Introduction

The principle of detection of electrically charged particles by discharge between two electrodes that have a large potential difference has been 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 unstable operation.

With the help of scintillator counters it became possible to apply only a potential difference 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 and later via electronic means.

The discovery that spark chambers could be used to determine the trajectory of electrically-charged particles came at an excellent moment, since the first particle accelerators just became operational. In the period from 1960 to 1970, many experiments at CERN and other accelerator laboratories have made use of this technique to detect the trajectories of secondary particles that are produced when beam particles hit a target.

The use of spark chambers around accelerators has now become history, 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 spark chamber. 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 that is easy to transport and, as a consequence, ideal for demonstrations, in particular at high schools in combination with a short lecture on cosmic rays.

On this page 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 spark chamber

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 grounded.

Above and below the box a scintillator counter is mounted producing 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. This 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 nicely; the more plates the better.

• Movies

Mechanics of the spark chamber

The sparkchamber is 450 mm wide, 300 mm deep and 300 mm height. Hard aluminum plates are used as electrodes. The dimensions are 400 x 250 mm and 1 mm thick; they have to be 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; here it is also 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 gas-tight 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 backside of the box.  The stack aluminum plates are mechanically fixed to the inside of the backside, with feedthroughs for the HV connections to the capacitors. The backside 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 backside 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.

• Drawings
• Assembly photos
• Detail photos

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 some of the atoms to be brought into 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

It is essential 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.

• Layout of electronic circuits
• Sparkgap

Cost estimate

The cost depends 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.

• Materials (PVC, Plexiglas, Aluminum 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.........................................100
• Gas (He/Ne 10 liter/150 atm, bottle)......................250