Radioactive atoms, direct detection

It has been hoped [20,21] that a method could be developed which would directly detect the radioatoms that are present in nature by an efficient ultra-sensitive mass spectrometer technique which would not itself depend upon the fact that the atoms being investigated are radioactive. The advantage of an efficient mass spectrometer system for long-lived radioisotopes can be seen from the equation for calculating the number of atoms present in a sample from its measured radioactive decay rate ... 

In contrast, the direct detection technique for 14C can count approximately one percent of the 14C atoms that are present in a 200 pg sample with virtually zero background in times that are of the order of a few hours. Thus, for milligram quantities of carbon, the improvement in sensitivity of direct counting over radioactivity is of the order 7.2 x 10s. 

The rapid expansion of lectin-based applications for the detection and quantification of glycoconjugates has been led by the development of commercially available, purified and chemically derivatized lectins, and in some cases, anti-lectin antibodies. Over 50 purified plant lectins are sold commercially by a number of producers and vendors, with this number growing annually. Equally important is the ease by which investigators can obtain lectins labeled with various fluorescent dyes, haptenic moieties, biotin, and radioactive atoms, as well as conjugated to enzymes and solid-phase supports. These derivatized lectins are useful for either direct or indirect detection and quantification techniques, or for the physical separation of particulate-bound or soluble glycoconjugates. Table 4 lists many of the commercially available lectin reagents and sources. 

In PET, radioactive substances that emit positrons are introduced into a patient s bloodstream. As the radioactive atoms decay, the positrons they emit collide with electrons, producing gamma rays that escape from the body and are detected by an array of instruments surrounding the patient. Computer analysis of the amount and direction of gamma ray production, and comparison of the data collected for people with and without certain brain disorders provides doctors with valuable information. For example, PET scans of the brain have been used to study the movement of the medication L-dopa in the brains of people suffering from Parkinson s disease. In these procedures, fluorine-18 atoms are attached to L-dopa molecules, which are then injected into a patient. Each flourine-18 decays and emits a positron that generates gamma rays when it meets an electron. 

In most laser studies of optical isotope shifts the crossed, laser-atomic beam method is used to avoid Doppler-broadening. In some cases short-lived radioactive isotopes are detected directly on-line, in others, by irradiating a target which is subsequently heated to form the source of an atomic beam. Figure 7 shows a spectrum obtained some years ago in Oxford on the resonance line of barium at 554 nm S] ... 

At its simplest, radioactivity counting involves a source, a suitable detector for the radiation emitted by the source, a means of counting those decay events that are detected and a timer. If we measure the rate of detection of events, we can directly relate this to the number of radioactive atoms present in the source. The basic premise is that the decay rate of the source (/ ) is proportional to the number of atoms of radioactive nuchde present (N), the proportionality constant being the decay constant, X. Thus ... 

Stable isotope compositions are useful tracers of the sources and transformations of marine materials however, they carry no direct information about the rates and dates of the associated processes. Such temporal distinctions are possible, however, with the many different naturally occurring radioactive isotopes (Fig. 5.1) and their wide range of elemental forms and decay rates. These highly dependable atomic clocks decay by nuclear processes that allow them to be detected at veiy low concentrations. Long-lived and Th... 

It was soon realised that this radioactivity is an atomic property with an intensity directly proportional to the concentration of the element yielding it and entirely independent of the state of chemical combination of that element. Not only do the rays affect a photographic plate, but they induce ionisation in air and thus assist the discharge of an electroscope. Hence a radioelement can be detected electroscopically no matter what chemical process it undergoes. This enormously simplifies the method of detection which is both rapid and delicate. 

X7500) from the atomic weapons test program conducted in Nevada, USA, from 1951 to 1962. Again, fallout of the isotope was directly finked with radioactivity detected in cattle (Van Middlesworth, 1956), and in later studies, air trajectory calculations of fallout from underground explosions showed how effectively and rapidly air-borne could be transported over large distances prior to deposition (MarteU, 1964). 

The chief advantage of radiotracer methods for catalytic investigations is the sensitivity in detecting small numbers of atoms or molecules when these are radioactive. This advantage is of equal importance in the two distinct types of experiment in which radiotracers are used. The first method involves the direct observation of a labelled molecule chemisorbed on a catalyst surface. In the second method labelled species are added to the reaction mixture to determine the mechanism of a catalytic reaction. 

One less common but attention-grabbing application of TRMS is related to nuclear physics, in the area of synthesis of new unstable nuclei. In order to synthesize heavy atoms, Pb or Bi targets are irradiated with a stream of charged particles [209]. The newly produced heavy ions are directed through quadrupole lenses and velocity Alters toward detectors. Their implantation energy is correlated with the subsequent radioactive decays in order to identify the generated nuclei [210]. Detection of new heavy elements is particularly difficult because they have very short half-lives. The data obtained from heavy ion detectors and silicon detectors are put together to match the characteristics of the new elements with the theoretical predictions. 

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