Rutherford backscattering spectrometry(RBS) is a technique used to determine the structure and composition ofmaterials. A beam of ions (typically helium ions) is accelerated to highenergy and aimed at a sample. By analyzing the distribution and energyof the ions that are scattered from (that is, deflected by collisions with)the atoms in the sample, researchers can determine the sample’s composition.To accelerate the ions to high energies, a tandem electrostaticaccelerator may be used. In this device, negative ions (He-) start at apotential V = 0 and are accelerated by a high positive voltage at themidpoint of the accelerator. The high voltage produces a constant electricfield in the acceleration tube through which the ions move. Whenaccelerated ions reach the midpoint, the electrons are stripped off, turningthe negative ions into doubly positively charged ions (He++). Thesepositive ions are then repelled from the midpoint by the high positivevoltage there and continue to accelerate to the far end of the accelerator,where again V = 0. For a particular experiment, helium ions are to be given a kinetic energy of 3.0 MeV. What should the voltage at the center of the accelerator be, assuming that the ions start essentially at rest? (a) -3.0 MV; (b) +3.0 MV; (c) +1.5 MV; (d) +1.0 MV.
Rutherford backscattering spectrometry
(RBS) is a technique used to determine the structure and composition of
materials. A beam of ions (typically helium ions) is accelerated to high
energy and aimed at a sample. By analyzing the distribution and energy
of the ions that are scattered from (that is, deflected by collisions with)
the atoms in the sample, researchers can determine the sample’s composition.
To accelerate the ions to high energies, a tandem electrostatic
accelerator may be used. In this device, negative ions (He-) start at a
potential V = 0 and are accelerated by a high positive voltage at the
midpoint of the accelerator. The high voltage produces a constant electric
field in the acceleration tube through which the ions move. When
accelerated ions reach the midpoint, the electrons are stripped off, turning
the negative ions into doubly positively charged ions (He++). These
positive ions are then repelled from the midpoint by the high positive
voltage there and continue to accelerate to the far end of the accelerator,
where again V = 0. For a particular experiment, helium ions are to be given a kinetic
energy of 3.0 MeV. What should the voltage at the center of the accelerator
be, assuming that the ions start essentially at rest? (a) -3.0 MV;
(b) +3.0 MV; (c) +1.5 MV; (d) +1.0 MV.
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