A 2D scintillator-based proton detector for high repetition rate experiments Download: 670次
1 State of the art
The advent of high power lasers (HPL) working at high repetition rate (HRR) is nowadays a reality and HRR proton sources are now routinely produced with energies ranging from a few to tens of MeV. Laser-driven proton sources are characterized by a divergence that in several measurements has been proved to be related to the energy of the protons and the spatial distribution of the proton beam[1].
Laser-driven proton beams are becoming more and more important for applications in different fields of physics[2], chemistry and materials science[3], as well as biology, medicine[4] and cultural heritage[5]. For this, spatial and energy characterization of the proton beams nowadays plays an important role for the potential use of such sources. The first demonstration of laser-driven proton production was carried out in laser systems working in the single-shot mode, and one of the most used diagnostic consists of a series of radiochromic films (RCFs)[6] placed one after the other and able to recover the spatial distribution as a function of the proton energy[7]. Figure
Fig. 1. Sample of Gafchromic HD-V2 radiochromic films irradiated by protons at the Spanish Center for Pulsed Laser (CLPU).
The possibility of extending this technique to the HRR mode of operation is nowadays a challenge in the laser–plasma community, and several laboratories and research groups are working on this. The main idea is to substitute the active RCF layers with scintillator detectors capable of transforming the ion energy deposition into light that can then be collected by an optical CCD camera. Several research groups have proposed special online configurations to imitate the RCF stack but, up to now, only a partial extension of the RCF capabilities was possible. During 2011 and 2012, two research groups from the United Kingdom and from Germany proposed scintillator-based detectors. The group from Rutherford Appleton Laboratory (RAL)[9] proposed using a detector sensitive to three different wavelengths. The group from Dresden[10] proposed a stack of scintillators placed one after the other, as in the RCF stack, with a readout system looking at the transversal scintillation emission. Such devices have been tested in a proton beam accelerator and are currently used in the Dresden laboratory. Both detectors can partially reproduce the working mode of the RCF stacks even though they increase the complexity of the viewing system and the data interpretation.
2 Detector design
We present a scintillator-based detector able to measure both the proton energy and its transversal spatial distribution along the propagation axis and of being set at HRR. It consists of a series of scintillators placed similarly to an RCF stack (shown in Figure
Fig. 2. 2D top view of detector; the proton beam solid angle is parametrized by the internal half-angle $\unicode[STIX]{x1D703}$ , the detector dimension $D$ is represented by the length of the scintillator plate $L$ and the relative half-angle between the plates $\unicode[STIX]{x1D719}$ , and $n$ is the number of layers. $L_{0}$ is the distance between the proton source and the detector, $d$ is the longitudinal dimension of the scintillator plate, and $T_{1},\ldots ,T_{n}$ represent the projections of the proton beam solid angle for each plate.
To assess the system in detail we assume a proton beam propagating in a symmetric cone emission with half-angle
The projection of the proton emission cone in the scintillator plate can be written as
The working condition can be written as
2.1 Case $L_{0}=0$
The case
Fig. 3. Case $L_{0}=0$ . Plot of the number of layers $n_{0}$ versus the half-angle between the plates $\unicode[STIX]{x1D719}$ (according to Equation (5 )) for different divergence half-angles $\unicode[STIX]{x1D703}$ . When the curves $n_{0}$ ($\unicode[STIX]{x1D719}$ ) are above a given fixed $n_{0}$ value, the design of the detector is such that proton energies corresponding to the $n_{0}$ value are detectable. As an example, for a proton beam with a 40$^{\circ }$ divergence ($\unicode[STIX]{x1D703}=20^{\circ }$ ), six layers can work with a maximum angle $\unicode[STIX]{x1D719}\sim 13^{\circ }$ , while for eight layers $\unicode[STIX]{x1D719}\sim 10^{\circ }$ . It is important to note that the proton energy corresponding to the $n$ th layer depends on the thickness and composition of the layer.
Figure
2.2 Case $L_{0}\neq 0$
Assuming a proton divergence with an half-angle
Fig. 4. The number of scintillator layers $n$ is represented as a function of the scintillator foil size ($L$ ) for different values of $L_{0}$ and for fixed values of $\unicode[STIX]{x1D719}=12^{\circ }$ and $\unicode[STIX]{x1D703}=20^{\circ }$ . Different values of $L_{0}$ are plotted, representing the distance between the detector and the interaction point position. As an example (dashed line in the graph), a detector with $n=5$ layers needs to be built with a size $L$ greater than: ${\sim}1.5~\text{cm}$ ($L_{0}=0.5~\text{cm}$ ); ${\sim}3.5~\text{cm}$ ($L_{0}=1~\text{cm}$ ), ${\sim}6.5~\text{cm}$ ($L_{0}=2~\text{cm}$ ); ${\sim}12~\text{cm}$ ($L_{0}=4~\text{cm}$ ).
The result is that increasing
3 Implementation and preliminary calibration of the detector
A first detector prototype has been designed and constructed at the Spanish Center for Pulsed Laser (CLPU) in Salamanca and tested at the Centro de Micro-Análisis de Materiales (CMAM) of the Universidad Autónoma de Madrid, where a collimated proton beam up to 10 MeV is available for user access.
Fig. 5. Lateral view of the detector with a longitudinal dimension of the base between the first and the last plate of approximately 90 mm ($L_{10}=90~\text{mm}$ with $L_{0}=0$ ). The plates are separated from each other with a relative angle $\unicode[STIX]{x1D719}=12.5^{\circ }$ and have a dimension of $L=20~\text{mm}$ .
Figure
Fig. 6. Top view of the interaction chamber with the detector placed inside and the camera set outside the chamber to record the signal.
Table 1. BC-400 scintillator main properties.
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The detector was placed in the middle of the interaction chamber, on the front part of a 4-axis goniometer, able to rotate
Fig. 7. The top picture represents configuration 1, in which an odd number of scintillator plates are imaged by the camera. The bottom picture represents configuration 2 with the imaging of the pair scintillator plates.
Fig. 8. Top view of a Monte Carlo simulation (using the FLUKA code) of the proton energy deposited in the scintillator layers of the detector. The proton beam arrives with an incidence angle of $12.5^{\circ }$ on each plate. To facilitate the simulation procedure, the scintillator foils are placed parallel to each other; this configuration is totally equivalent to the original one and does not affect the general results. The colour scale represents the amount of energy lost by a 10 MeV proton beam in each scintillator. The $x$ axis and $y$ axis represent the respective spatial distribution of the deposited energy (plate thicknesses are not shown to scale for easier visualization).
Fig. 9. (a) Experimental signal obtained by the CCD camera during irradiation with a 10 MeV proton beam, with the colour scale giving pixel values artificially overlaid on a 3D representation of the detector. (b) Example of Monte Carlo simulation (obtained with FLUKA) representing the transversally integrated deposited energy per particle for each scintillator plate irradiated by a 10 MeV proton beam. (c) Response of the scintillator (light output) to 10 MeV proton beam irradiation. We can observe a peak of energy in layer 2 (in panels (b) and (c)) due the thickness difference between layers 2 and 3 ($180~\unicode[STIX]{x03BC}\text{m}$ against $140~\unicode[STIX]{x03BC}\text{m}$ ). Then, far from the Bragg peak, a proton will deposit more energy in layer 2 than in layer 3. Each plate ($n$ ) has a different thickness (due to uncontrollable variation during the fabrication process): $n_{1}=120~\unicode[STIX]{x03BC}\text{m}$ ; $n_{2}=180~\unicode[STIX]{x03BC}\text{m}$ ; $n_{3}=140~\unicode[STIX]{x03BC}\text{m}$ ; $n_{4}=160~\unicode[STIX]{x03BC}\text{m}$ ; $n_{5}=190~\unicode[STIX]{x03BC}\text{m}$ ; $n_{6}=130~\unicode[STIX]{x03BC}\text{m}$ ; $n_{7}=150~\unicode[STIX]{x03BC}\text{m}$ (lines in panels (b) and (c) are visual guides and not fits).
The original design of the detector uses two cameras looking from opposite sides. Due to the experimental constraints and since the proton source was very stable, two configurations of irradiation were used to image the full detector with the same camera. The odd plates with the numbers 1, 3, 5, 7 and 9 were pictured when the goniometer was in the normal position (rotation axis at
Figure
Considering a laser–ion acceleration experiment, the divergence of the particle beam has to be considered in the detector design. Indeed, increasing the total size of the detector will induce an increase of the final beam size, which, in combination with the multiple scattering, will entail a reduction of the spatial and energy resolutions. In addition, the last layers will receive a reduced number of protons per unit area, also reducing the detector sensitivity. The optimum detector design must be defined by prioritizing one parameter with respect to the other, even if a good general rule is to keep the length of the detector short to maintain a small beam size for a given number of layers. This can be done either by reducing the angle
4 Discussion and conclusion
A scintillator-based 2D ion detector for HRR experiments has been designed and built at CLPU, and tested using a proton accelerator at the CMAM in Madrid. The scintillator detector, to our knowledge, is a diagnostic device that looks very similar to an RCF stack diagnostic. Throughout the detailed analysis reported here, we have shown that it is possible to account for the laser-driven proton divergence by maintaining a compact size of detector. We have also shown that the detector can be implemented with an additional permanent magnet to remove most of the electron population, although the effect it has on the proton flux distribution at each energy will have to be mitigated.
Finally, the presented design of this 2D ion detector is promising for replacement of the classical RCF stack detector for the HRR mode of operation. It represents a new class of online detectors to support laser–plasma physics experiments in the newly emerging high power laser systems operating at HRR.
Article Outline
M. Huault, D. De Luis, J. I. Apiñaniz, M. De Marco, C. Salgado, N. Gordillo, C. Gutiérrez Neira, J. A. Pérez-Hernández, R. Fedosejevs, G. Gatti, L. Roso, L. Volpe. A 2D scintillator-based proton detector for high repetition rate experiments[J]. High Power Laser Science and Engineering, 2019, 7(4): 04000e60.