Ratemeters

The counting-rate meter, as its name implies, is a device which indicates the average counting rate directly without requiring, as in the scaler-timer combination, separate measurements of the number of counts and the time. It does this by a circuit which, in effect, smooths out the succession of randomly spaced pulses from the counter into a steady current, whose magnitude is proportional to the average rate of pulse production in the counter. 

The heart of a ratemeter circuit is a series arrangement of a capacitor and resistor. To understand the action of a ratemeter, we must review some of the properties of such a circuit, notably the way in which the current and voltage vary with time. Consider the circuit shown in Fig. 7-25(a), in which the switch S can be used either to connect a to c and thus apply a voltage to the capacitor, or to connect bto c and thus short-circuit the capacitor and resistor. When a is suddenly connected to c, the voltage across the capacitor reaches its final value V not 

The relation between the x-ray intensity, i.e., the average counting rate, and the deflection of the recorder pen may usually be made linear or logarithmic, at 

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Fiq. 20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 nsec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37). 

Ludlum Measurements, Inc., Instruction Manual, Model 2200 Portable Scaler Ratemeter, 501 Oak Street, Box 248, Sweetwater, Texas 79556 (1982). 

This is an extension of the Allen and Svarovsky [13] x-ray gravitational sedimentometer. The x-rays are generated by an isotope source and, after passing through the suspension, they are detected by a scintillation counter. The signal from the counter passes to a pre-amplifier, and thence to a ratemeter, and a trace is recorded by a pen recorder. 

Fujino, K., Furo, K., Momoi, H. (1988). Preferred orientation of antiphase boundaries in pigeonite as a cooling ratemeter. Phys. Chem. Minerals, 15, 329-35. 

Absorption or scattering of radioactive radiation is applied in industry for measurement of thickness or for material testing. For example, the production of paper, plastic or metal foils or sheets can be controlled continuously by passing these materials between an encapsulated radionuclide as the radiation source and a detector combined with a ratemeter, as shown in Fig. 20.2. After appropriate calibration, the ratemeter directly indicates the thickness. The radionuclide is chosen in such a way that the radiation emitted is eflFectively absorbed in the materials to be checked. Thus, the thickness of plastic foils is measured by use of f emitters, whereas Cs or other y emitters are used for measuring the thickness of metal sheets. 

Another example of the application of encapsulated sources of radioactive radiation in technical installations is the control of material transport on conveyor belts. The counting rate of the ratemeter is a measure of the thickness or the density, respectively, of the material transported by the belt. By multiplication by the velocity of the conveyor belt, the mass of the transported material is obtained. In the same way, solids transported with gas streams can be determined. 

ISO. X and Gamma Reference Radiations for Calibrating Dosemeters and Dose Ratemeters. ISO 4037 (1979). 

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. 

Hence, the photographic film vaguely resembles a ratemeter because the intensity is extracted from the degree of darkening of the spots found on the film - the darker the spot, the higher the corresponding intensity because the larger number of photons have been absorbed by the spot on the film surface. The three most commonly utilized types of x-ray detectors today are gas proportional, scintillation, and solid-state detectors, all of which are true counters. 

Fig. 7-4 Block diagram of detfector circuits for a diffractometer. The ratemeter circuit actuates a meter, for a visual indication of the counting rate, and a chart recorder. The scalar and timer operate together.

The absorption of a quantum (photon) of x-rays in the active volume of a counter causes a voltage pulse in the counter output. Pulses from the counter then enter some very complex electronic circuitry, consisting of one or more pulse amplifiers, pulse shapers, etc. and, at the end, a scaler or ratemeter and, possibly, a pulse-height analyzer (Sec. 7-9). Let us call all the circuitry beyond the counter simply the electronics. Then we are interested not simply in the behavior of the counter alone, but in the behavior of the whole system, namely, the counterelectronics combination. 

Every effort should, of course, be made to measure line positions precisely. If one aims at a precision of 3 parts in 100,000, equivalent to +0.0001 A in the lattice parameter, then Eq. (11-1) shows that the 29 position of a line at 29 = 160° must be measured to within 0.02° and lower angle lines even more closely. It is better to determine the line profile by step-counting with a scaler than by chartrecording with a ratemeter. In parameter measurements it is usual to take the 29 value of maximum intensity as the line position strangely, curve-fitting techniques for establishing line position, which are standard in the field of stress measurement (Sec. 16-4), are practically never used. 

In one type of spectrometer, called single-channel, the analyzing crystal and counter are mechanically coupled, as in a diffractometer. Thus, when the crystal is set at a particular Bragg angle 0, the counter is automatically set at the corresponding angle 26. With the counter connected to a ratemeter and recorder, the whole spectrum can be continuously scanned and recorded. 

Line positions cannot be measured with sufficient precision on a chart recording made with a ratemeter. Instead, a scaler is used to determine the count rate at several positions on the line profile, and from these data the position of the line center is calculated. This procedure is particularly necessary when the lines are broad, as they are from hardened steel the line width at half-maximum intensity is then 5°-10° 29. If the line is 8° wide and the stress constant AT, is 86.3 ksi/ deg A20, as given above, a stress of 50 ksi will cause the line to shift by only 7 percent of its width when the specimen is turned through 45°. Measurement of such a small shift requires that the line center be accurately located at each angle ij/. 

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