as many of the tragic events in Japan in
the news we've we've seen and heard this
new term millisievert sort of used as a
unit of radiation so what is it so we
got our our millisievert unit and of
course well not surprisingly a
millisievert is one thousandth of 1:1
sievert of course but but what is the
sievert the sievert is the if you like
the the human response that's the
important thing it's the human response
to to radiation it contains all the
information it needs about the type of
radiation and the type of tissue in
other words where the radiation ends up
the physics of the of the measurement in
a sense comes from our unit of gray our
unit of dose and the unit of dose is
measured in 1 Joule per kilogram in
other words the energy that's deposited
in literally one kilogram of tissue
which is probably around about that size
to humanize the the response we clearly
need to know about the effect of the
radiation so the first thing to do is to
multiply the dose in grey by a radiation
factor and if they're out of particles
particularly destructive we we allocate
a factor of about 24 gamma only a
multiplier of 1 because we know that an
alpha source is more damaging than than
a gamma source actually most of the
gamma goes through you without being
absorbed
so it's whereas the alpha particles are
going to get stopped very easily by your
by your tissue that's liberating their
energy we want our measurement the
sievert to reflect how dangerous these
these particles are in some relative way
so at an alpha particle is 20 times more
dangerous than a gamma photon clearly we
want to know where what sort of particle
it was and but we also need to know
where it lands up now a radiation
deposited in certain types of tissue
like gonads the breast the thyroid and
so on are more likely
to have a negative response in other
words a development of cancer than than
bone or something like that so we have
to allocate different and apportion
different weightings to different
different tissues to to reflect their
likelihood of responding to radiation
so we then multiply our Rivlin dose by
another parameter which gives us the
effective equivalent dose and here we
are we've actually arrived at our R
sievert our measurement notice it's
still got the same fundamental unit
joules per per kilogram
what does one sievert actually actually
do to us well the one sievert given over
a lifetime in other words we add up our
annual dose over many years or our
little individual doses one sievert
would give us around about a 5% chance
of suffering from a fatal cancer so in
other words now we've we have humanized
the the unit into a probability and I
say stressed that it is only a
probability of getting of getting a
fatal cancer okay so let's try and put
this in some sort of some sort of
context what you know what sort of
quantity is actually as sievert and the
easiest way to do that is to just to
think about our natural radiation dose
in other words the dose that comes from
gamma rays coming out of the rocks very
small cosmic rays coming out of coming
out of the sky again very small actually
the carbon and the potassium inside us
also gives us our own gamma radiation
and so if we add all these things
together and assuming a sort of average
UK type of figure we end up with about
two and a half million annual average
dose for somebody in the UK now of
course you can see that you're unlikely
to live long enough to actually receive
a sievert of radiation unless of course
you have a medical x-ray or a lot of
dental x-rays you would not be able to
get to receiver so really the units of
sievert are only
irrelevant for nuclear industry clearly
the nuclear reactor that's gone wrong
will generate a lot of radioactive
material certainly if it gets out into
the environment cesium for instance a
gamma emitter I Dean and so on a beter
emitter and these can be taken up in the
body that can be breathed in and so on
and that will produce over a lifetime
quite a high dose or could produce quite
a high dose so it's right that people
are removed from the from the locality
unless they're wearing protective
protective equipment and taking the
appropriate appropriate precautions and
so on sievert is really designed in
terms of it units and it's probabilities
as to try and work out what lifetime
doses are due a long term doses clearly
with a very sudden and short dose and
then anything can happen and point of
fact if you get to the sort of level of
five to ten sieverts in total then or
five to ten gray that is the sort of
radiation dose that would make you
seriously ill and very unlikely to
survive still a scientific unit but in
the end we've got a certain uncertainty
in the fact that somebody having a
certain level of radiation one person is
going to get developed cancer one person
isn't it's really the probability that
that particle or that gamma photon
actually did some damage to our DNA
motors them go straight past without
having any effect whatsoever but some of
them will obviously cause damage and
then of course as a probability that
that damage will actually go on to
develop cancer and that's actually very
very low number well I thought I'd just
go and show you a brief demonstration of
how to measure how we measure gamma
radiation you may have seen that the
Geiger Muller tubes and the little land
counters that they were using to measure
sieverts and so on or the equivalent in
gamma so I'd like to just show you that
well clearly with radiation sources
being very dangerous we have to keep
them under lock and key it's going to
open the cupboard and inside this box
you can see right at the center of that
that box is a tiny dot and that's
caesium-137 and that's producing
radiation all the time now it's in this
box such that it's safe for me to handle
what we've got here is a is a gamma
detector is going to detect the gamma
radiation from the from the cesium it's
a special crystal inside the seed side
the can there and this is very similar
device to the thing that people have
been using to monitor monitor radiation
you might have seen people doing this
and this is this is the sort of
instrument that they would have been
using okay so what you're now seeing is
the gamma particles being received and
being counted well each one of those
gamma photons is carrying a certain
amount of a certain amount of energy and
of course we can adult that energy
assuming that it all ends up in one
kilogram of tissue then we can calculate
the dose and therefore the effective
dose in Seavers so effectively we've now
counted the number of gamma particles
that are arriving at our detector now of
course what we don't know is where all
the other particles were going in that
cesium sample because of course they're
going in all directions so again we've
got we've got to know something about
the geometry of where the where the
cesium particle is and so if it was on
the skin for instance then clearly
that's going to most of that half of it
is going to go into the skin if it's in
the air somewhere then clearly the the
gamma rays are going to go in all
directions I'm only going to get a small
part of that well what we've got here is
yet another method of measuring
radiation somewhat simpler than the
radiation method we've just seen the
gamma counter but this is sensitive to
alpha beta and gamma so if I then pick
up an americium source
which is contained in this in this
little container whose in this little
container then I place it near my Geiger
counter then you see how close I have to
get before this actually registers any
sort of count and really that shows us
that the alpha particles coming out of
here don't can don't travel very far in
air seem no effect they're really lots
of effect they're the sort of radiation
sources we're allowed to use in teaching
laboratories by their very nature are
going to be very low levels of radiation
they're enough to us to measure but
hopefully not enough for us to do any
serious serious damage with so this is
this is off the scale in terms of micro
sieverts and it would have to stay there
on my skin for instance for quite some
time before I've got anywhere near a
dangerous level but we do take very
great care with these samples because
they could potentially do somebody some
damage