What Is an Accelerator Mass Spectrometer?

An accelerator mass spectrometer is a laboratory device that uses a combination of magnetism and high voltage to measure radioactive elements. Though it was first demonstrated in the 1930s, the use of accelerators combined with mass spectrometry did not become commonplace until the 1970s. Mass spectrometry is the measurement of a molecule or atom's mass by separating different atoms by their weight or mass using magnetic fields.

Archaeology, the study of ancient civilizations, has used 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-14 present 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 required a large amount of material. The science of using radioactive carbon to determine the age of ancient artifacts is known as "carbon dating."

There are several different sections in an accelerator mass spectrometer, but the primary 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 electrical charge, exceeding millions of volts.

The accelerated particles then pass through an electron stripper, which is either a very thin layer of carbon or a specific gas. Electrons are removed from the particle, resulting in a positively charged ion. The ions are now accelerated again, because they are repelled by the high positive charge of the accelerator. This is why this part of the device is called a tandem accelerator, because it affects the ions twice using the effects of attraction and repulsion due the electrical charge.

Once the high-speed ions leave the tandem section, the remainder of the accelerator mass spectrometer is 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 electrical and magnetic strength to control the product stream. The detectors are extremely sensitive, and can sense one radioactive ion in many millions of non-radioactive ones.

Along with its use in archaeology 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 quantities in a drug taken by the patient. As the body absorbs the drug, the tracer element can be seen using samples introduced into an accelerator mass spectrometer. The ability of the spectrometer to see very small quantities of a radioactive element makes the technique valuable, because the patient sees very low levels of radioactivity that are not believed to cause health concerns.