Beta emission from carbon

Sample How old is a piece of ancient wood that is giving off beta emissions from carbon-14 at the rate of 1.9 disintegrations/minute/gram ... 

The end window of the tube must be thin enough to permit the weaker radiations to enter the tube (aluminium, 6-8 mg cm-2 mica, 2 mg cm-2) but even so alpha particles and very weak beta emissions are either completely or partially absorbed. The emissions from the biologically important isotopes of tritium and carbon-14 fall into this category and alternative detectors should be used for these isotopes. 

Beta particles are electrons emitted with very high energy from many radioisotopes. Positively charged counterparts of beta particles are called positrons. Positronic and electron emissions from radioactive atoms can be shielded by thin metal foils or one-quarter inch of plastic. Tritium ( H), phosphorus-32, and carbon-14... 

Beta (P) decay can occur in two different ways. In j8 decay, a neutron in the nucleus spontaneously decays into a proton with the simultaneous emission from the nucleus of an electron and a massless, neutral subatomic particle called an antineutrino ( v). Carbon-14 ( eC), for example, spontaneously decays into nitrogen-14 ( 7N) through the emission of an electron (j8 particle). In contrast, P decay (also called positron emission) occurs when a proton in the nucleus spontaneously decays into a neutron with the simnltaneous... 

If you assume that the ratio of carbon isotopes in the lower atmosphere has remained at the present level for the last 50,000 years (presently 1 out of 10 carbon atoms is carbon-14), you can deduce the age of any dead organic object by measuring the level of beta emissions that arise from the radioactive decay of carbon-14. This is the decay reaction ... 

Since the radioactive sample material from most methods of sample preparation is in intimate contact or in actual solution with the phosphor, the detection of emitted particles or radiation is highly efficient and may even approach 100%. Problems of self-absorption of the emissions are thus absent, or considerably smaller than those associated with planchette counting of solid samples. This is of particular importance for the measurement of low energy beta emitters such as tritium and carbon-14. On the other hand, the measurement method has intrinsic drawbacks such as quenching and chemiluminescence. 

R/S = 22.7 units/29.0 units = 0.783. From Figure 20.9, this corresponds to 0.35 half-life. The half-life of carbon-14 is 5.73 X ICP years. 0.35 half-lives X 5.73 X ICP years/half-life = 2.0 X 10 years. Radiocarbon dating indicates that the cloth is about 2000 years old, which places it at the beginning of the Christian era. 67. Emission of a beta particle would change a calcium atom into a scandium atom jJCa —> e -I- 2 Sc. 

---