Recognize the Decay Formula and the Decay Constant.
(l) ln2/t1/2.
In an atom prior to Beta Minus Decay, there will be excess neutrons in the atoms nucleus.
Materials with a low Atomic Number (such as water, plastic, and paraffin) are used in making neutron shields because their atomic size allow for greater transfer of the neutrons energy per collision.
Shallow dose equivalent applies to the external exposure of the skin or an extremity and is taken as a dose equivalent at a tissue depth of 0.007cm averaged over an area of 1 cm2.
Using the assumption that 2" of lead equals one tenth thickness, the amount of shielding required to reduce a Cs-137 gamma beam of 100 mr/hr intensity to 1 mr/hr would be 4". (1/100)
The mathematical expression which expresses the relationship between the radioactive decay constant and the half-life of an isotope is, T 1/2 =
ln2/ Decay constant.
The decay process emitting an alpha particle can be described as when a positively charged particle with a mass equal to a Helium nucleus is emitted from a nucleus during radioactive decay.
To determine shielding, build up factors is not a valid reason to perform a radiation survey.
If you shield a beta source with a higher atomic number shielding material, you will create more X-rays. (Bremsstrahlung).
Shielding material with a high atomic number attenuates radiation more effectively than a lower atomic number material because, more electrons are available for interactions.
The decay constant and the half-life are constants for a given radionuclide.
The effective half-life formula is Teff = (TR x TB) / (TR + TB). The result will always be shorter than the shortest half-life.
Beta shielding is better accomplished by aluminum than lead due to the production of Bremsstrahlung radiation.
The actual decrease in the intensity of a beam of gamma rays passing through a wall is less than the theoretical decrease because some gamma interactions result in other gamma rays of lower energy being given off, this is called buildup.
The shielding half value thickness for 1 Mev gamma of: lead is 0.5 inches, concrete is 4 inches, and water is 8 inches. The shielding tenth value thickness for 1 Mev gamma of: lead is 1.5 inches, concrete is 12 inches, and water is 24 inches. The shielding half value thickness for 6 Mev gamma of: lead is 0.7 inches, steel/iron is 1.3 inches, concrete is 8 inches, and water is 16 inches. The shielding tenth value thickness for 6 Mev gamma of: lead is 2 inches, steel/iron is 4 inches, concrete is 24 inches, and water is 48 inches.
(Please note that these numbers are incorrect, but they are what is
printed in the NEU study guide, and probably what is on the NEU test.)
For lines and equipment containing reactor water, or primary steam with hold-up times of less than five minutes, you would use 6 Mev gamma 1/10 shielding (lead 2", water 48").
