A meter proportional to gas is a Détecteur of particles conceived to detect the ionizing radiations. It has a very good output for the Photon S of weak energy.
Description
It is about a detector with gas ionization, like the Compteur Geiger-Müller but functioning with a weaker high voltage; thus, instead of being in saturation - each arrival of Photon or particle causes a maximum spark -, one is in the linear mode: the intensity of the electric arc created is proportional to the energy of the photon, from where the name of the detector.
The meter is thus composed:
- of a metal case with a window seals with the air, but transparent with the radiations; in general, it is about a metallized polymer (in order to evacuate the electric charges which is created during the use) posed on a grid of Silicium intended to give him a good mechanical resistance; the case contains a gas;
- of a filament passing in the center of the case, and isolated from this one;
- of a generator and regulator of high voltage, which creates an high voltage between the case and the filament; the filament is positively charged, and the case negatively;
- of an electronic chain of amplification, filtering and counting.
- the meters with gas flow : the case is connected to a bottle of gas, this one is renewed permanently;
- the sealed meters : the detector is closed, it should be changed when the quality of gas was degraded.
The gas used is in general a mixture of 90% of Argon and 10% of Méthane for the meter with gas flow; in the sealed meters, the gas shock absorber is in general a Halogène (Dichlore or Dibrome).
Principle
When an ionizing particle - in the broad sense, material particle or photon - cross the window, it meets molecules of gas and ionizes them by Effect Compton. These ions are then attracted towards the filament or the case because of high voltage, and cause a peak of tension when they reach them. These peaks of tension are entered. Under the ideal conditions of use, the “number of blows” (i.e. the pulse repetition frequency) is proportional to the number of particles having crossed the detector (certain particles cause several ionizations, of other cross it without interaction).
One uses a light gas, argon, because it ionizes easily by Compton effect but with difficulty by photoelectric effect.
The relatively weak high voltage and the presence of an organic gas, methane, make it possible to avoid the Townsend avalanche (ionizations in cascade). Indeed, the ionized electrons are accelerated too little to be able in their turn to cause ionizations, except for the area of the wire; the ions thus created having a slow coefficient of diffusion, they “stagnate” around the wire, creating a space charge, which makes it possible “to absorb” the “overflow” of electrons and to avoid the avalanche in this area. The organic gas thus plays the part “of shock absorber”: it ionizes easily and its molecules are heavy, therefore slow to diffuse, it is mainly him which will form this space charge.
Adjustment of the meter
It is necessary to regulate the high voltage of the detector and the profit of the chain of amplification in order to be in the linear part. For that, one subjects the meter to a “typical” radiation, in order to have a signal.
The first operation consists in tracing the number of blows a second detected according to the high voltage (HV stud) . This makes it possible to locate the zone of linearity. The typical value, for a mixture argon-methane (10%), is about 1 400 with 1 800 volts.
Then, one traces the distribution heights of impulses ( pulsates height analysis , PHA). If the detector does not receive particles having all same energy, there would be in the ideal a very fine peak, since the height of the impulses depends on energy. In the facts, there is a broad peak, élergissement being in particular due to the Effect Doppler (the Molécule S of gas are moving). There is thus a peak whose Largeur with middle height H is proportional contrary to the square Racine of energy E of the photon:
The high voltage is adjusted so that the top of the curve corresponds to the maximum of output of the detector, in general 1 to 2 volts. The high voltage thus depends on the energy of the particles which one wants to detect. In the case of a dispersive Analysis in wavelength, energy is determined by the angle of diffraction; the high voltage is then proportional to the sine of the angle (see Loi of Bragg ), one speaks sometimes “about sinusoidal amplifier”.
The detected impulses can be due to other phenomena: particle of close energy, electronic background noise… One thus filters the impulses which are “too high” or “too low”, while trying to preserve all the impulses due to the particles in which one is interested. It is the role of discrimination.
Lastly, the last important parameter is the Idle period: if the flow of particles is too high, the detector does not have time to go back at rest between two events, it “thus loses” counting. When the detector arrives at saturation, one can attenuate the incidental signal (by filtering it, or by decreasing the intensity of the tube with x-rays); if not, it is necessary to correct this variation with the linearity.
Precautions
The most important parameter is the cleanliness of the system: the impurities in gas produce parasites which prevent the effective use. When there is an impurity, there is often a typical profile with two modes (two peaks) in the distribution heights of impulses.
In the case of a sealed meter, this is guaranteed by the manufactures quality.
In the case of a meter with gas flow, it is necessary to employ a gas of high purity, a very clean pipe (copper or stainless without grease, polymer which does not degas). The critical points are the change of the bottle of gas and the changes of the window and the filament, which can give place to the introduction of impurities. If the presence of an impurity is noted, it is necessary to purge and clean the pipe, and to change the window and the filament.
Applications
In Nuclear medicine, the meters proportional are usually not used any more. They were primarily used to count the gammas and the β basic energy. They were replaced since by the Scintillation liquid.
On the other hand, they still are very much used to detect the X-rays, in particular in Spectrométrie of x-ray fluorescence.
See too
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