The present invention relates generally to a method and apparatus for detecting alpha particles, and more specifically to a detecting method and apparatus utilizing a semiconductor memory array.
An alpha particle is a small high-energy positively charged particle comprising a helium nucleus of two protons and two neutrons. The alpha particle exhibits an electrostatic charge of +2. Alpha particles are ejected spontaneously from the nuclei of some radioactive elements as they disintegrate. The alpha particle has a low penetrating power and a short range (a few centimeters in air). The most energetic alpha particles typically fail to penetrate the dead layers of cells covering the skin and can be easily stopped by a sheet of paper. However, alpha particles are hazardous when an alpha-emitting isotope is inside the human body.
In any area where radioactive materials are handled, it is imperative, both for the protection of personnel and to avoid contamination of the environment, to continuously monitor personnel, equipment, and clothing to identify the release of radioactive contamination and to identify the radioactive source. Alpha particle contaminants, such as plutonium, are particularly difficult to detect because of the limited penetration of alpha particles in air.
Three types of alpha particle detectors are known in the prior art: the scintillation counter, the gas counter, and the solid state junction counter.
In the early scintillation counter the scintillator material was viewed with a microscope to count the individual flashes of light produced when an alpha particle is stopped by a collision within the material. In the modern scintillation counter, scintillation material is deposited on the photocathode of a photomultiplier tube that amplifies the signal and provides information about the energy of the alpha particle, in addition to counting the alpha particles. The window through which the alpha particles pass into the scintillator material must be thin enough to allow transmission of the low energy alpha particles, but thick enough to form an opaque light seal.
Gas-filled alpha particle detectors use a specific gas as the detector material depending upon the operational mode, i.e., operation as a Geiger counter or as an ionization/proportional counter. The alpha particle enters the gas-filled ionization zone through a thin, fragile plastic or metal window. The output signal pulse is constant in the Geiger counter operation, and is related to the alpha particle energy in the ionization or proportional counter.
The semiconductor junction counter is a solid state p-n junction with a reverse bias that collects ionization charge from the passage of an alpha particle through the depletion layer. The alpha particle enters the counter through a metallic electrode detector window, which blocks ambient light while allowing the alpha particles to pass. At least one amplification stage is required to register an event and several stages of amplification are typically necessary to extract information representing the alpha article.
As charged particles, such as alpha particles, move through the p-n junction of a semiconductor device, they give up kinetic energy to electrons in the valence band of the semiconductor material. The electron moves through the band gap into the conduction band, where an electron-hole pair is formed. As is known, for electron-hole pairs created in a reverse biased junction, the electrons and holes are swept away by the electric field of the junction, creating a current representative of the density of the electron-hole pairs, and thus a current representative of the alpha particle density. Conventionally, a high sensitivity alpha particle detector comprises a plurality of parallel-connected diodes formed on a single die. However, such detectors can be costly and difficult to construct as they require low noise and high gain amplifications stages to detect individual alpha particles. Also, reverse biased diodes can exhibit considerable shot noise that limit their usefulness in detecting low alpha particle
A method and apparatus for detecting alpha particles using a memory array having a plurality of memory storage locations responsive to an operating voltage. Initial values are stored in the plurality of memory storage locations of the memory array. The operating voltage is reduced and the memory storage locations that maintain the initial stored value are determined. These memory locations are then available to detect the impingement of alpha particles. Individual memory cells consist of moderate gain feedback elements. Because the voltage is reduced below the nominal operating voltage of the memory array, and at this voltage the shot noise is low, a sensitive alpha particle detector is realized.