Unit 1: The Discovery of Radioactivity and Evolution of Atomic Theory

The dawn of modern nuclear science began not with deliberate theoretical planning, but through the rigorous investigation of anomalous luminescent phenomena. In early 1896, following Wilhelm Röntgen's discovery of penetrating X-rays emitted by cathode-ray vacuum discharge tubes, the French physicist Henri Becquerel hypothesized that luminescent salts might emit similar penetrating radiations upon exposure to solar illumination.

Becquerel's Serendipitous Investigation

Becquerel selected potassium uranyl sulfate ($\text{K}_2\text{UO}_2(\text{SO}_4)_2 \cdot 2\text{H}_2\text{O}$), a known phosphorescent compound. Placing crystalline specimens atop photographic emulsion plates wrapped in thick black paper (to shield optical photons) alongside copper silhouettes, Becquerel initially observed weak photographic fogging after solar exposure. In late February 1896, overcast Paris skies halted the solar experiments; Becquerel stored the wrapped plates in a dark cabinet with the uranium salt specimens resting directly on them.

Upon developing the plates on March 1, 1896, anticipating faint or absent images, Becquerel observed intense, sharp silhouettes of the copper objects. Crucially:

1. The emission of penetrating rays was entirely independent of external luminescent excitation (optical or thermal).
2. The radiation persisted indefinitely in total darkness, exhibiting no measurable attenuation over months.
3. The rays spontaneously ionized air, causing charged gold-leaf electroscopes to discharge at a rate proportional to the uranium mass.

Becquerel demonstrated that the emission was an inherent property of uranium itself, independent of whether it existed in elemental form, as uranyl nitrate, or as double sulfates.

Marie and Pierre Curie: The Atomic Hypothesis and Polonium/Radium Isolation

In 1897, Marie Skłodowska-Curie initiated a systematic quantitative survey of all known elements and mineral ores to determine whether elements other than uranium possessed spontaneous ionizing emissions. Employing a sensitive piezoelectric quartz electrometer designed by Pierre and Jacques Curie, she measured saturation ionization currents in air.

Ionizing Radiation
        │
        ▼
   Ion Pairs Created in Air (e⁻ + Ar⁺)
        │
        ▼
   Electric Potential Gradient (Parallel Plate Chamber)
        │
        ▼
   Saturation Ionization Current I = dq/dt
        │
        ▼
   Piezoelectric Quartz Electrometer Null-Balance Metrology

Marie Curie established that:

• Thorium compounds also spontaneously emitted penetrating ionizing rays, confirming that uranium was not unique. She coined the term radioactivity (*radio-activité*) to describe this atomic property.
• Natural uranium ores—specifically Joachimsthal pitchblende ($\text{U}_3\text{O}_8$) and chalcolite—exhibited specific activities 4 to 5 times greater than pure metallic uranium.

Synthesizing artificial chalcolite ($\text{Cu}(\text{UO}_2)_2(\text{PO}_4)_2 \cdot 8\text{H}_2\text{O}$) from pure laboratory salts, Marie Curie discovered it exhibited only normal uranium activity. This proved that pitchblende contained trace quantities of an unknown element far more radioactive than uranium.

Through arduous fractional crystallization and wet-chemical inorganic separations (precipitating bismuth-sulfide fractions followed by fractional sublimation), Marie and Pierre Curie announced the discovery of:

1. Polonium ($^{210}\text{Po}$, named after Marie Curie's native Poland) in July 1898, precipitating alongside bismuth sulfide.
2. Radium ($^{226}\text{Ra}$) in December 1898, separating alongside barium chloride via fractional crystallization of the chlorides, exploiting the lower solubility of radium chloride in boiling water and hydrochloric acid.

The Curies demonstrated that radioactivity is an intrinsic atomic phenomenon, unaffected by chemical bonding, valence state, temperature (from liquid hydrogen to blast furnaces), pressure, or intense magnetic fields.

Prior to 1911, the prevailing paradigm of atomic architecture was J.J. Thomson's "plum pudding" model (1904). In Thomson's conception, an atom of radius $R \approx 10^{-10}\text{ m}$ consisted of a continuous, diffuse sphere of positive electrostatic charge containing $Z$ negatively charged corpuscles (electrons) embedded like raisins in a dough. Because the positive charge was dispersed uniformly throughout the atomic volume, the internal electric field was weak, incapable of exerting massive Coulomb deflections on high-energy charged projectiles.

The Geiger-Marsden Scattering Experiment

At the University of Manchester, Ernest Rutherford directed Hans Geiger and Ernest Marsden to direct collimated alpha particles ($^{4}\text{He}^{2+}$ with kinetic energy $T_\alpha \approx 5.5\text{ MeV}$) from a bismuth-214 ($^{214}\text{Bi}$) source against ultra-thin gold foils ($\text{Au}$, thickness $t \approx 400\text{ nm} \approx 2000\text{ atomic layers}$). Scintillations produced by scattered alphas were counted visually via a microscope on a zinc sulfide ($\text{ZnS}$) phosphorescent screen in a darkened chamber.

Under Thomson's model, the maximum scattering angle $\theta$ suffered by an alpha particle traversing an entire gold atom was calculated via classical impulse theory:

Multiple independent stochastic deflections across 2000 atomic layers should follow a narrow Gaussian distribution, with probability of deflections exceeding $90^\circ$ bounded below $10^{-3500}$.

Astonishingly, Geiger and Marsden discovered that approximately 1 in 8,000 alpha particles suffered deflections greater than $90^\circ$, with some rebounding almost backward ($\theta \approx 180^\circ$). As Rutherford famously remarked: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Rutherford Scattering Differential Cross-Section Derivation

To explain the wide-angle deflections, Rutherford posited in 1911 that the entire positive charge $+Z e$ and virtually the total atomic mass $M$ are concentrated in an ultra-dense central core: the atomic nucleus, with radius $R_{\text{nuc}} \le 10^{-14}\text{ m}$.

Consider an alpha particle of mass $m$, charge $q_1 = +2e$, and incident velocity $v_0$ approaching a stationary nucleus of charge $q_2 = +Ze$ at an impact parameter $b$. Because the nucleus is far heavier than the alpha particle ($M_{\text{Au}} \approx 197 \gg m_\alpha \approx 4$), the center of mass frame coincides with the lab frame to high precision.

       Alpha (2e)
       ───────►              Impact parameter b
                             │
                             ▼
  - - - - - - - - - - - - - - - - - - - - - - - - - - 
                           \   θ (Scattering Angle)
                            \
                             Nucleus (+Ze)

The repulsive electrostatic potential is central:

1. Conservation of Angular Momentum:

1. Conservation of Linear Momentum (along axis of symmetry):

Let the angle between the position vector $\vec{r}$ and the apse of the hyperbolic orbit be $\phi$. The total transverse momentum transfer $\Delta p$ satisfies:

Substituting $dt/d\phi = r^2 / (v_0 b)$:

Dividing both sides by $2 \sin(\theta/2)$:

where $T = \frac{1}{2} m v_0^2$ is the projectile kinetic energy.

1. Differential Cross-Section:

The incident particles passing through an annular ring of area $d\sigma = 2\pi b |db|$ are scattered into a solid angle $d\Omega = 2\pi \sin\theta d\theta$:

Differentiating $b$ with respect to $\theta$:

Using $\sin\theta = 2 \sin(\theta/2) \cos(\theta/2)$ and substituting:

Yielding the foundational Rutherford Scattering Formula:

Distance of Closest Approach (Head-On Collision)

For a head-on collision ($\theta = 180^\circ$, impact parameter $b = 0$), the entire kinetic energy $T$ transforms into electrostatic potential energy at the distance of closest approach $d_0$:

For a $7.7\text{ MeV}$ alpha particle colliding with gold ($Z = 79$):

Because Rutherford scattering followed the pure $1/\sin^4(\theta/2)$ law up to this energy, the radius of the gold nucleus had to be strictly smaller than $30\text{ fm}$—over $10,000$ times smaller than the total atomic radius ($100,000\text{ fm}$). The atom is mostly empty space.

Rutherford's nuclear planetary model suffered a fatal classical instability: according to Maxwellian electrodynamics, an accelerating electron on a circular orbit must continuously radiate electromagnetic energy at the Larmor rate:

This continuous radiative loss would cause the electron to spiral into the nucleus within $\tau \sim 10^{-11}\text{ seconds}$, predicting an instantaneous collapse of all matter and a continuous emission spectrum.

The Bohr Postulates (1913)

Niels Bohr resolved this crisis by introducing quantum hypotheses:

1. Stationary States: Electrons occupy discrete circular orbits characterized by non-radiating stationary energy states.
2. Quantization of Orbital Angular Momentum:

1. Bohr Frequency Condition: Emission or absorption occurs only during discontinuous transitions between stationary states:

Equating the centripetal force to the Coulomb electrostatic attraction:

Substituting velocity into the orbital radius expression:

where $a_0 = \frac{4\pi\varepsilon_0 \hbar^2}{m_e e^2} \approx 0.529177 \times 10^{-10}\text{ m} = 0.529\text{ \AA}$ is the first Bohr radius of hydrogen.

The total mechanical energy in state $n$ is:

where the Rydberg constant for an infinitely massive nucleus is:

Moseley's Law and the Physical Meaning of Atomic Number Z (1913-1914)

Prior to 1913, elements were arranged in Dmitri Mendeleev's periodic table in order of increasing atomic weight $A$. This introduced notable anomalies:

• Argon ($A = 39.95$) preceded Potassium ($A = 39.10$).
• Tellurium ($A = 127.60$) preceded Iodine ($A = 126.90$).
• Cobalt ($A = 58.93$) preceded Nickel ($A = 58.69$).

Henry Moseley systematically bombarded 38 elements from aluminum to gold with high-energy electron beams in a vacuum tube and recorded their characteristic X-ray emission spectra via Bragg crystal spectrometry ($n\lambda = 2d \sin\theta$).

Moseley discovered that the frequency $\nu$ of the characteristic $K_\alpha$ and $L_\alpha$ lines satisfied a remarkably precise linear relationship with the integer position index $Z$ of the element:

where $a$ is a proportionality constant and $\sigma$ is an electrostatic screening (shielding) constant ($\sigma \approx 1$ for $K_\alpha$ transitions; $\sigma \approx 7.4$ for $L_\alpha$ transitions).

   √ν (Square Root of X-Ray Frequency)
     ▲
     │           /  K_α Line: √ν = C(Z - 1)
     │          /
     │         /
     │        /
     │       /
     │      /
     │     /
     │    /
     │   /
     └──┴────────────────────────► Atomic Number Z
        1  2  3  4  5 ...

For the $K_\alpha$ transition, an electron drops from the $n = 2$ ($L$-shell) to $n = 1$ ($K$-shell). Because one electron remains in the $1s$ orbital, the effective nuclear charge experienced by the transitioning electron is screened to $Z_{\text{eff}} = Z - 1$:

Taking the square root:

Moseley's law proved unequivocally that:

1. Atomic Number $Z$ is not an arbitrary sorting index, but the exact fundamental integer number of positive elemental charges $+e$ in the nucleus.
2. The inversions in the periodic table (Ar/K, Co/Ni, Te/I) were resolved because $Z$ was monotonically increasing even when atomic weight did not.
3. Moseley identified missing elements with precision, predicting the existence of unknown elements at $Z = 43$ (Technetium), $Z = 61$ (Promethium), $Z = 72$ (Hafnium), and $Z = 75$ (Rhenium).

Following the discovery that the nucleus possessed positive charge $+Ze$ while having an atomic mass $A$ approximately twice $Z$, physicists initially hypothesized the proton-electron model of the nucleus. In this view, a nucleus with mass number $A$ and atomic number $Z$ consisted of $A$ protons and $A - Z$ "nuclear electrons" bound tightly inside the core.

Fatal Flaws of the Proton-Electron Nuclear Hypothesis

By the late 1920s, quantum mechanics demonstrated that the proton-electron model was physically untenable:

1. Heisenberg Uncertainty Principle:

If an electron is confined inside a nuclear volume of diameter $\Delta x \approx 2 R \approx 10^{-14}\text{ m}$, its momentum uncertainty must satisfy:

For a relativistic electron ($E \approx p c$):

Electrons emitted in beta decay possessed kinetic energies of only $\sim 0.1\text{ to }3\text{ MeV}$, and no known nuclear potential well was deep enough to confine an electron with $>10\text{ MeV}$ zero-point energy.

1. Nuclear Spin Paradox:

Consider the nitrogen-14 nucleus ($^{14}\text{N}$, $Z = 7, A = 14$). Under the proton-electron model, it must contain 14 protons and 7 electrons, totaling $21$ spin-$1/2$ fermions. An odd number of fermions must possess a half-integer total spin:

However, molecular band spectra of $\text{N}_2$ (Rasetti, 1929) proved definitively that $^{14}\text{N}$ has an integer spin $I = 1$ and obeys Bose-Einstein statistics.

1. Nuclear Magnetic Moments:

The electron possesses a Dirac magnetic moment:

If electrons existed inside the nucleus, nuclear magnetic moments should be on the order of $\mu_B$. In reality, measured nuclear magnetic moments were $\sim 1000$ times smaller, scaling with the nuclear magneton:

The Experimental Sequence to the Neutron

In 1930, Walther Bothe and Herbert Becker bombarded light elements ($\text{Be}, \text{B}, \text{Li}$) with energetic alpha particles from a polonium source. Beryllium emitted an extraordinarily penetrating, electrically neutral radiation capable of traversing several centimeters of lead:

Bothe and Becker assumed this radiation was ultra-hard bremsstrahlung or high-energy gamma rays ($h\nu \sim 10\text{ MeV}$).

In 1932, Irène Joliot-Curie and Frédéric Joliot placed paraffin wax ($\text{C}_n\text{H}_{2n+2}$, rich in hydrogen atoms) in the path of the radiation. They observed that the mysterious neutral rays ejected high-energy recoil protons from the paraffin with kinetic energies up to $T_p \approx 5.7\text{ MeV}$. Assuming the rays were gamma photons ($m = 0$) undergoing Compton-like scattering with protons:

A $55\text{ MeV}$ photon could not be produced by a $5.3\text{ MeV}$ alpha reaction due to energy conservation limits.

James Chadwick's Breakthrough (1932)

James Chadwick repeated the experiment using an ionization chamber connected to an oscilloscope, measuring recoil velocities not only from paraffin (protons, $A = 1$) but also from helium ($A = 4$), lithium ($A = 7$), beryllium ($A = 9$), carbon ($A = 12$), and nitrogen ($A = 14$).

Chadwick modeled the interaction as a classical, non-relativistic elastic head-on collision between a neutral particle of mass $m_n$ and velocity $v_n$ and target nuclei of mass $M$ at rest. By conservation of momentum and energy:

Solving for the recoil velocity $V$:

Chadwick measured the maximum recoil velocity of protons ($V_p$) and nitrogen nuclei ($V_N$):

Substituting the experimental values $V_p \approx 3.3 \times 10^7\text{ m/s}$ and $V_N \approx 4.7 \times 10^6\text{ m/s}$, with $m_p \approx 1\text{ u}$ and $M_N \approx 14\text{ u}$:

Chadwick established the existence of a neutral baryon with mass nearly equal to the proton: the neutron:

The discovery immediately resolved all quantum paradoxes: $^{14}\text{N}$ contains 7 protons and 7 neutrons (14 total fermions), naturally giving integer spin $I = 1$ and Bose-Einstein statistics.

Natural radionuclides emit three primary categories of penetrating radiations, designated by Rutherford (1899) as alpha ($\alpha$), beta ($\beta$), and gamma ($\gamma$) radiation according to their penetrating power in matter.

1. Alpha Radiation ($\alpha$)

Alpha particles are helium-4 nuclei ($^{4}\text{He}^{2+}$) consisting of two protons and two neutrons tightly bound ($B \approx 28.3\text{ MeV}$).

• Charge: $q = +2e = +3.204 \times 10^{-19}\text{ C}$.
• Rest Mass: $m_\alpha = 4.001506\text{ u} = 6.644657 \times 10^{-27}\text{ kg} \approx 3727.38\text{ MeV}/c^2$.
• Emission Energies: Monoenergetic discrete lines typically between $4.0\text{ MeV}$ and $9.0\text{ MeV}$ (e.g., $^{238}\text{U} \to 4.198\text{ MeV}$; $^{212}\text{Po} \to 8.785\text{ MeV}$).
• Velocity: $v_\alpha \approx 1.4 \times 10^7\text{ m/s}$ to $2.1 \times 10^7\text{ m/s}$ ($\sim 0.05 c$).
• Linear Energy Transfer (LET): Extremely high, $\sim 100\text{ keV}/\mu\text{m}$.
• Range: Only $2\text{ to }8\text{ cm}$ in air; stopped completely by a single sheet of paper or the dead stratum corneum of human skin ($\sim 40\,\mu\text{m}$).

2. Beta Radiation ($\beta^-$ and $\beta^+$)

Beta particles are high-speed electrons ($\beta^-$) or positrons ($\beta^+$) ejected from the nucleus during weak force isobaric transitions.

• Charge: $q = -e$ ($\beta^-$) or $q = +e$ ($\beta^+$).
• Rest Mass: $m_e = 0.00054858\text{ u} = 9.10938 \times 10^{-31}\text{ kg} \approx 0.5109989\text{ MeV}/c^2$.
• Energy Spectrum: Continuous from zero to a well-defined endpoint energy $E_{\max}$ ($Q_\beta$), because decay energy is shared stochastically with a three-body partner (antineutrino $\bar{\nu}_e$ or neutrino $\nu_e$):

• Velocity: Highly relativistic, $v_\beta \approx 0.5c\text{ to }>0.999c$.
• Linear Energy Transfer: Low LET, $\sim 0.2\text{ keV}/\mu\text{m}$.
• Range: Several meters in air, a few millimeters in soft tissue; attenuated completely by a few millimeters of aluminum or Lucite acrylic. High-$Z$ shields must be avoided to prevent bremsstrahlung X-ray production.

3. Gamma Radiation ($\gamma$)

Gamma rays are high-energy, uncharged electromagnetic photons emitted during transitions between excited nuclear states following preceding alpha or beta decay.

• Charge: $q = 0$.
• Rest Mass: $m_0 = 0$.
• Velocity: Exactly $c = 2.99792458 \times 10^8\text{ m/s}$ in vacuum.
• Energy: Discrete photon energies typically ranging from $10\text{ keV}$ to $10\text{ MeV}$ ($\lambda \sim 10^{-11}\text{ to }10^{-14}\text{ m}$).
• Penetrating Power: Exponential attenuation in matter:

Requires centimeters of dense lead ($\text{Pb}$), depleted uranium ($\text{DU}$), or meters of reinforced concrete to attenuate by factors of $10^3\text{ to }10^6$.

In 1902-1903, Ernest Rutherford and Frederick Soddy published their revolutionary theory of atomic disintegration. Overturning the ancient chemical doctrine of elemental immutability, Rutherford and Soddy stated that radioactivity is an explosive subatomic transformation in which an atom of one chemical element spontaneously changes into an atom of a completely different chemical element.

The Fundamental Differential Rate Law

Radioactive decay is an intrinsically stochastic quantum phenomenon. The probability that a given unstable radioactive nucleus will undergo nuclear transition within an infinitesimal time increment $dt$ is a constant, denoted by the decay constant $\lambda$ (dimensions $\text{time}^{-1}$).

Because each nucleus decays independently of its neighbors and is unaffected by past history (a memoryless Poisson process), the net rate of disintegration $-dN/dt$ occurring in an ensemble of $N(t)$ identical radioactive nuclei is directly proportional to the number of surviving parent nuclei present at that instant:

Separating variables:

Integrating both sides from $t = 0$ (where $N = N_0$) to time $t$:

Exponentiating both sides yields the foundational Exponential Decay Law:

   Number of Surviving Nuclei N(t)
   N₀ ▲
      │\
      │ \
 N₀/2 │--\---- (Half-Life T₁/₂)
      │   \
 N₀/4 │----\-- (Two Half-Lives 2T₁/₂)
      │     \
      └──────┴──────┴────────────────► Time t
             T₁/₂   2T₁/₂

Derivation of Half-Life ($T_{1/2}$) and Mean Life ($\tau$)

The half-life $T_{1/2}$ is the time required for one-half of the initial population of radioactive atoms to disintegrate:

Dividing by $N_0$ and taking the natural logarithm:

The mean life (or average lifetime) $\tau$ of a radioactive nucleus represents the mathematical expectation value of the survival time $t$:

Integrating by parts ($\int u \, dv = u v - \int v \, du$ with $u = t, dv = e^{-\lambda t} dt$):

Therefore:

The relationship between half-life and mean life is:

During one mean life $\tau$, the population decreases to $1/e \approx 36.788\%$ of its initial value:

In radiochemical metrology, the primary quantity describing the source strength of a radioactive substance is radioactivity (or simply activity), denoted by $A(t)$. Activity is defined as the absolute rate of nuclear transformations (disintegrations) per unit time:

Units of Radioactive Activity

1. The Becquerel (Bq):

The derived SI unit of activity is the Becquerel, defined by the 15th General Conference on Weights and Measures (CGPM) in 1975:

Multiples include:

• Kilobecquerel ($\text{kBq} = 10^3\text{ Bq}$)
• Megabecquerel ($\text{MBq} = 10^6\text{ Bq}$)
• Gigabecquerel ($\text{GBq} = 10^9\text{ Bq}$)
• Terabecquerel ($\text{TBq} = 10^{12}\text{ Bq}$)
• Petabecquerel ($\text{PBq} = 10^{15}\text{ Bq}$)

1. The Curie (Ci):

The historical non-SI unit, originally established in 1910 to honor Marie and Pierre Curie, was defined as the activity of radon gas ($^{222}\text{Rn}$) in radioactive equilibrium with $1.000\text{ gram}$ of pure radium-226 ($^{226}\text{Ra}$). In 1953, the International Commission on Radiation Units and Measurements (ICRU) standardized the Curie as an exact physical definition:

Common subunits:

• Millicurie ($\text{mCi} = 10^{-3}\text{ Ci} = 37\text{ MBq}$)
• Microcurie ($\mu\text{Ci} = 10^{-6}\text{ Ci} = 37\text{ kBq}$)
• Nanocurie ($\text{nCi} = 10^{-9}\text{ Ci} = 37\text{ Bq}$)
• Picocurie ($\text{pCi} = 10^{-12}\text{ Ci} = 0.037\text{ Bq}$)

1. The Rutherford (rd):

A short-lived unit proposed in 1946:

Specific Activity Formulation

The specific activity ($a$ or $SA$) represents the activity per unit mass of a pure radionuclide or labeled chemical substance:

For a chemically pure, carrier-free radionuclide of molar mass $M$ ($\text{g/mol}$) and half-life $T_{1/2}$ ($\text{seconds}$), the number of atoms per gram is $N_A / M$, where $N_A = 6.02214076 \times 10^{23}\text{ mol}^{-1}$:

Converting specific activity to Curies per gram ($\text{Ci/g}$):

Notice the dramatic inverse proportionality: radionuclides with short half-lives exhibit enormous specific activities. A single microgram of carrier-free $^{99m}\text{Tc}$ possesses an activity of nearly $200\text{ MBq}$, sufficient for multiple clinical SPECT scans.

The initial decades of radioactivity research witnessed the invention of ingenious physical detection devices that revealed the discrete, particulate nature of subatomic phenomena long before electronic pulse amplifiers were conceived.

William Crookes and the Spinthariscope (1903)

In 1903, Sir William Crookes discovered that when alpha particles from a radium preparation struck a screen coated with phosphorescent zinc sulfide ($\text{ZnS:Cu}$), the emission did not appear as a continuous luminous glow under optical magnification. Instead, viewing the screen through a high-power objective lens revealed thousands of distinct, instantaneous points of scintillating light.

Crookes mounted a needle tipped with a microscopic speck of radium bromide a few millimeters above a $\text{ZnS}$ screen inside a small brass viewing tube with an adjustable focus eyepiece, inventing the spinthariscope (from Greek spintharis, "spark"). For the first time in human history, scientists could directly observe the macroscopic optical consequence of an individual single-atom nuclear disintegration!

Rutherford and Geiger utilized visual scintillation counting on $\text{ZnS}$ screens to count alpha particles individually. Observers sat in pitch darkness for 30 minutes to achieve dark adaptation, then counted flashes through a microscope for intervals of 60 seconds with hand tally counters. Despite physiological eye fatigue, this visual metrology yielded the experimental scattering cross-sections that proved the existence of the atomic nucleus.

Early Transmutation and the N-14 to O-17 Discovery (1919)

Prior to 1919, radioactivity was observed strictly as an immutable, spontaneous disintegration beyond human intervention. In 1919, Ernest Rutherford announced the first artificial nuclear transmutation: Collimating intense alpha particles from a polonium or bismuth-214 source into a gas-tight chamber filled with pure dry nitrogen gas, Rutherford observed scintillations on a $\text{ZnS}$ detector placed beyond the maximum stopping range of alpha particles in nitrogen ($R_\alpha \approx 7\text{ cm}$). Magnetic deflection proved that these long-range penetrating particles were fast protons ($^1\text{H}$):

Patrick Blackett (1925) confirmed this historic discovery by photographing 400,000 alpha tracks in an automatic Wilson cloud chamber, capturing eight historic bifurcated "forked tracks" where an alpha track ended abruptly, yielding an ultra-thin long proton track and a short, thick oxygen-17 recoil track.

Comprehensive Reference: The Four Primordial & Extinct Radioactive Decay Series

All natural heavy radioactive decay chains follow one of four series characterized by mass number $A$ modulo 4 ($A = 4n + k$, where $k \in \{0, 1, 2, 3\}$):

The neptunium series ($4n + 1$) is extinct in nature because the half-life of its longest-lived parent ($^{237}\text{Np}$, $2.14\text{ Ma}$) is vastly shorter than the age of the Earth ($4.54\text{ Ga}$). It was first synthesized artificially by Seaborg, McMillan, and Wahl in 1940.