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Response function of Imaging Plates to protons and alphas : experimental results and modelisation

Response function of Imaging Plates to protons and alphas : experimental results and modelisation. T.Bonnet, M.Comet, D.Denis-Petit, F.Gobet, F.Hannachi, M.Tarisien, M.Versteegen, M.M.Aleonard .

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Response function of Imaging Plates to protons and alphas : experimental results and modelisation

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  1. Response function of Imaging Plates to protons and alphas : experimental results and modelisation T.Bonnet, M.Comet, D.Denis-Petit, F.Gobet, F.Hannachi, M.Tarisien, M.Versteegen, M.M.Aleonard Instrumentation for Diagnostics and Control of Laser-Accelerated Proton (Ion) Beams: Second Workshop Centre d’EtudesNucléaires de Bordeaux Gradignan

  2. Outline • Imaging Plates (IPs) • Fading correction • AIFIRA accelerator experiment with protons and results • Modelisation of the response function to protons and alphas with Geant4 T.BONNET

  3. 1. Imaging Plates Presentation of IPs: IPs are plastic films sensitive to ionising radiations. Structure of an IP: Level scheme of the sensitive layer: Conduction Band • Ionisation of Europium by charged particles : the electron is trapped in F (Br-) site ( information storage ) • When reading with a scanner laser: • Excitation of F (Br-) by a 2.1 or a 2.5eV photon ( scanner ) • Coupling with an intermediate state • Charge transportation • Recombination and emission of a related photon (PSL ) of 3.2eV Ee- 35meV 2.5 eV 2.1 eV Eu3+ Eu3+ 3.2 eV 3.2 eV F(Br-) The number of PSL is related to the number of e-/hole pairs created. Eu2+ Eu2+ Valence Band T.BONNET

  4. 1. Imaging Plates Why using IPs for the detection of Laser-Accelerated protons? • Good spatial resolution (50 µm): • angular distribution measurements • Focal plane detector of spectrometers or Thomson parabola • Good sensitivity: • Able to detect few protons or ions • Insensitive to high EM fields T.BONNET

  5. 2. Fading correction Fading of signal: loss of signal by spontaneous recombination of e-/hole pairs We define f(t) as the probability for an e-/hole pair not to recombine before the reading time. (high rate laser shot, radioactive source) (single laser shot) Short Irradiation Δt : Long Irradiation τ: Irradiation from t=-τ to t=0. IP is irradiated at t=0 and thereading time is t=tl. • Y is the number of photostimulated photons (PSL/s) induced by a source per second. • We can only measure: We measure: For radioactive sources Y is independent of the time f(t) can be extracted T.BONNET

  6. 2. Fading correction An example of determination of f(t) with a β-source of 90Sr (Emax=2.28MeV): f(t) τ = 1 minute MS χ(tl) (PSL) Reading time tl (min) Reading time tl (min) T.BONNET

  7. 2. Fading correction Fading function for different radioactive sources IPs are irradiated by g and e- at different energies. f(t) f(t) SR MS Reading time tl (min) • For one type of films, fading is independent of nature and energy of incident particle. • Fading is quicker for SR films. Reading time tl (min) T.BONNET

  8. 2. Fading correction In order to check the validity of the correction, we calculate Y versus the reading time. Validation of the correction Source : 90Sr τ = 1 minute SR MS Reading time Reading time Y = 35.44 ± 1.11 PSL/s Y = 81.87 ± 1.09 PSL/s • The correction is good : the variation of Y is less than 3%. T.BONNET

  9. 3. AIFIRA accelerator experiment and results State of the art : signal induced by a proton Experiments with laser accelerated protons: Mančićet al. Rev. Sci. Instr. 79 073301 (2008) Choi et al. Meas. Sci. Technol. 20 115112 (2009) PSL/proton BAS-TR and scanner BAS-5000 1.60 BAS-TR and scanner BAS-1800II 0,185 These results are difficult to interpret since the protocols and the scanners are different. We need measurements with our own scanner and monoenergetic protons (accelerator). T.BONNET

  10. 3. AIFIRA accelerator experiment and results Experiment on AIFIRA accelerator: Rutherford BackScattering (RBS) of Proton ranging from 600 keV to 3.5 MeV on Ta target • Proton Energy on IP is fixed by the diffusion angle: ΔE/E~1% • Number of protons on IP is measured by a 25 mm² diode • IP BAS-SR, BAS-MS • TR were not available at the time of the experiment • reading with a scanner FLA-7000 • An aluminium shield is used to avoid reflection in the chamber. • An inserter allows to extract quickly the film; The IP is read in the 5 minutes after irradiation. • An aluminium sheet covers part of the IP to measure background signal from photons. T.BONNET

  11. 3. AIFIRA accelerator experiment and results Experimental results : fading and measured signal per incident proton Fading is measured for 2 different proton energies: 650keV and 2.9 MeV Measurements corrected for fading: As previously seen, the fading correction is independent on the particle energy. A mean fading correction is calculated for each film. MS films are more sensitive. T.BONNET

  12. 4. Modelisation of the response function with Geant4 Signal on IPs can be defined as : Response Function : modelisation • Hypothesis : the response function is proportional to the deposited energy: • Then: measurements and simulations allow to get α: Can be calculated with a Geant4 Monte-Carlo simulation. Have been measured αis the luminescence efficiency in PSL/keV T.BONNET

  13. 4. Modelisation of the response function with Geant4 Determination of the α parameter with the monoenergetic proton data: SR The signal per proton is: Y(PSL/proton) α=1.60e-4 PSL/keV R=0.956 Calculated Etotdep(keV) The measured signal is not proportional to the calculated deposited energy. MS Y(PSL/proton) α=3.61e-4 PSL/keV R=0.985 Calculated Etotdep(keV) T.BONNET

  14. 4. Modelisation of the response function with Geant4 Determination of the L parameter with the monoenergetic proton data: Absorption of incoming and emitted photons in the sensitive layer. SR Sensitive layer Y(PSL/proton) α=2.31e-4 PSL/keV L=113 µm R=0.992 Excited centre Scanner’s photons Calculated Eeffdep(keV) Sensitive layer MS Y(PSL/proton) α=4.36e-4 PSL/keV L=211 µm R=0.994 Calculated Eeffdep(keV) The absorption length L is determined with minimisation techniques. T.BONNET

  15. 4. Modelisation of the response function with Geant4 By varying the source-IP distance we vary the energy of the alpha particles. Measurements with αparticles from 239Pu 239Pu 3.77 kBq SR Y(PSL/s) α AIR IP Source-IP distance(mm) MS Y(PSL/s) Source-IP distance(mm) T.BONNET

  16. 4. Modelisation of the response function with Geant4 Until now, we considered : Decreasing of the luminescence efficiency for highly ionising particles SR α=2.31e-4 PSL/keV L=113 µm kB=0.06 µm/keV Y(PSL/s) calculated measured But for highly ionising particles, we need to take into account quenching luminescence, empirical Birks’ law gives: Source-IP distance(mm) MS α=4.36e-4 PSL/keV L=211 µm kB=0.05 µm/keV Y(PSL/s) B is the density of damaged molecules k is the fraction which will not lead to luminescence kB is determined with minimisation techniques. Source-IP distance(mm) The alpha data is well reproduced with 3 parameters: α, L and kB. T.BONNET

  17. Response functions of IPs: to protons to alphas SR MS PSL/proton PSL/alpha Incident proton energy(MeV) Incident alpha energy(MeV) T.BONNET

  18. Conclusions • Fading : • is independent of the energy and the nature of incident particles. • can be corrected for short and long irradiations. • Measurements were carried out with proton accelerator: • We have data for IPs SR and MS for protons energies ranging from 600 keV to 3.5 MeV. • The modelisation of the IP response functions works well: • for protons with 2 parameters : α the luminescence efficiency and L an absorption coefficient • for alphas we add a 3rd parameter : kB to take into account the diminution of the luminescence efficiency for highly ionising particles • A new campaign is planned on July on AIFIRA to measure the response function of BAS-TR to protons and alphas. T.BONNET

  19. Thank you for your attention Feel free to ask questions. T.BONNET

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