Bainbridge Mass Spectrograph

♦ Study about Bainbridge Mass Spectrograph , In the Bainbridge mass spectrograph , the positive rays are produced in a specially designed discharge tube ♦

In the Bainbridge mass spectrograph, the positive rays are produced in a specially designed discharge tube. The positive rays pass through the fine hole in the cathode C and enter the velocity selector through the slit S1. The velocity selector allows only those positive ions to pass through the slit S2 for which v = E/B. All other ions are deflected sideways. These positive ions coming out of the slit S2, all have the same velocity v, are subjected to perpendicular magnetic field ‘B’. The positive ions therefore move in circular orbits in the vacuum chamber and falls on the photographic plate. The images of slit are formed, for different isotopes. By measuring r1, r2 etc., the isotopes masses m1, m2 etc. are evaluated.

spectrograph

Velocity Selector:

The positive ions of different velocities v1, v2, enter through slit S1 into the velocity selector. There is an electric field E and a magnetic field B applied in this region. The electric field is obtained by applying suitable potential to plates P1 and P2. The magnetic field is perpendicular to the electric field (into the page, magnetic poles not shown). The force on a positive ion due to the electric and magnetic field are

spectrograph

FE = qE (towards left)

FB = qvB (towards right)

If FE > FB, the ion is deflected towards left if FE < FB, the ion is deflected towards right. Thus only those ions pass through the slit S2, for which the velocity is such that

qE = qvB   or   v = E/B

The velocity selector allows only those ions through the slit S2 for which v = E/B, whatever may their masses.