Transcript Photographic film

Who is this?
Marie Curie
Born 1867 Poland
1893 degree in physics, 1894 degree in mathematics.
1894 Denied a place in Krakow University because she was a woman
Then became first female professor at the Sorbonne
1896 Becquerel discovers that radioactive substances affect photographic plates
1898 After working on Becquerel’s discovery, she discovers new element
“polonium”
Established that radioactivity was a result of something in the atom.
1903 first woman to be awarded the Nobel prize for physics.
1906 Pierre Curie dies in road accident.
1911 first (and only so far) person to be awarded a Nobel Prize for Chemistry (and
Physics).
1934 After years of working with radioactive substances, whose dander was not
recognised at the time, with no safety procedures, she dies of a blood disorder
linked to radiation exposure.
All her research documents and even her cook book are too dangerous to handle
and are kept in a lead lined box.
Photographic film
1.
What happens to film when radiation is incident
upon it?
It darkens.
2.
Can photographic film tell you the type of radiation
incident upon it?
No, just the amount of radiation received.
3.
What can this be used for?
Can be used in radiation badges, that record the
exposure of workers to radiation. Different windows
detect different types of radiation.
Geiger-Muller Tube
The detector is a metal tube filled with gas. The tube has a thin wire
down the middle and a voltage between the wire and the casing.
Good at detecting alpha and beta, not as good at detecting gamma
(why?).
collision & ionisation
radiation
Argon
Argon gas
gas
mica window
When the radioactivity enters the tube,
it ionises the gas in the tube. This
produces a pulse of current which is
amplified and passed to a counter.
The Argon
contains a little
bromine to act as
a quenching
agent and
prevent
continuous
discharge.
counter
124
125
Cloud chamber
Cloud chambers show the actual paths of the ionising particles.
They rely on ionisation. The cloud chamber is cooled and then is
super-saturated with alcohol. If an ion is formed a droplet of
condensation appears. Best for alpha radiation as alpha most
ionising; then Beta which shows faint traces, but cloud chambers
are not as good for gamma as gamma is only weakly ionising.
Cooled alcohol vapour
Radioactive source
Solid carbon dioxide
Detection of radiation
• Photographic film
• The amount of darkening of the film shows the amount of
radiation. This was the early method of detection. Qualitative
only – tells us radiation is present but not which type. Used in
modern monitors in conjunction with metallic shielding, which
can stop alpha and beta particles but not the more penetrating
gamma rays.
• Cloud chamber
• The instrument contains a super saturated vapour of an alcohol.
When radiation passes through it, it ionises some of the air
particles, which causes them to act like nuclei onto which
alcohol droplets form. In this way the radiation is made visible:
alpha particles form dense white tracks, beta particles form less
dense zig-zag tracks, gamma rays form even fainter tracks.
Detection of radiation
• Geiger-Muller counter
• Uses the ionising power of particles, esp good for beta. The beta
particle is passed into a chamber where it hits a gas molecule –
usually argon. The argon is ionised and loses an electron. The
electron accelerates towards an electrode and as it does so hits
another argon atom causing it to ionise and lose an electron. This
results in a stream of electrons from lots of ionised argon atoms
reaching the electrode. This constitutes and electrical impulse
which is measured by either a digital counter or by generating
clicks. It allows us to determining amount but not type of radiation
present.
• Scintillation counter
• Certain substances will emit a flash of light when irradiated by
different types of radiation. Especially good for gamma rays. The
more energy the gamma rays have the brighter the flash of light,
which is detected by a photo-multiplier. This generates an electrical
signal, which can be counted. It is able to detect low intensities of
radiation and magnify them so they can be recorded electronically.