Diapositiva 1

1
A LASER DRIVEN ELECTRON SOURCE FOR THE
PRODUCTION OF RADIONUCLIDES
Andrea Gamucci, Marco Galimberti, Danilo
Giulietti, Leonida A. Gizzi, Luca Labate,
Gianluca Sarri, Paolo Tomassini, Antonio
Giulietti Intense Laser Irradiation Laboratory,
IPCF, CNR Campus, Pisa, Italy INFN, Sezione di
Pisa, Italy Nicolas Bourgeois, Jean-Raphaël
Marquès Laboratoire pour l'Utilisation des Lasers
Intenses, CNRS UMR 7605, Ecole Polytechnique
91128 Palaiseau, France Tiberio Ceccotti,
Sandrine Dobosz, Pascal Monot, Pascal D'Oliveira,
Horia Popescu, Fabrice Réau, Philippe
Martin CEA-DSM/DRECAM/SPAM Gif sur Yvette Cedex,
France David Hamilton, Jean Galy Institute for
Transuranium Elements, Karlsruhe, Germany
Abstract We present the results of an experiment
performed with 10 TW laser pulses focused onto a
helium gas-jet operating at different backing
pressures. The accelerated electrons, impinging
on a tantalum radiator, undergo bremsstrahlung
radiation emission and are capable of
radio-activating a gold sample put behind the
radiator. Electron bunches with energy peaks in
the 10 50 MeV region and angular divergences of
few tens mrad with a high-efficiency (1010
electrons of energy gt 8 MeV per Joule of laser
energy) were produced and gave rise to an
absolute reaction rate of photoactivations of
1.46 x 106 per Joule of incident laser energy.
Bunches of this kind can be employed for a
variety of nuclear studies, e.g. to perform
measurements of the differential photonuclear
cross section on radioisotopes, or to measure the
polarization of the electron bunches using a
technique known as Compton transmission
polarimetry.
The Source
Production of energetic electrons with a non
extremely high-intensity laser
A suitable radiator converts the laser-plasma
accelerated electrons via Bremsstrahlung process
in ?-radiation able to activate a sample with the
production of radioactive nuclei.
The experimental set-up
10 TW UHI-10 TiSa

• 65 fs CPA pulses
• Energy up to 0.8 J
• ?0 800 nm
• power Contrast Ratio gt 106

• Radiator
• 2mm Tantalum
• Sample
• 4mm 197Au

• F/5 OAP
• w0 ? 10 ?m
• I ?8.5x1018 W/cm2
• a0 ? 2

The Sample Radioactivation
The first step consist in irradiation and
activation of the gold foil

• To obtain the electron spectrum, their angular
  distribution and their number, 2 diagnostics have
  been used independently

• For photon energies in the range 10 15 MeV,
  the cross section for 197Au (?,n) 196Au reaction
  grows relatively due to a resonance in the
  nuclear photo-absorption amplitude, known as
  Giant Dipole Resonance (GDR)

• Several nozzles with different aperture sizes
  (from 2 to 6 mm) have been employed at a wide
  range of He backing pressures. The best data have
  been obtained with the 4mm nozzle _at_ 25 bar He
  pressure.

The energetic electrons produced in the
interaction have been carefully characterized

• A magnetic spectrometer before a scintillating
  Lanex screen has been used to get the shot to
  shot spectrum with 1D angular resolution along
  the slit axis

• SHEEBA (Spatial High Energy Electron Beam
  Analyzer) a set of radiochromic films spaced out
  by layers of different material and thickness

Electron spectra for the 2mm _at_ 8 bar and 4mm _at_ 25
bar nozzles

• SHEEBA detector has been used on sets of 10
  shots. The data show a high degree of collimation
  even after this integration (angular divergence
  is evaluated better than 100 mrad).

The produced electron energy fits very well in
this spectral interval!
M. Galimberti et al., Rev. Sci. Instrum. 76,
053303 (2005)
Only 106 laser shots to induce a non-negligible
amount of photonuclear reactions

• Ne (Egt3.2 MeV) ? 3.07875 ?1011

The Data Analysis

• From the number of 196Au nuclei produced by the
  photo-activation process, we can calculate the
  bremsstrahlung flux that originated these
  reactions.
• The comparison is made with a Monte Carlo
  simulation. Once we know the bremsstrahlung flux,
  then we can go back to the corresponding electron
  beam flux, if the electron spectrum and the 197Au
  (?,n) 196Au reaction cross section are known.

The next steps of the electron beam flux
measurement concern the post-irradiation gamma
spectroscopy and a detailed analysis of the data
coupled with dedicated Monte Carlo calculations.

• As a further self-consistency check the
  experimentally determined half-life has been
  calculated from the exponential count growth over
  the measurement period.
• A value of 6.17 days was taken from the nuclear
  data sheets for the half-life of 196Au.

• After irradiation, the decay photons from the
  gold sample have been detected in a high-purity
  germanium detector cooled to ? 80 K.
  (Efficiency at these photon energies 0.018 ?
  0.001)
• Post-irradiation period of 143 hours.
• The primary radioactive decay channel for this
  nuclide is via isomeric transition with the
  emission of two primary photons (333 keV and 355
  keV)

• The code also accounts for the experimental
  geometry

Results and Discussion
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• The goal of the Monte Carlo calculation is the
  experimentally determined (?,n) reaction yield.
  After the computation, that accounts also for the
  underlying physics processes, results for the
  entire data set of 106 laser shots and for
  corresponding values for a single laser shot are
  obtained.

• A nuclide with a well-known cross section
  (197Au) was used as an activation sample and
  irradiated in a bremsstrahlung flux generated
  from electrons produced in a laser driven
  accelerator.
• There a lot of possible way to efficiently
  employ this source

• Given the laser energy of ? 0.8 J, the absolute
  reaction rate is 1.46?106 per Joule of incident
  laser energy
• Its worth noting that these results are fully
  consistent with the other diagnostics ones (see
  SHEEBA!)

• Perform experiments on radioisotopes on a
  day-by-day basis
• Nuclear physics studies
• Photonuclear cross section measurements
• Biomedical employment of radioactive samples

Calculation uncertainties are below 0.1

• Furthermore, this source can be used for
  measurements of Compton transmission polarimetry,
  that for photons of a few MeV, is a well
  established method that relies on the fact that
  the transmission of a photon beam through iron
  depends on the polarization of the beam photons
  as well as on the magnetization of the iron
  target. Reversing the polarity of the magnetic
  field in iron results in an asymmetry of the
  transmission signal at the percent level.