Ionization chamber

A ionization chamber is a Détecteur of particles which locates the passage of a particle by measuring the total load of the electron S as of the Ion S produced during the ionization of the medium (gas) by the particle.

Description

To recover the electrons and the ions before they recombine in atoms, the presence of a Electric field is necessary to separate them and to make them derive towards electrode S. the loads (electrons and ions) while deriving induce currents on the electrodes. These currents are detected by a Amplificateur which produces an electrical signal.

The median number of pairs of electron-ion produced in a passage of a particle charged is given by the formula of Bethe - Bloch. The loads detected by the amplifier depends on several factors, above all the high voltage of the electric field. Once the tension is enough large to prevent the recombinations, the loads of ionization derive almost completely towards the electrodes. One obtains a signal which reflects the total load of ionization.

The detectors operating in this area, for example the rooms with liquid air have excellent a resolution in energy and a very good Linéarité. But the signals are rather weak since it there not of amplification of the loads in the detector.

Detailed description

This type of detector measures the load deposited by a particle charged crossing a ionizable medium, which can be a gas, a liquid, even a solid, each one having its advantages and its applications.

A sufficiently energy charged particle is able to tear off the electrons of the atoms of the crossed medium, it is the process of ionization. The median number of pairs of electrons and primary ions thus created by the passage of a particle charged is given by the formula of Bethe-Bloch: NR = -d.dE/dx/W where D is the thickness of the detector, and W average energy necessary to create a pair. In the gases W about 30 eV is.

In a detector with ionization, the medium is plunged in an electric field generated by a pair of electrodes, generally of cylindrical or plane geometry. The electrons lately created move then towards the anode and the ions, towards cathode. According to the type of wanted effect, the anode can take the shape of one or more very fine wire close which the electric field becomes very intense and where the electrons are accelerated until being able to ionize other atoms, creating secondary electrons , able in their turn to ionize atoms, this several times of continuation. It is the phenomenon of avalanche.

The electrons, approximately thousand times faster than the ions, are quickly captured by the anode, but the current of the ions deriving towards cathode induces a relatively important electrical signal on the electrodes, directly measured by a preamplifier which produces the electronic signal.

The signal detected by the amplifier depends on several factors, and above all the electric field applied between the electrodes and, in the case of a detector with gas, pressure.

The operational areas of the detectors with ionization are the following ones:

the area of recombination

When the electric field between the electrodes is weak, the electrons and the ions can recombine in atoms at once after their creation. Only a small fraction of the loads of ionization is detected by the amplifier.

the area of ionization and rooms with ionization

Once the electric field is enough strong to limit the recombinations, the loads of ionization derive almost completely towards the electrodes. One obtains a signal which reflects the total load of ionization. Detectors operating in this area, for example rooms with liquid (Ar) argon and detectors with semiconductors (If, Ge), have an excellent resolution in energy and the measured signal is already rather proportional to the load deposited (good linearity). The signals are rather weak because it there not of amplification of the loads in the detector, and the special amplifiers with low noise are necessary.

the area proportional

If the electric field is sufficiently strong (E ~ 10⁴ V/cm), the electrons are accelerated by the electric field and gain enough energy to produce secondary ionizations. Since the probability of a secondary ionization per unit of length (A) is constant for a given electric field, the full number of ionizations is proportional to the number of initial ionizations: NR = N₀ e^αd. The factor multiplication is given per M = e^αd = 10⁴ to 10⁸. The detectors operating in the area proportional are generally detectors with gas, because the gases make it possible to obtain a great multiplication factor at the time of the avalanche. The advantage of the rooms proportional is that they do not require electronics with low noise. They can be used for measurements of energy, but the precision is less good because of the fluctuation of the process of amplification and the factor multiplication depends on several factors of environment (tension, temperature, etc). The most important application of the rooms proportional is the measure of location, like the rooms proportional multi-wire ( Multi-Wire Proportional Chambers , or MWPC) and the rooms to drift. The rooms with drift are ideal as tracer in front of a calorimeter because the particles lose little energy in gases. The advantages of the gas chambers include a relatively low number of wire of anode, and a space good resolution, about 50 µm, and an easy construction allowing of the detectors of large surface.

the area Geiger

When the electric field is sufficiently strong, the primary education electrons are able to ionize other atoms very quickly and a very intense avalanche occurs. Moreover, one great number of photons are created in the process by de-energizing of the atoms. These photons initiate them also avalanches of ionization per photoelectric effect, with the length of the wire of anode where the electric field is strongest. These avalanches are sufficiently intense to generate an electric shock in gas, if powerful that it is audible. It is the principle of the Geiger counter. The discharge stops only when the space charge formed by the sheath of positive ions around the anode écrante sufficiently the electric field around this one so that the process of multiplication cannot continue any more. During this time the detector is not sensitive any more to ionizations primary education, it until the ions sufficiently migrated far from the anode. It is the origin of the idle period in the Geiger counter.

In a discharge, the current of anode is saturated. The amplitude of the signal is thus independent of the primary loads. The Geiger counters cannot measure the energy of the particles, but are used they to count the number of particles the beam, even with weak energies. This is useful for measurements of radioactivity. The measurable maximum rate is limited by the idle period.

the area of discharge

To increase the field beyond the Geiger area involves a continuous discharge. A detector is not useful any more if it is in this area.

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