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For $V(r)=A{e}^{-\kappa r}/r$, with real $A$ and $\kappa >0$, the first Born approximation gives the scattering amplitudeIndeed, orienting the polar axis along the momentum transfer gives the angular integral $4\pi \sin (qr)/(qr)$, and the remaining integral is ${\int }_0^{\infty }{e}^{-\kappa r}\sin (qr)dr=q/(q^2+{\kappa }^2)$. The differential scattering cross-section is $|f_B(q){|}^2$.

The limit $\kappa \to 0^{+}$ at fixed nonzero $q$ produces the Rutherford differential cross-section for a Coulomb potential. The unscreened interaction has long range, so this limiting calculation by itself does not establish the validity of a naive Born approximation to the full Rutherford scattering state. The exact Rutherford scattering amplitude has an additional phase, while its modulus agrees with the Rutherford differential cross-section.

The later angular test used an evacuated chamber with a radon source and scattering foil fixed in place. A diaphragm served as a collimator; a zinc sulfide screen and attached optical microscope rotated about the foil on a graduated platform. Particle scintillation counts sampled successive angles. Foil-out counts supplied background subtraction, with corrections for source radioactive decay and finite beam width. Shielding reduced interfering radiation, but screen glow limited large angles. This tested the Rutherford differential cross-section.^[ref]

For elastic scattering by the Yukawa potential, take $\kappa >0$ and use the first Born approximation in the plane-wave normalizationChoose the polar axis along the momentum transfer $\hslash \mathbf{q}$. The angular integral isThe radial volume element cancels the remaining powers of $r$, leavingFor real $A$, the differential scattering cross-section is thereforeThis also has a finite forward limit when $\kappa >0$. The Born amplitude for a Yukawa potential provides a screened route to the Coulomb potential by taking $\kappa \to 0^{+}$ at fixed $q>0$.

For an alpha particle of charge $2e$ scattering from a heavy nucleus of charge $Ze$, set $A=2Z{e}^2/(4\pi {ϵ}_0)$. Neglect recoil, so that the reduced mass is approximately the projectile mass, $E={\hslash }^2{k}^2/(2m)$, and elastic geometry givesThe Coulomb-limit scattering amplitude becomeswhich gives the Rutherford differential cross-sectionThe unscreened Coulomb potential has long range, so taking this limit does not justify the Born approximation for its full scattering state, particularly for a strongly charged target. The exact Rutherford scattering amplitude differs by a phase and has the same modulus. Thus the resulting differential scattering cross-section remains applicable to the ideal nonrelativistic Coulomb problem beyond the weak-potential regime.

To relate the detector counts to the atomic number, first compute the number of target atoms per illuminated area. With molar mass $M=0.197\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$, the number density integrated through the foil isThe detector's transverse area is already its area normal to the scattered ray. Its small solid angle isThere is no extra $\cos 45^{\circ }$ factor: the foil is traversed normally and the detector area is transverse to its line of sight.

For a thin foil with negligible multiple scattering and energy loss, the detected fraction is $f=\mathcal{N}(d\sigma /d\Omega )\Delta \Omega$. Thus the measurements implyUsing $E=7.68\times 10^{6}e\mathrm{J}=1.23047\times 10^{-12}\mathrm{J}$ in the Rutherford differential cross-section gives

The stated measurements do not reproduce gold's atomic number 79 in this model. The values and units above agree with the original PDF. Substituting $Z=79$ instead would predict $f\simeq 3.17\times 10^{-7}$ with the other inputs unchanged, rather than $3.7\times 10^{-7}$. No stated correction accounts for that discrepancy; forcing the answer to 79 would require changing the data or adding an experimental correction not specified in the problem.

Closest approach. In the heavy-target approximation, a head-on alpha particle of charge $2e$ converts its initial kinetic energy to repulsive Coulomb potential energy at its turning point. The nuclear radius bound from a Coulomb turning point is thereforeThus the point-projectile interpretation of the radius bound isIt is an upper limit from Coulomb scattering, not a measurement of the actual nuclear radius. If “size” denotes diameter, the associated bound is twice this value. Including nuclear recoil, $E_{\mathrm{cm}}\simeq [207/(207+4)]E_{\alpha }$, increases the closest separation by about two per cent to $4.4\times 10^{-14}\mathrm{m}$.

Count-rate estimate and normalization. The graph has approximate slope $\Delta \ln S/\Delta \ln A\simeq 15/10=3/2$. It therefore supplies the empirical log-log plot relation $S\propto A^{3/2}$. Calibrating its normalization with the lead observation givesThe Rutherford differential cross-section at fixed beam energy and scattering angle instead has a $Z^2$ dependence per target nucleus. More generally, the target normalization in Rutherford scattering rates iswhere $\mathcal{N}$ is the target nuclei per unit area. If the foils have equal $\mathcal{N}$, the theoretical estimate is $19.7$, or about $20$ particles per minute, close to the graphical estimate at its rough level. Equal foil mass per area would instead give $37.8$ per minute because $\mathcal{N}\propto 1/A$.

The source does not specify that target normalization. Thus $23$ per minute is the estimate from its supplied graph; an exact rate prediction from Rutherford theory alone is underdetermined without comparable foil areal densities. The empirical $A^{3/2}$ slope and an exact equal-$\mathcal{N}$ $Z^2$ law should not be presented as identical assumptions.

The Rutherford model places a concentrated central electric charge inside a much larger chemical atom. In 1911, Ernest Rutherford argued that rare large deflections were predominantly single close encounters, rather than accumulated small deviations in a diffuse-charge atom. He treated the central electric charge as positive for calculation while noting that the scattering deductions did not establish its sign. He explicitly deferred atomic stability.^[ref]

The physical inference is a compact region generating a strong electric field, not a miniature hard wall. Most impact parameters miss that region, while a small subset give large deflections. The model supplies a way to infer interior structure from outgoing angles, a central use of scattering throughout atomic physics and nuclear physics. This inference does not by itself specify Electron quantum states or explain spectral lines.

The repulsive model also sets a probing scale. At closest approach, conservation of energy with zero radial velocity givesFor the numerical model above, even a head-on projectile turns at $r_{\min }=2a=45.504\mathrm{fm}$. This is a calculated closest-approach distance, not a measured atomic nucleus radius. A point-charge fit can constrain how compact the scattering centre must be, but cannot directly resolve structure at distances the projectiles never reach.

The Rutherford differential cross-section must not be extrapolated to zero angle: the unscreened point model has a forward divergence, while real neutral chemical atoms exhibit the screening effect, finite beam acceptance, and accumulated small deflections. At very short separation, finite nuclear size and the nuclear force can change the interaction. Nuclear recoil changes the relation between laboratory and centre-of-mass angles for lighter targets; appreciable energy loss or repeated large deflections invalidates the simple thin-foil scattering counts. These limits delimit the calculation rather than supplying evidence for a nuclear contact collision in the early experiment.

At fixed incident flux, projectile energy and detector geometry, the Rutherford differential cross-section gives count rate $S\propto \mathcal{N}{Z}^2$, where $\mathcal{N}$ is the target nuclei per unit area. Equal $\mathcal{N}$ gives a $Z^2$ rate ratio. Equal foil mass per area instead gives $Z^2/A$, with $A$ the atomic mass number, while equal physical thickness also requires the material mass density. An empirical log-log plot with slope $s$ supplies a different, experimentally normalized relation $S\propto A^s$. These normalizations must be specified before comparing different target elements.