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.