The following points highlight the top 6 methods for deduction of radioactivity. The methods are: 1. Ionization 2. Ionization Chambers 3. Gesger Counters 4. Semi-Conductor Detectors 5. Scintillation 6. Autoradiography.
1. Ionization:
The passage of high energy radiation through matter results in the formation of ions as a result of collision of electrons with atoms. So much energy is transferred to the orbital electrons that on escape from the atom, it gives rise to a slow positively charged ions and a very fast secondary electron.
If the ions and electrons are not immediately separated by means of an electric field they recombine and remain un-decayed. Electrons slowed by multiple collision can be captured by reactive gas molecules and give rise to negative ions.
An ion strongly accelerated in an electric field may collide with a neutral gas molecule and thereby give rise to a fresh positive ion and electron. X- and y- rays must also first give rise to free electron before they can be detected and since the probability of ionization occurring decreases rapidly with increasing energy. Such a radiation is more difficult to detect.
2. Ionization Chambers:
While ionization chamber measurements form the absolute basis of descimetry, the method is too slow and insensitive for detecting short-lived radioactivity.
3. Gesger Counters:
These are gas-filled counters operating at reduced pressure. They do not measure continuous currents like ionization chambers but register collision ionization. The primary ions in the counter gas are multiplied by applying an electric field of 800 to 2,000 V.
In the range of 200 to 600 V, number of ions present is strictly proportional to the number of primary ions, and for this reason proportional counters can be used to distinguish p rays from highly ionizing particles. The life-time of a gas-filled counter is limited by the capacity of the gas to a total of 109 to 1010 collision discharges.
4. Semi-Conductor Detectors:
When silicon crystals are irradiated, ionization occurs and secondary electrons are released with the aid of electron donors (for example lithium). These can be conducted to electrodes and measured as current pulses. Such drift detectors are suitable for detecting corpuscular and low energy X- and y- rays at room temperature. On account of their extremely small size they can even be implanted.
5. Scintillation:
This is the name given to the light flashes emitted by luminescent substances when excited by high-energy radiation. The flashes can liberate photo electrons from photosensitive substances. The photo electrons are amplified 107 to 105 times by means of a photo tube multiplier before being converted into current pulses.
The pulse height depends on the energy of the original, γ-radiation and the pulses can be sorted by means of a discriminator. By using different discriminator or channels, the different radioactive substances in a mixture of isotopes can be determined either successively or simultaneously.
(a) Solid Scintillators:
The commonest type in use in nuclear medicine consists of single crystals of thallium activated sodium iodide. Since the decay time of fluorescence is only 0.25µs the scintillation crystals have resolving power about a thousand time greater than gas-filled counters.
Since there is no dissipation of the crystals the life-time of these scintillation detectors is limited only by that of the replaceable multiplier and its semi-permanent photosensitive layer. The wavelength of the luminescent radiation is about 410 nm.
(b) Liquid Scintillators:
The radiation from preparations emitting β-rays, soft X-rays or γ-rays can be measured with particularly high pulse yield if they are mixed directly with a scintillator solution. Only a few highly purified alkyl benzenes (mainly toluene and xylene) and ethers are suitable as solvents. They transmit the radiation energy to the scintillators by ionization via metastable excited states.
The first scintillator usually consists of a solution of 2, 5- diphenyloxazole in toluene. Its fluorescence has a maximum at 380 nm. Since the photocathodes of many multipliers develop optimum activity only at wavelength above 400 nm the spectrum of the primary fluorescence must be displaced to higher wavelengths by using a second fluorescent substance.
6. Autoradiography:
The second scintillator converts the ultraviolet radiation from the first by fluorescence into radiation with a wavelength of about 420 nm. If the radioactive preparation is insoluble in the solvent, other substances must be added to make it soluble.
The oldest method of detecting radioactivity is the photographic one. Both electromagnetic and corpuscular radiation cause electrons to be expelled from the halogen atoms in the silver halide grains in a gelatin emulsion.
Each electron reduces a silver ion to metallic silver. The sites at which this occurs in the silver halide grains constitute “development centres” where the developer begins the reduction of the whole grain to black metallic silver. The resulting degree of intensification is about 1012.
Macroscopic autoradiography is used in nuclear medicine mainly for radiation exposure monitors; another use is the localization of radioactivity in chromatograms and in organ sections of large surface area.
The film used is high-sensitivity X- ray film; for pure y-ray sources an intensifying screen is usually necessary, though this reduces the sharpness of the image.