Radioactivity MSK

8/6/2019 Radioactivity MSK

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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

Radioactivity MSK

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