tbonnet

Report
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
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
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1. Imaging Plates
Presentation of IPs:
IPs are plastic films sensitive to ionising radiations.
Structure of an IP:
Layer
BAS-SR
BAS-MS
BAS-TR
Protective
6µm
9µm
No protective layer
Sensitive
BaFBr:Eu2+
120 μm
BaFBr0,85I0,15:Eu2+
115 μm
BaFBr0,85I0,15:Eu2+
50µm
Support
188µm
190µm
250µm
Magnetic
160µm
160µm
160µm
Level scheme of the sensitive layer:
Conduction Band
Ee-
35meV
2.5 eV
2.1 eV
Eu3+
Eu3+
3.2 eV
F(Br-)
Eu2+
Valence Band
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3.2 eV
Eu2+
•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
The number of PSL is related to the number of
e-/hole pairs created.
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1. Imaging Plates
Why using IPs for the detection of LaserAccelerated 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
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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.
Short Irradiation Δt :
(single laser shot)
IP is irradiated at t=0 and the reading
time is t=tl.
Long Irradiation τ:
(high rate laser shot,
radioactive source)
Irradiation from t=-τ to t=0.
•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
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f(t) can be extracted
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2. Fading correction
f(t)
χ(tl) (PSL)
An example of determination of f(t) with a βsource of 90Sr (Emax=2.28MeV):
τ = 1 minute
MS
Reading time tl (min)
Reading time tl (min)
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2. Fading correction
Fading function for different radioactive sources
f(t)
f(t)
IPs are irradiated by g and e- at different energies.
SR
MS
Reading time tl (min)
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.
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2. Fading correction
Validation of the correction
• In order to check the validity of the correction,
we calculate Y versus the reading time.
Source : 90Sr
τ = 1 minute
SR
Reading time
MS
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%.
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3. AIFIRA accelerator experiment and results
State of the art : signal induced by a proton
PSL/proton
Experiments with laser accelerated protons:
0,185
Mančić et al.
Choi et al.
Rev. Sci. Instr. 79 073301 (2008)
Meas. Sci. Technol. 20 115112 (2009)
BAS-TR and
scanner BAS1800II
1.60
BAS-TR and
scanner BAS5000
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).
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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.
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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
As previously seen, the fading correction is
independent on the particle energy.
A mean fading correction is calculated for
each film.
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Measurements corrected for
fading:
MS films are more sensitive.
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4. Modelisation of the response function with Geant4
Response Function : modelisation
• 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 :
Can be calculated with a Geant4
Monte-Carlo simulation.
Have been measured
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α is the luminescence efficiency in PSL/keV
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4. Modelisation of the response function with Geant4
Y(PSL/proton)
Determination of the α parameter with
the monoenergetic proton data:
The signal per proton is:
SR
α=1.60e-4 PSL/keV
R=0.956
Y(PSL/proton)
Calculated Etotdep(keV)
The measured signal is not
proportional to the calculated
deposited energy.
MS
α=3.61e-4 PSL/keV
R=0.985
Calculated Etotdep(keV)
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4. Modelisation of the response function with Geant4
Absorption of incoming and emitted
photons in the sensitive layer.
Excited
centre
Sensitive layer
The absorption length L is determined with
minimisation techniques.
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SR
α=2.31e-4 PSL/keV
L=113 µm
R=0.992
Calculated Eeffdep(keV)
Y(PSL/proton)
Scanner’s photons
Sensitive layer
Y(PSL/proton)
Determination of the L parameter with the
monoenergetic proton data:
MS
α=4.36e-4 PSL/keV
L=211 µm
R=0.994
Calculated Eeffdep(keV)
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4. Modelisation of the response function with Geant4
3.77 kBq
AIR
239Pu
Y(PSL/s)
Measurements with α particles from 239Pu
SR
α
IP
Y(PSL/s)
Source-IP distance(mm)
• By varying the source-IP
distance we vary the energy of
the alpha particles.
distance(mm)
Eα(MeV)
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1
6
11
16
21
5
4.5
3.9
3.3
2.75
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MS
Source-IP distance(mm)
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4. Modelisation of the response function with Geant4
Decreasing of the luminescence efficiency for highly
ionising particles
Y(PSL/s)
Until now, we considered :
SR
α=2.31e-4 PSL/keV
L=113 µm
kB=0.06 µm/keV
calculated
measured
But for highly ionising particles, we need to
take into account quenching luminescence,
empirical Birks’ law gives:
Y(PSL/s)
Source-IP distance(mm)
MS
α=4.36e-4 PSL/keV
L=211 µm
kB=0.05 µm/keV
B is the density of damaged molecules
k is the fraction which will not lead to
luminescence
kB is determined with minimisation
techniques.
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Source-IP distance(mm)
The alpha data is well reproduced
with 3 parameters: α, L and kB.
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Response functions of IPs:
Incident proton energy(MeV)
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to alphas
PSL/alpha
PSL/proton
to protons
SR
MS
Incident alpha energy(MeV)
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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.
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Thank you for your attention
Feel free to ask questions.
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