A monoenergetic beam a alpha particle , each of them having kinetic E is incident on a sample of singly ionised Helium gas in ground state . The Helium gas in ground state. The Helium sample may start emitting radiation Since the Kinetic energy of incident alpha particle in sufficiently high . A part of this radiation (if any) is allowed to pass through atomic Hydrogen sample in ground state. A detector placed near the hydrogen records both radiation and electrons. Assume all`He^(+)` ions to be initially at rest.
If maximum Kinetic energy of electron intercepted by the detector is `27.2eV` , the K.E of particle must be lower than -

Options

A. `81.6eV`
B. `96.8eV`
C. `102eV`
D. there is no upper limit

Answer

B

Explanation

The `K.E` of alpha particle should not be so large that it excite electron in `He^(+)` to `2^(nd)` excited state.
i.e `KE_(alpha) lt 2xx2^(nd)` excitation energy of `He^(+) =2xx48.4=96.8eV`