Top 6 Methods for Deduction of Radioactivity

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.