Thin foils for beam-foil spectroscopy Page: 4 of 13
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more energy and loses this energy by decaying and giving off tome light.
It decays down to the ground state of each ion eventually, emitting light
of various wavelengths. Sine we are spectroscopists we're interested
primarily in this light, which is analyzed with a spectrometer and
electronic detection. In fact, with a microamp of beam through a
carbon foil at these energies you can put your eye next to the chamber
and you can see a lot of light. Nitrogen, for example, is a blue light.
In fig. 2 there is a rather nice example of a singly-charged
lithium beam. If you look carefully you can see that the light is blue
initially and then it's green further on down. In fact the blue light
comes from doubly-charged lithium and it gives off the light rather
quickly. It takes a few nanoseconds for the light to travel down beam
and we can say that this blue light decays within a fraction of a
nanosecond. It goes back down to the ground state of the doubly-charged
lithium ion whereas the green light decays very slowly. There is
quite a complex process going on which produces this light in the beam.
But it's all initiated by the foil. We look at various positions along
this beam and measure how fast the light decays.
The foils are about 5 g/cm2 and in the Argonne
arrangement we have 1/4" diameter with a pre-aperture of 5/16 of an
inch. We focus the beam through this aperture without a foil into the
Faraday cup and then we apply what we call rasters, horizontal and
vertical, to apply a high frequency electric field somewhere before
the bending magnet. It sweeps this beam spot so then we get a rather
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Berry, H.G. Thin foils for beam-foil spectroscopy, article, January 1, 1975; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc871712/m1/4/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.