[填空题]
X-rays of wavelengthl p ae81a9)fpj 0.133 nm fall on gq7co n)zns a+vehj) 4x+c1:oa crystal whose atoms, lying in planes, are spaced 0g)7cs) eq+noj z :hna+4v 1ocx.280 $nm$ apart. At what angle \phi (relative to the surface, Fig. Below) must the X-rays be directed if the first diffraction maximum is to be observed? $\phi $ = $^{\circ}\,$
