Gamma rays penetration

Gamma Ray, Penetrating—Short wavelength electromagnetic radiation of nuclear origin. 

Energetic x rays and gamma rays penetrate tissue well enough so that an external source of such radiations may deliver a relatively uniform dose to all parts of the body. Particulate radiations, on the other hand, penetrate much less readily, especially those with high mass and charge, such as alpha particles. With such radiations, irradiation of deep-seated tissues wUl occur only as a result of internal deposition. 

Alpha particles are the least-penetrating form of radiation and can be stopped by a sheet of paper. Beta particles readily pass through paper but not through a sheet of aluminum. Gamma rays penetrate several centimeters into solid lead. 

The moderator slows the neutrons to increase the efficiency of the fission reaction. The control rods absorb neutrons to slow or halt the fission reaction. 39. Even though gamma rays penetrate human tissue very deeply, they are very small and cause only occasional ionization of biomolecules. Alpha particles, because they are much more massive, are very effective at causing ionization of biomolecules and produce a dense trail of damage once they get inside an organism. 

The radiation source used in radiation therapy may be inside or outside the body. In almost all cases, radiation therapy uses gamma radiation emitted by radioisotopes. Any alpha or beta radiation that is emitted concurrently can be blocked by appropriate packaging. For example, is often administered as seeds consisting of a core of radioactive isotope coated with 0.1 mm of platinum metal. The platinum coating stops the alpha and beta rays, but the gamma rays penetrate it readily. The radioactive seeds can be surgically implanted in a tumor. 

Table VIII shows partial results obtained from a group of calculations involving thinner slabs and other than normal incident angles. To facilitate comparison with the work of Kirn et al, on gamma-ray penetration, the slab thicknesses were varied so as to keep the optical path lengths constant for different angles of incidence. The results are generally similar, in that obliquity decreases transmission for thin slabs and increases it in thicker slabs.

Gamma rays, because of their size, have the lowest ionizing power. However, several inches of lead or a thick cement block is necessary to stop gamma ray penetration. 

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