I was introduced to alpha particle spark detectors when I watched a Youtube Video  by Carl Willis of his home made spark detector. I like the detector so much I wanted to build one for myself. So I researched the topic and read the seminal work from 1944 by W.Y. Chang and S. Rosenblum .
The device is pretty simple. Take a thin tungsten wire, charge it to about 3000 volts with respect to ground. Position the wire approximately 1/16" (1.5 mm) above a smooth brass ground plate. The air acts like an insulator between the highly charged wire and ground plate.
When an alpha particle passes in between the wire and the electrode it ionizes the air between. The high voltage potential between the wires and the electrode, is triggered by the ionization of the alpha particle, which causes an electrical avalanche that we see as an electrical discharge (spark) between the wires and the electrode.
This detector is only sensitive to alpha (α) particles and will Not detect beta(β), x-ray or gamma(γ) radiation.
Our device operates on this same principle but there are a few changes. First I am using thinner copper wires (.003 diameter) that are held at ground potential. It is the metal plate that is charged to approximately 8000 volts. The wire is positioned approximately 0.1" (3mm) away from the metal plate.
Caution: This device uses a high voltage power supply and radioactive materials. Please follow the proper precautions.
The following is the parts list:
- (4) 3 KV capacitors Allied PN# 70016091
- (2) 6KV diodes Images PN# DI-6KV
- (1) 510 ohm 5W Allied PN# 70204781
- (1) Prototyping pcb Allied PN# 70219395
- (1) Adj HV power supply Images PN# HVT-07
- (1) Metal plate (see text)
- (1) .003 copper wire Images PN# CW.003
- Misc. 1/8" plastic, 22 ga. wire. copper foil tape.