Cosmic rays seen with a Sparkchamber.
E. van den Born, H. Tiecke
NIKHEF, Amsterdam
C.Brouwer, J. Dijkema Radboud University Nijmegen
Introduction
The principle of detection of electrically charged particles by discharge between two
electrodes that have a large potential difference is applied since 1950.
In the beginning this principle is used as a counter, i.e. just to detect the passage of a
particle, a task which is completely taken over by the development of scintillator material.
Originally one used a permanent voltage difference between the plates (electrodes) which
often was the cause of spontaneous discharge and by consequence an instable operation.
With the help of scintillator counters it became possible to apply only a potential differ
rence at the moment a particle passed the plates. One realized quickly that with the use
of metal electrodes, which were mounted parallel to each other, the trajectory of a
particle could be made visible in a spectacular way. And even more important was the
fact that more particles could be detected at the same time using photographic material or
later also via electronic means.
The discovery that sparkchambers could be used to determine the trajectory of elec
trically charged particles came at an excellent moment, since the first particle accelerators
became just operational. In the period 1960-1970, many experiments at CERN and other
accelerator laboratories have made use of this technique to detect the trajectories of
secondary particles which are produced when beamparticles hit a target.
The use of sparkchambers around accelerators has become history by now, although they
still play an important role for educational purposes.
The cosmic rays, i.e. the electrically charged particles which hit the earth coming from
outer space, are made visible in a beautiful way with a sparkchamber. It is fascinating to
observe the continuous ‘bombardment’ of these particles, whether we are outside or
sitting in a cellar.
We have now constructed a small chamber which is easy to transport and as a conse
quence ideal for demonstrations, in particular at high schools in combination with a short
lecture on cosmic rays.
In this report we will describe in short the working principle, the mechanics, a few details
on the required electronics and refer to web addresses where more information can be
obtained. We conclude with a table from which a cost estimate can be obtained.
The operation principle of a sparkchamber
One millimeter thick aluminum plates are stacked with spacing of 10 mm and electrically
isolated from each other. The stack is put in a small enclosed box with a transparent front.
The box is filled with a gas mixture of Helium and Neon (70%He/30%Ne). Every second
plate can be put on high voltage, the other one is earthed.
Above and below the box a scintillator counter is mounted which produces a light pulse
when a cosmic particle crosses. If a light pulse is observed at the same time in both
counters, a particle has crossed the chamber and with the help of some electronics and a
spark gap, a high voltage of approximately 5 kV is put on the plates. Ions are formed in
the gas along the trajectory of the particle. That is the location where the electrical
resistance is smaller and therefore a discharge will take place. The choice of the gas
mixture determines the brightness and color of the spark. Because of the fact that many
plates are put on top of each other the trajectory shows very nice; the more plates the
nicer.
Mechanics of the sparkchamber
For a good overview of the construction, a series of drawings and pictures can be viewed
at www.nikhef.nl/~h42/sparkchamber. The sparkchamber is 450 mm wide, 300 mm deep
and 300 mm height. Hard aluminum plates are used as electrode. The dimensions are 400
x 250 mm and 1 mm thick; they have to be nicely flat, no bends or scratches and the
corners should be rounded off to avoid as much as possible spurious sparks at the edges.
At the four edges a hole is drilled to position the spacers and mount a tierod; also here it
is important to take care of smooth edges. Twenty five plates are stacked and spacers at
the corners take care of a uniform distance; the four tierods make a solid stack. As can be
seen in the pictures, a small ‘tongue’ at one side of the plates makes the connection,
either to earth or to high voltage. Subsequently the stack has to be mounted in a gastight
box. The top-, bottom- and side plates are made of 20 mm thick PVC, while the front
plate is 10 mm thick plexiglass. All these parts are glued together in order to provide a
tight gas volume. The electronics is mounted on the outside of the backplane of the box.
The stack aluminum plates are mechanically fixed to the inside of the backplane, with
feedthroughs for the HV connections to the capacitors. The plane itself is also made of 20
mm thick PVC and is fixed with screws to the rest of the box. An O-ring takes care of
the gastight connection. Special connectors in the backplane provide the gas inlet and
outlet.
Finally it should be mentioned that we have just given an example for the material and its
dimensions. There are many more possibilities.
Scintillation counters
With the help of two scintillation counters one signals the passage of a charged (cosmic)
particle. A counter exists of a piece of scintillator material, a light guide and a light
amplifier (photomultiplier). The scintillator material is in fact plexiglass with a small
addition of a chemical element; it is mostly delivered as sheet material. If a charged
particle crosses the material, the electromagnetic interactions will cause that some of the
atoms will be brought in an excited state. These will decay back into the ground state by
emitting photons with a wavelength around 500 nm. The amount of light is small and
therefore has to be amplified with the help of a photomultiplier.
Electronic circuits
Essential is to choose the proper capacitors that transmit the high voltage to the aluminum
plates. Also essential is a sparkgap. Although this can be bought commercially (for
example at E2V Technologies, UK), one can save substantial costs producing a ‘home
made’ sparkgap with the help of a spark plug. Two printed circuit boards have been
designed and produced; one provides the high voltage (~5 kV) and the second one
discriminates the photomultiplier signals, contains the coincidence logic and provides the
trigger signal for the sparkgap. In addition it also can provide the low voltage for
photomultipliers that have a built in high voltage supply. All together a very nice
compact design. The layout of electronic circuits can be found at
www.nikhef.nl/~h42/sparkchamber.
Cost estimate
The cost depends somehow on the material choices and certainly on the availability of a
mechanical and electronic workshop. An indication of the material costs (in euros) is
given in the table below.
1)
2)
3)
4)
5)
6)
Materials (PVC, Plexiglas, Alu plates, spacers)………400
Two printed circuits with components………………. 275
Sparkgap commercial (home made)………………1100 (50?)
Two photomultipliers (with built-in HV generator)…1000
Scintillation counters…………………………………pm
Gas (He/Ne 10 liter/150atm,bottle) …………………..250
For more information please contact:
H. Tiecke at NIKHEF (tiecke@nikhef.nl)
C.Brouwer at Radboud University of Nijmegen (c.brouwer@hef.ru.nl)
For more pictures and short movies look at www.nikhef.nl/~h42/sparkchamber.