9 MeV protons.
Given: $B = 4.9$ MeV, $m = 938$ MeV
Using the formula $B = \frac{1}{\sqrt{1 - \beta^2}} - 1 \times m$, we can solve for $\beta$:
$4.9 = \frac{1}{\sqrt{1 - \beta^2}} - 1 \times 938$
Solving for $\beta$, we get $\beta = 0.101$
The group velocity is
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