Rutherford's gold foil experiment thin sheets

Question

I am struggling to understand why Rutherford needed to use extremely thin sheets in this experiment. I understand that gold was used, as it was relativley easy to achieve these thin sheets, but why do the sheets need to be thin?

Answer 1

The sheets were thin to maximize the amount of forward scattered signal. As AChem explains, even air would provide a barrier to subatomic particles (thanks @MattBlack for pointing out the glaring oversight), so it was necessary in addition to work in vacuo. The thinner the sheet, the greater the number of alpha particles that would scatter through the material and be detected. In addition, a thin sheet would reduce the probability of interaction of a alpha particle with multiple atoms, simplifying interpretation of the scattering pattern.

In any case, it is easy a posteriori to justify the design of an experiment and assume it arose as Athena did from Zeus' head, but in practice it is usually an iterative process involving trial and a lot of error. The final design is a result of necessity, based on available tools and materials, and of history and accident as more is learned about the system in question and features are tweaked. There is some analogy in the process of natural selection, except the iterative filter employs human reason. The word serendipity is also commonly used, although this refers to the plain luck in which one observes the unexpected. That would also have played a part. For instance, some alpha particles back-scattered. That was unexpected according to some atomic models of the time (such as the plum pudding model) and would have been simpler to interpret for a thin sheet (altogether eliminating the possibility of multiple scattering).

Finally, don't assume that something was not attempted simply because a textbook does not mention it. Certainly experiments were performed with thicker foils, and they should have been consistent with the principles gathered from the neater thin foil tests.

The following is an excerpt from a nice brief summary of the story of this discovery on the APS website:

In 1907 Rutherford returned to England, to the University of Manchester. In 1909, he and his colleague Hans Geiger were looking for a research project for a student, Ernest Marsden. Rutherford had already been studying the scattering of alpha particles off a gold target, carefully measuring the small forward angles through which most of the particles scattered. Rutherford, who didn’t want to neglect any angle of an experiment, no matter how unpromising, suggested Marsden look to see if any alpha particles actually scattered backwards. Marsden was not expected to find anything, but nonetheless he dutifully and carefully carried out the experiment. He later wrote that he felt it was a sort of test of his experimental skills. The experiment involved firing alpha particles from a radioactive source at a thin gold foil. Any scattered particles would hit a screen coated with zinc sulfide, which scintillates when hit with charged particles. Marsden was to sit in the darkened room, wait for his eyes to adjust to the darkness, and then patiently stare at the screen, expecting to see nothing at all

Answer 2

The main reason is that Rutherford and Geiger were both interested in exploring the interactions of alpha particles with thin sheets of matter. Mainly, they were rather fascinated by the causes of their deflection. It was known that alpha particles could be stopped by air in relatively short distance. Studying thin sheets of different metals (not only gold) and their interactions with radiation was a hot topic in early 1900s. Those physicists were developing calculations for scattering.

Almost all papers of Rutherford are available freely to the public. Use Google Scholar to read the original works.

Rutherford, Ernest. "LXXIX. The scattering of α and β particles by matter and the structure of the atom." The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 21.125 (1911): 669-688.

The scattering data for the β rays, as well as for the α rays indicate that the central charge in an atom is approximately proportional to its atomic weight. This falls in with the experimental deductions of Schmidt. In his theory of absorption of β rays, he supposed that in traversing a thin sheet of matter, a small fraction α of the particles are stopped, and a small fraction β are reflected or scattered back in the direction of incidence. From comparison of the absorption curves of different elements, he deduced that the value of the constant β for different elements is proportional to nA$^2$ where n is the number of atoms per unit volume and A the atomic weight of the element. This is exactly the relation to be expected on the theory of single scattering if the central charge on an atom is proportional to its atomic weight.

Note that in older times, atomic mass was called atomic weight.