how to add boxshadow only at the top side in flutter

I wanted to add shadow to a box inside BoxDecoration only on top side. Is it possible in flutter ? This is my approach so far but not getting the desired results. Thanks in advannce.

class SectionHeaderDelegate extends SliverPersistentHeaderDelegate {
  final String title;
  final double height;

  SectionHeaderDelegate(this.title, [this.height = 50]);

  Widget build(context, double shrinkOffset, bool overlapsContent) {
    return Container(
      alignment: Alignment.centerLeft,
      padding: EdgeInsets.only(left: 5 * SizeConfig.widthMultiplier),
      decoration: BoxDecoration(
        color: AppColors.colorWhite,
        borderRadius: BorderRadius.only(
            topRight: Radius.circular(30), topLeft: Radius.circular(30)),

         boxShadow: <BoxShadow>[
               color: AppColors.colorLightGrey,
               blurRadius: 20.0,
               offset: Offset(0.0, 1.0))
      child: Text(title, style: AppStyles.complaints),

  double get maxExtent => height;

  double get minExtent => height;

  bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) => false;