Four Ways of Reducing Radiation Exposure

The health of radiation professionals needs to be protected from over exposure to radiations. The four most effective ways of reducing exposure to radiation have been elaborated below:

1) Radiation protection shielding

When a sheet of absorbing material is placed between a radiation source and a detector, the radiation arriving at the detector decreases to an extent depending on the energy of the radiation and the nature and the thickness of the shield. For gamma rays (g-rays) lead is generally installed because for a given weight it absorbs more radiations than any other readily available shielding material. Its effectiveness for a particular radiation is usually indicated by the half value layer (HVL). The half value layer (HVL) is the thickness of the lead sheet capable of reducing the radiation to one half (50%) of the original. A second half value layer (HVL) will reduce the remaining half to half (i.e. 25%). A third half value layer (HVL) would
reduce the radiation to half of 25% (i.e. 12.5%) or exactly the ¹/₈th of the original and a fourth half value layer (HVL) would further reduce the radiation to ¹/₁₆th of the original. The thicknesses of HVL for some important radioisotopes used in medical field have been given below:

Radioisotope	HVL in cm Lead
Cesium-137	0.5
Chromium-51	0.2
Cobalt-58	0.7
Cobalt-60	1.2
Gold-198	0.3
Iodine-130	0.2
Iodine-131	0.3
Iodine132	1.0
Irridium-192	0.3
Iron-59	1.1
Potassium-42	1.2
Sodium-22	1.0
Strontium-90	1.2

2) Time

The exposure time is directly proportional to the time spent at a place of radiation. The necessary time for any procedure ought to be estimated well in advance, allowing a good safety margin. The equipments used in radiation applications should be simple to minimize the time of operation with a view to reduce the exposure time. But any necessary handling precaution should not be omitted to save time.

3) Distance-Inverse Square Law

The ionization radiation travels like the light and as we go away from a point source, the amount of radiation reaching a given area would decrease. The decrease would be proportional to the square of distance in centimeters. For example: A point at 40 cm from the source would receive ¹/₄th (25%) of the radiation reaching at a point at 20 cm from the source. As ^20x20/_40x40 = ¹/₄.

4) The Gamma Factor for Gamma Emitters

The dose rate at 1 cm from the point source of each gamma emitter radioisotope has been determined and with reference to this the dose rates are worked out in terms of roentgens(r). The roentgen (r) is that amount of radiation which delivers a dose of one rad. A roentgen (r) is unit based on ionization in air and rad is a unit of energy absorption, but both have corresponding amount of energy. For example: the gamma factor of Iodine-131 (¹³¹I) is 2.18, of Gold-198 is 2.35 and that of Sodium-24 is 18.4 which means that 1 milliCurie (mCi) of these radioisotopes would have a dose rate of 2.18 , 2.35 and 18.4 roentgens or rad respectively at 1 centimeter from point source. Now, if 200 mCi of Iodine-131 (¹³¹I) is placed on a table in the laboratory, its dose rate at 1 centimeter distance would be 200x2.18 entgens and at 40 cm distance it would be ^200x2.18/_40x40= 0.273 roentgens/hour or 273 milliroentgen/hour (by Distance-inverse square law). A dose rate of 15 milliroentgen/hour is considered safe at 40 cm from the point source. So to attenuate the radiation we need to place a shield of lead around the Iodine-131 (¹³¹I) container. The attenuation factor for Iodine-131 (¹³¹I) could be calculates as ¹⁵/₂₇₃=¹/18 of its value; which means we need a little more than 4 half value layers of lead shield. Four half value layers for Iodine-131 (¹³¹I) would be 4x0.3=1.2 centimeter thick lead shield.

Safe Handling of Radioisotopes

General health precautions while handling the radioisotopes or radiochemicals are must for all professionals associated with radioisotopes or radiochemicals. Regardless of the quantity of radioisotope or radiochemical, strict precautions and personal cleanliness awareness are indispensable. Some important tips in this regard are listed below:

1. Laboratory coat should be worn to protect the clothing.


2. Rubber or plastic gloves should be worn.


3. All handling of radioisotopes or radiochemicals should be done on the surfaces lined by absorbent material.


4. Dispensing of radiochemicals should be done over stainless steel, aluminium or plastic trays to contain any spillage.


5. Eating and smoking should be avoided in a hot laboratory (the laboratory where radioisotopes are used is called hot laboratory).


6. At the end of each procedure, the person responsible should cleanup his/her work space, dispose off any contaminated material in a suitable fashion.


7. If a spill occurs, that should be dealt with immediately and brought to the notice of Radiation Safety Officer.


8. The hot laboratory should be monitored periodically for unknown radiation from accidental spillage of radiochemicals.

What is the use of Iodine-131 in Medical Practice?

There are two main uses of Iodine-131 (¹³¹I) in medicine or medical practice.

1. Diagnostic application in some procedures


2. Therapy for thyroid disorders or thyroid cancer

The Iodine-131 (¹³¹I) has a radioactive half life of 8.1 days and its radiations are beta particles(b^- particles) and gamma rays (g-rays). This is most widely used radioisotope in the management of hyperthyroidism and thyroid cancer and thyroid function related diagnostic procedures. There are half a dozen investigations associates with the thyroid function which involve the oral or intravenous administration of a few microCuries (mCi) of ¹³¹I. Subsequent study of the patient, either by the direct measurement of ¹³¹I deposited in the thyroid gland through measurement of ¹³¹I excreted in the urine of the patient or by assessment of radioactivity in the blood samples drawn at different time intervals after the administration of ¹³¹I. The part of the ¹³¹I retained or excreted depends on the normal, hyperthyroid or hypothyroid conditions. The uptake or excretion of ¹³¹I exhibits a diagnostic parameter. After absorption of ¹³¹I by the thyroid gland the iodine is elaborated into the thyroid hormone which is discharged in the blood. The hyperactive gland produces too much hormone which would be detected in the blood samples taken at 24 to 96 hours after the administration of radioactive iodine (¹³¹I). The measurement of the iodine content is computed from the counts of radioactivity detected in the blood samples. The radioactivity is measured as gamma rays (g-rays) by a Geiger Muller counter or it may be measured as beta particles (b-particles) by a Scintillation counter at a 'Hot Laboratory'.

The therapeutic use of Iodine-131 (¹³¹I) could be culminated through optimal doses of this radioisotope with reference to the thyroid disorder and the age and weight of the patient. There are specialized clinics at the authorized medical centers having facilities for the Nuclear Medicine and associated research.

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 Radioactive Decay Or Half Life Of A Radioisotope

Radioactive isotopes or radioisotopes of an element are always in the process of nuclear disintegration in order to acquire the stable form. The major unit of radioactivity is Curie (Ci) which means 37x10⁹ disintegrations per second. 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). Other units of the radioactivity will be discussed in some other article. The radioactive chemical is being expressed in terms of radioactivity it possessed at the time (0 hour) of evaluation and labeling. Every radioisotope undergoes decay or nuclear disintegration at a uniform rate and the time after which it loses the half of its activity is called its Half Life.

Radioactivity is linked to per unit mass or volume of radioactive chemical. The radioactive Half Life could be a few hours, days or many years. For example ²⁴Na has a Half Life of 15 hours, ¹²⁵I has a Half Live of 60 days, ⁶⁰Co has a Half Live of 5.2 years and ¹⁴C has a Half Live of 5730 years. If 1gram of a radioactive chemical has 2Ci radioactivity at 0-hour, it would be reduced to 50% (1Ci) after the completion of 1st Half Life, 25% after completion of 2nd Half Life, 12.5% after the completion of 3rd Half Life and 6.25% after the completion of 4th Half Life and goes on reducing to 50% on the completion of successive Half Lives as depicted below, through the Radioactivity Decay Graph.

However the mass or volume of the radioisotope would not under go any change with the reduction in radioactivity due to passage of time and completion of successive Half Lives one after the other. Preparation of Radioactivity Decay Graph is must for the radioisotope users to workout the radioactivity at a particular time or date with respect to the Half Life of a radioisotope.

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.

How The Structure Of Matter Is Associated With Radioactivity

We know that the matter is made up of elements. The smallest part of any element is its atom. Atoms are composed of a positively charged nucleus containing protons (positively charged subatomic particles) and neutrons (inert particles), and around the nucleus, there are orbital electrons (negatively charged subatomic particles). In 1896 Bacquerel discovered the phenomenon of radioactivity in the atoms of some elements. The mass of an atom is represented by its nucleus that is the sum of protons (positively charged subatomic particles) and neutrons (inert particles) in the nucleus. Each element has been allotted a chemical symbol and its atomic number is fixed. The number of electrons is always equal to the number of protons in the nucleus of an atom and this number stands for the atomic number of an atom. Mass of an electron is ¹/₁₈₀₀ of the mass of a proton on atomic scale. The atoms in some of the elements have natural variation in mass number and that made them unstable or radioactive.

Isotopes: Atoms of a particular element not have to be exactly alike in terms of mass number. In such atoms the things that must be alike are the number of protons (positively charged subatomic particles) in the nucleus or the nuclear charge and the number of orbital electrons (negatively charged subatomic particles). But the number of neutrons (inert particles) may vary and hence atomic mass may vary in a narrow range. Atoms of the same element with same atomic number but with different number of neutrons (inert particles) are called isotopes. A single chemical symbol of an element is not sufficient to represent an isotope. The chemical symbol along with a superscript at the upper left or right, depicting the mass number and lower left depicting the atomic number, represents an isotope; however, it is not necessary to mention atomic number to depict an isotope, as ⁵¹Cr represents an isotope of Chromium. Each element has a unique atomic number but the mass number may vary depending on the number of the isotopes of that element. An element generally has only one stable isotope.

Radioactivity: Majority of the elements found naturally have stable atoms. The atoms of the elements never change unless they are attacked with subatomic particles from outside; however, some of the atoms in some heavy elements are inherently unstable. The unstable atoms are called radioactive isotopes or radioisotopes. The nucleus of the radioactive atom or radioisotope undergoes disintegration with the ejection of tiny particle accompanied by electromagnetic radiation. After disintegration the rest of the material of the nucleus rearranges itself and becomes the nucleus of a different element.