not calling javascript

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ASP.NET

Javascript

after this line

Rpt.ExportToHttpResponse(ExportFormatType.Excel, Response, true, "Report");

i wrote statment which call javascript method to enable button

string JScript = "javascript:Btn_Click()";
            ScriptManager.RegisterStartupScript(this, this.GetType(), "JScript", JScript, true);
             btnExcel.Enabled = true;

i.e.

Rpt.ExportToHttpResponse(ExportFormatType.Excel, Response, true, "Report");

string JScript = "javascript:Btn_Click()";
            ScriptManager.RegisterStartupScript(this, this.GetType(), "JScript", JScript, true);
             btnExcel.Enabled = true;

but javascript doesnot call why?