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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 > 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
10 TW UHI-10 Ti:Sa
The experimental set-up
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.
65 fs CPA pulses
Energy up to 0.8 J
0 = 800 nm
power Contrast Ratio > 106
Schematic layout of the equipment
Radiator
2mm Tantalum
F/5 OAP
Sample
w0  10 m
I 8.5x1018 W/cm2
a0  2
4mm
The first step consist in irradiation and activation of the gold foil
The energetic electrons produced in
the interaction have been carefully
characterized
The Sample Radioactivation
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 @ 25 bar He pressure.
SHEEBA (Spatial High Energy Electron Beam
Analyzer): a set of radiochromic films spaced out
by layers of different material and thickness:
To obtain the electron spectrum, their angular
distribution and their number, 2 diagnostics have
been used independently
Electron spectra for the 2mm @ 8 bar
and 4mm @ 25 bar nozzles
Raw Data
The produced
electron energy
fits very well in
this spectral
interval!
Ne (E>3.2 MeV)  3.07875
n.1
n.7
n.9
n.14
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 photoabsorption amplitude, known as Giant Dipole
Resonance (GDR)
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 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).
M. Galimberti et al.,
Rev. Sci. Instrum.
76, 053303 (2005)
Only 106 laser shots to induce a non-negligible
amount of photonuclear reactions
1011
n.19
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.
The Data Analysis
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)
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.
Nuclear data sheets value for the
half-life of 196Au: 6.17 days.
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)
From the number of 196Au nuclei produced by the photoactivation 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 code also
accounts for the
experimental
geometry
Results and Discussion
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.
Given the laser energy of  0.8 J, the
absolute reaction rate is 1.46106 per
Experimental
Yield per laser
and calculated
Joule of incident laser energy
shot
Calculation
uncertainties
are below
0.1 %
197Au
yields
N (,n)
(1.24 ± 0.05)108
(1.17 ± 0.05)106
N (817.5 MeV)
(2.78 ± 0.12)1010
(2.62 ± 0.11)108
N e( ≥ 8 MeV)
(7.76 ± 0.33)1011
(7.32 ± 0.31)109
N e( ≥ 3.4 MeV)
(3.34 ± 0.14)1012
(3.15 ± 0.13)1010
It’s worth noting that these results
are fully consistent with the other
diagnostics’ ones (see SHEEBA!)
Very Large!
High quality electron beams
Thank You!
(197Au)
A nuclide with a well-known cross section
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:
Perform experiments on radioisotopes on a day-by-day basis
Nuclear physics studies
Photonuclear cross section measurements
Biomedical employment of radioactive samples
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.
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