= Blackett-Occhialini triggered cloud-chamber experiment
{c}
<Patrick Maynard Stuart Blackett> and <Giuseppe Occhialini> began their cosmic-ray collaboration in 1931. They used the coincidence methods of <Walther Bothe> and <Bruno Rossi> to make a particle initiate its own recording. Blackett later reported tracks on about $80\%$ of their counter-controlled photographs, compared with roughly $2$–$5\%$ for random expansions in small chambers. Those figures describe the reported apparatus and conditions, not a universal <detection efficiency>.<Cloud Chamber Researches in Nuclear Physics and Cosmic Radiation by Patrick Maynard Stuart Blackett>[Cloud chamber researches in nuclear physics and cosmic radiation, lecture 13 December 1948, pp. 104–105: collaboration, coincidence precedent and photographic yield]{ref}
In their 1933 account, <Geiger counters> above and below the chamber fed a <coincidence circuit>. Its pulse operated a <thyratron>, magnet and <spring>-release <valve>, letting a light <piston> expand the <gas>. A <mercury-vapor lamp> then illuminated the drops. A water-cooled <solenoid> supplied the <magnetic field>; two oblique <cameras> allowed track reconstruction. Moving the lower counter changed which trajectories could be recorded.<Some Photographs of the Tracks of Penetrating Radiation by Patrick Maynard Stuart Blackett and Giuseppe Paolo Stanislao Occhialini>[Some Photographs of the Tracks of Penetrating Radiation, pp. 699–701, section 1 and Fig. 2; p. 719, Description of Plates: expansion sequence, cameras and alternate counter position]{ref}
\Image[/blackett-occhialini-triggered-chamber.png]
{title=Counter coincidence initiates expansion, illumination and photography in a functional reconstruction of the Blackett-Occhialini method}
{description=Original diagram, not a scale drawing or a historical photograph. The pictured track and positions are illustrative; magnetic curvature and event direction require further measurements.}
Two timing problems must be separated. The <coincidence circuit>'s window distinguishes nearly simultaneous counter signals. The mechanical expansion latency determines how much the existing <ionization> trail spreads before droplets form. For a chosen one-coordinate <diffusion> model with initial <variance> $\sigma_0^2$, <diffusion coefficient> $D$ and expansion delay $t_e$,
$$
\sigma_{\rm track}^2=\sigma_0^2+2Dt_e.
$$
Taking $\sigma_0=0$, $D=2.0\times10^{-6}\,\mathrm{m^2\,s^{-1}}$ and $t_e=20\,\mathrm{ms}$ gives $\sigma_{\rm track}\simeq0.283\,\mathrm{mm}$. This is an original illustrative calculation, not the historical track width. It shows why a fast electronic pulse does not by itself guarantee a sharp photograph: the subsequent mechanical response also matters.
The photographs revealed groups containing positive and negative electron-like tracks, complementing the <Anderson positron track experiment>. Their interpretation depended on track curvature, direction, <ionization> and <particle range>, not simply on the presence of a coincidence.<Some Photographs of the Tracks of Penetrating Radiation by Patrick Maynard Stuart Blackett and Giuseppe Paolo Stanislao Occhialini>[Some Photographs of the Tracks of Penetrating Radiation, pp. 702–708 and p. 718, Summary: associated tracks and evidence for positive electrons]{ref} Blackett explicitly noted that the counter control favored larger <particle showers>.<Cloud Chamber Researches in Nuclear Physics and Cosmic Radiation by Patrick Maynard Stuart Blackett>[Cloud chamber researches in nuclear physics and cosmic radiation, p. 108: multiplicity-dependent triggering and overrepresentation of large showers]{ref} The <multiplicity bias from a detector trigger> gives a small model of that effect: a recording method can improve the study of rare interactions while making its photographs unrepresentative of the incident population.
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