How did Rutherford arrive at gold foil as the best choice for his particle experiments?

Submitted by Poltibolsa t3_zzjf23 in askscience

I've been trying to find readings that track the progression of his thought and his experiments, why did he use this substance and not that substance, why gold foil (is it because gold is so malleable but so is lead and lead is much more dense....any thoughts or suggestions for readins. I am also very interested in learning the exact mechanics of the transition from alchemy to modern science.

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sgarn t1_j2cqvwm wrote

>is it because gold is so malleable but so is lead and lead is much more dense

Gold is much more dense than lead, at 19.3 vs 11.3 g/cm³, but the density of the metal isn't really a direct factor in Rutherford scattering in contrast to the atomic mass (more directly the atomic number, but this was prior to the discovery of the neutron) and atomic density. With gold being very close in atomic number to lead, and gold being more malleable and non-reactive than lead, and with gold foil more ubiquitous than lead foil, and with gold being more atomically dense than lead, there wouldn't have been an advantage to using lead over gold.

From Geiger's 1910 paper on the early experiments:

"Gold appeared to be the most suitable substance for such comparative measurements, since it can be obtained in very thin and uniform foils, and in addition its scattering power is higher than that of any other material available."

"Since thin and uniform sheets could only be obtained for gold, tin, silver, copper, and aluminium, the experiments were confined to these metals."

These and later experiments by Geiger and Marsden under the supervision of Rutherford ultimately showed that scattering per atom was proportional to the square of the atomic weight. That is, other materials were observed, but since gold was far more suitable and lead was very close in atomic weight to gold, it would not have yielded much more insight compared to lighter and more malleable materials. It's important to note that thin sheets and uniform thickness were important because the scattering was also observed as a function of the number of sheets used.

In fact, it appears that [lead was used in the early experiments] (https://royalsocietypublishing.org/doi/10.1098/rspa.1909.0054) but it was less effective than gold - this was speculated to be due to impurities in the lead but I believe atomic density would have played a role here, possibly also the relative difficulties in producing thin sheets of lead.

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adam12349 t1_j2domty wrote

One of the important things about these scattering experiments is the thickness of your target object. If the target is thin the interaction can be treated as individual scatterings on the atoms of your target object. If the object is thick the interaction at the front of the object effect the interaction at the back, so your scattering is no longer the sum of individual scatterings.

Knowing the crossection of the experiment you can calculate a distance that the particles of the oncoming beam can freely travel in your target. Which means that a collision isn't guaranteed. A thin target would let a lot of the particles through without interaction but some of them would end up colliding and those can be treated as independent collisions. If the target is thicker than that distance almost all particles of the beam would collide statistically even those that scattered at the front of the object scattering again in the target.

So you want your target to be as thin as possible and gold foil can be really thin like a few atoms thin. Which is perfect for scattering experiments but of course there are other options too.

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LoneGiggity t1_j2e2ok0 wrote

Replying to adam12349 (#1,225,758)

Is this also the reason why NEO radiation shielding is gold thin? From these experiments? I vaguely remember being taught that thick shielding is the exact opposite of what is needed in module shielding for near earth and lunar orbits. Its information 30 plus years old that randomly sits in my brain.

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VerrKol t1_j2edj0i wrote

Replying to LoneGiggity (#1,230,529)

Shielding effectiveness always increases with thickness. Thin/low weight shields are more desirable from a cost perspective.

Orbits where electron contributions dominate dose are more easily shielded which may be what you are remembering

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Putrid-Repeat t1_j2eiq4h wrote

I'll add since no one said this but gold does not oxidize. To get a metallic sheet so this and at this time it must be hammered thin. You can get gold so thin is transparent.

Being so thin the something made of really almost any other metal would be very oxidized.

Another difficulty is that most other metals work harden or are just much harder, for example platinum which does not oxidize well either does not forge into sheets like this easily.

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magibug t1_j2emajn wrote

gold is so malleable that with just a hammer and a lot of determination, you can make a sheet of pure gold which is only ~ 3 atoms thick!

(when this sheet was irradiated, Rutherford's team observed that >>99% of the electrons went right through the sheet like a too large sieve. and the percentage of reflected back/passing through is the size of the nucleus vs the atom and that the atom itself is mostly empty)

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enderjaca t1_j2emudj wrote

Replying to sgarn (#1,216,659)

>with gold foil more ubiquitous than lead foil

Just to clarify, that doesn't necessarily mean that gold is found more commonly than lead, it's that gold foil was relatively easy to make compared to lead foil? Is that still true today?

I recall selling and using adhesive lead foil for adjusting the weight and balance of tennis racquets, for example. Most modern commercial uses of gold foil I've noticed seem to be related to artistic projects or embellishing food at fancy restaurants.

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a_wild_redditor t1_j2eojab wrote

Replying to enderjaca (#1,237,519)

Gold leaf is readily available very thin, it can be well under 1μm. The lead sheet or foil I'm familiar with is much thicker since common uses require its density (counterweighting as you mentioned), its radiation shielding properties, and/or its easy formability by hand while holding its shape afterwards.

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LittleLostDoll t1_j2f3ufs wrote

Replying to enderjaca (#1,237,519)

if yoiu up lead baloon on youtube adam savage talks about when they wanted to see if they could get a lead baloon to float they were having trouble getting the lead thin enough, and two companies broke their machines even try

this ws about 15-20 years ago but id think even today it isnt much thinner than they were able to get back then

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Kraz_I t1_j2f8byk wrote

Replying to enderjaca (#1,237,519)

Gold is the most ductile and malleable of all pure metals (and probably alloys too, since they tend to be harder and more brittle). This is why it's used in computers and advanced electronics, even though silver and copper are more conductive. A very small amount goes a long way, it can still hold together at under 1 micron thickness.

If you tried to hammer lead into a foil that thin at ambient temperature, (which is the traditional way gold foil is made), it would crumble to bits.

Even in antiquity, artisans knew how to make very thin gold foil by hammering it thin, then folding it with paper between the layers to prevent cold welding, and continuing the process, with the number of layers doubling each time.

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Mountebank t1_j2flzjf wrote

Replying to Glaselar (#1,218,740)

I can’t believe how they’re using their bare hands when moving the stack of gold foil in that jackhammer press. Does Japan not have an equivalent to OSHA? Or does that not apply to small businesses like the one in the video?

For comparison, my workplace has a similar machine, but you need to press two buttons with both hands to start it and there’s a screen of lasers between the user and the press that shuts the machine down if the lasers are blocked.

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