[Physics] Will osmium or lead stop all high-energy photons in a shorter distance

Question

Consider the following scenario: there is a magic photon source that emits photons in both directions along a single axis. In one direction is an infinite amount of lead, in the other direction is an infinite amount of osmium. The emitted photons pass directly into these metals and do not interact with anything else, nor are any other forms of radiation present. The energies of these photons range from high ultraviolet to the most energetic gamma ray known to be possible, with every amount of energy being equally represented. The photons are emitted at sufficient intervals that any increase in temperature caused by a photon will dissipate before the next photon is emitted (though the base temperature is unspecified).

In this situation, will the photons, and any secondary energized particles, travel farther through the lead or the osmium?

The description of high-energy photon flux attenuation that I am familiar with says that a high-energy photon passing through conventional matter will lose energy by interaction with atomic nuclei that it passes, with more massive nuclei reducing the photon's energy by a greater amount. Thus lead, being the stable element with the highest atomic number, seems like an obvious choice for shielding against this type of radiation. The question arises because osmium has a close atomic number to lead but is much denser, meaning that a photon will interact with more atomic nuclei in the same distance. In a purely hypothetical sense, this might make osmium a superior shield.

Additionally, if any element or compound will stop these photons more effectively than either lead or osmium, I would like to know about that.

Answer 1

I tried answering this by going to the XCOM database where you can get a calculation of the stopping power of elements and compounds.

First - pick a few likely candidates. I found a table of elements with density which is a good place to start. The highest density elements are also among the highest Z ones:

                               proton
density  name        Z   m    density
-------+----------+---+-----+---------
15.4     Thorium    90  232    5.97
16.65    Tantalum   73  181    6.72
18.95    Neptunium  93  237    7.44
19.32    Gold       79  197    7.75
19.35    Tungsten   74  183    7.82
19.84    Americium  95  243    7.76
20.2     Uranium    92  235    7.91
21.04    Rhenium    75  185    8.53
21.45    Platinum   78  195    8.58
22.4     Iridium    77  191    9.03
22.6     Osmium     76  187    9.19

I got the first three columns from the above link, then added the atomic mass of the most stable (or one of the stable) isotopes, and used these to estimate proton density (from which electron density follows). Units of density and proton density are g/cm³.

I decided to plot the XCOM output for four elements: Pb, Os, Ir and U. You could repeat for others - but this gives a general direction. The result (log-log scale):

As you can see, lead is much worse than the other three - and the other three are almost indistinguishable at high energies, but have a little bit of difference at lower energies (where the binding energy of the electrons is comparable with the energy of the incoming radiation, and photoelectric effects dominate).

Whether Uranium or Osmium/Iridium "win" depends on the energy distribution of your "all energies" input beam - but this approach should give you an idea on how to tackle the question. Clearly, lead is worse than either of these three for most energies (it's better than Uranium just below Uranium's K edge...).