1. Radiation Detectors :
Detection actually means measurement of the radiation with its energy content and other related properties.
The detection system would, therefore, consist of a detector and a measuring circuit.
The detectors are to be chosen according to the requirement.
Photographic film is used for general diffraction studies.
The GM counter and proportional; counter which are gas ionization types, the scintillation counters and the semiconductor detectors are being increasingly used for detection purposes.
The semiconductor type is used for soft or low energy radiation detection.
The choice is to a large extent dependent on the X-ray energy to be detected.

2. The interaction of radiation in different types of detectors is of different nature. There are, two different methods of measurement after detection of the radiation.
One is the counter type in which the detector output, in the form of a series of pulses, is counted by a counter over a certain interval of time.
Each pulse represents an interaction of radiation with the detector element.
The other is mean level detection, where the pulses that occur are so high per unit of time that resolution to individual ones is not possible and an average effect, generally in the form of current output, is measured.
Set up of gas-tilled detector chamber
Ionization chambers including proportional and Geiger Muller counters employ gas filled chambers with varying applied voltages to the electrodes.
A central electrode separated from the chamber by insulator is kept at a potential E. with respect to the chamber wall and this is maintained through the shunt combination of resistnace R and capacitance C.
The RC product is chosen to be much larger compared to the time required by any ion in the chamber to be collected by appropriate electrode.
For a charge q across C, the voltage output
When radiation falls on the chamber, let -ion pairs be produced and let them drift towards the oppositely charged electrodes.
Then the pulse voltage magnitude, being the measure of the ions collected by the electrode at various supply voltages can be plotted as a function of voltage. It is shown in the diagram.

3. Radiation Detectors : V IV 1 III II 2 I
If the pulse voltages are plotted for various applied voltages, it is as shown in the figure.
The curve is divisible in to several regions with respect to applied voltages. V
Curve 1 is for a stronger sources
Curve 2. is for weaker sources.
The curve is divisible in to several regions with respect to voltage.
In region I, many ion pairs are lost by recombination.
With increase in voltage ion drift velocity increases and recombination possibility decreases.
In region II recombination is negligibly small and the collected charge is directly given by IV 1
III II 2 I
where e is the ion charge, and  is the number of ions  C is actually the sum of the external and internal capacitances of the chamber. Region II is the ionisation chamber region, also known as the saturation
Pulse amplitude as a function of applied voltage for the ionization type of detectors.

4. Region II is the ionisation chamber region, also known as the saturation region.
Region Ill As the voltage increases further, electrons, because of their large mobility are accelerated and acquire higher energy, in turn, producing secondary ionization.
This effect of secondary ionizationis dependent on what is known as gas multiplication which initially is constant and the output is seen to be proportional to the input.
This part is the region of proportional counter.
In the latter part of region Ill, the gas multiplication factor decreases with increased production of primary ion pairs and as the secondary ionization occurs close to the anode, formation of a positive sheath around it further limits the amplification factor.
This is a region of limited proportionality.
The region IV With a further increase in voltage, all the gas molecules close to anode are ionized and the output is fairly constant.
has a plateau called the Geiger plateau and the voltage at which this taris to form is called the Geiger threshold. This region is the Geiger Muller counter region.
When acceleration of electrons increases due lo further increase of voltage, secondary ionization is associated with the release of photons also, which in turn ionize the gas molecules.
Region V Finally, the entire sensitive gas volume is ionized and complete discharge sets in the chamber.
When radiation ionizes the gas in the ionization chamber the produced charged particles behave most normally but depending on the gas, the field and the chamber condition.
The faster electrons have a drift velocity that is a function of the gas pressure p and the electric field EF

5. Ionization chamber
A parallel plate ionization chamber is shown in the Fig.
The electrodes form a parallel plate capacitor as housed in the chamber.
The collector electrode is connected to the measuring circuit and has a varying potential depending on the ionization current through the chamber.
Proper insulator mounting and guard rings are provided to avoid leakage and produce a uniform field throughout the section.
The sensitive or active volume of the chamber is shown enclosed by the broken lines.
The ionization current is then expressed as the integral of the ion pairs number n0 produced per unit volume over the active volume V0 multiplied by charge e.
Thus

6. Proportional Counters.
When the electric field strength at the center electrode of an ionization chamber is increased above the saturation level, but under that of the Geiger region the size of the output pulse from the chamber starts to increase but remains proportional to the initial ionization.
A device operated in this fashion is called a proportional counter.
The pulse size in a cylindrical proportional counter is calculated from the consideration of construction and dimension of the chamber, its capacitance and contained charge etc.,
Proportional counters are generally used in pulse-type operation.
This is a gas ionisation chamber where gas multiplication is obtained by increasing the pulse size but making it independent of the primary ionization. Amplification as high as 106 can be obtained here.
Pulse formation is identical with that described for Geiger counters, but gas amplification is approximately 1000 times less.
So, a preamplifier ( x 10) is needed and is mounted close to the detector to avoid reducing the pulse size through capacitance in connecting cables.
The dead time is very short, about 0.25 sec.