Alpha Particle Spark Detector

Spark Detector

This apparatus was constructed using the machine shop after work summer 2014 and in the first couple months of my senior year.  The results are rather stunning as each spark is an ionization event initiated by an incident alpha particle emitted from the Americium-241 source held by the tweezers.  The physics department took interest and the device was used as a visceral demonstration of ionizing radiation and electric potential.

I submitted a picture and write up of the apparatus to Attic Salt, my university’s yearly interdisciplinary journal.  My work was accepted and was published in the 2014/15 journal.

The write-up below was published along with the above picture. As the article was destined for an interdisciplinary journal, the article was written in a ‘pop-sci’ style meant to appeal to people with or without a technical background.

Construction of the apparatus is detailed below the write-up.


A nano-meter beyond what the human eye perceives lies a universe of complexity outside our common experience.  A huge flux of subatomic particles passes through the human body every instant without interaction and without our slightest notice.  It is only by the cleverness and ingenuity of man that these invisible particles have been observed, named and understood.

Long before grand experiments such as the Large Hadron Collider at Cern or the Super-Kamiokande Experiment in Japan, physicists at the turn of the century used comparatively basic tools that could fit on a desktop to make fundamental discoveries about the nature of the world around them.

The device in the accompanying picture is able to illuminate the unstoppable and constant disassociation of the universe.  An intrinsic property of its structure, a radioactive Americium-241 source breaks apart by ejecting a heavy, energetic chunk of its nucleus in a process known as alpha decay.  These alpha particles cannot be seen, heard or felt; however, under the appropriate set of conditions, they can be made to create quite a light show.

I stumbled across an article published in 1945 detailing the operating principles and implementation of a small instrument capable of detecting alpha particles [1].  It is unique in the fact that it creates an audible and visible spark each time an alpha particle is incident upon the active area of the detector.  Being one that is inclined to build archaic physics apparatus, I set about amassing the materials and tools necessary to reproduce such a device.

The operating principle is simple.  Five hair thin tungsten wires are suspended above a two inch diameter aluminum plate known as a cathode.  The tungsten wires are grounded and a potential of -5000 volts is applied to the cathode.  An alpha particle passing between the wires and plate will ionize or knock off electrons from air molecules in its path.  These free electrons are accelerated by the strong electric potential towards the tungsten wires, creating an audible and visible spark in a process known as a Townsend Avalanche.

The image you see before you is the alpha detector in action.  Held by a pair of tweezers is the Americium-241 source.  Every spark captured in this image was initiated by an ejected alpha particle passing between the thin tungsten wires and the cathode of the detector.  A long exposure photograph was taken to image many such events as each little lightning bolt lasts only 2.40ms.  As soon as the Americium source is moved away from the detector, this discharge stops, yet the atomic nucleus decays persistently with or without our observation.

In summary, an atomic nucleus decays and a piece of the nucleus hurdles towards the detector where it initiates an electric discharge visible to the unaided eye.  Thus, we are provided with the opportunity to witness the normally invisible process of radioactive decay.  What great insight can be gained from such a tiny and supposedly “outdated” instrument.


Much care was taken in its construction.  Five 0.08mm tungsten wires are held roughly 2mm above the aluminum cathode.  The cathode was fashioned from a 2.0″ diameter aluminum rod. The rod was cut using a band saw then turned on a lathe to smooth the surface.

Even after the machining, rough spots on the cathode resulted in undesirable corona formation and spontaneous discharge.  Obtaining permission from the mechanical engineering department, I gained access to the material science lab and proceeded to polish the cathode to a 3.0 micron finish.  This was done through a laborious succession of polishing papers, diamond paste and a wet polishing wheel.


A potential -5000v is applied to the aluminum cathode.  The HV power supply was constructed using an inverter fed through a six stage Cockroft-Walton multiplier.  It is this high potential that necessitated the mirror smooth cathode for successful operation.

The distance between the wires and the cathode was adjusted until the device was sensitive to alpha particles.  If the wires are too close to the cathode, a continuous arc occurs.  If the wires are too far away, no spark occurs in the presence of the alpha source.

Inspiration provided by  Carl Willis:


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