// Coefficiente di assorbimento dei gamma
//
// fit con la funzione: N(x) = N(0)*exp[-mu*x] + bck  
//
Double_t fitf(Double_t *x, Double_t *par)
{
  Double_t fitval = par[0]*TMath::Exp(-par[1]*x[0]) + par[2];
  return fitval;
}

void esempio2() {
  gROOT->Reset();
  LNCanvas *ln1 = new LNCanvas("ln1");
  PInput p;
  TGraphErrors *gr = p.inpGraphErrors("assorb.dat");
  TF1 *func = new TF1("fit",fitf,1,60,3);
  func->SetLineWidth(1);
  func->SetParameters(20000,0.01,0.0);
  func->SetParNames("N(0)","mu","background");
  gr->Fit("fit","R");
  gr->Draw("AP");
  TPaveText *pt = new TPaveText(-4,32000,24,35000,"br");
  pt->SetFillColor(9);
  pt->SetTextColor(5);
  TText *text = pt->AddText("Gamma Absorption");
  pt->Draw();
  tex = new TLatex(27,22000,"#mu = 0.045 #pm 0.003");
  tex->SetTextSize(0.05);
  tex->SetLineWidth(2);
  tex->Draw();
  ln1->Modified();
  ln1->cd();
}