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’Etudes Nucléaires de Bordeaux Gradignan

2. Outline 1.Imaging Plates (IPs) 2.Fading correction 3.AIFIRA accelerator experiment with protons and results 4.Modelisation of the response function to protons and alphas with Geant4 7 juin 2012 T.BONNET 2

3. Presentation of IPs: 7 juin 2012 T.BONNET 3 LayerBAS-SRBAS-MSBAS-TR Protective6µm9µmNo protective layer SensitiveBaFBr:Eu 2+ 120 μm BaFBr 0,85 I 0,15 :Eu 2+ 115 μm BaFBr 0,85 I 0,15 :Eu 2+ 50µm Support188µm190µm250µm Magnetic160µm Structure of an IP: Level scheme of the sensitive layer: Conduction Band Valence 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 1. Imaging Plates IPs are plastic films sensitive to ionising radiations. Eu 2+ Eu 3+ F(Br - ) E e- 3.2 eV 2.5 eV2.1 eV 35meV The number of PSL is related to the number of e - /hole pairs created.

4. 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 7 juin 2012 T.BONNET 4 1. Imaging Plates

5. Short Irradiation Δt : Long Irradiation τ: 7 juin 2012 5 T.BONNET Fading of signal: loss of signal by spontaneous recombination of e-/hole pairs Y is the number of photostimulated photons (PSL/s) induced by a source per second. We can only measure: IP is irradiated at t=0 and the reading time is t=t l. Irradiation from t=-τ to t=0. We measure: 2. Fading correction We define f(t) as the probability for an e-/hole pair not to recombine before the reading time. For radioactive sources Y is independent of the time f(t) can be extracted (single laser shot) (high rate laser shot, radioactive source)

6. Reading time t l (min) f(t) Reading time t l (min) χ(t l ) (PSL) 7 juin 2012 T.BONNET 6 τ = 1 minute MS An example of determination of f(t) with a β - source of 90 Sr (E max =2.28MeV): 2. Fading correction

7. Fading function for different radioactive sources 7 juin 2012 T.BONNET 7 Reading time t l (min) f(t) Reading time t l (min) f(t) For one type of films, fading is independent of nature and energy of incident particle. Fading is quicker for SR films. IPs are irradiated by g and e - at different energies. SR MS 2. Fading correction

8. Validation of the correction 7 juin 2012 T.BONNET 8 Y = 35.44 ± 1.11 PSL/sY = 81.87 ± 1.09 PSL/s In order to check the validity of the correction, we calculate Y versus the reading time. Source : 90 Sr τ = 1 minute MSSR The correction is good : the variation of Y is less than 3%. 2. Fading correction Reading time

9. State of the art : signal induced by a proton 7 juin 2012T.BONNET 9 Mančić et al. Rev. Sci. Instr. 79 073301 (2008) BAS-TR and scanner BAS- 1800II BAS-TR and scanner BAS- 5000 PSL/proton 0,185 1.60 Choi et al. Meas. Sci. Technol. 20 115112 (2009) 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). 3. AIFIRA accelerator experiment and results Experiments with laser accelerated protons:

10. Experiment on AIFIRA accelerator: 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. 7 juin 2012T.BONNET 10 3. AIFIRA accelerator experiment and results Rutherford BackScattering (RBS) of Proton ranging from 600 keV to 3.5 MeV on Ta target

11. Experimental results : fading and measured signal per incident proton Measurements corrected for fading: 7 juin 2012T.BONNET 11 Fading is measured for 2 different proton energies: 650keV and 2.9 MeV 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. 3. AIFIRA accelerator experiment and results

12. Response Function : modelisation 7 juin 2012 T.BONNET 12 Signal on IPs can be defined as : Hypothesis : the response function is proportional to the deposited energy: Then: measurements and simulations allow to get α : Have been measured Can be calculated with a Geant4 Monte-Carlo simulation. 4. Modelisation of the response function with Geant4 α is the luminescence efficiency in PSL/keV

13. Determination of the α parameter with the monoenergetic proton data: 7 juin 2012T.BONNET 13 Y(PSL/proton) Calculated E tot dep (keV) Y(PSL/proton) Calculated E tot dep (keV) SR MS α=1.60e -4 PSL/keV R=0.956 α=3.61e -4 PSL/keV R=0.985 The measured signal is not proportional to the calculated deposited energy. 4. Modelisation of the response function with Geant4 The signal per proton is:

14. 7 juin 2012T.BONNET 14 Sensitive layer Scanner’s photons Determination of the L parameter with the monoenergetic proton data: The absorption length L is determined with minimisation techniques. 4. Modelisation of the response function with Geant4 Absorption of incoming and emitted photons in the sensitive layer. Sensitive layer Excited centre Y(PSL/proton) Calculated E eff dep (keV) Y(PSL/proton) Calculated E eff dep (keV) SR MS α=2.31e -4 PSL/keV L=113 µm R=0.992 α=4.36e -4 PSL/keV L=211 µm R=0.994

15. Measurements with α particles from 239 Pu 7 juin 2012T.BONNET 15 By varying the source-IP distance we vary the energy of the alpha particles. Y(PSL/s) Source-IP distance(mm) Y(PSL/s) Source-IP distance(mm) SR MS 4. Modelisation of the response function with Geant4 239 Pu 3.77 kBq AIR α IP distance(mm) 16111621 E α (MeV)54.53.93.32.75

16. Decreasing of the luminescence efficiency for highly ionising particles 7 juin 2012T.BONNET 16 Until now, we considered : But for highly ionising particles, we need to take into account quenching luminescence, empirical Birks’ law gives: Y(PSL/s) Source-IP distance(mm) Y(PSL/s) Source-IP distance(mm) SR MS α=2.31e -4 PSL/keV L=113 µm kB=0.06 µm/keV α=4.36e -4 PSL/keV L=211 µm kB=0.05 µm/keV The alpha data is well reproduced with 3 parameters: α, L and kB. 4. Modelisation of the response function with Geant4 B is the density of damaged molecules k is the fraction which will not lead to luminescence calculated measured kB is determined with minimisation techniques.

17. Response functions of IPs: 7 juin 2012T.BONNET 17 to protonsto alphas PSL/proton Incident proton energy(MeV) PSL/alpha Incident alpha energy(MeV) SR MS

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 3 rd 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. 7 juin 2012 T.BONNET 18

19. Thank you for your attention Feel free to ask questions. 7 juin 2012T.BONNET 19

20. RBS 7 juin 2012 T.BONNET 20

21. Measurements to evaluate fading at 2 energies: Measurements with protons at 650 keV and 2.9 MeV Mean fading: 7 juin 2012 21 T.BONNET

22. 7 juin 2012T.BONNET 22 Y(PSL/proton) Calculated E eff dep (keV) Y(PSL/proton) Calculated E eff dep (keV) SR MS α=2.31e -4 PSL/keV L=113 µm R=0.992 α=4.36e -4 PSL/keV L=211 µm R=0.994 Determination of the L parameter with the proton data: The proton data is well reproduced with 2 parameters: α and L 4. Modelisation of the response function with Geant4 Sensitive layer Scanner’s photons Excited centre Absorption of incoming and emitted photons in the sensitive layer.