In regards the secondary question, namely
Am I missing something here?
Yes; absolutely. You have neglected both the shielding effect of the 65 miles of air between Las Vegas and the bomb explosions, and the fact that the actual radiation intensity falls off as the square of the distance.
The study The Children of Atomic Bomb Survivors: A Genetic Study. by National Research Council (US); National Academy of Sciences (US); Neel JV, Schull WJ, editors. notes for Hiroshima:
On the other hand, both from these two curves and on the basis of our own observations regarding the shape of the distance-dosage curve, it seems likely that persons at distances in excess of 3,000 meters received little if any radiation. [right above Figure 4.3]
Air Shielding Attenuation
Consider the air-shielding effect first. To a first approximation the shielding material is irrelevant, except so far as different materials have different densities.
The shielding effect can be approximated by calculating the material density times the thickness. For Hiroshima survivors beyond 3,000 metres I will assume:
3000 metres of air at 1.2754 kg/m³ = 3,826 kg/m²
30 metres (1%) of pine at 420 kg/m³ = 12,600 kg/m²
total shielding effect = 16,400 kg/m²
For Las Vegas Bomb Tourists assume:
- 65 miles * 1600 m/mile at 1.2754 kg/m³ = 248,700 kg/m²; roughly 15 times that of Hiroshima survivors just outside the 3,000 metres "no significant radiation" suffered radius.
At a distance of 65 miles = 104,000 metres, roughly 34.7 times as far from the blast centre as the Hiroshima survivors beyond 3,000 metres, the radiation reaching Las Vegas *Bomb Tourists has been distance-attenuated 34.7² = 1200 times as much as for the Hiroshima survivors outside the 3,000 metres radius.
Total Attenuation (of Direct Radiation)
While the precise calculation is undoubtedly more complex, certainly both attenuation effects must be regarded as combining. This means that the direct radiation experienced by Las Vegas Bomb Tourists will have been somewhere in between 1/1200 = 0.08% and 1/(1200*15) = 0.0005% that experienced by Hiroshima survivors outside the 3,000 metres "experienced no significant radiation" radius.
Yes, this analysis ignores the effects of radioactive fallout - that calculation being so intrinsically dependent on meteorological particularities of each blast. However I hope the above makes clear that the distance of Las Vegas from the test site was far more than sufficient to protect occasional visitors. Permanent residents might be another story - but that's a different question.
From comments below, and expanded
The largest nuclear device ever detonated was the Russian Tsar Bomba, at 50 Mega Tons (and it was s research device, not an intended weapon). The largest device ever designed and detonated by the U.S. was the MK-41 at 25 Mega Tons: about 1600 times the Hiroshima bomb (15 Kilo Tons of TNT).
For back-of-the-envelope considerations, even the 1600 times yield increase from Hiroshima is much smaller than the 1200 * 15 = 18,000 times attenuation provided by sheer distance from the bomb blast.
However all such devices are thermonuclear, not pure atomic, devices with a second fusion stage triggered by a primary fission stage (and possibly also incorporating a third, fission) stage.
As thermonuclear weapons represent the most efficient design for weapon energy yield in weapons with yields above 50 kilotons of TNT (210 TJ), virtually all the nuclear weapons of this size deployed by the five nuclear-weapon states under the Non-Proliferation Treaty today are thermonuclear weapons using the Teller–Ulam design.
From here any discussion of radiation (and fallout) profile is very specific for each particular bomb design - by intent. One must distinguish between the different effects and penetrating power of all of:
Alpha particles - high energy helium nuclei;
Beta particles - high energy electrons (and possibly positrons);
Gamma rays - high energy photons;
High (and low!) energy neutrons; and
Radioactive fallout such as strontium-90 (a long-term environmental hazard only unless one is immersed directly in the speed of weather only disbursed radioactive cloud, and even that is constrained to spread roughly as the square of distance traveled).
However energy is energy; it spreads outwards from an epicentre; and the inherently geometric effects described above cannot be avoided.