mathspec 和 unicode-math 字符不显示

Question

我在文档中使用了mathspec和两个不同的数学符号。我按照这个使用了unicode-math如何设置表格主体数学部分的不同字体。但使用时$...$不会显示符号。我的 MWE 是：

\documentclass[a4paper,10pt]{article}
\usepackage{tabularx,lipsum}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}}
\usepackage[no-math]{mathspec}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont[Mapping=tex-text]{Times}
\setmathsfont{Times}
\usepackage{makecell}
\usepackage{unicode-math}
\setmathfont[version=table]{Helvetica}
\newfontfamily\tablefont{Helvetica}
 \usepackage{etoolbox}
 \AtBeginEnvironment{table}{\setmainfont[Color=black, Numbers={Proportional,OldStyle}]{HelveticaNeueLTStd-Cn}}%
 \AtBeginEnvironment{tabular}{\mathversion{table}\tablefont\fontsize{9}{11}\selectfont\renewcommand{\theadfont}{\bfseries}}%
    \begin{document}
            \title{Geometry}
    \maketitle
From the Egyptian pyramids to modern architecture, points, lines and   angles
are everywhere. $a^\circ$ Geometry is a $\alpha + \beta$ very visual    element of mathematics where the
designs of buildings and the orbits of planets can be studied using basic
objects like points, lines and circles.

   \begin{table}[htb!]
    \centering
    \begin{tabular}{L{2cm}|L{10cm}}
        \hline
       \thead{Variabili} & \thead{Descrizione} \\
        \hline
        \thead{$\alpha + \beta$} & \thead{$a+b$}\\
        \hline
    gdp\_us & GDP USA trimestrale nominale (dal Q1 1947 al Q4 2013), fonte: FRED2\\
        \hline
    \end{tabular}
    \end{table}

Thales (624--546~BCE) is known to be the founder of Greek geometry. He was
an astronomer and $a^\circ$ philosopher, and $\alpha + \beta$ records  show he was the first person to
use mathematical geometry to calculate the height of an Egyptian   pyramid
using the Sun's rays and to accurately predict the timing of a solar
eclipse.
    \end{document}