Background In Radioactivity Measurements:

In the field of radiation measurements, background or natural radiation plays an important role and must always be corrected. In simple words, it may be defined as the count rate measured in the absence of sample and must be subtracted from that for the sample counts. Background in radioactivity measurements may be compared somewhat with blank in chemical or instrumental measurements. It essentially arises from the sources other than the sample. Following principal sources of background radiation may be considered.

i) Cosmic ray background: The earth receives a steady stream of high-energy particles, largely protons from space. When these interact in the upper atmosphere, neutrons, mesons and gamma rays are produced. On the earth's surface, we are mainly concerned with γ-radiation.

ii) Natural radioactivity in the earth, air, and water: All the radioactive decay series and their decay products present in the laboratory emit radiations. Also rocks and minerals contain small amounts of ⁴⁰K, ⁸⁷Rb, ¹³⁸La and ¹⁴⁷Sm. Air may contain radon (²²²Rn) and its decay products. Interestingly human body contains equilibrium concentrations of ⁴⁰K, ¹⁴C and ²²⁶Ra.

iii) Radiations from medical uses, accidental contamination within laboratory and consumer products: Radioisotopes are finding increasing use in medicine for diagnostic as well as therapeutic purposes. For example ⁶⁰Co is used for radiation cancer therapy. Sometimes radioactive solution may be spilled in the laboratory or the consumer products may contain small amounts of radioactivity. All such radiations may raise the background level.

iv) Radiations from nuclear power and explosions: In many countries nuclear power is being used extensively but nuclear power stations are always located far away from the city or habitat area so that radiations may not affect general public. However, in a nuclear measurement laboratory, high radiation background is likely to be observed. Similarly many countries undertake testing of nuclear warfare thus increasing the radiation level in the area.