Detector Materials Specific types

A specific type of proportional counter that is used for accurate counting of a and P activity on smear samples, is the gas flow proportional counter. In some types of gas flow proportional counters, the sample is put inside the detector for greater sensitivity—there is no structural material to absorb the radiation before it can be detected. Figure 5.24 shows a diagram of the detector of such a gas flow proportional counter. Other gas flow proportional counters have very thin Mylar windows sealing off the detector s gas chamber. During counting, samples are positioned very close to the window. 

For all detector types, the transfer of energy to the detector material as the photoelectrons and recoil electrons slow down occurs through excitation or ionization of electrons in neighboring atoms within the detector structure. What occurs next depends on the specific molecular structure of the detector. We can distinguish between structures such as noble gases, photoconductors, fluorescent phosphors, and photostimulable phosphors. 

Notice that the dose has a strict definition of energy per unit mass of the absorber and, in principle, can be measured for a given radiation at a certain energy in a specific material. The equivalent dose is a relative unit in that a radiation weighting factor is applied to a measured quantity. The dose can be measured from ionization in an electronic radiation detector the equivalent dose must take into account the type of radiation causing the ionization. 

Specific detection is an analytical determination based on specific responses related to the chemical characteristics of a molecule excited by a certain type of irradiation. In this detection method, measurement of the molecule of interest may usually be performed without separation from matrix materials or from other ingredients if appropriate instrumental adjustments are made. The need for identification and structure elucidation for newly discovered compounds drives the progress of specific detection techniques with NMR and X-ray diffraction and MS. The UV-diode array detector often aids the recognition of chromatographic peak purity, ensuring complete resolution of a separation procedure. 

The first developments of this type of measurements were first introduced in 1974 by Keramidas et coll, and applied to the characterization of ceramic materials [47]. Later, this method was specifically applied to the characterization of oxygen storage materials for TWC and fully described by Bernal et al. [48]. Briefly, experiments consist in injecting 5%02/He pulses (0.25 ml) every 10 seconds into the inert gas flowing on the 200 mg sample (60 ml.min ). Gas composition is followed using a gas chromatograph equipped with a TCD detector. 

There are three types of detectors Thermal Conductivity Detector (TCD), Flame Ionization Detector (FID) and Electron Capture Detector (ECD). In these detector systems, EID exhibits the highest sensitivity for the detection of hydrocarbon materials and TCD has been used commonly for the detection of inorganic components and organic materials except for hydrocarbons. The ECD is used specifically for the detection of materials with large electronegativity, such as alkyl halides. Recently, GC has been equipped with a mass spectrometry system (GC-MS). 

Check proximity of flammable materials to any potential ignition sources. Open flames and devices that generate sparks should not be near fiammables. Pay special attention to devices placed in fume hoods that do not meet National Electrical Safety Code (U.S. DOC, 1993) Division 1, Group C and D explosion-resistance specifications for electrical devices. Stirrers, hot plates, Variacs, heat tape, outlet strips, ovens (all types), refrigerators, flame sources (e.g., flame ionization detectors (FIDs) and atomic absorption spectrometers), and heat guns constitute the majority of devices that do not typically conform to these code requirements (see section 8.C.6.1). 

Although not specifically delineated, the volume is also divided into three general sections. The first addresses the full spectrum of infrared detectors and contains a limited coverage of all the material presented in subsequent chapters. It serves as an introduction to the volume and presents to the reader an overall view of the present state of the infrared technology art. It also serves as the mortar between the more in-depth discussions which follow. The midsection. Chapters 3,4, and 5, is a detailed analysis of those detector types which are most widely used today thermal, photoconductive/photovoltaic and photoemissive. 

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