Penetration Power Of Radiation Energies

The penetration power of the radiation energy is related to the type of radiation. The radioactive chemicals emit radiation in the form of particles or rays and the penetration power of these particles or rays in the tissues of our body varies due to variation in the energy of these particles or rays. The alpha (α) particles can not penetrate more than a few micrometers in our body tissue, and are of little practical importance in medicine. The beta negative (β^-) and beta positive (β⁺) particles have penetrating power varying from 100 to 500 micrometers (mm) as in case of radiations of Carbon-14 (¹⁴C) and Sulphur-35 (³⁵S) to over a centimeter (cm) as in case of Yttrium-90 (⁹⁰Y) radioisotope. The beta negative (β^-) particles from Gold-198 (¹⁹⁸Au), Gold-199 (¹⁹⁹Au) and Iodine (Iodine-125, 130, 131, 132 etc) have a penetration power in tissues ranging from 1 to 3 millimeters (mm). The gamma rays (g-rays) are like x-rays and are usually very penetrating. The energy range of gamma rays (g-rays) is almost equal to that from 40 kilovolt (KV) to 3 megavolt (MV) x-ray machines. Negative beta (b^-) particles may or may not have accompanying gamma rays (g-rays). The gamma rays (g-rays) emitted by a particular radioisotope would always have the same penetrating power or energy. The positrons or positive beta (b⁺) particles in addition to possible gamma rays (g-rays) are always accompanied by 50 KV x-rays.

Quantity of radioactive material is always expressed in terms of radioactive disintegrations per second. The major unit of expression of radioactivity represents 37 billion (37 x10⁹) disintegrations per second and is called Curie (Ci). One thousandth (¹/₁₀₀₀) part of a Curie is called milliCurie (mCi) and one thousandth (¹/₁₀₀₀) part of a milliCurie is called microCurie (mCi). Brief description of these units is as below:

1. Curie (Ci): 37x10⁹ disintegrations per second


2. milliCurie (mCi): 37x10⁶ disintegrations per second


3. microCurie (mCi): 37x10³ disintegrations per second

There are quite many radioisotopes used in medical practice as a therapy and also in medical diagnostic procedures. The quantities of radioactive materials used in therapy are in milliCuries (mCi) and those used in diagnostic procedures is in microCuries (mCi).

What Is Meant By Radioactive Disintegration

Radioactive disintegration is a process of nuclear disintegration of a radioisotope in its effort to achieve a stable nucleus. We know that in the naturally available radioactive elements there are only two kinds of particles which could be ejected from its atoms:

1. The alpha (a) particle, which is really the nucleus of a Helium atom (⁴He) and carries away 4 mass particle and 2 atomic particles.
   
   _ZX^A - ₂a⁴ = _Z-2X^A
   
   Here X represents chemical symbol, Z is atomic number and A is mass number.
   
   
2. The other particle which could be ejected is beta (b) particle, which is an electron. It does not however, comes from an orbit, but from a neutron, which under certain circumstances, dissociates into a proton and an electron. The electron is not tolerated in the nucleus and is ejected immediately, but an extra positive charge (neutron replaced by a proton) is left in the nucleus. The new atom now has the same mass but the atomic number one higher than the old or previously possessed by it.
   
   _ZX^A - _-1b⁰ = _Z+1X^A

Uranium, Thorium and Radium are the best known naturally occuring radioactive elements. In 1934 it was documented that it was possible to create isotopes and radioisotopes by bombarding the stable elements with high energy subatomic particles. Except Hydrogen and Helium more than two isotopes have been created from every element by artificial manipulations. There are 21 isotopes of Iodine ranging from ¹¹⁹I to ¹³⁹I and out of these 20 are radioactive isotopes or radioisotopes except ¹²⁷I.

The artificially created radioactive isotopes or radioisotopes have the same radiations as those of natural ones. Some of them also emit protons or beta⁺ (b⁺) particles.

_ZX^A - ₊₁b⁰ = _Z-1X^A

Radioactive disintegration of radioisotopes results in the emission of only one type of above mentioned particles and radiation like x-rays called gamma (g) rays. The atoms of any particular radioactive element are destined to emit the same kind of radiation till its total disintegration; there is no way to switch on to any other type of radiation.