A mass spectrometer is being used to separate common oxygen-16 from the much rarer oxygen-18, taken from a sample of old glacial ice. (The relative abundance of these oxygen isotopes is related to climatic temperature at the time the ice was deposited.) The ratio of the masses of these two ions is 16 to 18, the mass of oxygen-16 is 2.66 ✕ 10−26 kg, and they are singly charged and travel at 2.90 ✕ 106 m/s in a 1.30 T magnetic field. What is the separation (in m) between their paths when they hit a target after traversing a semicircle?

0.092 m

Explanation:

A charged moving particle immersed in a region with magnetic field follows a circular trajectory at constant speed (uniform circular motion), since the magnetic forces acts perpendicular to the direction of motion of the particle.

Since the magnetic force acts as centripetal force, we can write:

where

q is the charge of the particle

v is its velocity

B is the strength of the magnetic field

m is the mass of the particle

r is the radius of the orbit

Solving the equation for r,

For the ion of oxygen-16, we have:

(it is singly charged)

So the radius of its orbit is

For the ion of oxygen-18, we have:

(it is singly charged)

So the radius of its orbit is

After each ion has travelled a semicircle, the separation between the two ions will be twice the difference in their radius, so:

protons in