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Radioactivity
Emission of energy from unstable nuclei which aretrying to become stable
Natural effect - radioactive isotopes are found
everywhere.
Radiation can cause ionisation (removal of electrons)of the atoms in our cells which can cause mutationsin DNA.
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Radioactivity
Radioactivity is the spontaneous disintegration of an atom that is
accompanied by emission of radiation.
There are many radioactive elements that are isotopes (having the same
atomic number but different atomic mass) of non radioactive elements.
An atom of a radioactive isotope has the same number of orbital
electrons as an atom of its non radioactive counterpart and, in general,
will behave chemically and biologically like the non radioactive species.
The difference between the radioactive and the non radioactive atoms of
identical elements is the number of neutrons in the nucleus, the number
of protons and electrons being the same for all. (Some elements havemore than two isotopes.)
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Radioactive decay - Mode
Units of RadioactivityActivity is expressed in terms of the Curie (Ci).
Where 1 Ci is 3.700 1010 disintegrations per second (dps).
Specific activityis the term used to describe the rate of radioactive
decay of a substance the energies of which are measured in millions of
electron volts, or mega electron volts (MeV).
It is expressed by disintegration per second per unit mass or volume, or
in units such as microcurie or millicurie per milliliter, per gram, or per milli
mole.
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Radioactive Emissions
Emission What? Penetration
Alpha E 2 protons2 neutrons
few cm in air. Stopped bypaper
Beta F electron 1 metre in air. Stopped bythin aluminium
Gamma K electromagneticwave
few metres of concrete willreduce their energy.Difficult to stop
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Absorption of RadiationAbsorption of Radiation
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Background Radiation
Radioactive isotopes occur naturally:
Cosmic rays we are protected from these by the
atmosphere, but airline pilots receive a higher dose.
Granite contains uranium, people in Cornwall receive a
higher dose than usual because there is a lot of granite
there.
Food, our bodies, buildings etc.
There are also made artificially:
Medical Uses cancer treatment, medical tracers
NuclearIndustry weapons industry, power industry
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Fundamental law of radioactive decay
Each nucleus has a fixed probability of decaying per unit time. Nothingaffects this probability (e.g., temperature, pressure, bonding
environment, etc.)[exception: very high pressure promotes electron capture slightly]
This is equivalent to saying that averaged over a large enough number
of atoms the number of decays per unit time is proportional to the
number of atoms present.
Therefore in a closed system:dN
dt! PN
N= number of parent nuclei at time t
P =decay constant = probability of decay per unit time (units: s1)
To get time history of number of parent nuclei, integrating
N t ! Noe Pt
No = initial number of parent nuclei at time t= 0.
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Radioactivity - Detectors
When resolution is the highest priority, measurement should be made
with a solid-state detector [such as lithium drifted Ge(Li) or Si(Li) or ultra
pure germanium] coupled to a multi channel analyzer system.
Scintillation Counters
A good match should exist between the emission spectrum of the
scintillator and the response curve of the photocathode of the multiplier
phototube.
Sodium iodide crystal doped with 1% thallium(I) iodide, a crystal useful
for counting beta particles but particularly useful for counting gamma
radiation.
When radiation interacts with a NaI(Tl) crystal, the transmitted energy
excites the iodine atoms. Upon their return to the ground electronic state,
this energy is reemitted in the form of a light pulse in the ultraviolet
region, which is promptly absorbed by the thallium atom and reemitted as
fluorescent light at 410 nm.
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Detectors
Proportional Counters
A sample of radioactive material can be placed inside the active volume
of a flow proportional counter, thus avoiding losses due to windowabsorption.
The chamber is purged with a rapid flow of counter gas (P-10 gas, a
mixture of 10% methane in argon) and the flow is maintained during
counting of samples.
Counter life is virtually unlimited. Such a counter is particularly suited
for distinguishing and counting low-energy alpha and beta particles.
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Detectors
Radioactivity Flow Detectors
Radioactivity emissions from the flowing sample are detected by
photomultiplier tubes on opposite sides of the flowing stream.
The resultant signals are summed, checked for coincidence, and sorted
by either a pulse-height analyzer or a multi channel analyzer.
The sample counts in a flowing system are dependent on the flow rate
and on the volume of the counting cell.
Sensitivity is maximized by increasing the flow-cell volume and
decreasing the flow rate.
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Isotopes Detection by LSC
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Isotopes Detection by LSC
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LSC
LSC Solvents
For liquid-scintillation counting (LSC), samples are dissolved in a liquid
cocktail that contains a suitable solvent, a primary scintillator andperhaps a secondary scintillator, and additives to improve water
miscibility and to permit counting at low temperatures.
Modern LSC cocktails must be designed to handle multipurpose
applications, most of which are of an aqueous nature.
These cocktails must have an efficient energy-transfer system that will
convert sample radioactivity into measurable light and, for aqueous
samples, must have a surfactant (emulsifier) system that will allow the
incorporation of a variety of aqueous sample types. Lipophilic samples donot require these surfactants.
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Detection of Radiation
Geiger counter
Photographic film
Cloud chamber