What is an alpha particle?
Alpha particles (radiation) are randomly generated positively charged energetic nuclear particles originating from the radioactive decay of heavy nuclides and from certain nuclear reactions. The alpha particle consists of 2 neutrons and 2 protons, so it is essentially the same as the nucleus of a helium atom.
Two protons and two
nuetrons of the Helium nuclei.
Because it has no electrons, the alpha particle has a charge of +2. This positive charge causes the alpha particle to strip electrons from the orbits of atoms in its vicinity. As the alpha particle passes through material, it removes electrons from the orbits of atoms it passes near. Energy is required to remove electrons and the energy of the alpha particle is reduced by each reaction. Eventually the particle will expend its kinetic energy, gain 2 electrons in orbit, and become a helium atom. Because of its strong positive charge and large mass, the alpha particle deposits a large amount of energy in a short distance of travel. This rapid, large deposition of energy limits the penetration of alpha particles. The most energetic alpha particles are stopped by a few centimeters of air or a sheet of paper.
lead based products (solder, paint, etc.), ceramics, and some plastics
are all well known alpha emitters. The most interesting
of the series is the uranium-238 series which decays via a chain containing
8 alpha decays and 6 beta decays to lead-206. This chain includes the longest-lived
isotopes of radium and radon: radium-226 and radon-222, respectively.
The emission in Pb (lead) is from Po (polonium) decay. Lead-210 will decay
with a half-life (no marked decrease in energy value) of 22 years via beta
decay to Bismuth-210 to Polonium-210. The Polonium-210 decays to Lead-206
producing a 5.3 MeV alpha particle. Please see the Thorium
Decay and Uranium Decay series.
In addition to these ancient radionuclides and their progeny, some radionuclides are being continually produced by cosmic rays. The most prominent of these is carbon-14, produced in the interaction of cosmic ray neutrons with nitrogen in the atmosphere.
What is the concern with an alpha particle?
Todays smaller semiconductor design geometries and charge sensitive circuitry can be adversely effected by energetic alpha particles. An alpha particle can cause what is known as a "Single Event Upset" or "Soft Error Upset" or "SEU". A 5.3 MeV Po210 alpha particle (a daughter in the Uranium decay family of lead) will travel approximately 4 centimeters (1.57 inches) in air at sea level (see Range of Alpha Particles in Air ). High energy alpha particles can penetrate silicon devices to a depth of approximately 25 microns. Along the track of the alpha particle, electrons are dislodged from the crystal lattice sites. If the abundance of electrons are collected by the “empty” storage well, then the cell can flip from “1” to a “0” logic state generating a soft error in the device. While usually no permanent damage occurs, the error can never be reproduced by random test pattern generation, potentially causing unecessary design changes, where a change in material components might be more in line. Once mainly limited to memeory devices, alpha particles are effecting various other devices due to increasingly smaller trace widths and lower voltage settings. And since several of the materials commonly used in the semiconductor industry naturally contain alpha particles (lead based solder, molding compounds, etc), efforts are underway to create "cleaner" versions of these critical materials which are lower in alpha particle emissions. If your design is sensitive to alpha particle induced upset, why surround it with materials high in alpha emission? The Model 1950 Alpha Counting System is used to measure and determine the alpha content of these materials.
The fact that a memory location could momentarily give an incorrect answer may seem trivial at first glance, but consider the effect of a soft error on the controller of a pace maker, or the guidance system of a passenger aircraft, or perhaps the cooling water control at a nuclear facility. The easiest and most common fix for this situation is to create a triple (or greater) redundancy, wherein 2 of the 3 outputs of your device agree. If one were upset by an alpha event, the other two would overrule. However, with todays smaller footprints and ultra-miniaturized circuit layouts, real estate becomes a serious factor. Currently, several semiconductor packaging groups are working on coatings or "shields" to protect the inner device working from potential alpha emissions created by packaging and/or mounting materials.
When were alpha particles "discovered" ?
The French physicists Pierre and Marie Curie performed much of the ground breaking research into radioactivity as we know it today. However, it is Ernest R. Rutherford, a New Zealand-born British physicist who was awarded the Nodel Prize at age 37, that is credited with discovering the true nature of alpha particles. He began investigating radioactivity in 1897 and had by 1900 found two kinds of radioactivity with different penetrating power. Alpha radiation (or alpha rays) was distinguished and named by Rutherford in 1909, who found by measuring the charge and mass of alpha particles that they are the nuclei of ordinary helium atoms. He named the two types of radiation alpha and beta, for the first two letters of the Greek alphabet. Later, when the third naturally occuring form of ionizing radiation was discovered, it was given the third letter of the Greek alphabet, Gamma.
Baron Ernest R. Rutherford
As an ironic side note: Rutherford was obviously a pioneer of modern atomic science, who by 1903 was able to explain the decay products of Thorium and that radioactivity is caused by the breakdown of the atoms to produce a new element. In 1904 Rutherford worked out the series of transformations that radioactive elements undergo and showed that they end as lead. He was the first to recognize the nuclear nature of the atom in 1911. He was awarded the Nobel prize in 1908 (did we mention he was only 37?). Knighted in 1914, and made a Baron in 1931. Rutherford produced the first artificial transformation, changing one element to another (1919), bombarding trapped nitrogen with alpha particles and getting hydrogen and oxygen. After further research he announced that the nucleus of any atom must be composed of hydrogen nuclei; at Rutherford's suggestion, the name 'proton' was given to the hydrogen nucleus in 1920. He speculated that uncharged particles (neutrons) must also exist in the nucleus. And yet, with all this incredible knowledge and foresight, he is actually quoted as saying:
"The energy produced by the breaking down of the atom
is a very poor kind of thing.
Anyone who expects a source of power from the transformation of these atoms is talking moonshine."
E.R. Rutherford, 1933
Wouldn't the good Baron be suprised today to learn that major European countries such as France and the UK use nuclear power as their number one source of energy, out-supplying oil and coal burning plants? Were the concepts of fission and fussion, incredible as it seems, beyond the imagination of such a visionary?
It wasn't until 1962 that the possibility of single event upset or soft error was postulated by Wallmark. The first actual satellite anomalies were reported by Binder in 1975, caused by space borne events. In 1978, May and Woods of Intel fame performed the early pioneering work for the semiconductor industry by investigating alpha induced soft errors. In their work the source of the alpha particles was not cosmic, but rather from the natural decay of trace (ppm) concentrations of radioactive materials present in the integrated circuit packaging materials.
How are alpha particles counted?
There are three types of proportional counters, which were developed several decades ago for use by the nuclear industry. These three consist of sealed detectors, Frisch grid detectors, and gas flow proportional counters. The sealed detector has a window, which is relatively thick, and consequently cannot be used for alpha detection, since the alphas cannot penetrate the window. The Frisch grid is used for spectroscopy applications where the energy of the particle is required for radioisotope identification. Much larger volumes of P-10 nuclear counting gas are required to fully absorb the energy of high energy alpha particles. This added volume of gas creates a much higher detector background, which limits its use for semiconductor packaging materials analysis where the lowest possible “Lower Limit of Detection” is required for rapid analysis. The ideal detector of choice would be the gas flow proportional unit.
The Model 1950 Alpha Counting System is a gas flow alpha measurement system, consisting of two critical components. The detector, to sense the radiation, and the electronics, to measure the radiation flux. The detector consists of two chambers, separated by a very thin Mylar film (window). The upper portion is the detector chamber, which contains both the anode and cathode electrodes. P-10 nuclear counting gas (90% Argon and 10%Methane) flows through the detector chamber with the anode at the top, and the metalized Mylar window below, which is the cathode. Several hundreds of volts bias is applied between the anode (+) and the cathode (-). This bias is used to collect the holes and electrons produced via ionization as the alpha particle passes through the gas. The lower chamber, which accepts the sample under analysis, is referred to as the sample chamber.
Proportional counters/detectors operate using a pulse mode principal wherein each individual quantum of radiation (alpha particle) interacts with the nuclear counting gas within the detector. The charge integral of each burst of current, or the total charge Q, is recorded. The energy deposited in the detector is directly related to Q. In the pulse counting mode, all pulses above an energy threshold of 1 MeV are registered from the detector, regardless of value of Q.
Heavily charged alpha particles interact with matter primarily through coulomb force between their positive charge and the negative charge of the orbital electrons within the absorber atoms ( P-10 gas, 90% Argon, 10% Methane) . The detector relies on the charge created by the electrons for their response. The alpha particle enters the absorbing media through the window and ionizes the P-10 gas; the electrons are accelerated by the anode bias while the holes are accelerated to the cathode (Mylar window). The alpha particle will lose energy in the gas until it is either fully absorbed within the gas, or it collides with the detector walls.
Since migration of the electrons in the gas filled detector is very important, the fill gas must be chosen from those species that do not exhibit an appreciable electron attachment coefficient. The proportional counter is designed to maintain the purity of the gas. In the Model 1950, the gas flows through the detector chamber and is then either vented to the atmosphere or to an alternative exhaust method.
The detector analog electronic system of the Model 1950 consists of a charge sensitive pre-amplifier, amplifier, discriminator, and high voltage power supply. A DC voltage of several hundreds of volts is applied between the anode and the cathode. The electrons are collected at the anode and the holes are collected at the cathode. This pulse of current is amplified and then discriminated where only those pulses of 1 MeV or greater are recorded by the scaler. A timer circuit is used to record the elapsed time of the measurement. The data reduction portion of the Model 1950 Alpha Counting System is computer based and includes customized software for data presentation on the monitor and for data reduction and manipulation.
What makes the ASI counters unique ?
Area: The Model 1950 features a large area detector window covering 11" x 14", which converts to 1000 square centimeters. A larger area allows for greater counting efficiency over a shorter period of time.
Efficiency: The Model 1950, again because of it's large detector area, features a higher level of efficiency as more data is collected based on the sample size. By using the sample tray height adjustment feet, the sample is brought to within 2 mm of the window. The Model 1950 is shipped from the factory with an average efficiency of 80 to 84% (2 pi).
Reliability: With proper care and maintenance, the Model 1950 will provide many years of service. By effectively managing the introduction of samples into the detector chamber, and being aware of the sensitive nature of the detector window, the user can expect very low operational costs, with consumables consisting of counting P-10 Gas, printer supplies, and in the case of powder samples, mylar film and double sided tape.
Return on Investment: Customers using the tool on a regular basis can typically expect a return on investment (ROI) of between one to two years, depending on the amount of samples being processed.
Software upgrades: Software upgrades to our customers are free for the life of the tool.
copyright Alpha Sciences Inc. 1999-2009