Download 8- Radioisotope tichniques

Dr Gihan Gawish
Atomic structure
(N)
(Z)
A=Z+N
Dr Gihan Gawish
Isotope
• Isotopes are any of the different types of atoms
(Nuclides) of the same chemical element, each
having a different atomic mass (mass number)
• Isotopes of an element have nuclei with the
same number of protons (the same atomic
number) but different numbers of neutrons.
• Therefore, isotopes have different mass
numbers, which give the total number of
nucleons, the number of protons plus neutrons.
Dr Gihan Gawish
Isotope
• About 339 nuclides occur naturally on
Earth, of which 250 (about 74%) are
stable.
• Counting the radioactive nuclides not
found in nature that have been created
artificially, more than 3100 nuclides are
currently known
Dr Gihan Gawish
Isotope
• Elements are composed of one or more
naturally occurring isotopes, which are
normally stable.
• Some
elements
(radioactive) isotopes
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have
unstable
Isotope
Elements in chemistry are represented by a symbol with two numbers
* Atomic number = number of protons or electrons
* Atomic mass = number of nucleons (protons + neutrons).
12
6
C
Change the atomic number = a different element.
Isotopes are atoms with identical atomic numbers but different mass numbers
1 2 3
12 13 14
32 31
131 127
1 1 1
6
6 6
15 15
53 53
H HH
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C C C
P P
I I
Isotope
• Some isotopes are stable, others are
unstable or radioactive
• Radiation is emitted when an unstable
nucleus spontaneously changes, or
disintegrates into more stable one.
• Every element in the periodic table has at
least one radioactive isotope.
• Radioactivity is a form of nuclear
reaction
(nucleus)
not
chemical
reaction (electrons)
Dr Gihan Gawish
Nuclear and chemical reactions
• A nuclear reaction involves changes in an
atom’s nucleus, usually producing a different
element. Chemical reaction never changes the
nucleus, it only rearranges the outer shell
electrons.
– Different isotopes of an element have
essentially the chemical reactivity (same
electrons), but often have completely different
behavior in nuclear reactions.
Dr Gihan Gawish
Nuclear and chemical reactions
• The rate of nuclear reaction is not affected
by the change in temperature, pressure or
addition of a catalyst, or the chemical form
(compound or element).
• The energy change accompanying a
nuclear reaction can be several million
times greater than that of a chemical
reaction.
Dr Gihan Gawish
Radioactive decay
• Radioactive decay is the process in
which an unstable atomic nucleus loses
energy by emitting ionizing particles and
radiation.
• This decay, or loss of energy, results in an
atom of one type, called the parent nuclide
transforming to an atom of a different type,
called the daughter nuclide.
Dr Gihan Gawish
Radioactivity
Nuclear decay or Radioactivity is the
spontaneous emission of radiation from a
nucleus.
– One element can change into another element via
radioactive decay or transmutation
– Discovered by Henry Becquerel in 1896.
• He concluded that uranium gave off some
radiation.
Dr Gihan Gawish
Radioactivity
• The radiation or radioactivity was later shown to be separable
by electric (and magnetic) fields into three different types:
1. Alpha (a); a helium nucleus, He2+, emitted as alpha particle.
2. Beta (b); an electron emitted from the nucleus
3. Gamma (g); radioactivity consisting of high-energy light waves.
Dr Gihan Gawish
Alpha emission
When an atom emits an alpha particle, the nucleus
loses two protons and two neutrons. Example
238
U
92
234
Th
90
4
+2He
Alpha Particle
Emission of an alpha particle from uranium-238
produces an atom of thorium-234
• The alpha particle is emitted by elements:
– of mass number greater than 140, or
– of atomic number greater than 83
• These elements are seldom used in Biochemistry
Dr Gihan Gawish
Beta Emission
• Most of the radioisotopes commonly used in Biochemistry
are Beta emitters. The beta particle is one of two types:
• Electron emission (b or b-):
– decomposition of neutron  electron + proton
• The nucleus ejects electron as a beta particle and retains the
proton.
– An example is the radioactive decay of carbon-14
C N +
14
– conversion of proton  neutron + b+. 6
• Positron emission (b+):
14
7
b
0
-1
• A positron has the same mass as an electron but a positive
charge.
– An example is the decay of Zn-65
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65
30
Zn Cu +
65
29
b
0
+1
Gamma emission
• A few radioisotopes of biochemical significance are
gamma emitters
• Gamma emission (g) causes no change in the mass
or atomic number
g-emission is often a secondary process occurring after
initial decay by a or b emission. Surplus energy is
sometimes emitted.
g-rays are high energy waves, corresponding to radiation
with a wavelength of about 10-12 m.
• The most dangerous kind of radiation for humans.
– Cobalt-60 is used in cancer therapy as a source radiation
that kills cancerous tissue.
• Example of g-emitters:
I b
131
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53
-
+
Xe
131
54
Xe + g
131
54
Ionizing Radiation
• Energy of a particles and g rays is fixed because they
are of specific composition or wavelength.
• Energy of b particles varies with the atom they
originate from.
– E.g. 32P releases high-energy b particles, while tritium 3H
release low-energy b particles during the decay.
a, b and g emissions are all ionizing radiation, because
they have the potential, upon encountering an atom, to
knock out its electrons, thereby creating ions. This is
why these radiations are harmful.
Dr Gihan Gawish
Ionizing Radiation
• Ionizing Radiation: A general name for
high-energy radiation of all kinds, such as
a particles, b particles, g rays, x-rays, and
cosmic rays.
– X-rays and g-rays are electromagnetic radiation.
– Cosmic rays: A mixture of high-energy particles –
protons and various atomic nuclei – that come from
space.
Dr Gihan Gawish
Radioactivity Half-Life
• Nuclear decay is a first order process 
dN
N
0.693
 N  Ln
 -t  t1 / 2 
dt
N0

• Rates of nuclear decay are measured in units of half life (t1/2),
defined as the time required for one half of the radioactive
sample to decay.
Isotope
Particle type
Half life
3H
b-
12.3 yr
14C
b-
5570 yr
32P
b-
14 days
22Na
b- & g
15 hr
125I
g
60 days
131I
b-
8 days
a
>billions yrs
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238
U
Units of Radiation
• The SI unit of radioactive decay (the
phenomenon of natural and artificial
radioactivity) is the becquerel (Bq).
• One Bq is defined as one transformation
(or decay) per second.
Dr Gihan Gawish
Units of Radiation
• In the meteric system, radioactivity unit is
Becquerel (Bp); 1 Bp= 1 disintegration per
second (dps).
•The basic unit of radioactivity is Curie (Ci), and its
subdivisions: mCi, mCi
•The two units can be interconverted:
– 1 Ci= 3.7 x 1010 Bp or dps.
Dr Gihan Gawish
Units of Radiation
• Instruments (b or g counters) report
radiation as cpm or (count per minute).
cpm = dpm * (counting efficiency of
machine)
dpm= disintegration per minute
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Detecting Radiation
Three methods are commonly
employed in Biochemistry to
detect radiation:
1. Geiger-Muller counters
2. Scintillation counters
3. Autoradiograph or photographic
exposure
Dr Gihan Gawish
Geiger-Muller Counter
• The most common devise to detect radiation,
particularly b-particles.
• Geiger counter is simply an argon-filled tube
with two electrodes. When radiation (b or g)
collide with gas atoms  ejection of electrons 
ions formation.
– Geiger counter produces electrical current in
proportion to the amount of ionizing radiation.
• Radiation produces a clicking sound in this
devise. The more radiation that enters the tube,
the more frequent the clicks. Intensity of radiation
Dr Gihan Gawish can also be registered on a meter
Geiger-Muller Counter
• This devise is seldom used for accurate
measurements, but extremely useful as
a survey meter to detect contamination,
exposure, and rough estimation of
radioactivity.
current
(i)
Dr Gihan Gawish
Scintillation Counter
• The most versatile method for measuring radiation in the
laboratory.
• In this devise, a radioactive substance is placed in a vial,
mixed with scintillation cocktail, and placed in the counter
– scintillation cocktail contains a solvent, usually aromatic, plus
fluorescent substances, usually PPO (2,5-diphenyloxazole) and
POPOP (1,4-bis-PPO)
• When radiation strike the solvent, a serious of reaction
take place that emit a flash of light. The number of flashes
are counted electronically.
Dr Gihan Gawish
Scintillation Counter
Dr Gihan Gawish
Autoradiography
• The simplest devise for detecting radiation
is a photographic film.
• If the film is protected from light, any
radiation striking the film will trigger the
formation photon- & electron-dense
location.
Dr Gihan Gawish
Autoradiography
• Extremely useful for all kinds of blots
(southern, northern, etc.), hybridization
studies, localization of biomolecules in
cell or organelles, monitoring the fate of
metabolites,
plus
many
other
applications.
Dr Gihan Gawish