= Chadwick neutron discovery experiment
{c}
<Walther Bothe> and <Herbert Becker> observed penetrating radiation from <beryllium> struck by <alpha particles> before <James Chadwick>'s interpretation.<The Existence of a Neutron by James Chadwick>[Section 1, page 692: Bothe and Becker]{ref}
<Irène Joliot-Curie> and <Frédéric Joliot> found that adding material containing <hydrogen> increased <ionization>, interpreting the ejected <protons> as <Compton scattering> recoils from extremely energetic <gamma rays>. <James Chadwick> tested this interpretation against <nuclear recoils> in other elements.<Possible Existence of a Neutron by James Chadwick>[Page 312, opening paragraphs: Joliot-Curie observation and counter]{ref}
The counter coupled a small <ionization chamber> to an <amplifier> and <oscillograph>: charged particles produced <ions>, and the electrical <signal> gave a deflection. With <Norman Feather>, <James Chadwick> also observed <nitrogen> recoil tracks in a <cloud chamber>.<Possible Existence of a Neutron by James Chadwick>[Page 312: valve counter and Feather's expansion-chamber observations]{ref}
The apparatus used <polonium> on <silver> facing <beryllium> inside an evacuable vessel. <Paraffin wax> before the <aluminium> counter window supplied recoil <protons>; intervening <aluminium> screens measured their <particle range>. Deflections were photographed on bromide paper. The first counter contained air; later fillings included <hydrogen>, <helium>, <nitrogen>, <oxygen> and <argon>. Counts with wax absent included gas recoils; wax insertion added entering <protons>. Thick lead between source and counter hardly changed the initial counts, testing penetration against the low background.<The Existence of a Neutron by James Chadwick>[Section 2, pages 694–697; figures 1–2]{ref}
\Image[/chadwick-neutron-apparatus-inference.png]
{title=From penetrating radiation to a charged recoil signal and a mass inference}
The diagram separates two routes to a <signal>: a <proton> entering from the wax and a <nuclear recoil> created inside the chamber <gas>. Its lower row is a logical summary, rather than a reconstructed experimental trace. In either route the observed <ionization> belongs to a charged secondary particle; identifying the incident radiation requires an additional collision model.
In the February letter, a <photon> interpretation requiring about $52\,\mathrm{MeV}$ for the <protons> predicted only about $0.4\,\mathrm{MeV}$ for <nitrogen> recoils, below the observed <ionization>. Here $1\,\mathrm{MeV}=10^6$ <electronvolts>. <James Chadwick> proposed a neutral particle near the <proton> <mass> and the <nuclear reaction>
$$
{}^{9}_{4}\mathrm{Be}+{}^{4}_{2}\mathrm{He}
\longrightarrow{}^{12}_{6}\mathrm{C}+{}^{1}_{0}n.
$$
These are the letter's approximate historical values, not a modern <mass> determination.<Possible Existence of a Neutron by James Chadwick>[Page 312: photon/nitrogen discrepancy and proposed reaction]{ref}
Why compare two materials? A <gamma ray> is a <photon>, so its incident <momentum> is $E_\gamma/c$, where $c$ is the <speed of light>. A slow massive projectile instead has <momentum> $mv$ and <kinetic energy> $mv^2/2$. The relationship between incoming <momentum> and <energy> therefore changes. An <elastic collision> model must explain the recoil endpoints for both target <masses> with the same incident projectile population. Calling the radiation neutral supplies no such agreement by itself.
The original measurements depended on <particle range> calibration, material in the path, detector response and whether a recoil was fully stopped in the sensitive <gas>. Large deflections alone need not specify a complete recoil <energy>. The calculations below expose the ideal collision assumptions behind the inference; applying them to a measured spectrum requires checking those assumptions.
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