Alpha particles instrument

Several high-pressure mass spectrometers were used at Alberta a high-pressure alpha-particle instrument capable of operating at pressures up to 200-300 Torr, " a 100-keV proton-beam mass spectrometer, and a 4-keV pulsed electron-beam instrument. 

The alpha-particle instrument is shown in Fig. 1. The gas, supplied from a conventional gas handling system, is irradiated in the ionization chamber. The radiation is supplied from an enclosed polonium alpha source of a few hundred millicuries. The irradiated gas bleeds through a leak out of the ion source and into the evacuated electrode chamber. There, the ions effusing from the leak are captured by the electric fields while the gas is pumped away. The ions are focused, accelerated, and then subjected to mass analysis and electron multiplier detection in a 90° sector field analyzer tube. 

MIMOS lla is an advanced version of the MER instruments (Klingelhofer et al. 2003) operating continuously since landing in January 2004. A new detector system has been implemented, inherited from the Alpha Particle X-ray Spectrometer (APXS), also part of the MER payload. This Si Drift Detector system provides higher energy resolution and increased... 

Ideally, measuring radioactivity in water assets in the field would involve minimal sampling and sample preparation. However, the physical properties of specific types of radiation combined with the physical properties of water make evaluating radioactivity in water assets in the field somewhat difficult. For example, alpha particles can only travel short distances and they cannot penetrate through most physical objects. Therefore, instruments designed to evaluate alpha emissions must... 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

The Mars Pathfinder rover carried an Alpha Proton X-ray Spectrometer (APXS), and the two Mars Exploration Rovers (MER - Spirit and Opportunity) carried Alpha Particle X-ray Spectrometers (also called APXS, but in this case more precise versions of the Pathfinder instrument, though without the ability to monitor protons for light element analyses). These instruments contained radioactive curium sources (Fig. 13.16) whose decay produced a-particles, which irradiated target rocks and soils. The resulting characteristic X-rays provided measurements of major and minor element abundances. The MER rovers also carried Mossbauer spectrometers, which yielded information on iron oxidation state. 

All Mars rovers to date have carried alpha-particle X-ray spectrometer (APXS) instruments for chemical analyses of rocks and soils (see Fig. 13.16). The source consists of radioactive curium, which decays with a short half-life to produce a-particles, which then irradiate the sample. Secondary X-rays characteristic of specific elements are then released and measured by a silicon drift detector. The Mars Pathfinder APXS also measured the backscattered a-particles, for detection of light elements, but the Mars Exploration Rovers measured only the X-rays. 

The cyclotron is a device in which positive ions are accelerated in a powerful magnetic field until they attain velocities of the order of thousands of miles per second. The use of such an instrument not only makes possible the use of particles other than the alpha particles from naturally radioactive elements but also serves to increase the fraction of effective impacts between ionic bullets and nuclear targets that bear like charges. 

Carrier or Tracer Addition. To quantify the purified final sample that will be measured by a radiation detection instrument (as compared to a mass spectrometer), a carrier or tracer is added to the sample. The carrier usually is the same element as the radioanalyte ( isotopic carrier ) and is standardized, typically at 5-20 mg/mL concentration. The carrier serves two purposes to provide macro quantities so that certain chemical steps (such as precipitation) may be performed on the sample, and to determine the chemical yield, usually by weight. A tracer serves only to determine the chemical yield of the process its nanogram quantities or less, comparable to the radioanalyte in the sample, prevent use as carrier. The tracer is measured by its characteristic radiation at the same time as the radioanalyte. An advantage in alpha-particle spectral analysis is that the activity of the analyte can be calculated from the activity of the tracer without knowledge of the detector counting efficiency, as discussed below. 

The first objective for the Sojourner was to show that it could function in the little-known environment on the surface of Mars and to observe its behavior in order to make design improvements in future rovers. Sojourner moved around the immediate area of the lander, butting the APXS up against rocks. Detectors measured interactions between a radioactive source in the APXS and the surface materials by obtaining an energy spectrum of the alpha particles, protons, and x rays produced by the exposure. This instrument could determine the chemical composition of materials, including the amounts present of most major elements except hydrogen. 

Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles interact with matter. 

There are many accounts of alpha particle detection instruments for the measurement of Rn (Sedlet, 1966). Lucas (1957) was one of the first to develop a very low background counting system. Higgins et al. (1961) adapted the method for well-water Rn and Ra surveys. Peacock and Williamson (1962) developed a shallow-borehole probe using ZnS(Ag) that could make in-situ Rn determinations without the use of a pump, but required a 5 cm diameter light-proof hole into which the ZnS(Ag)-coated Incite rod and photomultiplier assembly was inserted. Rushing et al. (1964) used a similar technique for the determination of Rn in effluents and environmental samples. For U prospecting, Dyck (1969) and Allen (1976) applied a Lucas-type cell to determine Rn in soil, lake waters and stream waters. 

The claim by G A that only one of these traditions developed techniques to imitate real-world conditions is quite misleading. Both traditions used the cloud chamber to manufacture an artificial environment that approximates known phenomena. For the Cavendish physicists, the cloud chamber became one of the defining instruments of particle physics, precisely because the laboratory phenomena were modeled on the movement of the charged particles. The knotty clouds blended into the tracks of alpha particles and the threadlike" clouds simulate beta-particle trajectories (Galison Assmus, 1989, p. 268). Of course, G A are correct that these physicists aspired to dissect nature into its fundamental components, reflecting the long tradition of the corpuscular conception of matter. 

The SSTR technique is based on the damage created in a solid along the path of a heavily ionizing particle such as an alpha particle or a fission fragment. The damage along the path, called a track, may become visible under an ordinary optical microscope after etching with suitable chemicals. The visible tracks are counted either by direct observation by a human or with the help of automated instruments. ... 

Alpha particles cause extoisive ionization in matter. If the particles are allowed to pass into a gas, the electrons released by the ionization can be collected on a positive electrode to produce a pulse or curr t. Ionization chambers and proportional counters are instruments of this kind, which permit the individual counting of each a-particle emitted by a sanq)le. Alpha particles interacting with matter may also cause molecular excitation, which can result in fluoresc ce. This fluorescence — or scintillation — allowed the first observation of individual nuclear particles. The ionization in semiconductors caused by a-particles is now the most common means of detection, see Ch. 8. 

The yield for a low-mass sample, e.g., 1 mg or less for alpha-particle measurement, can be determined with nonisotopic carrier in an aliquot taken before preparing the counting source. The analytical technique can be instrumental, such as colorimetry or atomic absorption spectrometry. Subsequent source preparation, by precipitation, evaporation, or electrodeposition, must be quantitative or highly reproducible so that a reliable yield value for this final step can be included in the total yield. 

The radiation detection systems employed in radioanalytical chemistry laboratories have changed considerably over the past sixty years, with significant improvement realized since the early 1980s. Advancements in the areas of material science, electronics, and computer technology have contributed to the development of more sensitive, reliable, and user-friendly laboratory instruments. The four primary radiation measurement systems considered to be necessary for the modern radionuclide measurement laboratory are gas-flow proportional counters, liquid scintillation (LS) counters. Si alpha-particle spectrometer systems, and Ge gamma-ray spectrometer systems. These four systems are the tools used to identify and measure most forms of nuclear radiation. 

Because the five senses are useless for detecting radiation, each facility must have readily available portable radiation detection instruments. These instruments should be selected to detect and quantify the three basic types of radiation alpha particles, beta particles, and gamma rays, as discussed in Chapter 2. In some cases, neutron detectors may be required. The RSO/RCM generally is responsible for calibrating the instruments at selected intervals, typically six months. The individual user is responsible for daily operational and source checks prior to each use. 

General environmental survey instruments (e.g., alpha particle meters) are available, but they are not specific for plutonium. The predominant analytical method for measuring plutonium present at or near background concentrations in both biological and environmental media requires radiochemical separation and purification in conjunction with a quantitative measurement technique (e.g., alpha spectrometry, liquid scintillation, or mass spectrometry). 

Sensitive methods for analysis of plutonium in urine are particularly important for estimating occupational plutonium body burdens. Routinely available instrumentation, such as the alpha spectrometer, can readily detect these low concentrations. More sensitive methods are commonly required for urine samples in order to assess chronic exposures to plutonium. These low detection limits were first achieved in the past by nuclear emulsion track counting (see Table 6-1). In this method, the electrodeposited sample is exposed to nuclear track film, subsequent to the isolation of plutonium. The alpha-particle emitting isotopes of plutonium will leave tracks on the film which are counted to quantify the amount of plutonium. Nuclear emulsion track counting has been used in the past to measure plutonium concentrations in the urine of workers at a nuclear reactor plant (Nielsen and Beasley 1980). A type of scintillation counting has been used to measure plutonium-239 and americium-241 in animal tissues (NCRP 1985). 

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