Bubble Chamber Photography
Technique of photographing nuclear particles by means of the bubbles they release in a super-heated liquid. Has in recent years become a specialized tool in nuclear research, quite distinct from the recording of tracks in nuclear emulsions, and the photography of the events in cloud chambers, although the latter problem is very similar to that of bubble chamber photography.
Basically a bubble chamber is a means of studying the interaction of charged particles (e.g., protons, electrons and other elementary parts of matter) with the atomic nuclei of the medium in the chamber. The particles are released into the chamber with extremely high energy, usually after acceleration by an electrostatic field in a proton-synchrotron.
The particles can enter the bubble chamber directly, or can be made to strike a thin metal target, so producing particles other than protons. The particles are traveling at speeds so close to that of light that the relativistic mass increase is about 30 times.
Photo credit: Image in the Public Domain
Types of Bubble Chamber
A bubble chamber operates on the principle that a charged particle passing through a super-heated liquid causes boiling to occur along its path. The analogous situation in a cloud chamber is that of an ionizing particle leaving a trail of droplets in a supercooled vapor. The advantage of a bubble chamber lies in the greater density of the medium, giving a much higher probability of a particle being stopped within the chamber, and ensuring a greater number of collisions with the atomic nuclei of the medium.
The most favored liquids for use in a chamber are hydrogen, helium, or a simple hydrocarbon such as propane. Hydrogen is considered particularly useful because the nucleus is very simple, consisting of one proton only. Propane, however, is more easy to work with, and presents less severe optical problems although having a more complicated nucleus.
For photographing particle tracks in a hydrogen bubble chamber, a condenser behind the chamber focuses an image of an electronic flash close to the camera lenses. Usually at least three cameras are used to obtain at least two good views over the whole of the chamber.
This provides dark-ground illumination with no direct light reaching the camera lenses. A bubble is visible on account of the small amount of light scattered sideways from the main beam. Hydrogen has a very low refractive index (about 1.09) so that at 10 from the main beam the scattered light has about t the intensity of that passing straight through the bubble, and at 20 it has dropped to 1/10. The image of the flash tube must therefore be very close to the camera lenses. Only the blue light from the flash is used (with a non-color sensitized film), partly to eliminate partial light spill into the camera lens by the chromatic aberration of the condenser, and partly to obtain maximum image resolution.
Because the liquid in the chamber is under high pressure, the transparent faces of the chamber must be of very thick glass of high optical quality to avoid distortion of the bubble tracks. The glass faces are coated to reduce the reflectance in the part of the spectrum used.
(The inner faces of the glass are not optically eliminated by being in contact with the liquid, as is usually the case, because of the very low refractive index of hydrogen.)
With propane, the light scattered out of the main beam is so much higher that the flash tubes can be placed at one side of the chamber, with the light refracted through approximately 90. Thus no rear window or condenser system is required, and a wider band of the spectrum can be used for recording. However, these advantages in the optical system are considered by many workers not to outweigh the advantages of the simple nucleus of the hydrogen atom.
For operation of the chamber, the pressure of the liquid is normally kept above that of the vapor. Just before the entry of the particle beam, the pressure is reduced, and after a delay of about 5 milliseconds, depending on the exact conditions and the availability of light, a flash exposure of about 100 to 200 microseconds duration is made of the bubbles, and the pressure immediately restored.
Bubble chamber photography needs a fast enough film, as the camera lenses are stopped down to f16 or even f32 for adequate depth of field through the chamber.
With a bubble size of less than 0.5 mm and a 15 times reduction, the film records merely the light points due to the bubbles. After formation the bubbles begin to grow, which to the camera lenses appears as an increase in brightness.
With a fast film the bubbles can be photographed immediately, before currents in the liquid distort the tracks, or the liquid as a whole begins to boil.
This film speed must be consistent with adequate resolution. The central part of the diffraction pattern of an optical system working at f16 is about 15 fl, and that from an f32 system is about 30 fl. The resolution of the film need not therefore be infinitely high in view of this low optical resolution. On the other hand the images of the bubbles must not be appreciably enlarged by light scatter in the emulsion or the individual images will be joined.
The recording of an interesting track or collection of tracks is partly a matter of luck.
It is customary to take many thousands of pictures, select the ones suitable for detailed examination and put them in a tracking machine which follows along a track with a photoelectric scanner, recording the coordinates on punched tape, ready for insertion into a computer. The film must therefore have many of the properties normally associated with cine film: freedom from static discharge when wound on rapidly, stability in storage both before exposure and between exposure and development, and tolerance to rapid processing- possibly at a high temperature. The positions of individual bubbles may have to be measured with an accuracy of about 2 fl.; this is not possible over several inches of film owing to the unpredictable distortions in the base. However, by placing fiducial marks on the glass windows of the chamber so that reference points appear every centimeter or so on the film, this accuracy can be achieved on ordinary commercial films using triacetate base. The tracks must also be of sufficient density to be detected by the tracking machines.
If the image is to be closely confined to the region of the original optical image, this calls for a film with as high a contrast as possible, consistent with accepting a wide enough range of exposures. Further, the granularity of the developed image must be sufficiently low to avoid errors in interpreting the positions of bubbles.
Usually the most satisfactory compromise between the various requirements of the sensitive material is a film of 40- 160 ASA, developed to a considerably higher contrast than that used in normal camera work. Sometimes a faster film with coarser grain can be used if the emphasis is on the positions of tracks rather than of individual bubbles.