Radioactive source instrumentation

To minimize experiment time a very strong Co/Rh source was used, with an initial source strength of about 350 mCi at launch. Instrument internal calibration is accomplished by a second, less intense radioactive source mounted on the end of the velocity transducer opposite to the main source and in transmission measurement geometry with a reference sample. For further details, see the technical description in Sect. 3.3. 

Health and safety requirements. Special licenses are required to operate an instrument that uses radioactive substances (e.g., the radioactive sources for the electron capture detector)... 

Energy dispersive instruments are used for qualitative analysis and routine quantitation (Fig. 13.5) and represent the first category of instruments. They are generally equipped with a low power X-ray tube instead of a radioactive source in order to eliminate constraints caused by legislation. 

A typical Mossbauer effect spectrometer consists of a transducer which imparts controlled motion of a radioactive source, the sample itself, a suitable radiation detector, and certain necessary electronic instrumentation. Such a system is diagrammed in Figure 2A. 

The reflectivity within the halo is not constant one observes a maximum close to the radioactive source with a decrease outwards. The tracing of reflectivity curves with a photocell, recorder, and moving stage has shown that the decrease curves are smooth but sometimes stepwise (Figure la, b). The width of the halos is difficult to estimate by a subjective method, and our instrumental measurements are still too few to allow, definite interpretation. However, the width generally varies between 20 and 50 microns. Preliminary observations suggest that the intensity of the halos is proportional to the quantity of uranium (Figure 4c). 

Cherenkov( erenkov)Radiation is the very faint emission of a bluish light from transparent substances(such as glass, water, etc) developed in the vicinity of strong radioactive sources. This phenomenon, first observed by M-me Curie ca 1910 and later by other workers in the field of radioactivity, was not understood until P.A.Cherenkov explained it after conducting exhaustive studies in 1934-1938, incl. He also developed an instrument("Cheren-kov counter ) which became useful for research... 

Another important application of this technique has been to determine the elemental composition of the lunar and Martian surfaces. Turkevich et al. (1969) constructed a rugged device to measure the backscattering of a particles from the lunar surface, which flew on three Surveyor missions in 1967-68 and yielded the first complete and accurate analysis of the lunar surface. The a particles came from a radioactive source (242Cm) that was part of the instrument package. The results of these experiments, which showed an unexpected and comparatively high abundance of Ti, were confirmed by laboratory analysis of lunar samples gathered in the Apollo missions. Since then, this technique has been used to study Martian rocks and soil. 

Determining the accuracy of the analytical methods for environmental samples and for calibrating radiation instrumentation requires that standard, certified radioactive sources with known concentrations of uranium. 

All instruments that use radioactive sources should be kept in a shielded enclosure and made up of lead-containing glasses, sheets, and bricks that attenuate the radiation to a permissible level radiation going outside the area should be continuously monitored... 

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. 

The discovery of the ECD dates back to a sensitive anemometer based on ions generated from a radioactive source made from radium extracted from the luminous dials of discarded aircraft instruments. This device was very sensitive but its response was perturbed by cigarette smoke. This was a drawback in the device and to discover its source, other compounds, such as halocarbons were tested. However, in 1948, this was something to merely note for the future. Looking back I realized that the key to invention is need. We did not at that time need a device to detect low levels of chloro-fluorocarbons and so the electron capture detector was in a sense prematurely invented. [2]... 

Energy dispersive instruments of small dimensions are reserved for both qualitative analysis and routine measurements (Figures 12.10 and 12.11). The spectrum is obtained by use of a detector installed quite close to the sample, which distinguishes the energy of each of the fluorescence photons captured. These instruments are equipped with either a small size and low power X-ray tube (around 10 W) or a radioactive source for field instruments. 

Figure 12.15 Portable field apparatus. This instrument represents a category of energy dispersive spectrometers equipped with an X-ray generator but no radioactive source (reproduced courtesy of Niton, USA). Type of information that can be obtained.

NOTE The P-particle source is 12 mCi of Ni electroplated on an electrode. Even though P-particles penetrate only 8-10 cm of air and the detector cover completely shields the source, in the U.S. this is considered to be a radioactive source and cannot be discarded when the instrument wears out. It must be labeled as being radioactive, it must be tested twice a year by wipe tests, and it must be sent to a licensed operator if it needs cleaning. It must be disposed of separately as a radioactive source and a record kept for its entire lifetime. This is not considered trivial by the U.S. Nuclear Regulatory Commission (NRC), even though common sense indicates otherwise. 

Rb, Mo, Ag. Ba, Tb). In this way a number of excitation lines can be produced. The advantages of these radioactive sources lie in their simplicity and small size as well as their independence ofelectrical power. Thus they are suited to portable instruments. Spectra obtained from fluorescent emission show much better resolution than primary X-ray spectra as they lack the continuum background, figure 8.27. Consequently sensitivity and precision are rather better for fluorescence methods than they are for electron excitation techniques. ... 

One of the drawbacks of the classical BCD for routine analyses, is the radioactive source it contains. A number of conditions must be met if the instrument, with its source, is to be shipped for field study or used in aircraft or mobile-source applications. Because of these problems and the oxygen and freon interferences with the BCD method, other detection schemes have been developed for PAN analysis to improve response time and avoid potentially hazardous materials. 

To measure an energy spectrum of a radioactive source means to record the pulse-height distribution produced by the particles emitted from the source, which is achieved with the use of an instrument called the multichannel analyzer (MCA). Multichannel analyzers are used in either of two different modes the pulse-height analysis (PHA) mode or the multichannel scaling (MCS) mode. 

Wavelength-Dispersive Instruments Wavelength dispersive instruments always contain tube sources because of the large energy losses that occur when an X-ray beam is collimated and dispersed into its component wavelengths. Radioactive sources produce X-ray photons at a rate less than 10 that of an X-ray tube the added attenuation by a monochromator would produce a beam that was difficult or impossible to detect and measure accurately. 

An ionization instrument for the analysis of gas has been developed in which the gas passes through a small chamber where it is irradiated by a small radioactive source. For a constant source of radiation, the ions produced in the gas d nd on the flow velocity of the gas and on its temperature, pressure and atomic composition. The dependence of the ionization on the atomic composition is a consequence of the different ionization potentials of the differrat types of atoms of the gas and the different probabilities for electron capture and collision. The ion current is collected on an electrode and measured. This current is a function of the gas pressure and velocity since the higher the pressure, the more ions form, while at higher velocity, the fewer ions are collected as more ions are removed by the gas prior to collection. Such ionization instruments are used in gas chromatographs and other instruments as well as in smoke detection systems (the normal radiation source is Am, usually 40 kBq), where secondary electrons condense on smoke particles, leading to lower mobility for the electrons and a decreased ion current. 

An on-line analyzer must be packaged much more robustly than a laboratory instrument to withstand the process environment which, for example, may have an explosive atmosphere and significantly variable ambient temperature. It must also be capable of continuous, unattended operation over long periods of time. Clearly, the simpler the instrument the better. Of the methods listed in Table 1, WDXRF, polarized EDXRF, and Pyro-microcoidometry have not been adsqrted to on-line process instrumentation, whereas the other methods have. The relative simplicity of Pyro-EC makes it particularly suitable for adaptation to process instrumentation. The sulfur dioxide sensor is a small, plug-in, low cost electrochemical cell, easily replaceable and with an expected lifetime of over one year. The UV lamp, UV optics, and photomultiplier used in Pyro-UVF are not required. The X-ray tube (or radioactive source). X-ray detector, and X-ray optics used in all the XRF instruments are not required. 

Specitications in purchasing detectors and tests of the received instrument systems can limit the contamination in detector materials and associated nearby components such as the sample holder, preamplifier, and radiation shield to acceptable levels. The background due to cross-contamination from other samples and placing highly radioactive sources near the detector can be prevented by careful laboratory practices. For example, solid and liquid radionuclide sources should be enclosed as thoroughly as is feasible when brought to the counting room. 

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