double exp_rand();

void rand_exp()
{

  TCanvas *c1 = new TCanvas("c1","c1");
  c1->SetGrid();
  c1->SetFillColor(10);

  TH1F *h1 = new TH1F("h1","",300,0,30);

  double value;
  for (int i=0; i<1000; i++) {
    value = exp_rand();
    h1->Fill(value);
  }

  h1->Draw();

}

double exp_rand()
{
  double t_const = 3.7*1.2/0.494;
  double x_max=30.;
  double y_max=1.;
  double x, y;
  double value;

  while (1) {
    x = gRandom->Uniform()*x_max;
    value = exp(-x/t_const);
    
    y = gRandom->Uniform()*y_max;
    if (y <= value)
      break;
  }

  return x;

}