Custom radical sign with plain XeTeX

Question

I would like to use Cambria Math font in mathematical equation, and I have a simple problem here. plain TeX uses many symbols for the root sign, so it can use the proper one depending on context, however the Unicode have only one (or maybe two) that is capable to replace big roots:
If the 3. family is set to Cambria Math, the line \def\sqrt{\XeTeXradical 3 `\⎷} does not produces the root sign at the proper place for example $\sqrt{a\over b}$.
How can I do this properly?

Answer 1

I think the `\⎷ is causing you trouble. Use the Unicode code point for the sqrt radical instead:

\font\x="Cambria Math:script=MATH" at 10pt
\textfont1=\x\textfont2=\x\textfont3=\x
\def\sqrt{\XeTeXradical 3 "0221A}
$$
\sqrt{1\over 2}
$$
\bye