Isotopes and Decay
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7/30/2019 Isotopes and Decay http://slidepdf.com/reader/full/isotopes-and-decay 1/5 Isotopes and decay Ionising radiations Radioactive isotopes decay into other elements. When they decay, they give out ionising radiations – so called because they cause ionisation of the medium that they travel through. There are three main types of radiation called alpha (), beta () and gamma (). They were first identified at the end of the 19th century and were classified according to their penetrating power (alpha is the least penetrative and gamma the most). We now know more about what they are and why they behave as they do. Picture 2.4 Alpha radiation. Alpha radiation - What is it? Alpha radiation is a stream of particles. These are the nuclei of helium – two protons and two neutrons. This means that they have a (relatively) large mass. Alpha particles from a given radioactive decay all have the same energy (of the order of a few MeV). What's its charge? The atomic number of helium is 2 so an alpha particle carries a double positive charge. What's its symbol? or How fast do they go? Alpha particles don’t travel very fast (compared with beta particles) because they have such a large mass. They carry energy away from a radioactive decay; this energy is of the order of a
Transcript of Isotopes and Decay
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Isotopes and decay
Ionising
radiations
Radioactive isotopes decay
into other elements. When
they decay, they give out
ionising radiations – so
called because they
cause ionisation of the
medium that they travel
through. There are three
main types of radiation
called alpha (), beta ()
and gamma (). They were
first identified at the end of
the 19th century and wereclassified according to their
penetrating power (alpha is
the least penetrative and
gamma the most).
We now know more about
what they are and why they
behave as they do.
Picture 2.4 Alpha radiation.
Alpha radiation -
What is it? Alpha radiation is a stream of particles. These are the nuclei of
helium – two protons andtwo neutrons. This means thatthey have a (relatively) largemass. Alpha particles from agiven radioactive decay all havethe same energy (of the order of afew MeV).
What's its charge? The atomic number of helium is 2so an alpha particle carriesa double positive charge.
What's its symbol? or
How fast do theygo?
Alpha particles don’t travel veryfast (compared with betaparticles) because they have sucha large mass. They carry energyaway from a radioactive decay;this energy is of the order of a
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few MeV and is given to thekinetic energy of the alphaparticle; they have a mass of 6.6x 10 –27 kg so their speed is about 7x 106 m.s-1 – i.e about a fiftieth of
the speed of light.How ionising arethey?
Their low speed means that theytend to collide with plenty of other atoms and cause a lotof ionisation on their way. Analpha particle can ionisethousands of air particles before itslows down to thermal speeds.
How far do theygo?
All these collisions mean that theylose energy quickly, so they havea short range in air.
What do they gothrough?
They are easily stopped byanything solid – even a pieceof paper will stop alpha radiation.
Effect of magneticfield
Alpha particles are deflectedslightly in a magnetic field. Their large mass means that they don'tget deflected much. They willtravel in the arc of a circle in auniform field (see page 19).
Effect of electricfield They are deflected slightly in anelectric field and will move in aparabolic curve in a uniform field.
Picture 2.5 Beta radiation.
Picture 2.6 Bending beta particles in
Beta radiation -
What is it? Beta radiation (the sort that you'llcome across in school) is astream of fast moving electrons. Beta plus radiation is a stream of particles called positrons. Betaradiation from a given radioactive
decay is given out with a range of energies (see page 16).
What's its charge? You will usually come across beta
minus (-) radiation, which has anegative charge.
What's its symbol?or
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magnetic field. They must take a curved pathto reach the detector.
How fast do theygo?
These particles have very littlemass (about 7, 000 times lighter than an alpha particle) and travelclose to the speed of light(300, 000 km.s-1).
How ionising arethey?
They tend to pass through the air and solid matter without manycollisions with other atoms. Sobeta radiation is only weaklyionising.
How far do theygo?
However, it means that it has along range in air.
What do they gothrough?
It will passthrough paper , aluminium andsteel. However, it is stopped
by lead or thick pieces of other metals.
Effect of magneticfield
Beta particles are easily deflectedin a magnetic field because of their small mass. They will travelin a circular path in a uniform field(see page 19). The direction of their deflection tells us whether they are positively or negativelycharged.
Effect of electricfield They are deflected in an electricfield and will move in a paraboliccurve in a uniform field.
Picture 2.7 Gamma radiation.
Gamma radiation -
What is it? Gamma radiation is at the highfrequency end of the electromagnetic spectrum. Ithas a veryshort wavelength (much less thanthe radius of an atom) and will
pass through atoms with very littlechance of being deflected or absorbed. Gamma radiation isoften given out with alpha andbeta radiation.
What's its charge? It has no charge.
What's its symbol?
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How fast does itgo?
As it is part of the electromagneticspectrum, it travels at the speedof light – 300, 000 km.s-1.
How ionising is it? It will tend to pass through matter without causing much ionisation.
How far does it go? It has an extremely long range inair but gets weaker with distance.Its intensity obeys an inversesquare law – getting weaker withthe square of distance, i.e.doubling the distance quarters theintensity. Tripling the distanceleads to a ninth of the intensity.
What does it gothrough?
It will get through thin samples of most materials without any
noticeable decrease in intensity.However, its intensity is reducedby lead or very thick pieces of other metals. The thicker thesample, the greater the reductionin intensity.
Effect of magneticfield
No effect because it has nocharge.
Effect of electricfield
No effect because it has nocharge.
Question 6
a) Gamma radiation obeys an inverse square law. Its intensity drops
ith the square of distance because the radiation spreads out. Explain
hy alpha and beta do not obey the inverse square law.
b) Look at picture 2.6. This experiment can be set up in a school
laboratory. It would be more difficult (or impossible) to set up a similar
experiment to show alpha particle deflecting. Explain why this is the
case.
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Isotopes and decay
Our experts' view is:
a) Alpha and beta cause much more ionisation of the air than gamma. This slows the particles down and
reduces the intensity more rapidly than an inverse square. In a vacuum, they would obey an inverse square
law.
b) Firstly, the range of the alpha particles is too short - they might not make it across the width of the
magnet. Secondly, they have a larger mass and are not deflected so much - it would be difficult to be sure
they were bending - unless a much stronger magnet were used.
