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  Principle and measurement of Radioactivity, Chemistry tutorial

Introduction:

There is need to device a method which can precisely measure (that is, qualify and quantify) the radioactivity and its effects. This will influence both application of the radioactivity and monitoring its hazards on man and atmosphere. Different kinds of equipment are used in the measurement of radioactivity and each makes use of different principles. Each has its limitation that account for preference for others.

Measurement of Radiation:

There are so many methods employed for qualitative (detection) and quantitative measurement of individual nuclear properties.

Sample Preparation:

Samples should be made up with care and effort should be ensured that it is reproducible if some samples case to be compared. Samples can be made in either liquid or solid form.

Liquid Sample:

In this kind of samples, the emitters are comprised in the detection system as it makes sure high efficiency and reproducibility. The counting of alpha particle and beta emitters are best accomplished in liquid sample system.

Solid Sample:

These are the sample methods which can be accomplished in variety of ways like precipitation, evaporation and electrolysis. The benefit of employing solid sample for counting is that, the sample can be made very robust and small, allowing the utilization of either very simple counting system (example:  Gieger Muller counter) or the utilization of commercial counting system. Though, care must be taken to make sure the uniform thickness solid media.

Qualitative and Quantitative Measurement:

Different methods and instruments are in vogue whenever it comes to doing both qualitative (detection) and quantitative measurement. 

The detection and counting device are linked altogether, therefore it is worthy to illustrate that the qualitative and quantitative measurement go on concurrently.

General Properties of Detector:

When a nuclear particle enters detector, it generates excitation and ionization, both of which can be employed for detection. The excitation, if followed via fluorescent de-excitation leads to the emission of light that can be registered via light sensitive devices like photomultiplier tube (PMT) that convert light to an elective current (i). 

I = ΔQ/Δt    Electric change/Time

If the current passes via a resistor 'R', it will generate a voltage pulse.

ΔV = R (ΔQ/Δt)

The pulse is or else termed as signal that can then be quantified. The given are the methods and instruments that are generally employed for the measurement of radioactivity.

Track Measurement:

Tracks are made by nuclear particles in cloud chambers, in solids and in the photographic emulsions. The track reveals individual nuclear reactions and radioactive decay methods.

The tracks made can be directly noticed via naked eye in cloud and bubble chambers. Though, because of short duration of tracks, it is significant to encompass a permanent record via photography. Tracks measurements are in different forms.

Cloud and Bubble Chambers:

This was introduced in the year 1911. A chamber consists of air saturated with vapor. Particles emitted from the radioactive substances ionize in air chamber. On cooling to droplet of liquid, such ions condense, leading to the production of frog-like tracks that might be photographed.

Solid State Nuclear Track Detector (SSNTD):

The main kinds of SSNTD are photographic emulsion, crystals, glasses and plastic. Because of high density, nuclear particle can read all KE in such detectors. Nuclear emulsion is identical to the optical photographic emulsion. It is employed generally for α particle measurement.

Gas Counter:

The principle of all gas filled counters in ion chambers. The ionization generated in ion chamber via a single particle is too low to be detected apart from for alpha particles. Though the ion made are multiplied greatly. General forms of gas ionization counter comprise:

Geiger-Muller Counter:

Radiation enters the tube via a thin window. Geiger-Muller counter can detect just β and γ radiations. Note that it is not appropriate for α particle because α particle can't penetrate the wall of the window.

Ion Chamber:

The ion chamber is a gas-filled space between the two electrodes. The electrodes might be two parallel plates have the in another design, cathode act as the hollow cylinder and anode acts as a thin wire in its center. The chamber is designed for recording radiation reaching it from the outside. This is employed to measure α particle or employed to compute the radioactive sub-particles within it.

Proportional Counter:

It is identical to ionization chamber. The gas multiplication is a function that differs with the applied voltage, and is constant at a given voltage. The detector pulse output is directly proportional to the primary ionization. Therefore, a proportional counter helps to differentiate α and β particles and between similar particles of various energies, once various form of primary ionization are produced. This method is as well employed to detect the neutrons.

Scintillation Detector:

Scintillating counting method was developed in the year 1908 by Rutherford and Geiger as a reliable process of counting α particle by observing visually the flashes of luminescence generated in a thin layer on ZnS via the α particle. Scintillation optical comprise of scintillator or phosphor optically coupled to a photomultiplier tube that generates a pulse of electric current whenever light is transmitted to the tube from the scintillator. General forms of scintillator comprise: gas scintillator, liquid scintillator and solid scintillator.

Gas Scintillator:

Several high purity gases are helpful scintillator notable N2, He, Ar, Kr and Xe. Apart from for N2, much of the emitted light is in UV range. Thus, photomultiplier tube which is sensitive to UV should be employed or a wave-length shifting gas such as N2 is added.  

Liquid Scintillator:

This has a broad utilization for routine measurement of emitter and can be employed for α emitters. The sample is dissolved directly in liquid scintillator solution and a light output measured via photomultiplier tubes. Liquid scintillating counting offers some benefits whenever measuring the low energy β emitters compared to other detectors with problems such as attenuation by detector window, self-absorption and backscattering are avoided. Though, introduction of sample to scintillator medium often decreases the light output greatly, a phenomenon termed as quenching. The technique as well measures α emitter.

Solid Scintillator:

Different solid media are employed in solid scintillating methods. Solid scintillator offers a great benefit in measuring virtually, all the emitters. ZnS (Ag) is a traditional phosphor for α detection, whereas anthrancene and stilbene can be employed for β particles detector. NaI with small amount of Tl (NaI(Ti)) is a most general phosphor employed in measuring γ rays.

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