Wire anode

Counter, Gas-flow Proportional (GPC)—P-particles are detected by ionization of the counter gas which results in an electrical impulse at an anode wire. If a sufficient amount of radiostrontium is present and the ionization efficiency is calibrated, the quantity of radiostrontium can be determined. 

The avalanche of electrons in proportional detectors is collected only on part of the anode wire. Furthermore, only a small fraction of the gas volume of the... 

To make the reference electrode, anodize a 5-cm-long silver wire (1 mm diameter) in a saturated KC1 solution to form AgCl. Introduce the anodized wire in a 250 pL micropipette tip through a syringe rubber piston. Fill the tip, which contains a low-resistance liquid junction, with saturated KC1 solution. 

The detector operates according to the delay-hne principle. Here a 20 pm thick anode wire and a cathode which consist out of parallel metal strips connected to a delay line are used. The charge creating event induces a signal in the cathode which propagates with a velocity of about 0.2 mm ns in both directions of the... 

The basic principle of operation of gas filled detectors, be it multiwire proportional chambers or linear devices, with a single anode wire, is the mechanism of gas amplification. This kind of detectors have — as its name implies, a filling of an... 

Thus, to a first approximation, the gas amplification depends exponentially upon the number of charge per unit length of anode wire, the logarithmic term being of less influence. 

The derivation for the gas amplification does not take into account the effect of the drifting positive ions in a proportional counter. Indeed, due to the narrow amplification region, around the anode wire, practically all the charges are created in a very small volume. These ions drift rather slowly away from the wire. They tend to reduce the electric field around the wire. If a second photon is detected at the same position on the wire, the field and consequently, the gas amplification will be lower. Since the density of the positive ions, in the anode-cathode gap increases... 

The sheath of positive ions causes the effective potential of the anode wire to decrease. 

Fig. 3 a and b. Pulse height (normaUzed to low rate pulse height) versus count rate per anode wire length. Parameter the number of charges per avalanche, a Standard proportional counter b Optimized high rate counter (From Ref. )... 

Since E, is very high in the vicinity of the anode wire (some 10 V/cm), the signal rises very rapidly, and then grows slowly until the positive ions reach the cathode. This can take up to several hundred psec. 

The resistive chain, shown in Fig. 8, can be a resistive anode wire itself, for instance in a linear position sensitive detector, or it can consist of resistors connecting the cathode read-out pads. 

As compared to the previous method, this technique uses the resistive chain as a component of a diffuse transmission line The resistive elements are replaced by a highly resistive anode wire which, together with its capacitance per unit length, forms a distributed RC-transmission line. The anode wire can be made as a quartz fiber coated with carbon. 

In fact, in this method there is no necessity of any other read-out electrode, as for instance in the case of the delay-line technique. This can be certainly an advantage. But unluckily, the highly resistive anode wire shows to be very sensitive to mechanical damage. An occasional high voltage breakdown either destroys the coating or, at least, changes its properties locally and thus the detector response. 

The RC-method, or rise-time method, is shown in Fig. 10. Charges injected into the wire by an avalanche at a location x cause a current to flow through the anode wire in both directions, until the initial charge is spread out over the entire length. The position of the avalanche is estimated by measuring and comparing the two currents at both ends of the detector. 

Table 2 shows the values as calculated for two anode-cathode gap thicknesses, anode wire diameters (2a) and spacings between them (s) (see Fig. 11). 

From the derivation for the gas amplification in section II-A, we know that to a first approximation, A is a function of CV = Q, the number of charges per anode length. From the values in Table 2 it becomes clear that, if the pitch s is made smaller, the capacitance decreases. This means that the operating voltage V, has to be increased to maintain a sufficient gas amplification. Another possibility is to choose finer anode wires. 

Since the avalanches are generated in the close proximity of anode wires, the spatial resolution of MWPC will be quantized in the direction perpendicular to the anode elements. This is one of the principal disadvantages of the MWPC as a position sensitive detector. 

The spatial resolution along the anode wires, on the other hand, can be very good in a MWPC, As we will see, there exist several positional read-out schemes which allow a better resolution than the distance between the cathode read-out elements. Perpendicular to the wires, the resolution is limited to the distance between them, since there is no signal generation possible, except on the anode wires. 

In many applications full two-dimensional information is necessary, and thus fast read-out systems have to be provided. The various techniques, which have been discussed before, have been applied with area detectors. The delay-line read-out has been used with two-dimensional detectors in synchrotron radiation laboratories. At the DORIS-ring in Hamburg an area detector with a 1 mm anode wire distance and a total area of 200 mm x 200 mm is currently in use for measurements of muscle diffraction patterns and for X-ray crystallography The spatial resolution is about (2,5x2,5)mm FWHM. 

In this method, the signal originating at each wire (both the anode wires and the cathode wires) is processed by an individual amplifier-discirminator circuit. This requires many amplifiers. For instance, for a two-dimensional detector with a resolution of 256x256, a total of 512 amplifier-comparator systems are necessary. Each circuit on its own, however, is not complicated, and large scale fabrication is not too difficult. 

Corresponding to E, there exist a critical radius r,., determining the critical volume around the anode wire, within which avalanche formation can take place. At the distance r from the centre, the potential is V. The mean free path, between ionization is, by definition... 

In In counters half of the total radiation emitted by the sample is recorded, whereas 47t counters are equipped with two anode wires and the radiation emitted by the sample can be counted quantitatively. These types of counters are used for the measurement of the absolute activity A of radioactive samples, because an overall counting efficiency / == 1.0 in eq. (7.3) can be obtained. 

A gas-filled detector is basically a metal chamber filled with gas and containing a positively biased anode wire. A photon passing through the gas produces free electrons and positive ions by the interactions previously described. The electrons are attracted to the anode wire and collected to produce an electric pulse (see Fig. 5.20). 

FIGURE 19.4 In a Geiger tube, radiation ionizes gas in the tube, freeing electrons that are accelerated to the anode wire in a cascade. Their arrival creates an electrical pulse, which is detected by a ratemeter. The ratemeter displays the accumulated pulses as the number of ionization events per minute. 

The electric field near the end of the anode wire is not uniform. Most proportional counters are now made with a side window, rather than the end window shown in Fig. 7-17, so that x-ray absorption can take place in a region of uniform field. 

This is a very recent development. It involves a side-window position-sensitive proportional counter (Sec. 7-5), a multichannel analyzer, and the rpeasurement of the angular positions of many diffraction lines simultaneously. The anode wire of the counter, which is long and curved, coincides with a segment of the diffractometer circle and is connected, through appropriate circuits, to an MCA. The powder specimen is in the form of a thin rod centered on the diffractometer axis. The geometry of the apparatus therefore resembles that of a Debye-Scherrer camera (Fig. 6-2), except that the curved film strip is replaced by a curved counter. 

When monochromatic radiation is incident on the specimen, it sends out diffracted beams at particular W angles. These beams enter the side window of the counter at particular points, causing pulse formation at those points. The times required for these pulses to travel to the end of the counter are converted to digital form, analyzed, and sorted by the MCA. The contents of the MCA memory are therefore number of pulses (counts) as a function of the position on the anode wire where the pulses originated (angle 26). A display of the contents of the MCA resembles the pattern recorded by a conventional diffractometer (Fig. 7-5). 

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