int TTree_simple () {
  Int_t det;
  Float_t energy , time;

  TFile f ("simple.root", "RECREATE");
  TTree t ("tree", "My tree");
  t.Branch ("Det" , &det   , "det/I");
  t.Branch ("En"  , &energy, "energy/F");
  t.Branch ("Time", &time  , "time/F");

  TRandom3 r; r.SetSeed ();
  for (int i=0; i<100; i++) {
    det    = r.Integer (24);
    time   = r.Rndm() * 20.;
    energy = r.Rndm() * 30.;
    t.Fill ();
  }
  t.Write();
  return 0;
}