void DriftEmit(){
  
  gROOT->Reset();
  
  //
  
  Double_t c = 299792458.0; // Speed of Light (m/s)
  Double_t e = 1.602176e-19; // Electron Charge (C)
  Double_t mec2 = 0.510998e+6; // Electron Mass (eV)

  Double_t B_cath = 0.2; // Cathode Solenoid Field (T)

  Double_t eOverme = 1.758820088e+11; // Electron Cyclotron Frequency/Field ( rad/(s T) )
 
  ////// 

  // For a Gaussian with sigma a0, <r^2> = 2 a0^2.
  // For a Top-hat distribution with radius a0, <r^2> = a0^2 / 2.
 
  // Cathode Magnetic Emittance (um) - Gaussian Beam
  cout << " For a Gaussian with sigma a0, <r^2> = 2 a0^2" << endl;
  Double_t a0 = 0.003; // Beam Gaussian sigma at Cathode (m) 
  Double_t mag_emit_cathode = 1.0e+6 * (eOverme * B_cath)/(2.0*c) * a0**2;
  cout << " Cathode Magnetic Emittance (Gaussian Beam): mag_emit_cathode = " << mag_emit_cathode << " um" << endl;

  //////
  
  cout << " -------------- " << endl;
  
  //////

  // Cathode Magnetic Emittance (um) - Top-hat Beam
  cout << " For a Top-hat distribution with radius a0, <r^2> = a0^2 / 2" << endl;
  Double_t a0 = 0.003; // Beam Top-hat radius at Cathode (m) 
  Double_t mag_emit_cathode = 1.0e+6 * (eOverme * B_cath)/(8.0*c) * a0**2;
  cout << " Cathode Magnetic Emittance (Top-hat): mag_emit_cathode = " << mag_emit_cathode << " um" << endl;
 
  //////

}

/*
root [0] .x DriftEmit.C
 For a Gaussian with sigma a0, <r^2> = 2 a0^2
 Cathode Magnetic Emittance (Gaussian Beam): mag_emit_cathode = 528.011 um
 -------------- 
 For a Top-hat distribution with radius a0, <r^2> = a0^2 / 2
 Cathode Magnetic Emittance (Top-hat): mag_emit_cathode = 132.003 um
root [1] .q
*/