[Historical Photographs] Photography Set-up for Precision Measurement of Nuclear Transmutation by P. M. S. Blackett

By the mid 1920's Blackett knew nuclear transmutations involving alpha particles can be observed directly through cloud chambers. In his previous experiment involving nuclear collisions, the tracks of interest were "forked alpha tracks"; he knew it was not easy to photograph them as the occurrence is considered very rare. Since it is not possible to "tweak" the probability of nuclear events without jeopardizing track quality, one has to take many photos in order to find one photograph containing a "forked track". Hence, there are some considerations in experiment setup - particularly on photographic methods - to optimize data taking and measurements. Improvements in this regard not only saves cost of photographic plates (or films), it also saves time in analyzing them.

The objective of optimization was:

1. Take as little number of photos where each photo contains as many alpha tracks as possible. This will help to reduce the processing and analyzing time. Do keep in mind that development of photographs with a dark room was a painstaking process back in the 20s, and analyzing photographs was done literally by rulers and eyes.


2. HOWEVER, the number of alpha tracks per photo cannot exceed a certain value of which would degrade the quality of collision / transmutation tracks if-and-when it happens. Imagine having too many alpha tracks within a given space inside the chamber, any forked track may be "covered" or distorted by other tracks of no interest.


3. The tracks should be as defined as possible. That means it must be all sharp in focus, and as thin as possible.


4. Photographs must be taken in such a way, measurement of scattering angle is meaningful. That is to say, with measurable and diminishing parallax error.

The solution was:

1. Use a pure alpha source, preferably mono-isotopic. The emissions were also collimated in such a way alphas are "fanned out" in a plane parallel to the view port of the cloud chamber.


2. Following the premise where two approximately parallel tracks is considered resolved if their physical separation is at least twice the width of an alpha track, then Blackett worked out the optimal number of tracks for a parallel beam of 1 cm width is 42. Some of the presented photos in the papers seems to contain more resolved tracks per photograph because of admitted bias.


3. Use a mixture of gases. For example, if nitrogen was intended as the target for alpha collisions, a mixture of X:Y:Z for nitrogen:oxygen:hydrogen is used. Both oxygen and hydrogen act as diluent to the target gaseous atoms, while specifically, oxygen appears to make alpha tracks thin while hydrogen increases the length of alpha tracks.


4. In order to measure the scattering angles with any degree of accuracy, two cameras was used, positioned perpendicularly to the cloud chamber. Both camera shutter was triggered simultaneously to get a stereo photograph pair. With two photos taken of known angle, it is able to work out the effect of parallax (solutions from trigonometry) and hence accurate determination of angles. Details on this will be posted shortly.


5. In order to take clear track photographs on all tracks, The plane of the lens is not perpendicular to the optical axis of the camera. It is tilted in such a way that all alpha tracks in the chamber is sharply focussed.

The following was Blackett's instrument set-up, and the resulting improved quality of alpha tracks:

The double cameras were mounted 90 degrees from each other. The chamber itself, at the "junction" of the camera lens tube was covered by a wooden black-box so a completely dark room was not necessary. The shorter silvery tube below the lens tube is a mercury arc lamp, which acts as a flash whenever the cameras are taking photos. You can clearly see the gas manifold too, showing the inlet control valves of three gas mixtures.

Using the double camera set-up with tilted lens, choice of radioactive source, and appropriate gas mixture in the cloud chamber; neat photographs of alpha tracks can be obtained. All tracks are pencil thin and crisp clear.

The alpha tracks are so fine, the limiting resolution now turns to the grain of the photographic plate. Here you can see two tracks are resolved at less than 0.1 mm physical separation. This is a negative photograph, so the tracks appears black. 

REFERENCE

P.M.S. Blackett, "On the design and use of a double camera for photographing artificial disintegrations", Proc. Roy. Soc. A, Vol. 123, 792 (1929)