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

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

Presentation of IPs:

7 juin 2012 T.BONNET 3

Layer BAS-SR BAS-MS BAS-TR

Protective 6µm 9µm No protective layer

Sensitive BaFBr:Eu2+

120 μmBaFBr0,85I0,15:Eu2+

115 μmBaFBr0,85I0,15:Eu2+

50µm

Support 188µm 190µm 250µm

Magnetic 160µm 160µm 160µm

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.

Eu2+ Eu2+

Eu3+ Eu3+

F(Br-)

Ee-

3.2 eV 3.2 eV

2.5 eV 2.1 eV

35meV

The number of PSL is related to the number of e-/hole pairs created.

T.BONNET 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

1. Imaging Plates

Short Irradiation Δt : Long Irradiation τ:

7 juin 2012 5T.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=tl.

Irradiation from t=-τ to t=0.

We measure:


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)

Reading time tl (min)

f(t)


χ(t

l) (P

SL

)


τ = 1 minuteMS

An example of determination of f(t) with a β- source of 90Sr (Emax=2.28MeV):


Fading function for different radioactive sources



f(t)


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.

SRMS


Validation of the correction


Y = 35.44 ± 1.11 PSL/s Y = 81.87 ± 1.09 PSL/s

• In order to check the validity of the correction, we calculate Y versus the reading time.

Source : 90Srτ = 1 minute MSSR

• The correction is good : the variation of Y is less than 3%.


Reading time Reading time

State of the art : signal induced by a proton


Mančić et al. Rev. Sci. Instr. 79 073301 (2008)

BAS-TR and scanner BAS-1800II

BAS-TR and scanner BAS-5000

PS

L/p

roto

n

0,1851.60

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:

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.



Rutherford BackScattering (RBS) of Proton ranging from 600 keV to 3.5 MeV on Ta target

Experimental results : fading and measured signal per incident proton

Measurements corrected for fading:


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.


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 :

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

Determination of the α parameter with the monoenergetic proton data:


Y(P

SL

/pro

ton

)

Calculated Etotdep(keV)

Y(P

SL

/pro

ton

)

Calculated Etotdep(keV)

SR

MS

α=1.60e-4 PSL/keVR=0.956

α=3.61e-4 PSL/keVR=0.985

The measured signal is not proportional to the calculated deposited energy.


The signal per proton is:


Sensitive layerSca

nn

er’s

ph

oto

ns

Determination of the L parameter with the monoenergetic proton data:

The absorption length L is determined with minimisation techniques.


Absorption of incoming and emitted photons in the sensitive layer.

Sensitive layer

Excited centre

Y(P

SL

/pro

ton

)

Calculated Eeffdep(keV)

Y(P

SL

/pro

ton

)

Calculated Eeffdep(keV)

SR

MS

α=2.31e-4 PSL/keVL=113 µmR=0.992

α=4.36e-4 PSL/keVL=211 µmR=0.994

Measurements with α particles from 239Pu


• By varying the source-IP distance we vary the energy of the alpha particles.

Y(P

SL

/s)

Source-IP distance(mm)

Y(P

SL

/s)


SR

MS


239Pu 3.77 kBq

AIR α

IP

distance(mm)

1 6 11 16 21

Eα(MeV) 5 4.5 3.9 3.3 2.75

Decreasing of the luminescence efficiency for highly ionising particles


Until now, we considered :

But for highly ionising particles, we need to take into account quenching luminescence, empirical Birks’ law gives:

Y(P

SL

/s)


Y(P

SL

/s)


SR

MS

α=2.31e-4 PSL/keVL=113 µmkB=0.06 µm/keV

α=4.36e-4 PSL/keVL=211 µmkB=0.05 µm/keV

The alpha data is well reproduced with 3 parameters: α, L and kB.


B is the density of damaged moleculesk is the fraction which will not lead to luminescence

calculatedmeasured

kB is determined with minimisation techniques.

Response functions of IPs:


to protons to alphas

PS

L/p

roto

n

Incident proton energy(MeV)

PS

L/a

lph

a

Incident alpha energy(MeV)

SRMS

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.


Thank you for your attentionFeel free to ask questions.


Documents

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