In a cyclotron, charged particles, or ions, are accelerated inside an evacuated tank, to prevent them from colliding with air molecules and being scattered. The vacuum tank is placed between the poles of an electromagnet, which creates a field that bends the ion beam into a circular orbit between two accelerating electrodes, or “dees.” Because the ions carry a positive electric charge, they are attracted toward the dee, which is electrically negative at the moment. Were it not for the magnetic field, the ions would be accelerated in a straight line; instead, they are deflected into a circular path back toward the dee gap. By the time the ions again reach the dee gap, the sign of the electric potential on the dees is reversed, so that now the ions are attracted toward the opposite dee. As this process of alternating the electric potential is repeated, the ions gain speed and energy with each revolution. This causes them to spiral outward in a regular manner. Finally, they strike a target inserted into their path or are extracted from the cyclotron for use as an external beam.

17. Based on the information presented in the passage, which of the following most closely represents the sequence of events describing the path of an ion in a cyclotron?

1) A positively charged ion is repelled from a negative dee, then attracted to a positive dee.
2) A negatively charged ion is attracted to a positive dee, then repelled by a negative dee.
3) A positively charged ion is attracted to a negative dee, then attracted by a magnetic field.
4) A positively charged ion is attracted to a negative dee, then attracted to a negative dee.
5) A positively charged ion is attracted to a negative dee, which reverses the ion’s electric potential.



18. In the context in which it appears, “regular” most nearly means

1) normal
2) agreeable
3) average
4) predictable
5) useful



19. The author of the passage mentions the magnetic field primarily in order to

1) argue that the magnetic field is less important than the dee gap in cycling the ions
2) suggest that ions would accelerate to dangerous speeds without a magnetic field
3) provide support for the notion that, in nature, ions tend to travel in straight lines
4) cast doubt on the hypothesis that ions tend to travel elliptically once deflected
5) indicate the necessity of deflecting the ions back toward the dee gap