Signal counter

Expansion of the network is guided by the principle that, when a new unit is added, it should be inserted at the position in the network where it can be of the greatest value this is determined by the local measure of success. There are several ways to define this location. We will consider the two most popular signal counters and local error. The difference between signal counters... 

If alternative 2 applies, the same unit might be selected by the local error measure for insertion of a new unit as would be picked by the signal counter because, in both cases, the unit is frequently chosen as BMU. Alternative 1, however, picks out units that have a low signal counter rather than a high one. It follows that the course of evolution of a GCS will depend on the type of local measure of success that is used. 

After the winning unit has been identified, say uh, the local measure at that unit is updated. If the local measure is a signal counter, the signal counter at the BMU is incremented by 1 ... 

When signal counters are used as the local measure of success, they are not allowed to grow without bound. Either after each pattern is shown to the network or at the end of each cycle, all signal counters are decreased by a small fraction ... 

The slow decay of the signal counter, unless it is boosted by fresh wins, serves a second purpose. While a large counter indicates a suitable region of the map for the insertion of a new unit, a very small value indicates the opposite. The unit may be of so little value to the network that it is a candidate for deletion (section 4.5). Unlike the SOM, not only can units be added in the GCS, they can also be removed, so the signal counter can be used to identify redundant areas of the network where pruning of a unit may enhance efficiency. 

If the local error is used instead of the signal counter, it is common to set the initial local error at the new unit to the average of the errors at all units to which it is connected ... 

In a completed map, every unit should have a similar probability of being the winning emit for a sample picked at random from the dataset. However, as the map evolves and the weights vectors adjust, the utility of an individual unit may change. Because the signal counter or the local errors are reduced every epoch by a small fraction, the value of this measure for units that are very rarely selected as BMUs will diminish to a value close to zero, indicating that these units contribute little to the network and, therefore, can be pruned out. 

Figure Bl.10.2. Schematic diagram of a counting experiment. The detector intercepts signals from the source. The output of the detector is amplified by a preamplifier and then shaped and amplified friitlier by an amplifier. The discriminator has variable lower and upper level tliresholds. If a signal from the amplifier exceeds tlie lower tlireshold while remaming below the upper tlireshold, a pulse is produced that can be registered by a preprogrammed counter. The contents of the counter can be periodically transferred to an online storage device for fiirther processing and analysis. The pulse shapes produced by each of the devices are shown schematically above tlieni.

The concentration of is determined by measurement of the specific P-activity. Usually, the carbon from the sample is converted into a gas, eg, carbon dioxide, methane, or acetylene, and introduced into a gas-proportional counter. Alternatively, Hquid-scintiHation counting is used after a benzene synthesis. The limit of the technique, ca 50,000 yr, is determined largely by the signal to background ratio and counting statistics. 

The methods for detection and quantitation of radiolabeled tracers are deterrnined by the type of emission, ie, y-, or x-rays, the tracer affords the energy of the emission and the efficiency of the system by which it is measured. Detection of radioactivity can be achieved in all cases using the Geiger counter. However, in the case of the radionucHdes that emit low energy betas such as H, large amounts of isotopes are required for detection and accurate quantitation of a signal. This is in most cases undesirable and impractical. Thus, more sensitive and reproducible methods of detection and quantitation have been developed. 

Liquid scintillation counting is by far the most common method of detection and quantitation of -emission (12). This technique involves the conversion of the emitted P-radiation into light by a solution of a mixture of fluorescent materials or fluors, called the Hquid scintillation cocktail. The sensitive detection of this light is affected by a pair of matched photomultiplier tubes (see Photodetectors) in the dark chamber. This signal is amplified, measured, and recorded by the Hquid scintillation counter. Efficiencies of detection are typically 25—60% for tritium >90% for and P and... 

In the voltage-frequency unit the unknown voltage is used to derive a signal at a frequency proportional to it. This frequency is then applied to a counter, the output of which is thus a measure of the voltage input. 

A Geiger counter monitors radiation by detecting the ionization of a low-pressure gas, as shown in the illustration. The radiation ionizes atoms of the gas inside a cylinder and allows a brief flow of current between the electrodes. The resulting electrical signal can be recorded directly or converted into an audible click. The frequency of the clicks indicates the intensity of the radiation. A limitation of Geiger counters is that they do not respond well to 7 rays. Only about 1% of the 7-ray photons are detected, whereas all the (3 particles incident on the counter are detected. Because the efficiency of a Geiger counter depends on the size of the tube, a counter used to monitor a wide range of activities usually contains two tubes of different sizes. 

A scintillation counter makes use of the fact that phosphors—phosphorescent substances such as sodium iodide and zinc sulfide (see Section 15.14)—give a flash of light—a scintillation—when exposed to radiation. The counter also contains a photomultiplier tube, which converts light into an electrical signal. The intensity of the radiation is determined from the strength of the electronic signal. 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

Spectroelectrochemical Cell Figure 5.4 shows spectroelectrochemical cells used in electrochemical SFG measurements. An Ag/AgCl (saturated NaCl) and a Pt wire were used as a reference electrode and a counter electrode, respectively. The electrolyte solution was deaerated by bubbling high-purity Ar gas (99.999%) for at least 30 min prior to the electrochemical measurements. The electrode potential was controlled with a potentiostat. The electrode potential, current, and SFG signal were recorded by using a personal computer through an AD converter. 

An interesting variant of a CEMS counter is the parallel-plate avalanche counter (PPAC) [18, 19], which carries the sample between parallel electrodes made of Perspex coated with graphite (Fig. 3.8, left). A counter gas is used to amplify the low conversion-electron current emitted by the sample, with an avalanche effect taking place between the plates. Compared with the CEMS proportional counters, PPAC gives a larger signal-to-background ratio, faster time response, simpler construction, and better performance at low temperatures. 

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