358
Rate Meter and Scintillation Counter.358
Rate Meter and Scintillation Counter.
EKCO Electronics Ltd, England
Box 25*10*20, Head 20*4D
The scintillation counter is a device used to measure radiation output, more
specifically gamma rays or photons and charged particles. The counter consists
of a portable metal box housing all the electronics, while the actual scintillation
device is in a 20cm x4cm cylinder that is attached by a metre long wire
to the measuring box.
A typical detector uses material (in this case
Sodium Iodide doped with Thallium) that emits tiny flashes of light or scintillations,
when struck by energy such as gamma rays, charged particles, etc. This light
hits the photocathode surface of a photomultiplier, ejecting photoelectrons
that are then accelerated to strike dynodes. If the velocity that the photoelectron
has is high enough, it can eject additional electrons. This process is called
secondary emission. Typically with secondary emission the multiplication factor
is between two to five. So for every electron hitting the dynode two to five
electrons are ejected. By having lots of dynodes in series, a single electron
can eject up to a billion more electrons. Scintillation counters usually have
a resolution approximately ten nano-seconds (i.e. ten billionth's of a second.)
and produce pulses whose amplitude is related to the energy of the radiation.
Scintillation counters are used in measuring the radioactivity of materials, or areas affected by nuclear tests (e.g. nuclear weapons tests). Also there are biological applications, where it is possible to keep track of radioactive isotopes through a chemical or biological process and even keep track of isotopes spreading through the environment.
The Scintillation Counter in the museum was originally used by the Zoology Department at the University of Queensland and donated to the museum in 1997
References.
M. Born, Atomic Physics, Blackies and Sons, 1937;
R.C Brown, A Textbook of Physics, Longmans, 1961;
A. Hudson, R. Nelson, University Physics, Harcourt Brace Jovanovich Inc., 1982;
T.A LDittlefield, N. Thorley, Atomic & Nuclear Physics, Van Nostrand, 1963.
D.A.
366 X-RAY SPECTROMETER (c. 1940)
Adam Hilger Ltd., London / No. E334 301 28210
Cubical iron base, encasing a spring wind-up device. Raised above the base is a hollow iron box and thin slit; followed by a crystal; a sheet of photographic film in an arc-shaped brass frame; enclosed by a perimeter of lead shielding.
The encased winding device, when wound, rotates a toothed wheel, which systematically rotates the crystal's position. An X-ray beam is designed to be shone through the crystal and the resultant diffraction pattern recorded on the film. The lead shield is for protection against the X-ray radiation.
This X-Ray spectrometer was used in the teaching laboratories at the University of Queensland for various laboratory experiments involving the measurement and observation of X-ray behaviour, diffraction patterns and crystal structures.
Device is also known as an X-ray Crystal Diffraction Camera.
Reference:
202 RUHMKORFF INDUCTION COIL (c 1910)
H.W. Cox Ltd/London
Patent No. /16926
61*33*33
Consists of two concentric coils of wire wound on a cylindrical
core of soft iron wires impregnated with paraffin wax, all mounted
on a hollow base containing a capacitor (which is constructed
from sheets of tin foil separated by paraffin paper). Contains
a flat spring contact breaker but also has terminals to which
a mercury break may be connected. A polarity change switch is
missing. The primary (inner) coil consists of two or more layers
of thick silk covered copper wire impregnated with paraffin wax
placed in an ebonite tube. The secondary coil is divided into
sections separated by ebonite disks (to prevent discharge within
the coil), each containing many turns of very thin, fine silk
covered copper wire impregnated with paraffin wax also encased
in ebonite. This is terminated in two discharging rods. When a
small voltage (12 volts, say) is placed across the primary coil,
a large one is induced across the secondary coil (105 volts).
It is possible because of the large number of turns in the secondary
coil, the concentrated magnetic field (due to the iron core and
the close of the coils), and the abrupt and rapid interruptions
(due to the contact breaker). The coil was invented in 1851 by
Heinrich Daniel Ruhmkorff (1803-1877), who was a German instrument
maker in Paris. It was popular for energizing discharge tubes
and in particular for generating x-rays (which were discovered
in 1895 by Roentgen). Harry Cox died of x-ray induced cancer.
References:
J.C.
244 FOCUS X-RAY TUBE (c 1900)
6/six/No 37592
33*23*15
Blown glass tube with anode, anticathode and focus cathode. Electrodes
are connected to soldered terminals by wires. Side discharge tube
with anode and cathode. Sealed, rubber capped exhaust tube. Glass
discoloured due to radiation damage or metal deposits. The cathode
focuses an electron beam to a small spot on the anticathode, where
they are absorbed. This excites atoms in the anticathode, which
relax by emitting X-ray photons. X-rays emerge from the side of
the tube and since they emanate from a small spot on the anticathode,
X-ray images are sharp. The extra anode allows a larger current
to pass without damaging the anticathode. The side tube is used
to release (previously occluded) gas into the main chamber. This
is to counteract the occlusion of gas that occurs during operation
and which lowers the pressure until the discharge ceases. Tubes
of this kind were used in medicine and for further X-ray experimentation.
References:
G.P
245 X-RAY TUBE, COLLIMATING TYPE (c 1900)
36*27*10
Has side arm with spark-gap, focus cathode, anticathode but no
anode and long side arm with X-ray transmitting glass end. Uses
a curved cathode to focus the electron beam onto a Tungsten anticathode.
The tube has a high lead content to absorb X-rays except for a
small transmission window at the end of a long tube. The combination
of small source on the anticathode and small window at the end
of the tube produced a collimated beam. Primarily used in crystallography,
x-ray spectroscopy, and radiography.
References:
T.E.
328 COOLIDGE X-RAY TUBE (circa 1930)
VICTOR X-RAY CORPORATION/CHICAGO, ILL, USA
60693
50*18D
Spherical glass bulb (purple from radiation damage) with cylindrical stems carrying the electrodes (cathode end broken).
The Coolidge Tube, first produced in 1913 by W. Coolidge, is the forerunner of all the types of x-ray tubes in common use today. The Coolidge tube was the first type of practical x-ray tube to employ the principle of thermionic emission.
A tungsten filament is used as the tube cathode, and during operation is heated to incandescence by passing a current through it. This causes the filament to emit electrons at a rate dependent on the temperature of the filament. The electrons are then accelerated towards the tube anode by the strong tube voltage. Upon hitting the anode, the electrons are decelerated very rapidly, and shed their excess kinetic energy mostly as heat, and partly as x-ray radiation. To prevent the electron beam from dispersing due to repulsive forces between the electrons, the cathode filament is surrounded by a metal focusing cup at a high negative potential, that has the effect of converging the beam to a relatively small focal area on the anode. X-ray tubes previous to the Coolidge tube (known as Gas Tubes), relied for their electron source, on the tube voltage being strong enough to 'pull' electrons from the cathode. These were accelerated towards the anode, and collided with residual gas molecules purposefully left in the tube, ionizing the molecules and causing the ejection of more electrons. In this way, the required electron beam was built up with a kind of 'avalanche effect'. However, the number of electrons in a beam produced this way, and their energy upon collision with the anode, were both dependent on the gas pressure within the tube, which was rarely stable, and difficult to control. In addition, the number of electrons produced in this way, was, by today's standards extremely small, and hence the intensity of the produced x-rays very low, leading to very long exposure times.
The Coolidge tube, using as it did thermionic emission to obtain a source of electrons, removed the dependency on residual gas for the number and energy of the electrons in the electron beam (an indeed in Coolidge tubes, almost all gas is removed). In fact, as the number of electrons produced depended on the current applied across the cathode filament, and the energy of the electrons on the tube voltage, the Coolidge tube made it possible to easily and independently vary the number of electrons (and hence the intensity of x-rays produced), and their energy (and hence the frequency of produced x-rays). Also, thermionic emission allowed a much higher bound on the numbers of electrons produced, and hence lead to a drastic reduction in exposure times.
References:
1. F. Jaudrel-Thompson & W.J. Asworth, X-Ray Physics and Equipment, Blackwell Scientific Publications, Oxford 1965, pp.401-411.
2. H. Pilon, The Collidge Tube: Its Scientific ASpplications, Medical & Industrial, London, Balliere, Tindall and Cox, 1920.
3. J. Thewlis (Ed), Encyclopaedic Dictionary of Physics, vol. 1, Permagnon Press, 1962.
4. http://www.invent.org/book/book.text/26.html
PF
336 X-RAY MACHINE, FOOT (circa 1945)
Maker unknown
125*78*94
Upright metal box painted white, with three black leather viewing holes on the top all facing inwards. The viewing holes have metal alloy surrounds. Under the middle viewing hole is a brass label with the words: 'PATENT PENDING". There are chrome-plated brass strips around the bottom and running up the front and top, parallel to two decorative handles, which are also made of chrome-plated brass. There is a raised metal platform at the front with a rubber foot plate and part of the main body is cut away to allow for the insertion of feet directly under an x-ray tube, which is hidden in the main body of the machine. Beneath the foot-plate is a fluorescent plate which may be seen from the viewing holes via two mirrors. In front of the viewing hole on the right hand side is a plastic switch connected to a power inlet, with a power cord extension coming out of the bottom right hand side of the box, and a high voltage transformer, leading to the x-ray tube. There is no lead shielding.
This exhibit is on loan from the Powerhouse Museum, Sydney.
Foot x-ray machines were used in shoe stores, during the 1940s
and 1950s, to check show sizes, especially for children. The radiation
dosages given out by these machines were approximately 10 Roentgens
per minute. Their use was discontinued around the latter half
of the 1950s.
References: Hilary Irvin, In the Footsteps of Röntgen, Hilary
Irvin Productions, Edgecliffe, pg 162-163.
MT
137 DISCHARGE TUBES, SET (c 1910)
54*54*13
A set of sixteen discharge tubes, including several Crookes' tubes and Geissler tubes in a mahogany and pine box with shaved wood padding.
A partially evacuated glass tube containing two metal electrodes, the Crookes' tube exhibits fluorescence when a high voltage is supplied through the electrodes. A few of these in the set were used to demonstrate fluorescence in various materials including calcite, ruby and sea shells. Crookes' tubes were also used to study the straight-line motion and momentum of cathode rays (electrons), best demonstrated by the Crookes' Maltese Cross tube, and their deflections in a magnetic field. Later experiments with Crookes' tubes led to the discovery of X-rays in 1895 and the electron in 1897.
A Geissler tube is a long thin glass tube containing two metal electrodes and a gas, in approximately 0.001 atmospheric pressure. When several hundred volts are applied to the electrodes, the gas ionizes, current flows, and the gas glows with a characteristic colour. Originally used as light sources for studying the spectra of gases, as well as general illumination, the modified Geissler tubes are now mainly used as neon signs.
References: Christie's Catalogue, "The Nicholas Webster Collection
of Geissler and Crookes' Tubes and Other Laboratory Apparatus",
London 1991.
KCM
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