Geant4 simulations on Compton scattering of laser photons on

EUROPEAN UNION
GOVERNMENT OF ROMANIA
Structural Instruments
2007-2013
Sectoral Operational Programme
„Increase of Economic Competitiveness”
“Investments for Your Future”
Extreme Light Infrastructure – Nuclear Physics (ELI-NP) - Phase I
Project co-financed by the European Regional Development Fund
Geant4 simulations on Compton scattering of laser
photons on relativistic electrons
Carphatian Summer School of Physics 2014
Dan Mihai FILIPESCU
 – ray beams
The  ray beam are produced by the inverse Compton scattering of laser
photons from relativistic electrons.
• Energy
E

L
Ee
Electron Beam
Laser
e
e
E 
L
1   cos    L 1  cos  L    E e
Continuous
spectrum!
Important parameters in the equation:
1. Electron energy
2. Laser photons energy
Experimental facility NewSUBARU – GACKO
• Electron energy injection from LINAC : 1 GeV;
• Electron energy in New SUBARU storage ring: 0.5 – 1.5 GeV;
• Electron beam intensity in storage ring: 500 mA;
• Linearly polarized and unpolarized  beam;
•Gamma energy:
– 7 – 34 MeV using Nd doped ZVO4
laser (1.064 m & 0.532 m corresp.
to I-st & II-nd harmonics)
– 1 – 4 MeV using CO2 laser (10.59
m);
• Collimated gamma flux (3 mm collimator) :
5·106 /s (E/E = 5% FWHM) for 200 mA
storage current and 4.2 W Nd laser input;
• Distance between the interaction point and
the collimator: 19 m.
• Gamma pulse width
- 60ns at 30kHz for 1064nm wave length
- 55ns at 30kHz for 532nm.
Energy spectra of Laser Compton Scattered (LCS) γ-ray beam
After collimation the LCS-γ ray beam becomes quasi-monochromatic. NewSUBARU
typical energy resolution: 2-3% .
Energy spectra of collimated LCS beam
depends on:
• Energy resolution of electron beam
• Energy resolution of laser beam
• Twiss parameters of electron beam
• Spatial distribution of laser beam
Continuous
spectrum!
Photon neutron cross section measurements
We performed a systematic measurement of photoneutron cross sections for stable Sm
isotopes in the vicinity of four radioactive Sm nuclei. The radioactive Sm nuclei belong to
the second peak of fission products centered around A ∼ 140 in the fission of nuclear
fuels, 235U and 239Pu. 151Sm is a (n,γ) / β decay branching point in the s-process
nucleosynthesis.
The photoneutron emissions studied constitute a part of the reaction network of the pprocess nucleosynthesis in which photodisintegration plays a primary role in reprocessing the preexisting nuclei produced by the s-process and r-process.
Photoneutron cross sections for two odd-N nuclei, 147Sm and 149Sm, are measured for
the first time. Those for 144Sm represent the destruction cross section for the p-process
nucleus.
Effect on photoneutron cross section measurements
Non-monochromatic corrections
Monochromatic approximation
Taylor expansion of c.s.
Taking into account  beam energy distribution
LCS γray beam energy profile measurements
• Large volume LaBr3(Ce) detector 4  4 inch;
• Spectra was measured at low laser power;
• Incident LCS γ-ray spectra were obtained by
reproducing with Geant4 simulations the
LaBr3(Ce) spectra.
LCS γray beam energy profile measurements
• Not all experimental spectra could be reproduced with the original LCS source
simulation code, EGS4, for either:
• Specific electron beam parameters (bad emittance values, high beam
divergence)
• Specific collimation geometries
LCS γray beam energy profile measurements
• Good reproduction of LCS experimental spectra for various electron beam emittance
and beam size parameters and also for various collimation geometries using the Geant4
code.
Corrected photoneutron cross sections
Laser scattering on electrons
Laboratory frame system (xe, ye, ze)
Taken from C. Sun PhD. Thesis
Klein-Nishina random generator for GEANT4
E (MeV)

0
1
2
3
4
5
3500
3000
2500
2000
1500
1000
500
5000
0
4000
3000
2000
1000
0
0
20
40
60
80
100
 (mili grad)
120
140
160
180
Laser scattering on electrons
Electron rest frame coordinate system (x’e, y’e, z’e)
Scattered photon energy in the electron rest frame coordinate system (x’e, y’e, z’e) is:
and it is in the range of:
The differential cross section is given by:
Taken from C. Sun PhD. Thesis
Compton scattering of laser photons on relativistic electrons
from
Laboratory frame coordinate system (xe, ye, ze)
to
Electron rest frame coordinate system (x’e, y’e, z’e)
Taken from C. Sun PhD. Thesis
Radii of electron and laser beams in the interaction region from
the long straigth beamline in the electron storage ring
NewSUBARU
K Horikawa et. Al, NIM A 618 (2010)
Electron beam Twiss parameters
Electron beam emittance:
Drift space coordinates and Twiss
parameters:
Laser modeling
Schematic view of laser beam
transverse beam size (hyperbolic
dependence along the beam axis)
w0 = beam size at focal point
zR = b/2 = Rayleigh length
Assuming a Gaussian beam, the laser intensity distribution is expressed as:
Simulation procedure
Generate interaction point:
- choose a z coordinate randomly
- generate transverse coordinates given by electron beam Twiss
parameters
- compute laser beam intensity distribution for interaction point
Generate electron and laser Lorentz vectors:
- apply rotations and Lorentz transformation described ahead
- compute Gamma photon Lorentz vector using Klein Nishina formula
- apply the inverse rotations and Lorentz transformation to gamma
photon
Propagate Gamma photon through matter:
- increment an energy histogram with the share given by the laser beam
intensity distribution computed above.
Gamma source geometry
Effect of linear polarization on gamma beam spot
Laser beam polarization
Stokes parametres
Laser beam polarization
Stokes parameters transformation
Polarization setup
Gamma ray beam spot size
Acknowledgements:
• Hiroaki Utsunomiya
• Florin Rotaru
• Octavian Sima
• Madalina Boca
• Ioana Adriana Gheorghe
• Franco Camera

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