formed by brass or nickel with 1-5 cm
diameter & 10-50 cm length.The inner electrode (anode) is a fine wire of
tungsten with 0.1-0.5 mm thickness. The anode is stretched along the axis of
the tube & well-insulated by ebonite plugs. When radiation enters the tube,
the gas is ionized rapidly under a high voltage power supply resulting in an
electric pulse. But, the discharge of the pulse terminates when large quantity
of positive ions get collected around the negative electrode & this prevents
the occurrence of further ionization.
To
accelerate the process of ionization, GM tubes contain a quenching gas.
However, presence of a quenching gas & the window through which the
particles pass into the tube restrict sthe entrance of radiation thereby reducing
the efficiency of the detector. To overcome this problem, GM tubes with very
thin windows or with no windows have been developed. But the windowless GM
tubes require a continuous flow of special quenching gas through the tube.
Counter Tubes
The
choice of counter tubes to be used depends on the nature of the sample to be
counted (whether the sample is solid or liquid, whether it is soft or high
energy β-emitter etc.)
Proportional Counter
It
is a modified GM tube which operates in a region of applied current where the
charge produced is proportional to the initial ionization. This device is more
advantageous than GM counter because it has greater stability &
reproducibility & can operate at reduced voltage.
The consumption
of quenching gas is lesser in proportional counter than in GM tube. This device
produces continuous pulses which enable very fast counting. The most popular
proportional counter is the ‘flow counter‘ in which a gas like argon or indane
is continuously.
Optimum voltage
The GM counter should be operated only
in the voltage region known as “Geiger – Muller region”. It is the voltage
range where a small increase or decrease in voltage does not affect the
counting rate. When the applied voltage is plotted against cout rate, the first
region is known as “recombination region”. As the voltage is low, the ionized
gas molecules recombine. When the voltage is increased further, the gas
molecules ionize and stay in their ionized state without showing any increase
in the count rate. This region is called “simple ionization region”. As the
voltage is increased still further, the count rate increases proportionately
with increase in voltage and hence this region is known as the “proprrtional
region”. The counters based on the region are known as proportional counters.
A further increase in voltage does not
affect the count rate. At this threshold voltage, the rise in count rate begins
to level off into plateau and is stable for about 300 volts length(the precise
length depends on a particular tube). This region is known as “Geiger muller
region” at which the Geiger muller counter is operated. Still higher voltages
lead to a sudden sharp rise in the count rate. At this voltages the potential
across electrodes is so high that spontaneous discharges occur in the tube
which are not due to radioactivity. This region is known as “continuous
discharge region”. The tube should never be operated at the continuous
discharge region because the tube will get irreparably damaged.
SCINTILLATION COUNTING METHODS
Scintillation
counting is based on the observation that certain chemical substances emit
flashes of light on exposure of radiation. The measurement of these flashes of
light is the basis of the method called scintillation counting. Depending upon
the type of scintillator used for counting the radioactivity, they are
classified into two types:
i.
solid
scintillation counting
ii.
liquid
scintillation counting
i.
Solid
scintillation counting
Solid
scintillation counter is ideal for gamma emitters. The counter consists of a
large crystal of sodium iodide containing small amount of thallium iodide and
an assembly of photomultiplier, preamplifier, a source of high voltage and a
scaler encased in an aluminum casing. The radioactive sample is placed into a well
shaped opening drilled into the crystal surface. The crystal absorbs much of
the energy of the gamma rays which causes excitation of electrons of the atoms
composing the crystal. The excited electrons rise to higher energy orbital. As
these electrons return to normal level, the absorbed energy is discharged as
flashes of light or scintillation.
The
light (photons) is converted into electrical signals by the photomultiplier
tube, amplified and is registered es counts by the scaler.
ii.
Liquid
scintillation counting
Liquid
scintillation counters are especially useful in measuring soft β emittors such
as 3H, 35S and 14C, as these compounds cannot
be measured efficiently using a G-M counter or a gamma counter.
A
liquid scintillation counter consists of two photomultiplier tubes one on
either side of a scintillation vial and enclosed in a light tight lead shield.
The photomultiplier tubes are connected to a scaler through a high voltage
power supply, a coincidence counting circuit and a pulse height analyzer.
The
radioactive sample to be counted is mixed in a glass or plastic vial with
“scintillation fluid”. The scintillation fluid contains three components:
i.
An
organic solvent
ii.
A
primary flour and
iii.
A
secondary flour
The
energy of the β particle is first transferred to the organic solvent which
causes excitation of solvent molecules. When the excited solvent molecules
returned to ground state, energy is transferred to the primary flour viz., 2,5
diphenyl oxazole (PPO) causing excitation of PPO. On returning to ground state,
the primary flour emits photons in the UV region. The photocathodes of the
photomultiplier tube are not sensitive enough to capture these photons. Hence a
secondary flour viz., 1,4 bis (5-phenyloxazole-2)benzene (POPOP), is used to
absorb the energy from the primary flour and reemit it as light of longer
wavelength, which is close to the maximum sensitivity region of the
photomultiplier tube. The size of the electrical pulse produced by the
photomultiplier tube is proportional to the energy of β-particle absorbed by
the scintillation fluid. The sequence of transfer of radiation energy from the
radioactive material.
Liquid
scintillation counters are ideal for low energy β-emitters e.g. 3H, 35S,
14C, 32P, etc. These counters can also be used for
counting dual or triple labeled samples.
Composition of Scintillation fluid
The
scintillation fluid contains 0.5% PPO and 0.01% POPOP in Analar scintillation
grade toluene. The fluid (̴
10 ml) is used per vial for counting. The radioactive samples to be counted are
usually spotted (1-50 µl) on Whatman No.3 discs and are dried in a hot air oven
or under infrared lamp. Then these discs are placed inside the vials containing
the scintillation fluid and counted. The radioactive samples in small volume
1-5 µl can also be counted, by directly pipetting the volume inside the
scintillation fluid. Large volumes cannot be counted directly because water
quenches the counting. Hence larger volumes (upto 100 µl) can be spotted on
Whatman No.3 discs, dried and then counted.
Cerenkov counting
Radioactive
isotopes which produce β-particles with >265 KeV can be counted directly in
liquid scintillation counter without the help of scintillation fluid. Modern
photomultipliers accept about one third of the light energy emitted by these
isotopes and is known as cerenkov radiation.
Background radiation
In
liquid scintillation counting, even in the absence of any radioactive material
added to the vials, some counts are always registered by the counter (̴ 50 counts).
This is known as background counts. The background counts are derived from a
number of sources such as
i.
Low
aptitude signals arising spontaneously within the photomultiplier tubes.
ii.
Naturally
occuring radio isotopes in the vial and in the scintillation fluid the organic
solvents may contain small amounts of 3H and 14C.
iii.
Due
to cosmic radiations.
Advantages of scintillation counting
§ The rapidity of
fluorescence decay (10-9 s), which when compared to dead time in a
Geiger-Muller tube (10-4 s), means much more count rates are
possible.
§ Much higher
counting efficiencies particularly for low energy β emitters; over 50%
efficiency is routine in scintillation counting and efficiency can rise to over
90% for high energy emitters. This is partly due to the fact that the negatrons
do not travel through air or pass through an end window of a Geiger-Muller tube
but interact directly with the flour; energy loss before the event that is
counted is therefore minimal.
§ The ability to
accommodate samples of any type, including liquids, solids, suspensions and
gels.
§ The ability to
count separately different isotopes in the same sample, which means dual
labeling experiments, can be carried out.
§ Scintillation
counters are highly automated, hundreds of samples can be counted automatically
and built in computer facilities carry out many forms of data analysis, such as
efficiency correction, graph plotting, radioimmunoassay calculations, etc.
Disadvantages of scintillation counting
·
The
cost per sample of scintillation counting is not insignificant; however, other
factors including versatility, sensitivity, ease and accuracy outweigh this
factor for most applications.
·
The
greatest disadvantage of scintillation counting is quenching. This occurs when
the energy transfer process. It can be any one of three kinds.
Ø Optical
quenching
Ø Colour quenching
Ø Chemical
quenching
Optical quenching
This
occurs if inappropriate or dirty scintillation vials are used. These will
absorb some of the light being emitted, before it reaches the photomultiplier.
Colour quenching
This
occurs if the sample is coloured and results in light emitted being absorbed
within the scintillation cocktail before it leaves the sample vial. When colour
quenching is known to be a major problem, it can be reduced, as outlined later.
Chemical quenching
It
occurs when anything in the sample interferes with the transfer of energy from
the solvent to the primary flour or from the secondary flour, is the most
difficult form of quenching to accommodate. In a series of homogenous samples,
chemical quenching may not vary greatly from sample to sample. In this case
relative counting using sample counts per minute can be compared directly.
However in the majority of biological experiments using radioisotopes, such
homogeneity of samples is unlikely and is not sufficiently accurate to use
relative counting. Instead an appropriate method of standardization must be
used. This requires the determination of the counting efficiency of each sample
and the conversion of counts per minute to absolute counts. It should be noted
that quenching is not such a great problem in solid scintillation counting.
Chemiluminescence: This can also
cause problems during liquid scintillation counting. It results from chemical
reactions between components of the samples to be counted and the scintillation
cocktail, and produces light emission unrelated to excitation of the solvent
and flour system by radioactivity. These light emissions are generally low
energy events and are rejected by the threshold setting of the photomultiplier
in the same way as is photomultiplier noise.
Phospholuminescence : This results
from components of the sample, including the vial itself, absorbing light and
re-emitting it. Unlike chemiuminescence, which is a once only effect,
phospholuminescence will occur on each exposure of a sample to light. Samples
that are pigmented are most likely to phosphoresce. If this is a problem,
samples should be adapted to dark prior to counting and the sample holder
should be kept closed through the counting process.
AUTORADIOGRAPHY
Ionizing radiation acts upon a
photographic emulsion to produce a latent image much as does visible light. For
an autoradiograph, a radiation source, (i.e. radioactivity) emanating from
within the material to be imaged (the object) is required, along with a
sensitive emulsion. Autoradiography is very sensitive and has been used in a
wide variety of biological experiments.
These unusually involve a requirement to locate the distribution of
radioactivity in biological specimens of different types. For instance, the
sites of location of a radiolabelled drug throughout the body of an
experimental animal. The techniques of autoradiography have become more
important with recent development in molecular biology. In general, weak
β-emitting isotopes (e.g. 3H, 14C and 35S) are
most suitable for cell and tissue localization experiments.
Choice of emulsion and film
A
variety of emulsion is available with different packing densities of the silver
halide crystals. Care must be taken to choose an emulsion suitable for the
purposes of the experiment, since the sensitivity of the emulsion will affect
the resolution obtained. X-ray film is
generally suitable for macroscopic samples. Liquid emulsions are prepared by
melting strips of emulsion by heating them to around 60°C. Then either the
emulsion is poured onto the sample or the sample attached to a support is
dipped into the emulsion. The emulsion is then allowed to set before being
dried. Such a method is often referred to as a dipping film method and is
preferred when very thin films are required.
Background
Accidental
exposure to light, chemicals in the sample, natural background radioactivity
(particularly 40K in glass) and even pressure applied during
handling and storage of film will cause a background fog (i.e. latent image) on
the developed film. This can be problematic, particularly in high resolution
work and care must be taken at all times to minimize its effect. Background
will always increase during exposure time, which should therefore always be
kept to a minimum.
Time of exposure and film processing
The
time of exposure depends upon the isotope, sample type, level of activity, film
type and purpose of the experiment. The sample applies to the processing of the
film in order to display the image. Generally the process must be adapted to a
given purpose, and a great deal of trial and error is often involved in
arriving at the most suitable procedures.
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. Quantitative
images are produced until saturation is reached. The approach provides high
resolution but limited sensitivity.
Intensifying screens
When
32P-labelled or γ-isotope-labelled samples, [e.g. (32P)
DNA or 125I labelled protein fractions in gels] are to be located,
the opposite problem to that presented by low energy isotopes prevails. These much
more penetrating particles and rays cause little reaction with the film as they
penetrate right through it, producing a poor image. The image can be greatly
improved by placing, on the other side of the film from the sample, thick
intensifying screen consisting of the solid phosphor.
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. Sensitivity of
films may be increased by preflashing. This involves a millisecond light flash
prior to the sample being brought into juxtaposition with the film and is often
used where high sensitivity is required or if results are to be quantified.
Quantification
Autoradiography
is usually used to locate rather than to quantify radioactivity. However, it is
possible to obtain quantitative data directly from autoradiography by using a
densitometer, which records the intensity of the image. This in turn is related
to the amount of radioactivity in the original sample. There are many varieties
of densitometers available and the choice made will depend on the purpose of
the experiment.
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