Accelerator mass spectrometers use magnetism and high voltage to measure radioactive elements. They are used in archaeology to date artifacts and in medical diagnostic testing. The device has a magnetic separator and a tandem accelerator that accelerates particles and directs them to a detector.
An accelerator mass spectrometer is a laboratory device that uses a combination of magnetism and high voltage to measure radioactive elements. Although first demonstrated in the 1930s, the use of accelerators combined with mass spectrometry did not become common until the 1970s. Mass spectrometry is the measurement of the mass of a molecule or atom by separating different atoms based on their weight or mass using magnetic fields.
Archaeology, the study of ancient civilizations, has made use of the presence of radioactive materials, primarily carbon, since the early 20th century. A scientist could take a sample from an archaeological site, measure the amount of radioactive carbon-20 in the sample, and estimate the age of the sample or artifact. Before the use of an accelerator mass spectrometer, measuring carbon-14 was time-consuming and material-intensive. The science of using radioactive carbon to determine the age of ancient artifacts is known as carbon dating.
There are several sections in an accelerator mass spectrometer, but the main equipment is a magnetic separator and a tandem accelerator. The first part of the unit uses a low power magnetic separator to remove unwanted particles or molecules from the sample stream. Then the sample enters the tandem accelerator, which first accelerates the particles by attracting the negatively charged particle with a high-energy positive electric charge, exceeding millions of volts.
The accelerated particles then pass through an electron stripper, which is a very thin layer of carbon or a specific gas. The electrons are removed from the particle, resulting in a positively charged ion. The ions are now accelerated again, because they are repelled by the accelerator’s high positive charge. That’s why this part of the device is called a tandem accelerator, because it strikes the ions twice by exploiting the effects of attraction and repulsion due to electric charge.
Once the high-velocity ions leave the tandem section, the rest of the accelerator mass spectrometer is made up of additional magnets that can direct the sample stream to a detector that counts the number of particles reaching it. Each section of the spectrometer is connected to computers that can adjust the electric and magnetic force to control product flow. The detectors are extremely sensitive and can detect one radioactive ion out of many millions of non-radioactive ions.
Along with its use in archeology and geology, an accelerator mass spectrometer can be used in medical diagnostic testing. A radioactive element, called a tracer, can be injected into a patient or included in small amounts in a drug the patient takes. As the body absorbs the drug, the tracer can be seen using samples introduced into an accelerator mass spectrometer. The spectrometer’s ability to see very small amounts of a radioactive element makes the technique valuable, because the patient sees very low levels of radioactivity that are not thought to cause health problems.
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